Shoulder pain is a common problem. In fact, around 30 percent of us will experience it at some point in our lives. Approximately 65 percent of all shoulder pain conditions are due to impingement syndromes.

This is becoming a more widely-recognized cause of persistent shoulder pain, but as a condition it is often not treated effectively. To treat the condition effectively, we need to understand it — from anatomy to movement patterns.

Anatomy of the shoulder joint in relation to impingement

The shoulder or glenohumeral (GH) joint is a ball-and-socket joint that permits a large range of motion and, as a result, is quite unstable. This is largely due to a large ball (the humeral head), trying to fit into a relatively small and shallow socket (the glenoid fossa of the scapula).

The coracoacromial arch is located above the GH joint. This is an arch of bone formed by the coracoid and acromion processes of the scapula and joined together by the coracoacromial ligament. It is often known as the roof of the shoulder joint. The space between the GH joint and the CA arch is known as the subacromial space.

During subacromial impingement, tissues are "pinched" within this space. There are three structures that occupy this space: the supraspinatus tendon, the subacromial bursa and the long head of biceps tendon. It is these structures that may become pinched if subacromial compression occurs. This compression refers to a reduction in volume of the subacromial space.

There are two groups of causes for subacromial compression. The first is intrinsic where the compression is due to changes within the SA space, i.e. tendon injuries, degeneration, bursal swelling.

The second is extrinsic, where external changes affect the space. These include postural changes to the thoracic spine, altered movement patterns, bony growths, tight posterior capsule, etc.

The height of the SA space on MRI scans in healthy shoulders is between 7-14 mm. In those with subacromial impingement syndrome, this space has been shown to be much smaller in the region of 3 mm.

Movement patterns and muscle imbalances

Understanding movement at the shoulder identifying both how it should be and what the faulty movement patterns present in each individual case is the key to successfully treating an impingement syndrome. There is a specific sequence of movement that should occur at the arm and shoulder as the arm is lifted out to the side (abducted). If this pattern is incorrect, impingement may occur.

Shoulder abduction is a complex movement that involves a partnership between the scapula, clavical and humerus, as well as several muscles including the four rotator cuff muscles, trapezius, serratus anterior and deltoid. Changes in the muscle balances around the shoulder affect the movement of the bones in question.

Here are the most frequent muscle imbalances and resultant movement dysfunctions seen in those with subacromial impingement, in order of occurrence from 0 degrees abduction upward.

In the early phases of abduction, a slight upward motion of the humeral head is normal but should be limited by the rotator cuff, acting to draw the head downwards against the pull of the deltoid. Decreased EMG activity of infraspinatus and subscapularis has been seen during 30-60 degrees of abduction in those with subacromial impingement.

Weakness or inhibition in these muscles allows the humeral head to lift upwards, toward the coracoacromial arch, thus reducing the subacromial space. Further translation of the humeral head, this time in a supero-anterior direction can be caused by tightness in the posterior capsule.

The further the arm moves into abduction, the closer the greater tuberosity of the humerus moves towards the coracoacromial arch. In order to prevent the two colliding, the humerus is laterally rotated. A lack of lateral rotation range of motion usually in the form of tight medial rotators (latissimus dorsi, subscapularis) can reduce the ability of the arm to do this. A healthy range of motion into lateral rotation is 90 degrees.

In another bid to avoid contact between the greater tuberosity and coracoacromial arch, the serratus anterior and upper and lower fibers of trapezius work together to upwardly rotate the scapula. Delayed onset of contraction and reduced total EMG in these muscles has been shown in people with rototar cuff tendinopathy as a result of subacromial impingement. As well as the reduced performance of the upward rotators, increased tone in the opposing downward rotators (levator scapulae, pec minor and the rhomboids) could limit this movement.

The final part of abduction requires the scapula to move laterally around the ribcage. A large thoracic kyphosis limits this movement as the rib cage cannot flatten sufficiently to allow this motion. Instead, the arm draws forwards into flexion, rather than pure abduction.

Putting it all together

While this is by no means an exhaustive list of problems that may contribute towards an impingement syndrome, it does cover the most frequently occurring situations.

When observing a patient with suspected (or confirmed) impingement, pay particular attention to the strength of the rotator cuff muscles, serratus anterior and lower fibers of trapezius. This can be achieved with manual muscle testing (if more accurate testing is not available), and of course observation of muscle tone, wasting and contraction onset.

Also pay close attention to ensure the flexibility of the posterior capsule, medial rotators, pec minor, rhomboids and levator scapulae are enough to allow sufficient GH lateral rotation and upward rotation of the scapula.

A combination of exercise rehabilitation to target muscle imbalances, along with soft-tissue work to tight structures and a suitable rest period will in most cases bring good results. If a thoracic kyphosis is present, depending on the extent, manipulations may be required alongside a suitable exercise program.