- Email: [email protected]
The impact of rotator cuff pathology on isometric and isokinetic strength, function, and quality of life
The impact of rotator cuff pathology on isometric and isokinetic strength, function, and quality of life Joy C. MacDermid, PhD,a,b,c Joanne Ramos, BScPT,d Darren Drosdowech, MD,a Ken Faber, MD,a and Stuart Patterson, MB, ChB,e London, Hamilton, Toronto, and Mississauga, Ontario, Canada, and Lakeland, FL
The purposes of this study were to determine the reliability of strength and self-reporting measures, the relationship of different strength measures to function, and the impact of rotator cuff pathology on patients’ quality of life. Patients with nonoperated rotator cuff pathology (n ⫽ 36) and unaffected control subjects (n ⫽ 48) were assessed by use of the LIDO dynamometer to determine isometric and isokinetic (concentric and eccentric) strength of the shoulder rotators. The Shoulder Pain and Disability Index and Short Form–36 were self-reported by patients. Intraclass correlation coefficients (ICCs) were used to assess reliability, and Pearson correlations and multiple linear regression were used to determine the relationship between strength and function. The findings of this study include the following: (1) measures of self-reported physical disability had high reliability (ICC ⫽ 0.89); (2) the LIDO dynamometer reliably measured internal and external shoulder rotation strength in both concentric and isometric modes of testing (ICC ⫽ 0.78-0.94), whereas eccentric muscle actions had lower reliability; (3) all shoulder rotation strength measures were predictive of disability, with isometric external rotation strength being the most predictive (r ⫽ 0.56); and (4) the presence of rotator cuff pathology was highly predictive of impaired physical health quality of life (R2 ⫽ 0.71, P ⬍ .001). (J Shoulder Elbow Surg 2004;13:593-598.)
From the Hand and Upper Limb Centre Clinical Research Laboratory, St Joseph’s Health Centre, London,a School of Rehabilitation Science, McMaster University, Hamilton,b Ontario Ministry of Health and Long-Term Care, Health Research Personnel Development Program,c Wellness Canada Inc, Mississauga,d and Central Florida Orthopaedic Surgery Associates, Lakeland.e Reprint requests: Joy C. MacDermid, PhD, Co-Director Hand and Upper Limb Centre Clinical Research Laboratory, St Joseph’s Health Centre, 268 Grosvenor St, London, Ontario, N6A 3A8 Canada (E-mail: [email protected]). Copyright © 2004 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2004/$30.00 doi:10.1016/j.jse.2004.03.009
D
egeneration of the rotator cuff is a common source of shoulder dysfunction. It has been demonstrated that chronic shoulder impingement results in significant functional disability and a reduction in quality of life.2 The impact of rotator cuff tendinopathy and its relationship to strength impairment has not been fully explored. A number of studies have quantified the importance of the rotator cuff muscles in shoulder rotation strength. It has been determined that the rotator cuff muscles are prime movers in shoulder rotation and also work to pre-set or stabilize the shoulder before activating motion.5 Studies performed while blocking the suprascapular nerve suggest that 22% to 33% of external rotation strength can be attributed to the supraspinatus.13 A number of studies have established that strength is impaired with rotator cuff tendinopathy or tears or improves with surgery.1,3,13,14,26,28,29 Isokinetic dynamometers have been used to quantify shoulder rotation torque and how it changes with surgical repair of the torn cuff.1,8,12–15,18,26,28 Previous studies have demonstrated that reliable measures of rotator cuff strength can be obtained with certain isokinetic dynamometers.7,9,16,17,19 The LIDO dynamometer has been shown to assess shoulder rotation strength reliably in men who do not have shoulder pathology, but there remains a need to determine whether reliable results can also be obtained in patients.9 A variety of self-reporting measures are used to assess shoulder patients. However, relatively little independent testing of these has been undertaken, particularly with reference to the subgroup of patients who have rotator cuff tendinopathy but have not progressed to surgery. Therefore, the purposes of this study were (1) to determine the test-retest reliability of self-reported disability and (concentric, eccentric, and isometric) shoulder rotation strength measures obtained with the LIDO dynamometer; (2) to determine how different strength measures relate to disability; and (3) to determine the extent to which rotator cuff pathology impacts on physical health quality of life.
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MATERIALS AND METHODS
Reliability
Subjects
A subsample of subjects (n ⫽ 43) was tested on a second occasion 2 to 7 days after the original test to determine the reliability of the test procedures. This sample size was selected based on a sample size calculation and our own previous experience with analyzing the reliability (variability) of strength measures.
A total of 84 subjects volunteered to participate in this study, which was approved by the University of Western Ontario Ethics Board. Twenty-four men and twelve women (mean age, 43.6 years) with chronic rotator cuff tendinitis or impingement were recruited from a pool of patients diagnosed by an orthopaedic surgeon (shoulder specialist) on the basis of clinical examination. Patients were eligible if they were engaged in conservative management of their problem through either a home exercise program or active therapy and had persistent symptoms for at least 3 months but not if surgery had been prescribed. Exclusion criteria for the injured group included any previous history of surgical intervention on the affected shoulder(s), inability to complete testing questionnaires, and age less than 18 years. A convenience sample consisting of 28 men and 20 women (mean age, 40.8 years) without shoulder problems, as determined by self-reporting, was recruited as a comparative group from clinic staff and patient escorts.
