Extrinsic or outlet impingement should be differentiated from internal impingement, which stems from contact of the articular side of the supraspinatus on the posterosuperior glenoid rim in throwing athletes during the late cocking phase of throwing.

Alternatively, intrinsic degeneration of the rotator cuff may lead to dysfunction of the cuff’s native glenohumeral stabilization mechanism. These pathomechanics result in elevation of the humeral head relative to the acromion and subsequent outlet impingement.

Bigliani et al⁴ described three types of acromial morphology: the type I acromion is flat, type II is curved, and type III is hooked. They noted that 70% of cadaver shoulders with rotator cuff tears had a type III acromion.

A type I acromion with an increased angle of anterior inclination may cause impingement of the rotator cuff by narrowing the supraspinatus outlet.

Os acromiale is a failure of fusion in one of the acromial ossification centers.

Stage I impingement lesions occur initially with excessive overhead use in sports or work. A reversible process of edema and hemorrhage is found in the subacromial bursa and rotator cuff. This typically occurs in patients younger than 25 years old.

With repeated episodes of mechanical impingement and inflammation, stage II lesions develop. The bursa may become irreversibly fibrotic and thickened, and tendinitis
develops in the supraspinatus tendon. Classically, this lesion is found in patients 25 to 40 years of age.

As impingement progresses, stage III lesions may occur, with partial or complete tears of the rotator cuff. Biceps lesions and alterations in bone at the anterior acromion and greater tuberosity may also develop. These lesions are found almost exclusively in patients older than 40 years.

Palpation of the point of Codman just anterior to the anterolateral corner of the acromion: Tenderness is frequently a sign of supraspinatus tendinitis, tendinopathy, or acute tear of supraspinatus tendon.

Range of motion: Patients with impingement may have limited internal rotation from posterior capsule contracture. Active motion is typically more painful than passive motion, especially in the descending, eccentric phase of the motion arc.

Painful abduction arc: Pain from 60 to 120 degrees (maximally at 90) suggests impingement. Patients may externally rotate at 90 degrees to clear the greater tuberosity from the acromion and increase motion.

Neer impingement sign: This maneuver compresses the critical area of the supraspinatus tendon against the anterior-inferior acromion, reproducing impingement pain.

Hawkins sign: This maneuver compresses the supraspinatus tendon against the coracoacromial ligament, reproducing the pain of impingement. It has high sensitivity but low specificity.

Impingement test: The injection of local anesthetic into the subacromial space followed by relief of pain on clinical exam. This test has greatly improved specificity for making the diagnosis of subacromial impingement. A positive test is also positively prognostic of a satisfactory outcome after subacromial decompression.¹⁴

AC osteoarthritis: This degenerative process may be clinically asymptomatic, but an inferior AC joint osteophyte can contribute to the etiology of impingement syndrome. If symptomatic, tenderness may be elicited at the AC joint with palpation and the cross-arm adduction test.

Rotator cuff tear: The history of traumatic injury is variable. Patients complain of deep shoulder pain at night and may complain of weakness in the affected shoulder. Strength testing will evaluate for rotator cuff tear and tear size.

Glenohumeral instability: Subluxation or dislocation of the humeral head while stabilizing the scapula (load and shift test) helps confirm the diagnosis of glenohumeral instability. Throwing athletes may have a complex pattern of pathology that includes anterior laxity and posterior capsular tightness, which may result in internal impingement. These patients typically have posterior pain with apprehension testing. Internal impingement must be differentiated from extrinsic outlet impingement. Although a posterior capsule contracture can predispose patients to outlet impingement, classic extrinsic outlet impingement is believed to be rare in throwing athletes.

Biceps pathology: Pain is typically in the anterior shoulder. Tenderness may be elicited in the area of the bicipital groove. Pain during resisted flexion of the arm with the elbow in extension and the forearm supinated (Speed test) indicates biceps pathology.

Glenohumeral arthritis: Pain is associated with movement below 90 degrees of elevation. Patients complain of pain at night. Cogwheel crepitus may be present when loading the glenohumeral joint during resisted arm abduction.

A supraspinatus or acromial outlet view is a transscapular view taken with the radiographic beam angled 15 to 20 degrees caudally (FIG 1).

The outlet view is the best plain radiographic technique to evaluate acromial morphology and aid in detection of inferiorly directed enthesophytes. With this information, the surgeon may accurately plan the amount of osseous resection required to convert the acromion to type I morphology.

An abnormal acromiohumeral distance has been shown to correlate with the clinical status of patients.¹⁸

Axillary lateral radiographs may be helpful in the diagnosis of os acromiale.

Magnetic resonance imaging (MRI), computed tomography (CT) scan, arthrography, and ultrasonography should be reserved for patients whose diagnosis of impingement syndrome is not completely clear from the history, physical examination, and radiographs. These other modalities will also help diagnose biceps, labral, and rotator cuff pathology.

The rehabilitation program should start by preventing overuse or reinjury with relative rest and activity modification.

As pain continues to decrease and range of motion improves, strengthening of the rotator cuff and periscapular musculature is initiated. This is achieved through progressive resistance exercises with elastic bands or free weights.

Patients should avoid overhead weight training (military press, latissimus pulldowns) and long lever arms (straight arm lateral raises) because these maneuvers can exacerbate impingement and produce undue torque on the rotator cuff and glenohumeral joint.

The most common cause of failure for arthroscopic subacromial decompression and anterior acromioplasty is error in diagnosis.¹