Are you currently experiencing shoulder pain, especially with reaching or lifting movements? It's possible you may have shoulder impingement syndrome (SIS), otherwise known as subacromial impingement syndrome or swimmer’s shoulder.
The condition refers to the mechanical compression of structures within the shoulder joint, however the label of SIS alone only tells one part of the story. It lets us know that impingement is occurring at the shoulder during activity, however we still need to know something very important: the cause.
Amongst people with shoulder pain, multiple studies have indicated impingement rates as high as 64%, making it highly important to understand the nature of its causes. Today we will be reviewing the general anatomy of the shoulder and subacromial space, the structural and biomechanical causes of impingement in that region, and physiotherapy treatment for the condition.
Anatomy of the Shoulder: the Subacromial Space
As you may already know, the shoulder joint is formed by the head of the humerus (arm bone) connecting with the glenoid of the scapula (shoulder blade). In the area where these two surfaces connect, there is a space between bony surfaces formed - known as the subacromial space (see the diagram below). The ‘roof’ is formed by the acromion process, which is a part of the scapula. The ‘floor’ is the humeral head, while the space is closed off by the coracoid process (also a part of the scapula) and the coracoacromial ligament.
As you can see in the diagram above, there are three key structures that fill the subacromial space: the subacromial bursa, the long head of the biceps tendon and the supraspinatus muscle. Typically, the compression of the supraspinatus muscle is the key feature of shoulder impingement. Additionally, the supraspinatus is part of the rotator cuff muscle group, which work together to keep the shoulder joint stable during elevated arm movements such as throwing a ball. Consequently, shoulder impingement is very common amongst overhead athletes such as swimmers or baseball players.
Causes of Impingement
Now that we are familiar with how the subacromial space is formed and the contents residing within it, let’s go over the causes of impingement in this area. These can be broadly categorised into two groups: causes due to structural abnormalities in the shoulder joint and causes due to poor movement patterns of the humerus and/or scapula.
Structural changes will affect the size or ‘distance’ between the borders of the subacromial space, resulting in impingement. Examples include development of osteophytes due to arthritis progression within the shoulder space, or maladaptive healing of skeletal structures after a fracture - causing altered positioning/size of the humerus and/or scapula. Additionally, inflammation of the bursa or tendons within the space can also cause impingement, due to the increased size of these soft tissue structures caused by swelling.
Impingement caused by deficits in shoulder biomechanics relate to movement deficits of the humerus and/or scapula. To perform overhead movement, both are required to move in a process known as scapulohumeral rhythm. During this movement, ideally the ratio of humerus to scapula movement is 2:1, eg. 120 degrees of humerus movement and 60 degrees of scapula movement (as illustrated above). Impairments in humeral control is usually due to weakness of the rotator cuff, while impairments in scapula control (known as scapula dyskinesis) is often caused by weakness of the serratus anterior, the muscle that keeps the scapula held against the rib cage during movement. In a 2012 analysis of shoulder impingement studies, it was determined that altered scapula position + movement was a key commonality amongst people with shoulder impingement. Alterations in movement can also be due to increased laxity of ligaments or the shoulder joint capsule, as well as muscular tightness.
As you may have already guessed, muscular control and strengthening exercises are a key treatment for impingement associated with biomechanical deficits. A 2012 randomized control trial found scapula based training to have significant benefit for impingement related pain and disability, with further analysis of studies in both 2012 and 2014 advocating for scapula control and rotator cuff strengthening as beneficial treatment options.
A good exercise for serratus anterior strength and control is the foam roller wall slide (pictured below), which can also be performed with or without a resistance band to adjust difficulty. A scapula push up is another good exercise for serratus anterior strengthening. Rotator cuff exercises can be performed with resistance bands or cable machines, while a kettlebell drag in a plank position can target the rotator cuff in a position requiring shoulder stability.
Treatment for impingement associated with structural abnormalities can vary. Swelling management via PEACE & LOVE is encouraged for soft tissue inflammation, while adjusting overhead activity to deload the joint is also important to help enhance recovery. Manual therapy and acupuncture are also worth trialing, as they are both a potential source of pain relief. If arthritic progression is particularly severe, then surgery may be an option worth considering after discussion with your physio and GP.
While shoulder impingement can be a frustrating condition, it is also well researched and understood. Determining the causes will be an important first step to recovery, enabling you to understand the best treatment for your individual presentation of the condition. With an effective rehab program organised by your physio, you will be able to make progress towards returning to pain-free overhead activity.
If you have any questions, do not hesitate to get in touch with us!
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