Femoral condyle dimensions: comparing natural anatomy to knee replacement hardware

Orthopedics companies have recently pursued sex‐specific total knee replacement (TKR) designs that account for sexual dimorphism to improve post‐operative outcomes for females. Prior research on knee anatomy has shown no significant differences in morphology between the sexes when controlling for size. For knee replacements to be effective they must functionally mimic natural skeletal anatomy, yet the variance in TKR appliances in relation to skeletal variation has not been thoroughly studied. The question remains if scaling knee replacement appliances to the individual sufficiently reflects the functional variation of knee morphology, or if additional variation in shape must be taken into account. This research analyzes 14 bilateral landmarks on the distal femur and orthopedic hardware for 17 individuals who received unilateral TKRs and 36 control individuals with no TKRs. Landmarks reflect the morphology of the distal articular surfaces, including dimensions of the condyles and their relative spacing. Ten interlandmark distances (ILDs) were extracted for analyses. We performed principal components analysis on the pooled sample. PC1 (63% of variance) describes size differences; all ILDs positively relate to it and it separates females from males. PC2 (14% of variance) indicates that measurements reflecting condylar morphology are inversely related to intercondylar measurements (overall condylar breadth, intercondylar width [ICW], and proximal and distal intercondylar distances [PID, DID]). Of these traits, ICW, which has the weakest relationship with body size, exhibits the strongest association with PC2. Further exploration of intercondylar measures reveals that PID and DID dimensions are significantly different (p<0.05) in males. An ANCOVA reveals that these two dimensions have different scaling relationships, which indicates the medial condyle deviates from a proximodistal orientation (increases its angle) with increased body size. PID and DID are comparable distances in the TKR hardware (p = 0.327) and pooled females (p = 0.159). Overall, no sexual dimorphism is present in the dimensions of the distal femur when accounting for size, and bilateral asymmetry in individuals with unilateral TKR is not outside the range of normal human variation. The only exception is that males show medial condyle asymmetry between their natural femur and replacement hardware. Further, the comparably‐sized PID and DID dimensions in femoral implants mimic the pattern observed in females, while males show significant differences in these dimensions. This analysis indicates that traditional unisex hardware dimensions may be more similar to female distal femur anatomy, and is likely not contributing to reduced post‐operative outcomes for females.