ANATOMY AND PATHOANATOMY

Knowledge of the coracoacromial anatomy is crucial for diagnostic accuracy, operative facility, and avoidance of complications. The shape of the acromion and contour of its undersurface is best evaluated with Neer’s supraspinatus outlet view. Bigliani and colleagues ³ have described three distinct acromial shapes: type 1, flat (divergent); type 2, curved (congruent); and type 3, hooked (stenotic). They found an increased correlation between the type 3 hooked acromion and underlying full-thickness rotator cuff tears (69.5% for type 3 and 3% for type 1). This radiographic view is also valuable for determining the overall slope and thickness of the acromion and in predetermining where and how much bone should be removed with acromioplasty, when indicated.

The greater tuberosity of the humerus forms the floor of the subacromial space. It is important to note its size, shape, any osteophytic overgrowth, sclerosis, erosion, and/or cysts. It is best evaluated radiographically with an anteroposterior (AP) view, with the arm in external rotation.

It is important to remember that the subacromial bursa is an anterior structure. It extends from the anterior half to one third of the acromion to just medial to the acromioclavicular (AC) joint to 1 to 2 cm anterior to the acromion and 2 to 3 cm laterally. The bursal wall is frequently thickened and troublesome posteriorly, and has been termed the posterior bursal curtain. This curtain frequently closes as one backs the arthroscope posteriorly to get a larger field of view of the subacromial bursa. It is frequently necessary to resect a portion of this structure when performing subacromial surgery.

The anatomy of the coracoacromial ligament (CAL) is pertinent to the technique of acromioplasty. It attaches to the front and undersurface of the acromion as a thick band and continues around the anterolateral corner to attach to the lateral ridge for a variable distance.⁴ Anteriorly, the posterior medial band of the coracoacromial ligament attaches to the anteroinferior edge of the acromion, whereas the deltoid fascia attaches more superiorly (Fig. 8-1).

FIGURE 8-1 Lateral view of coracoacromial ligament and deltoid fascia attachment on anterior acromion. A, Deltoid fascia. B, Deltoid muscle fiber. C, Coracoacromial ligament insertion on undersurface and lateral edge of acromion.

Anteriorly, the CA ligament attaches on the inferior acromial surface and falls away easily from the overlying anterior deltoid muscle and fascia when detached. Laterally, however, the anterior lateral band of the coracoacromial ligament blends inextricably with the deltoid muscle fascia along the lateral acromion.⁵ Care must be taken when releasing or ablating the anterolateral band of the CAL not to detach the fascia aggressively or resect too much bone laterally, because this may result in a deltoid detachment.

Edelson and Luchs ⁶ and others have noted variable degrees of transformation of the coracoacromial ligament into bone at its acromial insertion, most commonly in the anterolateral band.⁴ Gartsman⁷ has labeled this phenomenon anterior acromial protuberance. Rockwood and Lyons,⁸ in their open technique, recommend resecting 8 to 10 mm of full-thickness anterior bone and then reattaching the deltoid fascia. This technique of full-thickness anterior bone resection back to the level of the AC joint has been included in a description of arthroscopic subacromial decompression.⁹

For the most part, the anterior acromial protuberance is really an inferior extension of calcification into the anterolateral band of the coracoacromial ligament insertion. One does not need to resect full-thickness acromial bone anteriorly to remove it. In fact, great care should be taken not to resect too much superior anterior bone, because this could detach the anterior deltoid fascia, resulting in an operative disaster.

HISTORY AND PHYSICAL EXAMINATION

The clinical history is important. Pain with the cocking and acceleration phase of throwing is most likely secondary to an underlying instability or posterior superior impingement. Nocturnal and rest pain are often indicative of a rotator cuff tear, whereas patients with cuff tendinitis develop pain with progressive activity. Other causes of shoulder pain such as scapular thoracic bursitis, suprascapular nerve syndrome, cervical radiculopathy, and referred pain from the gallbladder, liver, lung, or heart also need to be differentiated.

Causes of Impingement

Much of the controversy regarding the need for arthroscopic subacromial decompression centers on the nonspecific diagnosis of impingement. There are numerous types of shoulder impingement and only a small proportion of these necessitate ASAD. Patients complaining of pain with overhead activities can be differentiated into one of the following categories:

1. Secondary impingement

2. Chronic secondary impingement with pathologic subacromial changes

3. Primary extrinsic impingement

4. Posterior superior internal impingement

5. Anterior subcoracoid impingement

Secondary Impingement

The concept of secondary impingement originated with Codman,¹⁰ who proposed an intrinsic tendinous degeneration as the essential lesion in rotator cuff disease. The microvascular studies of Rathbun and McNab¹¹ and Moseley and Goldie¹² support this concept. The subsequent pain and weakness of the supraspinatus compromises its function as a humeral head depressor and allows the superior vector forces of the deltoid to dominate, producing a secondary, superiorly directed impingement of the cuff into the acromion (Fig. 8-2 A, B).

