Scapulothoracic dissociation

Abstract

Scapulothoracic dissociation is a rare and potentially limb- and life-threatening injury, that results from high-energy trauma. Scapulothoracic dissociation has the potential to be overlooked in the acute setting, especially in the setting of polytrauma. Therefore, a careful search for this condition should be performed in all patients with high-energy shoulder girdle injuries. The goals of this article are to review the anatomy of the scapulothoracic articulation as well as the spectrum, imaging evaluation, differential diagnosis and management of scapulothoracic dissociations.

Introduction

Scapulothoracic dissociation (STD) is a rare and potentially limb- and life-threatening injury with a mortality rate of approximately 11%.¹ STD has the potential to be overlooked in the acute setting, especially in the setting of polytrauma (associated injuries have the potential to divert the clinician’s attention and result in delayed diagnosis and treatment of this potentially devastating injury).^2,3 Therefore, a careful search for this condition should be performed in all patients with high-energy shoulder girdle injuries. The goals of this article are to review the anatomy of the scapulothoracic articulation as well as the spectrum, imaging evaluation, differential diagnosis and management of scapulothoracic dissociations.

Anatomy

The shoulder is formed by three bones and four articulations. The bones consist of the clavicle, scapula and proximal humerus, and the articulations are comprised of the glenohumeral, acromioclavicular, sternoclavicular and scapulothoracic joints.⁴ Neighboring muscles and ligaments contribute to the smooth and coordinated movements of the shoulder complex.

The scapulothoracic articulation in an atypical joint and in contrast to the other shoulder articulations, there is no articular cartilage, synovium, capsule or ligament at the level of this joint.³ The close relationship of the scapula to the thorax is mediated by muscles and bursae and the only osseous link of the scapula to the axial skeleton, is indirectly through the clavicle as well as the acromioclavicular and sternoclavicular joints.⁵ Williams et al,⁶ in a cadaveric study divided the structures of the scapulothoracic articulation into superficial, intermediate and deep layers (Figure 1). The superficial layer includes the trapezius and latissimus dorsi muscles and an inconsistent bursa between the latissimus dorsi and the inferior scapular angle (Figure 1a). The intermediate layer is comprised of the levator scapulae, rhomboid minor and rhomboid major muscles, a variable scapulotrapezial bursa that lies between the trapezius and the medial aspect of the scapular spine (Figure 1a), and several important neurovascular structures (including the spinal accessory nerve).^6,8 The deep layer is formed by the soft-tissue apposition of the subscapularis muscle (which spreads across the costal surface of the scapula) and the opposing serratus anterior muscle (which originates from the superolateral surfaces of the upper eight or nine ribs and inserts along the medial border of the scapula) as well as two relatively constant bursae: The infraserratus (scapulothoracic) bursa that is located between the serratus anterior and the chest wall, and the supraserratus (subscapularis) bursa that lies between the serratus anterior and subscapularis muscles (Figure 1b) (additional adventitial bursae may be seen in the deep layer at level of the superomedial and inferior angles of the scapula).^6,7,9

Figure 1.

Anatomy of the scapulothoracic articulation according to Williams and colleagues.⁶ (a) Modified three-dimensional CT image of the posterior chest wall shows the superficial and intermediate layers of the scapulothoracic joint (the muscles of the superficial layer are depicted on the right, while both the superficial and intermediate layers are shown on the left side-of the image). The superficial layer includes the trapezius and latissimus dorsi muscles and an inconsistent bursa between the latissimus dorsi and the inferior scapular angle (short arrow). The intermediate layer is comprised of the levator scapulae (LS), rhomboid minor (Rho. min) and rhomboid major muscles, a variable scapulotrapezial bursa (long arrow) that lies between the trapezius and the medial aspect of the scapular spine, and several important neurovascular structures. (b) Modified axial T ₁ weighted MR image of the deep layer of the scapulothoracic joint: The deep layer is formed by the soft-tissue apposition of the subscapularis muscle and the opposing serratus anterior muscle (short arrows), as well as two relatively constant bursae: The infraserratus (scapulothoracic) bursa (arrowhead) and the supraserratus (subscapularis) bursa (long arrow). The infraserratus bursa (arrowhead) is located between the serratus anterior muscle and the bony chest wall, and is bordered by the origin of the serratus anterior muscle laterally, and the rhomboid muscles medially.⁷ The supraserratus (subscapularis) bursa (long arrow) lies between the serratus anterior and subscapularis muscles. Rho. major = Rhomboid major muscle.

Scapulothoracic dissociation: Spectrum of abnormalities

Scapulothoracic dissociation was originally described in 1984 as an injury consisting of a laterally displaced scapula with separation of the ipsilateral acromioclavicular joint, disruption of the ipsilateral subclavian vessels and brachial plexus, and an intact overlying skin.¹⁰ Currently STD refers to a spectrum of musculoskeletal, vascular and neurologic abnormalities that includes a laterally displaced scapula with an ipsilateral distracted clavicular fracture (most common type) (Figures 2 and 3), acromioclavicular joint separation or less commonly sternoclavicular joint disruption, without or with concomitant injuries to the ipsilateral subclavian or axillary vessels and the brachial plexus (Figure 3).^3,11 Bilateral and open injuries may also occur.¹² A closed STD with severe neurovascular injury is considered analogous to a closed forequarter amputation.²

Figure 2.

Scapulothoracic dissociation. Mildly rotated anteroposterior chest radiograph in a 38-year-old female shows a distracted left clavicular shaft fracture and apparent lateral displacement of the left scapula. In high-energy trauma patients obtaining a nonrotated chest radiograph is not commonly achievable and careful search for scapulothoracic dissociation is warranted especially in the presence of an ipsilateral distracted clavicle fracture, acromioclavicular joint separation or sternoclavicular joint disruption. Patient had associated ipsilateral subclavian artery injury and brachial plexopathy (not shown). There are displaced second to fourth left rib fractures.

Figure 3.

Scapulothoracic dissociation and brachial plexus injury. (a) Mildly rotated anteroposterior chest radiograph in a 25-year-old male shows a comminuted, displaced right midclavicular shaft fracture and lateral displacement of the right scapula. Right scapulothoracic dissociation was confirmed on CT angiography of the chest (not shown; patient did not have concomitant vascular injury). Axial T ₂ weighted MR images at level of the C7-T1 (b) and T1-T2 (c) neural foramina show pseudomeningoceles at level of right C8 and T1 nerve root avulsions. This patient had a complete right brachial plexus palsy and a flail right upper extremity.

Mechanisms of injury

Scapulothoracic dissociation results from high-energy distraction injuries, typically from a motorcycle or motor vehicle accident, and less commonly from a fall or industrial machine accident.^3,13 The mechanism of injury is either a direct distracting blunt force or sudden deceleration while the affected upper extremity remains fixed (e.g. secondary to gripping a steering wheel or motorcycle handlebar).³ Patients with STD have an increased risk of polytrauma, including severe life-threatening injuries to other body parts and concomitant injuries of the ipsilateral upper extremity.^3,11,14 The presence of associated injuries has the potential to divert the clinician’s attention and result in delayed diagnosis and treatment of this potentially devastating injury.¹¹

Initial clinical assessment

Initial assessment and treatment of patients suspected of STD follow the advanced trauma life support protocol. If the patient’s clinical situation permits, a comprehensive physical examination of the injured upper extremity and a thorough neurologic examination should be performed.³ Patients with STD usually demonstrate massive asymmetric swelling of the injured shoulder that is due to variable combinations of edema, hematoma and periscapular muscle tears (Figure 4).^2,3,11 Vascular injury (Figure 5) has been reported in 64–100% of patients with STDs and is usually secondary to thrombosis or extrinsic compression of the subclavian or axillary arteries.^1,10,15 Physical examination may reveal pulselessness, pallor and coolness of the affected extremity. Nevertheless, life-threatening hemorrhage and limb-threatening ischemia are relatively uncommon (the latter due to the presence of extensive shoulder girdle collaterals) and overall outcomes related to vascular injury are good.^3,11 If the patient’s clinical condition permits, a thorough neurologic evaluation should be performed for determination of the presence, extent, and location of the neurologic injury, which is the main determinant of the functional outcome after STDs (approximately 52% of patients with STD will have a flail nonfunctional upper extremity with complete loss of motor and sensory functions).^2,3,11

Figure 4.

Periscapular muscle tears. Coronal STIR MR image of the right brachial plexus in a 20-year-old male with right scapulothoracic dissociation. The trapezius (black arrows), supraspinatus (short white arrow) and subscapularis (long white arrow) muscles are torn and there is significant edema of the subcutaneous and axillary fat.

Figure 5.

