Acromioclavicular Separations: Soft Tissue (Weaver-Dunn Or Allograft) Techniques

Robert F. LaPrade

Corey A. Wulf

Injuries to the acromioclavicular joint (AC) are common. AC separations, subluxations, and dislocations represent 9% of all injuries to the shoulder girdle. They more commonly occur in males during the second and third decades (1). There is a high incidence among collision athletes. It is the third most common injury in Division I hockey (2) players. The reported incidence in American collegiate football players and National Football League quarterbacks is 41% and 40%, respectively (3).

Despite the commonality of AC separations, very little was known in regard to its kinematics and biomechanics until recently. This lack of fundamental understanding has resulted in the development of surgical procedures that failed to stabilize the AC joint and restore more normal kinematics and function of the joint. It is the goal of this chapter to review the anatomy, biomechanics, and kinematics of the AC joint while incorporating these principles into the selection of a surgical technique for reconstruction of the AC joint.

BASIC SCIENCE

The shoulder girdle is capable of complex movements through multiple articulations involving the chest wall, scapula, proximal humerus, and clavicle. Movements through the aforementioned articulations are powered by the 20 muscle/tendon units that originate or insert on it (4). The clavicle functions as a strut to maintain the lateral positioning of the glenohumeral joint relative to the thorax. The clavicle also provides the origin of the coracoclavicular (CC) ligaments, the conoid, and the trapezoid, which suspend the scapula and contribute to the stability of the AC joint. Bony prominences on the undersurface of the clavicle mark the origins of each ligament. The trapezoidal ridge marks the lateral extent of the trapezoid, whereas the conoid tubercle marks the posterior extent of the conoid. These landmarks can be useful when determining the correct placement for graft fixation while reconstructing the AC joint.

The AC joint is the point of articulation between the scapula and the clavicle. The AC joint is a diarthrodial joint composed of the distal, or lateral, end of the clavicle and the medial aspect of the acromial process of the scapula. Hyaline cartilage is present on the articulating surfaces of the acromion and clavicle. There is a meniscal homologue interposed within the articular space. The meniscal homologue is composed of fibrocartilage and is quite variable in size and shape. The function of the meniscal homologue is unknown and thought to be negligible because it undergoes degeneration in the second and third decades. The articular surfaces are surrounded by the joint capsule and capsular ligaments. The capsule and joint are dually innervated by both the suprascapular and the lateral pectoral nerves.

The AC joint is stabilized both statically and dynamically. The static stabilizers are composed of the joint capsule/ligaments (acromioclavicular or AC ligaments) and the CC ligaments. The fascia of the trapezius and deltoid are the predominant dynamic stabilizers of the AC joint. The AC ligaments are the primary stabilizers of the AC joint at low forces, with the superior ligaments being the strongest and thickest. The CC ligaments are the primary stabilizers at greater forces. They work in concert to stabilize the AC joint as well as to link movement between the clavicle and the scapula. The trapezoid ligament is the broader of the two. It originates on the undersurface of the clavicle, medial to the trapezoidal ridge, and inserts broadly on the posterior, dorsal half of the coracoid. The trapezoid ligament’s main function is to resist AC joint compression and posterior displacement of the clavicle during loading of the glenohumeral joint. The conoid ligament has an oval footprint as it originates anterior to the conoid tubercle on the undersurface of the clavicle. It narrows while passing inferiorly to insert on the posterior and dorsal most portion of the coracoid, including the angle.

Historically, the clavicle and AC joint together were thought to be relatively immobile and simple in regard to their kinematics. This has changed over the years as
biomechanical evaluation has demonstrated the intricate role that the clavicle and its articulation with the scapula play in movements of the upper extremity. The clavicle is not stationary in its relation to the acromion. Motion about the clavicle occurs in three axes: anterior-posterior, superior-inferior, and axial rotation around the anatomic axis of the clavicle. The clavicle hinges on the sternoclavicular joint, allowing up to 35° of motion in the anterior, posterior, and superior directions (4). The axial rotation of the clavicle is greater in relation to the sternum than it is to the acromion, 45° to 50° and 5° to 8°, respectively (4). Despite the multitude of stabilizing constraints, the clavicle may show moderate amounts of displacement in all planes. Debski et al. (5) demonstrated that the clavicle may translate up to 5 mm in the anterior, posterior, and superior directions during application of a 70-N force.

Most historic procedures failed to account for the normal kinematics of the AC joint and led to high rates of failure. Rigid constructs that inhibited AC motion and translation, such as CC screws, failed due to fatigue or pull out. Kirshner (K)-wire fixation tended to migrate. Soft tissue reconstructions restore more normal kinematics while maintaining the reduction of the AC joint.