Questionnaires All subjects completed a Short Form (SF)–36.30,31 The Physical Component Summary Score of the SF-36 was calculated as described by the authors by use of a computer program and was used to indicate overall physical health quality of life. The patients with rotator cuff pathology also completed the Shoulder Pain and Disability Index (SPADI).4,11,24,25,32 The scores were computed as described by the authors. All questionnaires and strength tests were completed on the same occasion.
Strength testing Strength testing was performed with the LIDO computerized isokinetic dynamometer (Loredan, Inc, Davis, CA). Subjects were seated upright and asked to keep their backs flat against a heavy straight-back chair throughout testing to prevent trunk compensatory movements. The plane of the scapula position was used for testing. By use of a standard goniometer, the test shoulder was positioned in 45° abduction and 30° forward flexion. The drum of the dynamometer was tilted 45° from the vertical, and the axis of rotation of the humerus was centered with the axis of rotation of the dynamometer arm. Subjects were asked to grasp a handgrip and maintain the forearm and wrist in neutral pronation/supination. Shoulder isokinetic internal and external rotation strength was measured separately by use of continuous, reciprocal concentric and eccentric contraction cycles at a speed of 75°/s. Strength was tested through 90° of motion starting from ⫺45° (ie, 45° of internal rotation) to ⫹45° (ie, 45° of external rotation) from the horizontal. Isometric strength was measured in a neutral internal/external position. Submaximal practice for 1 to 2 sets was used to familiarize patients with the test protocol. One maximal practice repetition was performed before data were collected for analysis. The average peak torque generated from 3 maximal contractions was calculated and recorded as the criterion for each strength measurement. Both shoulders were tested in each individual.
Data analysis All statistics were calculated with SPSS 11.5.21 The test-retest reliability of strength and self-reporting scales was examined by use of intraclass correlation coefficients (ICCs type 2,1) and their associated 95% CIs.27 The differences in strength between patients and the unaffected control group was evaluated by use of analysis of variance at an ␣ ⫽ .05. Pearson correlation coefficients were calculated to assess the univariate relationship between strength and functional measures. Multiple linear regression was used to determine the relative importance of different strength measures with regard to patient-reported pain and disability. Regression equations were computed in two ways. Forward regression indicated the most efficient predictor equation (fewest number of variables), whereas backward regression was used to reflect the most comprehensive model (more significant variables). The outcome (dependent) variables were both quality of life (SF-36 Physical Component Summary Score) and shoulder pain and disability (SPADI). Independent variables entered into regression models for both analyses included the strength variables (isometric, concentric, and eccentric for both internal and external rotation), as well as age and sex. To assess the impact of rotator cuff pathology on quality of life, a single indicator variable for the presence of rotator cuff pathology was added to the list of independent variables entered into the stepwise multiple regression analysis of the SF-36 Physical Component Summary Score. The level of significance was set at P ⫽ .05.
RESULTS All subjects completed the study protocol without difficulty. Test-retest reliability tended to be moderate to high (Table I) for strength scores and high for self-reported disability measures (Table II). Patients with rotator cuff pathology were significantly weaker than subjects without pathology in both internal and external rotation (Table III). The ratio between internal and external rotation forces varied according to the muscle action tested and presence of pathology but indicated that external rotation was more compromised that internal rotation strength. The correlations between strength measurements and SPADI total scores indicated that lower strength was associated with greater pain and disability (negative correlations), although the strength of this relationship ranged from low (⫺0.25) to moderate (⫺0.56). Isometric external rotation showed the strongest correlation with SPADI scores (r ⫽ ⫺0.56). All
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Table I Reliability (ICCs) for Strength testing patients with rotator cuff pathology using LIDO Subjects without pathology (n ⴝ 19)
Patients (n ⴝ 24) Measure Concentric External Rotation Eccentric External Rotation Isometric External Rotation Concentric Internal Rotation Eccentric Internal Rotation Isometric Internal Rotation
Affected
Unaffected
Dominant
Nondominant
0.86 0.76 0.83 0.79 0.83 0.78
0.93 0.70 0.79 0.79 0.73 0.85
0.89 0.76 0.90 0.87 0.74 0.88
0.91 0.83 0.93 0.86 0.82 0.94
Average peak torque measurements on two separate occasions were tested for reliability and intraclass correlation coefficients (ICCs) calculated. A benchmark of 0.75 has been used to indicate excellent reliability.
Table II Reliability of Patient Rating Scales
Scale SPADI (n ⫽ 24 patients with rotator cuff-pathology) SPADI Pain scale (5 items Mean Score ⫽ 290/500 or 58/100) SPADI Function scale (8 items; Mean Score ⫽ 365/800 or 46/100) Total SPADI Score (Mean Score ⫽ 52/100) SF-36 (n ⫽ 40, 24 with and 16 without shoulder pathology) Physical Function Physical Role Bodily Pain General Health Vitality Social Function Role Emotional Mental health Physical Component Summary Score Mental Component Summary Score
ICC
95% Confidence Interval
0.89 0.92 0.95
0.76–0.95 0.83–0.97 0.89–0.98
0.94 0.90 0.97 0.91 0.85 0.92 0.72 0.90 0.94 0.83
0.89–0.97 0.81–0.94 0.94–0.98 0.84–0.95 0.73–0.92 0.86–0.96 0.52–0.84 0.82–0.95 0.89–0.97 0.70–0.91
The ICCs for test-retest reliability of self-report and associated 95% confidence intervals indicates that measures of physical disability are highly reliable in these patients.
