ANATOMY AND PATHOPHYSIOLOGY

The acromioclavicular joint is subcutaneous and exposed to substantial biomechanical forces during shoulder motion. It is therefore susceptible to degenerative processes and acute trauma. Located between the distal end of the clavicle and the anteromedial aspect of the acromion, the AC joint is a diarthrodial joint. The joint is initially composed of hyaline cartilage articulations incompletely separated by a fibrocartilaginous disk¹; however, it undergoes degeneration to fibrocartilage by 40 years.¹

Although the exact dimensions are debated, the average size of the adult acromioclavicular joint is approximately 9 mm in the coronal plane and 19 mm in the sagittal plane.² There is tremendous variability in the coronal orientation of the joint.^1,3 Three coronal orientations have been described: an overriding clavicle (the clavicle overrides the acromion), a neutral AC joint, and an underriding clavicle (the acromion overrides the clavicle).⁴ The overriding clavicle is the most common variant, occurring in almost 50% of individuals.⁵

Stability of the AC joint is conferred by dynamic and static constraints. Dynamic stabilization arises from the contraction of muscles spanning the clavicle and acromion or humerus. The anterior head of the deltoid, originating off the lateral third of the clavicle and inserting into the deltoid tuberosity of the humerus, acts as a suspensory support for the shoulder girdle. The trapezius adds additional dynamic stability. The acromioclavicular and coracoclavicular (CC) ligaments work in concert to provide static constraints for the AC joint.

The CC ligaments include the medial conoid and lateral trapezoid ligaments; however, up to 1% of patients have a coracoclavicular bar.⁶ The average distance between the clavicle and coracoid is from 11 to 13 mm.^2,7 The CC ligaments are extracapsular and the average distance from the most lateral extent of the trapezoid ligament fibers to the joint is 15.3 mm (range, 11 to 22 mm).⁸ The CC ligaments are the primary constraints to superior and inferior translation, whereas the AC ligaments and capsule are the primary restraints to anteroposterior instability. However, the AC and CC ligaments have a dynamic interplay, whose contributions to stability vary with the directions and forces applied.⁹

Given its anatomy, the potential motion of the AC joint far exceeds that actually demonstrated during active motion of the shoulder girdle.¹⁰ Actually, very little motion occurs at the AC joint in comparison to that of the sternoclavicular joint; less than 10 degrees of rotation occur at the AC joint with full elevation of the arm.¹¹

MECHANISM OF INJURY

Acromioclavicular joint pathology can be stratified into two broad categories, instability and arthritic conditions. Either condition may be the sequel of direct or indirect forces applied to the upper extremity. Direct forces are the most common mechanism of injury, resulting from a fall on the point of the shoulder, usually with the arm adducted. Indirect forces may be transmitted through the upper extremity and dissipated within the acromioclavicular articulation, leading to ligamentous and cartilaginous injury. These forces may lead to an acute injury or, with chronic degeneration, an arthritic condition. An isolated subset of the arthritic population is the younger athlete or weightlifter whose repetitive exercises have induced a stress reaction in the distal clavicle, known as distal clavicular osteolysis. Instability may also be iatrogenic, caused by an overzealous previous distal clavicle resection.

HISTORY AND PHYSICAL EXAMINATION

The history of a patient with AC joint pathology typically reflects the mechanism of injury. In a patient with AC instability, the patient will report a traumatic injury with resultant pain, deformity, and mechanical symptoms dominating their clinical picture. Patients with arthritic conditions typically present with an insidious onset of pain, leading to disability with activities of daily life and night pain.

Whereas pain arising in the AC joint is often localized to the anterosuperior aspect of the shoulder, referred pain may exist, given the rich innervation of the joint by the suprascapular, lateral pectoral, and axillary nerves. With injection studies, Gerber and colleagues¹² have demonstrated that patients may complain of pain distributed over the anterolateral neck, posterior trapezius, and anterolateral deltoid. Painful motions will often include cross-body adduction, overhead activities, internal rotation, and extension of the arm behind the plane of the scapula. Common complaints are pain while putting on a coat, strapping a bra, or washing the contralateral axilla. Weightlifters and athletes with distal clavicular osteolysis describe soreness and pain with flat and inclined bench presses, shoulder presses, and dips. Special attention should be paid to patients in whom a previous distal clavicular resection has been performed. These patients may present for inadequate resection, instability, or a missed diagnosis.

Physical examination of the shoulder always begins with the cervical spine. The examination then progresses to inspection, palpation, and a neurovascular examination. Scars about the shoulder should suggest prior surgical interventions to the physician. The portion of the examination focused on the AC joint should include direct palpation of the joint to elicit tenderness and instability. The AC joint is located just anterior to the Neviaser triangle, the soft spot between the distal clavicle and scapular spine. Often, an overriding distal clavicle will manifest a bony prominence immediately medial to the AC joint. The shoulder should be put through a range of motion. Painful movements may include cross-body adduction, internal rotation and extension of the shoulder, and overhead forward elevation. Shoulder extension, adduction, and internal rotation increase contact pressures across the posterior AC facet, and may be useful in identifying patients with an incomplete resection of the distal clavicle.

