There are four cardinal radiographic features of degenerative joint disease in the hip:

These hallmarks of degenerative joint disease can be readily demonstrated on the standard projections of the hip (Fig. 13.2). In the past, tomography was used to demonstrate the details of the degenerative process; however, its application was not to make a specific diagnosis but rather to confirm or exclude possible complications. Computed tomography (CT) scanning may further delineate the characteristic features of osteoarthritis (Fig. 13.3).

As articular cartilage is destroyed and reparative changes develop, evidence emerges of a change in the relation of the femoral head with respect to the acetabulum, known as migration. Generally, three patterns of femoral head migration can be observed: superior, which may be either
superolateral or superomedial; medial; and axial (Fig. 13.4). The most common pattern is superolateral migration; the medial pattern is less common, whereas axial migration is only exceptionally seen. It should be kept in mind, however, that in inflammatory arthritis of the hip, such as rheumatoid arthritis, in which a previous axial migration of the femoral head is commonly associated with acetabular protrusio, degenerative changes might develop as a complication of the inflammatory process. Thus, one may see secondary osteoarthritis with axial migration (Fig. 13.5).

Occasionally, the degenerative process in the hip may run a more rapid course. This destructive arthrosis of the hip joint is known as Postel coxarthropathy, a condition characterized by rapid chondrolysis that may quickly lead to complete destruction of the hip joint. Originally described by Lequesne, and also by Postel and Kerboull in 1970, this unique hip disorder occurs predominantly in women, with age of onset at 60 to 70 years. In all cases, a rapid clinical course of hip pain is the consistent common symptom. The histologic findings are those of conventional osteoarthritis with severe degenerative changes in the articular cartilage. However, osteophyte formation is absent or minimal. Hypervascularity in the subchondral bone is a common finding. The bone trabeculae are either abnormally thickened or abnormally thinned. Occasionally, one can observe foci of fibrosis, interstitial edema and hemorrhage in the marrow spaces, focal marrow fat fibrosis, and focal areas of bone resorption. The precise pathogenesis of this condition remains unclear, although direct drug toxicity and the analgesic effects of nonsteroidal antiinflammatory drugs have been implicated. Some investigators have suggested that intraarticular deposition of hydroxyapatite crystals might lead to joint destruction. Others have proposed subchondral insufficiency fracture of the femoral head as a cause of this arthritis. Because of the rapidity of the process, the radiographic presentation of this condition is marked by very little, if any, reparative changes, mimicking infectious or neuropathic arthritis (Charcot joint) (Fig. 13.6). More recently, Boutry and colleagues reported magnetic resonance imaging (MRI) findings of this form of osteoarthritis. These included joint effusion, a bone marrow edema-like pattern in the femoral head, neck, and acetabulum; femoral head flattening; and cyst-like subchondral defects (Fig. 13.7).

Secondary osteoarthritis is often seen in the hip joint in patients with predisposing conditions such as previous trauma (Fig. 13.8), femoroacetabular impingement (FAI) syndrome (see following section), slipped capital femoral epiphysis, congenital hip dislocation, Perthes disease, osteonecrosis, Paget disease, and inflammatory arthritides.



The radiographic findings are the same as those described for primary osteoarthritis, but the features of the underlying process also can often be detected. Although the standard radiographic views are usually sufficient for demonstrating these changes, CT, arthrography, or MRI may at times be needed for a more accurate assessment of the status of the articular cartilage.

FAI results from incongruity of the femoral head and acetabulum and is one of the leading causes of precocious osteoarthritis of the hip joint. Two types of FAI have been described based on the predominance of anatomic abnormalities affecting either femoral head or acetabulum. In cam type, the nonspherical shape of the femoral head secondary to excessive bone formation at the junction of head and neck results in abutment against the acetabular rim. In pincer type, because of deep acetabulum (coxa profunda), acetabular protrusio, or acetabular retroversion, acetabular “overcoverage” of the femoral head limits the range of motion in the hip joint and leads to abnormal stresses on acetabular rim. In both types of FAI, the abnormal mechanism results in damage of the acetabular labrum, thus promoting secondary osteoarthritis. The diagnosis of FAI is based on (a) the patient’s clinical history of chronic pain, (b) physical examination revealing reduced range of motion in the hip joint, particularly flexion and internal rotation, and (c) imaging findings on conventional radiography, CT, and MRI. In cam type, conventional radiography demonstrates excessive bone formation at the femoral head/neck junction with loss of normal anatomic “waist” at this site (Fig. 13.9A), occasionally resembling the smooth hand grip of some pistols (“pistol grip deformity,” or a “cam effect”) (Fig. 13.9B); an os acetabulum, which more likely represents an osseous metaplasia of the cartilaginous labrum or a fragment of damaged acetabular rim; and a radiolucent lesion at the head/neck junction, formerly called synovial herniation pit, and now designated as fibroosseous lesion. CT shows these abnormalities even better (Fig. 13.10). MR arthrography (MRa), particularly the radial reformatted images, in addition to the findings listed previously, clearly demonstrates abnormalities of the fibrocartilaginous labrum at the anterosuperior portion of the acetabulum (Fig. 13.11; see also Fig. 2.43). In pincer type, particularly in case of acetabular retroversion, conventional radiograph shows “crossover” sign, when more lateral projection of anterior acetabulum, which normally should project medially to the posterior acetabulum, “crosses” the posterior acetabular outline (Fig. 13.12). MRI demonstrates acetabular version and depth of the femoral head coverage (Fig. 13.13). To determine the sphericity of the femoral head and the prominence of the anterior femoral head/neck junction, the alpha angle is calculated

on the oblique axial CT or oblique axial MR images (Fig. 13.14). Radial reformatted MR images are of particular value in this respect because they allow optimal visualization of the anterosuperior region of the femoral head/neck junction, where the most significant changes in the alpha angle occur (see Fig. 13.14B). The normal alpha angle should not exceed 50 degrees. The larger the alpha angle, the more pronounced is nonspherical shape of the femoral head, and the greater is predisposition for anterior FAI.

The knee is a complex joint comprising three major compartments—the medial femorotibial, the lateral femorotibial, and the femoropatellar—and

each of which may be affected by degenerative changes. The radiographic features of these changes are similar to those seen in osteoarthritis of the hip, including narrowing of the joint space (usually one or two compartments), subchondral sclerosis, osteophytosis, and subchondral cyst (or pseudocyst) formation. The standard anteroposterior and lateral projections of the knee are sufficient to demonstrate these processes (Fig. 13.15). If the medial joint compartment is affected, the knee may assume a varus configuration, which is best demonstrated on the weight-bearing anteroposterior view (Fig. 13.16A); involvement of the lateral compartment may lead to a valgus configuration (Fig. 13.16B). CT and three-dimensional (3D) reconstructed CT images may provide additional information as to the status of osteoarthritic process (Fig. 13.17). A frequent complication of osteoarthritis of the knee is the formation of osteochondral bodies, which can be demonstrated on the standard projections of the knee (Figs. 13.18 and 13.19); however, MRI may also be effective in this respect (Figs. 13.20, 13.21, 13.22). The femoropatellar joint compartment is also commonly involved in primary osteoarthritis. The lateral radiograph of the knee and axial view of the patella are the most effective means of visualizing degenerative changes of the femoropatellar compartment (Fig. 13.23).