Biomechanical impact of posterior glenoid wear on anatomic total shoulder arthroplasty.

Introduction: This study quantified bone removed to correct three different sizes of posterior glenoid defects and also quantified the change in rotator cuff muscle length resulting from correction of each defect using three different glenoid designs.

Methods: A 3-D computer model quantified the cortical and cancellous bone removed when correcting three sizes of posterior glenoid defects and simulated internal/external rotation to quantify changes in rotator cuff muscle length when correcting glenoid retroversion in three sizes of posterior glenoid defects using three different glenoid prostheses: 1. eccentric reaming using a non-augmented glenoid (Equinoxe standard pegged), 2. 8°, 12°, and 16° Equinoxe posterior augment glenoid (wedge), and 3. 3 mm, 5 mm, and 7 mm Global Step-Tech posterior augment glenoid (step).

Results: For small defects, the 8° wedge and 3 mm step posterior augment glenoids conserves 50% (1.295 cm 3 ) and 23% (1.704 cm 3 ) more bone than eccentric reaming (2.147 cm 3 ), respectively. For medium defects, the 12° wedge and 5 mm step glenoids conserves 69% more (1.295 cm 3 ) and 2% less (2.720 cm 3 ) bone than eccentric reaming (2.655 cm 3 ), respectively. For large defects, the 16° wedge and 7 mm step glenoids conserve 48% more (1.852 cm 3 ) and 36% less (4.343 cm 3 ) bone than eccentric reaming (3.736 cm 3 ), respectively. For each size defect, muscle shortening was observed for both eccentric reaming and each augmented glenoid design. Eccentric reaming medialized the humerus and resulted in additional muscle shortening (wedge: 2.0%, 2.9%, and 3.6%; step: 1.2%, 1.7%, and 1.7%) in each size defect, respectively.

Discussion And Conclusions: Both step and wedge augmented glenoid designs conserved more anterior glenoid bone and were associated with less muscle shortening than correction with eccentric reaming. However, wedge posterior augment glenoids medialized the humerus less and were observed to be more bone conserving than step posterior augment glenoids, particular in large glenoid defects.