AJSM Webinar Series: Posterior Shoulder Instability: Understanding the Dark Side of the Shoulder-Biomechanical Evaluation of Posterior Shoulder Instability With a Clinically Relevant Posterior Glenoid Bone Loss Model

Biomechanical Evaluation of Posterior Shoulder Instability With a Clinically Relevant Posterior Glenoid Bone Loss Model

Pdf Summary

This study by Waltz et al. evaluates the biomechanical effectiveness of posterior labral repair in cadaveric shoulders with varying degrees of clinically relevant posterior glenoid bone loss, using a novel robotic testing model permitting multiple at-risk arm positions. Posterior shoulder instability, less common but clinically important, often occurs during activities placing the shoulder in forward flexion and internal rotation. The role of posterior glenoid bone loss in instability and repair outcomes is not well defined, unlike anterior bone loss.<br /><br />Ten fresh-frozen cadaveric shoulders were sequentially tested in seven states: native, posterior labral tear, labral repair, and these conditions combined with small (~7%) and large (~28%) posterior glenoid bone loss created using 3D-printed CT-based templates. Biomechanical testing applied 75 N posterior-inferior force and compression at 60° and 90° of flexion and scaption, measuring posterior-inferior translation, lateral translation, and peak dislocation force.<br /><br />Key findings include: <br /><br />- Posterior labral repair significantly increased shoulder stability by raising peak dislocation force by 10.1 to 14.8 N and decreased posterior-inferior translation by 1.0 to 2.3 mm regardless of bone loss or arm position.<br /><br />- Small bone loss (~7%) led to a significant decrease in dislocation force but labral repair largely restored stability close to native levels.<br /><br />- Large bone loss (~28%) drastically reduced stability, and labral repair alone could not restore it to native levels.<br /><br />- The shoulder was most unstable at 60° scaption in the native state.<br /><br />Clinically, these results suggest that while posterior labral repair is effective in restoring stability in cases with small to moderate posterior bone loss, large bone defects (>25%) may necessitate bony augmentation for adequate joint stability. This biomechanical study uses a realistic posterior glenoid bone loss model and variable arm positions to better simulate clinical scenarios and guide surgical decision-making in posterior shoulder instability treatment. Limitations include lack of dynamic muscle activation and the use of cadaveric specimens.

Keywords

posterior labral repair

posterior shoulder instability

posterior glenoid bone loss

cadaveric shoulders

biomechanical testing

robotic testing model

shoulder stability

dislocation force

arm position effect

bony augmentation