Natural distribution of the femoral mechanical–anatomical angle in an osteoarthritic population and its relevance to total knee arthroplasty

A common surgical goal in TKA is to restore neutral alignment of the lower limb by making bone cuts perpendicular to the mechanical axes of the femur and tibia. Standard practice for many surgeons is to use the same distal femoral valgus resection angle for all patients, assuming little or no variation in the femoral mechanical–anatomical (FMA) angle between different patients’ knees. This study analysed 174 pre-operative hip–knee–ankle radiographs of osteoarthritic knees (157 patients, 87 female and 70 male, mean age 70 years and mean BMI 31.8). Measurements of mechanical femorotibial (MFT) and FMA angles were made. The mean FMA angle was 5.7° (SD 1.2°, range 2° to 9°). There was a statistically significant difference between the FMA angle for males and females with males tending to have larger FMA angles (p < 0.001). There was a statistically significant correlation between MFT and FMA angle (r = − 0.499) with varus knees tending to have larger FMA angles (p < 0.001). These results indicate a wide distribution of FMA angle in an osteoarthritic population. In terms of achieving appropriate coronal alignment in TKA the use of a fixed valgus resection angle is not suitable for all patients and it may be preferable to adjust the distal femoral cut according to individual FMA angles. However if this angle is not available the cut may be adjusted according to pre-operative coronal alignment, using 6° for neutral/mild varus, > 6° for more severe varus and < 6° for valgus knees.