The joint destruction and the deformities that occur in rheumatoid arthritis are the result of the interaction between the diseased synovial tissue and the normal joint tissue.
To understand the pathomechanics of the deformities that occur in rheumatoid arthritis, one must understand how the synovium affects the different parts of the musculoskeletal system (i.e., bone, cartilage, ligament, tendons, and muscles).
Rheumatoid synovium causes cartilage to lose its ability to absorb deforming forces. Eventually, this results in mechanical disruption of the cartilage surface.
Rheumatoid synovium destroys bone and creates bone erosions and/or cysts.
Rheumatoid synovium can infiltrate the capsular tissue leading to ligament injury and eventually joint instability.
Rheumatoid synovium can surround and invade flexor and extensor tendons, resulting in disruption of the normal architecture of the tendons and loss of normal tendon function.
Muscles are less commonly affected but can be injured by perivascular inflammation. The intrinsic muscles of the hand are particularly susceptible to this, resulting in intrinsic tightness and secondary finger deformities.
Rheumatoid arthritis (RA) is a chronic, systemic inflammatory condition affecting synovial tissue. The joint destruction and the deformities that occur in RA are the result of the interaction between the diseased synovial tissue and the normal joint tissue. To understand the pathomechanics of the deformities that occur in RA, one must understand how the synovium affects the different parts of the musculoskeletal system (i.e., bone, cartilage, tendons, and muscles).
Synovium is the membrane that lines the joint capsule. It is lined with mesenchymal cells one to two layers deep. There are two cell types in the synovial lining: type A and type B. The type A synoviocyte is derived from the bone marrow, whereas type B is mesenchymal in origin. Type A cells function like macrophages, and type B cells have a synthetic function similar to fibroblasts. Synovial cells secrete clear lubricating fluid into the joint.
In RA, the earliest histologic changes in the synovium are the occlusion of the microvasculature and swelling of the endothelium. The synovial cells then become hyperplastic and hypertrophic. The number of cell layers increases from 2 to 10 layers deep. Lymphocytes and plasma cells infiltrate the tissue. T lymphocytes make up more than 50% of the cells in this infiltrate. The T-cell infiltrate consists of small nodular aggregates of CD4 cells surrounded by a diffuse infiltrate. The diffuse infiltrate consists of CD8 lymphocytes. Plasma cells enter the synovium late in the disease process. The proliferation of blood vessels in the synovium nourishes the surrounding tissue.
Although RA synovium itself contains only a few neutrophils, it produces transforming growth factor beta and interleukin (IL)-8, both of which attract neutrophils into the synovial fluid and accounts for the increased number of neutrophils found in joint fluid.
Effect on Cartilage
Matrix-degrading enzymes, produced by multiple cell types, destroy both cartilage and bone. IL-1, tumor necrosis factor alpha (TNF-α), and transforming growth factor beta are cytokines secreted by macrophages that stimulate the synovial cells to produce these matrix-degrading enzymes.
These cytokines interfere with articular cartilage remodeling. IL-1 has been shown to increase the production of factors that stimulate cartilage matrix destruction as well as to inhibit the synthesis of type II collagen and proteoglycans.
Neutrophils also produce enzymes that destroy cartilage. Metalloproteinases and cathepsins are proteases that cleave the proteins in collagen and proteoglycans. Collagenase-3, a metalloproteinase, cleaves type II collagen in cartilage. These enzyme interactions are key to the pathophysiology of joint destruction.
IL-1 depletes the proteoglycan concentration in cartilage. With this depletion, the cartilage loses its ability to absorb deforming forces. Eventually, this results in mechanical disruption of the cartilage surface. Synovial fibroblasts in the presence of these cytokines contribute to cartilage destruction, as do mast cells that produce proteins that enhance collagenase activity ( Fig. 102-1 ).
Effect on Bone
As the disease progresses, the hyperplastic synovium extends over the articular cartilage, invading it and eventually the underlying subchondral bone. The diseased synovium adheres to cartilage and bone by the intercellular adhesion molecule 1 to form the “pannus” of RA.
Activated osteoclasts (derived from monocytes) and multinucleated giant cells destroy bone and create bone erosions and/or cysts. The degree of bone loss that occurs can range from thinned cortices and absent medullary cancellous bone to severe bone resorption such as seen in arthritis mutilans. Both IL-1 and TNF-α have been shown to play a role in the abnormal bone and cartilage remodeling seen in RA. Both of these cytokines affect the factors that lead to enhanced osteoclastic bone resorption. TNF-α stimulates the differentiation of osteoclast progenitor cells into mature osteoclasts. IL-1 acts directly on these osteoclasts to increase their bone-resorbing capacity. In contrast to osteoarthritis, no osteophytes develop and no new bone formation occurs.
Spontaneous fusion of the wrist and/or finger joints can occur as the result of the inflammatory process.
Effect on Ligaments
Rheumatoid synovium secretes fluid that distends the joint capsule and stretches the surrounding tissue. The synovium can infiltrate the capsular tissue as well, leading to ligament injury and eventually joint instability.
Effect on Tendons
Rheumatoid synovium can affect tendons as well as articular cartilage, bone, and the joint capsule. It can surround and invade flexor and extensor tendons, resulting in disruption of the normal architecture of the tendons and loss of normal tendon function.