values were statistically significant (P ⬍ .05) except for isometric internal rotation (Table IV). Table IV displays Pearson correlations between strength measurements and the SF-36 Physical Component Summary Score. These also ranged from low (⫺0.38) to moderate (⫺0.47), with all values reaching a level of statistical significance (P ⬍ .01). Overall, correlations were higher for external rotation strength than internal rotation strength across all modes of testing. Isometric external rotation was the best predictor of pain and disability during a forward regression analysis of the SPADI scores (R2 ⫽ 0.36), whereas the additional significant variables identified by a backward regression analysis also included isometric internal rotation and eccentric internal rotation (R2 ⫽ 0.45). The presence of rotator cuff pathology was the only significant factor affecting physical health quality
of life during a forward regression analysis of SF-36 Physical Component Summary Score. In fact, 71% of the variation between subjects’ health status was related to the presence of rotator cuff pathology. A small additional impact on physical health scores resulting from sex, weight, and eccentric external rotation strength was identified by the backward regression analysis (R2 ⫽ 0.78). DISCUSSION This study determined that strength, pain/disability, and physical health status can be measured reliably in patients with chronic rotator cuff tendinopathy. It further illustrated the substantial impact that rotator cuff tendinopathy has on quality of life and the importance of shoulder strength as a component of that disability. Although strength was important, there
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Table III Difference in Strength Between Patients and Subjects without pathology Strength Measure (Nm) External Rotation Concentric Eccentric Isometric Internal Rotation Concentric Eccentric Isometric Ratio (Internal/External) Concentric Eccentric Isometric
p value
Patients Asymptomatic Mean (SD) 12 (8) 19 (9) 17 (8)
Mean (SD) 21 (10) 28 (11) 27 (11)
⬍0.0001 ⬍0.0001 ⬍0.0001
15 (10) 21 (9) 24 (11)
22 (12) 28 (12) 33 (13)
0.004 0.004 0.001
1.25 1.10 1.41
1.04 1.0 1.22
Average peak strength scores in Nm were significantly lower in patients that in controls for all modes of strength testing and in both rotations. The higher ratios of internal/external strength in patients indicate relatively greater impairment in external rotation strength vs internal rotation in patients with cuff pathology.
Table IV Correlation between strength of the affected arm and SPADI Total Scores (n ⫽ 36)
Strength Measurement Concentric External Rotation Eccentric External Rotation Concentric Internal Rotation Eccentric Internal Rotation Isometric External Rotation Isometric Internal Rotation
Shoulder Pain and Disability
Physical Component Summary
⫺0.41* ⫺0.46** ⫺0.45** ⫺0.49** ⫺0.56** ⫺0.25
⫺0.38* ⫺0.42* ⫺0.38* ⫺0.40* ⫺0.38* ⫺0.31*
*p ⬍ 0.05, **p ⬍ 0.01 Negative correlation indicates that as strength decreases, self-reported pain and disability increases. A moderate relationship exists between strength and pain/disability. The relationship between strength measures and physical health quality of life was also significant and in a similar direction, although slightly less strong.
was no evidence suggesting that extensive isokinetic testing was more useful than isometric strength testing in evaluating these patients. Of the three muscle forces measured, eccentric contractions tended to demonstrate lower test-retest reliability. If 0.75 is taken as the benchmark for excellent reliability,6 then all isometric and concentric contractions achieved this status, whereas only 5 of 8 eccentric contractions did so. This finding is consistent with previous research.20 Several factors may have contributed to this finding. Some subjects were observed to have difficulty producing consistent eccentric contractions. These problems were more common in elderly and/or sedentary patients, suggesting that certain subgroups of patients may require additional familiarization with isokinetic equipment before they
can consistently produce maximal efforts during testing. A second factor to consider, when testing with the LIDO dynamometer, is that it restricts eccentric contractions to a percentage of the previous concentric contraction as a safety measure against injury. Therefore, when subjects did not perform maximally on their concentric contraction, this may have affected the ability to produce maximal forces on the subsequent eccentric contraction. As few subjects had difficulty performing concentric contractions, this was thought to be a minor component of measurement error in this study. Although, theoretically, one might assume that eccentric (lengthening) contractions of the external rotators provide the most information on functional integrity of the cuff muscles, theory must be supported by evidence. In our patient group, which focused on chronic (presurgical) degenerative cuff pathology, that was not true, and our reliability data suggested that this may be because of the greater measurement error associated with this form of strength testing. Establishing optimal testing positions and protocols for patients is an important issue, as it has been observed that only 30% of patients with shoulder pathology could tolerate testing with a Cybex protocol.23 Protocols that can be completed by a minority of patients will introduce biases in studies reporting clinical outcomes. Therefore, we tested patients while sitting with the arm positioned in the plane of the scapula. Proponents for testing the shoulder in this position contend that it allows for optimum lengthtension relationships of the humeral abductors and rotators, allows maximum joint congruency as the center of the humeral head and glenoid fossa coincide, maintains the capsular fibers of the glenohumeral joint in a relaxed position, and provides maximum safety and comfort during testing.9,22 Concerns regarding testing in the sagittal or frontal plane are associated with increased risk of impingement,10 instability, and compromise of the vasculature of the rotator cuff tendons.22 In addition, because elderly patients may have difficulty with the supine position used in studies testing unaffected subjects,16 we used the sitting position. Finally, a slower speed (75°/s) was selected in consideration of the age distribution of our patients, as well as to minimize risk during eccentric contractions. All of our volunteers were able to complete this test protocol, suggesting that it is appropriate for clinical studies. Despite our efforts to evaluate a viable clinical test protocol, isokinetic tests did not demonstrate any advantage over isometric tests, with regard to either reliability or its relationship to disability outcomes. Because isometric testing has lesser requirements in terms of both equipment and staff costs, the use of isokinetic measures is unnecessary, unless a substantive rationale is established. If isokinetic measures
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were more predictive of the need for surgery, responsive to measuring treatment effects or related to the ability to return to work, then the additional information might be justified. Until additional evidence supports more definitive conclusions on the relative utility of different strength measures, isometric measures are reliable and valid indicators. Our data demonstrate that patients had significantly lower strength than a convenience sample of subjects without shoulder pathology. This deficit was greater in external rotation but was also present in internal rotation. Involvement of the internal rotators and disuse atrophy may have contributed. Although clinicians and patients are aware of the impact of rotator cuff pathology, this study provided statistical evidence as to the substantial impact of rotator cuff pathology, both specifically on the shoulder (SPADI ⫽ 52/100) and also more globally on the patients’ overall physical health status. Other studies, also using the SF-36, have reported that shoulder pathology and other musculoskeletal conditions have a significant impact on an individual’s perception of quality of life.2 In fact, the size of this impact is comparable to the effects of conditions such as hypertension, congestive heart failure, diabetes mellitus, myocardial infarction, and clinical depression.31 A greater appreciation of the impact of individual musculoskeletal conditions on quality of life underscores the need for expedient and effective treatment for these disorders. Isometric measurement of shoulder rotation strength provides reliable information on the functional integrity of the rotator cuff muscles, which is significantly related to patients’ function and quality of life. Strength measures should be combined with selected reliable self-reported disability measures to provide a comprehensive description of outcomes in research and clinical reporting. Ismail Gangat and Ana-Mara Marinescu are recognized for their role in completion of this study, which included recruitment of subjects, patient testing, and contributions to initial drafts of the manuscript. REFERENCES
1. Barton MA, Wallace WA, Robinson N. Isokinetic strength of the shoulder after repair of a torn rotator cuff. J Bone Joint Surg Am 1988;70:1270-1. 2. Chipchase LS, O’Connor DA, Costi JJ, Krishnan J. Shoulder impingement syndrome: preoperative health status. J Shoulder Elbow Surg 2000;912–5. 3. Cofield RH, Parvizi J, Hoffmeyer PJ, et al. Surgical repair of chronic rotator cuff tears. A prospective long-term study. J Bone Joint Surg Am 2001;83:71-7. 4. Cook KF, Roddey TS, Olson SL, et al. Reliability by surgical status of self-reported outcomes in patients who have shoulder pathologies. J Orthop Sports Phys Ther 2002;32:336-46. 5. David G, Magarey ME, Jones MA, et al. EMG and strength correlates of selected shoulder muscles during rotations of the
