Surgical anatomy of the posterior shoulder: effects of arm position and anterior-inferior capsular shift.

The purposes of this study were to evaluate anatomically various surgical intervals to the posterior shoulder and to determine the effects of varying arm positions and anterior-inferior capsular shift (AICS) on the relation of the posterior neurovascular structures to fixed bony landmarks. Fourteen cadaveric shoulders were dissected. The posterior surgical anatomy was defined, and the distances from fixed bony landmarks to neurovascular and musculotendinous structures were determined with digital calipers. Measurements were made with the arm in various positions and repeated after AICS. The most direct anatomic approach to the posterior shoulder was through a deltoid split in the raphe from the posterolateral corner of the acromion (PLCA), followed by an infraspinatus (IS) splitting incision. The IS/teres minor interval was at the inferior aspect of the glenoid rim and was difficult to locate in all specimens. The distance to the axillary nerve from the PLCA averaged 65 mm and decreased by an average of 14 mm (22%) with abduction and by 19 mm (29%) with extension. The posterior humeral circumflex artery was located along the humeral neck and was vulnerable to injury during lateral capsular dissection. The suprascapular nerve had multiple branches to the IS with most penetrating the muscle at its inferior portion. The closest branch to the glenoid rim was an average of 20 mm medial from it. No branch entered at the level of the IS raphe. The anatomic relations of the suprascapular nerve were unchanged after AICS. On the basis of this study, surgical exposure of the posterior shoulder with a deltoid split from the PLCA, followed by an IS split, appears to be anatomically safe. The arm position should be in neutral rotation, especially if previous anterior capsular procedures have been performed, which can alter the posterior neurovascular anatomic relations.