B Ilharreborde
email: brice.ilharreborde@aphp.fr

• E Ferrero

• A Angelliaume

• Y Lefe

F Accadbled

A L Simon

• J Sales De Gauzy

• K Mazda

Selective versus hyperselective posterior fusions in Lenke 5 adolescent idiopathic scoliosis: comparison of radiological and clinical outcomes

Keywords: Adolescent idiopathic scoliosis, Lenke 5, Selective fusion, Short segment

Purpose Recent literature has reported that the ]progression risk of Lenke 5 adolescent idiopathic scoliosis (AIS) during adulthood had been underestimated. Surgery is, therefore, proposed more to young patients with progressive curves. However, choice of the approach and fusion levels remains controversial. The aim of this study was to analyze the influence of the length of posterior fusion on clinical and radiological outcomes in Lenke 5 AIS. Methods All Lenke 5 AIS operated between 2008 and 2012 were included with a minimum 2-year follow-up. Patients were divided into two groups according to the length of fusion. In the first group (selective), the upper instrumented level (UIV) was the upper end vertebra of the main structural curve and distally the fusion was extended to the stable and neutral vertebra, according to Lenke's classification. In the second group (hyperselective), shorter fusions were performed and the number of levels fused depended on the location of the apex of the curve (at maximum, 2 levels above and below, according to Hall's criteria). Apart from the fusion level selection, the surgical procedure was similar in both groups. Radiological outcomes and SRS-22 scores were reported.

Results 78 patients were included (35 selective and 43 hyperselective). The number of levels fused was significantly higher in the first group (7.8 ± 3 vs 4.3 ± 0.6). None of the patients was fused to L4 in selective group. No correlation was found between length of fusion and complication rate. Eight patients had adding-on phenomenon among which 6 (75%) had initially undergone hyperselective fusions and had significantly higher postoperative lower instrumented vertebra (LIV) tilt. In the adding-on group, LIV was located above the last touching vertebra (LTV) in 62.5% of the cases and above the stable vertebra (SV) in 87.5%. Patients in the selective group reported a significantly lower score in the SRS function domain. Conclusion Coronal alignment was restored in both groups. Hyperselective posterior fusions can be considered in Lenke 5 AIS, preserving one or two mobile segments, with similar clinical and radiological outcomes. However, selection of the LIV according to SV and LTV need to be accurately analyzed in order to avoid adding-on during follow-up.

Introduction

Recent literature has reported that the progression risk of Lenke 5 adolescent idiopathic scoliosis (AIS) during adulthood had been underestimated and that curves greater than 35°should be carefully followed after skeletal maturity. As a matter of fact, Pesenti et al. reported that 50% of these curves demonstrated significant progression after 20 years of follow-up [START_REF] Pesenti | Evolution of adolescent idiopathic sco liosis: results of a multicenter study at 20 years' follow up[END_REF]. Surgery is, therefore, more often considered in AIS patients with progressive curves greater than 40°, especially in case of clinical imbalance or cosmetic demand. However, the choice of the approach, the selection of fusion levels and more specifically the length of arthrodesis remain controversial. Some authors reported a greater loss of function with long fusions ending below L3 [2 5], but postoperative functional outcomes similar to those of asymptomatic subjects of the same age have also been found at 2-year follow-up [6 9].

Lenke 5 AIS correction can be addressed through posterior or anterior approach. In the latter, surgeons usually tend to fuse less mobile segments, following Hall's recommendations and, therefore, sometimes accept a residual lower instrumented vertebra (LIV) tilt in order to reduce the risk of future adjacent segment degeneration [START_REF] Hall | The anterior approach to spinal deformities[END_REF][START_REF] Yu | Comparison of correction outcomes after anterior or posterior approach with lowest and vertebra instrumentation in adolescent idiopathic thoracolumbar/lumbar scoliosis[END_REF][START_REF] Matsumoto | Postoperative distal adding on and related factors in Lenke type 1A curve[END_REF][START_REF] Koller | Criteria for successful cor rection of thoracolumbar/lumbar curves in AIS patients: results of risk model calculations using target outcomes and failure analy sis[END_REF]. In the past decade, the posterior correction of Lenke 5 has regained popularity due to the development of all-screw constructs and direct vertebral derotation technique. In most of the cases, fusions are selective, sparring the thoracic spine, but extend from the upper end to the lower end vertebrae of the main structural curve (selective fusion) [START_REF] Hall | The anterior approach to spinal deformities[END_REF]. Fusions down to L4 are, therefore, sometimes necessary, especially when the apex is located below L2, leaving only two mobile discs below the fusion mass [START_REF] Matsumoto | Postoperative distal adding on and related factors in Lenke type 1A curve[END_REF]. No long-term study has ever compared short segment anterior and selective posterior fusions. In fact, the feasibility of such a comparison can be questioned due to the bias represented by the different approaches.

Recently, some surgeons have tended to reduce their fusion length in posterior procedures as well (hyperselective fusion), using the criteria initially described for anterior surgery, because they strongly believed in the 3D correction potential of pedicle screws and were concerned about the future risk of adjacent degeneration [START_REF] Hall | The anterior approach to spinal deformities[END_REF][START_REF] Kim | Determination of the distal fusion level in the manage ment of thoracolumbar and lumbar adolescent idiopathic scoliosis using pedicle screw instrumentation[END_REF]. The aim of this study was, therefore, to compare the clinical and radiological outcomes of selective and hyperselective fusions in Lenke 5 AIS operated via posterior approach.

Materials and methods

Patients

After IRB approval, all consecutive patients with Lenke 5 AIS operated through posterior approach in two departments (5 senior surgeons) between July 2008 and October 2012 were included. Clinical, surgical and radiological data were collected with a minimum 2-year follow-up. Patients with previous spine surgery or significant lower limb deformity affecting the frontal pelvic tilt were excluded.

Operative procedure

Patients were divided into two groups according to the surgical strategy. In center 1 (selective group), fusions were performed aiming to optimize the restoration of both coronal and sagittal alignments [START_REF] Koller | Criteria for successful cor rection of thoracolumbar/lumbar curves in AIS patients: results of risk model calculations using target outcomes and failure analy sis[END_REF][START_REF] Li | Analysis of radiographic parameters relevant to the lowest instrumented vertebrae and postoperative coronal balance in Lenke 5C patients[END_REF]. The upper instrumented vertebra (UIV) was the upper end vertebra of the main structural curve. Attention was paid to stop the fusion below the apex of the thoracic kyphosis (below T6) in order to avoid proximal junctional kyphosis. The vertebra selected as LIV had to be neutral on standing films, crossed between its pedicles by the center sacral vertical line (CSVL) and located above a mobile disc on bending films [START_REF] Nash | A study of vertebral rotation[END_REF]. Fusions were never extended below L4, leaving at least two mobile discs under the fusion mass.

Short-segment fusions were performed in center 2 (hyperselective group), following the rules initially described for short-segment anterior fusion by John Hall [START_REF] Bernstein | Solid rod short segment anterior fusion in thoracolumbar scoliosis[END_REF]. When the apex was located on a vertebral body, fusion extended one vertebra above and one vertebra below; while if the apex was located on a disc, fusion extended from two vertebrae above to two vertebrae below (Fig. 1).

Apart from the fusion level selection, the surgical procedures were similar in both centers, using 5.5 Titanium rods and lumbar pedicular screws (Legacy Medtronic, Minneapolis, MN, USA), sometimes supplemented by sublaminar bands (Jazz Implanet, Bordeaux, France) at thoracic levels. The proximal fixation was either performed by supralaminar hooks or pedicle screws, depending on surgeon's preference. Rod derotation and in situ bending were the main techniques used for correction. Arthrectomy was performed at each fusion level. No patient underwent anterior release. The same perioperative blood saving strategy was used in all patients, associating intraoperative cell saver and tranexamic acid. Spinal cord monitoring was systematically performed.

Radiological parameters

Patients underwent low-dose biplanar radiographs using the EOS system (EOS imaging, Paris, France) preoperatively, postoperatively (within 1 month) and at latest follow-up, as previously described [START_REF] Faro | Evaluation of a functional position for lateral radiograph acquisition in ado lescent idiopathic scoliosis[END_REF]. Spinal measurements were performed using the dedicated SterEOS software, by an experienced independent spinal surgeon. Coronal parameters included Cobb angles of the main and thoracic curves, LIV tilt (angle between the lower endplate of the LIV and the horizontal line), ilio-lumbar angle (ILA, angle between the lower endplate of L4 and the line joining the 2 sacroiliac joints) and the offset between the vertical line from the center of the sacral endplate (center sacral vertical line, CSVL) and the center of C7 (Fig. 2). Location of the last touching vertebra (LTV, the most proximal lumbar vertebra that touches the CSVL) was determined according to Lenke's definition on standing films [START_REF] Lenke | Adolescent idiopathic scoliosis: a new classifi cation to determine extent of spinal arthrodesis[END_REF].

On sagittal radiographs, the following parameters were measured: T4T12 thoracic kyphosis (TK), L1S1 lumbar lordosis (LL) and pelvic parameters (pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT)). The sagittal vertical axis (SVA, distance between the vertical line from the center of C7 and the posterosuperior corner of S1) was also analyzed. SVA was considered positive if directed forwards and negative if directed backwards.

As described by Berjano et al., lumbar lordosis was considered adapted to the pelvic incidence if values respected the following equation: LL = PI ± 9°[ 20 22].

Functional outcomes and complications

Functional outcomes were assessed at follow-up using the Scoliosis Research Society SRS-22 questionnaires [START_REF] Lonjon | Reliability and validity of the French Canadian version of the scoliosis research society 22 questionnaire in France[END_REF]. All clinical and radiological complications, including unplanned surgeries, were recorded. Adding-on was defined as a progression greater than 5°of the LIV frontal tilt [START_REF] Wang | Distal adding on in Lenke 1A scoliosis: how to more effectively determine the onset of distal adding on[END_REF], while proximal junctional kyphosis (PJK) was defined by a progression of the sagittal Cobb angle between the lower endplate of the UIV and the upper endplate of the UIV ?1 greater than 10° [START_REF] Helgeson | Evaluation of proximal junctional kyphosis in adolescent idiopathic scoliosis following pedicle screw, hook, or hybrid instrumentation[END_REF][START_REF] Kim | Results of revision surgery for proximal junc tional kyphosis following posterior segmental instrumentation: minimum 2 year post revision follow up[END_REF].

Statistical analysis

Statistical analysis was performed using SPSS software, version 20 (IBM, Armonk, New York, USA). Normality of the distribution was appreciated using Shapiro Wilk Test. Continuous variables were expressed as means and standard deviations. Radiological and clinical parameters were compared between groups using Student t tests. Chi square or Fisher tests were used as appropriate to compare categorical variables. Pearson's tests were performed to evaluate the correlation between radiological parameters and SRS-22 scores. A p \ 0.05 was considered statistically significant.

Results

Patients

Seventy-eight patients were included (35 selective and 43 hyperselective), with a mean follow-up of 55 ± 22 months the LTV or below, while 84% of the short segment fusions ended proximal to the LTV.

Operative time was significantly shorter in hyperselective fusions (140 ± 41 vs 210 ± 35 min, p = 0.01), but the average blood loss was not significantly different (Table 1).

Coronal radiological parameters

Preoperative parameters are reported in Table 3. The postoperative correction of the main curve was significantly greater in the selective group (63 vs 55%, p = 0.02). Similarly, the spontaneous correction of the proximal thoracic contra-curve was significantly greater after longer fusion (45 vs 23%, p = 0.02). However, satisfactory coronal balance (\20 mm) was obtained in both groups postoperatively, and no significant difference was found regarding residual ILA and LIV tilts, immediately after surgery and at latest follow-up (Table 2). The number of patients with a residual postoperative tilt [5°was 15 (34%) in the hyperselective group and 18 (51%) in the selective one without significant difference (p = 0.08).

Sagittal radiological parameters

Both groups were comparable preoperatively. The surgical procedure did not significantly modify the LL or the TK in any of the groups (Table 3). However, TK significantly increased in both groups during the follow-up period (average 10°in selective patients, and 8°in hyperselective cases). Of note, preoperatively, 62% of the patients had a LL adapted to their PI; however, 21 patients (27%) had PI-LL \ -9°and 8 (10%) had PI-LL [ 9°. Postoperatively, 69% of the patients had a LL adapted to PI (66 and 70% in selective and hyperselective groups, respectively). At final follow-up, only 49% of the patients had a LL adapted to their PI (28 and 59% in selective and hyperselective groups, respectively). All the patients with a PI-LL [ 9°, showing a postoperative hypolordosis, were in the selective group (n = 5). In the hyperselective group, all the patients with unadapted LL (n = 18) were hyperlordotic postoperatively, with PI-LL \ -9°(Table 4).

Functional outcomes

No significant difference was found between the groups regarding SRS-22 total scores at follow-up. However, the function domain score was significantly better after shortsegment fusions (p = 0.03, Table 5). SRS-pain scores at latest examination were greater than 4 in both groups, without significant influence of the fusion length. (Table 1). As expected, the number of levels fused was significantly higher in the selective group (7.8 ± 3vs 4.3 ± 0.6 in hyperselective fusions) (p \ 0.01). In the latter, most of the patients (85.7%) had an UIV between T10 and T7, while five patients had an UIV located at T11 and T12. In the hyperselective group, the UIV was T10, T11 or T12 in all cases. Most of the selective patients (26, 74.3%) were fused to L4, while nine fusions ended in L3 (25.7%). None of the patients was fused to L4 in the short-segment group. In the latter, the most frequent LIV was L3 (29, 67%), while 14 patients (33%) were only fused to L2. The selection of the LIV in regard to the location of the LTV and the stable vertebra (SV) is reported in Table 2a, b. In the selective group, the LIV was located in most of the cases (91%) on the SV or 1 level above (SV-1), while the most frequent selected LIV (74%) was SV-2 in hyperselective cases. Most of the selective fusions (91%) were extended to

Complications

The overall complication rate was 7.7%, without difference between the groups. Two early surgical site infections and two mechanical failures (proximal hook dislodgement) required revisions. In addition, asymptomatic radiological PJK were observed in 15 patients (19%) (Table 6). Eight patients (10.2%) developed an adding-on during follow-up, among which six (75%) had initially undergone hyperselective fusions and two had selective fusions. In the addingon group, the LIV was located above the LTV in five cases (62.5%), with 1 LTV-2 and 4 LTV-1. Similarly, the LIV was located above the SV in 87.5% (1 SV-3, 3 SV-2 and 3 SV-1). Postoperative LIV tilt was significantly higher in the adding-on group (12°± 2 vs 7°± 1, p = 0.02). However, the incidence of AO was not correlated to postoperative frontal balance nor residual LIV tilt (r \ 0.100, p [ 0.05).

Discussion

Results of the current study show that short-segment posterior fusions can be considered in Lenke 5 AIS in order to preserve motion segments, without impairing postoperative balance. No significant difference was found between groups regarding the residual LIV tilt, but more adding-on cases were observed at follow-up after hyperselective fusion, especially if the LIV was located above the SV.

Goals of surgery

While risk factors for Lenke 5 AIS progression in adulthood are better understood [START_REF] Pesenti | Evolution of adolescent idiopathic sco liosis: results of a multicenter study at 20 years' follow up[END_REF][START_REF] Koller | Criteria for successful cor rection of thoracolumbar/lumbar curves in AIS patients: results of risk model calculations using target outcomes and failure analy sis[END_REF][START_REF] Li | Analysis of radiographic parameters relevant to the lowest instrumented vertebrae and postoperative coronal balance in Lenke 5C patients[END_REF][START_REF] Poumarat | Natural history of progressive adult scol iosis[END_REF], the selection of fusion levels remains controversial when surgery is considered. Predictive factors of good radiological long-term outcomes include a postoperative LIV tilt \5°, a greater reduction rate of the main structural lumbar curve and global coronal balance [START_REF] Poumarat | Natural history of progressive adult scol iosis[END_REF]. The aim of surgery, was, therefore, to satisfy these parameters, while preserving a maximal number of mobile segments. Surprisingly, only 42% of the patients had postoperative LIV tilt \5°and 55% had good coronal balance, without any difference between groups. However, the overall functional outcomes at 5-year follow-up were in accordance with previous literature. But no significant correlations between residual LIV tilt and SRS22 score were observed (r \ 0.150, p [ 0.05).

In the present study, patients who developed adding-on had significantly higher postoperative LIV tilt (12°± 2 vs 7°± 1, p = 0.02). Of note, all the patients with a wedged (5°) postoperative disc did not develop adding-on at last follow-up (Fig. 3). Global coronal alignment, LIV position in regard to SV and LTV did not impact adding-on occurrence.

Radiological outcomes

Sagittal and coronal alignments were both restored postoperatively in each group [START_REF] Jackson | Radiographic analysis of sagittal plane alignment and balance in standing volunteers and patients with low back pain matched for age, sex, and size. A prospective controlled clinical study[END_REF]. Cobb angle correction of the main and the proximal contra-curves were significantly better in the selective group. LIV tilt and ILA were significantly reduced postoperatively, without loss of correction at follow-up. No difference was observed between hyperselective and selective procedure in terms of LIV tilt and ILA correction and postoperatively, number of patients with a LIV tilt [5°was not significantly different between groups (20 in the selective group and 23 in hyperselective group). Moreover, LIV selection in regard to SV and LTV Nevertheless, the risk of adding-on was not different between groups. Other authors reported the importance of SV, LTV, LIV tilt analysis in the choice of LIV [ 29 31]. However, longer follow-up might be necessary to observe occurrence of disc degeneration below fusion [START_REF] Sudo | Long term outcomes of anterior dual rod instrumentation for thoracolumbar and lumbar curves in adolescent idiopathic scoliosis: a twelve to twenty three year follow up study[END_REF][START_REF] Green | Long term magnetic resonance imaging follow up demonstrates minimal transitional level lumbar disc degeneration after posterior spine fusion for adolescent idiopathic scoliosis[END_REF]. Moreover, neither selective nor hyperselective fusions significantly affected postoperative LL, which is in accordance with previous literature [START_REF] Kim | Determination of the distal fusion level in the manage ment of thoracolumbar and lumbar adolescent idiopathic scoliosis using pedicle screw instrumentation[END_REF]. However, five patients from the selective group had lumbar hypolordosis at follow-up (although these normative values were obtained from adult patients). This lack of lordosis might be explained by a previous tendency, noticed in center 1, to overbend the lumbar rods that led to more PJK in the past. Indeed, since 2/3 of the lumbar lordosis is below L4, surgeons must pay attention not to give too much lordosis in the instrumented proximal lumbar spine, leading to a posterior shift of the fusion mass [START_REF] Xu | Sagittal balance in adolescent idiopathic scoliosis: a radiographic study of spinopelvic compensation after selective posterior fusion of thoracolumbar/lumbar (Lenke 5C) curves[END_REF]. The significant increase in TK between the postoperative and the last follow-up in both groups was not reported in other study [START_REF] Sudo | Long term outcomes of anterior dual rod instrumentation for thoracolumbar and lumbar curves in adolescent idiopathic scoliosis: a twelve to twenty three year follow up study[END_REF][START_REF] Maida | Sagittal balance in adolescent idiopathic scoliosis: radiographic study of spino pelvic compensation after surgery[END_REF][START_REF] Yang | Pre and postoperative spinopelvic sagittal balance in adolescent patients with Lenke Type 5 idiopathic scoliosis[END_REF]. Larger TK might be explained by a functional adaptation above the fixed lumbar spine, explaining the 15 asymptomatic PJK (without any differences between groups).

Nevertheless, postoperative SVA in both groups remained within normative range (-20 mm \ SVA \ 20 mm) [START_REF] Potter | Prevention and man agement of iatrogenic flatback deformity[END_REF], consequently one can conclude that selective fusion does not alter global sagittal alignment.