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glenohumeral joint. Clin Biomech (Bristol, Avon) 2000;15:95102. Fleiss JL. Reliability of measurement. In: Fleiss JL, editor. The design and analysis of clinical experiments. Toronto: John Wiley and Son; 1986. p. 1-32. Frisiello S, Gazaille A, O’Halloran J, Palmer ML, Waugh D. Test-retest reliability of eccentric peak torque values for shoulder medial and lateral rotation using the Biodex isokinetic dynamometer. J Orthop Sports Phys Ther 1994;19:341-4. Grana WA, Teague B, King M, Reeves RB. An analysis of rotator cuff repair. Am J Sports Med 1994;22:585-8. Greenfield BH, Donatelli R, Wooden MJ, Wilkes J. Isokinetic evaluation of shoulder rotational strength between the plane of scapula and the frontal plane. Am J Sports Med 1990;18:124-8. Hageman PA, Mason DK, Rydlund KW, Humpel SA. Effects of position and speed on eccentric and concentric isokinetic testing of the shoulder rotators. J Orthop Sports Phys Ther 1989;11: 64-9. Heald SL, Riddle DL, Lamb RL. The shoulder pain and disability index: the construct validity and responsiveness of a region specific disability measure. Phys Ther 1997;77:1079-89. Iannotti JP, Bernot MP, Kuhlman JR, Kelley MJ, Williams GR. Postoperative assessment of shoulder function: a prospective study of full-thickness rotator cuff tears. J Shoulder Elbow Surg 1996;5:449-57. Itoi E, Minagawa H, Sato T, Sato K, Tabata S. Isokinetic strength after tears of the supraspinatus tendon. J Bone Joint Surg Br 1997;79:77-82. Kirschenbaum D, Coyle MP Jr, Leddy JP, et al. Shoulder strength with rotator cuff tears. Pre- and postoperative analysis. Clin Orthop 1993;288:174-8. Kronberg M, Wahlstrom P, Brostrom LA. Shoulder function after surgical repair of rotator cuff tears. J Shoulder Elbow Surg 1997; 6:125-30. Kuhlman JR, Iannotti JP, Kelly MJ, et al. Isokinetic and isometric measurement of strength of external rotation and abduction of the shoulder. J Bone Joint Surg Am 1992;74:1320-33. Leggin BG, Neuman RM, Iannotti JP, Williams GR, Thompson EC. Intrarater and interrater reliability of three isometric dynamometers in assessing shoulder strength. J Shoulder Elbow Surg 1996; 5:18-24. Leroux JL, Hebert P, Mouilleron P, et al. Postoperative shoulder rotators strength in stages II and III impingement syndrome. Clin Orthop 1995;320:46-54. Magnusson SP, Gleim GW, Nicholas JA. Subject variability of shoulder abduction strength testing. Am J Sports Med 1990;18: 349-53. Mandalidis DG, Donne B, O’Regan M. Reliability of isokinetic internal and external rotation of the shoulder in the scapular plane. Isokinet Exerc Sci 2001;9:65-72. Norusis MJ. SPSS/PC⫹. 2002[11.0]. Chicago: SPSS Inc; 2002. Poppen NK, Walker PS. Forces at the glenohumeral joint in abduction. Clin Orthop 1978;135:165-70. Rabin SI, Post M. A comparative study of clinical muscle testing and Cybex evaluation after shoulder operations. Clin Orthop 1990;258:147-56. Roach KE, Budiman-Mak E, Songsiridej N, Lertratanakul Y. Development of a shoulder pain and disability index. Arthritis Care Res 1991;4:143-9. Roddey TS, Olson SL, Cook KF, Gartsman GM, Hanten W. Comparison of the University of California-Los Angeles Shoulder Scale and the Simple Shoulder Test with the shoulder pain and disability index: single-administration reliability and validity. Phys Ther 2000;80:759-68. Rokito AS, Zuckerman JD, Gallagher MA, Cuomo F. Strength after surgical repair of the rotator cuff. J Shoulder Elbow Surg 1996;5:12-7.
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27. Shrout PE, Fleiss JL. Intraclass correlations: uses in assessing rater reliability. Psychol Bull 1979;86:420-8. 28. Walker SW, Couch WH, Sprowl DW. Isokinetic strength of the shoulder after repair of a torn rotator cuff. J Bone Joint Surg Am 1987;69:1041-4. 29. Walmsley RP, Hartsell H. Shoulder strength following surgical rotator cuff repair: a comparative analysis using isokinetic testing. J Orthop Sports Phys Ther 1992;15:215-22. 30. Ware JE, Snow KS, Kosinski M, Gandek B. SF-36 health survey
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manual and interpretation guide. Boston: The Health Institute, New England Medical Center; 1993. 31. Ware JE, Kosinski M, Keller SD. SF-36 physical and mental health summary scales: a user’s manual. Boston: The Health Institute, New England Medical Center; 1994. p. 1.1-10.12. 32. Williams JW Jr, Holleman DR Jr, Simel DL. Measuring shoulder function with the Shoulder Pain and Disability Index. J Rheumatol 1995;22:727-32.
The purposes of this study were to determine the reliability of strength and self-reporting measures, the relationship of different strength measures to function, and the impact of rotator cuff pathology on patients’ quality of life. Patients with nonoperated rotator cuff pathology (n ⫽ 36) and unaffected control subjects (n ⫽ 48) were assessed by use of the LIDO dynamometer to determine isometric and isokinetic (concentric and eccentric) strength of the shoulder rotators. The Shoulder Pain and Disability Index and Short Form–36 were self-reported by patients. Intraclass correlation coefficients (ICCs) were used to assess reliability, and Pearson correlations and multiple linear regression were used to determine the relationship between strength and function. The findings of this study include the following: (1) measures of self-reported physical disability had high reliability (ICC ⫽ 0.89); (2) the LIDO dynamometer reliably measured internal and external shoulder rotation strength in both concentric and isometric modes of testing (ICC ⫽ 0.78-0.94), whereas eccentric muscle actions had lower reliability; (3) all shoulder rotation strength measures were predictive of disability, with isometric external rotation strength being the most predictive (r ⫽ 0.56); and (4) the presence of rotator cuff pathology was highly predictive of impaired physical health quality of life (R2 ⫽ 0.71, P ⬍ .001). (J Shoulder Elbow Surg 2004;13:593-598.)
