An anatomic study on the mechanism of shoulder medial rotation contracture in obstetrical brachial plexus palsy

Abstract

Objective To observe the origin of the nerves innervating the subscapularis muscle and its anagonistic muscles at brachial plexus root or trunk level, and the blood supply of the subscapularis muscle in order to clarify the mechanism of subscapularis muscle contracture in obstetric brachial plexus palsy (OBPP). Methods 32 sides of adult human cadavers were used. The innervating nerve of the subscapularis muscle and its antagonistic muscles (the infraspinatus muscle, the teres minor muscle, the deltoid muscle and the supraspinatus muscle) were dissected retrogradely to trace its origin at the root and trunk level of the brachial plexus. The nerve fiber external diameter in the corresponding root and/or trunk was measured and the constituent ratios in the root and/or trunk of the brachial plexus were calculated. The nutrient arteries of the subscapularis muscle were observed to compare the number and distribution of the arteries to the upper parts of the subscapularis muscle and the lower parts. Results The nerve fibers innervating the subscapularis muscle mainly originated from the upper and middle trunk. The percentage in the upper trunk was (81.61 ± 6.85)%, while that in the middle trunk was (18.39 ± 6.85)%. The nerve fibers of its antagonistic muscles only originated from the upper trunk. Not only the number but also the diameter of the arteries entering the upper parts of the muscle was larger than those entering the lower parts. Conclusions The difference in the nerve fiber origin between the subscapularis muscle and its antagonistic muscles and the characteristics of the blood supply of the subscapularis muscle were the major causes of subscapularis muscle contracture in obstetric brachial plexus palsy.