Injury of the Thoracic Cage and Thoracolumbar Spine

Prevalence, Epidemiology, and Definitions

Injuries of the thoracic cage and the thoracolumbar spine are common—not only in patients with severe traumatic injuries following motor vehicle accidents (MVAs) or, for example, those who fall from great heights, but also in daily clinical practice. Collapsed thoracic and lumbar vertebrae are encountered every day in our routine work, often as incidental findings on chest radiographs or CT scans. These latter fractures are usually confined to the end plates of the vertebral bodies and usually occur in osteoporotic patients.

More extensive injuries to the thoracic cage and spine in patients with normal bone quality are usually not isolated; severe impact is required for these injuries, and these patients often need full work-up to detect other injuries (e.g., to the visceral organs) because they are usually part of a multiple injury syndrome. Sternal fractures almost always occur only after significant force to the anterior chest wall. On the other hand, isolated rib fractures are very common with only minor trauma in (osteoporotic) patients and are associated with less severe injury. In this chapter, the discussion is predominantly restricted to that of acute injuries of the thoracic cage and the thoracolumbar spine.

Anatomy

The thoracic cage consists of the sternum anteriorly and the thoracic spine posteriorly, which serve as anchor points for the arching ribs in between. This cage protects the lungs, heart, and great vessels that are situated in the thoracic cavity and also protects the liver and the spleen situated in the upper abdominal cavity. There are 12 ribs on each side. However, anatomic variants occur, often with dysplastic ribs at T12 or extra ribs at L1. The ribs have an osseous part and a cartilaginous part anteriorly. The thoracic vertebrae articulate with two ribs, the costovertebral joints. Each rib has two synovial articulations with the vertebra: one with the vertebra itself at the level of the facet joint, the other with the transverse process. Multiple ligaments secure a very tight attachment of the ribs to the vertebrae. The cartilaginous parts of the first 10 ribs articulate with the sternum anteriorly, the costosternal joints. Of these, the attachment of the first rib is a synchondrosis, and the attachment of the other ribs are synovial joints. Thus the rib cage not only protects the encased organs but also adds to the stability of the thoracic spine.

The sternum consists of three parts: the manubrium, the body (corpus), and the xiphoid process.

The thoracic spinal column usually has 12 vertebrae, consisting of the vertebral body anterior to the spinal canal, the pedicles, and the transverse processes at both lateral sides and the lamina and spinous processes posterior to the spinal canal. It forms a bony canal, called the spinal canal, containing the spinal cord. The normal shape of the thoracic spine is mildly curved convex to posterior (kyphosis).

The lumbar spine usually has five vertebrae, although anatomic variants at the lumbosacral junction are quite common. The curve of the lumbar spine is concave to posterior (lordosis), with a more pronounced curvature than the thoracic kyphosis. Because of the difference in curves of the thoracic and lumbar spine, the thoracolumbar spine has an S-shaped curve when viewed en face. The form and stability of the spine depends to a very large part on the ligaments of the spine, which are some of the strongest ligaments in the human body. The anterior longitudinal ligament extends along the anterior surfaces of the bodies of the vertebrae. The posterior longitudinal ligament is located within the vertebral canal extending from the posterior surfaces of the bodies of the vertebrae. The contribution of the posterior ligamentary complex to spinal stability has become clear in the past decade. This complex includes the supraspinous ligament, the interspinous ligament, the ligamentum flavum, the facet joint capsules, and the thoracolumbar fascia.

Together with the bones and muscles, these ligament structures support the body, in which the bones act as anchor points. Injury to vertebrae (especially in high-energy trauma situations) is usually accompanied by damage to the other surrounding structures. Whereas bones usually heal remarkably well, destruction of ligamentous structures and joints can cause considerable instability and loss of integrity of the supporting structures, with poor patient outcome.

