Xavier Flecher

Matthieu Ollivier
email: ollivier.matthieu@yahoo.fr

Pascal Maman

Sebastien Pesenti

Sebastien Parratte

Jean-Noël Argenson

Sébastien Pesenti

Sébastien Parratte

Long-term results of custom cementless-stem total hip arthroplasty performed in hip fusion

Keywords: Hip fusion, Hip arthrodesis, Total hip arthroplasty, Anatomy, Custom

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Fused-hip conversion to total hip arthroplasty (THA) provides reasonable results, improving function and reducing pain [START_REF] Jain | Arthrodesis of the hip and conversion to total hip arthroplasty: a systematic review[END_REF][START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Lustig | Total hip arthroplasty after hip arthrodesis performed for septic arthritis[END_REF]. However, the overall success and functional improvement following this procedure remain controversial compared with a conventional THA [START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Kilgus | Joint replacement for ankylosed hips[END_REF][START_REF] Fernandez-Fairen | Is total hip arthroplasty after hip arthrodesis as good as primary arthroplasty?[END_REF][START_REF] Richards | Conversion of hip arthrodesis to total hip arthroplasty: survivorship and clinical outcome[END_REF]. The procedure remains technically demanding and is associated with a high complication rate [START_REF] Kilgus | Joint replacement for ankylosed hips[END_REF][START_REF] Kreder | A population study in the province of Ontario of the complications after conversion of hip or knee arthrodesis to total joint replacement[END_REF][START_REF] Strathy | Total hip arthroplasty in the ankylosed hip. A ten-year follow-up[END_REF]. Previous studies analysed factors that affected post-operative outcomes [START_REF] Schuh | Results and experiences of conversion of hip arthrodesis[END_REF][START_REF] Morsi | Total hip arthroplasty for fused hips; planning and techniques[END_REF][START_REF] Reikerås | Total hip arthroplasty for arthrodesed hips. 5-to 13-year results[END_REF] and focused on the results of cemented femoral fixation, with survival rate without failure ranging from 74 to 96% at 10 years [START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Hamadouche | Total hip arthroplasty for the treatment of ankylosed hips : a five to twenty-one-year follow-up study[END_REF]. However, no study reported specifically results of cementless fixation in this specific indication. We previously reported the results of 3D, individually designed cementless stems for patient with abnormal anatomy and/or high functional demand [START_REF] Flecher | Custom cementless stem improves hip function in young patients at 15-year followup[END_REF][START_REF] Flecher | Threedimensional custom-designed cementless femoral stem for osteoarthritis secondary to congenital dislocation of the hip[END_REF][START_REF] Flecher | Cementless total hip arthroplasty using custom stem and reinforcement ring in hip osteoarthritis following developmental dysplasia[END_REF]. It was our hypothesis that custom cementless stems may be an interesting solution to address the particular anatomical features of fused hip. Therefore, we aimed to analyse:

1. Three-dimensional anatomy of the fused hip based on preoperative computed tomography (CT) scan 2. Functional outcomes based on Harris Hip Score (HHS) and Hip Disability and Osteoarthritis Outcome Score (HOOS)

Modifications of hip anatomy based on post-operative standardised radiography 4. Complication rate and long-term survivorship based on

Kaplan-Meier analysis

Materials and methods

After local ethic committee approval, we retrospectively identified in a prospectively collected database, 23 hips (23 patients; 13 men and ten women) who underwent conversion from fused hip to THA with a custom femoral implant [titanium (Ti) alloy, hydroxyapatite (HA)-coated stem; 3D CT scan based] between January 1992 and December 2005. Mean patient age was 49 ± 9 years (28-69) and body mass index (BMI) 25 ± kg/m 2 [START_REF] Postel | Replacement of a failed total hip prosthesis[END_REF][START_REF] Gruen | BModes of failure^ of cemented stem-type femoral components: a radiographic analysis of loosening[END_REF][START_REF] Engh | Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components[END_REF][START_REF] Brooker | Ectopic ossification following total hip replacement. Incidence and a method of classification[END_REF][START_REF] Bland | Survival probabilities (the Kaplan-Meier method)[END_REF][START_REF] Hardinge | Conversion of hip fusion to Charnley low-friction arthroplasty[END_REF][START_REF] Rajaratnam | Long term results of cementless total hip replacement for reversal of hip ankylosis[END_REF][START_REF] Peterson | Hip arthroplasty after previous arthrodesis[END_REF][START_REF] Kim | Total hip arthroplasty for the treatment of osseous ankylosed hips[END_REF][START_REF] Rittmeister | Hip and knee replacement after longstanding hip arthrodesis[END_REF][START_REF] Sirikonda | Role of hip arthrodesis in current practice: long term results following conversion to total hip arthroplasty[END_REF][START_REF] Schäfer | Total hip arthroplasty after arthrodesis of the hip joint[END_REF][START_REF] Villanueva | Conversion of arthrodesis to Total hip arthroplasty: clinical outcome, complications, and prognostic factors of 21 consecutive cases[END_REF][START_REF] Jauregui | Hip fusion takedown to a total hip arthroplasty-is it worth it? A systematic review[END_REF][START_REF] Besser | A muscle transfer to replace absent abductors in the conversion of a fused hip to a total hip arthroplasty[END_REF]. Mean time from fusion to THA was 32 ± 8 years (7-56). The initial indication for fusion included 13 infections (11 tuberculosis) with five spontaneous and eight surgical fusions, six post-traumatic, (of which three were surgical), three congenital dislocations of the hip (all surgical fusions), and one sequelae of Legg-Perthes-Calve disease. Thirteen patients (55%) had undergone at least one previous hip surgery. Six of those had retained hardware (four with screws between the former femoral head and acetabulum, and two with a plate). At the time of surgery, 13 patients (55%) presented with back pain: three with a fixed scoliosis, 15 (65%) with knee pain (nine ipsilateral, four bilateral, two contralateral) and four (17%) from contralateral hip pain. Before surgery, all patients underwent X-rays and CT scan for prosthesis design. The radiographic analysis included pelvic anteroposterior (AP) and lateral views of both hips and long-axis X-rays of both lower limbs. The AP view allowed the surgeon to choose frontal femoral offset targeting contralateral offset. In case of an abnormal contralateral hip, an abductor level ratio of 0.5 according to Amstutz [START_REF] Amstutz | Total joint replacement for ankylosed hips. Indications, technique, and preliminary results[END_REF] was set. Leg-length discrepancies were evaluated on long-axis X-rays to equalise leg length during THA. Pre-operative CT scan allowed 3D anatomy evaluation and implant design according to a previously validated and published method [START_REF] Flecher | Custom cementless stem improves hip function in young patients at 15-year followup[END_REF][START_REF] Flecher | Threedimensional custom-designed cementless femoral stem for osteoarthritis secondary to congenital dislocation of the hip[END_REF]. Two professional engineers analysed all CT scans using dedicated software, including artifact elimination from retained hardware that did not interfere with the process. Implant technical specifications were determined based on CT scan analysis to restore extramedullary parameters (lower-limb discrepancy, femoral offset, femoral neck version) and ensure primary implant stability.

All operations were performed through an anterolateral Watson-Jones approach with the patient in a supine position. A trochanteric osteotomy was performed when the preoperative CT scan showed a posterior location of the greater trochanter (related to an excessive version of the upper femur) or when tension of the abductor muscles was considered in-sufficient (nine cases; 37%). The same hemispheric Ti alloy

) uncemented
HA-coated cup (Hilock, Symbios®, Switzerland) was planned for all cases. Due to insufficient bone coverage, revision implants with a hook in the obturator foramen and superolateral screws were used in four cases (17%: Hilock rev, Symbios®, Switzerland). No bone grafting of the acetabulum was required. The medullary canal was prepared with a smooth, dedicated custom broach, which exactly mimics the shape of the final stem. Then, an uncemented custom Ti alloy HA-coated stem (Symbios®, Switzerland) was inserted, fitting the intramedullary proximal femoral anatomy and accommodating neck offset to the new centre of the joint for patient according to the 3D CT-based pre-operative planning [START_REF] Bland | Survival probabilities (the Kaplan-Meier method)[END_REF]. Figure 1a-e presents pre-operative radiograph/planning and postoperative imaging of a 45year-old female patient with left hip pain secondary to a tuberculosis-related hip fusion.

Post-operative analysis

All patients were evaluated at three months, one year, three years and every five years. Mean follow-up was 15 [START_REF] Schuh | Results and experiences of conversion of hip arthrodesis[END_REF][START_REF] Morsi | Total hip arthroplasty for fused hips; planning and techniques[END_REF][START_REF] Reikerås | Total hip arthroplasty for arthrodesed hips. 5-to 13-year results[END_REF][START_REF] Hamadouche | Total hip arthroplasty for the treatment of ankylosed hips : a five to twenty-one-year follow-up study[END_REF][START_REF] Flecher | Custom cementless stem improves hip function in young patients at 15-year followup[END_REF][START_REF] Flecher | Threedimensional custom-designed cementless femoral stem for osteoarthritis secondary to congenital dislocation of the hip[END_REF][START_REF] Flecher | Cementless total hip arthroplasty using custom stem and reinforcement ring in hip osteoarthritis following developmental dysplasia[END_REF][START_REF] Amstutz | Total joint replacement for ankylosed hips. Indications, technique, and preliminary results[END_REF][START_REF] Harris | Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An endresult study using a new method of result evaluation[END_REF][START_REF] Nilsdotter | Hip disability and osteoarthritis outcome score (HOOS)-validity and responsiveness in total hip replacement[END_REF][START_REF] Postel | Replacement of a failed total hip prosthesis[END_REF][START_REF] Gruen | BModes of failure^ of cemented stem-type femoral components: a radiographic analysis of loosening[END_REF][START_REF] Engh | Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components[END_REF][START_REF] Brooker | Ectopic ossification following total hip replacement. Incidence and a method of classification[END_REF] years. The patients were evaluated pre-operatively using HHS [START_REF] Harris | Traumatic arthritis of the hip after dislocation and acetabular fractures: treatment by mold arthroplasty. An endresult study using a new method of result evaluation[END_REF] and post-operatively using HHS and HOOS [START_REF] Nilsdotter | Hip disability and osteoarthritis outcome score (HOOS)-validity and responsiveness in total hip replacement[END_REF]. Range of motion (ROM) was evaluated with particular care. Abductor strength was evaluated using a standard scale for manual muscle testing (from 0 to 5 points) after conversion. Evaluation of impact of THA on back, knee or contralateral hip pain was recorded. Complications including dislocation, infection or nerve palsy were recorded.

