Demographic heterogeneity in valgus angulation of the proximal ulnar canal in relation to the flexion-extension axis

Background: The long-term outcome of total elbow arthroplasty remains unsatisfactory because of loosening and polyethylene wear, which could be caused by malpositioning of the ulnar component. When introducing an ulnar component, 2 different angles should be considered in the coronal plane: the valgus angulation of the proximal ulna in relation to the flexion-extension axis (FE-axis) and the intramedullary varus angulation in relation to the FE-axis. Currently, available TEA designs may not always be able to reconstruct the FE-axis because of the morphologic variability of the ulna. Hypothesis: This study aimed to determine the demographic variability of the ulna and the relation between the 2 angulations in the frontal plane based on 3-dimentional computed tomography (CT) reconstructions of the elbow joint of healthy volunteers. Methods: Computed tomography scans of 36 left elbows of healthy volunteers were obtained (20 men and 16 women). The scans were segmented and analyzed using the Mimics Research 20.0 software. A local coordinate system was created based on the FE-axis of the elbow and the ulna's longitudinal axis. The measurements were conducted using the 3-Matic Research 12.0 software. Results: The valgus angulation of the proximal medullary canal was on average 16 degrees in men but 12 degrees in women and ranged between 5 degrees and 21 degrees. The varus angulation of the medullary cavity could be determined at 57 mm in men and 64 mm in women from the FE-axis. This angulation was on average 10 degrees in men and 7 degrees in women. There was no significant correlation between this angle and the length of the ulna or the point of varus angulation. Conclusion: This study found a wide range of valgus angulation of the proximal ulnar canal in relation to the FE-axis. The available elbow implant designs are discordant with the mean valgus angulation of the proximal ulna found in this study, and the valgus laxity of the implants does not cover the variability in the population. Level of evidence: Anatomy Study; Imaging (c) 2023 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserved.