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Impingement Syndrome

Impingement can be defined as a trapping of the soft tissues in the subacromial space, between the acromion and the humeral head. The entrapment of the soft tissues when moving the shoulder may lead to a painful reaction. Athletes, including tennis players, swimmers, throwers, and weightlifters, who make repetitive movements of the arms above the horizontal plane, are at risk of developing this painful condition.

The soft tissues can become too large and may be pinched between
the head of the humerus, the acromion process of the scapula, and the coracoacromial ligament. The soft tissues in the limited
subacromial space include the tendons of the long head of the biceps, the supraspinatus, infraspinatus, teres minor, and subscapularis muscles, and the bursa overlying the supraspinatus tendon. The subacromial space can also be inadequate. If the ligament is thickened or calcified, or becomes inelastic, or if the anteriorinferior
edge of the acromion process over the acromioclavicular joint becomes irregular with bony outgrowths or spurs, the space can be further compromised. This occurs most often in elderly people.

The contour of the anterior-inferior acromial bone is important since it may affect the size of the space below the acromion. The shape of the anterior acromion is studied by plain radiographs (supraspinatus outlet or arch view). The contour of the anterior-inferior acromion is classified according to Bigliani as follows:
– Type 1: the acromion is flat on the undersurface with the anterior edge extending away from the humeral head.
– Type 2: the acromion is gently curved on the undersurface with the anterior edge extending parallel to the humeral head.
– Type 3: an inferiorly pointing, or hooked, anterior bone spur (osteophyte) narrows the outlet of the supraspinatus muscle and tendon.

When the upper arm is moved forwards and upwards (its usual functional position) to an angle of 90° to the body and the arm is then further internally rotated (inward), the soft tissues are compressed against the sharp edge of the coracoacromial ligament. During movement the tendons and the bursa rub against the ligament,
causing mechanical irritation that gives rise to painful inflammation. As inflammation is accompanied by swelling, the space is even further reduced and the condition may become progressively worse. Repeated loading causes thickening of the soft tissues and leads to a chronic inflammatory reaction. The subacromial bursa and the vulnerable areas of the supraspinatus and biceps tendons are most affected by this process.

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