From the Hand and Upper Limb Centre Clinical Research Laboratory, St Joseph’s Health Centre, London,a School of Rehabilitation Science, McMaster University, Hamilton,b Ontario Ministry of Health and Long-Term Care, Health Research Personnel Development Program,c Wellness Canada Inc, Mississauga,d and Central Florida Orthopaedic Surgery Associates, Lakeland.e Reprint requests: Joy C. MacDermid, PhD, Co-Director Hand and Upper Limb Centre Clinical Research Laboratory, St Joseph’s Health Centre, 268 Grosvenor St, London, Ontario, N6A 3A8 Canada (E-mail: [email protected]). Copyright © 2004 by Journal of Shoulder and Elbow Surgery Board of Trustees. 1058-2746/2004/$30.00 doi:10.1016/j.jse.2004.03.009
D
egeneration of the rotator cuff is a common source of shoulder dysfunction. It has been demonstrated that chronic shoulder impingement results in significant functional disability and a reduction in quality of life.2 The impact of rotator cuff tendinopathy and its relationship to strength impairment has not been fully explored. A number of studies have quantified the importance of the rotator cuff muscles in shoulder rotation strength. It has been determined that the rotator cuff muscles are prime movers in shoulder rotation and also work to pre-set or stabilize the shoulder before activating motion.5 Studies performed while blocking the suprascapular nerve suggest that 22% to 33% of external rotation strength can be attributed to the supraspinatus.13 A number of studies have established that strength is impaired with rotator cuff tendinopathy or tears or improves with surgery.1,3,13,14,26,28,29 Isokinetic dynamometers have been used to quantify shoulder rotation torque and how it changes with surgical repair of the torn cuff.1,8,12–15,18,26,28 Previous studies have demonstrated that reliable measures of rotator cuff strength can be obtained with certain isokinetic dynamometers.7,9,16,17,19 The LIDO dynamometer has been shown to assess shoulder rotation strength reliably in men who do not have shoulder pathology, but there remains a need to determine whether reliable results can also be obtained in patients.9 A variety of self-reporting measures are used to assess shoulder patients. However, relatively little independent testing of these has been undertaken, particularly with reference to the subgroup of patients who have rotator cuff tendinopathy but have not progressed to surgery. Therefore, the purposes of this study were (1) to determine the test-retest reliability of self-reported disability and (concentric, eccentric, and isometric) shoulder rotation strength measures obtained with the LIDO dynamometer; (2) to determine how different strength measures relate to disability; and (3) to determine the extent to which rotator cuff pathology impacts on physical health quality of life.
593
594
MacDermid et al
J Shoulder Elbow Surg November/December 2004
MATERIALS AND METHODS
Reliability
Subjects
A subsample of subjects (n ⫽ 43) was tested on a second occasion 2 to 7 days after the original test to determine the reliability of the test procedures. This sample size was selected based on a sample size calculation and our own previous experience with analyzing the reliability (variability) of strength measures.
A total of 84 subjects volunteered to participate in this study, which was approved by the University of Western Ontario Ethics Board. Twenty-four men and twelve women (mean age, 43.6 years) with chronic rotator cuff tendinitis or impingement were recruited from a pool of patients diagnosed by an orthopaedic surgeon (shoulder specialist) on the basis of clinical examination. Patients were eligible if they were engaged in conservative management of their problem through either a home exercise program or active therapy and had persistent symptoms for at least 3 months but not if surgery had been prescribed. Exclusion criteria for the injured group included any previous history of surgical intervention on the affected shoulder(s), inability to complete testing questionnaires, and age less than 18 years. A convenience sample consisting of 28 men and 20 women (mean age, 40.8 years) without shoulder problems, as determined by self-reporting, was recruited as a comparative group from clinic staff and patient escorts.
Questionnaires All subjects completed a Short Form (SF)–36.30,31 The Physical Component Summary Score of the SF-36 was calculated as described by the authors by use of a computer program and was used to indicate overall physical health quality of life. The patients with rotator cuff pathology also completed the Shoulder Pain and Disability Index (SPADI).4,11,24,25,32 The scores were computed as described by the authors. All questionnaires and strength tests were completed on the same occasion.
Strength testing Strength testing was performed with the LIDO computerized isokinetic dynamometer (Loredan, Inc, Davis, CA). Subjects were seated upright and asked to keep their backs flat against a heavy straight-back chair throughout testing to prevent trunk compensatory movements. The plane of the scapula position was used for testing. By use of a standard goniometer, the test shoulder was positioned in 45° abduction and 30° forward flexion. The drum of the dynamometer was tilted 45° from the vertical, and the axis of rotation of the humerus was centered with the axis of rotation of the dynamometer arm. Subjects were asked to grasp a handgrip and maintain the forearm and wrist in neutral pronation/supination. Shoulder isokinetic internal and external rotation strength was measured separately by use of continuous, reciprocal concentric and eccentric contraction cycles at a speed of 75°/s. Strength was tested through 90° of motion starting from ⫺45° (ie, 45° of internal rotation) to ⫹45° (ie, 45° of external rotation) from the horizontal. Isometric strength was measured in a neutral internal/external position. Submaximal practice for 1 to 2 sets was used to familiarize patients with the test protocol. One maximal practice repetition was performed before data were collected for analysis. The average peak torque generated from 3 maximal contractions was calculated and recorded as the criterion for each strength measurement. Both shoulders were tested in each individual.
Data analysis All statistics were calculated with SPSS 11.5.21 The test-retest reliability of strength and self-reporting scales was examined by use of intraclass correlation coefficients (ICCs type 2,1) and their associated 95% CIs.27 The differences in strength between patients and the unaffected control group was evaluated by use of analysis of variance at an ␣ ⫽ .05. Pearson correlation coefficients were calculated to assess the univariate relationship between strength and functional measures. Multiple linear regression was used to determine the relative importance of different strength measures with regard to patient-reported pain and disability. Regression equations were computed in two ways. Forward regression indicated the most efficient predictor equation (fewest number of variables), whereas backward regression was used to reflect the most comprehensive model (more significant variables). The outcome (dependent) variables were both quality of life (SF-36 Physical Component Summary Score) and shoulder pain and disability (SPADI). Independent variables entered into regression models for both analyses included the strength variables (isometric, concentric, and eccentric for both internal and external rotation), as well as age and sex. To assess the impact of rotator cuff pathology on quality of life, a single indicator variable for the presence of rotator cuff pathology was added to the list of independent variables entered into the stepwise multiple regression analysis of the SF-36 Physical Component Summary Score. The level of significance was set at P ⫽ .05.
