Arthroscopic Subacromial Decompression

Published on 11/03/2015 by admin

Last modified 22/04/2025

Print this page

This article have been viewed 2974 times

CHAPTER 8 Arthroscopic Subacromial Decompression

Arthroscopic subacromial decompression (ASAD) has become one of the most commonly performed surgical procedures of the shoulder. The arthroscopic technique has evolved significantly from the open anterior acromioplasty as described by Neer ¹^,² and others, and the learning curve should not be underestimated.

The indications for ASAD remain controversial. This chapter focuses on the indications and ontraindications for the procedure, technical aspects of the surgical procedure, and how to avoid complications in its implementation.

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.

Anterior Subcoracoid Impingement

Gerber and coworkers ¹⁹ have described this type of anterior impingement between the humeral head and coracoid process. The coracoid tip may be enlarged, fractured, or iatrogenically altered, such as occurs with a laterally positioned Bristow-Latarjet transfer of the coracoid tip onto the anterior glenoid rim. Fractures of the coracoid from the recoil of a rifle in hunters may heal with lateral displacement. These changes are best noted on axillary view x-rays or a computed tomography (CT) scan with the arm flexed 90 degrees and internally rotated.

Whatever the underlying cause, the tip of the coracoid is positioned more laterally than normal and, as the arm is brought into forward flexion, there is a compression of the anterior rotator cuff between the humeral head and tip of the coracoid. This produces pain with Neer’s forward flexion test, but it occurs usually between 80 and 130 degrees of flexion rather than at full flexion. Also, Hawkins flexion and internal rotation test is consistently positive, but the pain is lower and more anterior than with superior impingement. The patient also demonstrates decreased horizontal adduction, with pain similar to that found with AC disease (Fig. 8-5) but again, the pain is more at the tip of the coracoid and not at the AC joint. Selective injections in the region of the coracoid process can help verify the diagnosis.

FIGURE 8-5 Horizontal adduction test.

Subscapularis tendon degeneration and partial tearing are associated with this type of impingement and treatment needs to be directed at the coracoid process with a partial lateral resection. The roller wringer effect has been described; this represents the mechanism whereby the subscapularis is traumatized.

DIAGNOSTIC IMAGING

The best radiographic views for determining the amount of anterior acromial protuberance are the axillary and supraspinatus outlet views. The axillary x-ray is also an excellent view for evaluation of the AC joint, particularly for picking up posterior AC arthritis that may be missed on a routine AP view. MRI is helpful for evaluating associated soft tissue pathology and AC disease.

TREATMENT OPTIONS: INDICATIONS AND CONTRAINDICATIONS

One needs to treat all varieties of impingement with a diligent conservative treatment program, including scapular and rotator cuff retraining, range-of-motion (ROM) exercises to regain full internal and external rotation, nonsteroidal anti-inflammatory drugs (NSAIDs), differential subacromial-AC joint injection, and activity modification for 3 to 6 months in an effort to avoid operative intervention.²⁰

PEARLS& PITFALLS

Pearl 1:

Primary extrinsic or chronic secondary impingement with pathologic subacromial osteophyte formation that has not responded to aggressive conservative treatment should respond well to arthroscopic subacromial decompression. Furthermore, subacromial space changes, such as bursal cuff fraying and roughening of the CAL, should be present to confirm the diagnosis of impingement.

Pearl 2:

The goal of arthroscopic subacromial decompression is to convert a pathologic coracoacromial arch to a physiologic arch.

Pearl 3:

Preoperatively, one should determine the technique to be used for ASAD (cutting block vs. limited anterior resection) and the need for an AC resection. AC resection should be a preoperative determination based on physical findings and corroborative imaging.

Pearl 4:

Perform a glenohumeral examination with every ASAD to evaluate and treat any concomitant intra-articular pathology.

Pearl 5:

Resect anterior acromial spurs and hook without detaching the deltoid fascia attachment.

Pearl 6:

Assess the acromial bone resection from the posterior and lateral portals to verify that a smooth and flat or contoured surface is achieved according to preoperative planning.

Pearl 7:

Attempt not to violate the AC joint capsule during ASAD unless AC resection is clinically indicated preoperatively.

Pearl 8:

Have the anesthesiologist maintain as low a systolic pressure as is safe for the patient so that bleeding, fluid extravasation, and soft tissue swelling around the neck are minimized.

Pearl 9:

Maintain the burr flat on the posterior half of the acromion when performing the cutting block technique for ASAD.

