Carpal bone mechanics are complex and poorly understood. An anatomic model that explains observed kinematic results is yet to be achieved. The aim of this study is to determine if morphologic sub-typing of the STT and TH joints exists.

The study used 100 sets of dry disarticulated carpal bones and 50 cadaveric wrists. A digital microscribe was used to reconstruct and measure the articular surfaces of the STT and TH joints and distal lunate of all specimens. Ligaments were dissected, reconstructed and measured. Lunate typing based on the morphology of the distal articular surface allowed the specimens to be split into three groups: type one lunates (single facet; 30%), type two lunates (double facet; 42%) and unclear (intermediate; type three; 28%).

Type one and type two groups had significantly different (p < 0.05) mean measures at the STT and TH joints, suggesting clear differences in joint shape and hence joint motion. Type three had mean measures that were not significantly different (p>0.05) from either of the other groups. Two distinct ligament patterns were also observed. The mean measures of each ligament were significantly different (p< 0.05) between type one and two specimens. Type three specimens were split into those with a ligamentous pattern similar to type one specimens and those similar to type two, each significantly different from each other (p< 0.05). Type one specimens had ‘sling-like’ supports for either side of the wrist, whilst type two specimens had numerous attachments to the scaphoid and hamate, creating potential points of rotation.

These results suggest that bony typing gives a clear indication of potential carpal motion for 72% of cases. The remaining 28% are reliant upon ligamentous typing. Individuals are predisposed to structurally support one pattern of motion. Further investigation will relate these anatomic differences to observable motion.