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Evaluation of Isometric Shoulder Muscle Contraction during Awakening after Arthroscopic Rotator Cuff Repair
Korean J Sports Med 2023;41:138-146
Published online September 1, 2023;  https://doi.org/10.5763/kjsm.2023.41.3.138
© 2023 The Korean Society of Sports Medicine.

Su Hyun Lee1, Jong Won Lee2, Doo-Hyung Lee3, Seok Won Chung4, Jin Su Lee4, Hyun Tae Kim5, Kyung-Soo Oh4

1Department of Orthopaedic Surgery, Seoul Red Cross Hospital, Seoul, 2Department of Orthopaedic Surgery, H Plus Yangji Hospital, Seoul, 3Department of Orthopaedic Surgery, Ajou University School of Medicine, Suwon, 4Department of Orthopaedic Surgery, Konkuk University School of Medicine, Seoul, 5Department of Orthopaedic Surgery, Sarang Plus Hospital, Seoul, Korea
Correspondence to: Kyung-Soo Oh
Department of Orthopaedic Surgery, Konkuk University School of Medicine, 120-1 Neungdong-ro, Gwangjin-gu, Seoul 05030, Korea
Tel: +82-2-2030-7637, Fax: +82-2-2030-7748, E-mail: orthopaedics11@gmail.com
Received April 29, 2023; Revised July 19, 2023; Accepted July 19, 2023.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
 Abstract
Purpose: Most rotator cuff repairs are performed under general anesthesia, and the shoulder muscles undergo exertion during the patient’s awakening. These may lead to subsequent retear. The purpose of this study is to evaluate the characteristics of shoulder muscle contraction during awakening from general anesthesia after rotator cuff repair.
Methods: Twenty patients underwent arthroscopic rotator cuff repair. Surface electromyography was used to investigate the amplitude of shoulder (upper trapezius [UT] and biceps brachii [BB]) and body (rectus femoris, RF) muscles during awakening in the operating room and resting in the postanesthesia care unit (PACU).
Results: The mean maximum voluntary isometric contraction (MVIC) of the UT, BB, and RF during awakening were 28.00%, 27.84%, and 35.65%, and the mean durations of activation were 3.98, 2.50, and 2.71 seconds. In the PACU, the mean MVIC of the UT, BB, and RF were 27.18%, 25.03%, and 27.20%, and the mean durations were 2.72, 0.26, and 0.67 seconds. No correlation between muscle contraction and postoperative pain was identified.
Conclusion: Less than 10% of the involuntary muscle contractions of the UT and BB measured in this study exceeded 20% of the MVIC and the contractions lasted less than 4 seconds. As the percentage of the MVIC of the rotator cuff is typically lower than that of the UT and BB, strong contractions of the rotator cuff muscle with detrimental effects occur at a low frequency and short duration. Therefore, retear due to muscle contraction during awakening is unlikely.
Keywords : Awakening, Electromyography, General anesthesia, Isometric contraction, Rotator cuff
Introduction

Retear after rotator cuff repair is the most important factor in determining the success of the operation and an important topic in the field of orthopedics. The retear rate is typically reported as 11% to 57%. Even in the case of large and massive rotator cuff tears, there is a report that the retear rate is as high as 94% (17 of 18) 1 year after surgery1,2. The retear rate is dependent on time, and most studies include the first 6 months postoperatively3,4. Iannotti et al.5 evaluated the rotator cuff integrity at 2, 6, 12, 16, 26, 39, and 52 weeks postoperatively and found that 94.7% of retears which are measured 1 to 4 cm occurred within 26 weeks of surgery. Similarly, Miller et al.6 reported that retears occurred within 6 months after the repair of large rotator cuff tears. No data regarding the possibility of retears immediately after surgery have been reported, though orthopedic surgeons are concerned about shoulder muscle contractions that occur during the process of awakening after general anesthesia. During the awakening process, consciousness is unclear. Unconscious movements of the limbs occur, and hallucinations or violent behavior may also occur. Severe movements or hallucinations are termed emergence agitation or emergence delirium7 and are considered a major postoperative issue8. To reduce excessive tension on the repaired tendon, an immobilizing brace is used to keep the shoulder in abduction and external rotation prior to the awakening process9. However, voluntary or involuntary isometric shoulder muscle contractions during awakening are uncontrollable.