Between the vertebral bodies are the intervertebral disks; they act as cushions between the bony structures and, together with the facet joints, provide for range of motion of the spine. They consist of a weak core, the nucleus pulposus, surrounded by a tight fibrous structure, the annulus fibrosus. Although the disks are relatively resistant to traumatic injuries, in acute situations the annulus fibrosis can rupture and traumatic herniations of the core can occur, which can give rise to compression of the spinal cord.

Biomechanics

The fracture mechanism and the biomechanics of injury determine the type of damage that occurs to the bones and supporting structures. A direct blow to the sternum anteriorly will displace the sternum posteriorly, potentially fracturing the sternum and/or causing a contusion of the heart. A direct blow to a rib will displace the rib inwardly, potentially fracturing the rib and/or causing a pneumothorax or lung contusion. Hemorrhage from an intercostal artery can occur.

The spine can be damaged by a direct or indirect force. A distinction has to be made between axial loading and other types of injuries—that is, flexion, extension, torsion, or a combination thereof. In axial trauma (e.g., sustained during a parachute or other jump, or in a fall from a ladder or other great heights (e.g., suicide attempts), the forces are transmitted through the entire axial skeleton. Axial loading injury can be associated with spinal fractures at multiple sites (often noncontiguous) as well as lower extremity injuries. Flexion and extension injuries occur if the spine is bent forcefully either forward or backward with or without torsion components. All these mechanisms exert different forces to the spine, leading to specific types of injuries. Some injury patterns indicate mechanisms that should raise our index of suspicion for accompanying pathology; for example, a flexion-distraction mechanism resulting from seat-belt injury is commonly associated with abdominal visceral injury. Similarly, seemingly innocuous transverse process fractures can be associated with visceral injury. Because the human body has the tendency to retain and regain its optimal form and structure, it is not always clear which trauma mechanism was present when the injury occurred. The severity of the trauma cannot always be determined by the damage to the bones themselves, which may have been grossly displaced during the traumatic event but have subsequently spontaneously (nearly) reduced. The bones may look more or less normal and perfectly aligned, but the damage to the soft tissues could be extensive and the cause of acute or subsequent disability. A traumatic spinal cord lesion can result from transient displacement of the vertebrae during impact, with normal alignment at the time of imaging. It is necessary to realize that when injury to the skeletal system is evaluated, there will be additional and accompanying damage to the soft tissues even if one sees only the damage to the bones on radiography or CT. Other patients could be completely paraplegic as a result of a traumatic event that resulted in forceful herniation of the intervertebral disk into the spinal canal with normal-appearing vertebrae.

Treatment of thoracolumbar spine trauma is based on a systematic evaluation of clinical and radiologic information obtained during assessment of a trauma patient. In the literature, numerous classification systems for thoracolumbar injury have been proposed in an attempt to better define thoracolumbar trauma and aid in making treatment decisions. These systems are typically based on either anatomical structures (Holdsworth and Denis three-column system) or on proposed mechanisms of injury (Ferguson and Allen for the Arbeitsgemeinschaft für Osteosynthesefragen [AO] system).

Holdsworth identified two columns in the spine to classify spinal injury: the anterior column and the posterior column. In this concept, osseous as well as ligamentous injury was considered. Denis introduced the concept of middle column or middle osteoligamentous complex between the traditionally recognized posterior ligamentous complex and the anterior longitudinal ligament ( Fig. 5-1 ). Failure of the so-called middle column correlates both with the type of spinal fracture and with its neurologic injury. The Denis system includes four different major types of spinal injury: compression, burst, seat-belt–type injuries, and fracture-dislocations; each fracture is then further classified into 1 to 16 subtypes. When the three-column concept of Denis is used, the general idea is that single-column (usually anterior) injuries are stable injuries, whereas two- or three-column injuries are unstable.

The AO system is based on a progressive scale of increasing morphologic damage and morbidity and consists of three primary fracture types: A (compression), B (distraction), and C (fracture-dislocation), in combination with a total of 27 possible subcategories. However, until recently none of the existing classification systems had gained universal acceptance. Lack of acceptance is caused by lack of validity and reproducibility. In addition, some systems are too simple, lacking sufficient clinically relevant information, whereas others are too complicated, with an impractical number of variables.

