Anatomical Variations of the Extensor Carpi Ulnaris Groove: A New Computed Tomography-based Evaluation.

Background: This study aimed to develop a classification that precisely describes the extensor carpi ulnaris (ECU) groove morphology. Methods: Reconstructed axial plane computed tomography scans of the wrists of 200 patients were reviewed. Three groups of groove shape were created based on the position of the deepest point: deviated to the ulnar side (Type U), in the middle of the groove (Type M), and deviated to the radial side (Type R). Groove depth, width, carrying angle, and radius of the curvature were measured using the Picture Archiving and Communication System in a slice in which the ulnar head was the largest. Results: Type U was present in 88 patients (44%), Type M in 74 patients (37%), and Type R in 38 patients (19%). The average depth, width, carrying angle, and radius of curvature were 2.2 mm, 9.2 mm, 135.8 degrees, and 7.0 mm, respectively. Depth, width, and carrying angle were normally distributed. Both depth and width were statistically correlated with the carrying angle; groove depth and width were not correlated. ECU groove shape and depth showed excellent intra-and inter-observer reliabilities; the reliabilities for the width were poor. Therefore, depth subgroups were defined using cutoffs of +/- 2 standard deviations (SD): d1 (<=-2 SD), <= 1.0 mm; d2 (+/- 2 SD), 1.1-3.3 mm; and d3 (>=+ 2 SD), >= 3.4 mm. The 200 wrists were classified as follows: 0.5% Ud1, 40.2% Ud2, 1.3% Ud3, 0.8% Md1, 38.3% Md2, 0.8% Md3, 1.5% Rd1, 16.4% Rd2, and 0.2% Rd3. Conclusions: The detailed morphology of the ECU groove was classified using three major types and depth subgroups. The Type R ECU groove, which lacks a medial bony buttress, might be more prone to ECU-related injuries. This classification helps to understand the ECU tendon-related injury pathologies and may provide valuable information for treatment decisions; however, further research is necessary.