Firat Gulagaci

Christophe Jacquet

Matthieu Ehlinger

Akash Sharma

Kristian Kley

Adrian Wilson

Sebastien Parratte

Matthieu Ollivier
email: matthieu.ollivier@ap-hm.fr

A protective hinge wire, intersecting the osteotomy plane, can reduce the occurrence of perioperative hinge fractures in medial opening wedge osteotomy

Keywords: HTO, Hinge fracture, Postoperative outcomes, Patient specific cutting guide

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

Introduction

Osteotomy around the knee is well established as an effective treatment for offloading an osteoarthritic compartment, where there is an unfavorable alignment. There are options of opening or closing techniques in the distal femur and proximal tibia [START_REF] Flecher | A 12-28-year follow-up study of closing wedge high tibial osteotomy[END_REF][START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF][START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF]. The predominant technique is the medial opening wedge high tibial osteotomy and the most common indication is medial compartment osteoarthritis [START_REF] Fujisawa | The effect of high tibial osteotomy on osteoarthritis of the knee. An arthroscopic study of 54 knee joints[END_REF][START_REF] Hernigou | Proximal tibial osteotomy for osteoarthritis with varus deformity. A ten to thirteen-year follow-up study[END_REF][START_REF] Schwartz | Minimally invasive opening wedge tibia outpatient osteotomy, using screw-to-plate locking technique and a calcium phosphate cement[END_REF][START_REF] Sprenger | Tibial osteotomy for the treatment of varus gonarthrosis. Survival and failure analysis to twenty-two years[END_REF][8].

Over the last two decades, the outcomes have improved in osteotomy around the knee by redefining the indications, improving preoperative planning and by making the surgical technique more accurate, safer, and reproducible.

With preoperative and intraoperative planning to define the hinge point for the osteotomy, whether it is an opening or closing wedge technique, it is important to preserve and maintain the hinge, as this is critical to facilitate bone healing and to provide stability to the osteotomy [9,10]. If the hinge is compromised, the fixation may not be sufficient and secondary loss of correction may occur [11].

Dexel et al. [9] reported an incidence of 30.4% for lateral tibial hinge fractures related to opening wedge high tibial osteotomy (OWHTO), half of these occurred during the procedure and the other half after 6 weeks of follow up. Therefore, it can be assumed that finding a solution to preserve the hinge intraoperatively is crucial for an optimal outcome following OWHTO.

Several techniques that have been developed to minimize hinge point issues such as taking care with the saw depth cut and subsequent speed with which the wedge is then opened. Other authors have developed protective cutting systems [12,13] and used drilling techniques to protect the hinge by decreasing the stress at the hinge point by placing an anteroposterior protecting hinge point wire [14,15].

The use of patient-specific cutting guides [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF] (PSCGs) requiring preoperative CT scan templating during OWHTO has raised the possibility of making instrumentation specific to each patient. Using these guides, a cadaveric study [11] reported that positioning a K-wire to intersect the cutting plane at the theoretical lateral hinge location increases the lateral hinge's resistance to fracture during the opening.

The aim of this study was to evaluate the clinical relevance of the use of this K-wire and its benefits in terms of lateral hinge protection during OWHTO with PSCGs in daily practice.

We hypothesized that the addition of an intersecting K-wire decreases the incidence of intraoperative hinge fractures during OWHTO and augments bone healing, thus reducing lateral hinge fracture rate in the early phases of rehabilitation.

Materials and methods

After gaining local ethics committee's approval, a retrospective comparative study identified 206 patients who underwent OWHTO from January 2014 to December 2017. Among these patients, 71 had an additional K-wire positioned at their theoretical lateral hinge location (HK + group), whereas 135 did not (HK-group). All the surgeries were performed by two different surgeons who had practiced more than 20 OWHTO using PSCGs before January 2014 to avoid biases related to learning curve effect [16]. Between January 2014 and June 2016, no K-wire was used during the surgery. After June 2016, the K-wire was systematically used according to the experimental results of biomechanical cadaveric study [11].

Mean follow-up time was 2.3 ± 1.0 years (range 2-4.2). Inclusion criteria were: patients < 65 years of age with isolated medial knee osteoarthritis (Ahlbäck ≤ 2// Kellgren and Lawrence < 4) [17, 18] and significant metaphyseal tibial vara (medial plateau tibial angle MPTA < 85°).

Exclusion criteria were: advanced medial knee osteoarthritis (Ahlbäck > 2) evidence of symptomatic patellofemoral or lateral knee osteoarthritis, previous ipsilateral knee surgery (except meniscus related arthroscopic procedures), history of fragility fracture, evidence of osteopenia or osteoporosis, post-traumatic arthritis. Patients requiring simultaneous anterior cruciate ligament reconstruction, meniscus, or cartilage surgery were also excluded.

Patient consent was collected preoperatively after they were informed of the procedure in accordance with the principles of the Helsinki Declaration. For patients meeting the inclusion criteria, a matched pairs process identified 60 patients in the HK + group and selected 60 patients from the HK-group, matching criterion were age (± 3 years) and preoperative MPTA value (± 2 degrees reflecting the extent of correction to be performed).

The mean age of patients of HK + and HK-groups was 44.9 ± 7.7 and 45.8 ± 4.3 (p = NS), respectively. The groups were comparable in terms of demographic background (and radiological parameters (Table 1).

In the preoperative planning stage, the planned correction was first calculated by the surgeon using conventional radiographs (weight-bearing long leg, A/P, and lateral views) following Miniaci's method [19]. Subsequently, all patients underwent a CT scan and patient-specific cutting guides (PSCG) osteotomy following a previously published surgeon guided step-by-step procedure [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF] Fig. 1.

The lateral cortex was protected in the HK + group using a ⌀2.2 mm K-wire. It was placed from distal-toproximal from anteromedial-to-lateral to intersect the cutting plane at the theoretical hinge point (10 mm from the lateral cortex). Subsequently, the osteotomy cut was performed and was gradually opened/distracted with a laminar spreader until the pre-drilled screw holes were aligned with the holes in the plate. The plate was secured using eight screws, the size of which were pre-determined during preoperative planning.

The bone defect was filled with femoral head wedge allograft or injectable calcium phosphate cement (Quickset, Graftys ® , Aix-en-Provence, France). Figures 2 and3 illustrate an example of hinge's protection obtained using the intersecting K-wire for a 11 mm/12° opening.

Patients' evaluation and follow up

All lateral hinge fractures (LHF) were evaluated according to the Takeuchi classification [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF].

Patients were evaluated prospectively at postoperative first, third, and sixth months, and then every 6 months. Radiographic outcomes were evaluated with intraoperative and postoperative fluoroscopic imaging and with CT imaging at 6 weeks post OWHTO surgery. Every radiographs or CT scans were reviewed by two senior clinicians (one radiologist and one surgeon), in cases of different interpretation, a third evaluation was performed by the two clinicians to reach a definitive interpretation. The Kappa coefficient of the first evaluation was 0.95. The timing of LHF was defined as intraoperative fractures; diagnosed either during the procedure or immediately postoperatively with a plain radiograph (same day) or postoperative fractures diagnosed on CT images (not viewed on radiographs) at 6 weeks or fractures diagnosed on radiographs performed during subsequent follow up. Figure 4 exhibits CT examination of the hinge integrity.

The same postoperative regime was used in both the groups, including:

• Full weight-bearing if no LHF occurred intraoperatively.

• For Takeuchi types 1 and 2 diagnosed intraoperatively (Fig. 5); toe touch weight-bearing for 6 weeks aided with the use of crutches was recommended. Full weight-bearing was allowed after 6 weeks. • For Takeuchi type III diagnosed intraoperatively: the fracture was treated intraoperatively by two additional compression screws (Fig. 6) and no weight-bearing during 6 weeks followed by toe touch weight-bearing for 6 weeks aided with the use of crutches was recommended. Full weight-bearing was after allowed 12 weeks. • If LHF was diagnosed at 6 weeks' follow up, for the types I and 2, full weight-bearing was extended. For the type III a toe touch weight-bearing for 6 weeks was recommended before full weight-bearing.

Statistical analysis

The descriptive statistics were presented as mean ± standard deviation for quantitative variables. Mann-Whitney U test for two independent samples with 95% confidence interval (95% CI) was used to evaluate the differences between two variables, one-way ANOVA between more than two variables and multiple linear regressions for relationships. Paired student t test was used to estimate evolution of functional outcomes during follow up. The differences in proportions between two samples were estimated with z-test with 95% CI. Finally, a multivariate analysis evaluating factors related to clinical outcomes was performed. A post hoc power analysis demonstrated that 60 patients by group allow to demonstrate a twofold difference in term of fracture rate in the first six postoperative weeks (expected rate 30%). PASW statistics version 20 (SPSS, IBM Inc., Chicago, Illinois) was used for statistical analysis. The threshold for statistical significance was set at p < 0.05.