Scapulothoracic dissociation. (a) Axial chest CT angiography image in a 23-year-old male involved in a high-speed motor vehicle accident with left scapulothoracic dissociation (associated left acromioclavicular joint separation is not shown). There is posterior and lateral displacement of the left scapula and a large left chest wall hematoma that compresses the ipsilateral subscapularis, pectoralis minor (pec. minor), pectoralis major (pec. major) and serratus anterior (marked by short arrows on the contralateral side) muscles. The subscapularis and serratus anterior (short arrows) muscles are the main constituents of the deep layer of the scapulothoracic articulation, which is demarcated by double headed arrows bilaterally (there is dissociation and asymmetric widening of this space on the left side). The left axillary artery is not discernible. (b, c) Consecutive coronal reconstructed chest CT angiography images show abrupt cutoff of the proximal left axillary artery (long arrow). There is reconstitution of flow in the left axillary artery branches more distally (short arrows) via collaterals. Surgical exploration revealed left axillary artery transection. (d) Anteroposterior left brachial artery angiogram shows a transected left axillary artery with extravasation of contrast.

Imaging evaluation

Scapulothoracic dissociation may be associated with more distant life-threatening injuries that have the potential to divert the physician’s attention from the scapulothoracic articulation. Awareness and careful search for STD are warranted especially in the presence of a substantially distracted clavicle fracture Figures 2 and 3 or acromioclavicular and sternoclavicular joint disruptions. Anteroposterior chest radiography performed as part of the initial trauma series, is the initial imaging modality used for diagnosing scapulothoracic dissociations. On a nonrotated chest radiograph, assessment of lateral scapular displacement can be performed by measuring the distances between a midline thoracic spinous process and the medial borders of the scapulae on both the injured and uninjured sides. A difference > 1 cm is diagnostic of STD, while a scapular index (ratio of the distances of the injured and uninjured sides) >1.29 is consistent with STD until proven otherwise.^15,16 In the presence of a medial scapular fracture, the scapular index may be determined by measuring the distances between the coracoid processes and the sternal notch or the glenoid rims and the sternal notch.¹⁴ The disadvantages of these measurements are that they rely on a well-centered, nonrotated chest radiograph with symmetrically positioned arms (which is not commonly achievable in high-energy trauma patients) and comparison with an uninjured side (which is not possible with bilateral scapulothoracic dissociations).³ Orthogonal radiographs of any suspected upper extremity fracture or dislocation should be obtained additionally.

Immediate vascular imaging work-up is recommended in patients with suspected STD typically with CT angiography (Figure 5).^3,17 Additionally, CT of the affected shoulder girdle may be obtained de novo or reformatted from the chest CT scan obtained during admission.

Brachial plexus injuries usually occur with increasing severity of trauma.³ The extent of neurologic injury determines the functional prognosis after STD and can be determined with varying combinations of physical examination, imaging (CT myelography and MRI), electrodiagnostic studies and surgical exploration. CT myelography and MRI (especially MR neurography) (Figure 3) can be used for evaluation of brachial plexus injuries, if surgical exploration of the brachial plexus has not been performed at the time of an open vascular repair. Differentiation of preganglionic (i.e. nerve root avulsions) from postganglionic (Figure 6), and partial from complete brachial plexus injuries determines clinical outcomes.^3,11 Nerve root avulsions have limited potential for spontaneous recovery and complete brachial plexus avulsions are typically associated with a poor functional outcome and an ipsilateral flail, non-functional upper extremity with complete loss of motor and sensory functions.^3,11

Figure 6.

Nerve injury. Illustration shows anatomy of the spinal nerves (left side-of image), as well as simultaneous preganglionic nerve root avulsions (short arrows) and postganglionic nerve transection (long arrow; right side-of image).

Recognition of preganglionic injuries can be difficult on imaging and relies on varying combinations of direct and indirect findings. Direct signs include discontinuity and abnormal enhancement of the injured nerve root relative to the contralateral side. Pseudomeningoceles, cord edema and contralateral deviation of the spinal cord are indirect manifestations of preganglionic nerve root avulsions. A pseudomeningocele will usually invaginate into the affected neural foramen and may contain the detached nerve root. Intact nerve roots anchor the spinal cord centrally and nerve root avulsions on one side, result in unopposed traction and deviation of the spinal cord by the contralateral nerve roots. Nerve root avulsions and pseuodomeningoceles (Figure 3) are well depicted with both CT myelography and MRI.^2,11 CT myelography should be performed no sooner than 3 weeks after injury to allow pseudomeningocele formation and resorption of potentially dye-displacing hematomas.¹⁸

Imaging manifestations of postganglionic brachial plexus injuries include nerve edema, nerve discontinuity (which may be associated with clumping or retraction of the nerves), and a peri-plexus hematoma. Postganglionic injuries are best evaluated with MR neurography.

Classification

Scapulothoracic dissociation is most commonly classified using the Zelle et al¹⁵ modification of the Damschen et al¹³ classification (Table 1). This modified classification has some predictive utility in determining the functional recovery of the affected limb.