Radiographic post-operative evaluation was performed on AP views of the hip and pelvis and on a true lateral view of the hip. The first post-operative radiograph was then used as a baseline from which subsequent radiographs were interpreted. Leg-length discrepancy was assessed according to Postel [START_REF] Postel | Replacement of a failed total hip prosthesis[END_REF]. Stem osseointegration was analysed according to the seven zones described by Gruen [START_REF] Gruen | BModes of failure^ of cemented stem-type femoral components: a radiographic analysis of loosening[END_REF] and the corresponding seven zones on the lateral radiograph. Progressive radiolucencies, or radiolucencies > 2 mm wide and signs of osteolysis were recorded. Femoral component stability was evaluated by the criteria of Engh et al. [START_REF] Engh | Roentgenographic assessment of the biologic fixation of porous-surfaced femoral components[END_REF]. We considered a stem loose if subsidence was > 2 mm or if the angular position of the stem shifted > 2°. Ossifications were analysed and classified ac-cording to Brooker classification [START_REF] Brooker | Ectopic ossification following total hip replacement. Incidence and a method of classification[END_REF].

Statistical analysis

We described parameters of interest using means and standard deviations (SD) or medians and ranges for continuous vari-ables (age, BMI, HHS, HOOS, postoperative leg lengths) and counts (%) for categorical variables (gender, side, satisfaction score, radiolucencies, osteolysis). The influence of time from fusion, previous surgery and demographic parameters on post-operative outcomes has been analysed in a multivariate model. We performed a 15-year survival analysis using the Kaplan-Meier technique [START_REF] Bland | Survival probabilities (the Kaplan-Meier method)[END_REF] with 95% confidence intervals (CI) considering revision for any reason or radiographic loosening as the end point. Analysis was performed using SPSS software (Version 12; SPSS Inc., Chicago, IL, USA). All calculations assumed two-tailed tests.

Results

Pre-operative radiographic and CT scan analysis

According to the radiographic analysis, the recommended lengthening to equalise limb length was 21 ± 12 mm (-5 to +50 mm). The necessary lateral correction (offset) ranged from -12 to 24 mm. Anteversion evaluated on CT scan averaged 17 ± 11° (-29° to 80°); therefore, the necessary correction in the prosthetic neck to restore the desired neck anteversion (15°) averaged -2 ± 10° (-65 to 46°). HHS improved from 59 (40-84) to 89 (75-100) points (p < 0.0001). HOOS results are presented in Fig. 2. Twenty-one patients (91%) were pain free, and three (9%) had occasional pain. Back pain decreased in eight patients (62% with pre-operative back pain). Knee pain decreased in eight patients (42% with preoperative knee pain) and increased in seven (40% with preoperative knee pain) that further required a knee arthroplasty. Conversely, contralateral hip pain decreased in only one of four (25%) patients with pre-operative contralateral hip pain. ROM analysis and post-operative hip function are presented in Table 1.

In our multivariate model, previous surgery and time from fusion did not influence ROM (respectively, p = 0.12 and p = 0.18) or post-operative limping (respectively, p = 0.12 and p = 0.18).

Post-operative radiographic analysis

The mean leg-length discrepancy was 7.8 ± 2.3 mm (0-12 mm). Eighteen patients (78%) had a discrepancy < 5 mm. The abductor lever arm ratio averaged 0.61 ± 0.12 (0.5-0.7), with all patients having a ratio > 0.5. Three ossifications (13%) were observed (two grade 1 and one grade 3) according to Brooker [START_REF] Brooker | Ectopic ossification following total hip replacement. Incidence and a method of classification[END_REF]. One hip (4%) exhibited osteolysis in zone 7 according to Gruen [START_REF] Gruen | BModes of failure^ of cemented stem-type femoral components: a radiographic analysis of loosening[END_REF]. No radiolucency was recorded.

Complications and survival

No intra-operative complications occurred. One early infection occurred and was treated by early lavage-debridement without implant revision. No dislocation occurred. One zirconia head fracture (4%) occurred and required liner and femoral head exchange with components retained. One aseptic stem loosening was observed at 11 years. The Kaplan-Meier

Discussion

While some authors described conversion of hip fusion to THA as a satisfactory procedure [START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Hardinge | Conversion of hip fusion to Charnley low-friction arthroplasty[END_REF][START_REF] Rajaratnam | Long term results of cementless total hip replacement for reversal of hip ankylosis[END_REF], others found that ROM, muscle strength, persistence of limp and need for assistive walking aids are less satisfactory than in primary conventional THA [START_REF] Richards | Conversion of hip arthrodesis to total hip arthroplasty: survivorship and clinical outcome[END_REF][START_REF] Reikerås | Total hip arthroplasty for arthrodesed hips. 5-to 13-year results[END_REF][START_REF] Peterson | Hip arthroplasty after previous arthrodesis[END_REF]. It was our hypothesis that custom cementless stems may be an interesting solution to address the particular anatomical features of fused hip, allowing uncemented fixation in extreme anatomical abnormalities. Limited information is available for the surgeon regarding the femoral anatomy of fused hips. However, it could be useful to recognise the femoral abnormalities to anticipate the type of components to use [START_REF] Morsi | Total hip arthroplasty for fused hips; planning and techniques[END_REF][START_REF] Kim | Total hip arthroplasty for the treatment of osseous ankylosed hips[END_REF]. Kim et al. [START_REF] Kim | Total hip arthroplasty for the treatment of osseous ankylosed hips[END_REF] reported the use of cementless THA in patients with limited anatomic deformities and relatively normal-sized acetabula, whereas cemented THA were performed in patients with more severe anatomical deformities and small femoral canals.

The effectiveness of THA after hip fusion on pain has been shown by several studies. Kim et al. [START_REF] Kim | Total hip arthroplasty for the treatment of osseous ankylosed hips[END_REF] stated that 81 of 86 patients (94%) had complete or nearly complete relief of back pain post-operatively and that 28 of 38 patients (74%) had complete or nearly complete relief of knee pain post-operatively. This corroborates with our results, since back pain decreased in 62% of patients and knee pain in 42%. Regarding pain and function in the surgically treated hip: the literature shows that 79-85% of hips were pain free or with minimal pain [START_REF] Jain | Arthrodesis of the hip and conversion to total hip arthroplasty: a systematic review[END_REF][START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Lustig | Total hip arthroplasty after hip arthrodesis performed for septic arthritis[END_REF]. In our study, pain relief was obtained for 91% of hips, with a possible role of optimal femoral offset, hip versions and leg-length restoration allowed by our individual prosthetic design. Overall function of the operated hip and walking possibilities improved after conversion [START_REF] Rajaratnam | Long term results of cementless total hip replacement for reversal of hip ankylosis[END_REF][START_REF] Rittmeister | Hip and knee replacement after longstanding hip arthrodesis[END_REF], reaching a global HHS comparable with primary THA for overall indications. Thus, if pain relief and overall function represent the main goals of this procedure, the satisfaction rate for patients undergoing those procedure are not perfect [START_REF] Jain | Arthrodesis of the hip and conversion to total hip arthroplasty: a systematic review[END_REF][START_REF] Richards | Conversion of hip arthrodesis to total hip arthroplasty: survivorship and clinical outcome[END_REF][START_REF] Strathy | Total hip arthroplasty in the ankylosed hip. A ten-year follow-up[END_REF][START_REF] Morsi | Total hip arthroplasty for fused hips; planning and techniques[END_REF][START_REF] Sirikonda | Role of hip arthrodesis in current practice: long term results following conversion to total hip arthroplasty[END_REF]. Amstutz and Sakai [START_REF] Amstutz | Total joint replacement for ankylosed hips. Indications, technique, and preliminary results[END_REF] in an earlier report noted that function restoration (ROM, leg length and limp correction) was worse than those of conventional primary THA. Richards et al. [START_REF] Richards | Conversion of hip arthrodesis to total hip arthroplasty: survivorship and clinical outcome[END_REF] confirmed lower satisfaction scores in their fused patients compared with more standard indications ROM may be significantly improved, reaching 79% good to excellent, or obtaining an average post-operative hip flexion between 70 and 102° [START_REF] Lustig | Total hip arthroplasty after hip arthrodesis performed for septic arthritis[END_REF][START_REF] Morsi | Total hip arthroplasty for fused hips; planning and techniques[END_REF][START_REF] Rajaratnam | Long term results of cementless total hip replacement for reversal of hip ankylosis[END_REF][START_REF] Sirikonda | Role of hip arthrodesis in current practice: long term results following conversion to total hip arthroplasty[END_REF]. In our study, flexion averaged 88°, with 79% of patients having flexion > 70°. No relationship was found between post-operative flexion and presence of previous surgeries or time from fusion. Postoperative strength of the abductor is probably multifactorial in this population: in a series of 15 patients, Schafer et al. [START_REF] Schäfer | Total hip arthroplasty after arthrodesis of the hip joint[END_REF] found the post-operative Trendelenburg sign to be moderate to severe in seven (47%). In our study, eight patients (12.5%) had a post-operative Trendelenburg sign, and strength of the abductor was graded as 5/5 or 4/5 in 21 patients (87.5%). In the study of Fernandez et al. [START_REF] Fernandez-Fairen | Is total hip arthroplasty after hip arthrodesis as good as primary arthroplasty?[END_REF], none was graded as 5/5, and 30 patients were graded 4/5 (62%). This was significantly different compared with a primary THA control group, who presented better abductor muscle strength (5/5 in ten patients, 4/5 in 23). Eighteen patients (37%) had a positive Trendelenburg sign in their study. We hypothesised that individual restoration of both femoral offset and anteversion, with respect to lower limb equalisation, may positively influence our good results on muscle strength.

In our series, pre-operative CT-scan-based implants allowed a cementless fixation in all cases; recommended lengthening ranged from -5 to +50 mm, lateral offset correction from -12 to 24 mm and the necessary correction in the prosthetic neck to restore the desired normal neck anteversion from -65 to 44°. Post-operative complications were not rare (26%) and included three heterotopic ossifications, one deep infection, one zirconium head fracture and one aseptic loosening. Conversely, the rate of intra-operative complication was very low; e.g., no intra-operative fracture was observed. This is in contradiction with recent literature results [START_REF] Villanueva | Conversion of arthrodesis to Total hip arthroplasty: clinical outcome, complications, and prognostic factors of 21 consecutive cases[END_REF][START_REF] Jauregui | Hip fusion takedown to a total hip arthroplasty-is it worth it? A systematic review[END_REF], and we hypothesise that the use of custom prothesis designed to fit perfectly with intramedullary anatomy may explain those differences.

