The most important finding of this study is that, knee flexors result in a significantly greater increase in the PTS and counteracting the knee flexors by gravity with knee joints staying suspended and extended when steel plates are fixed is effective in solution of this problem.
Based on a systematic review of open-wedge HTO studies, attitudes towards counteracting knee flexors by gravity or not when steel plates were fixed are inconsistent. However, despite the procedure is of great significance for preventing unexpected increase in PTS, previous studies have not focused on it and osteotomy procedure is still not completely standardized and consistent. Thus, the purpose of this study was to evaluate the effects of counteracting knee flexors on PTS after OWHTO and to make the osteotomy technique more standardized.
Increase in PTS could result in the tibial forward movement relative to femur. Studies have shown that 1° increase in PTS was accompanied by 1.45 ° increase in the loss of knee extension[15]. Pressure distribution in the knee compartments rearranged when PTS increases by 5.5 °: peak pressure point moves backward 24% of the knee compartment, which may be an important cause of pain or surgical failure after OWHTO [16]. In addition, tibial forward movement resulted from the increase of PTS results in increased tension of the anterior cruciate ligament (ACL) which would eventually aggravate the degeneration of ACL[21]. Some other studies have even confirmed that the increase of PTS is a risk for postoperative anterior cruciate ligament injury, which would result in knee instability and accelerate knee degeneration[22]. Many studies reported that, increase in post-operative PTS would increase the amount of osteotomy on anterior tibial plateau during the secondary total knee arthroplasty (TKA) [9, 23–25]. It has also been reported that increase in PTS would be accompanied by the contracture of the patellar tendon and the secondary TKA would be more difficult to perform[26–29]. Previous studies have made several recommendations to avoid an increase in the PTS after OWHTO, including making a complete posterior osteotomy, maintaining an optimal gap ratio, maintaining an optimal hinge position and steel plate position and so on.
Lee et al. suggested that the gap ratio should be kept the anterior opening gap approximately 67% of the posterior[13] and Noyes et al. found that the optimal ratio to maintain the normal sagittal tibial slope was 50%[4, 17, 18]. Dong et al. reported that autologous tricortical iliac bone graft could be effect in promising the optimal gap ratio[30]. In other words, despite the results they reported are different, they all aim to keep anterior opening gap narrower enough to the posterior.
Previous studies reported that hinge position was significantly correlated with post-operative PTS. Joon et al. found that a lateral hinge instead of a posterolateral hinge contributes to a complete opening of the posterior opening gap [16]. And according to Marti et al, increase in PTS after osteotomy will occur if the posterior gap is incompletely opened or the posterior soft tissue is not fully released[3]. Ho‑Seung et al. suggested that standard height of hinge should be adopted to prevent the increase of PTS. Horizontal osteotomy line should be designed from 3 cm below the medial edge of tibial plateau towards to the fibular head. Lower hinge position could result in a significant increase in post-operative PTS and an increased risk of hinge fracture [31].
Therefore, keeping the posterior gap opened enough is a key surgical step which should be promised. However, knee flexors with strong strength are resistance to the posterior gap opening. Although distraction forceps are used to open the osteotomy gap during the operation, it only promises an appropriate post-operative force line on the coronal plane, rather than a promise of optimal sagittal tibial plateau slope. In this study, using gravity to counteract the strength of knee flexors is recommended, which can keep the entire posterior osteotomy gap evenly opened. In this proposal, a suspended and extended posture of the knee with placing a sterile cloth ball under the ipsilateral ankle to raise the ankle is adopted when steel plates are fixed. The advantage of our operation is that it is simple and easy to perform and is effective in preventing the increase in post-operative PTS. In particular, this operation is not standardly or consistently performed during surgery, therefore, one of the main purposes of this study is to suggest this procedure should be performed in OWHTO standardly and consistently.
There are several limitations in this study. First, there were only a limited number of subjects. Other factors such as the corrective angle were not considered. However, study limitations were minimized by selecting the optimal corrective angle and surgical protocols based on previously studied outcomes for OWHTO. Second, weight of lower limbs has not been measured. We only compared the different results of anti-knee flexor and non-anti-knee flexor. Fortunately, we have come to a very important and practical conclusion. Third, although the follow-up period in all patients is more than 1 year, the follow-up time is still not long enough, which requires a continued follow-up investigation.