Study Design
To analyze the association between rotator cuff strength and glenohumeral instability and decrease the risk of bias induced by other variables, we included a homogeneous group of patients with shoulder instability and a homogeneous representative sample from the population with normal shoulders as controls.
We used two groups for this observational controlled study: patients with recurrent anterior posttraumatic glenohumeral instability who were selected prospectively from a physical medicine and rehabilitation outpatient unit and a representative, matched control group of nonathletic healthy subjects. Informed written consent was provided by each subject before his participation in the study. The local ethics committee approved all procedures used in this study. The variable of sex was omitted by including only men, and the variable of age was represented by including only subjects between eighteen and forty-five years old, which is the age during which glenohumeral instability is most frequent1. Only right-handed subjects were included to decrease the risk of bias from differences in strength between the dominant and nondominant upper extremities.
Patients included in the recurrent anterior instability group were right-handed men between eighteen and forty-five years old who had symptomatic recurrent anterior unidirectional posttraumatic glenohumeral dislocation, were scheduled for stabilizing surgery, and had a first posttraumatic dislocation (group IV according to the classification system described by Kvitne and Jobe2), a healthy contralateral shoulder, no reported associated injury on computed tomographic (CT) arthrography, no prior participation in sports involving the upper limb, and no contraindication to isokinetic shoulder testing24. The diagnosis of shoulder instability was based on physical examinations by the same surgeon (L.B.) following the recommendations of Dodson and Cordasco1. Our study focused on traumatic anterior glenohumeral instability because dislocations are more frequent in the anterior direction and traumatic injury represents approximately 95% of anterior shoulder dislocations1. The need for surgical stabilization was an important inclusion criterion to indicate that the impact of recurrent anterior shoulder instability was major. Patients with voluntary instability or rotator cuff disease, as determined by medical examinations following the recommendations of the senior author1 and on CT arthrography, were excluded.
Participants in the control group were recruited from hospital staff members. Inclusion criteria were chosen so that the only variable that differed between the two groups was the shoulder instability. The control group consisted of voluntary participants who were right-handed men between eighteen and forty-five years of age with no history of shoulder pain or dysfunction and no prior participation in upper-limb sports.
Demographic Data
Between 2005 and 2008, one surgeon (L.B.) recommended surgical treatment of the anterior shoulder instability for sixty-three patients. Twenty-six patients, including six women, four patients with bilateral instability, five patients who were left-handed, and eleven who did not agree to participate in this study, were excluded. Thus, thirty-seven patients were included in the group with recurrent anterior instability of the shoulder. The mean age (and standard deviation) of the patients at the time of surgery was 24.5 ± 7.5 years, the mean height was 178.8 ± 6.4 cm, and the mean weight was 73.6 ± 9.1 kg. The dominant side was involved in twenty patients and the nondominant side, in seventeen. The number of dislocations before surgery was a mean (and standard deviation) of 3 ± 2 dislocations (range, one to ten dislocations). The mean time between the first dislocation and surgery was 4 ± 5 years (range, 0.5 to twenty years). No patient underwent rehabilitation before the study. The group with recurrent anterior shoulder instability was divided into two subgroups for comparison: patients with recurrent anterior instability on the dominant side and those with instability on the nondominant side. The controls were a representative group because they matched the study group in terms of age, weight, and height, with no significant differences between the two groups. The characteristics of the recurrent anterior instability group and the control group are presented in a table in the Appendix.
Isokinetic Testing Procedure
The dynamic strength of the internal rotator and external rotator shoulder muscles was evaluated by an isokinetic Con-Trex dynamometer (Con-Trex MJ; CMV, Dübendorf, Switzerland). The same examiner (P.E.) conducted the tests for all subjects. Data were acquired with use of a computer with Con-Trex software, and peak torque and shoulder motion were recorded. The isokinetic dynamometer with the computing software was calibrated before data collection.
Subjects were seated and stabilized uniformly as described in the Con-Trex manual for internal rotation and external rotation, with 45° of shoulder abduction in the scapular plane and a range of motion of 70° (see Appendix)25. The humerus was aligned with the rotational axis of the dynamometer. The elbow was supported in 90° of flexion, and the forearm was in neutral pronation-supination. Self-adhesive straps were placed horizontally across the chest and pelvis to stabilize the trunk to the seat15. The range of motion was 15° for internal rotation and 55° for external rotation, from a reference position of the forearm at 0° in the horizontal plane25. Isokinetic assessments of internal rotator and external rotator shoulder muscles in this seated position were shown to be valid and reliable16,25. This seated position was chosen to minimize stress and injury to the anterior capsule and rotator cuff and to decrease the apprehension of the subject and the risk of shoulder dislocation, which may occur in testing above the horizontal level with 90° abduction of the shoulder5,6,24,26.
Before testing, each subject was informed about the procedure, the effort required, and uniform commands that would be used to begin and finish each testing sequence. A global warm-up was performed on the upper-body ergometer (Ergotonic 4000; Sopur, Heidelberg, Germany) for six minutes at 50 kg·m/min of power with a frequency of seventy-five to ninety revolutions per minute. The subject's arm and the testing apparatus were statically weighed to provide gravity compensation data, and corrections were incorporated27. Artifacts were controlled during testing28.
