The shoulder is one of the most complex joints in the human body. It has an amazing amount of inherit mobility, which in turn requires an adequate about of dynamic and static stability to function. Then if you consider the manner in which we use our shoulders and the lack of anatomical support, it’s easy to see why stability is the most important factor to consider when treating shoulders.
To improve your shoulder assessment and treatment skills it is important to fully comprehend the details of static and dynamic stability. This will be a 3 post series that will hopefully clear up some confusion and allow you to understand stability on a deeper level.
Why We Need Stability
A couple quick facts about the shoulder…
The humeral head is 3 times larger than the glenoid fossa. The base of shoulder stability is a “floating” physiologic joint. The only true bony attachment is through the acromion process to the sternoclavicular joint. There are over 25 muscles that are involved with shoulder girdle movement. There is more movement (ROM) at the shoulder than any other joint. Which means there is a complex interaction with the muscle-length tension relationships and the sensorimotor system.
Take all this into account and you’ll quickly realize that the shoulder doesn’t have a whole lot going for it. Then add in the fact that we abuse our shoulders with respective motions, terrible postures, and athletic activities. The result is a joint that requires a great deal of coordinated dynamic and static support just to stay healthy.
Dynamic and Static Stabilizers
Understanding the complex interplay between the dynamic and static components of the shoulder is of paramount importance and a prerequisite for proper examination and intervention. Before one can consider how the dynamic and static components interact, it is necessary to understand the difference.
- The dynamic components include the contractile tissues (rotator cuff, deltoid, scapular muscles) and the sensorimotor system
- The static components include the connective tissue (labrum, capsule, ligaments)
This post series is intended to be a review of the anatomy and function stated in a concise form to use as a reference. There is a wealth of articles, websites, and posts regarding these concepts. For a more in-depth information please refer to the references listed at the end of this series.
Understanding Shoulder Movement
Osteokinematics / Arthrokinematics
Understanding the correlation between physiologic and accessory motion will allow one to better assess kinematic motion and capsular extensibility. Evaluating either shoulder motion or capsular integrity can help guide the clinician in their examination. For instance, a lack of range of motion in abduction may correlate with a decrease in inferior capsule extensibility. The interpretation of the osteokinematics and arthrokinematics can also help guide the clinician in their treatment by revealing where stability is needed.
It is important to keep in mind that changes in capsule or ligament integrity will alter the normal mechanics of the shoulder joint. For example, posterior capsule tightness can create an imbalance in the normal arthrokinematics and result in an obligatory anterosuperior migration of the humeral head and loss of internal range of motion (Ticker et al 2000, Sethi PM et al 2004).
Part 1 – Components and Motion
Part 2 – Static Stability
Part 3 – Dynamic Stability
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