Surface electromyography (EMG) has been used to evaluate muscle action potentials using surface electrodes10. EMG amplitude measured during muscle contraction provides quantitative data regarding muscle activation11. Several studies using EMG have been conducted to evaluate the effects of strength exercises on a repaired tendon after rotator cuff repair12,13. The effect of rehabilitation exercises on the rotator cuff can be predicted using measurements of the activity of the biceps brachii (BB) and upper trapezius (UT) muscles10,14. Shoulder exercises that apply a load of less than 20% of the maximum voluntary isometric contraction (MVIC) are appropriate during the initial rehabilitation stage after rotator cuff repair15,16. Therefore, it is theorized that shoulder muscle contraction should not exceed 20% of the MVIC during awakening after anesthesia. This study evaluates the characteristics of shoulder muscle contractions during awakening after the rotator cuff repair by measuring the amplitude and frequency of the contractions of the BB and UT using surface EMG.

Methods

This study was approved by the Institutional Review Board of Konkuk University Medical Center (No. KUMC2021-11-050), and all participants provided informed consent for their study participations and publication of all clinical images.

Twenty patients who underwent arthroscopic rotator cuff repair from March to August 2021 were included in this prospective, case series study. Patients with a full-thickness posterosuperior rotator cuff tear identified on magnetic resonance imaging were included. Patients with a partial thickness posterosuperior rotator cuff tear, an isolated subscapularis tear, and those who underwent previous rotator cuff surgery of the affected shoulder, concomitant surgery for glenohumeral joint instability, were excluded from the study. All patients underwent arthroscopic posterosuperior rotator cuff repair performed by one orthopedic shoulder surgeon. General anesthesia was used in all surgeries. A blind suprascapular and axillary nerve block was performed before the surgical procedure17.

1. General anesthesia

All anesthesia was performed by one anesthesiologist. After preoxygenation for 5 minutes, anesthesia was induced using propofol and intravenous remifentanil. Muscle relaxation was maintained using rocuronium, and tracheal intubation was performed. The inhalation of sevoflurane was discontinued at the end of the surgical procedure. The anesthesiologist waited until the train-of-four (TOF) count was 1 or 2, then administered sugammadex to reverse the effects of rocuronium. The TOF was checked every 15 seconds until the TOF ratio reached 90%. Remifentanil and ramosetron were administered intravenously to control postoperative pain, nausea, and vomiting. Extubation was conducted after the recovery of spontaneous respiration and motor response. All patients recovered in the postanesthesia care unit (PACU).

2. Electromyography equipment and recordings

Surface EMG signals were recorded using a TeleMyo DTS Desk Receiver (Noraxon), and all data processing was conducted using MR-XP software (version 1.08; Noraxon). As the activations of the supra- and infraspinatus muscles could not be recorded due to surgical dressings, the activations of the UT and BB muscles were measured. Activation of the rectus femoris (RF) muscle was also measured to monitor whole-body contractions independent of the surgical site and unaffected by pain.

The MVIC of each muscle was determined preoperatively. EMG signals recorded from the same muscle during an MVIC were used as the reference value for normalization. To minimize skin impedance, the skin surface was cleaned with an alcohol swab before placing the electrodes. Bipolar surface EMG electrodes were placed 2 cm apart at specific locations for each muscle (Table 1 and Fig. 1). A brief demonstration of the EMG equipment and the MVIC testing was provided for each patient. The MVIC testing was repeated in triplicate. Each measurement lasted 5 seconds and was obtained according to previously reported guidelines18. The patients were verbally encouraged and visual feedback of the signal amplitudes was provided to generate the maximum muscle force. At least 2 minutes of rest was permitted between the MVIC tests to minimize muscle fatigue. The maximum amplitude after three sets of contractions was used for normalization.

Table 1 . Description of MVIC procedures for each muscle

MuscleElectrode placementMVIC testing
Upper trapezius1/2 On the line from the acromion to the spine of vertebra C7Elevation of the acromial end of the clavicle and scapula
Biceps brachiiDistal 1/3 on the line from the medial acromion to the cubital fossaElbow flexion slightly less than or at a right angle, with the forearm in supination
Rectus femoris1/2 On the line from the anterior superior iliac spine to the superior part of the patellaHip flexion at hip joint angles of 90° in the supine position

MVIC: maximum voluntary isometric contraction.


Fig. 1. Photographs showing the bipolar surface electrodes placement for muscles of interest.

The activity of each muscle was measured during postoperative awakening in the operation room and during a resting state in the PACU. The TOF ratio was used to confirm that the muscle relaxant used for anesthesia had been reversed. After dressing the surgical site, an immobilization brace was applied to the shoulder and the electrodes were attached to each muscle. Muscle activation was measured in the operating room from the time of surgical dressings to leaving the room and measured once more during the first 5 minutes in the recovery room. Immediate postoperative pain was recorded retrospectively in the general ward using a visual analogue scale (VAS).

