A new look at the shoulder anterior capsuloligamentous complex complementing the insertion of the subscapularis tendon-Anatomical, histological and ultrasound studies of the lesser tuberosity enthesis.

Purpose: The aim of this study was to examine the relation between the anterior capsuloligamentous complex (ACLC - recognized as a single structure composed of the anterior capsule and its ligaments: medial glenohumeral ligament and anterior bundle of inferior glenohumeral ligament) and the subscapularis tendon with their footprints on the lesser tuberosity.

Methods: In this study, 19 fresh cadaveric shoulder specimens were examined: 13 in morphometric measurements and 6 were examined in a histological study. The subscapularis tendon and the ACLC were dissected until their insertion onto lesser tuberosity. Measurements of both dissected structures and their footprints on the lesser tuberosity were taken with a standard caliper. Six shoulders for histological examination were dissected without separation of the subscapularis tendon from the ACLC and longitudinal and transverse section samples were taken. Additionally, two of these six shoulders underwent ultrasound assessment before final sample preparation.

Results: Two well-isolated structures were clearly identifiable: muscle with its tendinous chords and the ACLC, forming together the anterior wall of the joint. The ACLC insertion complemented the tendon insertion - superiorly the thickest part of the tendon stayed in contact with the thinnest part of the ACLC and inferiorly the relation was opposite - the ACLC insertion reached its maximum transverse length. This reciprocal relation was similar to superior and posterior rotator cuff tendon-capsule complex. The footprint on the lesser tuberosity, being purely fibrocartilage enthesis, was composed in 45% of the ACLC insertion. Also the fibers run of the tendon and the ACLC were different: histologic assessment confirmed the tight fusion of the tendon and the ACLC, though those two layers were easily identified as their fibers never mixed.

Clinical Relevance: Better understanding of the anterior shoulder compartment anatomy simplifies an understanding of some arthroscopic procedures. The shape of the footprint, an importance of a broad superior part of the subscapularis tendon and its relation to the ACLC could be an anatomic proof and explanation for already existing surgical activities: subscapularis release and repair and soft tissue or bony procedures (Latarjet) in anterior shoulder instability. We declare that the experiments comply with the current law of the country in which they were performed (i.e. Polish law).