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This was a prospective study. All experimental procedures were approved by the Institutional Review Board. Informed consent was obtained from all patients.
From November 2015 to October 2017, we recruited 32 patients scheduled to undergo RSA after no response to conservative treatment for at least 6 months.
Patients were included if they consented to dynamic EMG measurement of the shoulder muscles before surgery and underwent primary RSA with a diagnosis of CTA or severe degenerative OA with a large rotator cuff tear using a reverse type prosthesis (Exactech, Gainesville, FL, USA). The exclusion criteria were any neurological deficit, such as axillary nerve palsy, cervical radiculopathy, or myelopathy; previous infection around the shoulder; a systemic disorder such as diabetes mellitus, rheumatoid arthritis, or thyroid dysfunction; and revision arthroplasty. Ultimately, 21 patients with CTA and 4 patients with OA with large rotator cuff tears were included. There were 20 women (80%). The mean age of the participants was 73 ± 6 years, their mean height was 153.8 ± 12.1 cm, and their mean weight was 55.6 ± 8.4 kg. The average follow-up period was 14 ± 3 months.
A single surgeon performed all RSAs. The glenohumeral joint was approached through the deltopectoral plane. The glenoid component was fixed with superior inclination of 10° and 0° retroversion and the humeral stem was fixed with 20°
retroversion.24 All surgeries were done without repairing the subscapularis, while the infraspinatus tear was repaired when possible.
All patients underwent the following tests before and after surgery as outpatients. Patients were asked to rate the intensity of their worst pain on a visual analogue scale from 0 (none) to 10 (maximal).15 Active ROM was measured using a goniometer with a long arm, including forward flexion, abduction, and external rotation. Active internal rotation was measured as the highest spine level that the patient could reach. Strength was measured at 90° of abduction using a handheld di
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gital dynamometer (JTECH Commander, UT, USA). However, in patients with active forward flexion ROM < 90° due to pseudoparalysis (n =13), muscle strength was measured at the maximum abduction angle. The Constant score and American Shoulder and Elbow Surgeons (ASES) Standardized Shoulder Assessment Form were used for functional evaluations.1,16
The day before surgery, dynamic EMG measurements of the deltoid and upper trapezius muscles were made during three motions: shrugging, forward flexion, and abduction. First, anodic surface electrodes were attached at a specific area for each muscle: the anterior, middle, and posterior deltoid and upper trapezius (Table 1). A disposable, self-adhesive, pre-gelled bipolar Ag/AgCl electrode was used with an interelectrode distance of 20 mm. Before attaching the electrodes, the skin covering the muscles was cleaned with alcohol to minimize impedance. EMG signals from the four muscles were collected using a BTS FREE EMG 300 system (BTS Bioengineering, MA, USA) (Fig. 1). Before EMG measurements, the three motions to be performed were explained. After each motion, the patient was allowed to recover for at least 2 min to avoid local muscle fatigue (Fig. 2). Shrugging was performed for 10 s and it was a relatively painless motion that was tolerated by the patients with CTA or severe OA. To measure the EMG signals during shoulder flexion, the arm was raised to 90° of forward flexion for 10 s. When it was difficult to maintain the arm at 90° of abduction due to pseudoparalysis (n=13), the patients were tested at the maximum degree of forward flexion that they could achieve.
Finally, to measure the EMG signals during shoulder abduction, the arm was abduct ed to 90° for 10 s. If it was difficult to maintain the arm at 90° of abduction, the measurement was performed in the same manner as the measurement of shoulder flexion. To reduce interference during the initiation or termination of each motion, the EMG signal for 3–6 s was analyzed. The signal was amplified with a gain of 500, noise < 1 μV, and a common mode rejection ratio of 100. The EMG signals were
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sampled at 1500 Hz and filtered at a bandwidth of 10,500 Hz. All signals were collected by a laptop computer equipped with a 16-bit analog-to-digital converter.
The EMG signals acquired for each motion were converted by full-wave rectification and filtered through a second-order Butterworth filter with a cutoff frequency of 3 Hz.14,34 Although maximal voluntary contraction (MVC) is the most common method for normalizing EMG amplitudes, we used the controlled reference voluntary contraction (RVC) method because of the massive rotator cuff tears. As a reference posture, the EMG amplitude was measured for 10 s while standing with both arms at the sides.26 %RVC was calculated as the normalized EMG amplitude upon comparison of the mean EMG values observed in the reference posture.
8 Table 1. Placement of electrodes for deltoid and upper trapezius
Muscle Placement of electrode
Anterior deltoid One finger width distal and anterior to the acromion
Middle deltoid The acromion to the lateral epicondyle of the elbow, which should correspond to the greatest bulge
Posterior deltoid About two finger widths posterior of the acromion
Upper trapezius 50% on the line from the acromion to the spine of vertebra C7
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Figure 1. Photographs showing the bipolar surface electrodes placement for four muscles (1: anterior deltoid, 2: middle deltoid, 3: posterior deltoid, 4: upper trapezius). A BTS FREEEMG 300 system (gray cuboidal shape) used to collect EMG signals was connected with the paired surface electrodes.
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A B
C D
Figure 2. Three motions and reference posture for EMG measurement. A. Standing without shoulder motion as reference posture B. Shrugging, C. Active flexion D.
Active abduction.
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For statistical analyses, partial correlations with confounding variables such as age and sex were used to evaluate the associations between preoperative EMG amplitudes of the deltoid and upper trapezius muscles and postoperative shoulder strength and active ROM; the results are presented as correlation coefficients (R).
Clinical data were compared between the preoperative assessment and final follow-up using the Wilcoxon signed-rank test. A P-value < 0.05 was considered to indicate statistical significance. Statistical analyses were performed using SPSS ver. 12.0.1 (SPSS, Chicago, IL, USA).
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