The Impact of Posterior Wear on Reverse Shoulder Glenoid Fixation

Achieving glenoid fixation with posterior bone loss can be challenging. The purpose of this study was to quantify the impact of two different sizes of posterior glenoid defects (10° and 20°) on reverse shoulder arthroplasty (rTSA) glenoid baseplate fixation and determine if utilizing different sizes of posterior augmented baseplates (8° and 16°) with off-axis reaming provides comparable fixation to using a standard baseplate with different amounts of eccentric reaming.

We quantified the impact of 10° and 20° posterior glenoid defects on rTSA baseplate fixation in composite scapulae using the ASTM F2028-14 rTSA glenoid loosening test method. Forty-two total implants (N = 7 for each size defect and for each type of baseplate) were tested at 750 N for 10,000 cycles. Baseplate displacement was measured before and after cyclic loading in the superior-inferior and anterior-posterior directions. Statistical analysis was performed with a two-tailed unpaired Student's t-test (significance defined as p < 0.05) to compare prosthesis displacements relative to each scapula (10° and 20° posterior defects for each type of baseplate versus the non-defect control) before and after cyclic loading.

All glenoid baseplates remained well-fixed after cyclic loading in composite scapulae without a defect and in scapulae with posterior defects. Increased pre- and post-cyclic displacement was observed with increased posterior defect size and differences in displacement were observed between standard and augmented baseplates. Augmented baseplates were observed to remove significantly less bone than standard baseplates when correcting posterior defects, regardless of size.

Both standard baseplates with eccentric reaming and two different sizes of augmented baseplates with off-axis reaming successfully maintained fixation following cyclic loading in composite scapula with corrected 10° and 20° posterior glenoid defects. Augmented glenoids may be more advantageous long-term from a fixation perspective as they preserve more subchondral glenoid bone due to the minimal reaming occurring by the off-axis method. Mid and long-term clinical follow-up comparisons of outcomes are necessary between these two techniques.