DO GENDER DIFFERENCES EXIST IN THE ROTATIONAL REFERENCE AXIS OF THE DISTAL FEMUR?

Abstract

Over 80% of patients are satisfied following total knee arthroplasty (TKA). Female gender was one of the factors found to be a predictor of poorer satisfaction. The landmarks commonly used to achieve correct rotation of the femoral component are the posterior condylar axis, the transepicondylar axes (TEA) & the anteroposterior axis (Whiteside’s line) of the distal femur. The design features of most conventional jig based TKA instrumentation assumes a constant relationship of 3 degrees external rotation between the posterior condylar axis & the epicondylar axis. However during TKA using computer assisted navigation, we observe that these rotational landmarks do not have a constant relationship & there is wide variation among the arthritic population & between the male & female rotational profile. We hypothesise no consistent relationship between the posterior condylar axis, the TEA & the anteroposterior axis of the distal femur.

125 Computerised Tomography (CT) scans of the knee were performed using a 3D helical CT scanner in subjects who did not have any pre-existing clinical & radiological evidence of knee arthritis. CT slices 3 mm in thickness were obtained over the distal femur from the level of the proximal pole of the patella. Standard protocols were established for identifying the bony landmarks & taking measurements. The posterior condylar axis, the TEA & the anteroposterior axis were constructed. The condylar twist angle (CTA), the posterior condy-lar angle (PCA) & the angles made by the TEA & the line perpendicular to the anteroposterior axis were then measured using the PACSWEB digital measurement tools. The data was analysed to determine the consistency of the angular relationship between the reference axes using the STATA data analysis & statistical software. Linear regression was used to investigate any differences in the angle measurements between genders.

125 CT scans of the knee were performed in 111 patients (60 males [65 knees] & 51 females [60 knees]). The mean age was 45 years (SD, 15 years). The results showed no significant difference between the rotational axes of the distal femur between men & women (CTA male(SD): female(SD): 5.9(1.6): 6.3(2.0) [p=0.317], PCA male(SD): female(SD): 2.3(1.5): 2.5(1.9) [p=0.648]). The results also showed it would be inappropriate to assume a constant relationship of 3 degress external rotation between the posterior condylar axis & the epicondylar axes (PCA mean (SD) 2.39(1.70) [p< 0.001], CTA mean (SD) 6.11(1.81) [p< 0.001]).

Our study suggests no significant difference between the rotational reference axes of the distal femur between men & women. Furthermore, most jig-based systems result in 3 degress external rotation of the femoral component. Our results show this is not consistent & may be responsible for the pain in 20% of patients post TKA because of abnormal patellar tracking.