The etiology of osteoarthritis of the hip: an integrated mechanical concept

Abstract

The etiology of osteoarthritis of the hip has long been considered secondary (eg, to congenital or developmental deformities) or primary (presuming some underlying abnormality of articular cartilage). Recent information supports a hypothesis that so-called primary osteoarthritis is also secondary to subtle developmental abnormalities and the mechanism in these cases is femoroacetabular impingement rather than excessive contact stress. The most frequent location for femoroacetabular impingement is the anterosuperior rim area and the most critical motion is internal rotation of the hip in 90 degrees flexion. Two types of femoroacetabular impingement have been identified. Cam-type femoroacetabular impingement, more prevalent in young male patients, is caused by an offset pathomorphology between head and neck and produces an outside-in delamination of the acetabulum. Pincer-type femoroacetabular impingement, more prevalent in middle-aged women, is produced by a more linear impact between a local (retroversion of the acetabulum) or general overcoverage (coxa profunda/protrusio) of the acetabulum. The damage pattern is more restricted to the rim and the process of joint degeneration is slower. Most hips, however, show a mixed femoroacetabular impingement pattern with cam predominance. Surgical attempts to restore normal anatomy to avoid femoroacetabular impingement should be performed in the early stage before major cartilage damage is present.

Level of evidence: Level V, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Figures

Fig. 1A–D

AP radiographs of (A) a normal hip and three forms of abnormalities associated with mild slipped capital femoral epiphyses called (B) “flattening,” (C) the “bump,” and (D) the “hook.” Reproduced with permission from Harris WH. Etiology of osteoarthritis of the hip. Clin Orthop Relat Res. 1986;213:22.

Fig. 2

Comparison of the mild deformity of the proximal femur secondary to a mild, unrecognized slipped capital femoral epiphysis and the contour of the grip of a pistol. Because of the similarity, these types of proximal femoral deformities are called “pistol grip deformities.” Reproduced with permission and copyright © 1975 by Elsevier from Stulberg SD. Unrecognized childhood hip disease: a major cause of idiopathic osteoarthritis of the hip. In: Cordell LD, Harris WH, Ramsey PL, MacEwen GD, eds. The Hip: Proceedings of the Third Open Scientific Meeting of the Hip Society. St Louis, MO: CV Mosby; 1975:212–228.

Fig. 3A–C

(A) FAI is shown in a 34-year-old man with an apparently normal AP radiograph. (B) The nonspherical femoral head leading to reduced offset at the neck and predisposition to cam FAI is visible on the lateral radiograph. (C) The MRI scan confirmed the labral tear and chondral injury resulting from FAI. Reproduced with permission from Ganz R, Parvizi J, Beck M, Leunig M, Nötzli H, Siebenrock KA. Femoroacetabular impingement. Clin Orthop Relat Res. 2003;417:115.

Fig. 4A–D

Diagrams illustrate the proposed mechanisms of joint damage in FAI. (A) In pincer FAI, linear impact due to the acetabular overcoverage occurs anteriorly. The persistent anterior abutment with chronic leverage of the head in the acetabulum sometimes results in chondral injury in the “contrecoup” region of the posterior-inferior acetabulum. (B) In cam FAI, the prominent femoral head/neck junction (C) is jammed into the acetabulum causing damage to the peripheral cartilage at flexion and internal rotation (D). Reproduced with permission from Lavigne M, Parvizi J, Beck M, Siebenrock KA, Ganz R, Leunig M. Anterior femoroacetabular impingement. Part I. Techniques of joint preserving surgery. Clin Orthop Relat Res. 2004;418:71.

Fig. 5A–B

Intraoperative photographs show acetabular cartilage damage during the treatment of young adult patients suffering from FAI. (A) In pincer FAI, the linear impact leads to substantial labral damage, while in early phases the adjacent cartilage remains intact. (B) In contrast, cam FAI frequently reveals deep-reaching flaplike cleavage lesions of cartilage from the subchondral bone early in the disease.