Results

Thirty six patients (30%) were found to have a LHF. Among these patients, 26 (72%) did not have an additional K-wire positioned at their theoretical lateral hinge location (HK-group) during the procedure. LHF rate for patients without additional K-wire group (HK-) was 43.3%, whereas it was 16.7% for the patients with an additional K-wire (HK +) (OR 3.8, 95% CI 1.6-8.3; p = 0.005) (Table 2).

According to the Takeuchi lateral hinge fractures classification [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF], 30 patients had type 1 LHF (83.3%), whereas three had type 2 (8.3%) and remaining three (8.3%) had type 3 (Table 2).

Among the 30 patients who had type 1 LHF, 23 (77%) did not have additional K-wire positioned at their theoretical lateral hinge location (HK-group). LHF types according to HK groups were summarized in Table 2.

Among all the 36 LHF, 13 (36.1%) were observed intraoperatively or on immediate postoperative imaging, 20 (55.6%) were diagnosed at the 6 weeks follow-up appointment with a CT scan, and three (8.3%) on further subsequent radiological analysis.

The number of recognized intraoperative fractures was higher in the HK-group (n = 11), (OR 6.5, 95% CI 1.4-30.7) as compared to HK + patients (n = 2).

The number of fractures detected at 6 weeks with CTbased imaging was higher in the HK-group (n = 14), (OR 2.7, 95% CI 1-10) as compared to HK + patients (n = 6).

There was no difference with regards to the number of fractures diagnosed during subsequent follow up (p = NS).

Patients' operative details and radiographic evaluation regarding the difference between planned and final measures (Δ) are summarized in Table 3.

The mean consolidation period of the osteotomy and the mean time to return to work was significantly longer for patients with type 2 or type 3 LHF (p < 0.0001 for both). The mean time to return to any kind of sports was significantly longer for patients with type 3 LHF (p < 0.0001). KOOS parameters were not statistically different between LHF groups. The relationship between patient outcomes and LHF types is summarized in Table 4.

Age, gender, mean operative time, number of fluoroscopic images taken during surgery, and mean follow-up times were similar between groups. The mean time to return to work, return to any kind of sports and bone union was significantly shorter for HK + group: mean time to return to work was 2.20 ± 1.1 months for HK + group and 2.95 ± 2.6 months for HK-group (p = 0.04); mean time to return to any kind of sports was 4.6 ± 1.4 months for HK + group and 5.9 ± 4.0 months for HK-group (p = 0.02); mean time for bone union on radiographs was 4.1 ± 0.9 months for HK + group and 4.9 ± 2.6 months for HK-group (p = 0.02) (Table 3).

Other complications

One nonunion was seen at 9 months after HTO in the HK + group, union was achieved 5 months after a revision procedure. A total of 11 hematomas were detected postoperatively (seven in HK-group, four in HK + group; p = NS). Allograft osteolysis was seen in two patients in HK-group and in one patient in HK + group (p = NS). In HK + group, three patients presented with surgical site infections (two superficial infections requiring antibiotic therapy and one deep infection treated by debridement) versus none in the HK-group (p = 0.3).

Discussion

The main finding of this study was that during OWHTO surgery positioning, a K-wire intersecting the cutting plane at the theoretical lateral hinge location reduces the rate of hinge fractures that are detectable in the first 6 weeks postoperatively.

The role of an intact competent lateral tibial hinge in OWHTO has been widely reported to be crucial in its importance for consolidation of the osteotomy and maintenance of angular correction in all three planes [23,24]. Takeuchi et al. [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF] reported a 25% rate of LHF during OWHTO, diagnosed either intraoperatively or on postoperative radiographs taken just after surgery. Dexel et al. [9] reported an incidence of 30.4%, half of which occurred during the procedure and the other half found on radiographs at the 6 weeks follow-up mark. In this study, the overall LHF rate was 30%, half of these fractures were diagnosed intraoperatively or on immediate postoperative radiographs. It is important to note that a number of studies do not report the specifics of the type of lateral hinge fracture rate occurring during follow up. It is accepted that secondary Takeuchi type 1 fractures found during follow up are almost certainly created at the time of the procedure, but are often not seen with either fluoroscopy or plain X-ray. These subtle fractures are cracks that propagate through the hinge during the process of sawing and distraction; probably would be detected if CT scan was used in immediate postoperative exams [25].

Lee et al.

[12] and Munier et al. [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF] focused on various cutting guides to achieve better correction and to reduce intraoperative complications during OWHTO. In their comparative study, Lee et al. [13] reported improvement in complications, including lateral hinge fractures using a releaser and a protective cutting complex. They reported superiority of their "protective cutting system" over conventional technique.

Diffo Kaze et al. [15] focused on the effect of hinge drilling to improve the resistance to fracture during OWHTO. Their results confirm that, for tibia vara corrective osteotomies < 5°, drilling a hole at the apex of the osteotomy reduces stresses in the lateral hinge and increases the critical angle before failure of the hinge, but only for corrections inferior to 5°. Similarly, for osteotomies superior to 9 mm of opening, Bujnowski et al. [14] recently demonstrated, in their ex-vivo study, that a pilot hole provided no significant decrease in the strains experienced at the lateral cortex, nor did it reduce the risk of fracture of the lateral tibial hinge.

Computer-assisted surgery (CAS) can also offer solutions to the surgeon by controlling the depth of the cut, however this alone does not prevent the hinge to be susceptible to fracture [26]. More recently, Dessyn et al. [11] in their comparative cadaveric study using patient-specific cutting guides (PSCG) [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF] demonstrated an improvement of the lateral hinges resistance to fracture during osteotomy opening using an additional K-wire intersecting the cutting plane at the theoretical lateral hinge location. These results are supported by Diffo Kaze et al. [15]; in these two studies, angulation, vertical displacement, and maximum load before breakage were all greater when the hinge was protected before applying an opening load. Our study justifies and advocates a protective K-wire, which acts as a saw-protector and strain absorber, which protects the hinge during opening. Using a locked medial plate system during OWHTO, Kim et al. [27] demonstrated in their retrospective study comparing a group without LHF and patients with LHF types I and II did not show any functional deterioration or radiological changes during midterm follow up. Their study confirmed that the occurrence of hinge fractures (Takeuchi types I and II) did not have comparatively worse KOOS subscores at their final follow up. However, patients with Takeuchi II and III fractures returned to their profession and sports activities later than patients with Takeuchi I or no LHF.

We recognize there are some limitations to this study. First, this is a retrospective analysis of data which have been recorded prospectively. As such the retrospective nature of this study might introduce a potential selection bias, however all the procedures in this study were done with a standardized technique in terms of preoperative planning. The surgical technique was carried out by two senior surgeons using the same fixation device and with standardized postoperative rehabilitation. The short-term follow up in this study may have avoided diagnosing late LHF, however previous published series report this complication to be rare after 6 weeks [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF]9].

One further potential bias is with the use of graft material placed in the gap during surgery as this may influence postoperative fracture rates. A more rigid graft might absorb strains that would have otherwise been transmitted to the hinge. In a cadaveric study, Scordino et al. [28] demonstrated that injectable calcium phosphate cement improves the initial maximal torsional strength and stiffness of high tibial osteotomy construct. Schwartz [START_REF] Schwartz | Minimally invasive opening wedge tibia outpatient osteotomy, using screw-to-plate locking technique and a calcium phosphate cement[END_REF] treated their patients suffering from isolated medial knee osteoarthritis secondary to genu varum with a minimally invasive OWHTO using injectable calcium phosphate cement. Among their 19 patients, 18 had union of their osteotomy within 2 months. There were no reports of infections in their study, but they did diagnose one type 3 LHF in their noted complications.

In this present study, patients were allowed to go onto full weight-bearing during their rehabilitation period, however this may have increased the number of LHF seen, the fracture rates of this study was similar to that reported in the literature [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF]9].

Finally, Lee et al.

[29] demonstrated that surgical experience may improve the surgeon's competency in the prevention of lateral hinge fractures, thus highlighting the need to be trained and experienced to fully respect the lateral tibial hinge. The results obtained by the two seniors surgeons involved (performing more than 50 HTO/year) might be different in non-experienced surgeon hands, but the advantages of the protective K-wire might also be more significant for the latter to provide additional radiological landmarks and a rigid sawing safeguard.

Conclusion

This study demonstrated that, during OWHTO, positioning a K-wire intersecting the cutting plane at the theoretical lateral hinge location reduced the number of intraoperative lateral hinge fractures. OWHTO using a protective K-wire exhibits shorter time to bone union, return to work and/or sport than patients operated on using exactly the same procedure, but without a K-wire.

