Impact of constrained humeral liner on impingement-free range of motion and impingement type in reverse shoulder arthroplasty using a computer simulation.

BACKGROUND: Dislocation is a major complication of reverse total shoulder arthroplasty (RSA). Humeral liner may be changed to the constrained type when stability doesn't improve by increasing glenosphere size or lateralization with implants, and patients, particularly women with obesity, have risks of periprosthetic instability that may be secondary to hinge adduction on the thorax, but there are few reports on its impact on the range of motion (ROM). This study aimed to determine the influence of humeral liner constraint on impingement-free ROM and impingement type utilizing an RSA computer simulation model.

METHODS: A virtual simulation model was created using 3D measurement software for conducting a simulation study. This study included twenty-five patients with rotator cuff tears and rotator cuff tear arthropathy. Impingement-free ROM and impingement patterns were measured during flexion, extension, abduction, adduction, external rotation, and internal rotation. Four different simulations of 2×2 were performed in each of the 25 cases, a total of 100 simulations: Glenosphere (38mm normal type vs. lateralized type) and humeral liner constraint (normal type vs. constrained type). There were four combinations: normal glenosphere and normal humeral liner, normal glenosphere and constrained humeral liner, lateralized glenosphere and normal humeral liner, and lateralized glenosphere and constrained humeral liner.

RESULTS: Significant differences were found in all impingement-free ROM in one-way ANOVA (abduction: p=0.01, adduction: p<0.01, flexion: p=0.01, extension: p=0.02, external rotation: p<0.01, and internal rotation: p<0.01). Tukey's post-hoc tests showed that the impingement-free ROM was reduced during abduction, external rotation, and internal rotation with the combination of normal glenosphere and constrained humeral liner compared to the other combinations, and improved by glenoid lateralization compared to the combination of the lateralized glenosphere and constrained humeral liner. In the impingement pattern, Pearson's chi-squared test showed significantly greater proportion of impingement of the humeral liner into the superior part of the glenoid neck in abduction occurring in the combination of normal glenosphere and constrained humeral liner group compared to the other groups (p<0.01). Bonferroni post-hoc tests revealed that the combination of normal glenosphere and constrained humeral liner was significantly different from that of lateralized glenosphere and constrained humeral liner (p<0.01). Using constrained liners resulted in early impingement on the superior part of the glenoid neck in the normal glenosphere, whereas glenoid lateralization increased impingement-free ROM.

CONCLUSION: This RSA computer simulation model demonstrated that constrained humeral liners led to decreased impingement-free ROM. However, using lateralized glenosphere improved abduction ROM.