Poster 188: The Effect of Forearm Supination Versus Pronation at the Time of UCL Reconstruction Graft Tensioning on Post-Surgical Medial Elbow Joint Gapping in a Biomechanical Cadaveric Model

OBJECTIVES: The anterior bundle of the ulnar collateral ligament (UCL) is the primary static valgus restraint of the elbow joint. In general, the effect of forearm supination and pronation on native and reconstructed UCL strain is poorly characterized. Specifically, the effect of placing the forearm in supination versus pronation during final graft tensioning at the time of UCL reconstruction has not been quantified. Therefore, the objectives of the current study were the following: 1) to determine the effect of forearm pronation and supination on medial elbow joint gapping with an intact native UCL and 2) to determine if placing the forearm in maximum supination or pronation at the time of UCL reconstruction graft tensioning affects subsequent medial elbow joint gapping under simulated valgus stress test at varying degrees of elbow flexion. Our hypotheses were that under valgus load: 1) medial elbow joint gapping will be larger when the forearm is in maximal pronation for both the native and reconstructed ligament and 2) tightening the UCL reconstruction graft in maximum pronation would allow larger gap formation during valgus load testing. METHODS: Eighteen full arm specimens were stripped of soft tissue, leaving only the elbow ligamentous and capsular structures intact. All specimens were statically tested at 30°, 60°, and 90° of elbow flexion with the forearm in full pronation, full supination, and neutral in an unloaded and loaded (3Nm valgus) configuration (Figure 1A). Medial joint gapping was measured using retroreflective markers attached to the ulna and humerus with a 3D motion capture system. Following intact testing, the UCL was transected and an ipsilateral flexor carpi radialis (FCR) tendon autograft was used to reconstruct the UCL with a standard docking technique (Figure 1B). For graft tensioning, all specimens were placed in 45 degrees of elbow flexion and randomized to either full supination or full pronation. Valgus testing was then repeated for medial joint gapping measurements. Following UCL reconstruction, the change in medial elbow joint gap under valgus load was calculated by subtracting the intact gap measurement from the reconstructed gap measurement for each specimen. A positive gap measurement was used to indicate an increase in joint gapping after reconstruction while a negative gap value indicated a smaller medial joint gap after UCL reconstruction. Two-way ANOVA were used to determine the effect of forearm position during reconstruction on change in gap and for each of the elbow flexion positions. Significance was set as p<0.05. RESULTS: Valgus load testing of the native UCL demonstrated no significant difference in medial elbow joint gapping between the pronated and supinated position at all elbow flexion angles (Figure 2). Following UCL reconstruction, this finding was maintained as there was no significant difference in medial elbow joint gapping during a valgus load test regardless of forearm testing position. However, forearm position (i.e. maximum supination or maximum pronation) at the time of UCL reconstruction graft tensioning significantly affected the difference of post-operative medial elbow joint gapping at 30-and 60-degrees, but not at 90-degrees of elbow flexion (Figure 3). Placing the arm in maximum supination at the time of UCL graft tensioning resulted in a larger post-operative medial elbow gap compared to specimens with a reconstruction graft tensioned in maximum pronation. CONCLUSIONS: Our hypothesis was disproven. In a cadaveric biomechanical model, forearm pronation or supination did not affect medial joint gapping with the native or reconstructed UCL during valgus load testing. However, securing the UCL reconstruction graft in maximal supination resulted in a larger difference in post-operative medial elbow gap at 30° and 60° of elbow flexion. While statistically significant differences were found, the post-operative gap difference was less than 1mm at each level of elbow flexion regardless of the forearm rotation during testing. It is unknown if this small amount of additional joint gapping following UCL reconstruction graft tensioning in maximum supination can result in a clinically notable difference in elbow function, strength or pain. Further research is necessary to determine the clinical significance of the current cadaveric model findings and whether other factors related to the native or reconstructed UCL can affect medial elbow joint gapping.