Author's contribution CJ and FG have made substantial contributions to acquisition of data or analysis and interpretation of data. ME, AS, KK, AW, and SP have been involved in drafting the manuscript or revising it critically. MO has made substantial contributions to conception and design or acquisition of data or analysis and interpretation of data. Every author has given final approval of the version to be published and agreed to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

A mid term comparison of open wedge high tibial osteotomy vs unicompartmental knee arthroplasty for medial compartment osteoarthritis of the knee. J Orthop Surg

Fig. 1

 1 Fig. 1 Preoperative 3D templating illustrating the operative technique of custom cutting guides for the HK + group. a, b Optimal positioning of the cutting guide with two K-wires (one to protect the lateral hinge during the osteotomy (HK + group) and one to guide the saw))/

Fig. 2

 2 Fig. 2 51-years-old patient suffering from painful right knee for more than 2 years. Preoperative examination exhibits an important metaphyseal tibial varus with Alback stage 2 arthritis on schuss X-rays (30° flexion PA weight-bearing)

Fig. 3 Fig. 4

 34 Fig. 3 Hinge K-wire protecting a 11 mm/12° medial opening before and after allografting the osteotomy's bone gap

Fig. 5

 5 Fig. 5 Example of a Takeuchi type 2 hinge fracture diagnosed postoperatively

Fig. 6

 6 Fig. 6 Example of a Takeuchi type 3 hinge fracture diagnosed and treated intraoperatively

Table 1

 1 Patient's characteristics HK K-wire positioned in theoretical lateral hinge location, MPTA medial proximal tibial angle, CPC calcium phosphate cement

	Parameters	HK + group (n = 60)	HK-group (n = 60)	p value
	Age	44.9 ± 7.7	45.8 ± 4.3	0.4
	Gender (male) (%)	32 (53.3)	35 (58.3)	0.3
	Active smoking (n)	6	4	0.7
	Preoperative MPTA (°)	82 ± 4	81 ± 4	0.4
	Planned correction MPTA (°)	9 ± 5	8 ± 4	0.5
	Operative time (mins)	26 ± 10	26 ± 11	0.8
	Fluoroscopic image (n)	5.1 ± 3.2	5.7 ± 4.5	0.4
	Osteotomy's gap grafting (allograft/CPC)	19/31	17/33	0.6
	Postoperative MPTA (°)	90 ± 4	90 ± 3	0.4
	Follow up (years)	2.2 ± 0.9	2.4 ± 1.0	0.1
	P significant if < 0.05

Table 2

 2 LHF types according to HK

	HK/LHF	No LHF	Type 1	Type 2 Type 3
	HK + group	50 (41.3%)	7 (5.8%)	1 (0.9%) 2 (1.6%)
	HK-group	34 (28.1%)	23 (19.8%) 2 (1.6%) 1 (0.9%)
	p	0.005	0.005	0.7	0.2
	Odd ratio (95% CI) 6.1 (2.6-14.3) 5 (2-13)	NA	NA
	Total (n)	84 (69.4%)
	HK K-wire positioned in theoretical lateral Hinge location, LHF lat-
	eral Hinge fracture

Table 3

 3 Patient information according to LHF types LHF lateral hinge fracture, MPTA medial proximal tibial angle, PPTA posterior proximal tibial angle, HKA hip-knee-ankle angle **Mean value of the parameter statistically different than no LHF group mean value

	Parameters/LHF type	No LHF (n = 84)	Type 1 (n = 30)	Type 2 (n = 3)	Type 3 (n = 3)
	Operation time (mins)	25.3 ± 1.0	22.6 ± 1.6	55.7 ± 5.2**	44.7 ± 5.2**
	Fluoroscopic image (n)	4.5 ± 0.2	5.2 ± 0.3	20.7 ± 1.1**	19.7 ± 1.1**
	ΔHKA	1.4 ± 0.1	1.7 ± 0.2	1.2 ± 0.5	1.2 ± 0.5
	ΔMPTA	0.5 ± 0.1	1.1 ± 0.1	1.2 ± 0.4	1.9 ± 0.4**
	ΔPPTA	0.5 ± 0.1	0.3 ± 0.1	3.6 ± 0.5**	2.1 ± 0.5**
	Follow up (months)	2.3 ± 0.1	2.1 ± 0.2	2.1 ± 0.6	2.3 ± 0.6

Table 4

 4 Comparison of clinical outcomes according to LHF types KOOS knee injury and osteoarthritis outcome score, ADL activity of daily life, QOL quality of life, UCLA University of California Los Angeles, LHF lateral hinge fracture **Mean value of the parameter statistically different than no LHF group mean value

	Parameters/LHF type	No LHF (n = 84)	Type 1 (n = 30)	Type 2 (n = 3)	Type 3 (n = 3)
	Bone union (months)	4.3 ± 0.2	4.3 ± 0.3	6.7 ± 1.0**	9.3 ± 1.0**
	Return to work (months)	2.4 ± 0.2	2.3 ± 0.3	4.7 ± 1.0**	7.3 ± 1.0**
	Return to sports (months)	5.0 ± 0.3	4.9 ± 0.5	5.7 ± 1.6	13.3 ± 1.6**
	ΔKOOS pain	31.5 ± 2.4	31.9 ± 4.2	33.0 ± 15.4	20.0 ± 21.8
	ΔKOOS symptom	32.7 ± 2.4	34.8 ± 3.9	30.0 ± 15.2	20.0 ± 21.5
	ΔKOOS ADL	33.1 ± 2.5	34.8 ± 4.3	24.0 ± 15.7	21.0 ± 22.3
	ΔKOOS sports	36.3 ± 3.2	26.0 ± 5.0	15.3 ± 15.2	5.0 ± 18.6
	ΔKOOS QOL	37.8 ± 3.5	52.2 ± 6.5	18.0 ± 21.2	2.5 ± 21.2

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(25.6%) 3 (2.5%) 3 (2.5%)

Funding No funding was required for this study.

Compliance with ethical standards

Conflict of interest Some of the authors disclosed potential conflict of interest.

Ethical approval

Local ethics committee approved our study protocol prior to investigation.


Xavier Nicolau

François Bonnomet

Grégoire Micicoi

David Eichler

Matthieu Ollivier

Henri Favreau

Matthieu Ehlinger
email: matthieu.ehlinger@chru-strasbourg.fr

Accuracy of the correction obtained after tibial valgus osteotomy. Comparison of the use of the Hernigou table and the so-called classical method

Keywords: Open wedge osteotomy, Navigation, Accuracy, Tibial osteotomy, Knee surgery Level of proof: retrospective study, III

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Introduction

Gonarthrosis is a main cause of disability [START_REF] Cross | The global burden of hip and knee osteoarthritis: estimates from the global burden of disease 2010 study[END_REF]. The risk of developing symptomatic osteoarthritis of the knee is favoured by genu varum present of painful patients [START_REF] Cahue | Varus-valgus alignment in the progression of patellofemoral osteoarthritis[END_REF]. The relationship between "varus deformity" on a healthy knee and "development of gonarthrosis" with an odd ratio of 2 has been highlighted [START_REF] Brouwer | Association between valgus and varus alignment and the development and progression of radiographic osteoarthritis of the knee[END_REF], explaining the fact that isolated medial gonarthrosis is ten times more frequent than isolated lateral femorotibial osteoarthritis [START_REF] Ledingham | Radiographic patterns and associations of osteoarthritis of the knee in patients referred to hospital[END_REF]. For Hanada et al., medial osteoarthritis is associated with varus deformity of the lower limb but also with the morphology of the proximal tibia, particularly its mechanical tibial angle (MTA) [START_REF] Hanada | Relationship between severity of knee osteoarthritis and radiography findings of lower limbs: a cross-sectional study from the TOEI survey[END_REF].

Valgus-producing tibial osteotomy (VTO) is indicated to treat isolated medial osteoarthritis of the knee, with acknowledged efficacy but unfortunately with deterioration over time [START_REF] Papachristou | Deterioration of long-term results following high tibial osteotomy in patients under 60 years of age[END_REF]. Survival at years is of 67% with known risk factors for failure such as age over 46 years, female, presence of comorbidities, and a history of arthroscopy [START_REF] Khoshbin | The effect of patient, provider and surgical factors on survivorship of high tibial osteotomy to total knee arthroplasty: a population-based study[END_REF]. One of the elements that can guarantee the success of a VTO is the correction obtained with the necessary over-correction and a valgus objective of 3 to 6° [START_REF] Hernigou | Open wedge tibial osteotomy with acrylic bone cement as bone substitute[END_REF].

The main objective of this work was to compare the accuracy of the correction obtained during a VTO using the Hernigou table (HT) [START_REF] Hernigou | Open wedge tibial osteotomy with acrylic bone cement as bone substitute[END_REF], based on a trigonometric calculation taking into account the correction objective set, but also the existing deformity and the length of the osteotomy line, or using the so-called classical method (CM) for which 1°of correction corresponds to 1°o f osteotomy opening. The secondary objective was to analyze the variations observed in tibial slope and patellar height. The working hypothesis was that the use of HT allowed a better accuracy to reach the target set than the CM with a modification of the tibial slope and patellar height for both groups.

Material and method

Study design

A retrospective study was conducted from January 1, 2009 to December 31, 2014 including medial opening VTOs performed by two expert surgeons (FB and ME).

