Are Shoulders with A Reverse Shoulder Prosthesis Strong Enough? A Pilot Study

Abstract

BackgroundIt has been suggested that limited active ROM of reverse shoulder prostheses relates to lack of strength. However, the postoperative strength has not been quantified.Questions/purposesWe therefore measured joint torques in patients with reverse shoulder prostheses and correlated torques with functional scores.MethodsWe recruited 33 patients (age, 72 ± 8 years) with a reverse prosthesis (37 shoulders, 21 primary and 16 revisions). We obtained Constant-Murley, DASH, and Simple Shoulder Test ([D]SST) scores, and performed two isokinetic protocols (abduction/adduction and external/internal rotation) at 60° per second. Minimum followup was 4 months (average, 23 months; range, 4–63 months).ResultsTwenty-three patients (24 shoulders; 13 primaries, 11 revisions) were able to perform at least one of the defined tasks. Mean abduction and adduction torques were 15 Nm ± 7 Nm and 16 Nm ± 10 Nm (19%–78% of normal shoulders). External and internal rotation tasks could be performed by only 13 patients (14 shoulders; nine primary, five revisions) generating 9 Nm ± 4 Nm and 8 Nm ± 3 Nm, respectively (13%–71% of normal shoulders). We found moderate correlations between Constant-Murley, DASH and (D)SST (D = Dutch translation) scores and abduction and external rotation.ConclusionsPatients with a reverse prosthesis had reduced strength when compared with normal values reported in the literature (only 65% of patients could perform the protocol). This effect was greatest for external rotation and might explain clinical outcomes with which a moderately strong relationship was observed. Our observations suggest limited strength is a major factor in reduced ROM.

Isokinetic strength differences between patients with primary reverse and total shoulder prostheses

“…Because most functional activities are dynamic, evaluating isokinetic shoulder strength may be more appropriate when relating strength to functional performance and clinical outcome. In 2012, isokinetic strength measurements after RSA have been described (Alta et al, 2012), but similar data have not yet been reported for TSA patients.…”

“…It is not completely clear if this difference is only caused by the type of prosthesis, or also by the (isokinetic) force generating capacity or a combination of both. Previous research (Bergmann et al, 2008) suggested that the limited glenohumeral motion of the RSA seems to be the result of a lack of joint torque generation, with moderate correlations between post-operative clinical outcome scores (Constant-Murley, DASH and SST) and the abduction and external rotation torques (Alta et al, 2012). Therefore, the evaluation of isokinetic shoulder strength after RSA in comparison to TSA patients may be of interest in modeling dynamic upper extremity function and may supply more specific information on the influence of the type of prosthesis on the force generating capacity, particularly where comparative data are only limited available (Puskas et al, 2013) for those two clinical scenarios.…”

“…Second, we limited our measurements to 60°/s. Isokinetic strength measurements have been performed at different velocities, mostly from 60°/s to 300°/s (Alta et al, 2012;Codine et al, 2005) with some exceptions at 30°/s (McMaster et al, 1991(McMaster et al, , 1992. In these measurements, the applied torque needs to increase above a threshold value to successfully perform a certain task at higher speeds.…”

“…To measure shoulder strength, we used the same measurement protocol as was used in our earlier paper (Alta et al, 2012). An isokinetic dynamometer provides constant velocity with accommodating resistance throughout a joint's RoM.…”

“…Bergmann et al (2008) previously described a better passive than active RoM in patients with a RSA and presumed that the limited glenohumeral motion resulted from a lack of joint torque generation rather than a structural limitation caused by the prosthetic design. We recently demonstrated a correlation between the postoperative Constant-Murley score and the abduction and external rotation torques in RSA patients (Alta et al, 2012). Similar data for TSA patients are not available and the comparison between RSA and TSA patients has not yet been described.…”