Effect of selective experimental suprascapular nerve block on abduction and external rotation strength of the shoulder.

The relative contribution to strength of the supraspinatus and infraspinatus to abduction and external rotation is not precisely known. Optimal testing positions for clinical assessment of supraspinatus and infraspinatus function are not precisely defined. We sequentially performed electromyographically controlled, complete nerve blocks of the suprascapular nerve, first at the spinoglenoid notch, to induce complete, isolated infraspinatus palsy. The suprascapular nerve was then blocked at the scapular notch, resulting in complete supraspinatus and infraspinatus palsy in 11 volunteers. Strength of abduction in the scapular plane and of external rotation was measured at 0 degrees , 30 degrees , 60 degrees , and 90 degrees of abduction, by use of a validated, instrumented strength-measuring device. Complete isolated infraspinatus palsy was obtained in 11 volunteers and complete combined supraspinatus and infraspinatus palsy in 9. Infraspinatus palsy alone led to a loss of approximately 70% of external rotation strength and approximately 45% of abduction strength. The degree of loss of strength was not dependent on the angle of abduction. Paralysis of the infraspinatus and the supraspinatus led to a loss of external rotator strength of approximately 80% and a loss of abductor strength of approximately 75%. The loss was again independent of the angle of abduction at which the testing took place. Testing of supraspinatus and infraspinatus strength can be performed at any angle between 30 degrees and 90 degrees without a change in specificity. The contribution of the infraspinatus to abduction strength is great. In the acute situation, the teres minor does not contribute more than 20% of external rotation strength at any degree of abduction.