Abstract
Introduction
Proper femoral component rotation is a crucial factor in successful total knee arthroplasty (TKA). Femoral component rotation using anatomic landmarks has traditionally been established by referencing the transepicondylar axis (TEA), Whiteside's Line (WSL), or the posterior condylar axis (PCA). TEA is thought to best approximate the flexion-axis of the knee, however WSL or PCA are commonly used as surrogates of the TEA in the operating room due to their accessibility. The relationship of these anatomic landmarks has been previously investigated in anatomic and computed tomography based studies. The relatively few knees evaluated have limited the power of these studies. Patient Specific Instrumentation (PSI) utilizing magnetic resonance imaging (MRI) is an emerging technology in total knee replacement. The purpose of this study was to use magnetic resonance imaging based planning software to assess the relationship of WSL and PCA to the TEA and to determine if the relationships were influenced by the magnitude of the pre-operative coronal deformity.
Methods
Five hundred sixty total knee replacements were performed in 510 patients utilizing PSI. The Materialize preoperative planning software was utilized to determine the rotational relationships of TEA, WSL, and PCA (Fig 1). The coronal plane deformity of each patient was also evaluated utilizing the MRI-based imaging and planning software.
Results
The WSL is externally rotated by 90.36 degrees (SD ±2.3 degrees) compared to the TEA and the PCA is 2.38 degrees (SD ±1.6 degrees) internally rotated compared to the TEA in the overall population (p<0.001). The relationship of WSL to TEA has more variability than the relationship of PCA to TEA. In the overall population only 77% of WSL and 74% of PCA based resection will be within 2 degrees of the TEA. The PCA is more internally rotated in females and in valgus knees (P<0.001) however is not affected by the degree of valgus deformity (p = 0.211).
Discussion
Femoral component rotation is determined based on one of three axis options. Classic studies have shown that the TEA is perpendicular to the WSL and the PCA is 3 degrees internally rotated to the TEA. However, there is wide variation in the relationships. Our MRI based evaluation shows that both WSL and PCA approximate the TEA in valgus knees regardless of the degree of deformity. Our study also shows that on average the PCA is 2.38 degrees internally rotated compared to the TEA, not the previously assumed 3 degrees. Our study indicates that the PCA is more internally rotated compared to the TEA in female patients and patients with valgus deformity. Males with varus knees may only require a 2 degree internal rotation correction rather than the historically established 3 degrees. WSL also shows more variability in its relationship to the TEA compared to the PCA. Advanced imaging can assist surgeons in assessing their options for femoral component rotation in TKA. Our data indicates that the relationships of axis options and historical assumptions may need to be reassessed as imaging technology advances.
