Diminished abductor muscular strength in patients with valgus-impacted femoral neck fractures treated by internal fixation: Clinical study and biomechanical considerations

Valgus-impacted femoral neck fractures treated with internal fixation occasionally result in unsatisfactory postoperative locomotive function, partially due to muscle shortening and a decrease in the moment arm. This study quantifies the degree of diminished abduction strength both clinically and biomechanically.Fifteen patients were enrolled in this study. Twelve patients with fracture healed in valgus-impacted position were further evaluated. Muscular strength around hip was examined, and values between the nonoperated and operated side were compared and analyzed. For the biomechanical study, two three-dimensional models were prepared: model I (control model without displacement) and model II (simulated malunion of a 15° valgus-impacted fracture). Two sets of hip flexion angles in each of the models were simulated with flexion angles of 0° and 23°.Mean and standard deviation values for muscle strength from the nonoperative/operative side among the valgus group are as follows: flexion strength was 9.2 ± 4.0/9.2 ± 3.2, extension strength was 5.8 ± 2.8/6.1 ± 3.2, abduction strength at 0° was 9.1 ± 3.7/7.4 ± 3.6, abduction strength at 10° was 6.7 ± 3.0/5.5 ± 2.2, and knee extension strength was 15.3 ± 6.2/15.1 ± 6.0 (kgf). When comparing values between the nonoperative and operative sides, statistical significance was only observed in abduction strength ( p < 0.01). The biomechanical models prove that valgus impaction decreases the moment arm by approximately 10% at both flexion angle.A significant decrease in abductor strength at 0° and 10° was observed in the valgus-healed group. This may be related to a decrease in the moment arm. Further research should be done to define the acceptable limit of deformity for the satisfactory postoperative functioning.