Clearly, we have not come to the end of all that can be known about human anatomy. One example of new discoveries involves the very area you described: the biceps muscle of the upper arm. This particular muscle has two parts to its tendon: a long and short head. They have different insertion points on the humerus (upper arm bone). The long head of the biceps becomes part of the labrum, a rim of fibrous cartilage around the shoulder socket (glenoid). The labrum helps give the shallow glenoid greater depth.
Cadaver studies, arthroscopic examinations, and MRIs have helped identify the presence of distinct anatomic variations in these structures (i.e., the tendon insertion points, labral attachments). For example, sometimes the long head of the biceps begins along the posterior (back) aspect of the labrum. In some people, the pattern of origination of the long head of the biceps is more toward the middle (anterior/posterior) of the labrum. And in others, it can be seen entirely along the anterior (front) labrum.
Likewise, there can be anatomic variations of the glenoid labrum. These are seen as a hole (referred to as a sublabral foramen). Instead of a continuous ring of tissue around the glenoid, there is a space where the labrum is absent. The foramen or hole also varies in size and shape. At some point (further along on the edge of the shoulder socket), the fibrous cartilage continues.
It appears that these anatomic differences affect slightly more than 10 per cent of the general population (based on a study of 100 cadavers). The changes are likely formed during embryologic development (as the body was formed in the uterus). Such differences may lead to rotator cuff injuries because they allow the arm to more into internal rotation more than is normal.
Injuries that affect the labrum (e.g., superior labrum anterior and posterior or SLAP tears) can also involve the long head of the biceps if the labral tear occurs at the insertion point of the tendon. But the presence of the sublabral foramen can be misleading when the surgeon is trying to make a diagnosis. The patient with this anatomic variation looks like he or she has a labral tear when, in fact, none is present.
There are other anatomic differences from person to person affecting the biceps tendon. These include: (1) shape and depth of the bicipital groove (indentation where the tendon rests along the front of the humerus), (2) presence and strength of connective tissue that holds the tendon in the groove (called the vinculum), (3) changes in the bone (e.g., bone spurs) that can affect the tendon as it moves up and down over the bone during repetitive motions.
Each of these individual anatomic variations must be taken into consideration when deciding what surgical procedure to choose. During surgery, the placement and number of anchors to reattach the labrum and the biceps may depend on the severity of the tear as well as the patient’s individual anatomic differences. It sounds like you were able to hear some of the discussion in the operating room around these decisions.