The inclusion criteria were medial opening VTO, one of the two surgeons mentioned above, correction method using the CM or HT, and complete pre-and post-operative radiological file.

Over this period of time, 39 VTOs performed by the 2 surgeons met the inclusion criteria. The patients were divided into two groups: HT group (n = 21) and CM group (n = 18).

The operative report (OR) of each patient was reviewed to collect the desired correction objective for each operator and for each patient.

No functional clinical outcome was reported, for the objective of this work was solely surgical correction accuracy. Only complications were collected.

Radiological measurement

A non-operator examiner (XN) carried out all measurements and evaluation on the PACS (picture archiving and communication system) [START_REF] Sorin | Reproducibility of digital measurements of lower-limb deformity on plain radiographs and agreement with CT measurements[END_REF]: posterior tibial slope according to the Brazier method [START_REF] Brazier | Evaluation of methods for radiographic measurement of the tibial slope. A study of 83 healthy knees[END_REF] and patellar height according to Caton-Deschamps [START_REF] Caton | Patella infera. Apropos of 128 cases[END_REF], MTA, and hip-knee-ankle angle (HKA).

Series

Table 1 shows the data for the whole series.

Surgical technique and planning method

A medial VTO was performed with an antero-medial skin approach centered on the pes anserinus. The proximal tibial metaphysis sub-periosteum was exposed by detaching in one piece the pes anserinus and the superficial bundle of the medial collateral ligament. A guiding pin was inserted in an oblique upward and lateral direction aiming at the lateral cortex of the tibia at the level of the tip of the head of the fibula or at the proximal tibiofibular joint, ensuring stability of the lateral hinge, while projecting above the tibial tuberosity. The patellar ligament was protected anteriorly as well as the neurovascular axis posteriorly. The osteotomy was then performed and completed with a chisel, keeping the lateral hinge intact.

Two methods for reaching the correction target were used. The use of the Hernigou table [START_REF] Hernigou | Open wedge tibial osteotomy with acrylic bone cement as bone substitute[END_REF] was fine-tuned by direct intra-operative measurement of the length of the guiding pin, which corresponded to the length of the osteotomy (Fig. 1). This length of the osteotomy was transferred to the measuring chart to choose the height of the wedge corresponding to the desired correction (Fig. 2). For the CM group, a 1°wedge opening was equivalent to a 1°correction wanted and decided by the surgeon according the pre-operative long X-ray (Fig. 3).

Only seven patients were operated on without tourniquet, all were in the HT group.

In both groups, osteosynthesis was performed using a Surfix® locking plate (Integra, Saint Priest, France). The opening was filled with a DuoWedge® wedge (Integra, Saint Priest, France) and with a DAOA® type wedge (Integra, Saint Priest, France) for the CM group, both made of tricalcium phosphate and hydroxyapatite. F female, M male, R right, L left An arthroscopy was systematically performed first when an unstable meniscal lesion was present. This was the case for two patients in the HT group (10%) for whom a resection was performed.

In both groups, free motion was permitted immediately after surgery. Weight bearing was limited to 10 kg in the CM group for 6 weeks, while it was forbidden for 6 weeks in the HT group.

Correction objective

A correction angle was specific to each patient. This angle written in the OR was considered as the overall desired correction goal. The surgeons, based on pre-operative radiological bipodal and monopodal telemetry data in the CM and HT groups, set the overall correction goal.

"Surgical correction" is the correction performed by the surgeon during the operation. It was calculated on the evolution of the MTA before and after surgery. It was also calculated for the HKA angle. Post-operative measurements were made on early images (< day 5) in all patients to avoid any bias due to possible loss of correction over time, to different fixation means used or to possible complications involving fixation stability.

"Surgical accuracy" [START_REF] Schröter | Surgical accuracy in high tibial osteotomy: coronal equivalence of computer navigation and gap measurement[END_REF] is the absolute value of the difference between the correction goal and the surgical correction performed (difference between post-operative and preoperative MTA or HKA = ΔMTA and ΔHKA). The closer the value is to 0, the better the accuracy.

Statistical analysis

Surgical accuracies were compared between groups using a Mann-Whitney test with a 5% significance level.

The correction objective set by the surgeon, as well as the comparison of the different quantitative variables between the groups, was carried out using a Mann-Whitney test with a 5% significance level. A univariate linear regression analysis was performed to evaluate the factors that may have influenced surgical accuracy according to ΔMTA and ΔHKA. A multivariate analysis was then performed, using a mixed linear regression model of surgical accuracy according to ΔMTA and HKA on variables that had a statistically significant association (p < 0.05) in univariate analysis.

The intraclass correlation (ICC) between ΔMTA and ΔHKA was calculated. The confidence interval and standard deviation of the differences obtained between the radiological measurements of surgical correction following ΔMTA and ΔHKA were calculated to represent the spread of differences according to the surgical accuracy measurement method. In order to evaluate and compare the evolution of the tibial slope and patellar height for each group before and after the VTO was performed, a univariate linear regression was performed.

Then, multivariate regressions with the tibial slope and the Caton-Deschamps index as the explanatory variable and the surgical correction performed according to ΔMTA and the planning groups were performed.

The number of patients within 3°of the objective was analyzed between the groups by an exact Fisher's test at the 5% threshold.

Fig. 2 Hernigou table used [START_REF] Lonner | Effect of Rotation and Knee Flexion on Radiographic Alignment in Total Knee Arthroplasties[END_REF]. For example (*), if the length (M) (measured during the procedure) of the osteotomy is 65 mm, and that the desired correction angle (β) is 16°, the opening wedge will be 18 mm 2 shows the data for the two different groups. There was a significant difference between the two groups concerning the correction objective (p < 0.001) but also concerning the pre-operative deformities (MTA (p = 0.002) and HKA (p < 0.001)), which were more important in the CM group. 2. Table 3 shows the result for the series and the two groups.

Surgical accuracy according to ΔMTA median was 3.5°[ 0.2-7.4] in the CM group versus 1.4°[0-4.1] in the HT group, with a significant difference (p = 0.006). Univariate analysis isolated pre-operative HKA as the only factor significantly associated (p = 0.0177) with surgical accuracy according to ΔMTA for each group and therefore potentially influencing it. The results of the multivariate regression of surgical accuracy according to ΔMTA as a function of preoperative HKA and the planning method used indicate that for the same objective, the CM has a significantly lower accuracy of 1.9°± 0.8 (R 2 = 0.23; p = 0.02).

The surgical accuracy according to ΔHKA was 3.1 [0.3-7.3] in the CM group and 0.8 [0-5] in the HT group with a significant difference (p = 0.006). The univariate analysis did not find any factor significantly associated with surgical accuracy according to ΔHKA, so no multivariate regression could be built.

Five procedures had been performed with a surgical accuracy < 3°(compared with the targeted objective) in the CM group (28%) versus 16 in the HT group (76%); the difference was significant (p = 0.004).

The median tibial slope increased after VTO in both groups. This increase was significantly greater in the CM group compared with the HT group, with a median increase of 5.5°[-0.3-13] versus 0.5°[-5.2-5.6] (p < 0.001), respectively.

The median Caton-Deschamps index [START_REF] Caton | Patella infera. Apropos of 128 cases[END_REF] decreased (patella lowered) in both groups after surgery, by -0.21 [-1.03; -0.05] and -0.14 [-0.4-0.16], but without significant difference (p = 0.19). In univariate analysis, changes in tibial slope and patellar height were not significantly related to frontal surgical correction performed according to ΔMTA, (R 2 = 0.07; p = 0.055) and (R 2 = -0.02; p = 0.54) respectively.

The ICC between ΔMTA and ΔHKA was 0.673. The assessment of the correction performed according to ΔHKA is overestimated compared with the assessment according to ΔMTA by an average of 2.01°± 2.17°. The scatter plot of the pre-and post-operative angle data pairs (MTA and HKA), obtained by the two different methods (HT and CM), shows that this overestimation is all the more important for angle values greater than 10°.

Fig. 3 On this long X-ray, the HKA angle showed a 6.5°varus. The surgeon wanted a 2.5°correction of valgus. So the opening wedge was 9° Discussion 1. The main result of this work is that the surgical accuracy of the HT group was statistically better than that of the CM group, confirming the working hypothesis. Although the initial deformity and therefore the correction goal were different between the CM and HT groups, the multivariate analysis found no influence of the initial deformity and the correction goal on surgical accuracy. For the secondary objective, the tibial slope increased and the patella lowered, again confirming the working hypothesis. The extent of the correction in the frontal plane had no influence on these two modifications. The CM group had a greater increase in tibial slope compared with the HT group. Finally, no difference was found between the two groups concerning the lowering of the patella. 2. There are strengths and limitations to this work.

Concerning the strengths, to our knowledge, this is the first work reporting surgical accuracy concerning the objective of correcting a VTO using a CM but also a HT, and especially, it is the first study comparing both techniques. Each operator was an expert in the use of one of the two techniques. A non-operating examiner performed the radiological measurements and analyses in a non-automated way, which may be a limitation of this work even if the software used was validated in the literature [START_REF] Sorin | Reproducibility of digital measurements of lower-limb deformity on plain radiographs and agreement with CT measurements[END_REF].

