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Dislocated Shoulder 23-Aug-2003 At 10:11:36 AM Julian

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Rich,

Yes, you are correct in that the longer it is dislocated (someone referred to 'sublaxation' earlier but this actually means a partial or incomplete dislocation) the greater the damage in general. Their are a number of different tissues which maintain the integrity and stability of the glenohumeral articulation. You make reference to the muscles. These do play a role though not in the sense you suggest. When the head of the humerous moves anteriorly off the 'saucer', as Tim so aptly described it, it is the ligaments and joint capsule (a surrounding membrane) which are most affected by the time factor. These tissues are made of collagen, a very strong substance, in fact the strongest biological substance known, which is fantastically strong and plastic if loaded for short durations. When force exerted on them is prolonged they exhibit far less strength and are subsequently damaged with comparatively minor loads; that is, their strength is highly dependent on time. Hence, the longer your shoulder is dislocated, up to a certain point, the more damage that is occurring.

You mentioned muscles. These do play a role in subsequent instability in that, through complex neural circuitry relating to pain, they take on a retarded firing pattern. The rotator cuff is not one muscle, or something that miraculously wraps around your shoulder holding it in place. It is actually 4 separate muscles which, collectively, are given this name. Imagine a set of cross-hairs superimposed over the 'saucer' and, for ease sake, the basket ball is now a golf ball. The rotator cuff (RC) muscles are attached to various points around the golf ball. One of their jobs in life is to keep the golf ball in the cross hairs. If it goes out of them and there is much force involved there is good chance you will hurt your shoulder, especially if your hand is above your head as that is the least stable position for your shoulder to be in.

As Tim suggested once again, the shoulder is very mobile, and although useful, the pay off is instability. The role of the rotator cuff is to maintain stability whilst the shoulder executes highly complex movements, often with great relative force. To make matters worse, in addition to this tenuous tug-of-war, individual RC muscles will also act as ‘prime movers’, that is, help move the arm in certain directions.

Now remember that the head of the humerous (golf ball) is strung between multiple opposing muscles/forces constantly talking to each other about where the golf ball is. We are talking about milli-second feedback loops. Although this sounds simple, the complexity of the neural circuitry required to monitor and combine information such as movement of the arm, speed, force, intended movement and actual movement, hand dexterity etc, and then altering future signals is mind boggling.

When you have a major trauma like a dislocation, the neural signals to these muscles are altered in the short term due to pain resulting from tissue damage. Unfortunately this can be a residual effect in the long term. This is far to complex to explain here but it basically relates to how the muscles fire both individually and collectively to maintain the integrity of the joint when the arm is moved. When the 'motor pattern' of the cuff is impaired even slightly, quite significant injury can result.

I hope this answers your question. I have tried to keep it concise but, as you can imagine, the information is endless. Suffice to say, there are many other factors which contribute to the stability (or instability) of the joint both prior to and post injury.

Julian