Few data are available on mid-to long-term results of cementless THA for hip fusion in the literature, long-term results being mostly available with cemented techniques [START_REF] Besser | A muscle transfer to replace absent abductors in the conversion of a fused hip to a total hip arthroplasty[END_REF][START_REF] Sochart | Long-term results of total hip replacement in young patients who had ankylosing spondylitis. Eighteen to thirty-year results with survivorship analysis[END_REF]. Survival of fusion takedown THAs ranges from 74% to 96% at ten years [START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Richards | Conversion of hip arthrodesis to total hip arthroplasty: survivorship and clinical outcome[END_REF][START_REF] Morsi | Total hip arthroplasty for fused hips; planning and techniques[END_REF]] and 73% at 26 years [START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF]. Sochart and Porter [START_REF] Sochart | Long-term results of total hip replacement in young patients who had ankylosing spondylitis. Eighteen to thirty-year results with survivorship analysis[END_REF] reported good long-term results of Charnley total hip replacement in patients with spontaneous fusion by ankylosing spondylitis. In their series, the probability that both components would survive was 91% at ten years, 73% at 20 years, and 70% at 30 years [START_REF] Jauregui | Hip fusion takedown to a total hip arthroplasty-is it worth it? A systematic review[END_REF]. However, in two reports at 7- [START_REF] Kilgus | Joint replacement for ankylosed hips[END_REF] and ten year [START_REF] Strathy | Total hip arthroplasty in the ankylosed hip. A ten-year follow-up[END_REF] follow-up, the rates of failure were high, particularly in younger patients and in patients who had one or more previous operations. Considering only aseptic loosening as an endpoint, survival of this cementless custom stem was 95.6% (CI 92.4-98.8) at 15 years, comparing favourably with the literature [START_REF] Joshi | Conversion of a fused hip to total hip arthroplasty[END_REF][START_REF] Kilgus | Joint replacement for ankylosed hips[END_REF][START_REF] Richards | Conversion of hip arthrodesis to total hip arthroplasty: survivorship and clinical outcome[END_REF][START_REF] Schäfer | Total hip arthroplasty after arthrodesis of the hip joint[END_REF]. Our results were not influenced by the nature of arthrodesis, conversely to the conclusion of Strathy and Fitzgerald [START_REF] Strathy | Total hip arthroplasty in the ankylosed hip. A ten-year follow-up[END_REF] in 80 patients, at a mean ten year follow-up. With a failure rate of 26%, they concluded that spontaneous arthrodesis had a better prognosis for longevity of the arthroplasty and higher hip scores. They also found an inverse relationship between failure rate and duration of arthrodesis. Patient age at the time of arthrodesis was another factor that influenced the results of conversion to arthroplasty. Patients > 50 years presented a lower failure rate and a better survivorship. Their findings were partly corroborated by Peterson et al. [START_REF] Peterson | Hip arthroplasty after previous arthrodesis[END_REF]. Thus, it would appear that lower age and longer interval between arthrodesis and arthroplasty have a substantially negative impact on long-term implant survival. These results have not been corroborated by our study, potentially due to the limited number of patients.

The limitations of this study include lack of control group to directly compare functional results with those of patients operated upon for primary osteoarthritis (OA) or dysplastic development of the hip. Secondly, the retrospective design of our series exposed our results to methodological bias, such as evaluation bias. Thirdly, the number of patient is low; however, this series represents a 13-year continuous inclusion period of a high-volume institution without a patient lost to follow-up.

The results of our study supported our hypothesis that custom cementless stems may be an interesting solution to address the particular anatomical features of fused hip at the time of THA, with an excellent survival rate at 15 years.

Compliance with ethical standards

Fig. 1

 1 Fig. 1 a-e Pre-operative radiographs/planning, and post-operative imaging of a 45-year-old female patient with left hip pain secondary to a left tuberculosis-related hip fusion a Pre-operative pelvic anteroposterior (AP) view of a left fused hip scheduled for total hip arthroplasty (THA), b Pre-operative planning showing the distance to be respected intra-operatively to restore hip anatomy. c Superimposing the three computed tomography (CT) views of the osteotomy level

Fig. 2

 2 Fig. 2 Results of last follow-up Hip Disability and Osteoarthritis Outcome Score (HOOS) analysis

Flexion

  = 2) 2/5 4% (N = 1) 1/5 Limping 35% (N = 8) 13% (N = 3) Trendelemburg 21% (N = 5) pelvic obliquity survivorship with revision for any reason as the end point was 95.6% (CI 92.498.8) at 15 years.

Table 1

 1 Post-operative range of motion and muscular evaluation

	Post-operative evaluation

Conflict of interest Jean-Noel Argenson is educational consultant for Symbios.

All of the other authors have nothing to disclose regarding the present manuscript.


Matthieu Ollivier
email: ollivier.matthieu@yahoo.fr

Yassine Bulaïd

Christophe Jacquet

Sebastien Pesenti

Jean-Noel Argenson

Sebastien Parratte

Fixation augmentation using calcium-phosphate bone substitute improves outcomes of complex tibial plateau fractures. A matched, cohort study

Keywords: Tibial Plateau Fractures, Augmentation, Osteosynthesis, Phosphocalcic cement, Clinical outcomes, CT-Scan

Introduction Injectable cements have been developed to improve fixation's stability and thus obtain early return to adequate joint function. We aimed to compare post-operative radiographic and clinical outcomes of patients suffering from a complex tibial plateau fracture (TPF) fixed with calcium-phosphate bone substitutes (CPBS) augmentation to a matched group of patients with identical fracture pattern, treated with the same fixation's type, but augmented with bone grafting. Methods After local ethic committee approval, we retrospectively identified in a prospectively collected database, patients with complex comminuted metaphyseal and epiphysial bicondylar TPF (Schatzker type VI) admitted in our emergency department between January 2011 and December 2013. From those, 23 patients (14 males, 9 females) were treated with CPBS (Quickset-CP®, Graftys, Aix-en-Provence, France) fixation augmentation. Patients' mean age were 44.4 years. We then created a control group using a 1:1 matching process on gender, age, fracture pattern, and method of fixation. Patients were evaluated prospectively at 3, 6, and then every six months using radiographic (AP/ML views) and clinical criteria (knee osteoarthritis outcomes score (KOOS) and EuroQOL-5D). Results Articular step-off and variation of articular step-off were significantly lower in the CPBS groups (mean step-off 1.4 ± 1.9 (0.5-6.5 mm) and mean step-off Δ = 0.3 ± 0.4 (0.5-2.2 mm)) than in the control group (mean step-off 3.6 ± 2.1 (1-7.5 mm) and mean step-off Δ = 2.2 ± 2 (0.5-7 mm) p < 0.01). At last follow-up, patients of the control group presented a higher rate of step-off > 2 mm and step-off Δ > 2 mm (respectively, 56 and 35%) than patients of the CPBS group (26 and 9%). Odd ratio of, respectively, 3.6 (95% CI (1.08-12.7) and p = 0.03) and 5.6 (95% CI (1.04-30.1) and p = 0.03).At mean follow-up of 29 months, KOOS pain subscore was significantly better in patients of the CPBS group (85.3 ± 12.1) than in control patients (74.2 ± 10.4 and p = 0.03). Conclusion The present study demonstrates that calcium-phosphate bone substitute used as synthesis augmentation improves mid-term radiological outcomes of patients suffering from complex tibial plateau fracture. Series reporting outcomes from a larger number of patients and longer follow-up must confirm clinical benefits and safety of this method.

Introduction

Tibial plateau fractures (TPF) are complex intra-articular and metaphyseal lesions, accounting for 5-8% of all fractures of the lower leg [START_REF] Petersen | Fracture of the tibial head[END_REF]. To prevent post-traumatic arthritis and stiffness, ideal management of those fractures must provide both an anatomical restoration of the joint surfaces to protect articular cartilage and a rigid fixation to permit early rehabilitation [START_REF] Manidakis | Tibial plateau fractures: functional outcome and incidence of osteoarthritis in 125 cases[END_REF][START_REF] Dang | Chondrocyte apoptosis after simulated intraarticular fracture: a comparison of histologic detection methods[END_REF][START_REF] Rademakers | Operative treatment of 109 tibial plateau fractures: five-to 27year follow-up results[END_REF]. Various treatment modalities can be used for the management of comminuted and/or depressed TPF. Conventional plate fixation with a single incision requires invasive exposure of the fracture zone, potentially harmful to soft tissue and ) epiphyseal vascularization, thus compromising post-operative outcome [START_REF] Hannouche | The arterial vascularization of the lateral tibial condyle: anatomy and surgical applications[END_REF].

To reduce soft tissue complications and deep infections, external fixation have been advocated; however, this technique is associated with high rates of non-union and pin track infections [START_REF] Messina | The use of arthroscopy to assist reduction of depressed tibial plateau fractures[END_REF][START_REF] Siegler | Percutaneous fixation of tibial plateau fractures under arthroscopy: a medium term perspective[END_REF][START_REF] Biggi | Tibial plateau fractures: internal fixation with locking plates and the MIPO technique[END_REF]. Recent developments in the techniques of internal fixation, including development of locked plating with raft screws and minimal invasive techniques have substantially decreased the complication rates observed [START_REF] Young | Complications of internal fixation of tibial plateau fractures[END_REF][START_REF] Basques | Adverse events, length of stay, and readmission after surgery for tibial plateau fractures[END_REF][START_REF] Chen | Arthroscopy-assisted surgery for tibial plateau fractures[END_REF][START_REF] Belanger | Compartment syndrome of the leg after arthroscopic examination of a tibial plateau fracture. Case report and review of the literature[END_REF][START_REF] Dall'oca | Tibial plateau fractures: compared outcomes between ARIF and ORIF[END_REF]. However, secondary loss of reduction remains a thorny issue [START_REF] Simpson | Outcome of tibial plateau fractures managed with calcium phosphate cement[END_REF][START_REF] Weigel | High-energy fractures of the tibial plateau. Knee function after longer follow-up[END_REF][START_REF] Lachiewicz | Factors influencing the results of open reduction and internal fixation of tibial plateau fractures[END_REF][START_REF] Lefkoe | Remodeling of articular step-offs. Is osteoarthrosis dependent on defect size?[END_REF]. Injectable cements have been developed to improve fixation's stability and thus obtain early return to adequate joint function [START_REF] Simpson | Outcome of tibial plateau fractures managed with calcium phosphate cement[END_REF]. Several recent studies have demonstrated that calcium-phosphate bone substitutes (CPBS) has a superior capability than cancellous bone grafts to prevent articular subsidence [START_REF] Trenholm | Comparative fixation of tibial plateau fractures using alpha-BSM, a calcium phosphate cement, versus cancellous bone graft[END_REF][START_REF] Russell | Alpha-BSM Tibial Plateau Fracture Study Group (2008) Comparison of autogenous bone graft and endothermic calcium phosphate cement for defect augmentation in tibial plateau fractures. A multicenter, prospective, randomized study[END_REF][START_REF] Bajammal | The use of calcium phosphate bone cement in fracture treatment. A metaanalysis of randomized trials[END_REF][START_REF] Goff | Use of bone graft substitutes in the management of tibial plateau fractures[END_REF]. We hypothesized that the use of CPBS instead of bone grafting in complex TPF would prevent from articular subsidence.

In this study, we aimed to compare post-operative radiographic and clinical outcomes of patients, suffering from a complex TPF (Schatzker VI), fixed with CPBS augmentation to a matched group of patients with identical fracture pattern, treated with the same fixation's type, but augmented with bone grafting.