Despite these strengths, this study was retrospective with a limited number of cases due to the relative rarity of these indications. Surgical accuracies presented as "average," without taking into account the distribution of values, must be interpreted carefully, as they give a false impression of "good surgical accuracy" and as "undercorrected" cases are then compensated by "overcorrected" cases. Converting these "relative values" into "absolute values" avoids this bias and allows comparison between different studies in the literature. Presenting the results as the deviation from an objective, whatever that objective may be, allows a better comparison of surgical accuracy between the different published works. The main element to be discussed in terms of limitation is the choice of radiological measurements. The evaluation according to ΔHKA tends to overestimate surgical accuracy compared with the surgical accuracy according to ΔMTA. This overestimation is even more important for angle values greater than 10°. These observations are confirmed by the literature: indeed the errors of evaluation of the mechanical angle according to HKA are all the more important as the deformation is important [START_REF] Brouwer | Association between valgus and varus alignment and the development and progression of radiographic osteoarthritis of the knee[END_REF][START_REF] Lonner | Effect of Rotation and Knee Flexion on Radiographic Alignment in Total Knee Arthroplasties[END_REF][START_REF] Yaffe | Radiographic and navigation measurements of TKA limb alignment do not correlate[END_REF]. According to Loner et al., this variability in measurement depending on the extent of knee deformation and bending is less noticeable in MTA measurements [START_REF] Lonner | Effect of Rotation and Knee Flexion on Radiographic Alignment in Total Knee Arthroplasties[END_REF]. It is also possible that this difference is due to the influence of weight bearing, although this has not been clearly demonstrated and remains controversial. For Kendoff et al. this difference is 0.4°for 50% of the body weight [START_REF] Kendoff | Influence of lower limb rotation in navigated alignment analysis: implications for high tibial osteotomies[END_REF], and for Specogna et al., there is a difference of 1.6°between supine and bipodal telemetry [START_REF] Specogna | Radiographic measures of knee alignment in patients with varus gonarthrosis: effect of weightbearing status and associations with dynamic joint load[END_REF]. Conversely, Siu et al. found no influence of weight bearing on the telemetry results of the HKA angle [START_REF] Siu | A standardized technique for lower limb radiography. Practice, applications, and error analysis[END_REF]. In any case, this difference should be included in the frontal plane laxity range that was established by Yaffe et al. at 2.8°± 1.1°, with more than 70% of the knees having an evenly distributed laxity with 1°valgus and varus [START_REF] Yaffe | Radiographic and navigation measurements of TKA limb alignment do not correlate[END_REF].

3.

The main objective of this work was to compare surgical accuracy for the correction of VTO using the CM or the HT. The literature on the accuracy of VTOs has shown rather poor results [START_REF] Den Bempt | How accurately does high tibial osteotomy correct the mechanical axis of an arthritic varus knee? A systematic review[END_REF]. The collected data ranged from 23 to 91% with target intervals that varied from one series to another (3°varus to 8°valgus).

Despite the common use of the CM [START_REF] Sorin | Reproducibility of digital measurements of lower-limb deformity on plain radiographs and agreement with CT measurements[END_REF], a few articles report its results. Opening the osteotomy by 1 mm for each degree of correction desired is an approximation that does not take into account the tibial width. According to Mihalko and Krackow [START_REF] Mihalko | Preoperative planning for lower extremity osteotomies[END_REF], using this correction rule leads to deviations between 2.9 and 4.4°, for an average correction of 15°, depending on the size of the subjects and the width of the tibial osteotomies. This correction rule would thus only be accurate for osteotomy widths of 5.65 cm. Marti et al. [START_REF] Marti | Accuracy of frontal and sagittal plane correction in open-wedge high tibial osteotomy[END_REF] used this method, but taking into account ligament laxity, using complementary stress images (varus and forced valgus), which they added to the correction objective. Despite this, they only had 48% of their corrections within their target range.

For the HT, the literature reported an accuracy around 75% on a series of 246 VTOs for a defined target of 3-6°valgus [START_REF] Hernigou | Open wedge tibial osteotomy with acrylic bone cement as bone substitute[END_REF]. The literature remains poor concerning the HT. It is possible to compare the HT method to the planning method known as "gap measurement," where the correction angle is planned on pre-operative radiographs by measuring the height of the osteotomy and carrying it out intra-operatively. With this method, Brower et al. [START_REF] Brouwer | Osteotomy for medial compartment arthritis of the knee using a closing wedge or an opening wedge controlled by a Puddu plate[END_REF] placed 56% of their cases in the correction target (0-6°of valgus) and Schröter et al. [START_REF] Schröter | Surgical accuracy in high tibial osteotomy: coronal equivalence of computer navigation and gap measurement[END_REF] 91% of VTOs within 4°. For Duivenvoorden et al. [START_REF] Duivenvoorden | Comparison of closing-wedge and opening-wedge high tibial osteotomy for medial compartment osteoarthritis of the knee: a randomized controlled trial with a six-year follow-up[END_REF], using a trigonometric table, different from the HT method, 52% of the VTOs were within the target range (2-6°of valgus). Our results are comparable. Both the CM and the HT are based on the fact that the opening angle of the VTO is equivalent to the desired correction on the HKA. However, the rotation in the frontal plane due to the opening of the VTO takes place at the hinge of the VTO located a few centimeters below the centre of the knee where the HKA angle is measured. This approximation leads to additional theoretical errors of 1-2°for corrections of 15° [START_REF] Mihalko | Preoperative planning for lower extremity osteotomies[END_REF] as we have seen previously. On the other hand, this approximation does not affect the evaluation of surgical accuracy according to ΔMTA.

Recently, the use of custom-made cutting guides has been introduced for VTOs [START_REF] Shi | Three dimensional patientspecific printed cutting guides for closing-wedge distal femoral osteotomy[END_REF] and also for distal femoral osteotomies [START_REF] Jacquet | More accurate correction using "patient-specific" cutting guides in opening wedge distal femur varization osteotomies[END_REF]. The objective is simple: ease of surgery, reproducibility, accuracy of correction, and safety of execution. Chaouche et al. [START_REF] Chaouche | Patientspecific cutting guides for open-wedge high tibial osteotomy: safety and accuracy analysis of a hundred patients continuous cohort[END_REF] carried out the largest series on this technique, with 100 patients suffering from isolated medial femorotibial tibial osteoarthritis on a varus tibial metaphysis. They found a mean accuracy of 1 ± 0.95°on ΔHKA, 0.54 ± 0.63°on ΔMTA, and 0.43 ± 0.8°on the posterior tibial slope, with no significant difference from the pre-operative planning performed. The measurements were performed on a scanner with 3D reconstruction. The use of CT scan to check the correction allows a high degree of accuracy but does not take into account the influence of weight bearing since the examination is performed in the supine position. When the evaluation was radiological, the studies reported frontal accuracies of 1-2° [START_REF] Shi | Three dimensional patientspecific printed cutting guides for closing-wedge distal femoral osteotomy[END_REF][START_REF] Jacquet | More accurate correction using "patient-specific" cutting guides in opening wedge distal femur varization osteotomies[END_REF]. The safety, reproducibility, and ease of use of these cutting guides have been demonstrated.

4.

The secondary objective of this work was to analyze and compare the evolution of the tibial slope and patellar height for each group.

The overall tibial slope was increased for the whole series as reported in the literature for medial-opening VTO [START_REF] Schröter | Surgical accuracy in high tibial osteotomy: coronal equivalence of computer navigation and gap measurement[END_REF][START_REF] El-Azab | Patellar height and posterior tibial slope after open-and closed-wedge high tibial osteotomy: a radiological study on 100 patients[END_REF]. This change in slope is not correlated with the importance of the frontal correction [START_REF] El-Azab | Patellar height and posterior tibial slope after open-and closed-wedge high tibial osteotomy: a radiological study on 100 patients[END_REF]. On the other hand, the increase in tibial slope was significantly greater in the CM group compared with the HT group. Several factors were identified, such as insufficient posterior osteotomy, insufficient posterior soft tissue release [START_REF] Marti | Accuracy of frontal and sagittal plane correction in open-wedge high tibial osteotomy[END_REF], triangular tibia [START_REF] Hernigou | Open wedge tibial osteotomy: combined coronal and sagittal correction[END_REF], and anterior plate positioning [START_REF] Hernigou | Open wedge tibial osteotomy: combined coronal and sagittal correction[END_REF]. To avoid a change in tibial slope, it has been recommended to perform the VTO parallel to the tibial slope to maintain the original slope [START_REF] Song | Navigated Open Wedge High Tibial Osteotomy[END_REF], to open the tibial osteotomy more posteriorly than anteriorly [START_REF] Hinterwimmer | Control of posterior tibial slope and patellar height in open-wedge valgus high tibial osteotomy[END_REF], to make a rectangular and symmetrical sagittal opening, or to position the hinge in the centre of the sagittal plane [START_REF] Ogawa | Effect of wedge insertion angle on posterior tibial slope in medial opening wedge high tibial osteotomy[END_REF].

