EFFECT OF FOOT-PROGRESSION ANGLE ON HIP-JOINT MOMENTS DURING GAIT

Loosening has emerged as the most serious long-term complication of to tal hip replacement and torsional loading of the femoral implant has b een implicated as a possible cause. In an effort to explore a strategy for minimizing this risk, the following hypothesis was tested: the fo ot-progression angle has a significant effect on the resultant hip mom ents, and particularly the internal-external rotation moment, during l evel walking. Twelve normal subjects performed a total of nine trials: three in which they walked normally with the right foot pointing appr oximately straight ahead; three in which they were told to walk with t he foot internally rotated approximately 30 degrees; and three in whic h the foot was externally rotated about 30 degrees. The inverse dynami cs approach was used to integrate the body segment parameter, kinemati c and force plate data, and to solve For the resultant moment at the r ight hip joint. In all three conditions-foot straight, foot in, and fo ot out-the subjects walked at the same average speed of 1.5 (+/-0.3)m s(-1). For the flexion-extension moment (where the maximum flexion mom ent of 95 Nm was in good agreement with other published data), there w as no significant difference between the three foot orientation condit ions. For the abduction-adduction moment, the foot straight condition exhibited the classic double peak pattern with a maximum abductor mome nt of 57 Nm, and there was no statistically significant difference bet ween the three curves. Although the three curves for the internal-exte rnal rotation moment were similar in shape, the foot out condition res ulted in a statistically significant (p < 0.05) reduction, from 10 to 5 Nm, in the external rotation moment at 40% of the gait cycle. The hy pothesis was therefore shown to hold true for the internal-external ro tation moment, but it is not certain whether this reduction of about 5 Nm has any clinical significance. The findings do suggest that foot p rogression and other strategies to reduce torsional loading on the Fem oral implant could be explored with patients who have undergone total hip arthroplasty.