ARTICULAR SURFACES

The femoral head articulates with the cup-shaped (cotyloid) acetabulum, its centre lying a little below the middle third of the inguinal ligament. (The profile of the anterior margin of the joint is parallel to the middle third of the inguinal ligament.) The articular surfaces are reciprocally curved but neither coextensive nor completely congruent (Fig. 81.2; see Figs 80.4, 80.15). The close-packed position of the hip joint is one of full extension, with slight abduction and medial rotation. As in the shoulder joint, the surfaces are considered ovoid or spheroidal rather than spherical, but this view is controversial. Evidence appears to suggest that the articular surface on the femoral head is a spheroidal, or slightly ovoid, surface in youth but that it tends to become almost spherical with advancing age. The femoral head is covered by articular cartilage, except over the rough pit where the ligamentum teres is attached. In front the cartilage extends laterally over a small area on the adjoining neck. Articular cartilage is, generally, thicker centrally than at the periphery. Cartilage thickness is maximal anterosuperiorly in the acetabulum and anterolaterally on the femoral head, the two areas that correspond to the principal load-bearing areas within the joint The acetabular articular surface, the lunate surface, is an incomplete ring, broadest anterosuperiorly where the pressure of body weight falls in the erect posture, and narrowest in its pubic region. It is deficient inferiorly opposite the acetabular notch. The lunate surface is covered by articular cartilage, which is thickest where the surface is broadest. The acetabular fossa, the central non-articular area in the floor of the acetabulum, is devoid of cartilage but contains fibroelastic fat largely covered by synovial membrane. The acetabular labrum, a fibrocartilaginous rim attached to the acetabular margin, serves to deepen the acetabulum and bridges the acetabular notch by attaching to the peripheral edge of the transverse acetabular ligament. The labrum is triangular in section; it is attached by its base to the acetabular margin and its acute free edge projects beyond the acetabular margin. The diameter of the acetabular cavity is constricted by the labral rim which embraces the femoral head, maintaining joint stability both as a static restraint and by providing proprioceptive information.

FIBROUS CAPSULE

The capsule is strong and dense. It is attached above to the acetabular margin 5–6 mm medial to the labral attachment, in front to the outer labral aspect and, near the acetabular notch, to the transverse acetabular ligament and the adjacent rim of the obturator foramen. From its acetabular attachment it extends laterally to surround the femoral head and neck, and is attached anteriorly to the intertrochanteric line, superiorly to the base of the femoral neck, posteriorly 1 cm superomedial to the intertrochanteric crest, and inferiorly to the femoral neck near the lesser trochanter. Anteriorly many fibres ascend along the neck as longitudinal retinacula, containing blood vessels for both the femoral head and neck. The capsule is thicker anterosuperiorly, where maximal stress occurs, particularly in standing. Posteroinferiorly it is relatively thin and loosely attached. It has two sets of fibres, circular and longitudinal. The circular fibres (zona orbicularis) are internal and form a collar round the femoral neck; although partly blended with the pubofemoral and ischiofemoral ligaments, these fibres are not directly attached to bone. Externally, longitudinal fibres are most numerous in the anterosuperior region, where they are reinforced by the iliofemoral ligament. The capsule is also strengthened inferiorly by the pubofemoral ligament, and posteriorly by the ischiofemoral ligament. Externally it is rough, covered by muscles and tendons and separated anteriorly from psoas major and iliacus by a bursa (see below). The capsular attachment to the femur lies well distal to the growth plate of the femoral head both anteriorly and posteriorly. Thus the upper femoral epiphysis is entirely intracapsular. The capsular attachment intersects the growth plate of the greater trochanter on the upper surface of the base of the neck (see Fig. 80.18).

Relations

The joint capsule is surrounded by muscles (Fig. 81.4). Anteriorly, lateral fibres of pectineus separate it from the femoral vein. Lateral to this the tendon of psoas major, with iliacus lateral to it, descends across it, partly separated from the capsule by a bursa. The femoral artery is anterior to the tendon of psoas major, and the femoral nerve lies deep in a groove between the tendon and iliacus. More laterally the straight head of rectus femoris crosses the joint with a deep layer of the fascial iliotibial tract, which blends with the capsule under the lateral border of the muscle. Superiorly, the reflected head of rectus femoris contacts the capsule medially, while gluteus minimus covers it laterally, being closely adherent. Inferiorly, medial fibres of pectineus adjoin the capsule and, more posteriorly, obturator externus spirals obliquely to its posterior aspect. Posteroinferiorly, the capsule is covered by the tendon of obturator externus, separating it from quadratus femoris and accompanied by an ascending branch of the medial circumflex femoral artery. Above this the tendon of obturator internus and the gemelli contact the joint capsule, separating it from the sciatic nerve. The nerve to quadratus femoris is deep to the obturator internus tendon, and descends medially on the capsule. Above this, the posterior surface of the joint is crossed by piriformis.

Fig. 81.4 Dissection to display the structures surrounding the hip joint. The femoral head has been disarticulated and removed.

LIGAMENTS

The ligaments of the hip joint (Fig. 81.3) are the iliofemoral, pubofemoral, ischiofemoral and transverse acetabular ligaments and the ligamentum teres. As the hip moves so the capsular ligaments, as capsular thickenings, wind and unwind, tightening around the hip, affecting stability, excursion and joint capacity. Joint capacity is maximal when the hip is held in a partially flexed and abducted position: a patient with an effusion in the hip joint is therefore most comfortable when the joint is held in a position of flexion and abduction. See Fuss & Bacher (1991) for details.