Young adult hip & non-arthroplasty: a free FRCS (Tr & Orth) Section 1 question

This is cam-type femoroacetabular impingement. In flexion and internal rotation the aspherical head-neck junction is forced under the anterosuperior acetabular rim, producing an outside-in shear that separates the labrum from the cartilage and peels the acetabular cartilage from the subchondral bone as a carpet delamination; the labrum itself is relatively preserved at first, and the damage may extend deep into the joint. Primary crushing, degeneration and ossification of the labrum with a narrow circumferential strip of cartilage damage is the pincer pattern. Posterior labral and posterior wall damage suggests posterior instability, ligamentum teres rupture follows a twisting injury or dysplasia, and inferomedial head wear is not an impingement lesion. Reference: Beck M et al., J Bone Joint Surg Br 2005; Ganz R et al., Clin Orthop Relat Res 2003.