The Effect of Sequentially Increased Polyethylene Constraint on Impingement in Reverse Shoulder Arthroplasty: A Biomechanical Investigation.

Background: The constraint of the polyethylene liner in reverse total shoulder arthroplasty (rTSA) can affect glenohumeral joint stability. However, its influence on glenohumeral range of motion (ROM) remains unclear. Therefore, it was the objective of this study to determine the effect of sequentially increasing polyethylene liner constraint on impingement-free ROM following rTSA. Additionally, these sequentially increasing constraint designs were evaluated with various humeral neck shaft angles (NSAs), glenosphere diameters, and a new variable termed the polyethylene rim width.

Methods: Twenty upper extremity cadavers were computed tomography scanned and manually segmented to developed three-dimensional models of the scapula and humerus. Each model was then virtually implanted with a generic rTSA implant. Nine different polyethylene constraint ratios (defined as the ratio between the polyethylene depth and polyethylene radius; with values ranging from 0.35-0.75 in 0.05 increments) were assessed, along with three NSAs (135°, 145°, and 155°), three glenosphere diameters (36mm, 39mm, and 42mm), and four polyethylene rim widths (1mm, 2mm, 3mm, and 4mm). This resulted in 108 different polyethylene designs and 324 different rTSA designs. All virtually implanted bone models were imported as rigid bodies into a custom motion software, in which six standard motions (abduction, adduction, forward elevation, extension, internal rotation, and external rotation) were conducted, followed by the assessment of global circumduction ROM. Impingement during each motion was automatically detected. The maximum impingement-free ROM for each implant configuration and motion pathway were then quantified and statistically assessed.

Results: Polyethylene constraint, polyethylene rim width, NSA, and glenosphere diameter were all found to significantly affect impingement-free ROM for all motions simulated (P<0.001). Increases in polyethylene constraint and rim width were found to significantly reduce impingement-free ROM (P<0.001). A 135° NSA with a 42mm glenosphere combination were found to maximize ROM during extension, adduction, internal rotation, external rotation, and global circumduction motion, while a 155° NSA with a 36mm glenosphere combination resulted in optimized abduction and forward elevation ROM.

Discussion: Increases in polyethylene constraint were found to significantly reduce impingement-free ROM for all motions evaluated. However, polyethylene constraint had the greatest impact on glenohumeral extension, adduction, internal rotation, external rotation, and global circumduction. Polyethylene rim width was also found to significantly affect impingement-free ROM for all motions. Further study is needed to determine the optimal value of polyethylene liner constraint in rTSA due to its impact on rTSA biomechanics, joint stability and ROM.