Mechanisms of circumduction and axial rotation of the carpometacarpal joint of the thumb.

Osteoarthritis of the carpometacarpal joint of the thumb (CMCJ) is a frequent clinical problem. The aim of the study was to discuss the mechanisms of circumduction and axial rotation of the CMCJ considering geometrical properties of the articulating surfaces and the configuration of the muscle system acting over the CMCJ. 28 CMCJ from 7 female and 7 male corpses (age: 81 yrs (median), 53-91 yrs (interval), which did not show any sign of arthrosis, were investigated. Contours in flexion/extension: in saddle point O, the contour of the proximal surface is stronger curved. For 23 of the 28 joints the contours showed an eye-catching difference. Contours in ab-/adduction: all 28 joints showed the respective incongruity. Straight lines and their included angles: in both articulating surfaces, the angles between the straight lines through the saddle point showed values which were close to 90 degrees. Out of neutral position a small axial rotation (maximal range: 3.5 degrees) is possible without that the contact at the saddle points is changed. But, when one of the straight lines of the proximal surface meets a respective straight line of the distal surface, the contact "point" is enlarged to a contact "line". When the axial rotation is further increased, the contact "line" splits into two contacts "points", which are located at outer areas of the articulating surfaces.