Increasing reverse total shoulder liner constraint in clinically planned cases causes decreased simulated range of motion and LIMIT, a novel phenomenon.

Background: Optimization of reverse total shoulder arthroplasty (rTSA) component design and positioning can improve outcomes. Polyethylene liner constraint (depth/radius) influences prosthetic stability and simulated shoulder range of motion (sROM). Increased constraint may reduce sROM due to earlier polyethylene-to-bone contact. Given that Minimal Clinically Important Differences in shoulder sROM can be small (external rotation = 3°, abduction = 7°, flexion = 12°), even subtle changes may meaningfully impact outcomes. The purpose of this study was to characterize simulated virtual passive sROM in a series of clinically planned rTSA cases while varying liner constraint. Methods: Clinically planned and performed rTSA cases by a single surgeon (R.U.H.) between August 2020 and September 2024 were analyzed using Blueprint software. For each case, polyethylene liner constraint was varied (45%, 55%, and 65%) while all other implant parameters were held constant. Cases indicated for diagnoses without significant osteophytes (massive rotator cuff tear, avascular necrosis, or rheumatoid arthritis), as well as rotator cuff tear arthropathy cases without osteophytes, were included. For each simulation, the mechanism responsible for impingement at each end-sROM was visually determined and recorded. The primary outcome was simulated passive sROM in 6 planes. Post hoc multiple comparisons were performed using the Tukey procedure. Results: Seventy-three cases were included in the final analysis. Polyethylene-to-bone impingement occurred most frequently during simulated internal rotation, adduction, and external rotation. Increasing liner constraint from 45% to 65% significantly reduced sROM in internal rotation, external rotation, adduction, and flexion (Δ = 25.0°, 23.3°, 13.8°, 12.2°, respectively). Smaller interval increases in constraint (45% to 55% and 55% to 65%) also resulted in significant reductions in sROM across multiple planes. In 50 cases, increasing constraint resulted in conversion of the impingement mechanism to polyethylene-to-bone contact. This transition was associated with reductions in sROM of up to 177°, with a mean reduction of 58.7°. Discussion: sROM was most frequently affected, and to a greater magnitude, in adduction, external rotation, and internal rotation. Increasing constraint resulted in a distinct mechanical phenomenon, which we termed "LIMIT (Liner-Induced Mechanical Impingement Transition)." LIMIT describes the conversion of impingement type to polyethylene-to-bone as constraint increases. This occurred in 50 cases and was associated with substantial reductions in sROM, particularly in abduction, flexion, and extension. Increased constraint may contribute to scapular notching through heightened impingement during external rotation, internal rotation, and adduction. Importantly, reductions in sROM exceeded the Minimal Clinically Important Difference in external rotation and flexion, suggesting clinical relevance.