Quantitative Assessment of Ulnohumeral Instability After K-NOW Unlinked Total Elbow Arthroplasty Using the Helical Axis: A Pilot Study.

BACKGROUND: Unlinked total elbow arthroplasty (TEA) is designed to mimic physiological kinematics and reduce stress at the bone-implant interface, yet postoperative instability remains a significant concern. The helical axis (HA) represents three-dimensional joint motion (rotation and translation) as a single axis, offering a potential tool to quantify joint laxity. This study aimed to evaluate and compare the kinematics of healthy elbows and those following unlinked TEA (K-NOW prosthesis) using the HA to quantify postoperative instability.

METHODS: Ten elbows from five healthy volunteers (mean age: 67.4 years) and five elbows from four patients after unlinked TEA (mean age: 69.0 years) were enrolled. Computed tomography scans were performed at 30° intervals during flexion-extension under three conditions: forearm neutral rotation, supination, and neutral rotation with a 500-g load. Three-dimensional surface models were reconstructed to calculate the HA. Main outcomes included translational magnitude along the HA and the standard deviation (SD) of the HA orientation angles (θ and φ).

RESULTS: The sum of absolute translational magnitude was significantly greater in the TEA group than in the healthy group under forearm neutral rotation (1.7 mm vs. 0.6 mm, P = .017) and supination (3.3 mm vs. 0.7 mm, P = .008). No significant difference was found under the 500-g loading condition (1.3 mm vs. 0.6 mm, P = .107). There were no statistically significant differences in the SD values for HA orientation angles between the TEA group and the healthy group. Within both groups, the 500-g load did not cause a significant increase in translational magnitude or orientation SD.

CONCLUSIONS: Elbows following unlinked TEA exhibit significantly greater translational displacement compared to healthy elbows, suggesting increased medial-lateral laxity. These preliminary, hypothesis-generating findings suggest that a 500-g load in the shoulder abduction position might not immediately exacerbate instability. Evaluation using the HA serves as a valuable quantitative method for assessing elbow kinematics and stability after TEA. This was a pilot study with a limited sample size; therefore, further large-scale research is needed to confirm these findings.