The patellar height has been reduced in this series as reported [START_REF] Tigani | Patellar height after high tibial osteotomy[END_REF], without any difference between the two groups. On the other hand, no correlation was found with the size of the correction, contrary to the literature [START_REF] Gaasbeek | The influence of open and closed high tibial osteotomy on dynamic patellar tracking: a biomechanical study[END_REF]. Biomechanically, medial opening VTOs lead to an increase in medial tilt through lateralization of the tibial tuberosity, which causes greater pressure of the lateral facet of the femoral trochlear groove on the lateral patellar facet. Since the direction of the quadriceps forces on the patella is not altered, lateralization of the tibial tuberosity results in a decrease in the medial rotation of the patella. The opening of the osteotomy also induces a lowering of the tibial tuberosity, leading to an increase in the tension of the patellar ligament and patellar retinaculum, thus increasing the pressure forces on the patellofemoral joint [START_REF] Gaasbeek | The influence of open and closed high tibial osteotomy on dynamic patellar tracking: a biomechanical study[END_REF].

Compliance with ethical standards

Fig. 1

 1 Fig. 1 Intra-operative measurement of the length of the osteotomy line using a wire positioned at the level of the osteotomy path

Table 1

 1 Data for global series

	Global series (n = 39)

Table 2

 2 Details of each groups

	TH and NA

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval This article does not contain any studies with human or animals performed by any of the authors.

Informed consent Informed consent was obtained from all individual participants by phone call.


Edouard Dessyn

Akash Sharma

Mathias Donnez

Patrick Chabrand

Matthieu Ehlinger

Jean-Noël Argenson

Sebastien Parratte

Matthieu Ollivier
email: matthieu.ollivier@ap-hm.fr

Adding a protective K-wire during opening high tibial osteotomy increases lateral hinge resistance to fracture

Keywords: High tibial valgus osteotomy, Patient-specific cutting guides, Hinge, Maximum load, cadaveric study

Purpose It was hypothesized in this in-vitro study that positioning a K-wire intersecting the cutting plane at the theoretical lateral hinge location would limit the cut depth and help preserve the lateral hinge during the opening of the osteotomy. Objectives were (1) to compare the mechanical resistance of the hinge and the protective effect of leaving the K-wire during the opening procedure (2) to check if the K-wire would limit the depth of the osteotomy. Methods An ex-vivo mechanical study, testing 5 pairs of fresh-frozen tibias, was designed. CT-scan based Patientspecific cutting guides were obtained to define the cutting plane and the location of the K-wire at the hinge, using standardized 3D planning protocol. In each pair, OWHTO was performed either with or without the K-wire. To evaluate the hinge's resistance to fracture, the specimens were rigidly fixed at the proximal tibia and a direct load was applied on the free tibial diaphysis to open the osteotomy. The maximum load at breakage, maximum permissible displacement and maximal angulation of the osteotomy before hinge failure was measured. To assess the preservation of an unscathed hinge (protected by the K-wire), the distance from the end of the osteotomy cut to the lateral tibial cortical was measured in mm.

Results

The maximum load to hinge breakage in the K-wires PsCG knees compared to the control group (48.3 N vs 5.5 N, p = 0.004), the maximum permissible displacement (19.8 mm vs 7.5 mm, p = 0.005) and the maximal angulation of the osteotomy before hinge breakage (9.9° vs 2.9°, p = 0.002) were all statistically superior in the K-wires PsCG knees compared to the control group. A mean distance of 10 ± 1 mm between cut-bone (saw-print) and lateral hinge cortical bone was found post-performing the osteotomy and the hinge failing.

Conclusion

The maximum load to breakage and the maximum permissible displacement were, respectively, 880% and 260% higher during the opening of the OWHTO in using K-wires compared to the non-K-wire control group. This confirms the mechanical advantage of using a K-wire for both stabilization and protecting the Hinge during OWHTO. This comparative cadaveric study shows an improvement of the lateral hinges resistance to failing during the opening of the osteotomy. This can be achieved by the placement of a K-wire intersecting the cutting plane at the theoretical location of the lateral hinge.

Introduction

Open Wedge High Tibial Osteotomy (OWHTO) remains an established surgical procedure [START_REF] Ahlbäck | X-ray classification and examination technics in gonarthrosis[END_REF][START_REF] Flecher | A 12-28-year followup study of closing wedge high tibial osteotomy[END_REF][START_REF] Lobenhoffer | Importance of osteotomy around to the knee for medial gonarthritis. Indications, technique and results[END_REF] for young and active patients with medial unicompartmental knee osteoarthritis secondary to varus mal-alignment of the lower limb. The success of an OWHTO depends on primarily following the correct indications, performing an accurate correction and maintaining the angulation and stabilization with a chosen method of an internal fixation, whilst keeping an intact lateral hinge [START_REF] Han | Ability of an intentionally smaller anterior than posterior gap to reduce the sagittal tibial slope in opening wedge high tibial osteotomy[END_REF][START_REF] Martin | Adverse event rates and classifications in medial opening wedge high tibial osteotomy[END_REF][START_REF] Stoffel | Open wedge high tibial osteotomy: biomechanical investigation of the modified Arthrex Osteotomy Plate (Puddu Plate) and the TomoFix Plate[END_REF]. Amongst these governing facets, preserving the lateral hinge plays a crucial role in improving postoperative outcomes, facilitating bone healing and stabilizing the osteotomy [START_REF] Bae | Medial cortical fractures in computer-assisted closing-wedge high tibial osteotomy[END_REF][START_REF] Dexel | Open-wedge high tibial osteotomy: incidence of lateral cortex fractures and influence of fixation device on osteotomy healing[END_REF][START_REF] Jo | The effects of different hinge positions on posterior tibial slope in medial open-wedge high tibial osteotomy[END_REF][START_REF] Nawas | Factors affecting the union of opening wedge high tibial osteotomy using a titanium wedge plate[END_REF]. If damaged, the fixation may not be sufficient and a secondary loss of correction may occur [START_REF] Bae | Medial cortical fractures in computer-assisted closing-wedge high tibial osteotomy[END_REF][START_REF] Dexel | Open-wedge high tibial osteotomy: incidence of lateral cortex fractures and influence of fixation device on osteotomy healing[END_REF][START_REF] Jo | The effects of different hinge positions on posterior tibial slope in medial open-wedge high tibial osteotomy[END_REF][START_REF] Nawas | Factors affecting the union of opening wedge high tibial osteotomy using a titanium wedge plate[END_REF]. Takeuchi et al. [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF] described a classification of hinge fractures in OWHTO. In their study, the authors reported an incidence of 25% for lateral tibial hinge fractures during OWHTO. Therefore, finding a solution to preserve the hinge unscathed seems critical to prevent a rather problematic consequence for both the patient and surgeon. Some Clinicians have tried to monitor their cut and opening using fluoroscopic images and protective cutting systems [START_REF] Lee | Openwedge high tibial osteotomy using a protective cutting system: technical advancement for the accuracy of the osteotomy and avoiding intraoperative complications[END_REF][START_REF] Lee | Comparative analysis of osteotomy accuracy between the conventional and devised technique using a protective cutting system in medial open-wedge high tibial osteotomy[END_REF]. Others have studied the influence of drilling the hinge through the saw cut to reduce the mechanical stress passing through the lateral tibial hinge [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF]. Computer-assisted surgery (CAS) can also offer solutions to the surgeon by controlling the depth of the cut, however this alone does not prevent the hinge to be susceptible to fracture [START_REF] Saragaglia | Role of computer-assisted surgery in osteotomies around the knee[END_REF].

In a previous study, Munier et al. [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF] demonstrated the use of patient-specific cutting guides (PsCG) offers an easy, reliable and reproducible procedure to create and maintain an optimal correction in all 3 planes during OWHTO. It was the aim of this research project to evaluate if PsCG could also offer an easy and reliable method to protect the lateral tibial hinge and increase its resistance to fracture. The hypothesis of this in-vitro study was that positioning a K-wire to intersect the cutting plane at the theoretical lateral hinge location as a result of using a PsCG would limit the cut depth and augment the strength of the lateral hinge during the opening. Using PsCG dedicated to OWHTO [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF], a comparative in-vitro study was set up using five matched pairs of cadaveric knees. Objectives were first to study the protective effect on the hinge's resistance to fracture when a K-wire was left in the lateral tibial hinge during the opening procedure and secondly to check if the K-wire would limit the cut depth. To our knowledge, a majority of published series focus on the diagnosis and treatment of OWHTO hinge fracture, and few provided reliable preventative measures to limit them, making this study original.

