The shoulder joint provides a very large range of motion. If you do no believe me, test your own shoulders! You can place your hand all the way up in the air, reach pretty high behind your back (like when you scratch those back itches), and turn your hand out to the side almost perpendicular to your body. So much motion is only possible because of the difference in size and geometry between the ball and the socket of the shoulder joint.
The shoulder joint has been compared to a golf ball on a tee. The humeral head (ball) is very large compared to the much smaller and almost flat glenoid socket (tee). Hence, the humeral head can spin on the glenoid to provide the very ample range of motion we just described. But as a consequence, the ball also can easily fall out of the socket. When the humeral head comes completely out of the glenoid, the term shoulder dislocation is used; when it moves to the point of almost falling off the socket, but not quite, the term shoulder subluxation is used. Shoulder instability includes the spectrum of shoulder subluxation and dislocation.
So how come the shoulder joint does not dislocate all the time?
In healthy shoulders, the humeral head stays well centered on the socket. This is partly due to the labrum and capsule, and partly due to the muscles and tendons around the joint. The labrum is a thick soft-tissue reinforcement,“glued” all around the perimeter of the bony socket. It makes the socket deeper and wider (if you are interested in superior labral tears you may like this post). When the labrum tears, the ball can scape the socket through this tear if the position of the shoulder reaches its limit. The capsule is like a bag of fibrous tissue that wraps around the joint space. It limits how much the ball can displace in reference to the socket. The rotator cuff muscles (supraspinatus, infraspinatus, subscapularis and teres minor) also help keep the shoulder stable by contracting in a coordinating fashion; they are particularly relevant when the shoulder is the mid-range of motion, as opposed to the extremes of motion. Because the socket (glenoid) is in one corner of the shoulder blade (scapula), instability can also result from poor function of the muscles that determine the position of the shoulder blade in space (periscapular muscles), since the glenoid will be poorly positioned as well.
Unidirectional, multidirectional, traumatic, recurrent, TUBS, AMBRI MDI,… all these names!
If you have read about shoulder instability, you have run into some of these terms, if not all! Let’s try to simplify. There are basically two major categories of unstable shoulders. In the first category, an injury tears the labrum and stretches the capsule either in the front of the shoulder or –less commonly– the back of the shoulder. Since the shoulder comes out in only one direction, this is called unidirectional instability: it dislocates either in the front (anterior dislocation) or the back (posterior dislocation). When the labrum tears in the front, the term “Bankart lesion” is used (Dr. Bankart described this labral tearing in anterior dislocations of the shoulder a long time ago). The acronym TUBS is commonly used by surgeons to identify this first category of shoulder instability: it is the consequence of a Traumatic injury, it is Unidirectional and Unilateral, it often results from a Bankart lesion, and it oftentimes requires Surgery.
Multidirectional instability (MDI) is different! This second category affects individuals with loose joints in general. Typically, the capsule of both shoulder joints is excessively large, allowing extra translation of the humeral head in every direction. Individuals with excellent control of their rotator cuff and periscapular muscles may still keep the ball centered on the socket. When these individuals relax their muscles, the ball can be moved a lot; but if there is no shoulder pain and the function is good with activity then their shoulders are considered lax, not unstable. MDI happens when the capsule is too large, elastic and the muscles are in suboptimal shape. Surgeons commonly use the acronym AMBRI, since there is no initial injury (Atraumatic), there is instability in multiple directions (Multidirectional), laxity usually is Bilateral, Rehabilitation (physical therapy) is oftentimes successful, and when surgery is needed the Inferior capsule of the shoulder joint needs to be shifted, as we will discuss below. Some individuals with marked laxity can dislocate and relocate their shoulders at will (voluntary instability).
What patients with MDI feel…
Imagine that you are planning to go for run and you do not tie your shoelaces on one side. As you try to run, your foot will be sliding inside the shoe, forth, back and sidewise. Your foot will feel unstable, and you will barely be able to run. Something similar happens in MDI: the humeral head (ball) glides in an uncontrolled fashion all over the socket, and it may actually come out fully in the front, or in the back or both, most of the time unexpectedly. As a consequence, use of the shoulder is limited, painful, and more importantly the patient cannot trust his or her own shoulder. Very limiting!
How is MDI assessed?
If your shoulder feels unstable all the time in all directions, you will benefit from a consultation with a shoulder specialist. In addition to having your shoulder examined for motion and strength, your surgeon will assess how much he or she can displace the humeral head forth and back manually. He may pull down on your arm looking for a so-called sulcus sign (excessive inferior displacement of the humeral head creates a grove on the side of the shoulder). You may also be assessed for excessive laxity in other joints (Can you get your elbows past straight? Can you bend your wrist so that your thumb lies on your forearm?) If you score high in a laxity scale, you may be recommended to be tested for a more substantial abnormality of collagen, the main fiber in joints and tendons. Ehlers-Danlos syndrome is an example of a collagen disease. The strength and coordination of the muscles around your shoulder blade will be assessed as well.
Plain radiographs may be normal or may show a socket that is too shallow, elongated, and sometimes abnormally angled. Magnetic resonance will show a very large capsule, especially if the images are obtained after injected a solution (contrast) in the shoulder joint.
What are the treatment options?
Many individuals with MDI improve with physical therapy exercises. The main goal of these exercises is to improve the strength and coordination of the muscles around the shoulder blade as well as the rotator cuff. These exercises can be learned in a few sessions, but must be practiced on a regular basis in order to be effective. Remember that the muscles around the scapula and the rotator cuff muscles are both important!
A small number of patients with really large capsules or extremely elastic and thin tissues will not be able to regain shoulder stability with physical therapy, and surgery may be considered. Correction of MDI requires surgically decreasing the volume of the shoulder capsule. This can be achieved using an open procedure called inferior capsular shift, or using an arthroscopic procedure called arthroscopic capsular plication. Arthroscopic shoulder surgery is performed through small skin cuts under visualization with a camera. Open surgery requires a longer incision. Occasionally, the tissues of patients with MDI are poor, and needs to be supplemented with tissue grafts from donors with healthy tissues. Your shoulder specialist will discuss with you the relative benefits and potential disadvantages of open and arthroscopic procedures to shrink the shoulder capsule.
Why is my MDI not getting better?
The most common reasons for MDI not to get better include overlooking the muscles around the shoulder blade, not being able to improve the coordination of the rotator cuff muscles, or having such compromised capsule that muscle power is just not enough to keep the shoulder joint stable.