Increased motion at the scaphotrapeziotrapezoidal (STT) joint and compensatory hypermobility of metacarpophalangeal (MP) joint contribute to the total abduction and adduction motion of the thumb after trapeziometacarpal (TM) joint arthrodesis. However, there were no detailed studies to evaluate the contribution of motion of each joint towards total thumb mobility.
We conducted a comparative study on thumb joint motion in 56 hands who underwent TM joint arthrodesis against that of 56 hands in normal subjects. Dynamic radiographs were performed and the angles subtended by the first (M1) and second (M2) metacarpals were to measure radial abduction and adduction, and volar abduction and adduction. In addition, angles subtended by the thumb proximal phalanx (P1) and M1 in abduction and adduction were measured to evaluate the hypermobility of the MP joint.
The average total arcs of M1M2 motion in normal subjects in radial and volar abduction-adduction planes were 24° and 35° respectively. The arthrodesis group averaged 9° of motion in the radial abduction and adduction plane and 8° in the volar abduction and adduction plane. P1M1 volar adduction angle was significantly larger in the arthrodesis group, suggesting that the arthrodesis group had larger compensatory motion of the MP joint in volar adduction compared to normal subjects.
These findings on the amount of hypermobility of MP joint after TM joint arthrodesis are valuable information for optimal postoperative rehabilitation protocol. To achieve good range of motion of thumb abduction and adduction following TM joint arthrodesis, emphasis must be placed in obtaining maximum potential motion of STT joint rather than hypermobility of MP joint.