RESULTS All subjects completed the study protocol without difficulty. Test-retest reliability tended to be moderate to high (Table I) for strength scores and high for self-reported disability measures (Table II). Patients with rotator cuff pathology were significantly weaker than subjects without pathology in both internal and external rotation (Table III). The ratio between internal and external rotation forces varied according to the muscle action tested and presence of pathology but indicated that external rotation was more compromised that internal rotation strength. The correlations between strength measurements and SPADI total scores indicated that lower strength was associated with greater pain and disability (negative correlations), although the strength of this relationship ranged from low (⫺0.25) to moderate (⫺0.56). Isometric external rotation showed the strongest correlation with SPADI scores (r ⫽ ⫺0.56). All
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Table I Reliability (ICCs) for Strength testing patients with rotator cuff pathology using LIDO Subjects without pathology (n ⴝ 19)
Patients (n ⴝ 24) Measure Concentric External Rotation Eccentric External Rotation Isometric External Rotation Concentric Internal Rotation Eccentric Internal Rotation Isometric Internal Rotation
Affected
Unaffected
Dominant
Nondominant
0.86 0.76 0.83 0.79 0.83 0.78
0.93 0.70 0.79 0.79 0.73 0.85
0.89 0.76 0.90 0.87 0.74 0.88
0.91 0.83 0.93 0.86 0.82 0.94
Average peak torque measurements on two separate occasions were tested for reliability and intraclass correlation coefficients (ICCs) calculated. A benchmark of 0.75 has been used to indicate excellent reliability.
Table II Reliability of Patient Rating Scales
Scale SPADI (n ⫽ 24 patients with rotator cuff-pathology) SPADI Pain scale (5 items Mean Score ⫽ 290/500 or 58/100) SPADI Function scale (8 items; Mean Score ⫽ 365/800 or 46/100) Total SPADI Score (Mean Score ⫽ 52/100) SF-36 (n ⫽ 40, 24 with and 16 without shoulder pathology) Physical Function Physical Role Bodily Pain General Health Vitality Social Function Role Emotional Mental health Physical Component Summary Score Mental Component Summary Score
ICC
95% Confidence Interval
0.89 0.92 0.95
0.76–0.95 0.83–0.97 0.89–0.98
0.94 0.90 0.97 0.91 0.85 0.92 0.72 0.90 0.94 0.83
0.89–0.97 0.81–0.94 0.94–0.98 0.84–0.95 0.73–0.92 0.86–0.96 0.52–0.84 0.82–0.95 0.89–0.97 0.70–0.91
The ICCs for test-retest reliability of self-report and associated 95% confidence intervals indicates that measures of physical disability are highly reliable in these patients.
values were statistically significant (P ⬍ .05) except for isometric internal rotation (Table IV). Table IV displays Pearson correlations between strength measurements and the SF-36 Physical Component Summary Score. These also ranged from low (⫺0.38) to moderate (⫺0.47), with all values reaching a level of statistical significance (P ⬍ .01). Overall, correlations were higher for external rotation strength than internal rotation strength across all modes of testing. Isometric external rotation was the best predictor of pain and disability during a forward regression analysis of the SPADI scores (R2 ⫽ 0.36), whereas the additional significant variables identified by a backward regression analysis also included isometric internal rotation and eccentric internal rotation (R2 ⫽ 0.45). The presence of rotator cuff pathology was the only significant factor affecting physical health quality
of life during a forward regression analysis of SF-36 Physical Component Summary Score. In fact, 71% of the variation between subjects’ health status was related to the presence of rotator cuff pathology. A small additional impact on physical health scores resulting from sex, weight, and eccentric external rotation strength was identified by the backward regression analysis (R2 ⫽ 0.78). DISCUSSION This study determined that strength, pain/disability, and physical health status can be measured reliably in patients with chronic rotator cuff tendinopathy. It further illustrated the substantial impact that rotator cuff tendinopathy has on quality of life and the importance of shoulder strength as a component of that disability. Although strength was important, there
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Table III Difference in Strength Between Patients and Subjects without pathology Strength Measure (Nm) External Rotation Concentric Eccentric Isometric Internal Rotation Concentric Eccentric Isometric Ratio (Internal/External) Concentric Eccentric Isometric
p value
Patients Asymptomatic Mean (SD) 12 (8) 19 (9) 17 (8)
Mean (SD) 21 (10) 28 (11) 27 (11)
⬍0.0001 ⬍0.0001 ⬍0.0001
15 (10) 21 (9) 24 (11)
22 (12) 28 (12) 33 (13)
0.004 0.004 0.001
1.25 1.10 1.41
1.04 1.0 1.22
Average peak strength scores in Nm were significantly lower in patients that in controls for all modes of strength testing and in both rotations. The higher ratios of internal/external strength in patients indicate relatively greater impairment in external rotation strength vs internal rotation in patients with cuff pathology.
