Osteoarthritis

degenerative joint disease affecting synovial joints. Although the term osteoarthrosis may be more appropriate because in this condition inflammatory changes are not pronounced, the term osteoarthritis is more widely accepted. In osteoarthritis, cartilage and bone are the primary sites of abnormality, with only mild alterations occurring in the synovial membrane. Typically radiographs reveal loss of joint space, eburnation, cyst formation and osteophytosis. Complications of this disorder include subluxation, joint malalignment, fibrous ankylosis and intra-articular osseous and cartilaginous bodies. Two special varieties of osteoarthritis (generalized osteoarthritis and inflammatory [erosive] osteoarthritis) have also been described but may not be discrete entities. Joint degeneration may result either from an abnormal concentration of force across a joint with normal articular cartilage matrix or from a normal concentration of force across an abnormal joint.

Among the causes of osteoarthritis are inherited factors, obesity, inflammation, trauma, mechanical factors, nutritional and metabolic status, osteoporosis and preexisting joint disease or deformity. In one way or another, these factors create a situation in which the joints can no longer resist the physical forces.

The pathologic changes in osteoarthritis are reflected on radiographs (Table 1).

Osteoarthritis, Table 1. Radiographic pathologic correlation in osteoarthritis.

Pathologic abnormalities Radiographic abnormalities
Cartilaginous fibrillation and erosion Localized loss of joint space
Increased cellularity and hypervascularity of subchondral bone Bone eburnation
Synovial fluid intrusion or bone contusion Subchondral cysts
Revascularization of remaining cartilage and capsular traction Osteophytes
Periosteal and synovial membrane stimulation Osteophytes and buttresing
Compression of weakened and deformed trabeculae Bone collapse
Fragmentation of osteochondral surface Intraarticular osseous bodies
Disruption and distortion of capsular and ligamentous structures Deformity and malalignment

Cartilage abnormalities include loss of cartilage, fibrillation and erosion, leading to loss of joint space. The changes in subchondral bone are sequential, characterized by a destructive phase with bone eburnation, cyst formation, flattening and deformity, and a productive phase in which osteophytes are formed. The cysts in osteoarthritis, however, must be differentiated from similar subchondral lucent lesions that may accompany other disorders. Osteophytosis is marked by presence of marginal (peripheral), central (interior joint), periosteal, synovial and capsular osteophytes. Buttressing may occur in some locations, especially the medial aspect of the femoral neck.

Various abnormal changes may complicate degenerative joint disease; the most important are joint malalignment, subluxation, fibrous and bony ankylosis, and intra-articular cartilaginous and osseous bodies (joint mice).