Table 1.

Zelle et classification of scapulothoracic dissociations

Type	Injury
1	Musculoskeletal (MSK) injury alone
2A	MSK injury + Vascular disruption
2B	MSK injury + Incomplete brachial plexus injury a
3	MSK injury + Vascular disruption + Incomplete brachial plexus injury a
4	MSK injury + Complete brachial plexus injury a

^aBrachial plexus injuries usually occur with increasing severity of trauma.³ The extent of neurologic injury determines the functional prognosis after STD and can be determined with varying combinations of physical examination, imaging (CT myelography and MRI), electrodiagnostic studies and surgical exploration.

Differential diagnosis

In addition to STD, post-traumatic asymmetric positioning of the scapula may be due to displaced scapular fractures, scapulothoracic dislocation and scapular winging (Figure 7).^2,19–21 Scapulothoracic dislocation is an extremely rare form of injury that is also referred to as scapular dislocation or locked scapula in the literature. Scapulothoracic dislocation may be extrathoracic or intrathoracic. Extrathoracic scapular dislocation is a post-traumatic disruption of the scapulothoracic articulation that results in lateral displacement of the scapula with deformity of the shoulder girdle and a locked scapula.^20,22 The condition is presumed to be secondary to muscle spasm and in contrast to scapulothoracic dissociation there is no evidence of associated fracture or dislocation of the other components of the ipsilateral shoulder girdle. The dislocated scapula usually reduces spontaneously with conscious sedation or muscle relaxants.²⁰ Extrathoracic scapular dislocation may be accompanied by transient ipsilateral upper limb motor, sensory or pulse deficits, that typically demonstrate complete recovery following scapular reduction. With intrathoracic scapulothoracic dislocations, there is anterior displacement of the inferior angle of the scapula into a post-traumatic or post-surgical posterior chest wall defect.² Intrathoracic scapular dislocations may follow low- or high-energy trauma or in the presence of post-surgical posterior chest wall defects occur in the absence of trauma.^19,20 This type of dislocation is typically treated with closed reduction in the acute setting.¹⁹

Figure 7.

Scapular winging. Axial (a) and three-dimensional reconstructed (b) CT images of the scapulae in a 24-year-old female with blunt left long thoracic nerve injury and medial left scapular winging. The medial border of the left scapula is lifted off the posterior thoracic wall when compared to the contralateral side (white lines demarcate the distances between the medial scapular borders and the opposing aspects of the posterior thoracic cage). In addition, there is medial and superior translation of the left scapula.

Scapular winging is a rare debilitating condition resulting from a variety of traumatic and non-traumatic processes.²¹ It is most often the result of varying combinations of paralysis of serratus anterior (most common cause), trapezius, and rhomboid muscles. Paralysis of any of these muscles results in lifting of the medial border of the scapula off the posterior thoracic wall (i.e. winging of the scapula).²¹ Additionally, the unopposed action of the other functioning scapular muscles may result in medial or lateral translation of the scapula, which is referred to as medial (serratus anterior paralysis) (Figure 7) or lateral (trapezius or rhomboid paralysis) winging of the scapula, respectively.²¹ Initial treatment is typically conservative, with surgery reserved for recalcitrant cases.²¹

Management

A multidisciplinary approach to management of STD is necessary due to the complexity of coexistent injuries, although there is no universally agreed upon treatment algorithm for this rare condition. Emergent surgery or endovascular intervention is typically performed for active arterial hemorrhage or limb-threating ischemia.³ Reduction and stabilization of ipsilateral upper limb fractures as well as acromioclavicular and sternoclavicular joint disruptions may be indicated, although little evidence exists to guide the timing of musculoskeletal interventions.¹¹ Urgent orthopedic intervention may be necessary when there is a progressive neurologic deficit or need for stabilization after a vascular repair. Scapulothoracic arthrodesis is usually not required for treatment of STDs. Surgical management of brachial plexus injuries (which may include nerve grafting, nerve transfer or above-elbow amputation) may be delayed if necessary.³ Above-elbow amputations (without or with glenohumeral arthrodesis) may be performed in some patients with complete preganglionic brachial plexus injuries due to the presence of a nonfunctioning, flail, anesthetic upper limb.^3,11

Conclusion

Scapulothoracic dissociation is a rare and potentially limb- and life-threatening injury. STD may be associated with more distant life-threatening injuries that have the potential to divert the physician’s attention from the scapulothoracic articulation. Therefore, awareness and careful search for scapulothoracic dissociation are warranted especially in the presence of a substantially distracted clavicle fracture or acromioclavicular and sternoclavicular joint disruptions.