Midcarpal Bone Structure and Force Distribution in the Normal Wrist

Purpose: Two distinct morphological wrist types have been defined based on the structure of the midcarpal joint: type: a lunate type 1 and a spherical proximal capitate and type 2: a lunate type 2 and a flat proximal capitate. Since distinct shapes will transfer forces differently, the purpose of this study was to evaluate the association between the forces transferred to the distal radius and ulna with morphological wrist type.

Methods: Forty-nine cadaver wrist radiographs were analyzed. These were tested for force distribution between the radius and ulna. Radiographs were evaluated for lunate, capitate shape and contact between the capitate and surrounding carpal bones. The percentage of compressive forces through the distal ulna and radius was determined by mounting load cells to the distal radius and ulna while tensile forces were applied to the extensor-carpi-ulnaris, extensor-carpi-radialis- brevis, flexor-carpi-radialis and flexor-carpi-ulnaris. Force transmission was tested in neutral, flexion-extension and radioulnar deviation.

Results: The mean ulnar force transferred for type 1 wrists was 27.6% and for type 2 wrists was 10.4% p=0.0003. Force transferred to the ulna was weakly correlated with other radiographic parameters. Table 1 Contacts between the capitate and the metacarpals were not associated with load transfer to the ulna.

Conclusions: 1) Wrist structure (defined by lunate type) affected percent transfer of force through the ulna in normal cadaver wrists. 2) Type 1 wrists and those with greater contact between the capitate and lunate and less contact with the scapholunate ligament transferred more force to the ulna. 3) Weak correlations between intracarpal structure and percent-load transfer may be explained by variations in soft tissues (TFCC and radiocarpal ligaments) which participate in load transfer between the radius and ulna. 4) Further study may improve our understanding of the relationship between wrist structure and transfer of forces.