Static and dynamic internal ulnar rotational stability in collegiate baseball pitchers

The primary soft tissue stabilizer of valgus stress across the elbow is the anterior oblique ligament. The posterior oblique ligament (POL) is a stabilizer of internal rotational stress. This study aimed to identify whether POL morphologic changes affect the ulnar internal rotational laxity of the elbow, and whether the elbow flexors restrain it. Fifty-three collegiate-level pitchers were recruited in this cross-sectional study. For ultrasound assessment, pitchers were positioned supine with 90° shoulder abduction, 90° elbow flexion, and the forearm in a pronated position. POL morphologic changes were evaluated, and the posteromedial ulnohumeral joint distance under the POL was measured at rest, under gravity-induced valgus stress, and under gravity-induced valgus stress with brachial flexor muscle contraction. A linear mixed-effects model was used to examine the effects of the group (abnormal vs. normal POL) and condition (elbow valgus [EV] vs. EV with brachial flexor muscle contraction). The prevalence of POL morphologic changes in all pitchers was 34% (18/53 patients). The abnormal POL group showed a significantly larger joint gap than the normal POL group under EV (difference: +0.63 mm, 95% confidence interval [CI]: [0.14, 1.12], P = .012). This between-group difference was not observed in EV with brachial flexor muscle contraction (difference: -0.05 mm, 95% CI: [-0.54, 0.44], P = .85). This study demonstrated that morphologic abnormalities of the POL are associated with increased ulnar internal rotational laxity of the elbow in asymptomatic collegiate baseball pitchers. In addition, activation of the elbow flexor muscles reduced posteromedial joint gapping under valgus loading, suggesting a compensatory role of dynamic muscular stabilization in POL-related rotatory instability.