The most important finding of the present study is that correction angle was a significant factor affecting the PTS and PH in patients who underwent OWHTO. The changes are highly variable and often the amount of change is too small to be clinically significant; however, the maximum assessed changes during this study (maximal PTS change 12°, maximal ISI change 0.9 and maximal BPI change 0.75 ) highlight the need to be vigilant for these problems.
To our knowledge, this is the first clinical study evaluating the effect of different amount of correction on PTS and PH in OWHTO to date. PTS is important for knee extension and flexion, for the correct function of the cruciate ligaments and normal knee kinematics30. In OWHTO,several factors could influence PTS, including the amount of correction20, optimal gap ratio in the sagittal plane31–33, hinge position31,32, hinge fracture, insufficient posterior osteotomy and release of soft tissue31,34. Many studies have demonstrated an increased PTS after OWHTO7,17,20,35−37.
In our study, we observed that the mean postoperative PTS increased. The change of PTS in MCA group and LCA group showed a significant difference compared with preoperative PTS. And, LCA group yielded tibial slopes that were significantly different from SCA group and MCA group, which indicates that the amount of correction performed is one of the key factors affecting the PTS after OWHTO. It is clear that the larger the correction, the more freedom is present for PTS or PH. Matthias et al.20 evaluated the influence of certain intraoperative factors of the degree of PTS change observed during OWHTO through a cadaveric model. They found that larger corrections yielded not only increased mean values for PTS, but also increased variability in the observed values. The changes noted in small corrections were generally below clinical significance; however, in cases with concomitant ACL deficiency, even small changes may be important. This result is consistent with another cadaveric study conducted by Rubino et al.19. El-Azab et al.38 found that PTS increased after OWHTO because of the geometry of the proximal tibial, which dictates corresponding sagittal plane changes in coronal plane ostetomies. In a recent meta-analysis, an unintentional mean postoperative slope increase of 2.02°has been detected in OWHTO39.
It has been suggested that increased PTS may result in knee instability and tibial translation including increased ACL strain, posteriorly shifted intra-articular cartilage peak pressure. In addition, those patients with increased PTS often underwent more bone loss of the tibial plateau posteriorly, which could make it more difficult to possible future total knee arthroplasty (TKA)40. However, the incorporation of specific surgical techniques such as controlling anterior/posterior osteotomy opening gap ratio between 2:341; a complete osteotomy of the posterior cortex42, putting the plate as far posteriorly as possible43, using reference K-wires for intra-operative PTS orientation44, applying a bicortical screw to fix the distal tuberosity to the tibia45 and avoiding larger corrections43 may helpful in minimizing PTS change. Nevertheless, of course,such an increased PTS can be beneficial in cases of posterior cruciate ligament deficiency, as it can lead to anterior translation of the tibial relative to the femur, especially if ACL deficiency is present46.
Ideally, any surgery on the knee should not significantly worsen the position of the patella, making more attention should be paid to avoid patellofemoral issues. A normal PH is an important determinant for knee function, otherwise, may result in anterior knee pain, decreased range of motion, patellofemoral arthrosis, and potentially complicated future TKA9,35. However, we observed that the mean postoperative patellar height decreased after OWHTO in our study. The mean change in ISI and BPI between the SCA group and LCA group are significant (p<0.05), thus we speculated that larger correction angle seems to decrease the patellar position and may have a negative impact on patellar height. In OWHTO, due to the attachment of the tuberosity to the distal fragment, the tibial insertion of the patellar is distalized. Also, a potential post-operative tendon shortening caused by scarring may occur.
Hence, in order to avoid a PH decrease, Gaasbeek et al.47introduced a biplanar descending OWHTO, of which sagittal osteotomy line was below the tibial tuberosity. Krause et al.2 conducted a study to gain insight into geometric changes of the PH and PTS after biplanar ascending OWHTO compared to descending OWHTO in patients with KOA. They found that descending OWHTO has proven useful to control PH and PTS, which is in line with other previous results44,48−50. However, this techniques are technically demanding and could lead to the potential for fracture of tibial tubercle fragment. Park et al.51 found two tibial tuberosity fractures with descending OWHTO in 33 patients, but they noted that this complication is preventable by leaving more than 1 cm thickness on the proximal tuberosity fragment.
There was a significant increase in the clinical score after OWHTO. Overall, OWHTO is an effective treatment for isolated medial compartment KOA in varus knees. Patients reported pain relief, improvement in whole-knee function and satisfaction with the clinical outcome. NO patellofemoral joint symptoms or other uncomfortable symptoms were observed. Although increased PTS and descent of the patella were observed, postoperative knee scores showed improvement compared with preoperative values, indicating changes of PTS and PH had no meaningful effect on clinical result in short term. In addition, there were no significant difference in terms of postoperative knee scores among three groups, suggesting the effect of different correction angles on knee function and pain was limited.
This study was not without limitations. First, the sample was relatively small and the follow-up was relatively short. Large sample size research and long-term follow-up were required to further evaluate the effect of correction angles on knee joint. Second, the slope was measured only with plain radiographs, and this may predispose to measurement error. But, two experienced observers evaluated radiographic parameters twice in a blinded fashion, which could possible to minimize potential bias from this limitation. Third, no single method of measuring PH has been accepted as the gold standard so far. Each method has it’s own particular limitations, which is probably why numerous previous studies have used two or three methods.