Scapulothoracic Dissociation

Introduction

Scapulothoracic dissociation is a rare and devastating injury of the shoulder girdle. Originally described by Oreck et al. in 1984, the injury was defined as a complete separation of the acromioclavicular (AC) joint with lateral displacement of the scapula and disruption of the brachial plexus and subclavian vessels with intact skin. It is most often the result of high-energy trauma to the shoulder girdle with a subsequent massive traction force applied to the affected upper extremity. In 1987 Ebraheim et al. expanded the definition to include the addition of either a distracted clavicle fracture or sternoclavicular (SC) joint separation. Currently, scapulothoracic dissociation is considered to be a spectrum of injury that includes AC joint separation (25%), clavicle fractures (55%), and/or SC joint separation (20%); vascular injury to subclavian or axillary vessels (64%); partial or complete disruption of the brachial plexus (40% have incomplete neurologic deficit, 36% with brachial plexus avulsions); severe soft-tissue swelling; and extensive disruption of the musculature surrounding the shoulder girdle ( Fig. 18.1 ). ^, This constellation of injuries is considered to be analogous to a closed forequarter amputation. It was originally reported that the mortality rate of scapulothoracic dissociation was 11%, but in reality, it is likely higher, due to many patients dying from associated injuries in the field prior to making it to the emergency room.

Relevant Anatomy

The relevant osseous anatomy of this injury pattern includes the scapula and the clavicle. The scapula itself has no direct osseous attachment to the axial skeleton. Instead, it has indirect attachments via the AC and SC joints. Additionally, the scapula has an indirect relationship with the thorax via infraserratus and supraserratus bursae.

Three muscle groups can be affected in this spectrum of injury: scapulothoracic, rotator cuff, and scapulohumeral. The scapulothoracic muscles include trapezius, serratus anterior, levator scapulae, rhomboid major, and pectoralis minor. The rotator cuff muscles include supraspinatus, infraspinatus, teres minor, and subscapularis. The scapulohumeral muscles include deltoid, latissimus dorsi, and triceps.

The vasculature of the upper limb is redundant, with extensive collateral flow. The subclavian artery arises from the innominate artery on the right and directly off of the aortic arch on the left. The subclavian artery has five main branches: the internal mammary artery, the vertebral arteries, the high thoracic artery, and the thyrocervical trunk. At the level of the first rib, the subclavian artery ends and becomes the axillary artery. The axillary artery has six main branches: the superior thoracic artery, the lateral thoracic artery, the thoracoacromial trunk, the subscapular artery, and the anterior and posterior circumflex humeral arteries. The axillary then becomes the brachial artery once it courses over the anterior shaft of the humerus at the inferior margin of teres major ( Fig. 18.2 ).

The nerve supply to the upper extremity originates from the C5-T1 nerve roots and is supplied mostly via the terminal branches of the brachial plexus ( Fig. 18.3 ). Scapulothoracic dissociation can present with preganglionic nerve root injuries, meaning they occur proximal to the dorsal root ganglion, or postganglionic nerve root injuries, meaning they occur distal to the dorsal root ganglion. The dorsal root ganglion contains sensory neurons that relay information from thermoreceptors, nociceptors, proprioceptors, and chemoreceptors to the central nervous system.

Pathoanatomy

Scapulothoracic dissociation, now understood to be a spectrum of injury, includes AC, clavicle, and/or SC injuries; damage to the subclavian or axillary vessels; brachial plexus injury/avulsion; as well as disruption of the musculature surrounding the shoulder, typically resulting from a high-energy traction mechanism. Traction injuries to the vasculature of the upper extremity can cause extensive internal bleeding and limb-threatening ischemia if not addressed in a timely manner. As noted above, there is extensive collateral vascular flow such that patients may have a perfused limb or even a palpable pulse even in the setting of a complete transection or thrombosis from intimal occlusion of the subclavian artery. Brachial plexus avulsions, while not necessary to treat emergently, dictate the long-term outcomes of this constellation of injuries.

With regard to the spectrum of osseous injury in scapulothoracic dissociation, it can be thought of within the framework of the superior shoulder suspensory complex (SSSC) originally described by Goss in 1993 and described in this text in Chapter 17 . The SSSC is a bone and soft tissue ring that comprises the glenoid, distal clavicle, acromion, AC joint, and coracoclavicular ligaments. This ring is held in place by two struts, which impart an element of redundancy: the superior strut is composed of the clavicle shaft and AC joint while the inferior strut is composed of the acromion, scapular spine, and lateral scapula. The SSSC concept posits that “double disruptions,” when the ring fails in two locations, lead to a degree of instability that has decreased potential to heal without surgical intervention. Additionally, this increased degree of instability can lead to further neurovascular compromise in a patient where some degree of vascular compromise or brachial plexus disruption likely already exists.

Initial radiographic evaluation should begin with a well-centered chest X-ray to look for lateral displacement of the scapula (a finding that is considered pathognomonic for scapulothoracic dissociation). Lateral displacement of the scapula is quantified by the scapular index. This is obtained by measuring the distance from the medial border of the scapula to the thoracic spinous process, then obtaining the ratio between the measurement of the injured side compared with that of the uninjured side. A ratio of ≥1.29 on chest X-ray without rotation is consistent with scapulothoracic dissociation. Rotation can be assessed by observing how the thoracic spinous processes line up with the sternum. Diagnosis is generally confirmed with dedicated radiographs of the scapula as well as computed tomography (CT) of the injured extremity ( Fig. 18.4 ).

On physical examination, this injury typically presents as severe swelling about the shoulder due to the hematoma formation that results from the significant soft tissue damage that is characteristic of this injury. Inspection of the shoulder girdle should be followed by a thorough vascular examination to assess perfusion of the limb. Initial assessment of perfusion should begin with palpation, Doppler evaluation, and brachial-brachial indices, followed by angiography to evaluate for injury to the subclavian, axillary, or brachial vessels.

Once determined that the patient is hemodynamically stabilized, a very thorough neurologic examination must be performed to determine the presence of incomplete or complete brachial plexus injury. As mentioned earlier, the vascular injury is a life- and limb-threatening event that often required emergent exploration and surgical treatment, but the severity of brachial plexus injury determines the long-term disability. Preganglionic neurologic injuries can present with weakness of the serratus anterior, rhomboids, and levator scapula, as well as with potential Horner’s syndrome (miosis, ptosis, and anhidrosis). Magnetic resonance imaging (MRI) or CT myelogram can be performed to evaluate for nerve root avulsion.