Direct synovial invasion of a tendon can occur, leading to tendon rupture as the tendon weakens. Alternatively, increased pressure on the tendon from proliferative synovitis may occur beneath the dorsal retinaculum, beneath the transverse carpal ligament, or within the flexor tendon pulleys and thereby contribute to tendon rupture. This pressure effect causes decreased blood flow to the tendon either directly or, in the case of the flexor tendons, by occluding the vincula, which supply the tendons. The resultant ischemic necrosis eventually allows the affected tendon to rupture. Tendons also can rupture by attrition, which is discussed later ( Fig. 102-2 ).
Effect on Muscle
Muscles are less commonly affected than joints and tendons but can be injured by perivascular inflammation. The intrinsic muscles of the hand are particularly susceptible to this, resulting in intrinsic tightness and secondary finger deformities.
As mentioned, tendon ruptures in the hand are a common complication of RA but do not always occur by the same mechanism. There are two types of tendon ruptures: attritional and ischemic.
Attrition ruptures occur as the tendon moves across roughened bone. Rheumatoid synovium erodes the joint capsule and infiltrates the bone, creating sharp bone edges. Ruptures occur as the tendon repeatedly passes back and forth over the bone edges. Attrition ruptures of the extensor tendons occur most frequently at either the distal end of the ulna or at Lister’s tubercle.
Although a single extensor tendon rupture can involve any finger, the small finger is affected most often. The tendon is destroyed as it rubs over the distal ulna. Often, after the rupture of the extensor tendon of the small finger, the ring finger extensor will rupture, followed by the long finger extensor, and so on. This occurs as the remaining intact tendons shift ulnarly and become abraded over the roughened edges of the distal ulna. Thus, the usual progression of extensor tendon ruptures is in a radial direction, affecting the index finger last. Such sequential ruptures may occur rapidly. This process was described by Vaughan-Jackson.
Ischemic tendon ruptures also occur in patients with RA in whom dorsal wrist tenosynovitis develops, as described previously. The tenosynovitis results in tendon compression, weakening the tendon by decreasing its blood supply, and ultimately results in tendon rupture.
Rupture of the extensor pollicis longus (EPL) is common in RA and can be caused by a combination of attrition and ischemia in the region of Lister’s tubercle ( Fig. 102-3 ).
Attrition ruptures of the flexor tendons occur on the volar aspect of the wrist, where they come in contact with the carpal bones, most commonly the scaphoid. Thus, the most common flexor tendon to rupture in RA is the flexor pollicis longus (FPL) from attrition by a bony spicule on the volar surface of the scaphoid that penetrates the volar wrist capsule. This was first described by Mannerfelt and Norman and has been referred to as Mannerfelt’s lesion . After the FPL ruptures, the flexor tendons of the index finger are usually affected next, then the long finger tendons, and so on, with the small finger flexor tendons usually the last to rupture. Thus, in contrast to the extensor tendons, the sequential rupture of flexor tendons usually occurs in an ulnar direction.
Rheumatoid synovitis in the wrist follows predictable patterns. The ulnar styloid, the ulnar head, and the midportion of the scaphoid are often the earliest to be involved by rheumatoid synovitis. Progressive synovial proliferation in these areas leads to the various patterns of wrist deformity.
Proliferative synovitis in the radiocarpal joint begins beneath the radioscaphocapitate, or “sling,” ligament. This ligament runs from the radial styloid across the volar scaphoid and inserts onto the head of the capitate, forming part of the volar wrist capsule. Loss of this ligament contributes to vertical rotation of the scaphoid and thereby wrist instability and collapse.
Synovitis in the scapholunate ligament area weakens and eventually disrupts the ligament, resulting in rotatory instability of the scaphoid. The scaphoid assumes a volar-flexed position, and there is secondary loss of carpal height and radial deviation of the carpus and metacarpals on the radius. The wrist now has shortened, and the extrinsic finger muscles that cross the wrist are working at suboptimal lengths. The resulting imbalance of the flexor and extensor muscle–tendon units crossing the wrists (extrinsic muscles) can be viewed as an extrinsic minus, or, in other words, a relative intrinsic-positive condition, and may be one factor in the development of intrinsic-plus deformity of the fingers ( Fig. 102-4 ).
The capitolunate joint is usually spared until late in the disease process (except in juvenile rheumatoid arthritis) because there are no ligaments between these bones. Synovium is more abundant in the region of ligaments, and because there are no ligaments, there is no significant synovitis ( Fig. 102-5 ).
Subluxation of the Carpus
Destruction of the ulnocarpal ligament complex of the wrist (the triangular fibrocartilage and its associated ligaments) by rheumatoid synovitis results in volar subluxation and supination of the ulnar side of the carpus. As this happens, the ulna becomes more prominent. The ulna is the fixed unit of the forearm and, as such, does not dislocate dorsally; it only appears to form the combination of rotatory subluxation of the scaphoid, volar subluxation of the ulnar carpus, and relative supination of the wrist in relation to the distal forearm. This common pattern of wrist collapse results in imbalance of the extensor tendons and radial shift of the carpus and tightly attached metacarpals. Compensatory ulnar deviation of the fingers completes the typical zigzag collapse deformity of the rheumatoid hand and wrist ( Fig. 102-6 ).