There are many conditions that can result in shoulder pain. Some of these conditions include the more traumatic ones like subluxations, fractures, tears in the rotator cuff muscles or the labrum (a structure that forms the socket) through an injury. The more insidious shoulder pain are usually frozen shoulder and impingement syndrome. In this article we will discuss the mechanical causes of an impingement syndrome, particularly the subacromial impingement syndrome.
What is an Subacromial Impingement Syndrome?
As the name implies, an impingement syndrome is a condition that results in pain and movement impairment because certain tissues are being impinged or compressed between 2 bony structures. One of the most common shoulder impingement occurs in the space under the acromion (see picture).
The impingement is most significant when the arm is elevated sideways to about 90 degrees. The structures that are impinged are soft tissues that lie under the acromion- which are your supraspinatus tendon (which is part of the rotator cuff) , the subacromial bursa, which lies under the the acromion. The pain felt can also be magnified by the irritation of the ligaments that surrounds the area. An untreated subacromial impingement can eventually result in a tear in the Supraspinatus tendon.
What are the contributing factors?
The cause of an impingement is anything that reduces the space through which the soft tissues lie. A structural fault such as a bony spur present under the acromion can reduce that space. However the presence of the spur itself may not be a primary cause of the impingement. Often, it is more of a result of the inability of the ball (head of the humerus) staying centre in the socket and or together with the bony anomaly in the acromion that result in the impingement. Hence, we need to look at what in a normal pain free shoulder holds the ball centre in the socket and without it will result in space reduction in the space under the acromion.
These structures are primarily the rotator cuff muscles, specifically the Subscapularis and the Infraspinatus. These two muscles are positioned lower and their function is to provide a downward force. Together with the other rotator cuff muscles including Supraspinatus and Deltoid muscles, they work together to stabilize the humeral head in the centre of the socket. When the ball jams up into the acromion, mechanically this means that the ball has moved upwards away from the centre in the socket. This is a result of the muscle imbalance between the muscles that pull the ball up (Supraspinatus and Deltoid muscles)and the muscles that hold the ball down (the infraspinatus and the subscapularis). When there is an imbalance, the ball rides up and jams into the acromion, hence reducing that space.
Knowing the above, amongst other exercises to correct faulty motor control patterns, the rehabilitation that you receive for a sub-acromial impingement must include the retraining of the lower cuff muscles- namely the Infraspinatus and Subscapularis.
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