External Rotation Strength After TSA in Osteoarthritic Shoulders with Eccentric Deformity Is Not Impacted by Posterior Rotator Cuff Deficiency

Abstract

Background: Patients with persistent glenohumeral osteoarthritis symptoms despite nonoperative management may pursue anatomic total shoulder arthroplasty (TSA). TSA revision rates are higher in patients with preoperative eccentric (asymmetric posterior erosion) compared with concentric (symmetric) glenoid deformity. If posterior rotator cuff deficiency demonstrated preoperatively in patients with eccentric deformity persists after TSA, it may manifest as relative weakness in external compared with internal rotation secondary to deficient activity of the shoulder external rotator muscles. Persistent posterior rotator cuff deficiency is hypothesized to contribute to TSA failures. However, it remains unknown whether rotational strength is impaired after TSA in patients with eccentric deformity. Our goal was to determine if patients with eccentric deformity exhibit relative external rotation weakness that may be explained by posterior rotator cuff deficiency after TSA.

Methods: Patients who were >1 year after TSA for primary glenohumeral osteoarthritis and had had preoperative eccentric or concentric deformity were prospectively recruited. Torque was measured and electromyography was performed during maximal isometric contractions in 26 three-dimensional direction combinations. Relative strength in opposing directions (strength balance) and muscle activity of 6 shoulder rotators were compared between groups.

Results: The internal (+) and external (-) rotation component of strength balance did not differ in patients with eccentric (mean internal-external rotation component of strength balance: -7.6% ± 7.4%) compared with concentric deformity (-10.3% ± 6.8%) (mean difference: 2.7% [95% confidence interval (CI), -1.3% to 6.7%]; p = 0.59), suggesting no relative external rotation weakness. Infraspinatus activity was reduced in patients with eccentric (43.9% ± 10.4% of maximum voluntary contraction [MVC]) compared with concentric (51.3% ± 10.4% of MVC) deformity (mean difference: -7.4% [95% CI, -13.4% to -1.4%] of MVC; p = 0.04).

Conclusions: A relative external rotation strength deficit following TSA was not found, despite evidence of reduced infraspinatus activity, in the eccentric-deformity group. Reduced infraspinatus activity suggests that posterior rotator cuff deficiencies may persist following TSA in patients with eccentric deformities. Longitudinal study is necessary to evaluate muscle imbalance as a contributor to higher TSA failure rates.

Level of evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Fig. 1

Experimental setup. Participants were seated with the trunk secured by straps while they performed maximal isometric contractions. The arm was fixed to a 6-degrees-of-freedom load cell via a premade fiberglass cast.

Fig. 2

Quantification of strength magnitude (SM) and 3-dimensional (3D) strength balance. Fig. 2-A Sample trajectory of the torque generated during a trial involving combined flexion and abduction, demonstrating the maximum torque (black dot) that was achieved in the target direction (red dotted channel). Fig. 2-B The points represent the maximal torque achieved in the 26 directions tested. Fig. 2-C Our measure of overall 3D strength (SM) was computed by first performing a principal components analysis of the 26 maxima, which yielded 3 principal axes defining the 3D space of achievable torques. The Euclidian norm of the 3 principal axis magnitudes represented the patient’s overall SM (in Nm) across all 26 directions. The weight-normalized overall SM (in Nm/kg) was then calculated by dividing by the patient’s weight. Fig. 2-D Our measure of relative strength in opposing directions (strength balance) was determined by computing the 3D center of the torque space, by first taking the vector mean of the patient’s 26 maximum torques. This vector mean was then normalized by the patient’s overall SM to obtain the 3D strength balance (% of unnormalized SM] along the 3 axes of interest). Strength balance along the internal-external rotation axis was of primary interest.

Fig. 3

Three-dimensional strength balance for each participant (smaller dots) and group means (larger dots), which did not differ between the eccentric and concentric groups. Both TSA groups demonstrated weakness in internal relative to external rotation compared with control participants. Shaded ellipses represent the 95% confidence intervals of the group means, and 2-dimensional projections of these intervals (onto the ADD/ABD-IR/ER and ADD/ABD-FLEX/EXT planes) are also shown. ADD/ABD = adduction/abduction; IR/ER = internal/external rotation; FLEX/EXT = flexion/extension.

Fig. 4

The mean weight-normalized strength magnitude, and 95% CI, for each group. After TSA, there was no difference between the patients with preoperative eccentric and concentric deformity, but the magnitude was reduced by at least 17% in both groups compared with control participants.

Fig. 5

Figs. 5-A and 5-B Group results for the mean muscle activity, and 95% CI, of the internal and external rotators based on linear mixed-effects models. Muscle activity was compared between the preoperative deformity groups across all 9 directions involving internal rotation for the internal rotators (Fig. 5-A), and across all 9 directions involving external rotation for the external rotators (Fig. 5-B). The only difference according to preoperative deformity was reduced muscle activity in the infraspinatus in the eccentric group. Figs. 5-C and 5-D Raw (un-modeled) group data for muscle activity of the teres major, an internal rotator, and the infraspinatus, an external rotator, across all 26 directions tested. Three 2D slices for each muscle demonstrate the direction combinations involving 1 or 2 directions. At the far right, the combinations involving 3 directions are shown. For the teres major, the directions involving internal rotation are designated by dashed circles or ovals. For the infraspinatus, the directions involving external rotation are designated by dashed circles or ovals. Lat = latissimus, Pec = pectoralis, MVC = maximum voluntary contraction, ADD/ABD = adduction/abduction, IR/ER = internal/external rotation, FLEX/EXT = flexion/extension.