Functional outcomes

In the current study, functional outcomes were very good according to the SRS-22 scores (selective group: 4.1 ± 0.4; hyperselective group: 4.0 ± 0.6). Despite no difference in terms of SRS-22 total scores, function was significantly better in short-segment fusions (selective 3.7 ± 0.7 vs 4.2 ± 0.9, p = 0.03). Eventhough 74% of the patients were fused to L4 in the selective group, the pain level at follow-up was not significantly higher (4 ± 0.6 vs 4.2 ± 0.9, p = 0.51), which is in accordance with previous literature [3, 6, 9, 37 39]. As a matter of fact, Sanchez-Raya et al. suggested that the amount of residual mobility and the LIV location had a moderate influence on longterm patients' quality of life after long fusions, in opposition to others variables such as age, preoperative functional status and time from surgery [START_REF] Sanchez Raya | Does the lower instrumented vertebra have an effect on lumbar mobility, subjective perception of trunk flexibility, and quality of life in patients with idiopathic scoliosis treated by spinal fusion?[END_REF]. Nevertheless, literature remains sparse about LIV selection in Lenke 5 AIS curves and few studies analyzed long-term adjacent segment degeneration according to the fusion length of fusion in such patients. The only relevant study is a recent analysis from Ding et al. [START_REF] Ding | Evaluation of quality of life in adolescent idiopathic scoliosis with different distal fusion level: a comparison of L3 versus L4[END_REF], who compared fusions to L3 and those to L4 at 3-year follow-up. No significant difference was found regarding postoperative Oswestry Disability Index (ODI), SRS-22, Short-Form 36 (SF-36) and Visual Analogical Scale (VAS) scores. However, only ten patients with Lenke 5 curves were included. While these findings suggest that extending a fusion to L4 can be considered safely, longer follow-up regarding radiological outcomes and SRS function scores remain necessary.

Limitations

This series is to date one of the largest dedicated to posterior surgery in Lenke 5 AIS with modern instrumentation, but several limitations can be mentioned. First, it is retrospective and a prospective randomized study would have provided a higher level of evidence. However, it is difficult to change a surgeon's preference regarding fusion levels selection and prospective studies are, therefore, difficult to carry on when comparing surgical techniques. Second, only 24% of the patients had more than 5-year follow-up. Nevertheless, results of this study need to be further confirmed with longer follow-up to compare occurrence of adjacent segment degeneration and clinical outcomes between groups [START_REF] Green | Long term magnetic resonance imaging follow up demonstrates minimal transitional level lumbar disc degeneration after posterior spine fusion for adolescent idiopathic scoliosis[END_REF]. Third, while our study analysed the location of the LIV in regard to SV and LTV, the preoperative flexibility and orientation of the discs were not assessed, and this parameter might play a key role in the development of adjacent complications when a fusion to L4 has to be considered.

Therefore, hyperselective posterior fusions can be considered in Lenke 5 AIS, preserving one or two mobile segments, with similar clinical and radiological outcomes than longer fusions. However, the selection of the LIV according to SV and LTV need to be accurately analyzed in order to avoid adding-on during follow-up.

Compliance with ethical standards

Conflict of interest All authors declared that they have no potential conflict of interest.

Fig. 1

 1 Fig. 1 Pre and postoperative X rays of patient of selective (a) and hyperselective group (b)

Fig. 2

 2 Fig. 2 Illustration of central sacral vertical line measurements (CSVL), lower instrumented vertebra tilt (LIV tilt) and ilio lumbar angle (ILA)

(n = 35

 35

Fig. 3

 3 Fig. 3 Radiographic example of adding on phenomenon

Table 1

 1 Demographic and surgical data (mean ± SD)

		Selective	Hyperselective	p
		(n 35)	(n 43)
	Age (years old)	15.6 ± 2	16.5 ± 2	0.03
	Sex	F 30, M 5 F 39, M 4	0.49
	Risser	3.0 ± 1	3.8 ± 1	0.11
	Number of	7.8 ± 3	4.3 ± 0.6	\0.001
	instrumented level
	Follow up (months)	55 ± 15	55 ± 29	0.32
	F female, M male, SD standard deviation
	Table 2 LIV selection in regard to the location of the (a) SV and
	(b) LTV
		Selective	Hyperselective
		(n 35)	(n 43)

Table 5

 5 not impact the postoperative residual LIV tilt in hyperselective group. Therefore, fusion to L4 did not provide more efficient LIV tilt correction or better global coronal alignment, although more segments were fused.

	Functional outcomes comparison between the two		Selective (n 35)	Hyperselective (n 43)	p
	groups (scoliosis research society score)	SRS 22 Total score SRS 22 Function	3.9 ± 0.4 3.7 ± 0.7	4.1 ± 0.6 4.1 ± 0.7	0.18 0.03
		SRS 22 Pain	4 ± 0.6	4.2 ± 0.9	0.51
		SRS 22 Self image	4.1 ± 0.4	4.1 ± 0.6	0.98
		SRS 22 Mental health	3.6 ± 0.8	3.9 ± 0.8	0.14
		SRS 22 Satisfaction	4.4 ± 0.5	4.5 ± 0.6	0.38

did


Julie Lebon
email: juliejebon@hotmail.fr

Cécile Batailler
email: cecile.batailler@chu-lyon.fr

Matthieu Wargny
email: matthieu.wargny@gmail.com

Elie Choufani
email: elie.choufani@ap-hm.fr

Philippe Violas
email: philippe.violas@chu-rennes.fr

Damien Fron
email: damien.fron@chru-lille.fr

Jerry Kieffer Kieffer

Franck Accadbled
email: accadbled.f@chu-toulouse.fr

Vincent Cunin
email: vincent.cunin@chu-lyon.fr

Jérôme Sales

De Gauzy

Magnetically controlled growing rod in early onset scoliosis: a 30-case multicenter study

Keywords: Magnetically controlled growing rod, Early onset scoliosis, Complication, Revision, Multicenter study

Purpose Preliminary results of magnetically controlled growing rods (MCGR) are encouraging. However, only short case series of MCGR for the treatment of early onset scoliosis (EOS) have been reported. Our aim was to eval uate its effectiveness and complications. Methods We report a 30-case retrospective, consecutive, multicenter series of MCGR. Effectiveness was judged upon: deformity correction and difficulties to achieve desired distraction. Secondary endpoints included compli cations and revision surgeries. Results Median age at surgery was 9.1 years (5 13). Mean follow-up was 18.4 months (12 33.9). Mean Cobb angle was 66 ° preoperatively and 44 ° at latest follow-up. MCGR bas avoided an average of 2.03 scheduled surgical

procedures per patient compared to traditional growing rod (GR). The intended total length gain was 40.1 mm per patient (5 140) and the to tal measured length gain was 21.9 mm (45.5% discrepancy). There were 24 complica tions: 7 proximal pull-outs of the books, 3 rod breakages, 6 failures of the lengthening of which 4 complete blockages and 2 complete blockages followed by backtracking, 1 proximal junctional kyphosis, 1 wound dehiscence, 1 superficial infection, 1 deep infection requiring implant removal, 1 pulmonary embolism, 1 pulmonary insuffi ciency, 1 secondary lumbar scoliosis, and 1 pai nful out patient distraction. Eight patients had a graduai Joss of effectiveness of distractions. There were 13 revision surg eries in 9 patients.

Introduction

Although conservative treatment remains the standard of care for early onset scoliosis (EOS), some patients require surgery [1,2]. Growing rods (GR) then represent a good alternative and provide satisfactory results [START_REF] Dannawi | Early results of a remotely operated magnetic growth rod in early onset scoliosis[END_REF]. However, they need repeated surgeries to allow spine and chest to grow every 6 9 months on average [4 7]. Of concern are significant complication rates, increased costs due to planned and unplanned procedures and psychological consequences [8 10]. The introduction of the magnetically controlled growing rod (MCGR), allowing non-invasive distraction represents a recent breakthrough. Preliminary results are encouraging in terms of effectiveness, security and comfort [START_REF] Dannawi | Early results of a remotely operated magnetic growth rod in early onset scoliosis[END_REF][START_REF] Akbarnia | Next generation of growth sparing techniques: preliminary clinical results of a magnetically con trolled growing rod in 14 patients with early onset scoliosis[END_REF][START_REF] Cheung | Magnetically controlled growing rods for severe spinal curvature in young children: a prospective case series[END_REF]. However, only relatively short case series were reported. Our aim was to evaluate the effectiveness and complications of this treatment. We report our experience in a retrospective consecutive multicenter series of 30 patients with 1 year minimum follow-up.

Materials and methods

We conducted a multicenter, consecutive, prospective series of patients operated on with MCGR for EOS, whatever the type of construct or the origin of the deformity, from October 2011 to January 2014. Indication was either failed conservative treatment or revision of a GR. Patients with less than 12 months of follow-up were excluded.

The first distraction was performed, on average, 3 months after the index procedure and, then, every 2nd or 3rd month, always in outpatient clinic. The distraction was monitored either by radiographs or ultrasound imaging, depending on the institution (Fig. 1), because the radiographic or ultrasound measures are reliable, reproducible and comparable [START_REF] Stokes | Reducing radiation exposure in early onset scoliosis surgery patients: novel use of ultrasonography to mea sure lengthening in magnetically controlled growing rods[END_REF][START_REF] Pe ´rez Cervera | Ultrasound control of magnet growing rod distraction in early onset scoliosis[END_REF].

The type of construct, the instrumented levels, the rate and amount of distractions (as performed with the external actuator and as actually measured on radiographs or ultrasound imaging) were recorded, as well as the number of distraction procedures per session (in case the desired length was not achieved with a single procedure), complications and revision procedures.

Radiographic analysis included preoperative, immediate postoperative and latest follow-up AP and lateral full-spine views and also radiographs performed in case of a complication and/or a revision procedure. Cobb angle, T4 T12 kyphosis, L1 L5 lordosis, T1 S1 and T1 T12 distances were measured by a single observer (JL).

Effectiveness was judged upon: deformity correction (Cobb angle, T1 S1 and T1 T12 distances), difficulties to achieve desired distraction and number of surgical procedures avoided by using non-invasive distraction. The desired distraction varied between 2 and 6 mm per procedure. It was a subjective choice of the surgeon, determined in outpatient clinic just before the distraction. It relied on: the total distraction already obtained, clinical examination (pain, stiffness), the difficulties during the previous distraction and, of course, Dimeglio's data according to the age of the patient [START_REF] Dimeglio | The growing spine: how spinal deformities influence normal spine and thoracic cage growth[END_REF].

For the calculation of the number of surgeries avoided by patient, we have counted one surgical distraction every 6 months with standard GR. Thus, for every period of 6 months without surgical resumption from the day of installation of MCGR (whatever is the indication of the surgery), we counted a surgery avoided per patient.

Secondary endpoints included: complications, revision surgeries, MCGR implant survival to 'revision' event, whatever the cause, and the presence of risk factors for difficulties with distraction procedures and for revision surgeries.

