2025 Basic Science Neer Award Winner: The impact of posterior and posterior superior labral injuries and the effect of their treatment on glenohumeral kinematics in the deceleration and follow-through phase of throwing: a biomechanical study

Abstract

Background: Combined posterior and posterior-superior (PPS) injuries (6:30-12:00; right shoulder) have a much higher incidence than isolated superior labral anterior to posterior (SLAP) injuries (11:00-1:00; right shoulder). However, it is unclear how PPS injuries affect glenohumeral (GH) biomechanics and how they should be treated. This study evaluated if (1) PPS injuries alter GH kinematics and joint characteristics, (2) standard repair techniques restore GH kinematics and contact characteristics toward their intact state, and (3) there are differences between repair techniques.

Methods: Ten fresh frozen male cadaveric shoulders (61.4 ± 5.7 years) without pre-existing shoulder pathology were evaluated using a custom shoulder testing system. After a PPS injury was created, a posterior repair technique reattached the labrum from 6:30 to 10:00, whereas a subsequent posterior-superior repair technique included an additional anchor at 11:00. The shoulder was placed in 90° of abduction and tested in the scapular plane at 10°, 20°, 30°, and 40° of horizontal adduction in the scapular plane at both 30° and 60° of external rotation using a deceleration load and 2 posteriorly directed unbalanced muscle loading conditions. These positions were chosen as they would occur during the deceleration and follow-through phases of pitching. All measurements were performed for the following conditions: intact, PPS injury, posterior repair, and posterior-superior repair. Humeral head (HH) position, GH translation, and GH contact area and pressure were measured.

Results: PPS injuries alter GH kinematics and GH joint contact characteristics. There was significant posterior translation of the HH in multiple positions of horizontal adduction in the scapular plane and GH rotation in both muscle imbalance conditions, significant decreases in joint contact area, and increases in joint contact pressure. Repair of PPS injuries by both repair techniques resulted in HH anterior and superior translation toward the intact state in multiple positions of horizontal adduction and GH rotation, increased joint contact area, and decreased joint contact pressure. Compared with posterior repairs, posterior-superior repairs resulted in significantly increased nonphysiologic anterior and inferior HH translation in relation to the intact state.

Conclusion: The PPS injury produces alterations in GH kinematics with implications for GH joint instability, increased GH joint loading, and potential joint damage. The posterior labral repair restores HH kinematics closer to intact, whereas the addition of a posterior-superior repair induces significantly increased nonphysiologic anterior and inferior HH translation at multiple positions of adduction and IR.

Keywords: Pitcher; deceleration; follow-through; kinematics; labrum; posterior; posterior-superior; repair.