Efficacy of a radial-based thumb metacarpophalangeal-stabilizing orthosis for protecting the thumb metacarpophalangeal joint ulnar collateral ligament.

STUDY DESIGN: Basic research (biomechanics).

INTRODUCTION: The high degree of motion that occurs at the thumb metacarpophalangeal (MCP) joint must be taken into account when immobilizing a partially torn or repaired thumb ulnar collateral ligament.

PURPOSE OF THE STUDY: To determine the efficacy of a radial-based thumb MCP-stabilizing orthosis in resisting abduction across the thumb ulnar collateral ligament.

METHODS: Ten fresh cadaveric hands were mounted to a custom board. An anteroposterior radiograph of the thumb was obtained with a 2 N preload valgus force applied to the thumb, and the angle between the Kirschner wires was measured as a baseline. Subsequently, 20, 40, 60, 80, and 100 N valgus forces were applied 15 mm distal to the MCP joint. Anteroposterior radiographs of the thumb were obtained after each force was applied. The angle of displacement between the wires was measured and compared with the baseline angle. The angles were measured with an imaging processing tool. A custom radial-based thumb MCP-stabilizing orthosis was fashioned for each cadaveric thumb by a certified hand therapist. The aforementioned loading protocol was then repeated.

RESULTS: The radial-based thumb MCP-stabilizing orthosis significantly reduced mean abduction angles at each applied load.

DISCUSSION: We found that our orthosis, despite being hand-based and leaving the thumb IP and CMC joints free, significantly reduced mean abduction angles at each applied load.

CONCLUSIONS: This investigation provides objective evidence that our radial-based thumb MCP-stabilizing orthosis effectively reduces the degree of abduction that occurs at the thumb MCP joint up to at least 100 N.

LEVEL OF EVIDENCE: n/a (cadaveric).