The inter-and intra-observer reliability of volar angulation measurements in a fifth metacarpal neck fracture

The fifth metacarpal neck fracture is the most common metacarpal fracture. The palmar angulation from the fracture displacement is critical for determining treatment, yet there is no consensus regarding the angulation measurement method or the surgical cut-off value. This study aimed to identify a reliable measurement method for assessing palmar angulation. We evaluated inter-observer and intra-observer validation of measuring palmar angulation in oblique plain X-ray and computed tomography (CT) sagittal cuts.We identified surgically treated patients for acute isolated fifth metacarpal neck fracture between January 1, 2008, and December 31, 2020, and obtained preoperative, opposite hand, and final follow-up oblique X-rays and sagittal computed tomography (CT) radiograms. The oblique radiograph was taken with a 45° posteroanterior pronation. The metacarpal neck palmar angulation was measured in the radiograms using the metacarpal neck-shaft center (MNSC) angle and the shaft articular surface (SAS) angle methods by three orthopedic surgeons in two sessions. For the CT radiograms, each measurer selected the sagittal slot at their discretion to measure the angle. The final palmar angulation was the average of six measurements (two sessions, three measurers per session).The study included 51 patients; the average age was 32.5 (range 18-73) years, with 46 men and 5 women. The MNSC angle inter-observer reliability was better than the SAS angle. The MNSC angle inter-observer reliability was better than that of SAS angle. Intraclass coefficients (ICCs) for the MNSC angle demonstrated an excellent inter-observer agreement among the three measurers in the first (0.93) and second (0.88) session compared to ICCs for the SAS angle in the first (0.81) and second (0.87) session. The MNSC angle intra-observer reliability was also better than the SAS angle, with higher ICCs. Preoperative CT radiograms were available for 42 patients. Using CT scans for measurements, in the two sessions, the MNSC angle inter-observer reliability was higher than that of the SAS angle [MNSC: 0.83; SAS: 0.35], second [MSNC: 0.85; SAS: 0.81]. The intra-observer reliability was also better in the MNSC angle. When comparing average value among obtained radiograms, the physiologic angulation of the opposite hand oblique X-ray had the smallest average value, followed by preoperative CT and preoperative oblique radiography. Overall, the SAS angle measurement had a slightly larger angle than the MNSC method in the fractured and non-fractured hand measurements. Finally, a serial comparison of the oblique X-rays (pre-and postoperative, final follow-up, and the opposite hand with closed reduction and internal fixation) indicated that the angulation significantly decreased, and the post-operative values did not differ from the final follow-up X-ray for either method.The palmar angulation measurement in 45° pronated oblique X-ray using the MNSC angle method had good-to-excellent reliability, with superior results to sagittal CT radiograms. Although the angle is likely overestimated, the MNSC method is reliable for judging the fracture degree and reduction adequacy after surgery compared to the non-fractured hand physiologic angulation.