Do Anatomic Variants in the Shoulder Predispose People to Injury?
After finishing all of the seasons of Trailer Park Boys and the latest season of Breaking Bad on Netflix I really had no legitimate reason to delay further studies into how the human body works. One of the reasons I love writing is because of the research I get to put into it. It might be tedious and could take hours to write a post like this but it is certainly worth it. To me the shoulder is by far the most interesting joint in the body to study. After visiting a human anatomy lab about a month ago I started to develop an interest in anatomic variations in the human body.
With this post I wanted to examine a few of the anatomic differences in the shoulder complex to see if they play a role in injury. I’m going to talk about the os acromiale which is a failure of one of the ossification centers to fuse in the acromion, variability of the biceps, and scapular variability
The acromion has four centers of ossification called the basi acromion, the meta acromion, the meso acromion, and the pre acromion. There are three different types of os acromiale: “the pre-acromion, the meso-acromion (commonest) and the meta acromion (3).
In children the acromion has a separate center of ossification and is joined to the scapula by cartilage. When that cartilage does not fuse it becomes a separate bone which is called the “os acromiale”. It is a rare condition and has been found in only 8% of people in cadaver studies (3). Os acromiale has been hypothesized to be associated with impingement syndromes but the evidence is mixed.
“The os acromiale anomaly is an uncommon one (1-8%) but can be an important cause of the impingement syndrome” (4). The unfused segments can tilt forward to cause pain.
In one study the authors state that, “Reports of os acromiale associated with subacromial pathology have been cited to imply that this entity is a cause of subacromial impingement; however, no study has demonstrated an increased frequency of os acromiale in patients with shoulder pain compared with the frequency in the general population.” (5)
The previously mentioned study was conducted in 2000. In 2006 a study called “Rotator cuff tears associated with os acromiale,” the authors goal was to determine the “coincidence of os acromiale and rotator cuff tears”(3). They found that the percentage of people with rotator cuff tears with os acromiale was the same percentage as people with rotator cuff tears without os acromiale. They also found that the presence of the os acromiale has no influence on the number of tendons involved in a rotator cuff tear and thus concluded that the os acromiale more than likely did not have a pathological effect.
The long head of the biceps originates from the radial tuberosity. The biceps have several functions which include elbow flexion, shoulder flexion, and wrist supination. The long head of the biceps also acts as a dynamic restraint in the shoulder, helping to prevent superior migration of the humeral head. The long head of the biceps is an important muscle for pitchers because it may “play a protective role by diminishing the stress placed on the inferior glenohumeral ligament”(1). The inferior glenohumeral ligament has been shown to be the “most important static structure providing anterior stability when the shoulder is abducted and externally rotated” (1)
In a study called “the Origin of the Long Head of the Biceps From the Scapula and Glenoid Labrum” the authors found that “40% to 60% of the biceps tendon arose from the supraglenoid tubercle and the remaining fibers were attached to the superior glenoid labrum”(1).
Upon further study they found four different types of labral attachments
1) All the labral part of the attachment was to the posterior labrum with none to the anterior labrum
2) Most was to the posterior labrum, but with a small contribution to the anterior labrum.
3) Equal contributions to anterior and posterior labrum
4) Most attached to the anterior labrum, with a small contribution to the posterior labrum.
Another anatomical variation that has to do with the biceps is the accessory head which is seen in “9.1% to 22.9% of the population”(2). More relevant to surgeons and less to trainers and PT’s is the importance of knowing about this variation so one does not mistake it for “a longitudinal split tear of the long head of the biceps tendon” (2).
As for injuries with biceps variation and I may be fishing for answers here, I found a case study on potential median nerve compression that may have been caused by an abnormal biceps muscle. They found that most of the fibers attached to the radial tuberosity but some continued in a tendon like fashion. The lateral aspect of this tendonous continuation compressed on the median nerve (6).
However, that is an n=1 case and it seems unlikely to affect any relevant percentage of the population. I could not find any evidence that biceps variability could be a contributing factor to injury.
A healthy scapula is crucial to the health of the shoulder complex as a whole. Dysfunctions in the muscles that attach to the scapula are often contributing factors to shoulder impingement. However, I’m more interested in talking about the variability in the bone structure of the acromion.
The acromion process is the most lateral aspect of the scapula. The acromion articulates with the collar bone via the acromioclavicular joint.
There are three different types of acromion with the second and third type associated with rotator cuff impingement. Type I is flat, type II is curved, and type 3 is hooked.
At this point, acromion variability is pretty common knowledge, most people have heard of the different types before. However it has been hypothesized that, “the hooked acromion is in fact an acquired form and increases an individual’s predisposition to rotator cuff pathology” (2).
In an examination of over 700 scapulas it was discovered that no hooked specimens were to be found in the bones of people under the age of 30. As age increased the hooks became more common and larger. The author found that the hook was formed by new bone growth at the insertion site of the coracoacromial ligament. The author concluded that, “the hooked acromion is not an anatomical variant but results from ossification in the attachment of the coracoacromial ligament” (7).
However, variant or not the hooked acromion is closely correlated with a higher incidence of rotator cuff tears.
Do anatomical variants predispose individuals to injury? From my research the answer seems to be closer to a no than a yes. I think this confirms what a lot of us already know, that imbalanced muscles, imbalances training, and repetitive movement patterns that cause chronic stress are more than likely the cause of injury and where we should look before we look at anatomic variations.
(1) the Origin of the Long Head of the Biceps From the Scapula and Glenoid Labrum
(2) Normal and Variant Anatomy of the Shoulder on MRI
(3) Rotator cuff tears associated with os acromiale
(4) The os acromiale: another cause of impingement.
(5) Os Acromiale: Frequency, Anatomy, and Clinical Implications
(6) A rare case of median never entrapment
(7) The Hooked Acromion Revisited