3. Electromyography analysis

The muscle signals were amplified using a gain of 400, noise less than 1 mV, and common mode rejection ratio of 100 dB. They were sampled at 1,500 Hz and filtered with a bandwidth of 10 to 500 Hz. To construct a linear envelope, full-wave rectification was performed. A Lancosh finite impulse response digital filter (Noraxon) was used to filter the raw signal. The bandpass filter frequency was between 10 and 350 Hz.

The EMG data were processed into root mean square (RMS) values in 150-millisecond windows. For normalization, three RMS measurements were obtained for each muscle. Resting EMG activity was used as baseline activity. The data were not normalized for body mass as body mass was similar between the patients (Supplementary Table 1). The activity level of each muscle was checked using the baseline MVIC. The number of times each muscle contracted to more than 20% of the MVIC and the duration of the muscle contractions were determined.

4. Statistical analysis

Continuous data are presented as mean±standard deviation. The Spearman correlation analysis was used to analyze the correlation between postoperative pain intensity and muscle contraction during awakening from anesthesia. All statistical analyses were performed using the PASW Statistics ver. 17.0 (IBM Corp.). Statistical significance was set at a p-value of <0.05.

Results

The final analysis included 20 patients (15 males and 5 females) (Table 2). Detailed information regarding individual patients is presented in Supplementary Table 1.

Table 2 . Anthropometric data of all subjects

CharacteristicData
No. of subjects20
Sex, male:female15竊5
Age (yr)58.7±10.5
Weight (kg )70.1±13.73
Height (cm )163.8±8.26
Body mass index (kg/m2)25.9±3.59
Operated side, right/left8/12
Time of anesthesia (min)163±33
Pain VAS
Preoperative6.65±1.22
Postoperative5.25±1.88

Values are presented as number only or mean±standard deviation.

VAS: visual analogue scale.



In total, the normalized EMG amplitude was more than 20% of the MVIC during awakening 12 times (12 of 96, 12.5%) in the UT, 6 times (6 of 77, 7.7%) in the BB, and 22 times (22 of 139, 15.8%) in the RF (Fig. 2A). In the PACU, the normalized EMG amplitude was more than 20% of the MVIC 4 times (4 of 72, 5.5%) in the UT, 1 time (1 of 75, 1.3%) in the BB, and 10 times (10 of 95, 10.5%) in the RF (Fig. 3A). During the awakening process, the average duration of muscle contraction more than 20% of the MVIC was 3.98±3.66 seconds in the UT, 2.50±3.35 seconds in the BB, and 2.71±4.17 seconds in the RF (Table 3). In the PACU, the average duration of muscle contraction of more than 20% of the MVIC was 2.72±2.45 seconds in the UT, 0.26 seconds in the BB, and 0.67±0.47 seconds in the RF (Table 4).

Table 3 . Electromyography activity during awakening from general anesthesia (n=20)

VariableMuscle
Upper trapeziusBiceps brachiiRectus femoris
Total muscle contraction9677139
Contraction over 20% of MVIC
Events12622
Mean duration (sec)3.982.502.71
MVIC rate (%)
Maximum40.5562.5455.53
Mean±SD28.00±4.6327.84±5.8235.65±7.64

MVIC: maximum voluntary isometric contraction, SD: standard deviation.


Table 4 . Electromyography activity while in postanesthesia care unit (n=20)

VariableMuscle
Upper trapeziusBiceps brachiiRectus femoris
Total muscle contraction727595
Contraction over 20% of MVIC
Events4110
Mean duration (sec)2.720.260.67
MVIC rate (%)
Maximum49.0529.4835.45
Mean±SD27.18±5.5225.03±0.0627.20±4.23

MVIC: maximum voluntary isometric contraction, SD: standard deviation.


Fig. 2. Electromyography activity during awakening from general anesthesia. UT: upper trapezius, BB: biceps brachii, RF: rectus femoris, MVIC: maximum voluntary isometric contraction.
Fig. 3. Electromyography activity while in postanesthesia care unit. UT: upper trapezius, BB: biceps brachii, RF: rectus femoris, MVIC: maximum voluntary isometric contraction.

The mean amplitudes of contractions that exceeded 20% of the MVIC during awakening were 28.00% of the MVIC in the UT, 27.84% of the MVIC in the BB, and 35.65% of the MVIC in the RF (Fig. 2B, Table 3). In the PACU, the mean amplitudes of contractions that exceeded 20% of the MVIC were 27.18% of the MVIC in the UT, 25.03% of the MVIC in the BB, and 27.20% of the MVIC in the RF (Fig. 3B, Table 4). Supplementary Tables 2 and 3 include detailed information regarding each muscle contraction of individual patients during awakening and in the PACU, respectively.