Materials and methods

Five matched pairs of knees (3 females and 2 males aged from 75 to 97, average age 88.8) were obtained from Aix-Marseille University Anatomical Department. Anatomical and biomechanical labs scientific and ethic committees validated our study protocol and ensured ethical use of de-identified specimens. Bones were completely dissected leaving only the patellar tendon insertion intact to allow positioning of the cutting guide [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF]. All specimens underwent a standardized CT-scan protocol (Discovery 710, GE Medical System, CERIMED, Marseille, France). 3D geometrical models of the tibias were created using Mimics 17.0 software (Materialise®, Leuven, Belgium). The 3D models were used to design a patient-specific cutting guide for each tibia [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF], defining the cutting plane and the insertion of two ⌀2.2 mm K-wires locations. It is accepted that the osteotomy cut should stop approximately 10 mm from the lateral cortex [START_REF] Munier | Can three-dimensional patient-specific cutting guides be used to achieve optimal correction for high tibial osteotomy? Pilot study[END_REF]. The appropriate positioning of the hinge is now known for prevention of an unstable lateral hinge fracture [START_REF] Nakamura | Appropriate hinge position for prevention of unstable lateral hinge fracture in open wedge high tibial osteotomy[END_REF]. To maintain and protect the lateral tibial hinge from an inadvertent cut from the sawblade, the first K-wire (Cut K-wire) was parallel and just below the cutting plane, whilst the second (Hinge's K-wire: HK) was placed from distal to proximal from antero-medial to lateral to intersect the cutting plane at the theoretical hinge point (10 mm from the lateral cortex). The Cut K-wire was used to control the accurate positioning of patient-specific cutting guide, and to maintain the guide rigidly to the bone during the process of drilling the pin holes. Importantly, it also helps in guiding the sawblade during the second part of the cut when the guide is taken off. The PsCG has 4 drill guides: 2 proximal and 2 distal to the cutting plane to improve the PsCG stability using ⌀3.9 mm pins. The two holes proximal to the cutting plane were also made to predict the position of the tibias on the experimental setup and to increase the rigidity for mechanical testing as described later. For each tibia, the PsCG was placed on the antero-medial tibial cortex. K-wires were inserted, and the four holes were drilled to insert the ⌀3.9 mm pins. The osteotomy was then performed and the cut was limited by the the buttress in the slot for the saw guide. The thickness of the sawblade used was 1.27 mm. The next step was to remove the PsCG including the pins except for the HK, the cut was then extended until the saw touches the HK (Fig. 1).

The proximal tibial epiphysis was fixed in PVC tubes using two rods inserted through the tubes and through the two proximal drill holes proximal to the cutting plane. The location of the rods was pre-planned on 3D models to have the resin parallel to the cutting plane, to ensure a reproducible position of the tibia inside the PVC tubes. The proximal tibial epiphysis was embedded in PVC tubes with Fascast polyurethane resin (Axson Technologies, Cergy, France).

All the tibias followed the same preparation process with the same parameters (cutting plane location and same Hinge pin location in all tibias). The experimental conditions were strictly identical and reproductible for each pair of tibial specimen being tested. The positioning of the PsCG, the creation of the osteotomy cut, the fixation of the tibia in the experimental setup, the anatomical location of the osteotomy cut and its depth in the epiphyseal tibia was strictly the same. Two factors only were modified in each pair of tibia: the lateralization of the tibia (right vs left) and the maintenance or the removal of the HK during OWHTO.

For mechanical testing, two groups were defined: in each matched pair, the HK was removed from only one tibia randomly chosen, defining the two groups: a "HK group (HK+) and a "No HK" group (HK-). To accurately reproduce surgical opening, the tibial epiphysis was fixed and a load was applied on the tibial diaphysis. When ready, tibias were placed in an Instron loading platform (INSTRON 5566A, Norwood, MA, USA). Load was applied on the diaphysis in a single load-to-failure test. Quasi-static loading with prescribed displacements at 10 mm/min was applied up until failure, according to a previously published protocol [START_REF] Takeuchi | Fractures around the lateral cortical hinge after a medial opening-wedge high tibial osteotomy: a new classification of lateral hinge fracture[END_REF] (Fig. 2). For every tibia, the presence of a unscathed lateral tibial hinge was visually assessed before the testing. Visually, the hinges were classified as broken or not in each group. For each tibia after fixation within the experimental setup, opening angles, vertical displacement in mm and load applied in N on the diaphysis was recorded. Failure was considered just before a significant drop occurred on the Force/Displacement curve. Force at failure (ML) and corresponding displacement (DML) were identified. Opening angles at failure α were also calculated based on DML value (Eq. 1, Fig. 3):

In each pair, the influence of the HK was evaluated by comparing DML, ML and α between the HK+ and the HK-. All tests were carried out until the fracture of the lateral hinge. The Load-Displacement curves were available for each tibia (Fig. 4a,b). Based on 3D models used to design the PsCG, distances x, y, z were measured and the opening angles at failure were calculated using this method of reading the graphical coordinates of the ML and the DML. After Mechanical testing including fracture of the lateral hinge, an evaluation of the distal part of the (1)

= 1 -2 = (tan (-1) ((y∕x)) -(tan (-1) ((DML -y)∕x)).
Fig. 1 3D schematization of a tibia showing the crossing of the two K-wires: the upper one parallel and just below to the cutting plane, the second one ascendant from antero-medial to lateral that intersect the cutting plane at a theoretical hinge point, identifying the lateral tibial hinge that would be protected from the saw Fig. 2 Final experimental model in the experimental device with application on the tibial diaphysis of the load and start of the two cameras to film the experiment osteotomy was made by 2 of the authors (ED and MD). The purpose of this evaluation was to measure, using a caliper, the distance in mm between cut (saw-print) and the lateral aspect of hinge's cortical bone (Fig. 5). The intra and extra-observer correlation was evaluated using the Spearman test. One of the authors (trained engineer: MD) analysed the results on our load-displacement curves. The precision reported was fixed to one

Statistical evaluation

A post-hoc power analysis was performed for opening angles, DML and ML. Non-parametric Wilcoxon test was performed to compare opening angles, DML and ML between HK+ and HK-groups. p < 0.05 was considered a threshold of statistical significance. To distinguish maximal angulation > 5 ± 3°, opening > 10 ± 6 mm and Load > 40 ± 30 Newtons before breakage between pairs, 5 pairs of knees were required by our sample evaluation. All calculations assumed two-sided test with alpha of 0.05 and 1-Beta of 0.8.

Results

Analysing the graphical coordinates of the ML and the DML, the final results were obtained (Table 1). On average, opening angles were found 339% greater in the HK+ compared to the HK-(P = 0.019). Indeed, α was on average at 9.9° in the HK+ compared to 2.9° in the HK -group (Table 2). In the same manner, tibias of the HK+ showed a 265% increase for the DML compared to tibias of the HK-group (P = 0.05). Finally, a significant increase of 882% (P = 0.04) was found for the ML again between the two groups. All these results are detailed in Table 2.

For each tibia, the cutting guide was accurately placed according to the pre-OWHTO planning provided and the osteotomy was performed as planned. During the second part of the osteotomy cut (PsCG removed, HK still in place), the sawblade reached the HK and could not cut further. After Mechanical testing, an evaluation of the distal part of the osteotomy made by 2 of the authors confirmed that the HK

