The most important finding of this study was that the postoperative KAJA significantly correlated with the KJLO. The contribution of KAJA was stronger than that of previously reported factors. The rate of ≧ 5。 KJLO achieved 68% when the postoperative KAJA exceeded 10。. Therefore, double osteotomy at the distal femur and proximal tibia should be considered if ideal alignment cannot be achieved even when the KAJA reach 10。.
Non-anatomic knee joint obliquity can cause excessive shear force on the articular cartilage. When the obliquity angle is 5。, the shear force in the medial compartment elevate to almost twice as high as the normal knee[10]. The force value becomes even higher as the obliquity angle increases. Although some studies have shown that there was no difference of short-term outcome with high-degree obliquity[8, 23, 24], it is reasonable there would be a long-term adverse effect on the articular cartilage[12]. Lee et al found that the change of KJLO was significantly less than that of anatomical geometry of the proximal tibia. This phenomenon can be explained by the compensation of the ankle joint[8, 18]. However, the capacity of compensation differs among individuals[9, 15, 25], and currently there is no information about the maximum of the capacity for most of the patients undergoing MOWHTO. Understanding when the knee obliquity will be “decompensated” is essential to ensure a satisfactory outcome. In this study, although the postoperative KAJA moderately correlated with the KJLO, the difference of the mean KJLO between the ≦ 5。Group and 5。-10。group was as small as 0.3。, which may be lack of clinical relevance. The mean KJLO became substantially higher when the KAJA was larger than 10。. Therefore, these results suggest that the change of the anatomical geometry of the proximal tibia caused by MOWHTO could not be well compensated by the mobility of the subtalar joint once the KAJA exceeded 10。. When performing MOWHTO, the 10。KAJA as a critical value should be kept in mind.
In accordance with previous studies, higher preoperative mHKA and MPTA were also predictors of higher postoperative KJLO[9, 14]. It was reasonable that higher mHKA might require a larger correction, which was supposed to result in greater obliquity. However, to our surprise, the correction amount evaluated by the change of WBL ratio was not a significant factor. The postoperative mean WBL ratio was 63.3% in this study, which was remarkably close to the classic aim at 62.5%[1]. Therefore, correcting a lower limb from a higher mHKA to the classic surgical goal, adherence the WBL to the Fujisawa point, tended to have greater postoperative obliquity. We speculated that the change of WBL ratio can be affected by multiple factors, such as different anatomical variance of the femur and tibia. Thus, it did not necessarily represent a large angle correction. In contrast, a higher preoperative mHKA could be a more representative factor. Furthermore, a higher preoperative MPTA implied that the proximal tibia attribute less to the varus deformity of the lower extremity. Therefore, MOWHTO on the knees with higher MPTA may result in a less physiologically oriented joint line.
The KAJA of all cases increased after MOWHTO. Although postoperative KAJA contributed the most to the postoperative KJLO, preoperative KAJA did not correlated to it. Similarly, after multiple linear regression, preoperative KJLO did not contribute to it. In other words, only a sufficient KAJA increase really resulting in high postoperative KAJA caused high-degree KJLO regardless of the preoperative value of KJLO and KAJA. These results suggest that the KAJA be assessed carefully during the operation to avoid exceeding the critical 10。 KAJA. Because a full-tibia image cannot be obtained by fluoroscopy, we recommend assessing the intraoperative KAJA by measuring the angles between the tibia articular surfaces and a reference rod at the knee and ankle joints. KJLA can be simply calculated by subtraction of these two angles. The measurements do not add additional tasks because they can be done along with the assessment of the mechanical axis by the “cable method”[26] or “alignment rod” method[22].
The limitations of this study are as follow: first, avoiding ≧ 10。KAJA did not prevent all high-degree KJLO. Further studies are required to identify other possible factors affecting knee joint obliquity. Second, this study only focused on knee joint obliquity. Whether the compensation caused further adverse effect on the ankle joint was not clear. A prospective long-term study may be needed to clarify this issue. Third, since the capacity of the ankle joint compensation differs by individual, the health status of the subtalar joint may play an important role in knee joint obliquity after MOWHTO. There was no such data in this study, and it merits further prospective studies to investigate its effect. Furthermore, because previous studies have analyzed short term clinical outcome, we did not provide such information since the post-operative period was also short.