Patients and methods

After local ethic committee approval, we retrospectively identified in a prospectively collected database, patients with complex comminuted metaphyseal and epiphysial bicondylar TPF (Schatzker type VI, AO/OTA type 41 C2 and C3) admitted in our emergency department between January 2011 and December 2013. For this study, inclusion criteria were patients age between 18 and 85 years, presenting an acute, closed, bicondylar metaphyseal, and epiphysial multi-fragmentary (Schatzker type VI, AO/OTA type 41 C2 and C3) TPF. The exclusion criteria were skeletally immature patients, pathological fractures, and significant pre-existing degenerative joint disease, severe systemic illness that contraindicated surgery or a neurological condition that would interfere with rehabilitation. From the 67 patients meeting both inclusion and exclusion criteria, we then identified 23 TPF treated with a CPBS (injectable resorbable Bone substitute, Quickset-CP®, Graftys, Aix en provence, France) fixation augmentation. Patients mean age was 44.4 ± 10.7 years (18-71), 14 of them being males and nine females.

In the same database, a control group of patients were selected for having similar inclusion criteria but a different osteosynthesis augmentation based on allogeneic or autogeneic bone grafting. Those control patients were then included following a 1:1 matching process on gender, age (±5 years), fracture pattern (AO classification), and type of fixation (dual plates or plate + screws).

A tourniquet was routinely used during synthesis proce-dures; all patients were operated-on in a supine position, with the injured leg in semiflexion. Twenty-one patients had surgery performed within 48 h. A staged protocol for soft tissue man-agement was used in the other 25 patients; the fracture was initially stabilized using transcalcaneal traction device and con-verted to open reduction and internal fixation (ORIF) after the skin wrinkle sign was observed as described by Egol et al. [START_REF] Egol | Staged management of high-energy proximal tibia fractures (OTA types 41): the results of a prospective, standardized protocol[END_REF]. In both groups, all surgeries consisted of the following standardized surgical steps in each case: (1) As recommended by Freeman et al. [START_REF] Freeman | The movement of the normal tibio-femoral joint[END_REF], medial column restoration was performed first with either a medial incision starting on posteromedial border of the tibial metaphysis or a more central incision cen-tered on the tibial tuberosity. (2) Reduction of the fragment and temporary fixation was done using K-wires. [START_REF] Dang | Chondrocyte apoptosis after simulated intraarticular fracture: a comparison of histologic detection methods[END_REF] The surgeon decided either to fix the fracture using a locking plate (LCP® Depuy-synthes Inc., Warsaw, IN, USA or Perilocking® Zimmer Inc., Warsaw, IN, USA) or screws depending on the fracture pattern. (4) An anterolateral approach (through either a second skin incision or the same central incision) of the lateral articular surface was then performed including a transverse submeniscal arthrotomy allowing direct visualization of the articular surface's reduction. (5) Depressed fragments were elevated and supported with a compression clamp or tempo-rary K-wires to obtain anatomic reduction. [START_REF] Messina | The use of arthroscopy to assist reduction of depressed tibial plateau fractures[END_REF] As on the me-dial side and depending on the fracture pattern, a locking plate (LCP® Depuy-synthes Inc., Warsaw, IN, USA) or Perilocking® Zimmer Inc., Warsaw, IN, USA) or cannulated cancellous bone screws were used. [START_REF] Siegler | Percutaneous fixation of tibial plateau fractures under arthroscopy: a medium term perspective[END_REF] Meniscus, cruciate lig-aments, and tibial spines injuries were repaired when possible. (8) Augmentation of the fixation was performed using 16 cm 3 of CPBS or morcelized bone allograft or autograft to fill the metaphyseal defect. (9) Finally, watertight closures were per-formed in layers with drain aloof from the grafted zone. Fluoroscopy was used at each step to assess reduction.

All patients had a similar postoperative regimen: the post-operative mobilization scheme included toe-touch weight bearing using two crutches for four to six weeks, followed by progressive increase to obtain a full-weight bearing at three months; active knee motion was encouraged on the third day after surgery. Thromboprophylaxis (low molecular weight heparin (tinzaparin 4.500 IU)) was administered in subcutane-ous route post-operatively until full -eight bearing was authorized. The first injection was performed at the hospital the day after the operation and then by a nurse at home or in the rehabilitation facility (Figs. 1, 2, 3, 4, 5 and 6).

The data collection pre-operatively included demographics, mechanism of injury, initial fracture pattern, time to surgery, and method of fixation (Table 1).

Patients were evaluated prospectively at three, six and then every six months. Radiographic outcomes were assessed using weight-bearing (when possible) anterior-posterior and lateral radiographs. Clinical union was defined as painless full-weight bearing and radiological union as the presence of bridging callous of two cortices visible on two x-ray views (evidence of presence of bone healing by direct or indirect means in at least two planes on x-ray). Potential articular step(s)-off, medial proximal tibial angle (MPTA), posterior proximal tibial angle (PPTA), tibial plateau widening (TPW), existence of post-traumatic arthritis, and structural void filler support were noticed and summarized using Heyney-Redfern (H-R) scale at last available follow-up [START_REF] Heiney | Subjective and novel objective radiographic evaluation of inflatable bone tamp treatment of articular calcaneus, tibial plateau, tibial pilon and distal radius fractures[END_REF]. Patients were also evaluated clinically in order to identify treatment complication and appreciate range of motion, Fig. 2 Clinical case: 40-year-old patient. Closed Schatzker 6 TPF. Preoperative CT SCAN rehabilitation using the knee osteoarthritis outcomes score (KOOS) and EuroQOL-5D [START_REF] Ornetti | Cross-cultural adaptation and validation of the French version of the knee injury and osteoarthritis outcome score (KOOS) in knee osteoarthritis patients[END_REF][START_REF] Giannoudis | Long-term quality of life in trauma patients following the full spectrum of tibial injury (fasciotomy, closed fracture, grade IIIB/IIIC open fracture and amputation)[END_REF].

This study sample size (min 20 patients/groups) was designed to detect a 2-mm difference (reported standard deviation 2.5 mm) in terms of articular step-off between groups with alpha = 0.05 and 1-b = 0.8.

Normality of distributions was tested using Kolmogoroff-Smirnoff test; parametric tests were used to compare normally distributed parameters (paired student t test: demographic parameters, radiological outcomes); Wilcoxon tests were used to compare clinical scores; and Fisher exact tests were used to compare categorical parameters. All statistical analysis assumed two-tailed test, PASW Statistics version 20 (SPSS, IBM Inc., Chicago, Illinois) was used. The threshold for statistical significance was set at p < 0.05.

Results

No patient was lost to follow-up at mean follow-up 29.3 ± 11.2 months (12-44).

Radiographic outcomes

Post-operatively and at last follow-up, we found no difference between groups in terms of MPTA, PPTA, structural void 2 and3). However, articular step-off and variation (post-operative to last follow-up) of articular step-off were significantly lower in the CPBS groups (mean step-off 1.4 ± 1.9 (0.5-6.5 mm) and mean step-off Δ = 0.3 ± 0.4 (0.5-2.2 mm)) than in the control group ((mean step-off 3.6 ± 2.1 (1-7.5 mm) and mean step-off Δ = 2.2 ± 2 (0.5-7 mm) and both p < 0.01).

The mean H-R score was 12.3 ± 3.8 points in the CPBS group at the time of the last radiographic evaluation.

At last follow-up, patients of the control group presented a higher rate of step-off > 2 mm (56%) than patients of the CPBS group (26%), odd ratio 3.6 (95% CI (1.08-12.7) and p = 0.03).

At last follow-up, patients of the control group presented a higher rate of step-off delta > 2 mm (35%) than patients of the CPBS group (9%), Odd ratio 5.6 (95% CI (1.04-30.1) and p = 0.03).

Clinical outcomes

At mean follow-up 29 months, KOOS symptoms, activity of daily living, sports/recreational activities, and quality of live subscores as well as EuroQOL-5D evaluations were similar in both groups (Table 4). We found significant difference between groups in terms of KOOS pain subscore, as patients of the CPBS group presented lower pain score (85.3 ± 12.1 range (50-100)) than control patients (74.2 ± 10.4 (45-100), p = 0.03). All of the patients with ongoing professional activities before the trauma (34 patients) have resumed their professional activities at least one year after surgery.

Complication

We found no difference in terms of complications rate (CR) between CPBS group (CR = 22%; 5/23) and control group (CR = 17%; 4/23) (p = 0.4). Superficial wound infection occurred in five cases (three in CPBS group and two in control group, all of them were classified as Tscherne grade 2 or more), all patients were managed and healed with a course of oral antibiotics for a week. Deep infection occurred in two patients; the first case was a 40-year-old male of the CPBS group with persistent wound drainage 15 days after trauma which was treated with debridement (a methy-S staph. Aureus was found), and antibiotic course for six weeks (bone union was achieved radiologically and clinically at 5 months). The second patient was a 51-year-old active smoker, female, that presented a non-union and wound complication six months after trauma due to the deep infection (bone biopsy revealed a Methy-S staph. Aureus deep infection); she was treated with debridement, plate removal, and external fixator for six weeks, then treated as a septic bone non-union. We found two cases of thromboembolic complications in this series (one in each group, both were deep vein thrombosis that required curative treatment based on low molecular weight heparin (tinzaparin 10.000 IU).

Discussion

The main finding of this study is that TPF treated with ORIF and CPBS augmentation result in better radiological outcomes than TPF operated-on with the same synthesis but augmented with bone grafting. Several limitations can be outlined in our study. First, the retrospective design of the series limits extrapolation of our results, but the prevalence of Schatzker VI tibial fracture in the general population also limits a prospective data collection with a sufficient number of patients. Our sample size allowed us to compare radiological parameters between groups but might be not enough to evaluate clinical outcomes, morbidity, and revision rate. We performed a secondary power analysis, and our sample size was sufficient to detect the minimum clinically important differences regarding KOOS and EuroQOL scoring system. Moreover, we did not randomize fixation and augmentation process, but retrospectively matched patient based on surgeons' decision during surgery. Thus, patients were selected to receive one-or two-stage operative fixation based on the severity of their soft tissue presentation more than from a protocol decision. Complex TPF management is quite impossible to systematize due to of their different clinical and radiological presentations.

Concerning the modality of bone grafting in the control group, our extensive use of allogeneic bone graft to fill bone loss in knee revision have led some surgeons in our institution to propose it as an alternative to autologous bone graft in case of complex bone trauma as presented in this study. This duality led to potential bias due to two different types of graft in the control group.

