Propensity for hip dislocation in gait loading versus sit-to-stand maneuvers: implications for redefining the dome of the acetabulum needed for stability of the hip during activities of daily living.

Abstract

Current recommendations relating to the treatment of acetabular fractures are based on studies that evaluate the loading patterns associated with normal gait despite the fact that the forces on the acetabulum are significantly greater during sit-to-stand activities. We hypothesize that this increased force will lead to greater instability when an acetabular fracture occurs, and our goal was to compare cadaveric hip stability during single-leg-stance (SLS) and sit-to-stand (STS) maneuvers using a transverse acetabular fracture model. Seven fresh-frozen cadaveric hemipelvic specimens with proximal femurs were dissected of all soft tissues. Transverse acetabular osteotomies were created in 5-mm increments from distal to proximal. The roof arc angle and decrease of articular surface area were measured after each osteotomy, and the specimens were tested in SLS and STS. A 1200-N load was applied and visible dislocation was recorded for each loading orientation. The average roof arc angle needed to dislocate in the SLS position was 46.1° in the anteroposterior, 71.9° in the iliac oblique, and 25.2° in the obturator oblique views compared with 90.9° in anteroposterior, 101.4° in iliac oblique, and 67.3° in obturator oblique views for the STS orientation (P < 0.003 for all radiographic views). The decrease in articular surface area needed to dislocate the hip was significantly less for the STS group (10.9%) than the SLS group (36.4%) (P = 0.003). There is significantly greater hip instability seen with STS loading of a transverse acetabular model than with simple SLS loading. This would suggest that some fractures previously deemed stable may show significant instability during common activities of daily living, and reassessment of nonoperative treatment may be indicated.