As in the hip, the diagnosis of a disorder of the knee depends foremost on the evaluation of true joint space involvement. This evaluation is made most accurately through an anteroposterior (AP) standing view of the knee and a flexed lateral view (see Chapter 1 ). The joint consists of three compartments: the medial tibiofemoral compartment, the lateral tibiofemoral compartment, and the patellofemoral compartment. The various arthropathies are characterized by the manner in which they affect these compartments. They separate into the following categories: those that affect all three compartments, those that preferentially involve a specific compartment, and those that initially do not involve any compartment.
Total compartment involvement
Total compartment narrowing indicates that all three compartments of the knee are involved in a uniform manner. This implies a primary abnormality of the underlying cartilage leading to loss of joint space. This loss may be caused either by aggressive destruction from inflammation or by slow degeneration secondary to deposition of foreign substance into the cartilage. The latter is seen in the late phases of the crystalline arthropathies, ochronosis, and acromegaly. The early phases of these arthropathies are discussed elsewhere in this chapter. However, once there is uniform loss of the cartilage, osteoarthritic changes are seen in the adjacent tibia, femur, and patella. Although it is difficult to distinguish one arthropathy from another in this osteoarthritic phase of disease, the uniform involvement of all compartments will separate these arthropathies from common mechanical osteoarthritis.
The arthropathies that produce total compartment involvement by aggressive destruction are the inflammatory arthropathies. Each has specific radiographic characteristics that distinguish it from the others.
Rheumatoid Arthritis ( Figs. 5-1 and 5-2 )
Rheumatoid arthritis is a bilateral symmetrical disease producing uniform loss of all compartments of the knee and generalized osteoporosis. Despite the fact that the knee is a weight-bearing joint, there is little evidence of bone repair or osteophyte formation in response to the cartilage loss ( Fig. 5-1 ). Erosive changes may be present but are not a prominent part of the radiographic picture. Large synovial cysts may be present. A cyst may become so large that it resembles a bone neoplasm ( Fig. 5-2 ). However, observation of uniform loss of joint space as well as smaller cysts in the adjacent bone should prevent an erroneous diagnosis. Occasionally there will be preferential narrowing of the lateral compartment. However, the relative lack of bone repair to this loss (osteophyte formation and subchondral sclerosis) should direct one away from the diagnosis of a mechanical osteoarthritis and toward the correct diagnosis of rheumatoid arthritis.
Psoriatic Arthritis or Reactive Arthritis ( Fig. 5-3 )
Psoriatic arthritis and reactive arthritis present similar radiographic changes. Both are bilateral but asymmetrical diseases involving one knee more than the other or involving one portion of the knee more than another. Unlike rheumatoid arthritis, bone mineralization is maintained. Also unlike rheumatoid arthritis, there is usually evidence of bone proliferation in the form of bone excrescences at ligamentous and tendinous attachments or a periosteal reaction.
Ankylosing Spondylitis ( Fig. 5-4 )
Knee involvement in ankylosing spondylitis is uncommon. However, when knee involvement is present, ankylosis is the predominant part of the radiographic picture. Early in the disease process, small erosions with adjacent bone sclerosis will be present. However, in a relatively short period of time the joint may ankylose, leaving a normal contour to the ghost joint margins.
Juvenile Idiopathic Arthritis ( Fig. 5-5 )
Most frequently juvenile idiopathic arthritis presents as unilateral disease, but with total compartment involvement of the affected knee. The most prominent feature is overgrowth of the femoral and tibial epiphyses as well as overgrowth of the patella; with the overgrowth of the femoral condyles, the intracondylar notch appears widened. Joint space narrowing may not be prominent in the mono- or pauciarticular form of the disease. Because the cartilage is thicker in the child than in the adult, erosive disease and cyst formation, if present, are late manifestations. It may be difficult to distinguish these radiographic changes from those produced in hemophilia.
Hemophilia ( Figs. 5-6 and 5-7 )
Hemophilia ( Fig. 5-6 ) also produces overgrowth of the epiphyses and patella and widening of the intercondylar notch. There tends to be more cyst formation in the knee of the patient with hemophilia, secondary to intraosseous bleeding, than in the knee of a juvenile with rheumatoid arthritis. It has also been observed that the overgrown patella becomes square in hemophilia ( Fig. 5-7 ) and elongated in the inflammatory arthropathy of childhood. The effusion of the hemophilic arthritis may be relatively dense secondary to chronic hemosiderin deposition in the synovium.
