Plate-related results of opening wedge high tibial osteotomy with a carbon fiber reinforced poly-ether-ether-ketone (CF-PEEK) plate fixation: a retrospective case series of 346 knees

Open wedge high tibial osteotomy (owHTO) is an established surgical procedure to treat patients with medial osteoarthritis of the knee in combination with varus deformity [1‐3]. By reducing the effective load through the medial compartment and eduction or reversion of the adductor moment, the procedure aims to decelerate the degeneration progress, to relieve pain, and to correct the lower extremity alignment. While owHTO is a standard procedure, it bears the risk of postoperative hardware failures and lateral cortical hinge fractures [4‐6]. Different clinical studies reported lateral hinge fractures in 18–39% of patients [7‐10], which in turn can lead to an accelerated osteoarthritis, non-union, or a loss of correction accuracy [6, 11, 12]. Requirements for fixation systems in owHTO are high. They have to withstand the high forces of early full weight-bearing programs and minimize the risk of delayed and non-union in case of a lateral cortex fracture. At the same time, they have to allow for sufficient interfragmentary movement (IFM) to achieve a faster gap healing time [11, 13, 14]. In the past, several angle-stable locking plates, such as the Position HTO plate (Aesculap, Tuttlingen, Germany), the Puddu Plate (Arthrex Inc., Naples, FL, USA), or the TomoFix Plate (Synthes Medical, Oberdorf, Switzerland), have been developed aiming for an increased loading capacity and residual stability after failure of the lateral cortex [13]. Clinical results for patients treated with these implants have demonstrated a high stability and lower rates for loss of correction [5, 15, 16]. However, other studies have shown that a high stiffness of a locking plate may suppress callus formation and fracture healing [17, 18]. The authors suggested increasing the IFM of the implants in order to avoid incomplete bone healing.

Addressing the disadvantages of present implants for owHTO the PEEKPower HTO plate (Arthrex Inc., Naples, FL, USA) was introduced. The plate consists of carbon poly-ether-ether-ketone reinforced fibers. A biomechanical study, comparing the Tomofix plate composed of titanium to the PEEKPower HTO plate, showed that the PEEKPower HTO plate resists higher dynamic loadings and had a higher static flexural rigidity [19]. However, when followed-up in a clinical setting the 1st generation PEEKPower HTO plate resulted in a higher rate of implant-related complications than the TomoFix plate after 24 months [20]. This finding led to the development of the 2nd generation PEEKPower HTO plate. An initial prospective study provided promising outcomes of the 2nd generation PEEKPower HTO plate, showing comparable complication rates to commonly used fixation systems [21]. The study is, however, limited by the low number of patients (n = 28) and a maximum osteotomy gap size of 12 mm. Also, reliable mid- to long-term clinical outcomes of the 2nd generation PEEKPower HTO plate are missing.

The aim of this study was to evaluate the radiologic outcomes and safety of the 2nd generation PEEKPower HTO plate for valgus owHTO. Retrospectively, we analyzed the number of lateral cortex fractures, the quality and time of gap healing, the correction accuracy, and implant-related complications of 324 patients (346 knees) with a follow-up of 1 year after owHTO with the PEEKPower HTO plate.

The ethics committee of the University of Lübeck approved this retrospective study. The postoperative rehabilitation program and the radiographic evaluation after owHTO were standard of care procedures for all patients. After implant removal, the medical treatment ended. For the analysis of the outcomes, all personal patient data were anonymized.

Several plate fixation systems are available to fix the osseous gap at the proximal tibia, which arises during owHTO. Retrospective studies have shown satisfactory results for many of them with survival rates of 78–90% at 10 years and 56–68% at 15 years after HTO [1, 3, 30‐32]. However, frequent complications of owHTO are hardware failures and lateral cortical hinge fractures, which can lead to a delayed union, a non-union, or a loss of correction accuracy [5, 6, 11, 12].

The main finding of this study was that the PEEKPower HTO plate led to an osseous consolidation in all patients with a mean gap healing time of 4.0 ± 1.7 months independent of the presence of hinge fractures or BMI. Despite a hinge fracture rate of 30%, there was no case of non-union and only 4 patients (1.2%) had a loss of correction accuracy with a clinical relevant loss of ≥ 3 mm.