We intended to evaluate post-operative outcomes of patients suffering from Schatzker VI tibial fracture, depending on the type of Bfixation augmentation^ techniques (CPBS and bone grafting). Regarding radiological outcomes, we found that mean articular step-off was significantly lower, at a mean follow-up of 29 months, in the CPBS groups than in the control group. Some authors have shown that resorbable calciumphosphate cement provides more support of the articular surface than does cancellous bone grafting. Two cadaveric studies demonstrated that in Schatzker type-II fracture, the rate of displacement was 68% lower for subchondral defects filled with calcium-phosphate cement than for those filled with cancellous bone graft. In an animal model, Welch et al. [START_REF] Welch | Experimental tibial plateau fractures augmented with calcium phosphate cement or autologous bone graft[END_REF] also concluded that collapse and resorption of the autogenous graft material occurred almost immediately in the post-operative period, conducing to an articular collapse and a fixation failure. The collapse was significantly lower with calciumphosphate cement compared to cancellous bone grafting (p < 0.05). Our results are also consistent with recent literature comparing these two types of augmentations [START_REF] Simpson | Outcome of tibial plateau fractures managed with calcium phosphate cement[END_REF][START_REF] Russell | Alpha-BSM Tibial Plateau Fracture Study Group (2008) Comparison of autogenous bone graft and endothermic calcium phosphate cement for defect augmentation in tibial plateau fractures. A multicenter, prospective, randomized study[END_REF] these two authors studied all type of fractures [START_REF] Simpson | Outcome of tibial plateau fractures managed with calcium phosphate cement[END_REF][START_REF] Russell | Alpha-BSM Tibial Plateau Fracture Study Group (2008) Comparison of autogenous bone graft and endothermic calcium phosphate cement for defect augmentation in tibial plateau fractures. A multicenter, prospective, randomized study[END_REF] of which only a few complex TPF (n = 2 for Russel et al. [START_REF] Russell | Alpha-BSM Tibial Plateau Fracture Study Group (2008) Comparison of autogenous bone graft and endothermic calcium phosphate cement for defect augmentation in tibial plateau fractures. A multicenter, prospective, randomized study[END_REF], n = 0 for Simpson et al. [START_REF] Simpson | Outcome of tibial plateau fractures managed with calcium phosphate cement[END_REF]). Our follow-up was 29 months while the mean follow-up of these two studies was one year.

With the number available, we did not find any difference between groups regarding clinical outcomes (except for pain subscore of the KOOS), these results are consistent with the literature, as no study demonstrates clinical superiority of resorbable cement on autologous bone grafting. However, diversity of scoring systems used by other authors complicates comparison of our clinical outcomes [START_REF] Bajammal | The use of calcium phosphate bone cement in fracture treatment. A metaanalysis of randomized trials[END_REF][START_REF] Goff | Use of bone graft substitutes in the management of tibial plateau fractures[END_REF]. The results of these studies are summarized in Table 5.

The total complication rates were similar between groups in our retrospective evaluation; high rate of complications have been described based on initial soft tissue damage and fixation technique. Deep wound infection is often observed in patients with high energy trauma: with dual plates through two incisions in the series of Jiang et al., an incidence of 4.7% deep infection was reported [START_REF] Jiang | A comparative study of less invasive stabilization system (LISS) fixation and two-incision double plating for the treatment of bicondylar tibial plateau fractures[END_REF]. Despite using staged management with a temporary traction wire in high-risk patients, we found seven sepsis in our series: five superficial and two deep infections.

The present study demonstrates that calcium-phosphate bone substitute used as synthesis augmentation improves midterms radiological outcomes of patients suffering from complex TPF. Series reporting outcomes from a larger number of patients and longer follow-up must confirm clinical benefits and safety of this method as compared to autologous bone grafting.

Compliance with ethical standards

This article does not contain any studies with human participants performed by any of the authors. Informed consent was obtained from all individual participants included in the study.

)Fig. 1

 1 Fig. 1 shoudl appear at the end of methods part. thanks... Clinical case: 40-year-old patient. Closed Schatzker 6 TPF. Pre-operative x-rays

Fig. 3

 3 Fig. 3 Clinical case: 40-year-old patient. Closed Schatzker 6 TPF. Preoperative CT SCAN: 3D reconstructions

Fig. 4

 4 Fig. 4 Clinical case: 40-year-old patient. Closed Schatzker 6 TPF. Postoperative x-rays

Fig. 5 Fig. 6

 56 Fig. 5 Clinical case: 40-year-old patient. Closed Schatzker 6 TPF. CT SCAN at last follow-up

Table 1

 1

	Pre-operative patients' demographics, fracture patterns,	Parameters		CBPS group (n = 23)	Control group (n = 23)	p value
	and procedure details
		Age (years)		43.9 ± 10.3 (21-72)	44.8 ± 11.3 (18-71)	0.7
		Gender (M/F)		14/9	14/9	NA
		Fracture pattern	AO 41 C2	17	17	NA
			AO 41 C3	6	6
		Tscherne classification	0	2	1	0.5
			1	10	12
			2	8	8
			3	3	2
		Mechanism of injury		Sport: 9	Sport: 7	0.5
				Vehicle collision: 13	Vehicle collision: 16
				Fall: 1
		Time to surgery (days)		4.7 ± 3.9 (0-11)	3.9 ± 4.6 (0-14)	0.3
		Patients with time to		10	11
		surgery < 48 h
		Fixation				0.8
		Dual plating		14	13
		Plate + screws		9	10
		Augmentation type		CBPS 23	Bone allograft 10	NA
					Bone autograft 13

)

Table 2

 2

	post-operative radiographic analysis	Parameters	CBPS	Control	p value
		Articular step-off (mm)	1.1 ± 1.2 (0.5-6)	1.4 ± 1.5 (0.5-5)	0.7*
		Number	18	17	0.2 #
		<2 mm	4	6
		2 -5 mm	1
		>5 mm
		MPTA (°)	87.2 ± 7.1 (78-92)	86.4 ± 9.1 (77-92)	0.6*
		Number	17	16	0.8 #
		85°-90°	5	6
		80°-84° or 91°-94°	1	1
		< 80° or > 94°
		PPTA (°)	7.1 ± 4.3 (1-10)	7.6 ± 3.7 (3-10)	0.4 *
		Number
		6°-12°	15	17	0.7 #
		3°-5° or 13°-15°	7	6
		< 3° or > 15°	1
		Structural void support
		Number
		Sufficient	19	15	0.2 #
		Partial	4	7
		Insufficient		1
		Time to union (months)	5.2 ± 1.1 (4-7)	4.8 ± 1.5 (3-7)	0.6

*

Articular step-off: If more than one, due to fragmentation, the biggest one is reported here MPTA medial proximal tibial angle, PPTA posterior proximal tibial angle * Paired student t test # Exact 2 × 3 Fisher Articular step-off: if more than one, due to fragmentation, the biggest one is reported here Tibial plateau widening: variation of the distance measured between two parallel lines (both perpendicular to the join line) that pass from the most lateral and most medial part of the tibial plateau MPTA medial proximal tibial angle, PPTA posterior proximal tibial angle *Paired student t test # Exact 2 × 3 Fisher For EuroQOL-5D, numbers (%) reported are number of patients that report moderate to extreme issues regarding the parameters KOOS knee injury and osteoarthritis outcome score, ADL activity of daily living, QOL quality of life )

  . Our results are poorer regarding the rate of articular step-off > 2 mm, as compared to the Russel et al. and Simpson et al. study. However, we included only Schatzker 6 fractures while

Table 5

 5 Results of different series from the literature

	Studies	Series	Mean Age (range) FU	Type of fracture	Radiographic analysis	Infection rate
	Simpson et al.[14]	26 patients:	51.5 (21-71)	12 months AO:	Mean step-off = 4	0%
		-13 BG			B2.2 = 3, B3.1 = 10	vs 0.7 mm
		-13 CBPS (SRS)
	Trenholm et al.[18]	20 specimens:			Schatzker:	Mean step-off:
		-10 autogreffes			I = 1, II = 52,	3.8 vs 1.2 mm
		-10 CBPS (BSM)			III = 26, IV = 11,
					V = 28, VI = 2
	Russel et al. [19]	119 patients:	43	12 months		Step-off > 2 mm:	1.2%
		-38 BG				30 vs 9%
		-82 CBPS
	Bajammal et al.	455 patients	N/A		Schatzker:	Reduction failure:	n/a
	(meta-analysis) [20]				I = 1, II = 52,	CPBS vs BG RR 0.7
					III = 26, IV = 11,
					V = 28, VI = 2
	Goff et al.	672 patients	50 (15-89)			Step-off > 2 mm:	Stratisfied by
	(meta-analysis) [21]					-8.6% bone substitute	graft type:
						-5.4% HA	-3.4% bone
						-3.7% CPBS	substitute
						-11% calcium sulfate	-5.4% HA
							-2.9% CPBS
							-5.4% calcium
							sulfate
	Our study	46 patients:	44.4 (18-71)	29 months Schatzker VI = 46	Step-off > 2 mm:	15%
		-23 BG				56% BG
		-23 CPBS				26% CBPS

Conflict of interest

The authors declare that they have no competing interests.


Marc Jarry

Laurent Bruxelles

Grégory Dandurand

Dominic Justin Stratford

Benjamin Lans

Francis Duranthon

Archaeological surveys in Aha Hills (NE Namibia): first results

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.


Nicolas Serres
email: nicolas.serres@insa-strasbourg.fr

Huan He
email: huan.he@insa-strasbourg.fr

Thierry Meylheuc
email: thierry.meylheuc@jouy.inra.fr

Françoise Feugeas
email: francoise.feugeas@insa-strasbourg.fr

Influence of a bioadmixture on standardized parameters of cementitious materials

Keywords: biosurfactant, porosity, surface roughness, slump loss, mechanical strength

A lipopeptide bioadmixture was used to modify the surface bio-contamination of concrete and their durability. Experimental investigations were intended to establish the influence of this bioproduct regarding different properties of mortar and/or cement paste samples. Standardized tests, such as slump test, flexural and compressive strength measurements as well as porosity determination, aim to confirm the ability of this bioproduct to be used as an admixture in cementitious materials. The bioadmixture introduced an effect on the workability of fresh samples with a plasticizing action. The bioproduct can improve the mechanical strength of Portand cement mortars depending on its content in samples and allows to reach resistances consistent with the standard strength class of samples. If the bioadmixture does not influence the total porosity available to water of the cement pastes, the mercury injection porosimetry has revealed the existence of a threshold from which the porous network of samples changes. The evolution of the surface roughness is also impacted by the bioadmixture, due to both curing time and interactions of the surface with water. The bioadmixture inhibits the development and the formation of crystals on the sample surfaces and modifies hydric transfers within the porous network of the samples.

