Does medial-to-lateral femoral posterior condylar offset difference effect accuracy of established reference axes for determining femoral component rotation in total knee arthroplasty?

AIMS AND OBJECTIVES: The femoral posterior condylar offset (PCO) has been viewed with increased significance for knee joint movement patterns and has been discussed for its possible implication for femoral component rotation in total knee arthroplasty (TKA). However, a great inter-individual variability in medial and lateral PCO size has also been demonstrated. Though the medial and lateral PCO seem closely related to the functional flexion axis (fFA), determined by the radius curvature of the medial and lateral femoral condyle, the relationship of both parameters considering their impact on the accuracy of established reference axes for determining femoral component rotation in TKA remains unknown. The objective of this paper was, therefore, to compare the individual fFA with the anatomical and surgical transepicondylar axis (aTEA; sTEA) and with the posterior condylar axis (PCA) considering the medial and lateral PCO size. It was hypothesized that the disparity of the PCO influences the accuracy of the sTEA, aTEA, and PCA for determination of femoral component rotation in TKA. MATERIALS AND METHODS: MRI investigations of 56 consecutive non-arthritic knee joints (male/female 28/28; mean age 22.8 years; range 16-59 years) were used for this study. Coronal, sagittal and transverse MRI images were used to measure the medial and lateral PCO and to determine the fFA, aTEA, sTEA, and PCA for each subject as described previously. A paired two-tailed t-test was used to test for differences between the medial and lateral PCO sizes. Deviation of the aTEA, sTEA and PCA from the fFA were analyzed with a one-sample t-test. Correlation analysis (Pearson r) was used to determine the relationship between the PCO ratio (medial-to-lateral PCO) and the deviation of the aTEA, sTEA and PCA from the fFA in each subject. The level of significance was set at 0.05. RESULTS: The mean medial PCO was 34.0 mm (90%CI 28.72-30.55 mm; range 26.3 to 44.7 mm) and the lateral PCO averaged 29.64 mm (90%CI 30.3-31.4 mm; range 14.3 to 39.1 mm) (p<0.0001). The medial-to-lateral PCO ratio was 1.16 (90%CI 1.13 -1.19; range 0.93 to 1.85). The aTEA showed an increased external rotation in relation to the fFA throughout the whole PCO ratio range (mean deviation 4.2°; 95%CI 3.8°-4.6°; range -4.2° to 10.1°; p<0.0001), whereas the sTEA tends towards a slight but significant internal rotation throughout the PCO ratio range (mean deviation -1.6°; 95%CI -2.1°- -1.2°; range -8.1° to 4.8°; p<0.0001). The PCA showed the best conformity with the fFA (mean difference -0.2°; 95%CI -0.5°-0.2°; range -6° to 5.3°; p=0.36) and was most robust against medial-to-lateral PCO variations. A weak but significant positive correlation between the PCO ratio and the deviation from the fFA was solely found for the sTEA (r=0.28; p=0.042). CONCLUSION: Differences of the medial and lateral PCO size are a common finding. The PCA had the best match with the fFA, regardless of medial-to-lateral PCO disparity. Only the sTEA was influenced to a small extent by variation of the PCO-ratio.