Differences in valgus and internal-rotation instability after sequential distal ulnar collateral ligament release: a cadaveric study using Thiel-embalmed cadavers.

BACKGROUND: Distal ulnar collateral ligament (UCL) tears in overhead athletes have poorer outcomes with conservative treatment than proximal tears, potentially because of limited vascularity and altered ulnohumeral alignment. Previous in vivo studies have demonstrated excessive internal rotation malalignment in distal UCL tears. Nonetheless, it remains unclear whether distal tears produce greater internal rotation instability compared with valgus instability. This cadaveric study aimed to quantify valgus and internal rotation instabilities after sequential distal UCL tears using ultrasonography in Thiel-embalmed cadavers.

METHODS: Twelve elbows from six Thiel-embalmed cadavers were evaluated. A distal UCL tear model was created by sequentially releasing the distal footprint of the anterior bundle: intact, 5 mm release (partial tear), and approximately 10 mm release (complete tear). Valgus instability was assessed by measuring the medial joint space (MJS) with the elbow at 90° flexion under a 1 kg valgus load. Internal rotation instability was quantified by measuring the posteromedial joint space (PMJS) with the forearm maximally pronated under the same load. Three images per condition were obtained, and mean values were analyzed. The rate of change in MJS and PMJS was calculated by dividing the MJS or PMJS value (mm) under each UCL-release condition by the corresponding value in the intact condition. The resulting rate of change was unitless. Conditions were compared using one-way analysis of variance with Bonferroni correction, paired t-tests with P values adjusted using the Holm method were used to compare the rates of change between MJS and PMJS.

RESULTS: MJS and PMJS increased significantly from intact to partial and complete tears (all P < .01), with effect sizes >0.8. PMJS showed a significantly greater rate of change than MJS in both partial tears (1.46 ± 0.17 vs 1.29 ± 0.19; P = .018) and complete tears (1.94 ± 0.33 vs 1.65 ± 0.21; P = .018).

CONCLUSION: Distal UCL tears increased both valgus and internal rotation instability; however, internal rotation instability increased more. Therefore, functional factors, particularly increased internal rotation instability, may contribute to the characteristic internal rotation malalignment and poor prognosis of distal UCL tears.

LEVEL OF EVIDENCE: Basic Science Study, Biomechanics.