The anatomy of the attachment of the long head of the biceps onto the glenoid has been described as "posterior" or "posterior-dominant" in roughly 70% (55-82%)of patients in studies by Tuoheti et al and Vangsness et al. The most common type is posterior dominant. Only 25% (17-33%) of shoulders have an equal distribution of anterior and posterior attachment to the glenoid, and very rarely ~5% (0-8%) is the attachment anterior only. Histologic examination demonstrate that the structural fibers of the long head of the biceps attach directly to the superior glenoid tubercle in nearly 50% of shoulders while in the other half the biceps tendon inserts predominantly into the posterior glenoid labrum which in turn attaches to the glenoid.
Vangsness CT Jr, Jorgenson SS, Watson T, Johnson DL. The origin of the long head of the biceps from the scapula and glenoid labrum. An anatomical study of 100 shoulders. J Bone Joint Surg Br. 1994 Nov;76(6):951-4.
PMID:7983126 (Link to Abstract)
Tuoheti Y, Itoi E, Minagawa H, Yamamoto N, Saito H, Seki N, Okada K, Shimada Y, Abe H. Attachment types of the long head of the biceps tendon to the glenoid labrum and their relationships with the glenohumeral ligaments. Arthroscopy. 2005 Oct;21(10):1242-9.
PMID:16226654 (Link to Abstract)