FIGURE 8-2 A, Balanced deltoid and supraspinatus forces with medial vector. B, Superior migration of the humeral head from a dominant superiorly directed deltoid vector with weakened supraspinatus.

Jobe and associates ¹³ expanded this concept to include patients with underlying anterior glenohumeral ligament instability. As the humeral head subluxes anteriorly, the cuff is secondarily compressed against the coracoacromial arch. In addition, Matsen and coworkers¹⁴ have noted that posterior inferior capsular contracture produces an obligate anterior superior humeral head translation, with arm elevation and subsequent superior impingement.

Secondary impingement is more prevalent in a younger patient population actively involved in sports activities that entail overhead arm motion, and should be suspected when the bony architecture is unremarkable. Cuff tears associated with secondary impingement from tendinosis degeneration are most commonly articular-sided or intratendinous.

Chronic Secondary Impingement with Pathologic Subacromial Changes

If the underlying cause of the secondary impingement is not rectified with a diligent rehabilitation program (e.g., scapular alignment, posterior capsule stretching), the cuff and humeral head elevate. The decreased acromial–humeral head distance results in inflammation and anterior mechanical irritation in the subacromial space, with the development of a traction spur in the acromial attachment of the CAL. This anterior acromial osteophyte formation can lead to awning impingement with arm elevation and subsequent bursal side cuff fraying and roughening. Even if subacromial space changes are present on a secondary basis, débridement, including a possible subacromial decompression, must be considered.

Primary Extrinsic Impingement

Neer ¹ introduced the concept of extrinsic impingement of the anterior acromion, coracoacromial arch, and AC joint on the underlying rotator cuff and biceps tendon. He also emphasized that forward flexion of the arm is the dominant functional position and that anterior decompression, not lateral acromionectomy, is the appropriate operative approach for significant cuff degeneration.

Neer’s impingement sign is determined with the patient seated in front of the examiner, who stabilizes the scapula as the arm is elevated slightly lateral to the midline to impinge the tuberosity against the acromion (Fig. 8-3).

FIGURE 8-3 Neer’s impingement sign.

Pain thus produced is eliminated by injecting 10 mL of 1% lidocaine (Xylocaine) into the subacromial bursa beneath the anterior acromion (impingement injection test) to confirm the diagnosis. Hawkins and Kennedy ¹⁵ have described a second impingement sign, in which the arm is flexed forward 90 degrees and then forcibly internally rotated, jamming the supraspinatus tendon against the anterior edge of the coracoacromial ligament to produce pain (Fig. 8-4).

FIGURE 8-4 Hawkin’s flexion–internal rotation impingement sign.

Patients with primary extrinsic impingement frequently have a bony architecture with an anterior acromial hook (stenotic type 3 acromion), or a CAL spur that presses directly on the cuff and biceps with forward elevation of the arm; these are associated with bursal side rotator cuff tears.

Posterior Superior Internal Impingement

Walch and colleagues ¹⁶ and Jobe and associates¹⁷ have described a variety of secondary impingements noted in overhead athletes that occur when the arm is maximally externally rotated while abducted and extended, such as in the cocking phase of throwing. In this position, the posterosuperior articular surface fibers of the supraspinatus are placed under tension and shear but are also compressed between the humeral head and adjacent glenoid rim, resulting in posterior superior synovitis and partial undersurface tears.

Morgan and Rajan ¹⁸ have described a subset of throwers with glenohumeral internal rotation deficit (GIRD). This posteroinferior capsular contracture produces a posterosuperior humeral head translation, superior labral separation, and internal impingement. Whether any underlying anterior instability is a factor in this type of compression needs to be determined with each individual case. In long-standing cases of internal impingement and superior labral pathology, obligate anterior instability can be encountered and may need to be addressed.

Although easily confused with primary or secondary anterior impingement, careful examination usually demonstrates pain more at the posterosuperior aspect of the rotator cuff with the arm abducted and externally rotated and extended. In this subgroup, rotator cuff tears are articular-sided and frequently associated with peel-back superior labrum anteroposterior (SLAP) lesions. The use of dynamic assessment arthroscopy in these difficult diagnostic situations is recommended. With the arthroscope in the posterior portal, the arm is released from balanced suspension and placed into the abducted, externally rotated throwing position. Areas of internal impingement, as well as any superior labral peel-back, are readily visualized and assist the clinician in arriving at a working diagnosis.