Abstract
Background:
Little is known about scapular kinematics in patients with reverse total shoulder arthroplasty (RTSA). Understanding how RTSA affects shoulder function may help refine its design, use, and rehabilitation strategies. The purpose of this study was to quantify motion in the reverse shoulder. The scapulohumeral rhythm (SHR) of the RTSA shoulder was calculated using 3d-2d image registration techniques. SHR was compared to normal subjects in literature to asses kinematic changes post RTSA.
Methods:
26 subjects were recruited for an institutional review board approved study. Subjects who were ≥ 6 months post unilateral RTSA. Subjects were prompted to do abduction in the coronal plane with and without a 3 lb. weight. Three dimensional to two dimensional image registration techniques were used to derive orientation and position measurements for the humerus and scapula from dynamic x-ray. Tukey Honest differences statistics were used to assess significance differences between groups.
Conclusion:
SHR in RTSA subjects was found to be significantly lower than normal subjects found in the literature. Below 30° of abduction RTSA SHR was found to be highly variable from 17: 1–2: 1. Due to the variability results were represented with the range of 30° to max elevation. Above 30° RTSA SHR was found to average to be 1.8: 1 for un-weighted abduction and 1.2 for weighted abduction (Figures 1, 2). RTSA max range of motion (ROM) during weighted trials was significantly lower than un-weighted trials. RTSA SHR standard deviation decreased with increasing elevation.
Discussion:
RTSA subjects have significantly modified kinematics from normal subjects. In general RTSA SHR (1.8: 1) was significantly lower than normal (4: 1) subjects found in literature. In mid-range the normal and RTSA SHR is the same (Figure 1). The RTSA SHR is significantly different in lower and higher angles (Figures 1, 2). There is more glenohumeral rhythm in the normal group versus the RTSA group. More scapulothoracic motion was observed in the RTSA group at high levels and low levels of arm elevation. With this understanding of the motion of the reverse shoulder improvements can be made to the design and configuration of the implant to optimize RTSA outcomes.
