An image-based model of the wrist was developed for a biomechanical analysis of a limited intercarpal fusion for the treatment of Kienbock's disease . The model was used to quantify the wrist joint contact pressure distribution and ligament tension for multiple intercarpal fusion procedures. The goal of the study was to determine how the procedures alter the force transmission through the wrist joint, in general, and to determine if any of the procedures reduce the load applied to the lunate, in particular. A capitate-hamate fusion, a scapho-trapezial-trapezoidal fusion, and a scaphocapitate fusion all were modeled using the discrete element analysis technique.
Graphical models of 10 human wrist joints were created from CT data of cadaver specimens (Fig. 4). Each model included 27 joint surfaces. A rectangular grid was constructed on each joint surface, and a compressive spring was placed on each rectangular element, with a minimum spring density of one spring per square millimeter. Three to six tensile springs modeled each of the carpal ligaments, with a total of 48 tensile springs used in each model. External loads were applied to the fingers to model a grip maneuver. Intercarpal fusions were modeled by increasing the spring stiffness of the cartilage between the fused bones from 22.6 N/mm to infinity. The pressure distributions for the applied loading conditions were displayed on the graphic wrist models.
Scaphocapitate and scapho-trapezial-trapezoidal fusions significantly decreased the joint force at the radio-ulnate joint and the luno-capitate joint, compared to the intact wrist. In contrast, these fusions significantly increased the joint force at the radioscaphoid joint, in comparison with the intact wrist. In the midcarpal joint, scaphocapitate fusion also increased the joint force at the scapho-trapezio-trapezoidal joints and the triquetral-hamate joint, whereas scapho-trapezial-trapezoidal fusion increased the joint force at the scapho-capitate joint. Capitate-hamate fusion yielded no significant changes in the joint forces through the entire wrist joint. In the analysis of ligament tension, scaphocapitate and scapho-trapezial-trape-zoidal fusions significantly decreased the tension only in the dorsal scapholunate ligament. These findings demonstrate that scaphocapitate and scapho-trapezial-trapezoidal fusions are effective in decompressing the lunate. By contrast, capitate-hamate fusion is ineffective in reducing lunate compression. Although scaphocapitate and scapho-trapezial-trapezoidal fusions are recommended for the treatment of Kienbock's disease, the increase in force transmission through the radio-scaphoid and the midcarpal joints may lead to early degenerative changes.
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