Introduction

This work is motivated by environmental considerations in purpose to develop environmentally friendly admixtures, which aim to replace products subjected to REACH authorization, in order to develop eco-friendlier concrete. Some bacterial species secrete extra-cellular substances, in the aim of protection especially when their environment becomes aggressive and to increase their adhesion on material surface by constituting biofilms [1]. Many bacteria are able to produce amphipathic substances i.e. 'biosurfactants', which are potentially interesting compounds for many fields of study, due to having both hydrophilic and hydrophobic moieties in the same molecule [2]. Nature and properties of biosurfactants produced by microorganisms depend on the type of microorganism, tested strain, available nutrients and culture conditions. Several studies have demonstrated antimicrobial activity for some of them, such as lipopeptides (surfactin, iturin, etc.) produced by Bacillus and Pseudomonas [3] or amphisin compounds which possess strong antifungal activity [4]. In natural ecosystems, these biomolecules are often involved in the biological control of many plant pathogens [5] acting as an antagonism mechanism for controlling plant pathogenic germs (zoospores) by creating ducts in cytoplasmic membrane of the infectious germ [6]. Biosurfactants also have the advantage of being the most efficient and stable (Bacillus subtilis [7]) in a wide range of environmental conditions (pH, temperature, salinity). Some are able to maintain their surface activity at temperatures up to 90°C [8] or salinity up to five times higher than those tolerated by chemical surfactants [START_REF] Donaldson | SPE Annual Technical Conference and Exhibition[END_REF]. The emulsifying properties of surfactants synthesized by Serratia marcescens are also preserved in temperatures in the range of 10 -120°C and pH values ranging from 2 to 12 [START_REF] Pruthi | [END_REF]. In addition, unlike most chemical surfactants, biosurfactants exhibit better biodegradability and lower toxicity [11].

The biosurfactants are already used in many applications in different industrials fields, such as cosmetic, pharmaceutical, food, ink, paint or coating [12]. However, according to the authors' best knowledge, there is no current application as admixture for cementitious materials, with an incorporation as mass product in the concrete formulation, using a biosurfactant. A lipopeptide biosurfactant (viscosinamide) derived from Pseudomonas fluorescens was used in this work. The active principle of this bioproduct consists of bacterial exo-products, i.e. extracellular substances. This product has already improved antibacterial properties and inhibition of the adhesion of pathogenic Listeria monocytogenes in case of stainless steel [13]. In this study, the use of such product will allow to control the surface biocontamination of concrete and their durability, by increasing their resistance to natural environments, characterized by external agents. In a biodeterioration process, these external agents can be biological elements, such as microorganisms, or chemical compounds secreted by microorganisms. In the case of a biological attack, i.e. a localized attack, the chemicals are secreted inside a biofilm covering the material and/or inside the porous network. Thus, the decrease of the pore sizes seems an interesting target regarding the improvement of the characteristics of a bioadmixtured concrete in relation to this risk. This paper aims to evaluate the efficiency of the bioproduct as admixture for concrete using cement pastes (CEM I) and mortar samples thanks to implementing standardized tests, such as slump test, flexural and compressive strength measurements as well as porosity determination. Some characterizations have been carried out to analyze the efficiency of the new admixture on the porosity and surface roughness of the material. Capillary imbibition tests were also implemented to show the influence of such admixture on the hydric transfers inside the material bulk.

Material and methods

Sample preparation

Cement pastes were produced according to the EN 196-1 standard. These samples were made with a water-cement (W/C) ratio of 0.4 allowing the complete hydration of the cement and a suitable rheology for handling the fresh material. These samples were manufactured -with an ordinary Portland cement CEM I 52.5 R CE CP2 NF (98.5% clinker and 1.5% secondary constituents) -as blocks of 5×5×5 cm 3 , removed from the mould after 24h and stored in a conservation room (28 and 120 days) at 23 ± 2°C with a relative humidity (RH) higher than 90%. The bioadmixture content in the samples was ranging from 0.25 to 5% of the cement mass and the adopted formulations are presented below (Table 1). Mortars were produced according to the NF EN 196-1 standard. The amount of sand was constant (1350 g), for 450 g of cement. The amount of water (+ bioadmixture) was adjusted to reach W/C = 0.5 to obtain completely hydrated samples. The bioadmixture content in the samples was ranging from 0.25 to 2.5% of the cement mass and the quantity of the constituents is detailed below (Table 2). Mortar samples were manufactured -with the same Portland cement CEM I 52.5 R CE CP2 NF -as blocks of 4×4×16 cm 3 , removed from the mould after 24h and stored in a conservation room (28, 120 and 180 days) at 23 ± 2°C with RH > 90%. The bioadmixture was produced according to a technique based on cultivation of bacteria [13]. This product was considered as a liquid phase made of water plus surfactant and the amount of supplementary water added was deducted, as for all admixtures, in order to comply with the W/C ratio. This bioadmixture is a lipopeptide biosurfactant derived from Pseudomonas fluorescens. Its active principle consists of bacterial exo-products (extracellular substance). The bioadmixture was incorporated as mass product in the sample formulations.

Slump test

Slump values of the fresh mortar samples were obtained using a mini-cone with a top diameter of 50 mm, bottom diameter of 100 mm and height of 150 mm. The slump was measured at the room temperature, i.e. 21 ± 2℃. The slump was determined as the difference between the initial and the final heights of mortar after removing the mini-cone. After the first (initial) slump was measured, slump heights (slump values) were regularly measured at intervals of 15 minutes. The whole test period was 150 min.

Mechanical tests

Both compressive strength and flexural strength were performed using an Instron 3384 floor model universal testing system. A load cell with a capacity of 150 kN was used. The load measurement accuracy of machine is ±0.5% of reading values. A test speed of 2 mm/min and 1 mm/min for compressive strength and flexural strength were used, respectively, with an accuracy of ±0.2%. For flexural strength experiment, three mortar samples from each mixture group were tested according to the NF EN 196-1 standard. Then, the compressive strength tests were conducted using six broken pieces of mortar from flexural strength test.

The flexural strength of mortars, σ f (MPa), was calculated according to (Equation 1):

𝜎 # = %.' × * + × , -× . /
(Eq. 1) with F f (N) the load at the fracture point, L (mm) the length of the support span, b (mm) the width and d (mm) the thickness.

The compressive strength of mortars, σ c (MPa), was calculated according to (Equation 2):

𝜎 0 = * 1 2
(Eq. 2) with F c (N) the applied load and A (mm²) the area of the clamping plates (40 mm²).

Evaluation of the porosity

Available water porosity measurements have been carried out according to the NF EN 18-459 standard. This technique allows estimating the total porosity of the material, i.e. the ratio between the total volume of voids on the volume of the tested sample. Cement pastes and mortars were weighed under water and in the saturated surface-dry (SSD) conditions. Each sample was dried in an oven at 50℃ until constancy of weight. The porosity was calculated in accordance with the difference in weight between the water-saturated state and oven-dry conditions (Equation 3). (Eq. 3)

with p (%) the porosity, W w (g) the weight of the sample in water-saturated, W ssd (g) the sample weight in the saturated surface-dry condition and W d (g) the dry sample weight after the oven treatment.

Capillary imbibition tests were also carried out on cement pastes to evaluate the influence of the bioadmixture on their porous network. The capillary absorption method consists in observing the natural penetration of the wetting liquid (distilled water) into the porous network of the dry sample. The tests were adapted from the NF EN 480-5 standard. The sample was placed on a draining geotextile, which allows the contact between the wetting fluid and the bottom of the sample. The capillary absorption coefficients were evaluated by considering the absorption phase as a linear function of the square root time (Equation 4). ∆< = = 𝐴 𝑡 (Eq. 4) with ∆W/S (g.cm -²) the absorbed water, t (s) the time and A (g.s -1/2 .cm -2 ) the capillary absorption coefficient.

The mercury injection porosimetry (MIP) allows obtaining much information about the porous network (pore size, volume, distribution, etc.) of the cement pastes and its architecture (geometry) by the pores access radius. It consists in injecting a non-wetting liquid (mercury) in a porous material under controlled pressure. The first mercury injection was followed by a desorption and a second injection allows the evaluation of the total, free and trapped porosity. The pressure applied allows having access to porosities with a range of access diameters from 300 µm up to 3.6 nm. The porosity error margin is about 4% whatever the measured value. The radius of the pore class where the mercury was injected (r p ) was determined from the Washburn formula (Equation 5).

𝑟 A = - C × D EF × GHI θ J EF (Eq. 5)
with σ Hg the surface tension of the mercury (0.480 < σ Hg (N/m) < 0.485), θ the contact angle between mercury and the surface of the sample (130 < θ (°) < 141.3) and P Hg the mercury pressure.

Surface roughness

In order to observe the evolution of cement paste surfaces with the increasing of curing age (28 and 120 days), their roughness was measured with a non-contact 3D measurement system (STIL ® Micromesure station) equipped with an optical pen (axial resolution of 10 nm and lateral resolution of 2 µm). A 20×20 mm² area, selected in the centre of the sample was scanned. Cement pastes were first polished through 1200-grit using abrasive paper to ensure the reproducibility of the surface roughness (regarding porosity). The polished samples were next scanned with the 3D measurement device and then put into water until a curing age of 120 days in the conservation room (T = 23 ± 2°C) before another scan of the surface. Samples were not polished again before this second measurement. The curing conditions were identical for all samples (temperature, quantity/quality of the water) (Table 3).

Microscopic observations

The surfaces of the samples have been observed (without specific preparation) with a scanning electron microscope Philips XL30 ESEM coupled with EDS analysis. The samples have been analyzed under 0.9 Torr (120 Pa) water vapour partial pressure using at least 15kV electron accelerating voltage, to provide enough energy to the electron of the beam to excite the elements during EDS analysis.

Results and discussion

Workability of the fresh mortars

Slump tests have been carried out to evaluate the influence of the bioadmixture regarding the workability of Portland cement mortars at fresh state (Figure 1). The slump loss values of the mortars follow a similar trend, i.e. the values decrease with the elapsed time, which is normal because the setting of the cement 'stiffens' the mixture after one hour. Whatever the bioadmixture content inside the samples, the values converge, i.e. the slump of mortars strongly decreases in the early 30 minutes, and then slows down moderately later on. The admixture rate seems to have an effect on the workability of mortars, because only samples containing 1.5 and 2.5% of bioadmixture show a significantly higher slump in comparison with the unadmixtured mortar. A dosage of 1.5% of bioadmixture allows to reach a maximum threshold of initial slump bigger than 43 mm. This result suggests that the bioadmixture incorporation improves the workability of the fresh mortar and its optimal concentration could be 1.5%. The slump loss evaluation over a period of 150 minutes reveals that the bioadmixture does not trouble the slump loss rate of fresh mortars (Table 4). Indeed, mortars show a stable value during 30 to 60 minutes, whatever the admixture content, even if for 30 minutes, mortars containing 2.5% of bioadmixture have a lower slump loss rate. Some admixtures (water reducers and superplasticizers) are used to modify the workability of the fresh material (its rheology). The experimental results allow to see that the bioadmixture acts as a plasticizer on mortars because it increases their workability for a constant given watercement ratio. This effect seems variable according to the amount added.

The adding 1.5% of bioadmixture induced a reduction by at least 5% of the water input in order to reach the same slump than the control sample (without bioadmixture) (Table 5). This water supply decrease of 7.5 % allows to qualify the bioadmixture as a plasticizer thanks to the recommendations of the NF EN 934-2 standard. The chemical composition of the Portland cement (C 3 A, C 4 AF, C 3 S and C 2 S) is responsible of the rate of slump loss observed in presence of admixture during the first 30 minutes of mixing: C 3 A and C 4 AF would absorb the large amount of admixture, decreasing the zeta potential of cement particles, which is why a type of admixture like a lipopeptide biosurfactant could also increase the rate of slump loss [14]. The hydration of tricalcium silicate (C 3 S: 60 -65% in the clinker chemical composition) allows to form calcium silicate hydrates (C-S-H), that are the main hydration products of Portland cement, with calcium hydroxide (CH) that control the mechanical strength in the material. It is possible that the chemical composition of the bioproduct alters the kinetics of cement hydration.

