Does proximal versus distal injury location of the medial ulnar collateral ligament of the elbow differentially impact elbow stability? An ultrasound-guided and robot-assisted biomechanical study

Background: The location (proximal vs. distal) of elbow medial ulnar collateral ligament (MUCL) tears impacts clinical outcomes of nonoperative treatment. The purposes of our study were to (1) determine whether selective releases of the MUCL could be performed under ultrasound (US) guidance without disrupting overlying soft tissues, (2) assess the difference in medial elbow stability for proximal and distal releases of the MUCL using stress US and a robotic testing device, and (3) elucidate the flexion angle that resulted in the greatest amount of medial elbow laxity after MUCL injury. Methods: Sixteen paired, fresh-frozen elbow specimens were used. Valgus laxity was evaluated with both US and robotic-assisted mea-surements before and after selective MUCL releases. A percutaneous US-guided technique was used to perform proximal MUCL re-leases in 8 elbows and to perform distal MUCL releases in their matched pairs. The robot was used to determine the elbow flexion angle at which the maximum valgus displacement occurred for both proximally and distally released specimens. Open dissection was then performed to assess the accuracy of the percutaneous releases. Results: Percutaneous US-guided releases were successfully performed in 15 of 16 specimens. The proximal release resulted in greater valgus angle displacement (11 degrees +/- 2 degrees) than the distal release (8 degrees +/- 2 degrees) between flexion angles of 30 degrees and 70 degrees (P < .0001 at 30 degrees, P < .0001 at 40 degrees, P = .001 at 50 degrees, P = .005 at 60 degrees, and P = .020 at 70 degrees). Valgus displacement between release locations did not reach the level of statistical significance between 80 degrees and 120 degrees (P = .051 at 80 degrees, P = .131 at 90 degrees, P = .245 at 100 degrees, P = .400 at 110 degrees, and P = .532 at 120 degrees). When we compared the values for the mean increase in US delta gap (stressed - supported state) from before to after MUCL release, the proximally released elbows had larger increases than the distally released elbows (5.0 mm proximal vs. 3.7 mm distal, P = .032). After MUCL release, maximum mean valgus displacement occurred at 49 degrees of flexion. Conclusions: US-guided selective releases of the MUCL can be performed reliably without violating the overlying musculature. Valgus instability is not of greater magnitude for distal releases when compared with proximal releases. This findings suggests there must be alternative factors to explain the difference in clinical prognosis between distal and proximal tears. The observed flexion angle formaximum valgus laxity could have important implications for elbowpositioning during US or fluoroscopic stress examination, aswell as surgical repair or reconstruction of the MUCL.(C)2022 Journal of Shoulder and Elbow Surgery Board of Trustees. All rights reserve