Relationship between femoroacetabular contact areas and hip position in the normal joint: an in vitro evaluation

PURPOSE: Different approaches have been proposed to diagnose femoroacetabular impingement (FAI) condition and hip instability. It is still debatable which test is the most effective to make a correct diagnosis. The true mechanics of the hip during particular physical examination manoeuvres is unknown. METHODS: Eight fresh frozen hips were passively taken through 3 different commonly used positions for FAI diagnosis and hip instability: 90° Flexion-Adduction-Internal Rotation, Hyperextension-Adduction-External Rotation and Hyperextension-Neutral-External Rotation. Kinematics and anatomical data were acquired by an optoelectronic system. The contact areas between acetabulum and femoral head were analysed to determine whether these tests are able to localize regions of the hip that may give patients pain. RESULTS: In the hip positions where the femur was in Hyperextension-External Rotation, the contact area was mainly concentrated in the posterosuperior area of the acetabulum, while during 90° Flexion-Adduction-Internal Rotation position, there was a wider distribution of contact, not specific to the anterolateral acetabulum. CONCLUSIONS: The results confirm the ability of the Hyperextension-External Rotation tests to particularly analyse the posterior region of the acetabulum. Placing the hip in 90° of Flexion-Adduction-Internal Rotation allows for testing a wider zone of the acetabulum and is not specific to abutment of the femoral head-neck region against the anterolateral acetabulum.