In order to compare the hydration state of cement samples, some SEM images were taken using backscattered electrons mode (Figure 2) in a low-vacuum mode. The cement grains are more or less hydrated: hydrates appear bright-white in colour due to their higher atomic masses, while the unhydrated grains are grey (voids are black). The presence of the unhydrated particles decreases from 5.7 to 1.8% according to the bioadmixture content (Table 6). The low-vacuum mode allows the analysis of samples without dehydration. Figure 2 provides information on the chemical composition and morphology of the particles. It seems worth to point of that the cement hydration products in mortars have a different grey-level resolution according to their backscattering coefficients. The accuracy of the results depends therefore both on image quality and operator choice regarding the grey-level threshold selection. Thus, results of 2.2, 2.1 or 1.8% of unhydrated particles cannot be considered as showing differences of grains hydration in the whole sample. However, it appears clearly that from a 1.5% concentration of bioadmixture, the unhydrated particle rate decreases according to the bioproduct content (Table 6), because the values change from 5 to 2%.

0% 1% 2%

Figure 2 -Morphology and EDS analysis of C-S-H according to the bioadmixture content, after 28 days of curing.

The morphology of the C-S-H gel changes with the bioadmixture content, as its density increases (Figure 2). Regarding the EDS spectra, the peaks of the calcium, silicon and oxygen are clearly distinguished this confirms that C-S-H are observed. However, it is not possible to see any difference according to the bioadmixture rate using EDS with the low vacuum mode in the chamber of the microscope for this measurement. Both this semi-quantitative analysis and the low-vacuum mode induce uncertainties preventing a precise evaluation of the quantities of the chemical elements according to the peak height.

Compressive and flexural strengths

An essential feature regarding a hardened cementitious material is the mechanical strength at a given age. The compressive strength (Figure 3a) of the Portland cement mortars is in the range The bioadmixture content has no effect on the compressive strength of the mortars after 28 days of curing, even if the strength decreases to 58.6 MPa for a concentration of 2.5%, reflecting an influence (negative) of the bioproduct on the mechanical strength of mortars when its content is high. After 120 days of curing, for bioadmixture concentrations ranging from 0.5 to 1.5%, the compressive strength values are equivalent to those of the mortar without bioadmixture (Figure 3a), with an average value of 65.3 ± 1.0 MPa. In contrast, at concentrations of 2 and 2.5%, the compressive strength increases to 68.3 and 68.1 MPa, respectively. Unlike the previous case, these high bioadmixture concentrations lead to an increase in the compressive strength of mortars. After 180 days of curing, the results are more heterogeneous insofar as high bioadmixture concentrations (2 and 2.5%) also lead to an increase in the compressive strength (versus control sample) -as the concentration of 1% -whereas for concentrations of 0.5 and 1.5%, the results are equivalent to the control sample (without bioadmixture) (Figure 3a).

The bioadmixture concentration does not therefore lead to the same effect according to the curing time. High bioadmixture content allows to increase the mechanical strength of mortars after 120 and 180 days of curing time, whereas after 28 days, the compressive strength decreases with high bioadmixture concentration. However, whatever the curing time, a bioadmixture concentration of 1.5% seems sufficient to achieve resistances of the same order of magnitude (slightly higher after 180 days) than the sample without bioadmixture.

The flexural strength (Figure 3b) of the unadmixtured mortars slightly decreases according to the curing time between 28 and 120 days. Between 120 and 180 days, the flexural strength values are close. For the samples containing bioadmixture at concentrations ranging from 0.5 to 2.5%, the values are in the range of 7.5 -9.2 MPa at 28 days; 8.9 -9.3 MPa at 120 days and 9.0 -9.9 MPa at 180 days. A decrease in flexural strength for high bioadmixture concentrations (2 and 2.5%) after 28 days of curing is observed. However, the strengths are similar whatever the bioadmixture content at 120 days and an increase is even observed with a concentration of 2.5% at 180 days. Thus, the flexural strength seems to depend on the curing time.

These results suggest an influence of the bioadmixture regarding the flexural strength of the mortars. Again, high bioadmixture concentrations in samples are not always favourable to improve the flexural strength, especially at 28 days. However, bioadmixture concentrations ranging from 0.5 to 1.5% provide equivalent results to those obtained for the sample without bioadmixture, whatever the curing time.

The modification of compressive strength of mortars may be due primarily to the formation and structure of C-S-H (the principal hydration product). The silicate chain could produce bond forces and electrostatic forces which are the main factors increasing mechanical strength of these materials. Thus, the biosurfactant could affect the mechanical strength through modifying the hydration kinetics of cement. In addition, it appears that the bioadmixture delays the hydration of the cement grains when it is incorporated at more than 1.5% at 28 days (Table 6).

The differences observed (for the compressive strength, with a slight increase) at 120 and 180 days can be explained by a hardening of the cement related to the slower hydration of the cement particles when the bioadmixture rate exceeds 1.5%.

Porosity measurements

The total porosity available to water of the mortars is ranging from 13 to 15% (Table 7). These values are lower than those usually measured (16 to 24%) [15]. However, they remain in accordance with some results of the literature, where the porosity of such samples is ranging from 12 to 22% [16] or 14 to 22% [17]. The standard deviation between experimental values is very low, which validates the comparative procedure and allows to highlight the effect of the bioadmixture on this parameter. No influence of the curing time or the concentration of bioadmixture is noticed.

The total porosity available to water of the cement pastes is ranging from 33 to 37% (Table 7), which is in accordance with the current literature, where the porosity of such samples is generally ranging from 20 to 40% [START_REF] Mehta | Concrete: Microstructure, Properties, and Materials[END_REF]. Again, no influence of the curing time or the concentration of bioadmixture is noticed. Thus, it is necessary to evaluate if the porosity available by another liquid (mercury) is influenced by the addition of the bioadmixture. The porosimetry with water or mercury does not provide the same values of available porosity because of the different characteristics of the fluids. A non-wetting liquid provides more information about the architecture of the porous network. The mercury injection porosimetry (MIP) allows to measure the dimensions of the orifices, i.e. the access thresholds, through which the fluid must pass to penetrate the material. The volume of mercury which has penetrated into the samples (the mercury access threshold) and the porosity (the ratio between the total volume of the explored pores on the volume of the sample) of the cement pastes (CEM I), according to the diameter of the porosity, are presented below (Figure 4). For the volume of mercury per unit of mass of cement pastes (Figure 4a) and the distribution of the porosity (Figure 4b), the pattern of the curves remains similar, regardless of the bioadmixture content. However, it is possible to identify two groups of samples. For low concentrations (0.25 and 0.5%), the distribution of the porosity according to the pore diameter are undistinguishable to the reference (without bioadmixture), and present a peak for a critical pore size (percolation threshold) of 0.045 µm (Figure 4b). In such concentrations, the bioadmixture does not seem to change the geometry of the porosity of the cement pastes. This result corresponds to capillary pores (0.01 < Ø (µm) < 1). A substantial part of the porosity, accessible via pore diameters lower than 3 nm is not filled by the mercury (Figure 4a). For concentrations higher than 0.75%, the distribution of the porosity as a function of pore diameter remains identical, but the percolation threshold is equal to 0.025 µm (Figure 4b). The curves which represent the volume of mercury per unit of mass of cement pastes (Figure 4a) are offset from the first set of values, while following the same progression. At these concentrations, there is also small peaks, near 0.0075 µm (Figure 4b), representing gel pores (Ø < 0.01 µm), which may reflect an effect of the bioproduct on the capillary pores from a concentration of 0.75%. There are less capillary pores (0.01 < Ø (µm) < 1) and more gel pores inside samples containing higher concentrations of bioadmixture.

The capillary imbibition test provides additional information to evaluate the hydric transfer properties in cement pastes. The capillary absorption kinetic of the CEM I cement pastes is represented by the variation of the amount of water absorbed versus time, according to the curing time (Figure 5). The water absorption in samples is influenced by the bioadmixture because of a slow-down of the water absorption. Whatever the curing time, the bioadmixture incorporated inside cement pastes reduces the kinetic absorption. From the first 200 minutes, a linear relationship is established between the amount of the absorbed water and the time. The weight change per imbibition area unit (∆W/S in g.cm -²) is divided into two sections as a function of time. From 0 to 200 minutes, there is saturation of the open porosity and the capillary absorption velocity is higher. After 200 minutes, the slope of the curves decreases, indicating that the capillary pores are finer, i.e. the capillary absorption velocity is lower. This illustrates the filling of capillary pores with thinner diameters, until full saturation. Moreover, whatever the bioadmixture content and the curing time, the mass increase normalized by the imbibition area are constant after 1600 minutes.

For a bioadmixture content of 1.5%, the absorption coefficients decrease by about 50% after 28 or 120 days of curing time (Table 8). The bioadmixture modifies the geometry of the porosity so as to limit the capillary imbibition. Although the total porosity available to water is not changed by the bioadmixture, it appears that the decrease of the amount of capillary pores (0.01 < Ø (µm) < 1) influences the hydric penetration behaviour. The porosity measurements have shown that it is only at the nanoscale that a modification of the cement paste has been carried out with the bioadmixture from an incorporation of 0.75% of the cement mass. The macropores (1 -40 µm) are not modified by the addition of the bioadmixture, but the gel pores (Ø < 0.01 µm) present two different behaviours depending on whether the cement pastes are formulated with low or high bioadmixture content, i.e. they are fewer at low concentrations. The nature of the physicochemical interactions between the bioadmixture and the hydrates differ according to the admixture content in the cement paste. This bioproduct is a surfactant, i.e. an amphipathic molecule having both a hydrophilic part and a hydrophobic part. In a polar medium such as water, the concentration of the surfactant could lead to micelles formation. The bioadmixture can self-aggregate in the bulk solution. In other words, adding a high bioadmixture content could lead to a saturation of the capillary pores (0.01 < Ø (µm) < 1) by micelles that are intruded inside pores. As a consequence, the number of gel pores increase. The formation of such aggregates depends on the properties and composition of the host liquid. The mode of action of the bioadmixture in pore fluid could be different that its mode of action in pure water, like before its incorporation as mass product in the cement paste formulation [START_REF] Rajabipour | [END_REF].

Capillary imbibition tests have shown that the bioadmixture decreases the imbibition velocity of the cement pastes whatever the curing time (Table 8). Thus, the bioadmixture could modify the porous network of the samples, because the diameters of the capillary pores are decreased, which influence the hydric behaviour of the cement pastes. The volume of the capillary pores will tend to decrease because of its filling by C-S-H gels and other hydrates. Thus, the connectivity between the capillaries is reduced and the progression of the water inside the porous network of the cement pastes is blocked.