The mean gap healing time of 4 months demonstrated in this study is slightly faster compared to similar devices, as Brosset et al. observed that bone union occurred at 4.5 months on average after owHTO with the TomoFix plate [26]. Reason for the shorter healing time of the PEEKPower HTO plate might be the higher IFM. In our study, 26.5% patients had a delayed osseous healing, which was defined as absence of bony healing on plain radiographs after 4 months according to Miller [10]. Nevertheless, the bone gap healed in this cohort without further surgery 1 year after owHTO in all cases. There was no case of non-union. Differently, Cotic et al. reported for the first generation PEEKPower HTO plate non-union in 12% of the patients (n = 26) [20]. In a prospective study (n = 28) with the 2nd generation PEEKPower HTO plate, they found only one case of non-union without bone grafting [21]. Noteworthy, in this study, Cotic et al. excluded patients who had a gap size larger than 12 mm because in their opinion additional bone grafting seemed to be necessary using the 2nd generation PEEKPower plate. Several studies suggested that gap filling could protect lateral cortex fracture and loss of correction [33‐35]. In our study, patients had an osteotomy gap size ranging between 5 and 17 mm (average 10 ± 2.7 mm), in 58 cases, the gap size was ≥ 12.5 mm. The use of gap filler (Osferion) in 85% of the patients had no significant influence on the bony healing. However, it could be shown that with increasing gap sizes bony healing was significantly prolonged (p < 0.001). Overall, our results suggest a clear improvement of the 2nd generation PEEKPower HTO plate compared to the 1st generation regarding the outcomes of bony union. Although it must be noted that studies are difficult to compare because the criteria for delayed union and non-union are not defined uniformly, the 2nd generation PEEKPower plate seems to provide better results when compared to the non-union rate of other owHTO fixation devices. Warden et al. sent questionnaires to all members of the Australian Knee Society asking for the rate of united, delay-united, and non-united owHTOs they performed [11]. Of the reported 182 owHTOs using different fixation plates, 6.6% were classified as delay-united and 1.6% non-united. A large case series after owHTO (n = 245) using an internal fixation plate system was performed by Hernigou [31]. There were two patients with delayed unions for whom weight-bearing was delayed for 3 months, and one patient with non-union, which required removal of acrylic bone cement. For the TomoFix HTO plate Brosset et al. reported a non-union rate of 7% [26] and Meidinger et al. of 5.4% [36]. The latter showed that risk factors with a statistically significant influence on the development of a non-union included smoking, body mass index, and fracture of the lateral cortical hinge. In the present study, the majority (89%) of the patients were overweight or obese, but the BMI of the patient had no influence on the positive outcomes. This indicates that owHTO using the PEEKPower plate leads to a bony consolidation also in severely obese patients.

A typical, well-known complication during owHTO is the fracture of the lateral cortex. It increases stress forces and leads to higher demands on fixation systems. Takeuchi et al. presented in 2012 a new classification system for hinge fractures which was used also in this study [8]. However, Takeuchi determined all fractures during surgery or immediately afterwards. In our study, the hinge fracture was identified using the first radiograph taken 4 weeks postoperatively. Previous studies reported hinge fracture rates between 18% and 39% [7‐10]. We could show a comparable rate (30%) of lateral hinge fractures compared to the studies mentioned before. Most of our patients had a Takeuchi type I fracture (22%). Similarly to the findings of Takeuchi, we found type II in 5.5% and type III in 2% of the patients [8]. Our study data indicated a significantly increased risk for a hinge fracture with raising osteotomy gap size (p = 0.0041). Furthermore, we were able to show that Takeuchi I and II hinge fractures led to increased bony healing times after owHTO with the PEEKPower plate. None of the patients with a hinge fracture exhibited non-union. Mentionable, patients with fracture type III had no significant delayed bone healing time, which may be due to the low number of patients observed with lateral hinge fracture type III. Overall, our results demonstrate that owHTO using the PEEKPower plate leads to satisfactory results even in case of a lateral hinge fracture. Furthermore, we claim that the risk of a lateral hinge fracture is not a typical plate-related result but rather related to the surgical technique.

Many research studies did not focus on the postoperative correction accuracy, but in our opinion, this aspect is one of the most important quality feature for a fixation plate system. Unfortunately, there is no consistent method for measuring the loss of correction height.

Agneskirchner et al. have reported about loss of correction height as a comparison between prescript and achieved correction. However, they did not describe the method of measurement [13, 15, 37]. The present study introduces a new reproducible method to determine the loss of correction in PA radiographs. To our knowledge, this measurement has never been used before. According to this method, 298 patients (87%) had no loss of correction 1 year after owHTO, even those whose implant was removed early after 4.5–6 months because of deep infection. Of the 13% with a loss of correction, only 1.5% had a clinical relevant loss of ≥ 3 mm. We assume that especially the new design of the 2nd PEEKPower HTO plate is responsible for these excellent outcomes. Stoffel compared two different implants (Puddu Plate versus TomoFix Plate) and stated that in case of lateral cortex fracture only the Puddu Plate needed an additional screw fixation in contrast to the rigid long plate fixation system [38]. Agneskirchner concluded the same in his study comparing the biomechanics of four different implants [13]. In particular, he determined that shorter plates provided less stability, leading to lateral cortex fractures and loss of correction. This conclusion is not applicable for this current study.

In our study, overall 7 cases with deep infection (2%) were identified, which is comparable to other studies [8, 9, 26]. Over the entire follow-up period, none of the patients had neither an injury of nerve structures with sensory or motor deficit nor a screw breakage or material failure at the removal of the plate material. During the removal surgery, nearly all patients showed a black tissue discoloration around the plate without signs of inflammation or local infection (Fig. 5). Cotic et al. showed in a previous study that this discoloration has no cytotoxic effect [20].

Open wedge HTO using the PEEKPower HTO plate for patients with medial osteoarthritis of the knee in combination with tibial varus deformity leads to excellent bony consolidation also in cases with a hinge fracture, a gap size > 12 mm, and for severely obese patients.