Both shoulders were tested, beginning with the uninvolved shoulder24, to establish a database to compare with the involved shoulder, to practice initial isokinetic familiarization, and to decrease the subject's apprehension24. As an initial familiarization with isokinetic movements and specific warm-up, subjects performed three series of six graded submaximal repetitions at an intermediate angular velocity of 120°/s. Data were recorded at three different angular velocities: a high angular velocity of 180°/s, an intermediate angular velocity of 120°/s, and a low angular velocity of 60°/s. Ten repetitions were performed at 180°/s, five at 120°/s, and three at 60°/s. A one-minute rest separated each series of movements. Subjects were orally supported without visual feedback. Each subject followed the same standardized procedure.
At each angular velocity, the dynamic strength of the internal rotator and external rotator muscles was evaluated by the peak torque normalized to body weight (peak torque to body weight). Use of a normalized measure such as peak torque to body weight allows for comparison between subjects of different morphological conditions within similar test populations12,16,29-31. Previous isokinetic research examining normative values in various muscle groups in men and women expressed peak torque values as a percentage of body weight29. The ratio of external rotator peak torque to internal rotator peak torque (external rotator to internal rotator ratio) was then calculated for each angular velocity.
Statistical Analysis
Means and standard deviations were calculated for all variables. The association between isokinetic internal rotator and external rotator muscle strength and glenohumeral joint instability was analyzed by both side-to-side comparisons and comparisons with a control group. At each angular velocity, two-way analysis of variance (ANOVA; the two factors analyzed were side-to-side differences and recurrent anterior instability) was used to test the effect of the side-to-side differences and the effect of recurrent anterior instability on the internal rotator and external rotator peak torque to body weight and the external rotator to internal rotator ratio. When a main effect of the two factors (side-to-side differences and recurrent anterior instability) or an interaction between the two factors was significant, additional statistical analyses were performed: one-way ANOVA and the Fisher protected least significant difference post hoc test to determine the side-to-side differences effect in each group (dominant recurrent anterior instability group, nondominant recurrent anterior instability group, and control group) and to determine the recurrent anterior instability effect for each shoulder side in comparison with a control group. Concerning the recurrent anterior instability effect, for the dominant recurrent anterior instability group, the dominant pathological shoulder side was compared with the healthy dominant shoulder side of control subjects. For the nondominant recurrent anterior instability group, the nondominant pathological shoulder side was compared with the healthy nondominant shoulder side of control subjects. These analyses involved use of StatView software (Abacus Concepts, Berkeley, California). The level of significance was set at p < 0.05.
Source of Funding
There was no external funding source.
The interaction between the two factors (side-to-side differences and recurrent anterior shoulder instability) was significant for the internal rotator and external rotator peak torque to body weight for all angular velocities (p < 0.05). The effect of the side-to-side differences on the internal rotator and external rotator peak torque to body weight and external rotator to internal rotator ratios was significant for all angular velocities (p < 0.05). The effect of recurrent anterior shoulder instability on the internal rotator and external rotator peak torque to body weight and external rotator to internal rotator ratios was not significant at each velocity. Peak torque to body weight values for the internal rotator and external rotator and the external rotator to internal rotator ratios are reported in Table I. The differences (in percentage) between the nondominant and dominant sides for the internal rotator and external rotator strength are shown in Table II.
With regard to the effect of the side-to-side differences for the control group, both internal rotator and external rotator peak torque to body weight were significantly higher on the dominant side than on the nondominant side at all angular velocities (p < 0.05) (Tables I and II), with no significant difference for the external rotator to internal rotator ratios. For the group with recurrent anterior shoulder instability on the dominant side, the external rotator peak torque to body weight was significantly lower for the dominant shoulder with pathological changes than for the healthy contralateral nondominant side at 180°/s and 120°/s, and the external rotator to internal rotator ratios were significantly lower for the dominant shoulder with pathological changes at 180°/s (p < 0.05) (Tables I and II). For the group with recurrent anterior glenohumeral instability on the nondominant side, the internal rotator and external rotator peak torque to body weight values were significantly lower on the nondominant shoulder with pathological changes than on the healthy contralateral dominant side at all angular velocities (p < 0.05), and the external rotator to internal rotator ratios were significantly higher on the involved nondominant shoulder with pathological changes than on the healthy, contralateral dominant side at all angular velocities (p < 0.05) (Tables I and II).
Concerning the effect of recurrent anterior instability (comparisons with a control group), one-way ANOVA revealed a significant effect on the internal rotator and external rotator peak torque to body weight at 180°/s and 120°/s (p < 0.05). Internal rotator and external rotator peak torque to body weight were significantly lower on the side with pathological changes in the shoulder than on the healthy shoulder side of controls (Table I and Fig. 1).
Internal rotator (IR) and external rotator (ER) peak torque to body weight in the group with recurrent anterior posttraumatic glenohumeral instability on the dominant side (DomRAIG), the group with instability on the nondominant side (NDomRAIG), and the control group (CG).