Influence of the bioadmixture on the surface roughness of the cement pastes

The cement paste is an aggregate of particles having different composition and hydrated states with voids between the grains. As a consequence, the surface of the cement paste presents asperities and cavities, which constitute its roughness The topography of the samples is shown below (Figure 6). After 28 days of curing time, the topography of the surfaces is mainly influenced by the polishing operation, whereas at 120 days, all surfaces are much more heterogeneous. At 28 days, the surface topography is identical (Table 9), which is consistent with results regarding porosity: the macropores (1 -40 µm) are not modified by the addition of the bioadmixture. This micron scale corresponds to that is accessible by the 3D measurement device (Ra ≃ 0.8 µm). There is no influence of the bioadmixture concentration on the surface topography at 28 days. After 120 days of curing, a decrease of the mean roughness is observed according to the bioadmixture content (Table 9). In spite of mean roughness standard deviations, due to the heterogeneity of the surfaces, the average Ra values change from 2 to about 1 µm. Both curing time and interaction of the cement surface with water explain the observed differences at 28 and 120 days. The curing operation induces surface heterogeneity, associated with a development of crystals on the cement surface (Figure 7) coming from the hydration of the cement particles or from a calcium carbonate layer formed in water. The bioadmixture appears to inhibit the development and the formation of crystals on the surface of cement pastes, as the Ra values decrease for a given curing time, according to its concentration (Table 9). The roughness parameter is important but not sufficient to predict the biocolonization behaviour or the inhibition of the development of biofilms. Many surface parameters, such as surface energy, are also affected by the polishing operation endured by the samples after 28 days of curing, which amends the crystal surfaces (abrasion, splitting, etc.). Depending on the nature of the microorganisms present in the environment and their size, there is a synergy between this size and the roughness dimension, which control the bioreceptivity. That is why mastering the roughness parameter is a requirement for evaluating a potential biocolonization.

The bioadmixture seems inhibit the growth and the formation of crystals on the sample surfaces. Both this characteristic combine with the inhibition action of bacterial growth already demonstrated on stainless steel [20] with the same bioproduct, can potentially induce a limitation of the development of biofilms on admixtured surfaces. Another parameter impacting the biocolonization of cement surfaces is linked to its porosity, the cement paste acting as a water tank keeping favourable conditions for the development of microorganisms even in dry environment. The microbial colonization and the development of biofilms on cementitious surfaces can lead to the formation of stains (bioalteration) and/or to the failure of the structure (biodeterioration). The hostility of a biofilm depends both on the metabolic activity of the microorganisms that compose it and the penetration of biological compounds inside the porous network. Although Portland cement (CEM I) is only composed of mineral phases, the mixing water and the bioadmixture used to manufacture the cement pastes can contain organic matter that may furnish nutrients for the microorganisms. Furthermore, different bacteria secrete acid compounds that react with the cement matrix and induce the formation of expansive and/or soluble deterioration products [21]. The porosity of the cement pastes influences the colonization and the resistance of samples to biological colonization, so, decreasing the porosity like it was shown (Figure 4b) by increasing the bioadmixture content inside samples leads to a better resistance of the cement paste to this type of colonization. The studied lipopeptide biosurfactant derived from Pseudomonas fluorescens contains, among other, extracellular substances such as colloidal carbohydrates and proteins [22]. The exopolymers can clog the porous network and then limit the exchange with the outside. It is possible to limit the porosity of cementitious material or to limit the material biodeterioration by adding polymers in the fresh cement paste [23], [24]. The presence of polymers allows clogging the porosities and filling the cement matrix microcracks. These polymers do not protect concrete against acid attack of biological origin, but they improve the material cohesion and can delay erosion. The clogging of the porous network can also delay the penetration of aggressive compounds [25].

Conclusions

The bioproduct introduced an effect on the workability of fresh mortar samples with a plasticizing action, allowing to reduce by at least 5% the water supply, at constant slump, at a dosage level of 1.5% of bioadmixture. Furthermore, the bioadmixture can act positively to improve the mechanical strength of Portland cement mortars (beyond 120 days): an increase of the compressive strength was observed for the mortars containing 2 and 2.5% of bioadmixture (by weight of cement), in comparison with the unadmixtured samples. Whatever the curing time, a 1.5% bioadmixture content seems sufficient to reach similar resistances than samples without bioadmixture and consistent with the strength class (52.5) of the samples. Under the same curing conditions, no significant difference was observed for the flexural strength of mortars. Whatever its dosage level, the bioadmixture does not influence the porosity accessible to water of the mortars, according to the curing time.

Whatever its content, the bioadmixture does not influence the total porosity available to water of the cement pastes. In contrast, the mercury injection porosimetry (MIP) has revealed the existence of a threshold (between 0.5 and 0.75% of bioadmixture) from which the porous network of the cement pastes is changed. For the low admixtured samples, the pore radii are bigger, for a same volume of mercury by cement paste mass unit. For the upper concentrations than 0.75% of bioadmixture, the percolation threshold decreases to a radius of 0.025 µm. The structure of the porous network of the cement pastes, allowing the penetration of external agents (aggressive or not), is modified by the bioadmixture. Moreover, the open porosity is included in the surface roughness: the evolution of the surface roughness is also impacted by the bioadmixture. The study of the surface roughness of cement pastes has allowed to characterize the evolution of their surface states after different curing time. After 28 days of curing time, the surfaces exhibit a topography mainly influenced by the polishing operation carried out on abrasive paper. After 120 days, the surfaces are more heterogeneous. Both curing time and interactions of the cement surface with water explain these differences. The arithmetic average of the roughness profile (Ra) of the cement paste (CEM I) surfaces decreased. The curing procedure induces surface heterogeneity, with the development of crystals on surface originate from the hydration of the cement particles or a carbonate calcium layer from water. The bioadmixture appears to inhibit the development and the formation of the crystals on surfaces, because the Ra values decrease for a given curing time, according to its content. Capillary imbibition tests also reveal that the bioproduct influences hydric transfers within the porous network of the cement pastes, modifying the capillary pores as shown with the MIP results indicating that the diameter of the critical pore size decreases from 0.75% of bioadmixture content.

Figure 1 -

 1 Figure 1 -Evolution of slump of CEM I based mortars according to the time.

Figure 3 -

 3 Figure 3 -Effect of the bioadmixture content on compressive (a) and flexural (b) strengths of mortars (CEM I) according to the curing time.

Figure 4 -

 4 Figure 4 -Volume of mercury per unit of mass (a) and distribution of the porosity (b) of cement pastes (CEM I) according to the pore diameter.

Figure 5 -

 5 Figure 5 -Mass increase normalized by the imbibition area versus square root of time for the cement pastes after 28 (a) and 120 (b) days of curing time.

Figure 6 -

 6 Figure 6 -Topography of the CEM I cement pastes according to the curing time.

Figure 7 -

 7 Figure 7 -Observation of growing crystals on the surface of a cement paste (CEM I), after 120 days of curing in a conservation room (T = 23 ± 2°C, RH > 90%).

Table 1 -

 1 Cement pastes formulation.

	Cement (kg/m 3 )	Water (kg/m 3 )	Bioadmixture (kg/m 3 )
	180 ± 1	72 -63 W/C = 0.4	0 -9 BA/C = 0 -5 (%)

Table 2 -

 2 Mix design of mortar samples.

	Cement (g) Sand (g)	Water (g)	Bioadmixture (g)
	450 ± 1	1350 ± 5	225 -213.8 W/C = 0.5	0 -11.2 BA/C = 0 -2.5 (%)

Table 3 -

 3 Quality of curing water and sample surfaces.

Table 4 -

 4 Slump loss rate evolution of CEM I based mortars with time.

	Bioadmixture concentration (% by weight of cement) 0 -30 min. 0 -60 min.
	0	71%	77%
	0.5	70%	78%
	1.5	75%	79%
	2.5	59%	74%

Table 5 -

 5 Water supply decrease, at constant slump, of Portland cement mortars.

		Water (g) Water supply decrease (%)
	Control sample (CEM I)	225	-
	Mortar (CEM I) containing 1.5% of bioadmixture (by weight of cement)	208	7.5 ± 2.0

Table 6 -

 6 Relationship between the bioadmixture content and unhydrated particles (SEM observations) on the surface of Portland cement mortars.

	Bioadmixture concentration (%) Unhydrated particle area (µm 2 ) Unhydrated particle rate (%)
	0	62.7	5.7
	0.5	60.3	5.4
	1	56.8	4.2
	1.5	48.0	2.2
	2	52.6	2.1
	2.5	54.7	1.8

Table 7 -

 7 Available water porosity (%) of Portland cement mortars and cement pastes (CEM I) according to the curing time.

	Bioadmixture concentration (%)	28	Mortar 120	Curing time (days) 180	Cement paste 28 120
	0	14.2 ± 0.1	14.1 ± 0.1	14.0 ± 0.3	36.2 ± 0.1	34.8 ± 0.2
	0.5	14.2 ± 0.3	14.2 ± 0.2	13.9 ± 0.1	36.3 ± 0.1	36.2 ± 0.1
	1	14.3 ± 0.2	14.1 ± 0.1	13.9 ± 0.2	36.2 ± 0.1	36.0 ± 0.1
	1.5	14.5 ± 0.1	14.1 ± 0.2	14.1 ± 0.3	36.3 ± 0.2	33.9 ± 0.3
	2	14.4 ± 0.2	13.9 ± 0.1	13.9 ± 0.3
	2.5	14.3 ± 0.1	14.1 ± 0.2	13.9 ± 0.1

Table 8 -

 8 Absorption coefficients of the cement pastes (CEM I).

	Curing time (days)	Bioadmixture concentration (%)	Absorption coefficient (g.cm -2 .min -0,5 )	Curing time (days)	Bioadmixture concentration (%)	Absorption coefficient (g.cm -2 .min -0,5 )
		0	0.152 ± 0.002		0	0.134 ± 0.006
		0.25	0.132 ± 0.001		0.25	0.132 ± 0.002
	28	0.5	0.105 ± 0.003	120	0.5	0.125 ± 0.002
		1	0.108 ± 0.002		1	0.127 ± 0.003
		1.5	0.071 ± 0.001		1.5	0.076 ± 0.001

Table 9 -

 9 Mean roughness values of the cement pastes (CEM I).

	Curing time (days)	Bioadmixture concentration (%)	Ra (µm)	Curing time (days)	Bioadmixture concentration (%)	Ra (µm)
		0	0.90 ±0.25		0	1.72 ±0.36
	28	0.25 0.5	0.80 ±0.05 0.80 ±0.09	120	0.25 0.5	1.38 ±0.17 1.35 ±0.15
		1.5	0.80 ±0.04		1.5	1.14 ±0.06

Acknowledgments

This work was financially supported by the French National Research Agency (ANR) under the framework of the SEPOLBE project.