Table IV Correlation between strength of the affected arm and SPADI Total Scores (n ⫽ 36)
Strength Measurement Concentric External Rotation Eccentric External Rotation Concentric Internal Rotation Eccentric Internal Rotation Isometric External Rotation Isometric Internal Rotation
Shoulder Pain and Disability
Physical Component Summary
⫺0.41* ⫺0.46** ⫺0.45** ⫺0.49** ⫺0.56** ⫺0.25
⫺0.38* ⫺0.42* ⫺0.38* ⫺0.40* ⫺0.38* ⫺0.31*
*p ⬍ 0.05, **p ⬍ 0.01 Negative correlation indicates that as strength decreases, self-reported pain and disability increases. A moderate relationship exists between strength and pain/disability. The relationship between strength measures and physical health quality of life was also significant and in a similar direction, although slightly less strong.
was no evidence suggesting that extensive isokinetic testing was more useful than isometric strength testing in evaluating these patients. Of the three muscle forces measured, eccentric contractions tended to demonstrate lower test-retest reliability. If 0.75 is taken as the benchmark for excellent reliability,6 then all isometric and concentric contractions achieved this status, whereas only 5 of 8 eccentric contractions did so. This finding is consistent with previous research.20 Several factors may have contributed to this finding. Some subjects were observed to have difficulty producing consistent eccentric contractions. These problems were more common in elderly and/or sedentary patients, suggesting that certain subgroups of patients may require additional familiarization with isokinetic equipment before they
can consistently produce maximal efforts during testing. A second factor to consider, when testing with the LIDO dynamometer, is that it restricts eccentric contractions to a percentage of the previous concentric contraction as a safety measure against injury. Therefore, when subjects did not perform maximally on their concentric contraction, this may have affected the ability to produce maximal forces on the subsequent eccentric contraction. As few subjects had difficulty performing concentric contractions, this was thought to be a minor component of measurement error in this study. Although, theoretically, one might assume that eccentric (lengthening) contractions of the external rotators provide the most information on functional integrity of the cuff muscles, theory must be supported by evidence. In our patient group, which focused on chronic (presurgical) degenerative cuff pathology, that was not true, and our reliability data suggested that this may be because of the greater measurement error associated with this form of strength testing. Establishing optimal testing positions and protocols for patients is an important issue, as it has been observed that only 30% of patients with shoulder pathology could tolerate testing with a Cybex protocol.23 Protocols that can be completed by a minority of patients will introduce biases in studies reporting clinical outcomes. Therefore, we tested patients while sitting with the arm positioned in the plane of the scapula. Proponents for testing the shoulder in this position contend that it allows for optimum lengthtension relationships of the humeral abductors and rotators, allows maximum joint congruency as the center of the humeral head and glenoid fossa coincide, maintains the capsular fibers of the glenohumeral joint in a relaxed position, and provides maximum safety and comfort during testing.9,22 Concerns regarding testing in the sagittal or frontal plane are associated with increased risk of impingement,10 instability, and compromise of the vasculature of the rotator cuff tendons.22 In addition, because elderly patients may have difficulty with the supine position used in studies testing unaffected subjects,16 we used the sitting position. Finally, a slower speed (75°/s) was selected in consideration of the age distribution of our patients, as well as to minimize risk during eccentric contractions. All of our volunteers were able to complete this test protocol, suggesting that it is appropriate for clinical studies. Despite our efforts to evaluate a viable clinical test protocol, isokinetic tests did not demonstrate any advantage over isometric tests, with regard to either reliability or its relationship to disability outcomes. Because isometric testing has lesser requirements in terms of both equipment and staff costs, the use of isokinetic measures is unnecessary, unless a substantive rationale is established. If isokinetic measures
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were more predictive of the need for surgery, responsive to measuring treatment effects or related to the ability to return to work, then the additional information might be justified. Until additional evidence supports more definitive conclusions on the relative utility of different strength measures, isometric measures are reliable and valid indicators. Our data demonstrate that patients had significantly lower strength than a convenience sample of subjects without shoulder pathology. This deficit was greater in external rotation but was also present in internal rotation. Involvement of the internal rotators and disuse atrophy may have contributed. Although clinicians and patients are aware of the impact of rotator cuff pathology, this study provided statistical evidence as to the substantial impact of rotator cuff pathology, both specifically on the shoulder (SPADI ⫽ 52/100) and also more globally on the patients’ overall physical health status. Other studies, also using the SF-36, have reported that shoulder pathology and other musculoskeletal conditions have a significant impact on an individual’s perception of quality of life.2 In fact, the size of this impact is comparable to the effects of conditions such as hypertension, congestive heart failure, diabetes mellitus, myocardial infarction, and clinical depression.31 A greater appreciation of the impact of individual musculoskeletal conditions on quality of life underscores the need for expedient and effective treatment for these disorders. Isometric measurement of shoulder rotation strength provides reliable information on the functional integrity of the rotator cuff muscles, which is significantly related to patients’ function and quality of life. Strength measures should be combined with selected reliable self-reported disability measures to provide a comprehensive description of outcomes in research and clinical reporting. Ismail Gangat and Ana-Mara Marinescu are recognized for their role in completion of this study, which included recruitment of subjects, patient testing, and contributions to initial drafts of the manuscript. REFERENCES
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