Glenoid component cyclical failure decreases with increasing baseplate contact: a biomechanical study

Background

Glenoid baseplate loosening remains a common mode of failure in reverse shoulder arthroplasty. One of the key factors to baseplate stability is theorized to be maximization of baseplate backside contact. The purpose of this biomechanical study is to investigate the role of varying degrees of backside bony glenoid support in component stability for reverse total shoulder arthroplasty.

Methods

Twenty synthetic scapular models were divided into 3 test groups of 5 scapulae with glenoid baseplate contacts of 40%, 60%, and 75%, and one control group with glenoid baseplate contact of 100%. Standardized application of a commercially available glenoid baseplate and glenosphere was performed. The scapulae were mounted on a linear bearing with a humeral component and polyethylene liner which were affixed to a biaxial servohydraulic fatigue testing system. Each specimen was loaded for 10,000 cycles or to failure, about a 55° arc along the glenosphere at a rate of 1 Hz as a 750 N compression load was applied. Failure was defined as fracture of the scapula with implant fixation compromise. Before and after loading, stability of the baseplate was assessed by quantifying the total motion between the model and the baseplate with digital calipers as a ramp load between 0 and 150 N was applied. Two-sample unpaired t-tests were performed with significance set at P < .05.

Results

Baseplate contacts of 40% (1623 ± 227, P = .0001), 60% (3299 ± 1170, P = .0001), and 75% (5615 ± 1587, P = .0077) demonstrated statistically significant decrease in the average number of cycles to failure in all cohorts compared to our control (8641 ± 1070). Cycles taken for initial cracks to progress to failure showed no significant differences; 40% contact (862 ± 452, P = .4751), 60% contact (1651 ± 996, P = .4318), 75% contact (2882 ± 1347, P = .0620), and 100% control (1166 ± 657). Baseplate contacts of 40% (6150.4 ± 444.0, P = .0006), 60% (4647.1 ± 552.3, P = .0072), and 75% (2927.8 ± 918.5, P = .2573) demonstrated increasing micromotion (pre-post cyclical loading) in all cohorts compared to our control (2074.7 ± 1164.6) with statistical significance at 40% and 60%.

Conclusion

These biomechanical tests demonstrate that decreasing glenoid baseplate backside contact leads to increased micromotion and fewer cycles to failure. This supports the surgical goal of achieving maximal glenoid baseplate backside contact, suggesting that decreased glenoid baseplate support could contribute to significant loosening.