Relative Metacarpal Shortening as a Radiographic Measure of Fourth and Fifth Carpometacarpal Fracture Dislocation.

Purpose: Fourth and fifth carpometacarpal fracture dislocations may be missed on plain x-ray. The purpose of this study was to describe a radiographic measurement, the relative metacarpal shortening (RMS), to evaluate for fourth and fifth carpometacarpal fracture dislocations on posteroanterior radiographs. We hypothesize that the RMS will increase in patients with fourth and fifth carpometacarpal fracture dislocations relative to controls.

Methods: A retrospective review identified posteroanterior hand radiographs of skeletally mature patients with fourth and/or fifth carpometacarpal fracture dislocations. Comparisons were made of normal radiographs identified from patients presenting to the outpatient orthopedic hand clinic for complaints other than fracture or dislocation. Three independent reviewers performed measurements, with two of these reviewers measuring the RMS for each patient radiograph. The fourth and fifth RMS were calculated by dividing the length from the fourth/fifth distal metacarpal head to a tangential line to the third metacarpal head by the third metacarpal length. Measures were compared between the two cohorts. Agreement was assessed by intraclass correlation coefficients between reviewers.

Results: In total, 42 controls and 41 fractures were included. The fracture group included 18 patients (43.9%) with isolated fifth metacarpal involvement and 23 patients (56.1%) with both fourth and fifth metacarpal involvement. Fractures were more commonly right-sided, younger, and men relative to controls. The fourth and fifth RMS were higher for fractures (0.16 and 0.31) than for controls (0.12 and 0.26), respectively. The intraclass correlation coefficient agreement test was almost perfect for all measurements (range: 0.82-0.94).

Conclusions: The RMS is a radiographic measurement with almost perfect agreement between reviewers and is increased in fourth and fifth carpometacarpal fracture dislocations. Fourth RMS >0.13 and fifth RMS >0.28 should increase the index of suspicion for carpometacarpal fracture dislocation in a patient with corresponding ulnar hand pain.

Type of study/level of evidence: Diagnostic IV.