We evaluated the influence of the following factors: origin of EOS, history of GR treatment, age at surgery, type of construct (single or dual rod), proximal and distal limits of instrumentation, preoperative Cobb angle, T4 T12 kyphosis angle, L1 L5 lordosis angle and total number of distraction procedures.

All statistical analyses were carried out using the statistical software R Ò . Characteristics of the population are given by group size (percentage) for categorical variables. Means and standard deviation (±SD) are generally provided for quantitative variables. A paired Student's T test was used to compare the means. All comparisons were two-sided. p \ 0.05 was considered significant.

Results

Thirty patients were included (16 boys, 53%, 14 girls, 47%). Mean age at surgery was 9.1 years [5 13] and mean follow-up was 18.4 months (12 33.9).

Six centers with one experimented surgeon per center participated in the study: Toulouse (nine cases), Marseille (six cases), Lyon (five cases), Luxembourg (five cases), Lille (four cases), Rennes (one case).

Diagnoses were sorted in four groups: neuromuscular (11 cases, 37%), syndromic (9 cases, 30%), idiopathic (7 cases, 23%) and congenital (3 cases, 10%). Five patients (17%) had been previously treated with GR prior to the use of an MCGR. Single rod construct was used in 20 cases (67%) and a dual rod in 10 (33%). There was no recommendation for single or dual rod constructs (it was a personal choice of the surgeon). Standard titanium rods (diameter 5.5 mm) were used in all the cases. Mean proximal level of instrumentation was T2 (T1 T4) and mean distal level was L3 (T10 S1). There were two selective instrumentations ending in T10 and T12. The construct included pedicular screws and hook claws in 25 cases (83%), pedicular screws and hooks in three cases (10%), pedicular screws only in 1 case (3%) and sublaminar bands combined with screws in 1 case (3%). We used three types of hook claws (sublaminar pedicular, laminolaminar, pediculo-transverse). Pedicular screws were used for distal fixation in all cases.

Any patient had brace after Magec Ò rod insertion. Sports activities, excepting contact sports, were allowed for some patients (swimming, badminton, running).

Analysis of primary endpoint: effectiveness of MCGR

Reliability of distraction procedures

All patients had at least one distraction session except for one whose MCGR was explanted 3 months postoperatively with no further implantation. It was a patient who underwent two surgical revisions for loosening of the proximal instrumentation in the first 3 months after rod insertion (3 weeks and 2 months after rod insertion). Thus, it was decided to stop the treatment by Magec rod. A total of 171 distractions were performed among the 29 other patients, corresponding to an average of 5.9 per patient (1 13). A total of 278 procedures were necessary, corresponding to an average of 1.6 per session, with 1 4 procedures per session. It was decided to repeat the procedure when the desired length gain was not obtained and deemed insufficient. Mean time between two scheduled distraction sessions was 89 days (70 192 days). The graduai loss of effectiveness of distractions, 4 (14%) had a complete and permanent blockage and 2 (7%) presented a blockage followed by a backtracking causing loss of length gained from previous distractions (Table 1; Fig. 3).

Distractions number

Fig. 4 Satisfactory distraction in a 7 year old boy with idiopathie scoliosis treated with a submuscular single rod MCGR. Thirteen distraction sessions provided a total 42.5 mm gain at the latest follow up

Graduai loss of effectiveness started between the 3rd and 6th distraction sessions, most often after the 4th (4 cases). Cases of blockage occurred at the 2nd, 3rd, 5th and 8th distraction sessions. The two cases of backtracking were noticed at the 3rd and 9th sessions.

Figure 4 shows a case of satisfactory distraction and Figs. 5, 6 and 7 display three examples of graduai loss of effecti veness.

Deformity correction (Tables 2; 3) Mean Cobb angle was 66° (SD ±18) preoperatively, 40° (±14) postoperatively (p < 0.001) and 44° (±14) at latest follow-up (p = 0.013) (Fig. 8).

Mean thoracic kyphosis was 39° preoperatively, 35°

postoperatively (p = 0.015) and 42° at latest follow-up (p < 0.01). Mean Tl SI distance was 290 mm preoperatively, 349 mm postoperatively (p = 0.004) and 355 mm at latest follow-up (p = 0.582) (Fig. 9).

Scheduled surgical procedures avoided

On the basis of a surgical distraction scheduled every 6 months with traditional GR, the use of MCGR bas avoided a mean of 2.03 scheduled surgical procedures per patient [CI 95 % = (1.56; 2.51)) during the total observation period.

Analysis of secondary endpoint: safety of treatment

Complications

There were 24 complications in 17 patients (57% ): 7 proximal loosening in 5 patients, 3 rod breakages in 2 patients, 6 failures of the lengthening of which 4 complete blockages and 2 complete blockages followed by backtracking, 1 proximal junctional kyphosis (PJI() in spite of non-aggressive distractions, 1 wound dehiscence with implant exposure, 1 superficial infection, 1 deep infection 12 ~------------------

~Intended distraction 10 +----------------j=----<>- ~Measured distraction 2+------------------- o~------------------ 1 2 3 4 5
Distractions number Fig. 7 Thini example of Joss of effectiveness of repeated distractions over time in a 9 year old boy with idiopathie scoliosis treated with a single rod submuscular MCGR. The discrepancy between intended and measured Jength gain started at the 4th session requiring implant removal, 1 pulmonary embolism at day 3 treated with thrombectomy, 1 pulmonary insufficiency requiring tracheotomy, 1 secondary lumbar scoliosis, and 1 painful outpatient distraction.

Revisions (Fig. JO)

There were 13 revision surgeries in 9 patients (30% ), 5 patients had 1 and 4 patients had 2 revisions: 2 for wound excision, 5 for book loosening, 3 for rod breakage, 2 for non-functioning rod, and 1 for proximal junctional kyphosis.

Four patients had their MCGR removed at revision: three had a spinal fusion and one was treated with bracing. The five remaining patients had their MCGR treatment resumed.

Complications and revision surgeries were both more frequent in syndrornic and neuromuscular scoliosis, although not significantly (Table 4).

The survival analysis in our series indicated a survival rate without revision from 0.5 to Tl.9 months. ln other words, the probability to have any revision at 27.9-month follow-up was 50% (Fig. 11).

Risk factors for difficult distractions and revision surgeries

Mean postoperative thoracic kyphosis was higher in cases where difficulties with distraction (blockage, backtracking, loss of effectiveness) were encountered (41 ° vs. 30.8°, not significant). Difficult distractions were more frequent in idiopathie (36%) and neuromuscular (36%) scoliosis (not significant) (Table 5). Revision (n = 9) and revision-free (n = 21) groups were not different in terms of the following factors: age at MCGR implantation, preoperative Cobb angle, thoracic kyphosis and lumbar lordosis, the type of construct (single or dual rod), history of GR treatment, the number of distraction sessions and the instrumented levels (Table 6).

All five patients previously treated with standard GR did not need revision. Dual rod constructs tended to have fewer revisions (not significant). Idiopathic and neuromuscular scoliosis tended to have more revisions (not significant).

Discussion

Fusionless spinal instrumentation for EOS was first introduced in 1963 by Paul Harrington [START_REF] Harrington | Scoliosis in the growing spine[END_REF]. Twenty years later, Moe reported the first GR [START_REF] Moe | Harrington instrumentation without fusion plus external orthotic support for the treatment of difficult curvature problems in young children[END_REF]. Many different techniques have been developed ever since, but mechanical GRs remain the most popular worldwide [START_REF] Dannawi | Early results of a remotely operated magnetic growth rod in early onset scoliosis[END_REF][START_REF] Akbarnia | Next generation of growth sparing techniques: preliminary clinical results of a magnetically con trolled growing rod in 14 patients with early onset scoliosis[END_REF]. The goal is to correct the spinal deformity while allowing growth of the spine and chest. However, GR requires repeated surgeries for lengthening and the complication rate is correlated to the number of lengthening procedures [ 16 18]. Repeated general anaesthesia in children bears the risk of developing post-traumatic stress, speech disorders and long-term cognition disorders [START_REF] Suliman | Cumulative effect of multiple trauma on symptoms of posttraumatic stress disorder, anxiety, and depression in adoles cents[END_REF][START_REF] Backeljauw | Cognition and brain structure following early childhood surgery with anesthesia[END_REF].

MCGR is, therefore, promising on this regard as it allows non-invasive outpatient distraction procedures [START_REF] Caldas | General anes thesia, surgery and hospitalization in children and their effects upon cognitive, academic, emotional and sociobehavioral devel opment a review[END_REF]. It also seems to be cost-effective in saving hospitalization costs [START_REF] Charroin | Direct costs associated with the man agement of progressive early onset scoliosis: estimations based on gold standard technique or with magnetically controlled growing rods[END_REF][START_REF] Rolton | Magnetic controlled growth rods versus conventional growing rod systems in the treatment of early onset scoliosis: a cost comparison[END_REF].

Very few studies have investigated the results of MCGR. Cheung et al., in 2012, reported on the effectiveness and safety in a two-patient case series with 2-year follow-up [START_REF] Cheung | Magnetically controlled growing rods for severe spinal curvature in young children: a prospective case series[END_REF]. Cobb angle was improved, on average, from 67°to 29°with no implant-related complication and a superficial infection. They found that the healthcare costs of MCGR are substantially lower than with traditional GR. Although MCGR instrumentation costs more (HK$50,000; US$6451) than traditional GR (HK$25,000; US$3225), the latter is associated with further costs due to frequent operations, spinal cord monitoring, use of general anaesthesia, hospital stays, drug use, manpower, consumables, and time off work for the parents. Akbarnia et al. confirmed these encouraging results in 2013 in a 14-patient prospective series. Cobb angle was improved, on average, from 60°to 31°at 10-month followup with a superficial infection and a painful rod [START_REF] Akbarnia | Next generation of growth sparing techniques: preliminary clinical results of a magnetically con trolled growing rod in 14 patients with early onset scoliosis[END_REF].