Many of these signs and symptoms may overlap with those of other shoulder pathologies, including rotator cuff and superior labrum anteroposterior (SLAP) tears. A complete shoulder examination should be completed on all patients with complex pathology. If confusion remains regarding the source of the pain, injections of local anesthetic into the AC joint or subacromial space may assist the surgeon in differentiation of the diagnoses. Relief of pain after an AC joint injection has an excellent prognostic value for resolution of symptoms after arthroscopic distal clavicle excision. However, relief with an AC injection should be interpreted with caution if more than 1 to 2 mL of fluid is accepted without palpable distention and resistance to further insufflations. In these cases, there may be a defect in the inferior AC joint capsule (seen especially after prior surgery or with significant rotator cuff tears) such that injected anesthetic may freely fill the subacromial bursa, also relieving pain from rotator cuff and bursal conditions.

DIAGNOSTIC IMAGING OF ACROMIOCLAVICULAR JOINT PATHOLOGY

Radiographic evaluation of the acromioclavicular joint should be used to confirm rather than diagnose most AC joint pathologies, so a standard shoulder series of four x-rays should be obtained. An anteroposterior (AP) of the shoulder, an AP of the glenohumeral joint (orthogonal to the scapular plane), a scapular Y, and an axillary x-ray are the minimum views necessary to evaluate a shoulder thoroughly. Unfortunately, the AC joint only requires half the x-ray penetration used for standard shoulder views and thus may be overpenetrated. Depending on its exact plane and morphology in the coronal plane, the AC joint space can usually be best visualized on the AP view of the shoulder. The axillary view can be very useful in assessing the AP position, and the most commonly missed area of prior distal clavicular resections, the posterior AC joint. Additional dedicated AC joint views, such as the Zanca view (x-ray beam is centered over the joint and angled 10 to 15 degrees cephalad to isolate the clavicle from the scapular shadow), have been described; however, we have found them of limited usefulness.

If AC joint stability is in question, an AP of bilateral shoulders including the AC joints should be taken to compare the coracoclavicular distances with the identical tangential view. Stress views may also be obtained, but are not usually required.¹³

Magnetic resonance imaging is not typically necessary for diagnosing acromioclavicular pathology; however, it is useful to evaluate for masses or cysts about the AC joint and concomitant shoulder pathology that may need to be addressed. Unfortunately, findings of increased T2 signal intensity can be nonspecific for symptomatic pathology¹⁴ and therefore must be correlated with physical findings that support the diagnosis of AC pathology.

TREATMENT OPTIONS

Depending on the severity and duration of symptoms, distal clavicle arthrosis may be treated with conservative or surgical modalities. With mild to moderate symptoms that do not dramatically alter a patient’s activities of daily life, it is prudent to begin with activity modifications. Weightlifters and athletes may be able to alter their workout regimen to decrease stress across the AC joint. A local injection of corticosteroid inside the AC joint has a dual purposed; it is not only diagnostic, but also therapeutic.¹⁵ The anesthetic helps the clinician determine what percentage of shoulder pain and dysfunction is directly attributable to AC pathology. The corticosteroid injection may break the cycle of inflammation and allow for resumption of activities. In our experience, physical therapy does not dramatically alter the natural history or symptoms of AC joint arthrosis; however, it may alleviate concomitant rotator cuff–based symptoms.

Since 1941, when conservative management was shown to not be effective, distal clavicle excision has proven itself as a successful surgical intervention for AC arthrosis.¹⁶ Distal clavicle resection may be accomplished through an open or arthroscopic approach. Open distal clavicle resection requires detachment and subsequent repair of the deltotrapezial fascia from the clavicle. Arthroscopic distal clavicle management has been shown to provide reliable results^17,18 with a less invasive approach than open techniques,^19,20 and may be performed in conjunction with the arthroscopic management of other shoulder pathology.

Arthroscopic Distal Clavicle Resection

Indications and Contraindications

The indications for operative intervention vary with pathologic entity. Our discussion will not include acromioclavicular dislocations, in which ligamentous damage may require reconstruction and the clavicle may require stabilization. Care must be taken to pick up subtle (e.g., grade 2 injuries) cases. We have found AP drawer testing, often after an anesthetic injection, more reliable than comparison weight-bearing views. Confining this discussion to arthritic conditions, surgical intervention may be considered once conservative management has failed. AC arthritis and distal clavicular osteolysis are self-limiting diseases and therefore should not be considered until the patient cannot tolerate their pain and/or disability. AC symptoms that fail conservative management can be managed effectively with arthroscopic distal clavicle resection through a subacromial or direct superior approach.^17,18 Patients with prior failed AC resection may also be managed with revision arthroscopic débridement if bony impingement or inadequate resection is confirmed by imaging and clinical evaluation. However, in some cases, especially when associated AC instability is evident or when lesions such as large AC cysts are present, open resection and reefing of the superior ligaments and deltotrapezial fascia may be indicated. Such scenarios may represent relative contraindications to arthroscopic management.