Pearl 10:

Keep the burr opening facing upward into the acromion to avoid injury to the rotator cuff muscle or tendon.

The underlying cause of secondary impingement (normally seen in younger individuals with normal bony anatomy) should respond to aggressive scapular-thoracic rehabilitation and stretching or subsequent surgical intervention directed at the primary laxity or contracture of the shoulder. The causes of internal impingement and anterior subcoracoid impingement need to be addressed directly at the capsular or coracoid level and do not respond to anterior superior decompression. Also, isolated AC disease and massive cuff tears with significant underlying glenohumeral arthritis likely to necessitate future arthroplasty should not be decompressed.

The need for arthroscopic subacromial decompression at the time of rotator cuff repair remains controversial. A recent double-blind prospective study by Milano and colleagues ²¹ comparing rotator cuff repair with and without ASAD showed no significant difference in final outcome, although the authors did not evaluate or compare those patients with clinical and radiographic evidence of bursal impingement versus those without. In contrast, MacDonald and associates²² found improved results in the cuff repair plus ASAD group if the acromion was type 2 or 3.

In view of this ambiguity, the most rational indication at present for arthroscopic subacromial decompression is patients demonstrating clinical, radiographic, and arthroscopic evidence of extrinsic or chronic secondary bursal side impingement on the supraspinatus and corresponding anterior acromion or CAL osteophyte. In addition, full-thickness rotator cuff tears with chronic bony impingement may need to be addressed earlier surgically, because continued long-term conservative care can lead to gradual enlargement and further retraction of the tear, compromising later surgical results.

Arthroscopic Subacromial Decompression

The arthroscopic technique for subacromial decompression was first described by Johnson ²³ in 1986. Ellman²⁴ presented the first series with follow-up and detailed description of the operative technique. Paulos and Franklin²⁵ presented one of the largest early series (80 patients) and introduced the use of the midlateral subacromial portal. Sampson and coworkers²⁶ first described the cutting block technique for acromioplasty wherein the scope is placed laterally and the burring instruments are advanced from the posterior portal, using the posterior half of the acromion as a guide for resection. The authors also emphasized the importance of the supraspinatus outlet x-ray in both preoperative planning and postoperative evaluation and the benefits of evaluating the flatness of the cut from the lateral and the posterior portals.

Preoperative Planning and Decisions

If diligent nonoperative care has been unsuccessful and the correct type of bursal side impingement has been diagnosed, one must then perform the arthroscopic surgical technique correctly to arrive at a successful result. Arthroscopic subacromial decompression is not just an arthroscopic replication of the open procedure as described by Neer ¹^,² and Rockwood and Lyons.⁸ Arthroscopically, one avoids taking full-thickness bone off the anterior acromion so as not to detach the deltoid fascial attachment or resecting the CA ligament.²⁷ Fortunately, long-term results for arthroscopic subacromial decompression are found to be superior to open acromioplasty.²⁸

Whether one fashions a type 1 flat divergent surface or a type 2 congruent surface, both of which are physiologic variants, depends on the shape and thickness of the acromion and the preference of the surgeon based on arthroscopic findings. The most commonly cited concern with subacromial decompression is iatrogenic harm to the coracoacromial arch and destabilization of the glenohumeral joint with anterior superior subluxation of the head.²⁹ Although not uncommonly seen in open decompression surgery when the anterior deltoid is detached and fails to heal, it has only rarely been reported as a clinical problem with the arthroscopic technique,³⁰ even though slight elevation has been noted in cadaver studies.³¹ In fact, this superior migration is not commonly reported, even with large os acromial resections. This may be related to fact that the CAL essentially re-forms after release or partial resection.³² Some decrease in acromion-humeral distance after ASAD has been reported, however, when associated cuff tears are left unrepaired.³³ If one cannot repair a large or massive cuff tear or the acromion is very thin, then resection of the impinging osteophyte and smoothing, rather than flattening, the acromion would be the appropriate intervention. Acromioplasty in the younger overhand athlete with secondary impingement-related pain only serves to destabilize any existing instability further.

The AC joint is a common but sometimes overlooked source of shoulder pain. Degenerative disease of the AC joint frequently accompanies extrinsic impingement and cuff deterioration. Osteophytic overgrowth on the undersurface of the distal clavicle and medial acromion can impinge on the underlying supraspinatus muscle and tendon. The pain of an arthritic or osteolytic joint may mimic that of anterior impingement and one must decide preoperatively, using physical findings, radiography, MRI, bone scanning and differential injections, as necessary to determine whether AC resection is warranted.

