Patterns of flexibility, laxity, and strength in normal shoulders and shoulders with instability and impingement

Imbalance of the internal and external rotator musculature of the shoulder, excess capsular laxity, and loss of capsular flexibility, have all been implicated as etiologic factors in glenohumeral instability and impingement syndrome; however, these assertions are based largely on qualitative clinical observations. In order to quantitatively define the requirements of adequate protective synergy of the internal and external rotator musculature, as well as the primary capsulolabral restraints, we prospectively evaluated 53 subjects: 15 asymptomatic volunteers, 28 patients with glenohumeral instability, and 10 patients with impingement syndrome. Range of motion was evaluated by goniometric technique in all patients with glenohumeral instability and impingement. Laxity assessment was performed and anterior, posterior, and inferior humeral head translation was graded on a scale of 0 to 3+. Isokinetic strength assessment was performed in a modified abducted position using the Biodex Clinical Data Station with test speeds of 90 and 180 deg/sec. Internal and external rotator ratios and internal and external rotator strength deficits were calculated for both peak torque and total work. Patients with impingement demonstrated marked limitation of shoulder motion and minimal laxity on drawer testing. Both anterior and multidirectional instability patients had excessive external rotation as well as increased capsular laxity in all directions. Sixty-eight percent of the patients with instability had significant impingement signs in addition to apprehension and capsular laxity. Isokinetic testing of asymptomatic subjects demonstrated a 30% greater internal rotator strength in the dominant shoulder. Comparison of all three experimental groups demonstrated a significant difference between internal and external rotator ratios for both peak torque and total work. Conclusions are that there appears to be a dominance tendency with regard to internal rotator strength in asymptomatic individuals. Impingement syndrome and anterior instability have significant differences in both strength patterns of the rotator muscles and flexibility and laxity of the shoulder. Isokinetic testing potentially may be helpful in diagnostically differentiating between these two groups in cases where there is clinical overlap of signs and symptoms.