Simulated Bennett fracture treated with closed reduction and percutaneous pinning. A biomechanical analysis of residual incongruity of the joint.

Abstract

Contact area and contact pressure within the trapeziometacarpal joint were measured during static and dynamic loading in a cadaver model. The measurements were repeated after a Bennett fracture had been simulated with use of an osteotomy of the metacarpal base. The beak fragment was recessed two millimeters and was fixed in close apposition to maintain the integrity of the palmar beak ligament, and contact area and contact pressure were measured during lateral pinch. Despite the resulting articular incongruity, the mean total contact area of the joint surface increased from 15.8 to 25.8 square millimeters (63 per cent) in all specimens after simulation of the Bennett fracture (p = 0.02), and it shifted dorsally in moderately osteoarthrotic and non-osteoarthrotic specimens. The mean contact area in the palmar region of the joint surface decreased, from 58 to 25 per cent of the total area (p = 0.04); that in the central region increased, from 28 to 52 per cent (p = 0.05); and that in the dorsal region increased, but not significantly with the numbers available, from 14 to 24 per cent (p = 0.18). No pathological concentration of contact pressure was seen at the margin of the articular step-off. The changes in area and pressure reflect an unloading of the metacarpal beak, where osteoarthrotic degeneration most commonly occurs. There is no biomechanical basis for predisposition to post-traumatic osteoarthrosis after a Bennett fracture with a small palmar-beak component and a residual two-millimeter articular step-off, provided that the fragments of the shaft and the beak heal in close apposition. This suggests that reduction of the metacarpal shaft relative to the trapezium and the beak fragment, rather than strict anatomical restoration of the joint surface, should be the priority of treatment. Bennett fractures that can be reduced with articular incongruity of no more than two millimeters may be treated satisfactorily with closed reduction and percutaneous pinning without the need for open anatomical reduction and internal fixation of the joint surface.