Imbalance of the internal and external rotator muscu lature of the shoulder, excess capsular laxity, and loss of capsular flexibility, have all been implicated as etio logic factors in glenohumeral instability and impinge ment syndrome; however, these assertions are based largely on qualitative clinical observations. In order to quantitatively define the requirements of adequate pro tective synergy of the internal and external rotator musculature, as well as the primary capsulolabral re straints, we prospectively evaluated 53 subjects: 15 asymptomatic volunteers, 28 patients with glenohu meral instability, and 10 patients with impingement syndrome. Range of motion was evaluated by gonio metric technique in all patients with glenohumeral insta bility and impingement. Laxity assessment was per formed 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 lim itation of shoulder motion and minimal laxity on drawer testing. Both anterior and multidirectional instability pa tients had excessive external rotation as well as in creased capsular laxity in all directions. Sixty-eight per cent of the patients with instability had significant im pingement signs in addition to apprehension and capsular laxity.
Isokinetic testing of asymptomatic subjects demon strated a 30% greater internal rotator strength in the dominant shoulder. Comparison of all three experimen tal groups demonstrated a significant difference be tween internal and external rotator ratios for both peak torque and total work.
Conclusions are that there appears to be a domi nance 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 flex ibility and laxity of the shoulder. Isokinetic testing po tentially may be helpful in diagnostically differentiating between these two groups in cases where there is clinical overlap of signs and symptoms.
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