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 ).
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.
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.
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.
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.
Ankylosing Spondylitis ( Figs. 4-7 and 4-8 )
Ankylosing spondylitis causes bilateral symmetrical axial migration of both hips, without producing the severe acetabuli protrusio seen in patients with rheumatoid arthritis. At first mineralization is maintained, and a cuff of osteophytes is seen at the junction of the femoral head and neck along with osteophytes at the superior and inferior borders of the acetabulum ( Fig. 4-7 ). Unlike rheumatoid arthritis, ankylosing spondylitis is an ossifying disease. Erosive or cystic changes may not play a significant role in the changes in the hip. Instead the hip tends to progress to true bone ankylosis. The ankylosed femoral head tends to be almost normal in contour. Once ankylosis takes place, the surrounding bone structures become osteoporotic ( Fig. 4-8 ).
Calcium Pyrophosphate Dihydrate Crystal Deposition Disease ( Figs. 4-9 and 4-10 )
In calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, both hips are involved either symmetrically or, more commonly, asymmetrically. Before axial migration occurs, one may observe calcification of the articular cartilage ( Fig. 4-9 ). The axial migration rarely progresses to the extensive acetabuli protrusio seen in patients with rheumatoid arthritis. Unlike the inflammatory arthropathies, there is degeneration rather than active destruction of the cartilage. Therefore, the process is more indolent, and secondary osteoarthritic changes develop in the surrounding bones. Subchondral sclerosis, osteophyte formation, and cystic changes are seen ( Fig. 4-10 ). The osteophytes formed tend to be smaller than those formed in osteoarthritis. Because there is no incongruity between the femoral head and the acetabulum, the large medial osteophyte seen in mechanical osteoarthritis is not seen in CPPD arthropathy. Cyst formation is more prevalent in CPPD arthropathy than in mechanical osteoarthritis. Severe CPPD arthropathy of the hip may resemble the changes of a neuropathic hip with no semblance of a joint space, massive bone repair, excessive osteophytosis, and bone debris.
Septic Arthritis ( Figs. 4-11 and 4-12 )
Although the literature describes initial widening of a joint space with septic arthritis, usually we first see uniform narrowing of the joint space. The adjacent bony structures are osteoporotic. The diagnosis is clear when absence of the white cortical line along an extensive portion of the femoral head is observed ( Fig. 4-11 ). Normally, as the entire white cortical line is lost and the underlying bone is destroyed, secondary reparative bone will be laid down behind the destruction. However, with an aggressive septic arthritis and resultant osteomyelitis, the entire femoral head and acetabulum can be destroyed without any evidence of repair ( Fig. 4-12 ).
