The shoulder is one of the most mobile joints in the body and allows movement in a number of different directions. We need adequate mobility in our shoulders to enable us to do various activities in our daily lives – hanging the washing on the line, putting on a shirt or even throwing a ball. Unfortunately the shoulder sacrifices stability for this mobility and this is why it is vulnerable to injury and the most commonly dislocated joint in the body. However there are some things we can control to improve stability of the shoulder joint through movement.
The anatomy of the shoulder
The shoulder joint includes the collar bone (clavicle), the shoulder blade (scapula) and the upper arm bone (humerus). It includes a number of joints that work together to produce movement. The glenohumeral joint exists between the humerus and the scapular and is the main joint in the shoulder complex responsible for arm movements.
Extensive musculature surrounds the glenohumeral joint to help move the arm effectively. Ligaments and the joint capsule attach around the glenohumeral joint and become tight when the arm gets into the extremes of motion to reduce the chance of subluxation or dislocation. In addition, the glenoid labrum makes the shoulder socket deeper and provides a better fit for the head of the humerus.
What is glenohumeral instability?
The inability to maintain the humeral head centered in the glenoid fossa.
If the shoulder becomes increasingly unstable, the risk of complete separation of these articular surfaces without spontaneous relocation increases. This action is considered a dislocation. In addition, subluxations can occur whereby a symptomatic translation of the humeral head occurs within the glenohumeral joint without complete separation of the articulating surfaces.
What are the symptoms of shoulder instability?
- Sensations of looseness in the shoulder joint as if the shoulder partially comes out of place
- Pain with certain activities and movements sometimes associated with clicking or clunking
- Secondary conditions may develop (bursitis and tendinopathies) as a result of excessive movement within the shoulder joint
Why is my shoulder unstable?
The shoulder socket (glenoid fossa) is shallow and provides little support for a large humeral head. It is similar to a golf ball on a tee. In comparison to other joints within our body, it is easy to see why the shoulder is the most unstable but mobile joint in our body. The acetabular fossa of the hip is much deeper which stabilises the joint well but in doing so restricts mobility.
The ability of the humeral head to stay in contact with the glenoid fossa is achieved through static and dynamic factors.
Static factors
These include the structures discussed previously including the labrum, ligaments and capsule surrounding the glenohumeral joint. In athletes, instability can occur as the result of specific movements or impacts affecting the integrity of these structures supporting the shoulder joint.
- Contact sports often lead to structural changes within the labrum (macroinstability) from trauma.
- Overhead sports and throwing sports (swimming, tennis, baseball) lead to adaptations of the glenohumeral joint capsule and ligaments, which get more lax over time.
Dynamic factors
Instability can develop due to poor control and weakness of the shoulder blade muscles and the rotator cuff muscles.
- The rotator cuff muscles position the head of the humerus within the glenoid fossa during movements of the arm. It is important to restore functional strength of the rotator cuffs through full range of movement to correct the displacing forces from the principal shoulder movers, especially in end of range positions. If weakness is apparent in this muscle group the shoulder is less stable and movement will not be as well controlled.
- Regaining strength and control of the shoulder blade provides a stable base from which the arm can move, similar to a crane and it’s base. If there is poor control of the shoulder blade and weakness of the muscles that move it, the head of the humerus will lose contact with the glenoid fossa resulting in instability.
How can you improve stability to prevent shoulder injuries/ dislocations?
- Relieving stiffness of any muscles or joints involved within the shoulder complex to develop a stable resting and dynamic posture
- Restore functional strength of the rotator cuffs through range of movement
- Regain strength and control of shoulder blade movements
References:
- Escamilla RF, Yamashiro K, Paulos L, Andrews JR. (2009). Shoulder muscle activity and function in common shoulder rehabilitation exercises. Sports Med.; 39(8):663-85.
- Labriola, J.E., Lee, T.Q., Debski, R.E., McMahon, P.J. (2005) Stability and instability of the glenohumeral joint: the role of shoulder muscles. J Shoulder Elbow Surg; 32-38
- Lee SB, Kim KJ, O’Driscoll SW, Morrey BF, An KN. (2000). Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavera. J Bone Joint Surg Am.; 82(6):849-57.
- Abboud JA, Soslowsky LJ. (2002) Interplay of the static and dynamic restraints in glenohumeral instability. Clin Orthop Relat Res. (400):48-57. Review.
- Borsa PA, Laudner KG, Sauers EL. (2008). Mobility and stability adaptations in the shoulder of the overhead athlete: a theoretical and evidence-based perspective. Sports Med.;38(1):17-36.
