The shoulder joint is the most mobile joint in the body. This makes sense because we need to be able to reach overhead, reach in front of our bodies, reach behind our heads and reach behind our backs as part of our everyday function. Because it needs to be able to move in so many directions, the shoulder joint does not have a high degree of stability. Why does this matter?
The shoulder joint is one of the most dislocated joints in the body. To understand why, let’s talk about the anatomy of the shoulder. When thinking of the shape of the shoulder and its ‘socket’, it is helpful to compare it to a golf ball sitting on a tee. The golf ball is much larger than the tee that it sets on. This is also true of the shoulder joint. The head of the arm bone (humerus) sits a small concavity called the glenoid. In our comparison, the humeral head is the golf ball and the glenoid is the tee. It does not take much force at all to knock the golf ball from its tee, right? Thankfully, the shoulder has some additional stabilizing structures to keep us from dislocating so easily:
The labrum is a ring-like structure on the periphery of the glenoid that deepens the ‘socket’ and acts like a suction cup to keep the humerus in place.
The capsule is the name given to the connective tissue/fascia that surrounds and envelops the shoulder joint (much like Saran-wrap).
Ligaments of the shoulder connect the humerus to the scapula and provide stability.
Injury to any of these built-in stabilizing features of the shoulder can create increased undesired motion of the humerus, or instability, resulting in pain and dysfunction. When the static stabilizing features of the shoulder are compromised, the dynamic stabilizers become increasingly important. Dynamic stabilizers of the shoulder (i.e. the rotator cuff) are muscles that contract in a fashion to keep the humerus centered in the glenoid when using the arm.
It makes sense that injury to the labrum, the joint capsule or the ligaments of the shoulder can cause instability, but not all instability is traumatic. Some individuals are just more lax than others. We’ve all seen someone who is ‘double-jointed’ and can bend their bodies in ways that we never thought possible. Individuals who perform repetitive motions in extremes of ranges of motion (such as baseball pitchers) can also stretch ligaments or portions of the capsule without traumatic injury and develop atraumatic instability.
How does Physical Therapy Help?
If instability is not corrected, it can lead to further injury down the road. Your physical therapist will perform a thorough evaluation of your shoulder motion, strength, and function, and prescribe a plan of care designed to restore stability. This plan of care will differ based on whether the instability is traumatic or atraumatic.