Techniques

Basic Arthroscopic Setup.

Arthroscopic distal clavicle resection may be completed in the lateral or beach chair position; the choice should be predicated on concomitant shoulder pathology and surgeon comfort. Standard arthroscopic instruments, including a 4.2-mm motorized shaver, 4.0 acromionizer burr, and arthroscopic cautery, are used. The forequarter is draped free, with care taken to ensure access to the AC joint. Osseous landmarks are marked on the skin and the AC joint is delineated with localization confirmed by needle placement (Fig. 9-1).

FIGURE 9-1 Location for potential arthroscopic portals. A, Posterior portal for direct distal claviculectomy approach; B, standard posterior portal; C, standard anterior portal; D, anterior accessory portal; E, standard lateral portal.

Bursal Approach.

Once prepped and draped, a routine shoulder arthroscopy is begun. A standard posterior portal is created in the posterior soft spot (see Fig. 9-1). There are two options for anterior portals In anticipation of AC débridement, the standard anterior portal can be made in line with the demarcated AC joint to enable glenohumeral access via the rotator interval and access to the AC joint. Alternatively, the standard anterior portal in the rotator interval can be established, and a separate anterior accessory portal can be established for ease of decompression.²¹

After concomitant intra-articular pathology is addressed, the arthroscope is redirected through the posterior portal into the subacromial space. A lateral portal is created under the direction of an 18-gauge spinal needle. The vector of this portal should be parallel to the undersurface of the acromion to facilitate a technically facile subacromial decompression. A decompression and thorough bursectomy are completed; proper viewing of the AC joint from the posterior portal requires a medial bursectomy just posterior to the joint.

The next step is to identify the AC joint; this is completed with needle localization or by manually depressing the distal clavicle. Pressing on the distal clavicle effectively delivers more of the clavicle inferior to the acromion, thereby aiding in visualization from the subacromial space. Placement of a needle in the AC joint may help with identification of the joint arthroscopically and externally, as a triangulation landmark (Figs. 9-2 and 9-3). Cautery is used to strip the inferior AC capsule off the AC joint.

FIGURE 9-2 Needle localization of the anterior accessory portal as viewed from outside the joint.

FIGURE 9-3 Needle localization of the anterior accessory portal as viewed from the subacromial space after decompression and stripping of the inferior ligaments.

While visualizing from the posterior portal with a 30-degree arthroscope and superolaterally oriented optics, either the anterior portal or accessory anterior portal is used as a working portal into the anterior AC joint. If an accessory portal is to be used, it is localized along the anterosuperior aspect of the AC joint, and may enable ease of working within the AC joint²¹ (see Figs. 9-2 and 9-3). The in-line vector of a properly placed anterior or accessory anterior portal enables the burr to be controlled with uniplanar vertical sweeping motions (Fig. 9-4).

FIGURE 9-4 Proper visualization of the distal clavicle from the subacromial space. Note that the camera is pointed medially, with the optics pointing superolaterally. The shaver is introduced from an anterosuperior vector to allow for easy burring of the distal clavicle.

Soft tissue within the AC joint is removed with a motorized shaver or cautery exposing the distal clavicle. Care is taken to expose the superior anterior and posterior corners of the joint while retaining the superior AC ligament and/or capsule. Once exposure is obtained, the distal clavicle is resected with a motorized burr in a sequential fashion from anteroinferior to posterosuperior via the anterior portal. In general, 8 to 10 mm of distal clavicle is resected to enable sufficient resection and relieve any bony impingement while ensuring preservation of the medial CC ligaments.²²

During resection, it is imperative to preserve the superior ligaments to prevent iatrogenic anterior and posterior instability and to address the posterosuperior corner of the clavicle (Fig. 9-5). This area is the most common underresected portion of the clavicle. We routinely switch our arthroscope to the anterior accessory portal to visualize our resection at the completion of our resection (Fig. 9-6). In rare cases, an accessory posterosuperior portal may be needed to resect the posterosuperior distal clavicle fully, especially early in a surgeon’s learning curve.

FIGURE 9-5 A, AC joint ligamentous incompetence may lead to excessive posterior translation of the distal clavicle. Impingement of the posterior clavicle against the acromion may lead to a residually painful shoulder. B, Arthroscopic distal clavicle resection preserves the superior ligaments, thereby preserving joint stability.

