The active and passive kinematic difference between primary reverse and total shoulder prostheses.

Background: Reverse shoulder arthroplasty (RSA) and total shoulder arthroplasty (TSA) effectively decrease pain and improve clinical outcome. However, indications and biomechanical properties vary greatly. Our aim was to analyze both active and passive shoulder motion (thoracohumeral [TH], glenohumeral [GH], and scapulothoracic [ST]) and determine the kinematic differences between RSAs and TSAs.

Methods: During 3 range-of-motion (ROM) tasks (forward flexion, abduction, and axial rotation), the motion patterns of 16 RSA patients (19 shoulders), with a mean age of 69 ± 8 years (range, 58-84 years), and 17 TSA patients (20 shoulders), with a mean age of 72 ± 10 years (range, 53-87 years), were measured. The mean length of follow-up was 22 ± 10 months (range, 6-41 months) for RSA patients and 33 ± 18 months (range, 12-87 months) for TSA patients. Kinematic measurements were performed with a 3-dimensional electromagnetic tracking device.

Results: All patients showed better passive than active ROM. This difference was significantly larger for RSA patients than for TSA patients (TH in sagittal plane, 20° vs 8° [P = .001]; GH in sagittal plane, 16° vs 7° [P = .003]; TH in scapular plane, 15° vs 2° [P < .001]; GH in scapular plane, 12° vs 0° [P < .001]; and ST in scapular plane, 3° vs -2° [P = .032]). This finding also showed that in the scapular plane, TSA patients showed hardly any difference between active and passive ROM. Furthermore, TSA patients had 16° to 17° larger active TH motion, 15° larger active GH motion, and 8° larger active ST motion compared with RSA patients. The GH-ST ratios showed similar figures for both types of prostheses.

Conclusion: TSA patients have larger active TH motion because in the scapular plane, they completely use the possible GH motion provided by the prosthetic design. This larger active ROM in TSA patients only applies for elevation and abduction, not for axial rotation or passive ROMs.