No single safe zone exists for the valgus cut angle to reproduce neutral mechanical alignment in the presence of femoral bowing in total knee arthroplasty.

Purpose: The aim of this study is to report the prevalence of femoral bowing in the Egyptian arthritic knees as a representative of the middle eastern population and to study the correlation between the femoral bowing and the degree of arthritis, varus deformity and the distal femoral valgus cut angle (VCA).

Methods: This is a single-center observational cross-sectional study. Out of 562 knees Scheduled for TKA, 124 knees were excluded leaving 438 knees eligible for the study. The following angles were measured by two independent orthopedic surgeons: Femoral bowing angle (FBA), HKA angle, LDFA, MPTA and VCA.

Results: Out of 438 knees, 21knees (4.8%) had medial bowing (< - 3°), 111 (25.3%) had normal bowing (+ 3° to - 3°) and 306 (69.9%) had LFB of which 111 (25.34%) had mild LFB (+ 3 to + 5°) and 195 (44.52%) had severe LFB (> + 5°) bowing. LFB was more in older age group (p = 0.005), in females (p < 0.001), and in grade 4 OA, (p < 0.001). Also, there was a significant positive correlation between FBA and age and increasing varus HKA, and with varus orientation of the distal femur and the tibial plateau. The mean and the 95% confidence interval of the VCA for the medial bowing group was 3.43 (3.01-3.85°), for the normal bowing group was 5.42 (5.15-5.68°), for the mild lateral bowing was 6.74 (6.47-7°), and for the severe bowing group was 9.23 (8.89-9.55°).

Conclusions: There is no single safe zone for the VCA to reproduce postoperative neutral coronal alignment especially in cases of severe lateral femoral bowing in TKA. However, the VCA should be analyzed in term of how much femoral bowing exists. In other words, for each subset of femoral bowing there is a safe zone for the VCA.