Understanding Metacarpal Morphology and Practical Surgical Applications

Curved hand long bone morphology is understudied. Our study aimed to quantify the metacarpal dorsal and volar radius of curvature, canal and isthmus diameter, and compare the differences between the five metacarpals. A clear understanding of the radius of curvature (ROC) may help to better inform metacarpal biomechanical function, joint alignment, stability, and guide techniques that restore near-anatomical alignment during fracture fixation.

One hundred five computed tomography scans of cadaver hands were used to measure the volar and dorsal ROC, canal diameters, and isthmus dimensions of all metacarpals. Statistical analysis showed excellent interrater and intrarater reliability with intraclass correlation coefficients of 0.95 and 0.98, respectively.

The volar ROC of each metacarpal was smaller, and therefore more curved, when compared with the dorsal surfaces for all metacarpals. The index and small finger metacarpals had the largest and smallest ROCs, respectively. The thumb metacarpal had the largest intramedullary diameter. The fourth metacarpal had the smallest intramedullary diameter. The index metacarpal was the longest.

The thumb and small finger metacarpals had the smallest volar radii of curvature, which may reflect the thumb and small fingers' unique role in grasping movements and opposition. The dorsal surface is less curved with a large ROC in all metacarpals and therefore can be considered as relatively straight.

Based on our findings, we recommend surgeons use the nearly straight dorsal cortex as a guide when preparing and inserting intramedullary fixation. The volar metacarpal ROC varies between metacarpals and should be considered during fixation of fractures involving the volar surface. Restoring the anatomical arch may be important in optimizing intrinsic muscle function.