Carpometacarpal (CMC) Joint Subluxation: Causation Analysis and Impairment Evaluation

Carpometacarpal (CMC) joint subluxation refers to the changes that occur in the CMC joint as seen on x-rays and observed during physical examination. The CMC joint is the most commonly involved arthritic joint in the hand, and arthritis may appear in localized or systemic forms. A diagnosis of thumb-CMC arthritis is based on symptoms of localized pain, tenderness, and instability on physical examination and radiographic evaluation. The AMA Guides to the Evaluation of Disease and Injury Causation provides a protocol for assessing causation and requires that all three of the following criteria must be met: 1) the patient has an illness compatible with a disease-producing agent or an injury; 2) the worker's exposure in the occupational environment potentially caused the disease or is a plausible mechanism of injury of sufficient magnitude to cause the condition; and 3) the preponderance of evidence supports the disease or injury as occupational in origin. If any one of the three is possible but not probable, causation has not been established. The authors review several published articles and conclude that, based on the clinical facts and current science, CMC joint subluxation is unrelated to work and instead is reflective of aging. The article concludes with a comparison of impairment ratings of CMC-related disability using the fifth and sixth editions of the AMA Guides to the Evaluation of Permanent Impairment, both of which lead to identical impairment ratings but by different means.