Classification system for isolated arthritis of the scaphotrapeziotrapezoidal joint.

We have developed a classification system for osteoarthritis (OA) of the scaphotrapeziotrapezoidal (STT) joint that can be used on posteroanterior, lateral, and oblique wrist radiographs. It can be used to communicate the degree of severity of arthritis, and to study arthritis in the wrist further. Currently we know of no classification system for OA of the STT. We devised a classification system and made an initial study to assess the reproducibility of the system. It was compared with a classification system similar to the Eaton classification of carpometacarpal OA of the thumb. Cohen's kappa test and the sign test were used for comparison. Thirty-seven of the 46 wrists evaluated (80%) showed radiographic evidence of osteoarthritis of the scaphotrapeziotrapezoidal joint. Most kappa values for intrarater and inter-rater reliability lay between 0.87 and 0.95. There was absolute agreement between both systems about the existence of osteoarthritis. When we compared the two classification systems, ours tended to underestimate the arthritic stage. This classification may be helpful for communication, comparison, and evaluation of osteoarthritis of the STT joint among surgeons. Further study is needed to define the clinical and mechanical relevance of osteoarthritis in this joint.