(From Flatow EL. The biomechanics of the acromioclavicular, sternoclavicular, and scapulothoracic joints. Instr Course Lect. 1993; 42:237-245.)

FIGURE 9-6 Viewing the posterosuperior corner of the clavicular side of the acromioclavicular joint (left) from the anterior portal, it is clear that a small ridge of bone still needs to be resected to complete the distal claviculectomy symmetrically.

Once the resection is completed, a symmetrical and adequate resection is confirmed by measuring with an instrument of known dimensions. At our institution, we use a rasp measuring 8 mm in width, which must easily fit into the new joint space (Fig. 9-7).

FIGURE 9-7 After the distal claviculectomy, measure the new effective joint space with an instrument of a known width. In this case, we have used an 8-mm rasp.

Direct Approach.

A direct superior arthroscopic approach is indicated in patients with isolated AC pathology, most frequently in cases of distal clavicular osteolysis.^17,20 The direct approach avoids violation of the subacromial space and bursa and therefore may be less traumatic in select patients. However, patients typically present with concomitant subacromial and AC pathology, and therefore are more frequently managed via a bursal approach to the AC joint.

Portals are identified along the anterior and posterior line of the AC joint, 0.5 cm from the joint edge (Fig. 9-8; see Fig. 9-2). The posterior portal is developed bluntly and the arthroscope is introduced into the posterior AC joint. The location of the anterior portal is then confirmed with needle localization and developed. Electrocautery or a motorized shaver is used to remove intra-articular tissue and visualize the AC joint. Careful dissection with cautery is then performed to visualize the anteroinferior and anterosuperior corners of the AC joint. The superior AC ligament and capsule should be left intact and not be violated.

FIGURE 9-8 Two portals are created for a direct distal clavicle resection. The anterior portal (A) is just anterosuperior to the AC joint. The posterior portal (P) is just posterior to the AC joint.

(From Flatow EL, Cordasco FA, Bigliani LU. Arthroscopic resection of the outer end of the clavicle from a superior approach: a critical, quantitative, radiographic assessment of bone removal. Arthroscopy. 1992;8:55-64.)

Using the burr, the anterior distal clavicle is resected in a sequential fashion, from inferior to superior, and then progressing more posteriorly. Care is taken to ensure a smooth osseous resection without ridges and the superior corner of the clavicle must be visualized. The arthroscope is then switched to the anterior portal to allow for improved posterior joint visualization. The previously resected anterior clavicle serves as a template for posterior resection. In general, a minimum of 8 to 10 mm of bone is resected to ensure no persistent bony impingement. The anterior and posterior superior corners are carefully visualized to ensure no persistent ridges or overhangs of bone remain. Similar to the bursal approach, at the end of the resection, we use an 8-mm rasp brought into the joint to confirm appropriate resection width (see Fig. 9-6).

Closure and Postoperative Protocol.

At the conclusion of the procedure, the joint is irrigated and bony debris removed with a motorized shaver. Hemostasis is obtained. Portals are closed with buried absorbable simple sutures and the skin is dressed with adhesive strips and sterile dressing. The arm is placed in a sling for comfort. In cases of isolated AC pathology, patients are started on a gentle active and passive therapy protocol to minimize postoperative stiffness. Patients are seen at approximately 2, 6, and 12 weeks from surgery. At 2 weeks, the wounds are inspected and range of motion is examined. By 4 to 6 weeks, the soft tissues have healed and patients may resume all activities, although we encourage delay of heavy lifting or weight training, especially bench presses, until 3 months to avoid stretching weakened AC ligaments.

PEARLS& PITFALLS

PEARLS

• Always examine the AC joint preoperatively to avoid misdiagnoses or missed diagnoses.

• Carry out a thorough bursectomy.

• Ensure proper anterior accessory portal placement.

• Measure resection with a standardized instrument to prevent underresection.

• Resect the posterosuperior corner of clavicle.

• Visualize the resection from the posterior and anterosuperior accessory portals at the conclusion of the procedure to ensure that it has been done adequately.

PITFALLS

• There is misdiagnosis or a missed diagnosis.

• There is poor visualization because of an inadequate bursectomy.

• The posterosuperior corner has been missed.

• There is overresection of the clavicle leading to instability; remember the anatomy.

RESULTS OF ARTHROSCOPIC DISTAL CLAVICLE EXCISION

Arthroscopic distal clavicle resection has established an extremely successful and predicable track record in patients with preoperatively stable, arthritic AC joints.^17,20–24 Arthroscopic resection enjoys the same excellent long-term pain relief as open resection, but the benefits of less soft tissue violation and a faster return to painless full range of motion and athletic activities. With meticulous surgical technique and exacting attention paid to a smooth even resection, surgeons can expect good to excellent outcomes in 88% to 100% of patients with isolated acromioclavicular arthrosis.^18,21,25