Discussion

The main finding of this study demonstrated that positioning a K-wire intersecting the cutting plane at the theoretical lateral hinge location 1/limits the cut depth, thus preserving sufficient lateral hinge thickness. 2/increased the mechanical resistance of the hinge to failure. In this in-vitro study, influence of the HK on preserving the lateral hinge during the OWHTO opening was assessed into matched pairs of tibias coming from 5 subjects, allowing equivalent testing conditions into each pair. As each tibia underwent the same preparation process, and the location of the cutting plane and HK were defined based-on Patient-specific cutting guides, the study design only compared the influence of the HK presence during OWHTO opening. Subjects of the HK+ group tolerated much higher ML and DML than those of the HK-group. In the same way, the angulation α allowed at the maximum load before breakage was greater when using a hinge K-wire during OWHTO opening. Recently, Lee et al. [START_REF] Lee | Predictive factors for and detection of lateral hinge fractures following open wedge high tibial osteotomy: plain radiography versus computed tomography[END_REF] proved that the incidence of lateral hinge fractures of the knee after OWHTO was significantly higher on CT scans than on plain radiographs. The medial opening gap width was the only predictor of lateral hinge fractures, suggesting a need for care when spreading the gap to avoid lateral hinge fractures. On the other hand, Lee et al. [START_REF] Lee | The learning curve for biplane medial open wedge high tibial osteotomy in 100 consecutive cases assessed using the cumulative summation method[END_REF] showed that surgical experience may improve the surgeon's competency in prevention of lateral hinge fractures, highlighting the need to be trained and experienced to fully respect the lateral tibial hinge. Moreover, the pivotal role of a competent lateral tibial hinge in tibial osteotomies in terms of bone consolidation and maintenance of angular correction in all 3 planes has been widely reported [START_REF] Dexel | Open-wedge high tibial osteotomy: incidence of lateral cortex fractures and influence of fixation device on osteotomy healing[END_REF][START_REF] Lobenhoffer | Importance of osteotomy around to the knee for medial gonarthritis. Indications, technique and results[END_REF][START_REF] Saragaglia | Role of computer-assisted surgery in osteotomies around the knee[END_REF][START_REF] Weng | The effects of tibia profile, distraction angle, and knee load on wedge instability and hinge fracture: a finite element study[END_REF]. These studies suggest that the lateral tibial hinge must be kept unscathed. However, to our knowledge and to date, very few studies [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF][START_REF] Jang | Role of an anatomically contoured plate and metal block for balanced stability between the implant and lateral hinge in open-wedge high-tibial osteotomy[END_REF] have investigated the resistance of this tibial hinge to fracture and/or on potential operative strategies to improve this mechanical resistance. In their numerical study, Diffo Kaze et al. [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF] focused their study on the effect of hinge drilling to improve the resistance to fracture during OWHTO. Their results confirm that, for tibial varus correction osteotomies < 5°, drilling a hole at the end of the osteotomy reduces stresses in the lateral hinge and increases the critical angle before failure of the hinge but only for corrections inferior to 5°. Indeed, for osteotomies superior to 9 mm of opening in cadaveric specimens, Bujnowski et al. [START_REF] Bujnowski | A pilot hole does not reduce the strains or risk of fracture to the lateral cortex during and following a medial opening wedge high tibial osteotomy in cadaveric specimens[END_REF] recently demonstrated that a pilot hole showed no significant decrease in the strains experienced at the lateral cortex, nor did it reduce the risk of fracture of the lateral tibial hinge. According to Diffo Kaze et al. [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF], the ductility of the cortical bone is the decisive parameter for the critical angle of opening, defined as the angle from which the tibial hinge breaks. These results, go in the same direction as ours: in the two studies, angulation, vertical displacement and maximum load before breakage were all greater when the hinge was protected before applying opening load. The maximal angular deformation seems to be greater using the HK method described compared to hinge drilling method by Diffo Kaze et al. [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF] (fractures occurred at a mean of 9.9° in this study and always < 5° for all Diffo Kaze [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF] et al. study's specimens). Moreover, it is important to note that the Cut K-wire was situated just below the cutting plane, so when viewing the projection of both K-wires in the frontal plane (antero-posterior flouroscopy), the location of the hinge was just above this Cut K-wire, and just lateral to the hinge K-wire. Neither of these two K-wires follow the same placement as the pilot hole described by Diffo Kaze [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF] et al. and Bujnowski et al. [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF]. In this manner, the lateral tibial hinge was kept unscathed during our experiment, whereas in comparison it was drilled in the one described by Diffo Kaze et al. [START_REF] Kaze | Mechanical strength assessment of a drilled hole in the contralateral cortex at the end of the open wedge for high tibial osteotomy[END_REF]. For each tibia, the HK limited the cutting depth by leading the saw to "touch but never to breach the HK". In this manner, the K-wire acted as a constraint to the depth of the osteotomy cut allowing preservation of a lateral tibial hinge by a width of 10 mm.

To our knowledge, no reliable method to keep the hinge unscathed yet exists. The sole use of flouroscopic images does not prevent hinge fractures and does not accurately ensure the precision required to stop the saw from damaging the hinge. Using a traditional surgical technique, an incidence of almost 30% of hinge fractures has been reported, most of which are not recognized during surgery but rather identified at the radio-clinical 6 week follow-up [START_REF] Dexel | Open-wedge high tibial osteotomy: incidence of lateral cortex fractures and influence of fixation device on osteotomy healing[END_REF]. Even so, for Dexel et al. [START_REF] Dexel | Open-wedge high tibial osteotomy: incidence of lateral cortex fractures and influence of fixation device on osteotomy healing[END_REF], locking plates with extended diaphyseal fixation decreases the importance of an intact lateral hinge. Kim et al. [START_REF] Kim | Extraarticular lateral hinge fracture does not affect the outcomes in medial open-wedge high tibial osteotomy using a locked plate system[END_REF] demonstrated in their retrospective study that lateral hinge fractures, when occurring in OWHTO using a medial locking plate, showed no radiological changes or functional deterioration during midterm follow-up when compared to a group that endured OWHTO with the same plates but with no lateral hinge breakage appearing peri or post operatively. However, our study investigates a potential solution to protect the hinge intra-operatively and does not study the measures required when the hinge is fractured during or after the index surgery. In the same way, even if larger plates decrease the importance of protecting the hinge, the advantage of mini-invasive surgery remains important and should encourage the surgeon to deal and anticipate hinge failure with a minimally invasive approach. Lee et al.

[14] also showed that further evaluation with CT scanning was almost obligatory after OWHTO because of its higher detection rate (24.5%) of lateral tibial hinge compared with simple radiographs (16%). They importantly recognized the coronal osteotomy slope was steeper and the opening gap was larger when the hinge was broken. They concluded their work by stating that lateral tibial hinge fractures should be managed conservatively to prevent postoperative correction loss. CAS-OWHTO [START_REF] Bae | Computerassisted navigation decreases the change in the tibial posterior slope angle after closed-wedge high tibial osteotomy[END_REF] does not prevent hinge failure, but rather permits to optimize the position of the hinge's Axis, thus avoiding hinge related complications (Hinge Fractures, Bone healing impairment and secondary loss of correction). This experiment has some limitations. First, the cohort used is small with 5 samples in each group. However, our study was powered to distinguish clinically relevant differences between the two groups [START_REF] Ogden | Changes in load distribution in the knee after opening-wedge or closing-wedge high tibial osteotomy[END_REF]. Moreover, the study model is based on tibial pairs, which allows direct comparison and limits potential confounding factors such as cadaveric bone quality, specimens age, and conservation process, as the only difference between the groups was the positioning of a hinge K-wire during osteotomy opening. As an ex-vivo study, it only approximates in-vivo conditions. The authors appreciate that this is mechanical testing on bone rather than bone and the surrounding soft tissues which may improve the stability of the lateral hinge [START_REF] Lee | Effect of soft tissue laxity of the knee joint on limb alignment correction in open-wedge high tibial osteotomy[END_REF].

This comparative cadaveric study demonstrates that an improvement of the lateral hinge resistance to fracture during osteotomy opening can be obtained with an addition of a K-wire intersecting the cutting plane at the theoretical lateral hinge location. Therefore, the protective nature of the K-wire in-vivo, if applied in reality could substantially decrease the rate of hinge facture and prevent subsequent OWHTO adverse outcomes.

Conclusion

This comparative cadaveric study showed that an improvement of the lateral hinge resistance to fracture during osteotomy opening could be obtained by the addition of a K-wire intersecting the cutting plane at the theoretical lateral hinge location.

Fig. 3 Fig. 4 a

 34 Fig. 3 Schematic representation of α, x, y, and z

Fig. 5

 5 Fig. 5 Measurement of the distance between cut-bone and hinge lateral cortical

Table 1

 1 SummaryML maximum load before breakage of the hinge, DML displacement corresponding at the ML before breakage of the hinge, α angulation permitted before breakage of the hinge, HK-no hinge K-wire group,

	of the results obtained and calculated for each	Mechanical parameters Specimen 1	Specimen 2	Specimen 3	Specimen 4	Specimen 5
	pair of knee tested; d: distance		HK-HK+ HK-HK+ HK-HK+ HK-HK+ HK-HK+
	between cut (saw-print) and hinge lateral cortex	d (mm)	9.2	9.4	10.1	10.3	10.9	11.2 10.3 10	11.4 11.6
		ML (N)	10.1	8.4	1.0	47	2.9 117.7	6.4 12.6	7.3 14.1
		DML (mm)	12.2 31.4	3.1	16.7	7.5	31.2	6.1	6.8	8.6	6.3
		α (°)	4.4 15.7	1.1	6.0	2.7	12.6	2.7	3.0	3.8	2.7
		HK+ Hinge K-wire group

Table 2

 2 Summary of maximum loads, DCM and mean allowable angulations for all knees with and without pins ML maximum load before breakage of the hinge, DML displacement corresponding at the ML before breakage of the hinge, α angulation permitted before breakage of the hinge, HK-no hinge K-wire group, HK+ Hinge K-wire group, SD standard deviation

	Mechanical param-	Mean (SD)		Mean difference	95% IC of difference	Absolute difference	P value
	eters at failure	HK+	HK-
	ML (N)	48.3 (40)	5.5 (4.2)	42.9	[6.6-97.1]	+ 882%	0.04
	DML (mm)	19.8 (11.4)	7.5 (3.3)	12.3	[7.2-17.2]	+ 265%	0.05
	α (°)	9.9 (5.2)	2.9 (1.3)	7	[3.4-9.5]	+ 339%	0.02

Funding No funding was received to complete this study protocol.

Compliance with ethical standards

Conflict of interest

Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Informed consent This study involved de-identified human anatomical specimens.