Decision making with regard to surgical intervention is frequently dependent on the presence of spinal instability. In 2005, Vaccaro and colleagues proposed a new model to classify thoracolumbar spine injuries: the Thoracolumbar Injury Classification and Severity Score system (TLICS) . This classification system is determined by fracture morphology, the integrity of the posterior ligamentary complex (PLC), and the neurologic status of the patient. The system assigns numeric values to each injury of these three different categories depending on the severity of the injury ( Table 5-1 ) .

Finally, a sum score is calculated by counting the individual scores of the various subcategories. The TLICS cites the integrity of the PLC as one of the primary determinants of the need for surgical intervention. A total score of 5 or above suggests operative treatment of the patient, whereas a sum score of 3 or less suggests nonoperative treatment. Management might be conservative or operative in a patient with a total score of 4 ( Fig. 5-2 ). Previous studies have demonstrated that the TLICS is reproducible among different observers.

The focus on the middle column as in the previous classifications has for the most part been abandoned because the integrity of the PLC is the most important determinant of stability. Because stability of the thoracic spine is partially provided by the thoracic cage, the presence of rib fractures at the levels around the spinal fracture should be included in the consideration of thoracic spinal injury.

Imaging

Rib Fractures

Radiography

Rib fractures are usually detected first on chest radiographs that are acquired routinely in trauma. The chest radiograph is not specifically made to detect rib fractures but rather predominantly to get a rapid overview of the ventilatory system, more specifically the lungs, to detect damage that may influence oxygenation and circulation. The x-ray technique used is also suboptimal to detect rib fractures because it is predominantly made to exclude large abnormalities to the lungs, heart, and mediastinum. In less severe trauma, low-kV techniques (50 to 70 kV, based on the energy absorption characteristics of calcium) are used that yield better sensitivity to detect rib fractures. Even so, that sensitivity remains low; up to 50% can be missed at radiography. Not only are rib fractures often missed on radiography, but they can also be difficult to detect clinically and are almost impossible to distinguish from rib contusions. The circular structure of the thoracic cage and the many locations where rib fractures occur make it unlikely that ribs can be imaged such that the fracture is always perpendicular to the radiographic beam on radiographs, and thus rib fractures will often be missed. Fractures through the cartilaginous part of the ribs are never visible on radiography but can be identified with CT or MRI ( Figs. 5-3 and 5-4 ).

The number and location of rib fractures can be an indication of a significant traumatic event for the patient ( Fig. 5-5 ).

Single rib fractures usually have little clinical consequence, and diagnosing the fracture itself is not essential to management. However, rib fractures can lead to complications, including lacerations of the pleura, lung, or intercostal arteries, causing pneumothorax or hemothorax. A flail chest is present when the bony continuity of the chest wall is disrupted by fractures of two or more ribs in two or more places. This instability of a segment of the thoracic cage leads to paradoxical movement during respiration, which can cause respiratory insufficiency and can lead to atelectasis and diminished oxygenation. Furthermore, almost invariably there is significant injury to the underlying lung. Instability of the chest wall can also be caused by a combination of rib fractures or costochondral fractures and a sternal fracture.

Rib fractures of the first three ribs can give rise to an extrapleural hematoma that is located at the apex of the lung and is called an apical/pleural cap. It can, however, also be the result of injury to the subclavian artery or extrapleural extension of a mediastinal hematoma. Therefore, it is an indication to perform a CT angiogram of the thorax to exclude traumatic vascular injury.

Multidetector Computed Tomography

In the setting of multiple trauma, MDCT is the standard of care in the workup of severely injured patients. Because of the low sensitivity of radiography for rib fractures many more fractures are usually seen on CT scans. Suspicion of a flail chest is an indication for a CT scan.

Ultrasonography

Ultrasonography is excellent for detection of rib fractures but is not commonly used in clinical practice.

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