The current series demonstrates satisfactory coronal deformity correction over time, and little improvement of T1 T12 and T1 S1 distances with MCGR, in accordance with the literature [8 10, 24 27], although with a higher complication rate. Indeed, we report 57% of complication in our series against 38.8% in the series of Choi et al. in 2016 (about 55 cases with a mean follow-up of 19.4 months) [START_REF] Choi | Implant complications after magnetically controlled growing rods for early onset scoliosis: a multicenter retrospective review[END_REF]. Concerning the revision rate, it is quite similar in both series with 27.8% in Choi et al.'s, vs 30% in ours [START_REF] Choi | Implant complications after magnetically controlled growing rods for early onset scoliosis: a multicenter retrospective review[END_REF]. This high rate of revision resulted in a revisionfree survival of only 50% at 27.9 months in our study. However, with the non-invasive distraction of MCGR, an average 2.03 surgical procedures per patient were avoided at the end of follow-up if compared to a standard GR treatment.

Alike in Choi et al.'s study [START_REF] Choi | Implant complications after magnetically controlled growing rods for early onset scoliosis: a multicenter retrospective review[END_REF], most of the revisions and complications (rod breakage and proximal pull-outs) were not related to the MCGR implant itself. The distraction system failed in two cases only in our series. It was a backtracking of the rod with, both times, complete loss of the gained length. To the best of our knowledge, failure of the system with backtracking has been reported twice before in the literature: one case by Ridderbusch et al. [START_REF] Ridderbusch | Preliminary results of magnetically controlled growing rods for early onset scoliosis[END_REF] and six cases by Choi et al. [START_REF] Choi | Implant complications after magnetically controlled growing rods for early onset scoliosis: a multicenter retrospective review[END_REF].

As regards the other complications and revisions, we were not able to identify any risk factor, and these complications frequently occur both in MCGR and GR treatments [START_REF] Akbarnia | Dual growing rod technique for the treatment of progressive early onset scoliosis: a multicenter study[END_REF][START_REF] Choi | Implant complications after magnetically controlled growing rods for early onset scoliosis: a multicenter retrospective review[END_REF][START_REF] Blakemore | Submuscular Isola rod with or without limited apical fusion in the management of severe spinal deformities in young children: preliminary report[END_REF][START_REF] Thompson | Comparison of single and dual growing rod techniques followed through definitive surgery: a preliminary study[END_REF].

Some authors found a higher rate of rod breakage with single rod constructs [START_REF] Akbarnia | Dual growing rod tech nique followed for three to eleven years until final fusion: the effect of frequency of lengthening[END_REF][START_REF] Akbarnia | Dual growing rod technique for the treatment of progressive early onset scoliosis: a multicenter study[END_REF]. This hypothesis was not verified in the current series with a total of three breakages, of which two occurred in the same patient with a dual rod construct. Rod breakage rates in GR varied from 15% for Yang et al. [START_REF] Yang | Growing rod fractures: risk factors and opportunities for prevention[END_REF] to 24% for Bess et al. [START_REF] Bess | Complications of growing rod treatment for early onset scoliosis: analysis of one hundred and forty patients[END_REF]. Hickey et al. reported a single breakage in a six-patient series (13%) [START_REF] Hickey | Early experience of MAGEC magnetic growing rods in the treatment of early onset scoliosis[END_REF].

There was a gradual loss of effectiveness over the course of treatment in half of the cases. The so-called 'law of diminishing returns' described in GR seems applicable to MCGR from the 4th distraction session onward. The phenomenon was first reported by Sankar et al. [START_REF] Sankar | Lengthening of dual growing rods and the law of diminishing returns[END_REF] and represents the gradual decrease in length gain with each subsequent lengthening and over time, despite an increased distraction force applied. It may be explained by tissue scaring and stiffening of the instrumented segment [START_REF] Sankar | Lengthening of dual growing rods and the law of diminishing returns[END_REF][START_REF] Noordeen | In vivo distraction force and length measurements of growing rods: which factors influence the ability to lengthen?[END_REF].

Rolton et al. [START_REF] Rolton | Radiological and clinical assessment of the distraction achieved with remotely expandable growing rods in early onset scoliosis[END_REF] found also loss of effectiveness and incomplete distractions. In his study, the true to intended distraction ratio was calculated as 0.33; and the patients who had undergone previous surgery gained less distraction than the others. It was not statistically significant in our study, but difficult distraction (blockage, backtracking, loss of effectiveness) was more frequent in case of marked postoperative thoracic kyphosis and in neuromuscular and syndromic cases. Syndromic and neuromuscular cases also tended to be more prone to surgical revisions.

We acknowledge several weaknesses to this study: the constructs and levels of instrumentation were heterogeneous, distractions were monitored either with radiographs or ultrasound imaging and the follow-up was relatively small. However, it represents the largest series of MCGR to date.

Conclusion

MCGR is a reasonable option in case of contraindicated or failed conservative treatment in EOS. It provides satisfactory deformity correction and avoids repeated surgical procedures for lengthening. However, it has substantial complication rate. Although less frequent than in GR, the law of diminishing returns also applies to MCGR.

Compliance with ethical standards

Conflict of interest There is no conflict of interest or funding or grants in this study.

Ethical approval Ethics Committee gave its agreement for this work.

Informed consent Informed consent was obtained from all individ ual participants included in the study.

Fig. 1

 1 Fig. 1 Radiographic (a) and ultrasound (b) monitoring of the distraction before and after a distraction session

•Complet

  Fig. 2 lntended and measured distractions, per session and per procedure. N 29, one patient being excluded because of 3 month explantation

Fig. 5 Fig. 6

 56 Fig.5Fîrst example of loss of effectiveness of repeated distractions over time in a 9 year old girl with syndromic scoliosis treated with a dual rod submuscular MCGR. The discrepancy between intended and measured length gain started at the 5th session and worsened from the 7th

Fig. 8

 8 Fig. 8 Cobb angle at different times of follow up

  ----: ------------------------:----•----------: --------Time since surgery (months) Blue line : survival curve (dotted line : 95% Confidence lnterval). Red line : median survival

  1.Harrington PR (2002) Treatment of scoliosis: correction and internal fixation by spine instrumentation. J Bone Joint Surg Am 84 A(2):316 2. Karol LA, Johnston C, Mladenov K, Schochet P, Walters P, Browne RH (2008) Pulmonary function following early thoracic fusion in non neuromuscular scoliosis. J Bone Joint Surg Am 90(6):1272 1281

Table 1

 1 Total intended and

	total measured distraction per patient with the difficulties	Patients	Total intended distraction (mm)	Total measured distraction (mm)
	observed during the follow up	1	48	45
		2	78	23
		3	19	19
		4	8	8
		5	120	36
		6	83	33.J
		7	27.5	24.6
		8
		9	45.5	43.4
		10	49	31.5
		Il	28	4.6
		12	54	38.5
		13	35	6.4
		14	40
		15	65
		16	29
		17	24
		18	28
		19	5
		20	12 .5
		2 1	50
		22	13
		23	5
		24	28
		25	52
		26	9 1
		27	13
		28	40
		29	49
		30	24.2
		Total	1163.7
	intended total length gain (which corresponds to the
	addition of the 278 desired distractions done) was
	1163.7 mm corresponding to the 171 sessions for the 29
	patients, representing an average of 40.1 mm per patient
	(from 5 to 140) and 6.8 mm per session (2 35). Total
	measured length gain was 634.4 mm, representing an
	average of 21.9 mm per patient (5 43.4) and 3.7 mm per
	session (0 18.3) (Table 1). Total measured length inclu-
	ded only the distractions done during magnetically
	induced lengthenings. The gain obtained during insertion
	surgery was excluded. This represented a 45.5% discrep-
	ancy between the length gain commanded to the remote
	control and the length gain actually measured either on

radiographs or ultrasound imaging (Fig.

2

). Measured length gain equaled commanded length gain in 15 patients (52%), with no difficulty encountered throughout followup. ln the 14 remaining patients (48%), 8 (28%) had a

Table 2

 2

	Radiographie results	Preoperative	Postoperative	Latest follow up
		Mean	SD	Mean	SD	Mean	SD
	Cobb angle ( 0 )	66	± 18	40	± 14	44	± 14
	Thoracic kyphosis ( 0 )	39	± 17	35	± 17	42	± 16
	Lumbar lordosis ( 0 )	48	± 15	4 1	± 16	48	± 15
	Tl Tl2 distance (mm)	184	± 28	218	± 30	220	± 26
	Tl S I distance (mm)	290	± 4 1	349	± 36	355	± 34
	SD standard deviation
	Table 3 Comparison of radiographie results	Preoperative/postoperative		Postoperative/latest follow up
		Difference	%	p	Difference	%	p
	Cobb angle	26°	39	<0.001	+40	+9	0.0 13
	Thoracic kyphosis	40	JO	0.015	+70	+17	0.001
	Lumbar lordosis	70	15	0.025	+70	+15	0.025
	Tl Tl2 distance	+34mm	+16	<0.001	+2mm	+l	0.727
	Tl S 1 distance	+59mm	+17	0.004	+6mm	+2	0.582

Table 4

 4 Complications and revisions according to the diagnosis of EOS

		Idiopathic		Congenital		Syndromic		Neuromuscular	p
		N 7	%	N 3	%	N 9	%	N 11	%
	Revisions	0	0	1	33	4	44	4	36	0.19
	Complications	4	57	1	33	5	56	7	64	0.95

Fig. 11 Revision free survival of MCGR

Table 5

 5

	Influence of postoperative kyphosis and		Trouble free distraction (n 15) Problematic distraction (n 14) p
	diagnosis upon distraction	Postoperative kyphosis (°) 30.8	±8	4 1	±1	0.18
		Diagnosis					0.52
		Idiopathic	2	13%	5	36%
		Congenital	2	13%	1	7%
		Syndromic	6	40%	3	21%
		Neuromuscular	5	33%	5	36%

Table 6

 6

	Risk factors for revision surgery	Revision free	Revision		p
		N 21	Range	N 9	Range
	Age at MCGR implantation (years)	9	(7; 11)	9	(8; 11)	0.42
	Preoperative Cobb angle (°)	66	(52.2; 80)	73	(56.2; 77)	0.88
	Preoperative thoracic kyphosis (°)	44.4	(33.9; 49.7)	32.5	(14.5; 40.8)	0.21
	Preoperative lumbar lordosis (°)	50	(37; 59)	50.5	(31; 61.2)	0.90
	Dual rod construct	8	38%	2	22%	0.67
	History of GR treatment	5	24%	0	0%	0.29
	Number of distraction sessions	5	(4; 8)	5	(1; 6)	0.19
	Number of procedures per session	8	(5; 14)	6	(1; 11)	0.12
	Proximal level instrumented					0.24
	T1	6	29%	1	11%
	T2	7	33%	7	78%
	T3	5	24%	1	11%
	T4	3	14%	0	0%
	Distal level instrumented					NA
	T10	1	5%	0
	T12	0		1	11%
	L1	0		1	11%
	L2	2	10%	1	11%
	L3	4	19%	2	22%
	L4	10	48%	3	33%
	L5	2	10%	0
	S1	2	10%	1	11%
	Diagnosis					0.19
	Idiopathic	7	33%	0	0%
	Congenital	2	10%	1	11%
	Syndromic	5	24%	4	44%
	Neuromuscular	7	33%	4	44%


P Ouagne

D Soulat

C Tephany

J Gillibert

Measurement of the Appearance and Growth of Tow Buckling Defect in the Frame of Complex Shape Manufacturing Process by Using Fringe Projection Technique

Keywords: composite manufacturing processes, fringe projection, textile forming, tow buckling

During the manufacturing of composite complex shape parts, defects such as tow buckles characterised by out of plane elevation may appear. The parameters controlling the appearance and growth of the defect are not completely understood and need to be investigated. A device capable of reproducing tow buckles has been used to study the tow buckling phenomenon. Several techniques able to measure out of plane elevations are discussed to detect the appearance and evaluate continuously the growth of the tow buckle in relation to its size and shape. The fringe projection technique was chosen as it gives the best compromise between the size of the defect to measure and its resolution.

