The intermetacarpal angle screening test for ulnar-sided carpometacarpal fracture-dislocations

Ulnar-sided carpometacarpal injuries can be difficult to diagnose radiographically. We hypothesized that the resting position of the normal hand during lateral radiography provides a consistent relationship between the rays and that dorsal subluxation of the metacarpal base in fracture-dislocations increases the angle between the uninjured index and long metacarpals and the injured small metacarpal.

A control group of 100 consecutive patients with normal hand radiographs and a series of 12 patients with known carpometacarpal fracture-dislocations were examined. Angles between the index and small metacarpal shaft (I-S IMA) and between the long and small metacarpal shaft (L-S IMA) were measured on the lateral hand radiograph.

In the control group, the mean I-S IMA and L-S IMA were both 6°. In the study group, the mean I-S IMA was 18°, and the mean L-S IMA was 16°. Intraobserver and interobserver reliability was good to excellent for both groups, and a statistical difference existed between the normal and study groups. Based on box-plot analysis of normal and abnormal IMAs, a natural dividing line existed at 10°. With this dividing line, the I-S IMA had a sensitivity of 92% and a specificity of 81%, and the L-S IMA had a sensitivity of 83% and a specificity of 84%.

Both the I-S IMA and the L-S IMA were useful screening measurements on lateral hand radiographs for detection of ulnar-sided carpometacarpal fracture-dislocations. When evaluating posttraumatic ulnar-sided hand pain, advanced imaging should be considered if the I-S IMA or the L-S IMA is greater than 10°.