Tensile strain in the anterior part of the acetabular labrum during provocative maneuvering of the normal hip

Background: Injury of the acetabular labrum is a well recognized cause of hip pain in the young, active patient. The exact mechanism of these injuries remains a subject of speculation, although femoroacetabular impingement and twisting maneuvers have both been proposed as critical factors. We examined the hypothesis that torsional maneuvers of the morphologically normal hip joint generate mechanical strain within the acetabular labrum, particularly in areas that are prone to injury. Methods: Seven human cadaver specimens were loaded during five separate maneuvers with external rotation or abduction torques applied to the hip in neutral alignment and in moderate flexion or extension. Tensile strain within the acetabular labrum was measured with use of the technique of roentgen stereophotogrammetric analysis. Results: Substantial tensile strains were generated within the labrum during each of the loading maneuvers, with no significant difference in strain being noted between the maneuvers. Maximum strain in the anterior part of the labrum averaged 13.6% ± 7.8% in the axial direction and 8.4% ± 3.0% in the circumferential direction. The highest mean and maximum strain values were found at the two o'clock position of the labrum, with the highest strain concentration at the bone-labrum interface. Conclusions: External rotation and abduction maneuvers of the morphologically normal human hip joint in moderate flexion or extension can generate substantial tensile strains in the anterior part of the acetabular labrum. This finding supports the hypothesis that injury to the anterior part of the labrum may occur from recurrent twisting or pivoting maneuvers of the hip joint in moderate flexion or extension without femoroacetabular impingement. Clinical Relevance: The substantial amounts of tensile strain generated during loading of the morphologically normal hip are indicative of a mechanical process that may accelerate the onset of degenerative disease.