In your younger days, you swam or threw baseballs and footballs and you never had shoulder pain. Now you reach for the highest cabinet in the kitchen, and you can barely manage to lift your arm up all the way. Maybe you are currently active in a sport or job that requires repetitive arm motion and you are concerned about the health of your shoulder joint. The shoulder is a very complex area that allows a great degree of mobility. Because of this mobility, some stability of the area is sacrificed. The complexity of the joint and the compromise in stability often leads to injuries. Shoulder impingement, a very common syndrome, can occur at any time in a person’s life, and can become a persistent, nagging problem. Learning the shoulder’s anatomy and biomechanics will help you understand what shoulder impingement is, how it is caused, and how it can be prevented and treated.

A Little Anatomy
The bones that form the shoulder are the clavicle (collarbone), scapula (shoulder blade), and humerus (arm). The scapula is a flat, triangular bone that has two prominent projections on the top called the acromion process and the coracoid process. You can feel your acromion process by following your clavicle out to the end of your shoulder. That bony projection is the acromion process, and the joint located there is the acromioclavicular (AC) joint. Just below and medial to the AC joint is the coracoid process. On the side of the scapula is a slight cavity where the head of the humerus is located. A bony projection on the side of the humerus is called the greater tuberosity and it is a site for muscle attachment. The front of the humerus contains a groove where the head of the biceps tendon rests.
Ligaments are strong bands of tissue that connect bones. There are many ligaments that add degrees of stability to the shoulder, but the coracoacromial ligament is primarily involved in shoulder impingement. It connects the acromion process to the coracoid process and makes a roof inside the shoulder. The shoulder capsule is a fibrous but loose sleeve that surrounds the humeral head and is reinforced with ligaments.
The shoulder muscles can flex (move the arm up), extend (move the arm backwards), abduct (move the arm away from the side of the body), adduct (move the arm toward the center of the body), and rotate the arm internally or externally. The small rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, teres minor) are responsible for shoulder internal and external rotation. Their tendons (the end of the muscle that attaches to the bone) run over the top of the humeral head.
Bursas are small fluid filled sacs that protect tendons from the hard surfaces of bones, allow them to glide smoothly during movement, and provide nutrition. There are several located in and around the shoulder, but the subacromial bursa located underneath the acromion process is the bursa involved in shoulder impingement.

A Little Biomechanics
The space beneath the acromion is called the subacromial space. The coracoacromial ligament is the roof of the space and the top of the humeral head is the floor. The subacromial space available varies with shoulder movement, but one study on cadavers showed the mean space to be 11.1 mm without arm movement and decreased to 5.7 mm when the arm is elevated to 90 degrees [1]. Located inside this space are the subacromial bursa, the rotator cuff tendons, and the long head of the biceps tendon.

What is Shoulder Impingement?
Shoulder impingement is the trapping of the rotator cuff tendons (primarily the supraspinatus), the subacromial bursa, and/or biceps tendon in the subacromial space [2]. The syndrome progresses in three stages [3]. Stage I is characterized by inflammation and hemorrhage of the subacromial bursa and rotator cuff tendons. It is usually found in people less than 25 years old and can be reversible. Stage II is found in people from 25-40 years old and is a progression of Stage I symptoms that leads to irreversible thickening of the bursa and tendinitis of the rotator cuff. Stage III is seen in people over 40 years old who now have rotator cuff tears and bony changes like spurs on the acromion process. Signs and symptoms of shoulder impingement are anterior shoulder pain during movements of flexion, abduction, or rotation, weakness of the rotator cuff muscles, and limited range of motion.

What Causes Shoulder Impingement?
As with many syndromes, the causes can be multifactorial and one problem frequently leads to another. The causative factors can be intrinsic and pertain to the muscles of the shoulder joint, or be extrinsic and apply to other structures like bones, ligaments, and capsules [4]. ntrinsic factors are muscular weakness, muscular overuse, and muscular degeneration. Extrinsic factors are acromion shape, the coracoacromial ligament, capsular tightness or laxity, and degeneration of the acromioclavicular joint.

Intrinsic Factors
Muscular weakness or imbalances do not stabilize the humerus. This instability can cause the top of the humeral head to “bump” into the coracoacromial ligament and put pressure on the tendons and bursa in the subacromial space. Muscular overuse that leads to inflammation of tendons also decreases this already small space. In Stage III impingements, the muscle degeneration leads to tears, which weakens the joint. This allows more movement of the humeral head and more trauma.

Extrinsic Factors
Acromion processes are found to have three different shapes: flat, curved, and hooked [5]. The hooked acromions are associated with a higher prevalence of rotator cuff tears [5]. The coracoacromial ligament has been implicated as a major contributor to shoulder impingement syndrome [6]. The subacromial structures get compressed against the ligament, and this causes pain and inflammation. A key part of impingement surgery is to remove the ligament to allow for more space [2]. When the ligaments and capsule of the shoulder are lax, the humerus is unstable. Just like with weak muscles, this instability allows the humeral head to squeeze and damage the rotator cuff tendons and bursa. On the other hand, if the capsule and ligaments are too tight, there won’t be enough space available in for all of the structures to move without compression. Arthritis that causes degenerative spurs to form on the underside of the acromion can be caused by repeated impingement and leads to further impingement [2]. With bony projections hanging down from the acromion, there is even less space available for movement without impingement.

Treatment
The goal of any treatment program is to reduce the compression and friction that leads to pain and dysfunction [7]. In acute impingement syndromes, rest, ice, nonsteriodal anti-inflammatory drugs (NSAIDS), and strengthening and flexibility are beneficial and may reverse the syndrome. Physical therapy should focus on the reason for the impingement. If it is an overuse syndrome, rest and alterations of lifestyle are needed. If muscle weakness or imbalance is the culprit, specific exercises will address strengthening the rotator cuff and other stabilizer muscles. Non-operative physical rehabilitation is always the first choice, but sometimes surgery is indicated in the form of an arthroscopic or open subacromial decompression, also called an anterior acromioplasty.

References
1. Flatow, E.L., et al., Excursion of the rotator cuff under the acromion. Patterns of subacromial contact. Am J Sports Med, 1994. 22(6): p. 779-788.
2. Neer, C.S.d., Anterior acromioplasty for the chronic impingement syndrome in the shoulder: a preliminary report. J Bone Joint Surg [Am], 1972. 54(1): p. 41-50.
3. Neer, C.S.d., Impingement lesions. Clin Orthop, 1983(173): p. 70-77.
4. Bigliani, L.U. and W.N. Levine, Subacromial impingement syndrome [see comments]. J Bone Joint Surg Am, 1997. 79(12): p. 1854-1868.
5. Bigliani, L.U., D.S. Morrison, and E.W. April, The morphology of the acromion and its relationship to rotator cuff tears. Orthop Trans, 1986. 10: p. 228.
6. Burns, W.C.d. and T.L. Whipple, Anatomic relationships in the shoulder impingement syndrome. Clin Orthop, 1993(294): p. 96-102.
7. Wilk, K.E., et al., Shoulder rehabilitation, in Physical Rehabilitation of the Injured Athlete, J.R. Andrews, G.L. Harrelson, and K.E. Wilk, Editors. 1998, W.B. Saunders Company: Philadelphia. p. 478-553.