Shoulder Instability – Part 2 of Shoulder Series.


We see many patients with shoulder instability thanks to either genetics giving them ligamentous laxity (you can’t pick your parents) or due to trauma causing a subluxation or dislocation (You have to live life!).

 
What is shoulder instability?


Recall the last post in which I described the bony structure of the shoulder as a basketball on a tea cup saucer – built for mobility. In an effort to keep the basketball centered in the tea cup saucer you have the rotator cuff muscles (“SITS” – Supraspinatus, Infraspinatus, Teres Minor, Subscapularis) functioning to control the biomechanics of the glenohumeral joint. The long head of the biceps muscle runs over the top of the humeral head (the ball) and attaches at the 12 o’clock position on the glenoid fossa (the socket). In addition, there are very strong ligaments within the joint capsule. This joint capsule surrounds the ball and socket like a rubber boot that encases the springs of a car – the entire joint is within the capsule. The ligaments within the capsule are the superior, middle and inferior glenohumeral ligaments. Then we have the labrum, a triangular shaped ring of cartilage around the glenoid fossa (the socket). It deepens the socket.

Such an incredible structure must have specific functions. Broadly categorized they are:

1. Create static stability – this is done by the ligaments, the capsule, the bony/labral structure.

2. Create dynamic stability – done by the rotator cuff and the long head of the biceps muscle.

Shoulder instability occurs when either static or dynamic stability is constrained. Let us look at two broad categories of shoulder instability:

 
1. Atraumatic Instability – failure of the static and dynamic stabilizing forces due to genetic ligamentous laxity and/or rotator cuff malfunction (weakness, poor proprioception). You may be aware of someone who does that age-old party trick of subluxating (partially dislocating and relocating) his/her humeral head inferiorly (downward) under his/her own muscle power. This creates a sulcus sign – a divot beneath the acromion (recall from the previous post?) as the humeral head drops downward. Why? The above listed static restraints are looser than normal and hence there is more movement in the joint.

 
2. Traumatic Instability – need I say this one really hurts!? If there is a blow to the proximal humerus (upper arm) forcing the arm into combined extension (backwards), abduction (away from the body) and external rotation (hand rotated outwards) there may be an anterior dislocation. The humeral head pops out of the glenoid fossa to the front. If there is axial loading of the humerus (a force directed upward from the elbow to the shoulder like falling forwards onto your outstretched hand with elbows locked or falling onto your elbows) while the humerus is adducted (across your body) and internally rotated (hand turned inwards), the shoulder will dislocate posteriorly (backwards). Very rarely is there an inferior dislocation and even less so a superior one – we will not cover these in this post.

 
What does physical therapy do for a patient with either an atraumatic or a traumatic dislocation?

 
1. Atraumatic dislocation: The primary goal in physical therapy is to maximize the rotator cuff function so that the dynamic stabilizers can compensate for the failed static stabilizers. Yes, this means improving the rotator cuff strength. This is, however, not enough. The rotator cuff muscles must work in concert with each other and with the primary (larger and stronger) muscles which move the shoulder e.g. deltoid, pectoralis major. Rhythmic stabilization exercises are done to train the rotator cuff to keep the humeral head centered in the glenoid fossa while performing a variety of both static and dynamic tasks in the clinic. The rotator cuff muscles must contract together to compress the humeral head into the glenoid fossa thus creating a secure scapulohumeral connection (recall last post?). They must also be able to contract selectively to resist the forces created by the primary movers of the shoulder so that the humeral head does not move off center too much. If it does move off center the shoulder could subluxate, dislocate or simply impinge tendons on the acromion thus causing pain and dysfunction.

 
2. Traumatic dislocation: The primary goal after a traumatic dislocation is to avoid surgical stabilization. However, this mechanism of injury frequently requires surgery due to damage to stabilizing structures. However, a well constructed rehabilitation program can prove beneficial in avoiding surgery or, at least, can maximize the range of motion and rotator cuff strength preoperatively which, in my experience, results in a significantly better outcome following surgery. The physical therapist will help the patient regain normal range of motion and strength while protecting any traumatized (torn?)(unstable!) structures. Techniques used by the therapist will be joint and soft tissue mobilization to improve the accessory motions of the humeral head within the glenoid fossa, appropriate stretching to regain full range of motion while protecting traumatized structures and then progressive strengthening of the rotator cuff musculature in a selective manner – for example, anterior dislocations need stability anteriorly so more focus will be placed on tightening up and strengthening anterior structures.

 
In summary:
1. If you need to choose between atraumatic or traumatic shoulder instability I would suggest the atraumatic variety! This typically responds well to an expertly designed shoulder stabilization program (which you do for the rest of your life!).
2. Traumatic dislocations of the shoulder frequently require surgical intervention.
3. Shoulder joint function is dependent on static and dynamic stability mechanisms functioning in concert.
4. When you go to physical therapy for shoulder instability you should be prepared to work really hard both in the clinic WITH the physical therapist AND at home doing a comprehensive home exercise program specifically designed for your shoulder and your needs.

 
YOUR CALL TO ACTION!
1. Please forward this on to your friends, family members and co-workers.
2. Call us with any questions you may have regarding this post or stop by one of our clinics to get your questions answered.
3. POST something on this blog. Custom Physical therapy would love to hear from you.

NEXT POST: SHOULDER IMPINGEMENT

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