If the in-plane bending angle is the main criterion at the origin of the tow buckle appearance and growth, it is not the only one. This work shows that the fabric architecture such as the space between the tows perpendicular to the one showing the buckle is also crucial to control the buckle's appearance and growth. It also shows that the differential bi-axial loading of the fabric as well as the stiffness of the tows in the three main directions greatly influences the appearance of the defect.

KEY WORDS: complex shape,

Introduction

During the manufacturing process, defects may occur in composite parts. Potter et al. [START_REF] Potter | Variability, fibre waviness and misalignment in the determination of the properties of composite materials and structures[END_REF] have presented a list of defect states that may occur during the composite manufacturing processes. Some of the defects appearing during the first step of the RTM process (preforming stage) may have adverse impacts on the performance of composite parts and should be prevented [START_REF] Long | Composites Forming Technologies[END_REF][START_REF] Gereke | Experimental and computational composite textile reinforcement forming: A review[END_REF]. At the scale of the preform, denoted macroscale, it is possible to investigate if the preform shape is well obtained, if wrinkles appear, if there is non-homogeneity of the fibre density in tows, or if a discontinuity of the preform due to sliding of tows takes place [START_REF] Bloom | Damage progression and defect sensitivity: an experimental study of representative wrinkles in tension[END_REF][START_REF] Lee | The effect of blank holder force on the stamp forming behavior of non-crimp fabric with a chain stitch[END_REF][START_REF] Boisse | Simulation of wrinkling during textile composite reinforcement forming Influence of tensile, in-plane shear and bending stiffnesses[END_REF][START_REF] Zhu | Theoretical modeling of large shear deformation and wrinkling of plain woven composite[END_REF][START_REF] Ouagne | Analysis of defects during the preforming of a woven flax[END_REF]. At this scale, a lot of studies [START_REF] Pierron | Application of full-field measurement techniques to composite materials and structures[END_REF][START_REF] Lomov | Full-field strain measurements in textile deformability studies[END_REF][START_REF] Lomov | Full-field strain measurements for validation of meso-FE analysis of textile composites[END_REF][START_REF] Willems | Drape-ability characterization of textile composite reinforcements using digital image correlation[END_REF][START_REF] Lomov | Picture frame test of woven composite reinforcements with a full-field strain registration[END_REF][START_REF] Cao | Characterization of mechanical behavior of woven fabrics: experimental methods and benchmark results[END_REF][START_REF] Pierce | A low-cost digital image correlation technique for characterising the shear deformation of fabrics for draping studies[END_REF] have been published on the use of full field optical measurements of strains. They provide an ideal instrument to quantify the strain levels in different deformation modes (tension, in-plane shear). At the scale of the tows, mesoscale, other defects may appear. Tow misalignment in the plane of the fabric, [START_REF] Potter | Variability, fibre waviness and misalignment in the determination of the properties of composite materials and structures[END_REF] or out-of-plane misalignment also called tow buckles [START_REF] Bloom | Damage progression and defect sensitivity: an experimental study of representative wrinkles in tension[END_REF][START_REF] Ouagne | Complex shape forming of a flax woven fabric; analysis of the tow buckling and misalignement defect[END_REF][START_REF] Capelle | Complex shape forming of flax woven fabrics: Design of specific blank-holder shapes to prevent defects[END_REF], can be investigated. All these defects have a strong influence on the resin flow impregnation because they modify the in-plane and through-the thickness permeability components [START_REF] Walther | The effect of fabric and fiber tow shear on dual scale flow and fiber bundle saturation during liquid molding of textile composites[END_REF][START_REF] Arbter | Experimental determination of the permeability of textiles: a benchmark exercise[END_REF][START_REF] Hou | Gas transport in fibrous media: application to in-plane permeability measurement using transient flow[END_REF][START_REF] Ouagne | Continuous transverse permeability of fibrous media[END_REF][START_REF] Ouagne | Continuous measurement of fiber reinforcement permeability in the thickness direction: experimental technique and validation[END_REF], and they probably impact the performance of the composite part because the fibres are not aligned anymore. In the case of the fibre waviness defect that may also lead to out of plane bending of tows, Bloom et phenomenon and the appearance of the wrinkling defect mainly caused by the locking of warp and weft yarns [START_REF] Davidson | Effect of Fibre Waviness on the Compressive Strength of Unidirectional Carbon Fibre Composites. 53 rd structural materials comference[END_REF][START_REF] Hayman | The effect of face sheet wrinkle defect on the strength of FRP sandwich structures[END_REF]. During the forming process, the appearance of wrinkling is not only due to in-plane shear but also to the tension of the membrane and the bending stiffness. Several authors therefore decided to study independently of the process this defect by designing specific in-plane shear characterisation protocols using either the bias extension test or the picture frame test [START_REF] Potter | Variability, fibre waviness and misalignment in the determination of the properties of composite materials and structures[END_REF][START_REF] Cao | Characterization of mechanical behavior of woven fabrics: experimental methods and benchmark results[END_REF][START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF][START_REF] Rozant | Drapability of dry textile fabrics for stampable thermoplastic preforms[END_REF][START_REF] Launay | Experimental analysis of the influence of tension on in-plane shear behaviour of woven composite reinforcements[END_REF][START_REF] Willens | Optical strain fields in shear and tensile testing of textile reinforcements[END_REF][START_REF] Harrison | Constitutive modelling of impregnated continuous fibre reinforced composites micromechanical approach[END_REF]. Some other authors proposed different approach to model the appearance of defects such as the wrinkling defect [START_REF] D'agostino | Continuum and discrete models for structures including (quasi-) inextensible elasticae with a view to the design and modelling of composite reinforcement[END_REF][START_REF] Dell'isola | A two-dimensional gradient-elasticity theory for woven fabrics[END_REF][START_REF] Ferretti | Modeling the onset of shear boundary layers in fibrous composite reinforcements by second-gradient theory[END_REF][START_REF] Harrison | Modelling the forming mechanics of engineering fabrics using a mutually constrained pantographic beam and membrane mesh[END_REF]. Kirkwood et al. [34] also used the same approach to study independently of the process the yarn pull-out phenomenon by designing an especially dedicated set-up.

Non-destructive evaluation (NDE) techniques have been employed to inspect and characterise composite structures [START_REF] Willens | Optical strain fields in shear and tensile testing of textile reinforcements[END_REF]. Various NDE methods, such as the optical techniques of holography, electronic speckle-pattern interferometry, shearography and time-domain moiré interferometry, are used for thermoplastic or thermoset composites and are described by a number of authors [START_REF] Heslehurst | Assessment of local damage in composite structures by optical methods[END_REF][START_REF] Ambu | Assessment of NDT interferometric techniques for impact damage detection in composite laminates[END_REF][START_REF] Crammond | A point-wise approach to the analysis of complex composite structures using digital image correlation and thermoelastic stress analysis[END_REF][START_REF] Wang | Qu Weibo measurement of strain concentration factor of composites plate using moiré interferometry[END_REF][START_REF] Pezzoni | Laser-shearography for nondestructive testing of large area composite helicopter structures[END_REF][START_REF] Abou-Khousa | shearography and through-transmission ultrasonic methods for inspection of honeycomb composites[END_REF][START_REF] Post | High Sensitivity Moire´: Experimental Analysis for Mechanics and Materials[END_REF]. These techniques provide wide-area qualitative imagery in the in-and out-of-plane displacement at the surface of a structure. However, their effectiveness at inspecting structures of any significant thickness for buried defects is normally vastly reduced [START_REF] Ibrahim | Nondestructive evaluation of thick-section composites and sandwich structures: A review[END_REF]. If the use of all these methods is well described for composite parts, the detection of defects at the scale of fabrics, before injection of resin, and associated to this preforming stage, has not really been studied.

In this paper, an experimental device to reproduce the tow buckling phenomenon is used and is associated to the fringe projection technique to quantify the out-of-plane defect.

Design of the Instrumented Tow Buckling Device

Up to date knowledge on the tow buckle generation

The bending of tow in its plane has already been studied in the frame of the robot lay-up of pre-impregnated ribbons. Beakou et al. [START_REF] Beakou | Modelling slit tape buckling during automated prepreg manufacturing: A local approach[END_REF] modelled the appearance of such defects. However, the appearance of tow buckles in woven fabrics is more complex. In the first studies reporting the appearance of the tow buckling defect during the forming process, the main mechanism without which the tow buckling defect cannot appear has been identified for different carbon, glass and flax-based flat tow woven fabrics by several authors. One of the first descriptions was given by Harrison et al. in 2002 [29]. This work was more recently completed by some pieces of work performed at Orléans [START_REF] Ouagne | Analysis of defects during the preforming of a woven flax[END_REF][START_REF] Ouagne | Complex shape forming of a flax woven fabric; analysis of the tow buckling and misalignement defect[END_REF][START_REF] Capelle | Complex shape forming of flax woven fabrics: Design of specific blank-holder shapes to prevent defects[END_REF]. This mechanism is the bending in their plane of flat tows imposed by the geometry of the shape (Figure 1).

