Motion and morphology of the thumb metacarpophalangeal joint.

Purpose: To determine the variability in the flexion/extension motion of the thumb metacarpophalangeal (MCP) joint in a normal volunteer population and to determine any correlation between the shape of the thumb metacarpal head, gender, age, thumb interphalangeal (IP) joint range of motion (ROM), and thumb MCP joint ROM in a population of fresh-frozen cadaver arms.

Methods: The ROM of the thumb MCP joints of 100 volunteers (200 thumbs) was measured. The thumb MCP joints of 70 fresh-frozen cadaver arms were examined. Posteroanterior and lateral radiographs were taken of 64 MCP joints. Passive flexion and extension at the MCP and IP joints were measured with a standard goniometer starting at neutral (0 degrees ) with the metacarpal, proximal, and distal phalanges axially aligned. The distance from the volar to the dorsal edge of the articular surface (A) and the radius of curvature of the articular surface (r) of the metacarpal head were measured on the lateral view. The shape of the metacarpal head was given a value using the A/r ratio. Regression analysis was used to determine the correlation between the head shape and joint motion. The thumbs were categorized into a round group (A/r ratio > 1.7) or a flat group (A/r ratio of < 1.7) to facilitate statistical analysis.

Results: The volunteer population mean thumb MCP maximum flexion was 77 degrees range, 40 degrees -126 degrees ). Range of motion in hyperextension varied from 0 degrees to 72 degrees, with a mean of 35 degrees. Total ROM was from 55 degrees to 176 degrees, with a mean of 110 degrees. In the cadaver population studied the average MCP joint ROM was 94 degrees (mean flexion, 70 degrees; mean extension, 24 degrees ). The A/r ratio had a range of 1.1 to 2.2. There were 37 round and 27 flat thumb metacarpal heads. Regression analysis showed a significant correlation between metacarpal head shape and ROM; MCP joints with rounder metacarpal heads had greater motion. Round and flat metacarpal heads had significantly different motion arcs averaging 106 degrees and 77 degrees, respectively. Female gender was associated both with significantly greater MCP joint ROM (99 degrees women/87 degrees men) and a significantly higher incidence of round metacarpal heads (66% of women/36% of men). No significant correlation existed between specimen age, MCP, and/or IP joint ROM.

Conclusions: There is a wide range in the magnitude of the thumb MCP joint ROM and the normal shape (round vs flat) of the thumb metacarpal head. A rounder thumb metacarpal head has greater thumb MCP joint ROM than a flatter thumb metacarpal head. Clinically we have found this information helpful in predicting posttraumatic recovery of thumb MCP joint ROM and selecting candidates for and predicting patient satisfaction with thumb MCP joint arthrodesis.