Muscle activation patterns during active external rotation after reverse total shoulder arthroplasty: an electrophysiological study of the teres minor and associated musculature

BACKGROUND

Preoperative teres minor insufficiency has been identified as a risk factor for poor restoration of external rotation (ER) after reverse total shoulder arthroplasty (RTSA). However, there has been little investigation regarding muscle activation patterns generating ER. This prospective study sought to determine the timing and activation levels of the shoulder girdle musculature during ER in well-functioning RTSAs with an intact teres minor using a lateralized design.

METHODS

Patients who underwent RTSA ≥1 year previously with functional ER, an American Shoulder and Elbow Surgeons (ASES) score >70, superior rotator cuff deficiency, and an intact teres minor were identified. Electrophysiological and kinematic analyses were performed during ER in the modified neutral position (arm at side with 90° of elbow flexion) and in abduction (AB) (shoulder abducted 90° with 90° of elbow flexion). Dynamometer-recorded torque and position were pattern matched to electromyography during ER. The root-mean-square and integrated electromyography (in microvolts × milliseconds with standard deviation [SD]), as well as median frequency (MF) (in hertz with SD), were calculated to determine muscle recruitment. Pair-wise t test analysis compared muscle activation (P < .05 indicated significance).

RESULTS

After an a priori power analysis, 16 patients were recruited. The average ASES score, visual analog scale pain score, and ASES subscore for ER in AB ("comb hair") were 87.7, 0.5, and 2.75 of 3, respectively. In AB, muscle activation began with the upper trapezius, middle trapezius, and latissimus dorsi, followed by the anterior deltoid activating to neutral. With ER beyond neutral, the teres major (9.6 μV × ms; SD, 9.2 μV × ms) initiated ER, followed by the teres minor (14.1 μV × ms; SD, 18.2 μV × ms) and posterior deltoid (11.1 μV × ms; SD, 9.3 μV × ms). MF analysis indicated equal contributions of the teres major (1.1 Hz; SD, 0.5 Hz), teres minor (1.2 Hz; SD, 0.4 Hz), and posterior deltoid (1.1 Hz; SD, 0.4 Hz) in ER beyond neutral. In the modified neutral position, the upper trapezius and middle trapezius were not recruited to the same level as in AB. For ER beyond neutral, the teres major (9.5 μV × ms [SD, 9 μV × ms]; MF, 1.1 Hz [SD, 0.5 Hz]), teres minor (11.4 μV × ms [SD, 15.1 μV × ms]; MF, 1.1 Hz [SD, 0.5 Hz]), and posterior deltoid (8.5 μV × ms [SD, 8 μV × ms]; MF, 1.2 Hz [SD, 0.3 Hz]) were activated in similar sequence and intensity as AB. No differences in muscle activation duration or intensity were noted among the teres major, teres minor, and posterior deltoid (P > .05).

CONCLUSION

Active ER after RTSA is complex and is not governed by a single muscle-tendon unit. This study establishes a sequence, duration, and intensity of muscle activation for ER in well-functioning RTSAs. In both tested positions, the teres major, teres minor, and posterior deltoid function equally and sequentially to power ER.

Superior humeral head osteophytes are associated with rotator cuff insufficiency in glenohumeral osteoarthritis: a retrospective analysis

PURPOSE

The primary goal of this study was to investigate whether superior humeral head osteophyte (SHO) size is associated with rotator cuff insufficiency, including rotator cuff tear (RCT), supraspinatus tendon thickness, and fatty infiltration of the rotator cuff muscles.

METHODS

Patients  ≥ 18 years who were diagnosed with glenohumeral osteoarthritis were retrospectively reviewed. SHO size was determined by radiograph. MRI measured SHO and RCT presence, type, and size; supraspinatus tendon thickness; and fatty infiltration of rotator cuff musculature.

RESULTS

A total of 461 patients were included. Mean SHO size was 1.93 mm on radiographs and 2.13 mm on MRI. Risk ratio for a RCT was 1.14. For each 1-mm increase in SHO size on radiograph, supraspinatus tendon thickness decreased by 0.20 mm. SHO presence was associated with moderate-to-severe fatty infiltration of the supraspinatus with a risk ratio of 3.16.

CONCLUSION

SHOs were not associated with RCT but were associated with higher risk of supraspinatus FI and decreased tendon thickness, which could indicate rotator cuff insufficiency.

LEVEL OF EVIDENCE

IV.