No correlations between pain score and the number of contractions in each muscle, duration of muscle contraction, mean percentage of the MVIC, or maximum percentage of the MVIC were observed (Table 5).

Table 5 . Correlation analysis between pain VAS and muscle contractions

VariableMuscle
Upper trapeziusBiceps brachiiRectus femoris
Pearson coefficientProbabilityPearson coefficientProbabilityPearson coefficientProbability
Total contraction0.0040.980−0.0610.708−0.2820.077
Contraction of over 20% of MVIC
Events0.2160.181−0.1850.252−0.1350.406
Mean duration (sec)−0.6130.143−0.5140.376−0.1580.517
MVIC rate (%)
Maximum0.0490.917−0.2600.673−0.3580.132
Mean±SD0.641±−0.2230.121±0.631−0.038±−0.6830.952±0.204−0.055±−0.1030.824±0.674

VAS: visual analogue scale, MVIC: maximal voluntary isometric contraction, SD: standard deviation.


Discussion

Shoulder muscle contractions with sufficient intensity to adversely affect the repaired rotator cuff occurred rarely in this study. In addition, pain was not related to the frequency, duration, or intensity of shoulder muscle contractions. As no previous study has evaluated shoulder muscle contractions during awakening after general anesthesia, criteria to classify the severity of shoulder muscle contractions have not been established. Therefore, the intensity levels of shoulder muscle contractions used to determine the appropriateness of rehabilitation exercises were used in this study. Rehabilitation exercises are classified as low (0%–20% of the MVIC), moderate (21%–40%), high (41%–60%), and very high (>60%) intensity according to the muscle activity level15,16. Low muscle activity levels during low-load, closed chain, and elevation exercises are used to train the shoulder with minimal rotator cuff loads during rehabilitation14. Therefore, muscle contractions of more than 20% of the MVIC were considered harmful to the operated shoulder in this study. Muscle contractions of more than 20% of the MVIC occurred 16 times (9.5%) in the UT, 7 times (4.6%) in the BB, and 32 times (13.6%) in the RF in this study. The mean number of muscle contractions of more than 20% of the MVIC in each patient was 0.80 in the UT, 0.35 in the BB, and 1.6 in the RF. The maximal muscle contraction was classified as high to very high. However, such muscle contractions were rare. The average amplitude of the muscle activity was 27.77% in the UT, 27.27% in the BB, and 32.98% in the RF, which are all moderate muscle activities. Therefore, UT and BB showed moderate-intensity muscle contraction on average, and there was high- or very high-intensity muscle contraction, but it occurred very rarely, with the number less than one per person. In addition, these two muscles had lower muscle contraction amplitude and less frequency than RF. In this study, since RF was defined as a control representing whole-body muscle contraction that is not affected by pain, it can be inferred that the muscle contraction of UT and BB was weaker than that of other muscles of the whole body that are irrelevant to pain. Therefore, it can be implied that even in muscles with pain, the actual effect of muscle contraction may not be that great.

The activity of the rotator cuff muscles was not monitored in this study. Because surgical site dressing was applied to the skin, the electrodes could not be placed over the rotator cuff muscles. However, shoulder joint movement requires the interaction of several muscles, such as the rotator cuff, UT, and BB. During rehabilitation exercise, the percent of the MVIC of rotator cuff muscle contractions are the same or lower than that of the large muscles such as the deltoid, UT, and BB19. In this study, strong contractions of the UT and BB with high loads occur at a low frequency and within short durations. Therefore, although the muscle activity and integrity of the rotator cuff were not directly measured, they are assumed to be lower than those of the UT and BB and would not be sufficient to adversely affect the newly repaired rotator cuff.

Postoperative pain and irritation felt by the patient after surgery were pointed out as possible risk factors for muscle contractions exceeding 20% of the MVIC. It was hypothesized that after the patient recovered consciousness, severe pain may occur and induce a strong muscle contraction over 20% of the MVIC. Therefore, this study investigated the correlation between pain and muscle contraction. After awakening from general anesthesia, patients remained disoriented in the operating room, though consciousness was recovered. However, the patients’ orientation was typically nearly intact in the PACU; therefore, pain immediately postoperatively was evaluated retrospectively when the patients returned to the general ward. No association between pain score and muscle contractions exceeding 20% of the MVIC was identified in this study. Previous studies have focused on postoperative pain control after arthroscopic rotator cuff repair20-22, though few have addressed the effect of postoperative pain on the integrity of the rotator cuff repair site23. This is the first study to examine the relationship between pain and muscle contractions exceeding 20% of the MVIC.

