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Osseous Injuries of the Shoulder Girdle
Prevalence, Epidemiology, and Definitions
Osseous injuries that affect the shoulder girdle are common in adults. The acromioclavicular joint is the primary connection between the arm and the thorax, and the glenohumeral joint is inherently unstable, owing to the disproportionate size of the articular surfaces of this spheroidal joint. The majority of injuries occur either from a direct impact on a focal point on the shoulder or from a fall on an outstretched hand, resulting in both fractures and dislocations. Shoulder injuries have no particular relation to the sex of the patient but appear to be age dependent. For instance, glenohumeral joint dislocation and acromioclavicular joint separation occur in young people, whereas fractures most frequently occur in the elderly. This chapter reviews the osseous injuries of the shoulder that affect the acromioclavicular, sternoclavicular, and glenohumeral joints and presents some of the more commonly used classification systems that describe related injury patterns.
Manifestations of the Disease
Evaluation of the shoulder girdle begins with a radiographic inspection of the osseous structures and joints. There are limitations to the radiographic evaluation. Inspection of the clavicle and proximal humerus, as well as the acromioclavicular and glenohumeral joints, is adequate, but assessment of the scapula is limited. Indicators of instability such as glenoid rim fractures are difficult to visualize. In addition, complex fractures and fracture-dislocation complexes, owing to the displacement of osseous fragments, are more optimally characterized with CT. The availability of multidetector CT has enabled expeditious and accurate evaluation of the shoulder, and the depiction of acute osseous abnormalities in an infinite number of imaging planes has rendered CT an indispensable modality for assessment of bone integrity and joint configuration.
Although CT is an excellent imaging tool for demonstrating the integrity of the osseous cortex, it does not depict the connective and cartilaginous tissues of the shoulder girdle as well as MRI, even in the setting of arthrography. The main advantage of MRI is the ability to directly visualize the joint capsule, labrum, articular cartilage, tendons, and supporting ligamentous structures. MR arthrography has further improved the ability to assess the components of the glenohumeral joint, including the attachment of the capsule and labrum, the glenohumeral ligaments, and the articular side of the rotator cuff. MRI is outstanding for assessing shoulder instability, but in the setting of acute trauma it is also useful for depicting marrow edema associated with contusions and occult fractures.
Acute Osseous Injuries of the Clavicle
The clavicle serves as the only osseous bridge between the arm and the thorax. It is vulnerable to trauma, particularly during childhood and adolescence, and accounts for nearly 50% of fractures in children younger than 10 years. A fall directly on the shoulder during play or as a result of an athletic activity is the etiology of the fracture in more than 90% of cases. A small percentage of fractures occur from a direct blow to the clavicle, such as those occurring in a motor vehicle–related trauma or an assault, and rarely from a fall on an outstretched hand. A clavicular fracture is also a common fracture of childbirth ( Fig. 7-1 ).
Radiography
Eighty percent of clavicular fractures affect the middle third of the bone, and about 15% involve the lateral third ( Fig. 7-2 , 7-3 ). Only 5% of fractures involve the medial third of the clavicle. These may be difficult to visualize, owing to the overlap of the spine and ribs, and may require additional imaging by CT, particularly if there is a concomitant question of sternoclavicular joint subluxation or dislocation.
Acute Osseous Injuries of the Acromioclavicular Joint
The acromioclavicular (AC) joint is a common site for dislocations, accounting for about 12% of shoulder dislocations. The mechanism of injury is either a direct fall on the point of the shoulder or a fall on an outstretched hand. The force applied to the shoulder girdle during the fall determines the spectrum of pathology that develops within and around the joint. Generally, it is an injury of young people between 15 and 40 years old.
Radiography
As the AC joint separates, the initial injury is a strain of the superior AC ligament. A rupture of the trapezoid and conoid ligaments occurs with increased force. With complete rupture of the coracoclavicular (CC) ligaments, the clavicle is allowed to detach, resulting in injuries to the insertion of the deltoid and trapezius muscles. A variation that occurs in young people is preservation of the CC ligaments with avulsion of the coracoid process at its base. This injury typically occurs in patients younger than 25 years, because fusion of the coracoid ossification center can occur as late as 21 to 25 years of age.
The classification proposed by Tossy and colleagues is the most widely used classification for describing injuries to the acromioclavicular joint. In a type 1 separation, conventional radiographs appear normal or may show some soft tissue swelling over the joint ( Fig. 7-4 ). In a type 2 injury, there is disruption of the AC ligament and partial disruption of the CC ligaments, so that the clavicle is allowed to migrate superiorly. Displacement measures less than 5 mm or 50% of the width of the clavicle on weight-bearing views. In type 3 injuries, the AC and CC ligaments are completely disrupted, and there is clavicular migration exceeding 5 mm or 50% of the bone width. Rockwood and colleagues proposed three additional grades to the classification. In type 4 injuries, the clavicle is displaced posteriorly into or through the trapezius muscle. In type 5 injuries, superior clavicle migration is more pronounced than in a type 3 separation. In type 6 injuries, the clavicle dislocates inferiorly below the coracoid or acromion process, an injury that may also be associated with rib fractures. Types 4 to 6 are extremely rare injury patterns. They are usually discovered at the time of surgical exploration and may not be necessarily detected radiographically.
Magnetic Resonance Imaging and CT
Occasionally, further evaluation of an acromioclavicular joint separation by MRI is required. The strength of MRI is in the ability to directly visualize the joint capsule, interstitial soft tissues, and integrity of the superior AC ligament as well as the CC ligaments ( Fig. 7-5 ). CT may be considered in patients who have a concomitant fracture of either the coracoid or acromion process.
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