The main finding of this study was that patients with recurrent anterior instability have rotator cuff weakness. The relative value of this weakness as a function of side-to-side comparison was dependent on whether the dominant or nondominant upper extremity was involved. Side-to-side differences were increased when the nondominant side was involved and decreased when the dominant side was involved.
For healthy subjects in the control group, the internal rotator peak torque to body weight values and the external rotator peak torque to body weight values were comparable with the results for healthy subjects from other studies5,6,11,15,17,26,32-35. The external rotator to internal rotator ratio was comparable with the usual ratio of 0.60 to 0.80 on the healthy shoulder5,6,11,15,17,26,32,33, indicating our control group was representative of healthy subjects.
Classic, recurrent anterior shoulder instability is often associated with an internal rotator strength deficit because, during external rotation and abduction movements, the humeral head cannot be maintained in the glenoid fossa2,11,13. In a review of isokinetic shoulder strength assessment, Codine et al.11 reported a more consistent internal rotator to external rotator ratio close to 1.0 in glenohumeral instability (for a usual ratio of 0.60 to 0.80 on the healthy shoulder5,6,11,15,17,32,33) that showed a trend of internal rotator strength deficit. Our results are in agreement with this. Warner et al.6 and Tsai et al.13 reported an internal rotator strength deficit in patients with recurrent anterior shoulder instability prior to surgery. Moreover, our results reveal a global weakness of the rotator cuff strength concerning internal rotator and external rotator strength. In contrast, Rupp et al.7 reported higher internal rotator strength in swimmers with apprehension compared with healthy control subjects. Bak and Magnussen15 and Dauty et al.9 reported no side-to-side difference in internal rotator and external rotator strength in patients with unilateral shoulder instability. Prior reported results are controversial because of differences in study aims, populations, and strength assessment. Our study may have avoided the confounding influence of the side-to-side comparisons by the use of a control group because, for many patients with impingement or instability, the contralateral shoulder cannot be considered a normal referent side17. Additionally, the 10% to 15% side-to-side strength difference that is considered not pathological16,18-20 can constitute statistical bias for side-to-side comparisons15,17.
Concerning the relationship between the external rotator to internal rotator ratio and recurrent anterior shoulder instability, our results showed no differences for the external rotator to internal rotator ratios between the recurrent anterior shoulder instability group and the control group although the study may have lacked sufficient statistical power. Our results are in agreement with those of Codine et al.5 and Edouard et al.12 who suggested that a modification in the ratio should not be systematically considered a pathophysiologic factor12. The variability of external rotator to internal rotator ratios among individuals in one group of subjects with identical demands or stresses on the shoulder muscle was greater than the intergroup variability5,6,10,15,24. Caution should be taken in interpreting the external rotator to internal rotator ratio because of the low reliability18,34.
There were several limitations of our study. First, the number of patients and controls could be considered too few. Second, an eighteen to forty-five-year-old age group could be considered as too wide an age range. Third, concerning the link between glenohumeral instability and rotator cuff strength, determining whether the weakness is a cause or a consequence of the instability is difficult. The causes of this weakness are unclear and could be due to atrophy with disuse, pain, limited shoulder motion, apprehension, and/or anxiety19. Fourth, the internal rotator and external rotator muscles are not the exclusive muscles contributing to glenohumeral stability, since the deltoid and scapular muscles also have an important role3,23. The strength of dynamic stabilizers cannot be considered the exclusive or major factor of dynamic stability. Muscular activation, coordination, and coactivation may be important4. The coordination function of the rotator cuff muscles provides a net vector of compression to stabilize the humeral head in the glenoid4. The disruption of the rotator cuff strength balance could cause a deficient centering of the humeral head and could promote shoulder instability if there are insufficient passive stabilizers4,11. Determining and analyzing the center of rotation36 could be of interest to better understand the pathophysiology of glenohumeral joint instability and for rehabilitation; however, in clinical practice, isokinetic rotator cuff strength assessment can be relevant to guide and optimize the strength rehabilitation and to help in the decision to return to sports9,35. Moreover, although surgical stabilization is currently considered the standard treatment for recurrent anterior glenohumeral instability to restore the static stabilizer components1,9,37, rehabilitation programs may optimize dynamic stabilization to help to avoid preoperative or postoperative dislocations1,4,9,14,21-23.
In conclusion, our results showed that weakness in internal rotator and external rotator strength was associated with recurrent anterior shoulder instability. Shoulder instability was associated with side-to-side differences of rotator cuff strength. The side-to-side differences were increased when the nondominant side was involved and decreased when the dominant side was involved. We found no association between recurrent anterior instability and the external rotator to internal rotator ratio. Our results suggest that a control group is most appropriate in analyzing the influence of constraints on shoulder strength because the contralateral side often cannot be a normal reference. Further prospective studies are necessary to determine whether the weakness is a contributing etiology or consequence of recurrent instability. Other dynamic variables, such as muscular activation, coordination, coactivation, and the center of rotation displacement, should be analyzed as factors of glenohumeral joint dynamic stabilization.