As this mechanism cannot be or can only be partially accommodated, another mechanism consisting in deforming the tow out of its plane takes place and a tow buckle appears as shown in Figure 2.

The measurements performed on a face of a tetrahedron where tow buckling appears show that globally, the bending angles remain similar all along the buckling zone with lower values (more severe bending) towards the top of the shape [START_REF] Ouagne | Complex shape forming of a flax woven fabric; analysis of the tow buckling and misalignement defect[END_REF]. This bending angle quasi-continuity means that the displacements of the tows exhibiting the buckles are almost similar and symmetrical against the curvature point line. This mechanism is the same as the one observed for ribbon lay-up.

However, the in-plane bending of the tow is not the only parameter influencing the appearance of the tow buckles. Reducing the tension of the tows perpendicular to the ones exhibiting the buckles delays and reduces the size of the buckles to a very low extent [START_REF] Capelle | Complex shape forming of flax woven fabrics: Design of specific blank-holder shapes to prevent defects[END_REF]. This is particularly true for the tetrahedron shape where the tension is maximum in the zone exhibiting buckles. As a consequence, the device should be able to impose different in-plane bending angles to the tow showing the buckles and to apply different magnitudes of tension to the perpendicular tows. Other parameters related to the meso and macro architecture of the fabric are also very important and should be studied with the device. The device should therefore permit the study of different types of reinforcement with different yarn or tow architectures and geometries. The tow buckling device According to the previous section, the buckling defect is mainly the result of a combination between the bending of tows in their plane and level of tension applied to the tows perpendicular to the ones showing the buckles. The tension imposed to the tows showing the buckles is also probably very important and should also be studied. A device has therefore been designed to impose separately in-plane bending angles to the tows showing the buckles and variable tensions in the perpendicular tows. The design of the mechanical part of the device was described into details in [START_REF] Tephany | Development of an experimental bench to reproduce the tow buckling defect appearing during the complex shape forming of structural flax based woven composite reinforcements[END_REF] and is summarised in Figure 3. The fabric samples are maintained by jaws in the horizontal and vertical directions. Adjustable tensions can be applied in both directions. In Figure 3a, the horizontal jaws can bend in their plane following a circular translation so that the tows bend on a given point and stay straight as shown in Figure 1. To perform the circular translation, the deformable parallelogram technique has been used (Figure 3b).

Semi-cross geometry samples have been chosen as these ones prevent the appearance of wrinkles or tow sliding on the contrary of square samples or cross shape samples, respectively. The device is instrumented. Load sensors, Linear Variable Differential Transformer, optical measurements using mark tracking technique and fringe projection measurement techniques are used during the test to provide all the necessary information required to study the tow buckling phenomenon. The device instrumentation is summarised in Figure 4.

Measurement of the buckle's height

The detection of the buckles appearance and the measurement of its amplitude variation during its growth are critical parameters that need to be carefully and accurately measured. Particularly, the buckle's size, or buckle's height (Figure 5) should be evaluated by a noncontact technique to avoid any interaction between the measurement and the buckle's appearance and growth.

The determination of the buckle's height consists in evaluating the topography of the reinforcement at given times during a test. As an example, the size of the buckle should be measured during the test for any modification of a device parameter such as the bending angle, or the tension applied on the tows in each direction. As a consequence, the measurement tool should be able to scan a relatively large zone on the surface of the sample test, and the acquisition and treatment times should be reasonable.

Several methods were investigated to measure this parameter. The first one is the stereo-image correlation with 3D mark tracking, which permits to evaluate the displacement field of the reinforcement [START_REF] Lomov | Full-field strain measurements in textile deformability studies[END_REF][START_REF] Tephany | Development of an experimental bench to reproduce the tow buckling defect appearing during the complex shape forming of structural flax based woven composite reinforcements[END_REF]. However, this method was rejected as it is not possible to follow the marks through the totality of the test because of the incidence angle of the cameras. This is due to the fact that during the appearance of the buckles, some zones of the fabric, visible at the beginning of the test cannot be observed at the end of the test, and the profile of the buckles cannot be entirely recorded. The second tested method is the shadow detection method. It consists in observing the evolution of the size of the shadows caused by the buckle's appearance under glancing light conditions. If this method could be easily used for the determination of the profile of a unique buckle, the profile of the shadow during the test is disturbed by the elevation of adjacent buckles. The evaluated buckle's size is therefore not representative of the true height of the defect. This method was also rejected.

The third tested method is fringe projection. This method is particularly interesting for our case of study because it permits to measure the topography of the surface of an object without any contact, which could modify the state of the surface [START_REF] Hariharan | Basics of Interferometry[END_REF][START_REF] Leandry | Adaptation de la Méthode Deprojection de Franges pour la Mesure du Relief de Grands Objets et Pour la Modelisation Anthropometrique: Application à l'étude de Flotteurs sous Pression et au Suivi de Pathologie de L'abdomen[END_REF][START_REF] Surrel | Les techniques optiques de mesure de champ: essai de classification[END_REF][START_REF] Jahne | Handbook of Computer Visional Applications[END_REF]. Moreover, its resolution and zone of study correspond to the requirements of this work. Table 1 compares the resolution and extent of the zone of study of different interferometry methods and the Fringe projection techniques. The fringe projection technique is also particularly interesting as it does not necessitate that the material is conductor (scanning tunnel microscopy or scanning electron microscopy), and the size of the observation zone is much larger than the one of atomic force microscopy, which requires long acquisition times.

In our case of study, the size of the buckles is situated in a range between 0 and 1.5 mm. The use of fringe projection is therefore the most appropriated technique. The global principle and the performance of this measurement technique were reviewed by Gorthi et al. [START_REF] Gorthi | Fringe projection techniques: Whither we are[END_REF], and no further description will be given in this paper.

Experimental Procedure

Description of the used fringe projection apparatus

The measurements performed by using the fringe projection technique have been completed at the Science et Ingéniérie des Matériaux et Procédés of Grenoble (http://simap. grenoble-inp.fr/accueil/). The used experimental device is composed of three main elements: a CCD camera, a video projector and software elements H3SensorDigit3D coupled to the FringeAnalysis4®. The H3SensorDigit3D is a piece of software developed by the HOLO3 Company (Saint Louis, France) dedicated to the 3D digitalisation using optical ways. The FringeAnalysis4® is a software dedicated to the analysis of fringes images used in the holography, speckle or structured light interferometry. A video projector is used to project calibrated computer elaborated fringes.

Materials

The flax fabric used in this study is a plain-weave fabric with an areal weight of 280 ± 19 g/m 2 , manufactured by Groupe Depestele (France) from untwisted tows. The fabric is not balanced, as the space between the weft tows (1.59 ± 0.09 mm) is different from the one between the warp tows (0.26 ± 0.03 mm). The width of the warp and the weft tows are respectively 2.53 ± 0.12 mm and 3.25 ± 0.04 mm. As a consequence, there are 360 warp tows and 206 weft tows per metre of fabric.

Results

Initial test: profile and size of the buckles at the initial and final states

The profile of altitude studied in this work is recorded along the red and black lines shown in Figure 6a1 andb1. The different colours presented in Figure 6a1 represent different level of altitudes. At the initial state, light blue colour is observed along the width of the horizontal tows. Dark blue colour is observed between these ones. As globally, two colours are observed; a square periodic profile of tow elevation is obtained by processing the fringe projection measurements along the vertical line (Figure 6a2). This is due to the nature of the plain weave fabric manufactured with flat tows interlaced following a plain weave pattern (Figure 6a3). By performing approximate measurements with a Vernier calliper directly on the fabric, it appears that the differential elevation between the warp and the weft tows is about 0.4 ± 0.1 mm.

At a state where buckles are well visible, the same procedure has been used. A colour gradient can be observed across the width of the horizontal tows (Figure 6b1). The altitude of the tow where buckles are formed show increasing altitude colours from one side of the tow to the other one up to a maximum value. A chevron type profile as presented in Figure 6b2 is well in accordance with the colour evolution and with the experimental observation (Figure 6b3). The maximum elevation recorded using the fringe projection technique has been compared with the one measured on the shape using a Vernier calliper. The measured altitude (1.55 ± 0.07 mm) is in good agreement with the ones given by the fringe projection technique.

This preliminary test therefore indicates that the fringe projection technique, and the chain associated to the signal processing is validated. As a consequence, the profile evolution can be used to investigate into details the appearance and the growth of the buckles in the following section. Profile of the buckles: evolution as a function of the in-plane bending angle

Figure 7a shows the evolution of the buckle's profile and altitude for four different in-plane bending angles. The colours presented in Figure 6a1 have been processed through in house MATLAB routines in order to get access to the actual elevation values. The buckle's profile progressively changes from a square profile (α = 0°) to a chevron type profile (α = 40°) as observed on the edge of a tetrahedron shape (Figure 6b3). The change of the profile shape is progressive. A square type profile with different elevations than at the initial state is still observed for in-plane bending angle of 20°. For an in-plane bending angle of 30°, the square profile of the buckle disappears and is replaced by a chevron type profile. At the final state (α = 40°), the profile remains similar, but the maximum elevation of the buckle is increased up to about 1.5 mm.

Figure 7a also shows that negative peaks of altitudes are observed before and after the rise of the buckles for each bending angle. These ones are due to the fact that the tow perpendicular to the one showing the buckle passes underneath it, and a drop of altitude is observed at its edge. However, the negative peak is more pronounced on the side of the buckle that does not show any altitude increase particularly for high value bending angles. This phenomenon is probably due to the fact that the tow exhibiting the buckle imposes a compression load on the tow passing underneath as a consequence of its rise. This phenomenon is due to the rotation of the buckle around itself in the buckle's zone as illustrated by a simple test on a single tow in Figure 8.

This phenomenon can also be observed on the profiles presented in Figure 7b for an orientation 90°of the fabric. The proportion of this negative decrease of altitude is actually larger in this case as no increases of altitude are observed between the initial and the maximum bending angle that the device can possibly impose. On the contrary, the altitude of the tow that still shows a square profile decreases. For high bending angles, a plateau of altitude is still observed, with a lower altitude. Even if the horizontal tows are submitted to equivalent bending angles, these ones do not show any increase of altitude like it is the case for the same test with a 0°orientation.