CLINICAL EVALUATION

Injuries and separations of the AC joint do not typically present a diagnostic dilemma to the evaluating clinician. The mechanism of injury, deformity, and location of pain are reliable findings that lead the clinician to the correct diagnosis. The most common mechanism of injury is through direct trauma with an impact onto the acromion while the arm is in an adducted position. However, it may also occur with a fall onto a hand or elbow that drives the humerus into the undersurface of the acromion. The proposed mechanism for the rare, inferiorly displaced clavicle involves a force applied axially to the upper extremity while the arm is in hyperabduction and external rotation with the scapula in a retracted position. Patients often complain of pain over the superior aspect of the shoulder in the region of the AC joint, but may note pain that radiates into the anterior portion of the neck. Deformity about the AC joint is also a common finding in both the acute and the chronic settings. Prominence of the distal clavicle in the superior, or cranial, direction in relation to the acromion is the most common finding, but displacement in the anterior-posterior planes may be found in association with superior displacement. Less commonly, patients may sustain an AC joint separation with inferior displacement of the clavicle in relation to the acromion.

Evaluation of the patient starts with a detailed history. The key elements include the mechanism, previous injuries, and identification of any unrecognized or masked injuries. A detailed history is followed with a thorough physical examination. Inspection often identifies the deformity about the AC joint. Palpation elicits pain over the AC joint, but one should palpate the entire length of the clavicle and sternoclavicular joint, as concomitant injuries have been reported. Range of motion (ROM) is often limited by pain, especially when the arm is placed in adduction and forward elevation. The cross-body adduction test is performed with the arm in 90° of forward elevation and passively adducting the arm. A positive test is present when pain is reproduced superiorly at the AC joint. The O’Brien’s test is helpful in differentiating patients with Superior Labrum Anterior and Posterior (SLAP) lesions from those with AC joint injuries. However, approximately 18% of patients with type V AC joint injuries had concomitant SLAP lesions (6). It is also important to palpate around the coracoid to evaluate for avulsion fractures of the coracoid. Neurovascular documentation is important, especially in the setting of an inferiorly displaced clavicle.

Imaging

Radiographic evaluation is directed by the clinician’s physical exam findings. Standard shoulder series including AP, scapular Y, and axillary views are routinely ordered. Historically, visualization of the AC joint on a standard shoulder series has been difficult due to the overpenetration that can occur with more superficial structures. We have found this to be less of an issue with the introduction of digital X-rays, which allow the user to adjust the contrast and brightness for better visualization. A Zanca view can be added to allow for optimal visualization of the AC joint. It is performed with the X-ray beam centered over the AC joint and angled 10° to 15° cephalad while using half the penetration strength of a standard AP. CT scans more accurately define bony abnormalities or fracture patterns, both of which are uncommonly encountered in the authors’ practice. As such, a CT scan is rarely obtained since it adds very little information to the plain radiographs that would affect treatment. MRI may be considered if there is concern for associated or concomitant glenohumeral joint pathology. We do not routinely obtain MRIs as a part of the initial evaluation or index procedures, but it may be of benefit in the setting of failed treatment due to persistent or recurrent pain in the absence of ongoing AC instability.

Classification

AC joint separations are commonly classified using the Tossy/Rockwood system. There are six types based on the amount and direction of displacement. The types correlate with the structures injured. Type I represents a sprain of the AC ligaments without appreciable displacement of the clavicle in relation to the acromion. Type II injuries involve complete disruption of the AC ligaments while the CC ligaments remain intact. The clavicle is usually subluxated superiorly in relation to the acromion. The clavicle is unstable upon direct stress. Type III injuries
present as a dislocation of the AC joint with 100% subluxation displacement of the clavicle relative to the acromion. Type III injuries represent tears of the AC and CC ligaments, whereas the trapezial and deltoid fascias remain intact. The clavicle is unstable in the horizontal and vertical planes. Type IV injuries are characterized by complete dislocation of the AC joint as a result of the disruption of the AC and CC ligaments and posterior displacement of the clavicle through the trapezial fascia. The posterior displacement of the clavicle is best visualized on the axillary radiograph. Type V separations result in displacement of the clavicle between 100% and 300% of the width of the clavicle in relation to the acromion due to failure of the AC ligaments, CC ligaments, and deltotrapezial fascia. Finally, type VI injuries are rare injuries with complete inferior displacement of the clavicle into a subacromial or subcoracoid position.

TREATMENT

Nonoperative

Conservative treatment remains the mainstay for type I, II, and III injuries. However, the treatment of type III injuries remains controversial and an area of intense debate. In a systematic review, Spencer (7) performed an analysis of the English-language literature to determine whether grade III AC joint separations were best treated operatively or nonoperatively. The author concluded that nonoperative treatment was deemed more appropriate than traditional operative treatments because the results of the latter were not clearly better and were associated with higher complication rates, longer convalescence, and longer time away from work and sports. This conclusion was based upon a relatively low level of evidence with the most data coming from level IV studies with very few level I and II studies reported. We prefer to initially treat patients with type III injuries nonoperatively, especially if the patient is in an athletic season, and reserve surgical management for those who fail conservative treatment.

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