Anatomy Monday: The Rotator Cuff

The discussion always starts the same way. A client comes in with shoulder movement restricted and painful. The pain has been the result of a traumatic event, a fall or a particular, sudden and typically unusual movement. The injury is often pretty old and sometime has been diagnosed as a problem with the rotator cuff.
My following question is typically: “which rotator cuff muscle(s) did your doctor diagnose/repair/operate on?”
The answer I get more often then not is a blank stare.
There isn’t any single muscle called the rotator cuff in the human body. What is referred to as the rotator cuff is a collection of muscles that I will present to you in this post.
Anatomically speaking, the shoulder is the area where the arm is attached to the trunk (or thorax). It is a interestingly complex joint providing somewhat contradictory functions. It needs to be flexible to allow for a very wide range of movements but it also needs to be strong and fixed to allow for example lifting or pushing .

There are, in effect three, joints involved in the movement of the arm at the shoulder. The primary one, and the one that interests us is called the glenohumeral joint. It is where the humerus (the large bone in the arm) articulates with the glenoid cavity of the shoulder blade (the scapula).

A ball and socket joint, the head of the humerus is two to three times larger than the glenoid cavity. As opposed to the hip joint, the humerus is not encased into the glenoid fossa. This allows for a very mobile joint but also a much less stable joint. To insure the stability of the shoulder, a group of four muscles originating from the shoulder blade and attaching to the arm bone creates a cuff around the joint. These muscles as a unit are referred to as the rotator cuff.

Probably the most powerful of those muscles and one of the least visible and accessible to the untrained person is the subscapularis. The subscapularis (or subscap as massage therapists like to call it) originates in the anterior surface of the shoulder blade. If you’re looking at someone’s back, it sits ‘underneath’ the shoulder blade, closer to the ribs. It attaches to the front of the humerus (technically on the lesser tubercule of the humerus). It is responsible for the internal (medial) rotation of the arm. The internal rotation is when you rotate the bone towards the midline of the body.

The next rotator cuff muscle is the supraspinatus. The supraspinatus originates on the top aspect, above the palpable ridge of the shoulder blade (technically in the supraspinous fossa – literally above the spine of the scapula). The muscle has a long tendon that connects to the highest point of the head of the humerus (technically on the superior and medium facet of the greater tubercle of the humerus). The tendon passes under the joint between the shoulder blade and the collar bone (the acromioclavicular joint to be precise). That tendon is often the source of many restrictions of shoulder movements. The principal action of the muscle is abduction of the arm or lifting the arm away from the body’s midline.

Next is the infraspinatus. The infraspinatus is the muscle sitting immediately below the palpable ridge in the shoulder blade (or in the infraspinous fossa). It attaches to the greater tubercle of the humerus, a little behind where the supraspinatus attaches (to be precise, at a point posterioinferior to the insertion of the infraspinatus, the middle facet of the greater tubercle). The principal action of the muscle is to externally (laterally) rotate the arm. It does the opposite action to the subscapularis.

And last but not least, the fourth rotator cuff muscle is the teres minor. Teres minor originates from the middle half of the lateral border of the shoulder blade. If you consider the shoulder blade as a right triangle with its right angle closer to your neck, teres minor attaches to the middle half of the hypotenuse. It attaches to the greater tubercle of the humerus, just under the insertion of the infraspinatus. Its primary action is also the lateral rotation of the arm and is solicited when the arm is slightly behind the body.

The rotator cuff muscles’ main function is to keep the head of the humerus in the glenoid cavity during various movements. It is interesting to note that the front and lower aspect of the joint (the anteriorinferior aspect) is not reinforced by the rotator cuff. When you dislocate the shoulder, it is often during a downward and forward thrust movement, such as when swimming freestyle for example.

Knowing which of the rotator cuff muscles is compromised is important to tailor the treatment to your injury. A lot of work around strengthening and rehabilitation work can be done to help restore a lot of the shoulder’s function. So the next time your physician diagnoses a problem with your rotator cuff, ask which muscle is involved!

Olivier;

Tagged as: anatomy, frozen shoulder, infraspinatus, rotator cuff, shoulder, subscapularis, supraspinatus, teres minor