aDepartment of Orthopaedic Surgery, Beth Israel Deaconess Medical Center, 330 Brookline Avenue,
E/CC2, Boston, MA 02215, USA hHarvard Medical School, 260 Longwood Avenue, Boston, MA 02115, USA
The human thumb assumes 50% of the workload of the human hand , and is therefore the most important digit. As such, the thumb has a propensity for the development of osteoarthritis. Moreover, the thumb is also often diseased, in anywhere from 68% to 80% of patients who have rheumatoid arthritis (RA) [1,2].
Much attention over the years has been given to the carpalmetacarpal (CMC) joint of the thumb, whereas the metacarpophalangeal (MP) joint of the thumb remains largely unstudied. The purpose of this article is to review the etiology of thumb MP joint arthritis, and discuss the different treatment options of this condition.
Although range of motion of the thumb MP joint in healthy individuals is only a few degrees [3,4], disease of the joint, whether due to instability or pain, can bring about significant disability and loss of function to patients . Pain about this joint is usually a result of arthritis, whose etiology may be either degenerative, post-traumatic, or inflammatory—most commonly RA. Isolated degenerative arthritis of the thumb is very rare, and when seen, a specific etiology must be sought . Osteoarthritis of the MCP joint is usually related to prior injury , whereas in the
* Corresponding author. Department of Orthopaedic Surgery, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, E/CC2, Boston, MA 02215.
E-mail address: [email protected] (C.S. Day).
rheumatoid thumb, it is usually a result of liga-mentous and capsular involvement as well as erosion of the joint surfaces .
RA, which affects roughly 6% of the general population , is the primary cause of MP arthritis. RA is a condition that may start as an antigen-dependant activation of T-cells with an array of downstream effects, such as activation of proin-flammatory cells from bone marrow and circulation, secretion of cytokines and proteases by macrophages and fibroblast-like synovial cells, active synovial tissue proliferation, and finally, auto-antibody production . This autoimmune type of inflammation results in the key feature that separates RA from osteo- or post-traumatic arthritis— soft-tissue damage that renders the joint unstable. As a result, treatment must be geared to not only pain relief, but also to return stability to the joint.
MP arthritis in the rheumatoid thumb typically starts with a boutonniere deformity [1,2]. Boutonniere deformity is a debilitating and painful condition whose feature is MP flexion and interphalangeal (IP) hyperextension of the thumb . As part of the disease progression, active extension of the MP joint is lost as the central slip over the MP joint subluxes around the joint below its axis of rotation, eliminating the physiological pulley system to extend this joint. As a consequence, the extension force is then transmitted to the IP joint, where it hyperextends this joint. Subsequently, MP and IP joint deformity may become fixed and contracted, possibly leading to joint dislocation and destruction [1,2]. Dislocation in the rheumatoid thumb is attributed to the destruction of the supporting soft-tissue structures, such as the collateral ligaments, whose function is to provide static joint stability. In addition, articular cartilage is usually eroded as well, as a result of the rheumatoid pannus formation in the joint . Therefore, when treating the rheumatoid thumb, it is paramount to address both paint and stability of the joint.
Osteoarthritis (OA) of the MP joint is a rare disease. OA in general is thought to be a multifac-torial process involving biomechanical, biochemical, genetic, and metabolic factors that lead to secondary inflammation at the joint. Osteoarthritic conditions in the thumb are usually a result of a mechanical process, such as trauma or repeated microtrauma, which results in joint surface mal-tracking, and in combination with the release of degenerative enzymes by chondrocytes  it causes articular surface degeneration. The key distinction of the osteoarthritic thumb as compared with the rheumatoid thumb is that the osteoar-thritic thumb is inherently stable—its collateral ligaments are generally intact. Therefore, treatment of this condition is usually geared toward pain relief only rather than stability.
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