Careful preoperative x-ray evaluation is necessary to determine the appropriate operative approach and to avoid complications. AP, outlet, and axillary views are the key to evaluating the acromion. The AP view demonstrates the orientation and/or disease of the AC joint and the lateral slope of the acromion. The outlet view is used to determine the shape of the acromion (type 2 or 3) and overall thickness. On the outlet view, lines are drawn on the undersurface of the acromion, one from the front tip of the acromion to the posterior edge and a second along the posterior half of the undersurface of the acromion, extending out anteriorly. The distance between these two lines at the anterior margin approximates the amount of undersurface anterior bone that would be resected if one desires to flatten the undersurface of the arch completely (Fig. 8-7).

FIGURE 8-7 Preoperative planning for ASAD. A, Cutting block line.

The axillary view is used to determine the shape of the acromion (cobra versus square-tipped) and whether there is any anterior acromial protuberance or subtle AC arthritis. If, on the outlet view, one notes a very thin or curved acromion, the flat cut line on the undersurface of the posterior half of the acromion may actually exit the superior aspect of the acromion, taking off too much anterior bone (Fig. 8-8).

FIGURE 8-8 Thin curved acromion where cutting block technique would not be appropriate. A, Original cutting block line; B, modified cut, resecting only the anterior hook, preserving the deltoid fascia and not producing a type I flat acromion.

In these cases, the cutting block technique, as described by Sampson and colleagues,²⁶ would be inappropriate. Instead, the lateral approach would be more applicable, removing less anterior bone and not producing a type I flat acromion but a more feathered type 2 devoid of osteophytes. The procedure may be performed using the beach chair or lateral decubitus position.

TECHNIQUES

Glenohumeral Diagnostic Arthroscopic Technique

The anatomy of the shoulder is outlined with a marking pen prior to the operative procedure and the portals marked. The glenohumeral joint is then examined completely from a posterior and high anterior portal, established at the superior aspect of the rotator interval. This will later be the anterior portal for the subacromial bursoscopy. Any pathology within the glenohumeral joint is appropriately addressed.

Partial undersurface or small complete rotator cuff tears are frequently marked with a tag suture placed through an 18-gauge needle introduced from superiorly into the joint and retrieved out the anterior portal. This suture marker is beneficial later when subacromial bursoscopy is performed, because it provides a quick reference to the questionable cuff area from the superior view.

The arthroscope is then removed from the glenohumeral joint and through the same posterior skin portal, redirected at a 10-degree caudad angle to the acromion into the subacromial bursa and far enough anteriorly to enter the bursal chamber. If the bursa is easily entered and distended, then the inflow is brought in at the arthroscope with a pump and a lateral portal is then made on the basis of an accurately placed 18-gauge needle.

If the bursa is significantly inflamed or not easily distended, with poor visualization, the trocar and sheath are brought directly out anteriorly, just lateral to the coracoacromial ligament, to exit from the previously made high anterior skin portal. The outflow cannula is placed on the tip of the trocar and pushed back into the subacromial space so that it lies under the anterior half of the acromion. The sheath is separated slightly, the arthroscope is inserted into the posterior cannula, and flow and visualization are established. A lateral portal is then directed with an 18-gauge needle.

The bursa is then viewed from posteriorly and débrided from the lateral portal until good visualization is established. Any suspicious areas of the rotator cuff that may have been previously identified with a suture marker are débrided and examined from the posterior and lateral portals.

Lateral Approach: Limited Anterior Resection Technique

Preoperatively, one will have decided whether to use a lateral approach—a limited anterior resection—or a cutting-block approach for the decompression.²⁴ If the patient has a thin curved acromion and a lateral approach is appropriate, place the lateral portal 3.5 to 4 cm lateral to the acromion and about midway between the midportion of the acromion and anterolateral corner. Use an 18-gauge needle as a guide to determine if the portal permits access for instruments along the anteroinferior edge of the acromion so that the motorized instruments can be directed slightly upward at the acromion for ease in burring and shaving.

The soft tissue on the undersurface of the anterior half of the acromion is then débrided with an aggressive shaver and/or a cautery ablation system (Fig. 8-9). Care should be taken with either instrument to stay on the undersurface of the bone and not pop off anteriorly or laterally into the deltoid fibers, which are very vascular. The anterolateral corner of the acromion is identified with an 18-gauge needle directed from superiorly; the débridement is started at this point, progresses medially toward the AC joint and then subsequently from this point posteriorly, along the lateral edge of the acromion for about 1 cm.