Meanwhile, a blind suprascapular and axillary nerve block was used preoperatively to help control the postoperative pain in this study17. However, the nerve block was not effective to control postoperative pain as most patients reported moderate pain in the PACU (Supplementary Table 1). This may be due to the fact that the ropivacaine was washed away during the arthroscopic surgery. As the anatomic bony landmark used for the nerve block was obscured in the swollen shoulders postoperatively, the nerve block procedure was conducted right before the main procedure. Therefore, the measurement of rotator cuff activity was likely not directly affected by the nerve block procedure and was still meaningful even after the nerve block. Ironically, the effect of pain on muscle contraction was evaluated well with a less effective nerve block. An alternative and effective local anesthetic for immediate postoperative pain control has not yet been identified, and it is beyond the scope of this study to determine whether the brachial plexus block was an effective method of postoperative pain control.

Besides postoperative pain, several risk factors can induce emergence agitation, including old age, long operation time, obesity, type of inhaled anesthetics, neuromuscular block, and reversal method24. In this study, the effects of some risk factors were reduced during anesthesia. Rocuronium-sugammadex was reported to reduce the severity of agitation following electroconvulsive therapy more effectively than succinylcholine by decreasing the plasma lactate level25. The incidence, duration, and intensity of emergence agitation during closed reduction of nasal bone fractures were decreased in patients administered rocuronium- sugammadex compared to those in patients administered succinylcholine26. Therefore, rocuronium was used for muscle relaxation and sugammadex was used to reverse the neuromuscular block in this study. The use of these neuromuscular blockers and reversal agents may have reduced the occurrence of muscle contractions exceeding 20% of the MVIC in this study.

Most patients in this study did not have a paroxysmal reaction during extubation. Remifentanil, which is widely used as an anesthetic adjuvant during general anesthesia, was used in this study. The use of remifentanil effectively suppressed the number of cough occurrences during extubation in a previous randomized controlled study27. And also it is a selective μ-opioid receptor agonist that suppresses the cough center of the central nervous system28. Therefore, it might have reduced the occurrence of muscle contractions.

This study has several limitations. First, this study is a case series with a limited number of patients. Muscle contractions of more than 20% of the MVIC were observed in eight patients in the operating room and two patients in the PACU (Supplementary Tables 2 and 3). While these results may indicate that muscle contractions exceeding 20% of the MVIC did not occur frequently during the anesthesia recovery process, they may also be a result of bias. No significant correlation between pain and muscle contraction was observed, though this may be due to the small patient population. Therefore, a future study including more patients is needed. Second, activations of the rotator cuff muscles were not measured directly in this study as the electrodes could not be directly attached to the surgical site. Therefore, the UT and BB muscle activities were assessed. However, as the muscle activity of the rotator cuff is not higher than that of the UT and BB19, the muscle activity of the cuff is expected to be lower than the activities recorded in the current study. Therefore, this limitation does not significantly affect the results. Finally, this study analyzed muscle contractions immediately postoperatively. As the rotator cuff integrity was not evaluated using magnetic resonance imaging or ultrasonography, the effect of the muscle contractions on cuff integrity or retear is unknown. Therefore, further studies are needed.

Involuntary muscle contractions of the UT and BB that are strong enough to affect the integrity of the repaired rotator cuff are infrequent and short during the awakening period after arthroscopic rotator cuff repair. As the muscle activity of the rotator cuff muscles is typically lower than that of the UT and BB, it is unlikely that rotator cuff muscle contractions during awakening will adversely affect the integrity of the repaired rotator cuff. No association between postoperative pain and muscle contraction in more than 20% of the MVIC was detected. Therefore, retears due to isometric muscle contractions during awakening from anesthesia after arthroscopic rotator cuff repair are unlikely. In addition, further research should be continued on the direct evaluation of rotator cuff muscle contraction itself and its effect on postoperative rotator cuff healing.

Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Author Contributions
Conceptualization: JWL, DHL, SWC, JSL, KSO. Data curation: JWL, DHL, SWC, HTK, KSO. Formal analysis, Investigation: JWL, JSL, HTK, KSO. Methodology, Resources, Supervision: DHL, SWC, KSO. Project administration: SWC, KSO. Validation, Visualization: SHL, JWL, KSO. Writing–original draft: SHL, JWL, KSO. Writing–review & editing: SHL, JSL, KSO.
Supplementary Materials

Supplementary Materials can be found at https://doi.org/10.5763/kjsm.2023.41.3.138.

kjsm-41-3-138-supple.pdf
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