Osteoarthritis is the most common arthropathy. Although many arthropathies lead to secondary osteoarthritic changes, this chapter deals with primary osteoarthritis and osteoarthritis secondary to alteration of normal mechanics across a weight-bearing joint. The radiographic hallmarks of osteoarthritis are as follow:
- 1.
Normal mineralization
- 2.
Nonuniform loss of joint space
- 3.
Absence of erosions
- 4.
Subchondral new bone formation
- 5.
Osteophyte formation
- 6.
Cysts
- 7.
Subluxations
- 8.
Unilateral or bilateral asymmetrical distribution
- 9.
Distribution in hands, feet, knees, and hips; sparing of shoulders and elbows
Except for the type of joint space loss, all of these features may also be seen in osteoarthritis developing secondary to an underlying cartilage problem. In secondary osteoarthritis the joint space loss is uniform; in primary or mechanical osteoarthritis the joint space loss is nonuniform.
The hand
Primary osteoarthritis in the hand involves the distal interphalangeal (DIP) joints and proximal interphalangeal (PIP) joints with relative sparing of the metacarpophalangeal (MCP) joints ( Fig. 13-1 ). The soft tissue swelling around the DIP joint associated with osteophyte formation is called a Heberden node ( Fig. 13-2 ); that around the PIP joint is called a Bouchard node. There is nonuniform loss of the joint space with subchondral sclerosis and osteophyte development in the area of greatest loss of cartilage. The osteophyte must not be confused with either the new bone formation of psoriasis or the saucerized flared edge of the bone caused by erosion of psoriasis. An osteophyte is an extension of a normal articular surface. In the interphalangeal (IP) joints the osteophyte extends laterally or medially and proximally toward the body ( Fig. 13-3 ). Erosion and ankylosis, manifestations of inflammatory disease, are not present. Cyst formation is relatively rare in the digits.
Primary osteoarthritis of the wrist involves only two joints: the joint between the base of the first metacarpal and the trapezium and the joint between the trapezium and the scaphoid (triscaphe joint) ( Fig. 13-4 ). There may be radial subluxation of the base of the first metacarpal in relationship to the trapezium ( Fig. 13-5 ). Large osteophyte formation is seen between the base of the first and second metacarpals. Eburnation and cyst formation are present. Osteoarthritic changes involving any other joint in the wrist must be considered secondary to another arthropathy, (e.g., calcium pyrophosphate dihydrate crystal deposition disease).
Erosive osteoarthritis is a close relative of primary osteoarthritis and should be discussed at this time. Erosive osteoarthritis is seen primarily in postmenopausal females. It has the same distribution in the hand that primary osteoarthritis has, with involvement of the DIP and PIP joints in the fingers ( Fig. 13-6 ) and the first carpometacarpal joint and the triscaphe joint in the wrist ( Fig. 13-7 ). It is distinguished from osteoarthritis in that it has an inflammatory component superimposed on osteoarthritic changes. Therefore, in addition to osteophyte formation, erosive disease is present and ankylosis can occur ( Fig. 13-8 ). Occasionally confusion exists between erosive osteoarthritis and psoriasis. Psoriasis has no osteophyte formation, and the erosions are marginal. Erosive osteoarthritis has osteophytes, and the erosions are more central in location.
Martel has likened the appearance of the erosive osteoarthritic joint to that of a seagull and the appearance of the psoriatic arthritic joint to that of mouse ears ( Fig. 13-9 ). The only other joints involved with erosive osteoarthritis are the IP joints of the feet. The presence of erosions and osteophytes in any other joint in the body indicates an underlying inflammatory arthropathy with secondary osteoarthritis, not erosive osteoarthritis.
The feet
The most common joint of the foot involved with osteoarthritis is the first metatarsophalangeal (MTP) joint. This is usually seen in association with a hallux valgus or a hallux rigidus deformity of the big toe. There is nonuniform loss of the joint space. Osteophyte formation, subchondral bone repair, and cyst formation are present ( Fig. 13-10 ). In a hallux valgus deformity, the sesamoids appear lateral to their normal relationship with the metatarsal head. There may be thickening of the lateral cortex of the first metatarsal as weight is placed on this area ( Fig. 13-11 ). Osteoarthritic changes may be seen elsewhere in the foot wherever the normal mechanics are changed ( Fig. 13-12 ). For instance, a person with a tarsal coalition may develop osteoarthritis in the tarsal joints that are not congenitally fused.
