The main finding of this report is that high FA is prevalent, especially in OA knees with severe valgus deformity. On the other hand, increased FA is associated with flattening of the femoral trochlea.
It is well known that rotational deformities of the distal femur in varus or valgus OA knees should be considered for determining the femoral component rotation when performing TKA [4]. However, only limited data are available about the relationship of rotational deformities of the distal femur to the overall lower-extremity rotational alignment [4, 20]; specifically, quantitative data on the influence of FA during TKA, which are important for interpreting the rotational ability of the distal femur relative to the hip. There is no consensus on the relationship between the mechanical alignment of the lower extremity and the degree of FA in patients with OA knees in the few studies that address this issue [4, 19]. Moon et al. [4] assessed the degree of FA using the transepicondylar axis (tFA) instead of the conventional method of using the posterior condylar axis to eliminate the influence of the posterior condyle [1, 9, 17]. They found no correlation between the coronal alignment and the degree of FA, so they believe that the correlation between mechanical alignment and FA can be mainly attributed to the relationship between the posterior condylar geometry and the coronal alignment. However, Yoon et al. [19] reported that the tFA angle decreased as the degree of varus deformity increased. It should be noted that Moon et al. only generally divided coronal alignment deformities into varus and valgus groups, whereas Yoon et al. further divided the varus group into three subgroups, and almost one-third of the included OA patients had knees with excessive varus deformity (> 10°) in their study. Therefore, it is reasonable to speculate that the change in FA with mechanical alignment may be more evident in knees with excessive varus or valgus deformities. Accordingly, in the current study, we further divided the coronal deformities into five groups, including an excessive valgus group, which was not reported in the aforementioned studies. Our results verified our speculation that tFA was significantly greater in the excessive valgus group than in the other groups. In addition, over half (58.8%) of the OA patients in the excessive valgus group had excessive FA (≥ 20°) [6, 7], and this ratio was significantly greater than for other groups (around one-fourth to one-third). These findings demonstrate that high FA is prevalent, particularly in severe valgus knees. Despite substantial improvements in surgical technologies, the reported incidence of patellofemoral complications still varies from 5 to 12% [11, 23]. Increased FA has been considered a predisposing factor for patellar maltracking, which is one of the major reasons for patellofemoral complications after TKA [2, 5, 13]. Therefore, the prevalence of high FA in OA knees found in the current study may contribute to the high incidence of patellofemoral complications after TKA.
However, only limited knowledge is currently available on the clinical implications of variations in FA for TKA. Mobile-bearing TKA may have potential clinical applications because it can theoretically adjust for rotational malalignment through its self-aligning feature and improve patellar tracking. It has also actually been used for complex patellar maltracking in patients with valgus knee osteoarthritis with permanent patellar dislocation [8]. Additionally, as femoral derotational osteotomy for the treatment of patellar dislocation has been shown to achieve good clinical results [14], it is feasible to implement correction osteotomy at the distal femur for OA knees, as required during TKA operation.
Restoration of the femoral trochlear anatomy influences the functional outcomes of OA knees after TKA [15]. Besides coronal deformity of the lower extremity [24], FA has also been reported to relate to the morphology of the femoral trochlea in previous studies [5, 9, 12]. Liebensteiner et al. [9] investigated the trochlear sulcus and LTS, and their results showed that healthy knees have a shallower trochlear sulcus and a flatter LTS with a higher FA. Reikerhs et al. [12] and Diederichs et al. [5] found a weak or no correlation between FA and the trochlear sulcus or slope in a limited patient group with increased femoral anteversion. Most of these studies focused on healthy knees or knees with femoral trochlear dysplasia, but there has been little study of OA knees in this context. Our results showed that, apart from coronal deformities, increased FA is another important contributor to the flattening of the trochlea, and femoral trochleas with high FA (≥ 20°) were significantly flatter than those with normal FA. This flattening of the trochlea in OA knees with increased FA may be an adaption to the resulting increase in lateral patellofemoral stress. However, during TKA, when a surgeon performs a standard resection of the anterior cortex and uses a prosthesis with a uniform anterior femoral thickness regardless of the variable morphology of the femoral trochlea induced by FA, patellofemoral overstuffing or understuffing may occur after TKA [10, 16]. The present results may provide some useful information for the patellofemoral morphology design of femoral prostheses to accurately replicate femoral trochlea discrepancies according to FA for TKA.
This study had some limitations. First, we included only Asian patients. Racial disparities in FA or in morphological parameters related to the patellofemoral joint may exist, and caution should be used when applying the current results to populations of different ethnicities. Second, the current research lacked clinical results and did not analyze patellofemoral kinematics. Clinical and kinematic analyses of how FA affects the patellofemoral joint after TKA for OA knees should be further investigated to verify the clinical meaning of the current results.