FORCE AND PRESSURE TRANSMISSION THROUGH THE NORMAL WRIST - A THEORETICAL 2-DIMENSIONAL STUDY IN THE POSTEROANTERIOR PLANE

Force transmission through the wrist in the normal population was inve stigated using the rigid body spring modeling (RBSM) technique (assumi ng carpal bones are rigid bodies interposed by series of springs simul ating articulating cartilage and constraining ligaments), One-hundred and twenty normal wrist posteroanterior X-rays of adults (evenly divid ed to represent both genders and two age groups) provided the anatomic al data. Reaction forces between the carpal bones were modeled using a system of compression linear springs, representing cartilage and subc hondral bone, and of tensile linear springs, representing ligaments. T he spring constants were determined based on the material properties o f wrist cartilage and ligaments. Assumed axial loads were applied alon g the metacarpals to simulate a grasp strength of 10 N with active sta bilization of the wrist in neutral position. The force transmission ra tio at the radio-ulno-carpal joint was 55% through the radio-scaphoid and 35% through the radio-lunate joints. The remaining 10% of the load was passing through the triangular fibrocartilage with minor differen ces between genders. Among the intercarpal joints, a large percentage of the load of the wrist was transmitted to the scaphoid. The peak pre ssure was highest at the proximal pole of the radio-scaphoid, with a r adio-scaphoid versus radio-lunate peak pressure ratio of 1.6. The most important ligaments in terms of load transmission were those opposing ulnar translation of the carpus. The wrist morphology had little infl uence on the magnitude and pattern of load distribution. There was no effect of age on wrist force distribution.