To study the profile evolution into details as a function of the increasing bending angle, the different recorded profiles have been aggregated. The bending angle at which the buckle appears for the orientation 0°can be investigated. The evolution of the profile is shown for increasing bending angles in Figure 9a for a sample placed with an orientation 0°(with the weft tows placed in a fixed position and warp tows submitted to in-plane bending) and in Figure 9b for a sample placed with an orientation 90°.

The maximum elevation of the buckle has also been recorded for each bending angles and is reported in Figure 10 for both orientations.

For the 0°orientation, it appears that low elevation is recorded below in-plane bending angles of 28°. For bending angles in the range [0-28°], the elevation slowly rises from its initial value (elevation 0.40 ± 0.04 mm) to reach a value of 0.6 mm. Above bending angles of 28°and up to 35°, an important rise of elevation is observed, from 0.6 to 1.45 mm. This is for our testing conditions (7.5 N and 15.3 N tensions applied in the X and Y directions) within this range of bending angle that the buckle rises in one side of the tow and decreases in altitude in the other side as described in Figure 8. For the highest bending angles tested in this work, the elevation still increases with a lower slope. It is also interesting to note that the full chevron profile is not observed below angles of about 30°. This means that the buckle does not exhibit a continuous rising profile (chevron) for in-plane bending angles lower than about 30°. Up to this bending angle, the tow globally shows a plateau shape profile that progressively rises. Above 30°, instead of showing a plateau, the tow starts to rotate around itself and the buckle appears before quickly growing up to values of about 38°. Above this bending angle value, it probably becomes more difficult for the buckle to rise following a rotation around itself as this one needs to overcome compression resistance of the tow situated below it. The compression phenomenon and its magnitude are probably depending on the compression behaviour of the underneath tow as well as the lateral stiffness of the tow showing the buckle.

For the orientation 90°, an increase in altitude of the horizontal tow submitted to in-plane bending is observed up to about 28°(Figure 10). The altitude rise is very similar to the one observed for the 0°orientation up to this bending angle. However, above 28°, the altitude of the tow does not rise anymore. It remains to a value of about 0.2 mm up to 38°. This elevation of altitude is visible on Figure 11. It shows that the tows submitted to in-plane bending do not keep their initial elevation. A small elevation with a square profile can be observed.

No appearance and growth of buckle with a rotation around itself takes place in these conditions. For the largest in-plane bending angles, the elevation of the horizontal tow decreases up to values of altitudes lower than the one of the initial state to reach an elevation of 0.22 mm. This corresponds to a decrease of about 45%. The fact that no buckle appeared with the orientation 90°is not really surprising as this had already been observed in a formed shape using the same plain weave In the 90°o rientation, the weft horizontal tows passes up and down the warp tows that are close to each other. The space between two adjacent warp tows is very small (0.26 mm). The length of the horizontal weft tow in the above position is therefore reduced to the width of the warp tows, 2.53 mm. This length is not sufficient for the buckle to appear in our test conditions. Moreover, the reduction of elevation for high in-plane bending angle is probably due to the fact that the tow, instead of rising to form a buckle imposes a compression load on the underneath warp tow.

On the contrary, for the 0°orientation, the space between two adjacent weft tows is larger (1.59 mm), and the width of the weft tows are larger (3.25 mm) than the warp tows. The length of the horizontal warp tow in the above position is therefore larger (about 4.8 mm). This is 1.9 times larger than for the weft tows.

The influence of the tension of the tows perpendicular to the ones exhibiting the buckles (vertical tows) has been also investigated. Figure 12 shows a comparison of the evolution of the buckles height as a function of the in-plane bending angle for orientation 0°for two different loadings (250 N and 8 N) in the X direction (direction of the tows perpendicular to the ones exhibiting the buckles) (Figure 3). In both cases, the global shapes of the curves are similar with a slow increase of the buckles height followed by an important rise and a plateau for the highest bending angles. However, in the case of the higher applied load in the X direction, the increase of the buckles height is observed for lower bending angles. The main rise starts from an in-plane bending angle of 20°instead of 28°in the case of the lower applied load. The final height of the buckle in both cases is equivalent (1.45 mm), but this height is reached for a lower angle, ~30°instead of 35°in the case of the low applied load in the X direction. This therefore means that a low tension in the X direction should be preferred if one want to delay the appearance of tow buckles in the frame of the fabric forming.

The variations of profile exposed in this work are the result of the buckle's appearance of the flax fabric tested in this work. It is expected that relatively similar behaviours are observed for other types of fabrics constituted from flat type tows. Buckles have already been observed on complex shapes elaborated from aeronautical grade carbon interlock fabric [START_REF] Allaoui | Experimental preforming of highly double curved shapes with a case corner using an interlock reinforcement[END_REF]. The profile of the buckles (presence of a plateau before the chevron full profile), their maximum elevation and the in-plane bending angle appearance depends on the different stiffness of the tow. Particularly, the lateral Bending angle ( 0 )

• 0 Tension X=8 N • 0 0 •• 0 • 0 • 0 ~00000 •!Jl'!l"lal"l••M!J•••
The bi-axial state of the fabric (the tension of the horizontal and vertical tows) has been investigated. This work shows that the magnitude of the tension in the direction perpendicular to the tows showing the buckles is also a crucial parameter as a higher tension favours the appearance of buckles for lower in-plane bending angles. As one wants to avoid the appearance and the growth of the buckles, a low tension in the direction perpendicular to the one of the tows that may exhibit the buckles should be favoured. In this case, and with the tows and the reinforcement used in this work, an in-plane bending angle of 28°could be obtained in the face of a shape without any buckle appearance.

The characteristic in-plane bending angle values evaluated in this work probably also depend on the mechanical properties of the tows constituting the fabric. The compression stiffness of the tow underneath and the lateral compression of the tow showing the buckles are other probably influence the in-plane bending at which the buckle appears and then the speed at which it grows.

The influence of the fabric architecture at the mesoscale as well as the mechanical properties of the tow constituting the fabric and the bi-axial state of the fabric should be studied more by using the validated device to understand in a better extent the tow buckling defect. This would lead to tendencies that could be used to elaborate a multifactor criterion to predict the appearance and growth magnitude of the defect. stiffness of the tow probably controls the profile of the buckle. It would be expected that the chevron type profile is observed for lower bending angle in the case of tows with high lateral stiffness.

Conclusions

A device capable of reproducing tow buckles has been used to investigate the tow buckling phenomenon. Several techniques able to measure out of plane elevations have been discussed to detect the appearance and growth of the tow buckling phenomenon in relation to its size and its shape. The fringe projection technique was chosen as it gives the best compromise between the size of the defect to measure and its resolution. A complete description of the technique as well as the equipment used in this work is given.

The appearance and the growth of the tow buckle have been studied as a function of the in-plane bending angle that was identified as the essential parameter at the origin of the tow buckling phenomenon. For the reinforcement studied in this work, the tow supposed to show the buckle slowly rises up to an in-plane bending angle of 28°. The rise does not overcome the value of 0.2 mm. The profile of the tow remains with a square geometry. Above bending angles of 28°, the tow buckle itself appears (chevron profile), and its size quickly increases in the case of the orientation 0°. The tow buckle growth mechanism has been identified as a double rotation around itself. During its rotation, the tow exhibiting the buckle imposes a compression load on the tow underneath. Above a value of 35°, the buckle rises with a lower slope. This is due to the fact that the rotation around itself of the buckle is limited by the compression phenomenon taking place during this rotation.

As the plain weave fabric used in this study is not balanced, the 90° orientation was tested. In this case, the tow does not rise above a very limited value of 0.2 mm similarly to the orientation 0° up to in-plane bending angles of 28°. Above 28°, no more elevation is recorded. The tow does not rotate around itself as in the 0° case. This is due to the fact that the tow submitted to in-plane bending does not have enough space to initiate and grow a buckle. The space between two adjacent warp tows is very small (0.26 mm). The length of the horizontal weft tow in the above position is therefore reduced to the width of the warp tows (underneath in vertical position), 2.53 mm. This length is not sufficient for the buckle to appear in our test conditions.

If the in-plane bending angle is the main criterion at the origin of the tow buckle appearance and growth, it is not the only one. This work shows that the fabric architecture such as space between the tows perpendicular to the one showing the buckle is also crucial to control the buckle's appearance and growth.

Figure 1 :

 1 Figure 1: Bending in their plane of tows

Figure 2 :

 2 Figure 2: Tow buckles on an edge and on a face of a tetrahedron shape

Figure 3 :

 3 Figure 3: Principle of the tow buckling device

Figure 4 :

 4 Figure 4: Instrumentation of the tow buckling device

Figure 5 : 6 -

 56 Figure 5: Definition of the buckle height

Figure 6 : 8 X

 68 Figure 6: Expected profile of the buckles

Figure 7 :

 7 Figure 7: Evolution of the buckles profile for different in-plane bending angles: (a) Orientation 0°, (b) orientation 90°F igure 8: Rotation of the buckle around itself imposing a compression load

Figure 9 :

 9 Figure 9: Profile evolution of a buckles: (a) orientation (0°with warp tows submitted to bending in its plane) and (b) orientation 90°with weft tows submitted to bending in its plane)

Figure

  Figure Maximum elevation of tows for different orientations of the plain weave fabric

Figure 11 : 6 E 1 •

 1161 Figure 11: Elevation of the weft tows submitted to in-plane bending solicitation: orientation 90°F igure 12: Influence of the tension in the tows perpendicular to the ones showing the buckles

  al. and Davidson et al. showed

doi: 10.1111/str.12206

Table 1 :

 1 Dimensional characteristics of different topography measurement techniques[START_REF] Jahne | Handbook of Computer Visional Applications[END_REF]

		Real time holography	Dynamic holography	TV-holography	Speckles	Fringe projection
	Surface of analysis	16 cm 2 -1 m 2	16 cm 2 -10 m	2	10 mm 2 -1 m 2	25 mm 2 -1 m 2	100 mm 2 -1 m	2
	Measurement range extent	0.1-20 μm	0.1-20 μm		0.1-10 μm	0.1-10 μm	several centimetres
	Resolution for displacement measurements	0.01 μm	0.01 μm		0.1 μm	0.1 μm	0.1-0.4 mm

Acknowledgements

The authors would like to thank the ADEME (French Environmental agency) and Region Centre for their financial support. The authors also would like to thank the SIMaP laboratory of Grenoble for the use of their fringe projection equipment.