Approach to the Hip - Clinical Tree

The assessment of hip pain depends on a systematic review of a quality anteroposterior (AP) view of the pelvis. The femurs should be internally rotated approximately 15 degrees, the pelvis should not be oblique, and the sacrococcygeal junction should not project more than 4 cm superior to the superior pubic ramus. The following should be systematically assessed on the AP view of the pelvis: 1) hip joint space, 2) shape of the femoral head and femoral head neck junction, 3) geometry of the acetabulum, 4) evaluation of the sacroiliac joints and pubic symphysis, and 5) evaluation of the pubic rami, sacrum, and iliac wings for insufficiency fracture.

Joint space

The diagnosis of hip disease depends foremost on evaluation of the actual joint space. In some disorders the joint space is initially unaffected or even widened. Eventually, the femoral head migrates in one of three directions within the acetabulum, producing a specific pattern of joint space narrowing. The joint space narrows in a superolateral direction, a medial direction, or an axial (superomedial) direction ( Fig. 4-1 ).

Figure 4-1, AP view of a normal hip. Arrows show direction of femoral head migration with cartilage loss: superolateral (SL), axial (A), or medial (M).

Superolateral Migration

Superolateral migration of the femoral head within the acetabulum indicates a nonuniform loss of cartilage. The cartilage loss is confined to the upper outer portion of the articulation. This is usually secondary to change in the normal mechanical stress across the hip joint and is characteristic of osteoarthritis ( Fig. 4-2 ). With this cartilage loss, subchondral bone, or reparative bone, as well as small osteophytes, are formed on the lateral aspect of the femoral head and acetabulum. Weight bearing is then shifted from the center of the femoral neck to the medial cortex of the femoral neck. As a result, new bone is laid down in apposition to the medial cortex. As the disease progresses, a large medial osteophyte forms on the femoral head to fill the lack of congruity between the acetabulum and the femoral head. Cystic changes are also part of osteoarthritis.

Figure 4-2, AP view of the hip demonstrating changes of osteoarthritis: nonuniform loss of cartilage with superolateral narrowing, osteophyte formation, subchondral sclerosis, and new bone apposition along the medial cortex of the femoral neck ( arrow ).

Medial Migration

Medial migration of the femoral head within the acetabulum can be seen in patients with primary osteoarthritis or in patients who have sustained an acetabular fracture ( Fig. 4-3 ). Regardless, change in stress across the hip joint produces medial migration of the head in the acetabulum. The superolateral portion of the joint must widen, and the superior joint lines lose congruity when the head moves medial in the joint.

$Figure 4-3, AP view of the hip showing fracture through the acetabulum. As a result, there is medial migration of the femoral head within the acetabulum, with osteoarthritic changes.$

Axial Migration

Axial migration, also called superomedial migration, of the femoral head within the acetabulum indicates symmetrical uniform loss of cartilage. When the cartilage is affected uniformly, the earliest narrowing occurs along the axis of weight bearing on the axis of the femoral neck, as illustrated by a line drawn just superior to the fovea ( Fig. 4-4 ). Axial migration is seen in any disease that destroys the cartilage in a uniform fashion. This includes the inflammatory arthropathies, the crystalline arthropathies, and other deposition arthropathies such as ochronosis and acromegaly. Upon observation of axial migration, one must evaluate the specific bone changes around the joint, such as mineralization, calcification, erosions, subchondral sclerosis, osteophyte formation, and cyst formation. The common arthropathies that produce axial migration are described here, with emphasis on their differential changes.

Figure 4-4, A, AP view of a normal hip. B, AP view of the same hip 1 year later demonstrating axial or superomedial migration. The narrowest portion of the hip joint is along a line just superior to the fovea.

Rheumatoid Arthritis ( Figs. 4-5 and 4-6 )

Rheumatoid arthritis is a bilateral symmetrical disease progressing from axial migration ( Fig. 4-5 ) to acetabuli protrusio ( Fig. 4-6 ). The bony structures are osteoporotic. There is little if any subchondral bone sclerosis. There is no osteophyte formation and no bone apposition along the inner aspect of the femoral neck. Erosions, when present, are relatively small. Synovial cysts may or may not be present. The hallmark is bilateral symmetrical axial migration with osteoporosis and lack of any evidence of bone repair.