Subaxial Cervical Injuries: Current Concepts in Classification and Treatment

Alpesh A. Patel

Paul A. Anderson

Alexander R. Vaccaro

Subaxial cervical spine injuries account for more than 66% of cervical fractures and 75% of cervical dislocations (1). The implications, most notably spinal cord injury, are so severe that radiographic and clinical evaluation of the cervical spine is routinely performed on all patients presenting with traumatic injuries. Additionally, there has been a proliferation of surgical techniques and spinal instrumentation specifically to address these injuries.

In light of these implications and despite a vast amount of clinical experience, the diagnosis, classification, and treatment of subaxial cervical trauma remain exceedingly inconsistent (2). This is reflected in the large number of classification systems and treatment options that have been reported. Instead of improving our understanding of cervical trauma, this has had the opposite effect. The same injury can be categorized by multiple descriptors, assigned to a number of classification systems, and treated in vastly different ways depending upon geographic location, institutional preferences, and on-call schedules.

The purpose of this chapter is to briefly review the relevant anatomy of the subaxial spine and previously reported classification systems and to update current concepts in classification and treatment of subaxial spine injuries.

ANATOMY

The anatomy of the cervical spine allows for greater motion than the thoracic and lumbar spine. This greater degree of freedom also makes it more vulnerable to injuries affecting both the osseous and ligamentous structures. The osseous and ligamentous anatomy of the cervical spine has been described as belonging to two, three, or four columns (3, 4, 5 and 6). While each of these descriptors defines the relevant anatomy, the four-column concept, described by Anderson et al. (6) may more accurately account for the various injury patterns that can be seen. The anterior column consists of the vertebral body, the intervertebral disk, the uncinate processes, the transverse processes, and the supporting ligaments (anterior longitudinal ligament [ALL] and posterior longitudinal ligament [PLL]). The left and right lateral columns include the pedicles, the lateral masses, the facet joints, and the facet capsules. The posterior column includes the lamina, spinous process, and the dorsal ligaments (ligamentum flavum, interspinous ligament, and the ligamentum nuchae).

Although a column system has also been applied to the thoracic and lumbar spine, there are a number of unique anatomic characteristics of the cervical spine, including the lateral masses and facet joints (7). The lateral masses are located between the inferior and superior facets along the dorsolateral surface of the vertebral body. The subaxial cervical facets are oriented 45 degrees in the sagittal plane and play a central role as a load-bearing structure in the cervical spine. The facet orientation allows for a large range of motion in flexion, extension, and lateral bending. The bony articulation and the capsule of the facets act as the major structures resisting forward subluxation with the dorsal ligaments and muscles functioning as secondary restraints.

The other bony and ligamentous structures of the cervical spine have been shown to also resist hypermobility in different planes (8). The ALL, ventral anulus, and facets resist extension, while the dorsal bony and ligamentous structures resist flexion. Compression is resisted by the vertebral bodies, intervertebral disk, and facet joints while tension (distraction) is resisted by the ALL, anulus, PLL, and dorsal ligamentous structures. Lateral displacement is limited by the uncinate processes, which are present only in the subaxial cervical spine. These anatomic restraints, and their modes of failure, lead to the development of a number of classification systems.

TRADITIONAL CLASSIFICATION SYSTEMS

Traditionally, classification systems for cervical trauma have been based upon mechanistic criteria and/or descriptive
radiographic findings. The mechanistic systems combine radiographic findings with biomechanical information on modes of failure to produce a presumed mechanism of injury. The subjective nature of these systems, lacking any standardization, has lead to significant variability—what may be a tear-drop fracture to some can be a fracture dislocation, a compression-flexion injury, or even a facet dislocation to others. Nonetheless, these systems have added to our understanding of subaxial cervical spine trauma and lay the foundation for recently described classification systems.

HOLDSWORTH

Sir Frank Holdsworth (4) is generally credited with providing the first comprehensive classification system for spinal column injuries based on his experience with over 2,000 patients. His paper, published a year after his death, was one of the first attempts to classify spinal trauma according to the mechanism of injury. He identified categories of simple wedge fracture, dislocation, rotational fracture dislocation, extension injury, burst injury, and shear fracture. Although he did not discriminate between cervical, thoracic, or lumbar injuries, he was the first to identify the importance of the dorsal ligamentous structures in spinal stability.

ALLEN AND FERGUSON

Subsequently two other classification systems evolved specific to the subaxial cervical spine. In 1982, Allen et al. (9) proposed a mechanistic classification system of subaxial cervical spine injuries based on their experience with 165 patients. Mechanism of injury was inferred from the recoil position of the spine assessed on plain radiographs. Six categories were defined: compressive flexion, vertical compression, distractive flexion, compressive extension, distractive extension, and lateral flexion. Increasing numerical values were assigned to each category to account for progressive degrees of spinal instability. The Allen and Ferguson system remains widely utilized today and created terminology that is incorporated into other classification systems.

Four years later, Harris et al. (10) proposed modifications to the Allen and Ferguson system, including rotational vectors in flexion and extension at the expense of the distractive forces detailed in the Allen and Ferguson scheme. Here too, six mechanisms were identified: flexion, flexion and rotation, hyperextension and rotation, vertical compression, extension, and lateral flexion. Both the Harris and the Allen-Ferguson systems used radiographic descriptions (fractures, dislocations, alignment) to categorize an injury pattern based upon a presumed mechanism of action. Neither system can account for neurologic status of the patient, nor does either provide prognostic (degree of injury severity) or treatment criteria.

WHITE AND PANJABI

White and Panjabi, through mechanical in vitro studies, have defined cervical instability as characterized by a 3.5-mm horizontal displacement of one vertebra in relation to an adjacent vertebra or greater than 11 degrees of angulation difference between adjacent vertebra on a lateral x-ray (8). The authors used these biomechanical findings along with clinical data to create a point-based classification system for the middle and lower cervical spine (Table 48.1) (11). The system has not gained widespread utility—the descriptors are either too subjective (ventral and dorsal element “destruction,” dangerous anticipated loads) or are impractical to obtain clinically. The stretch test, for example, involves traction applied through cranial tongs with sequential radiographic evaluation; a positive test is defined as distraction of greater than 1.7 mm or a change in alignment of 7.5 degrees (11).

Though not routinely clinically applicable, the White-Panjabi system is the first classification to acknowledge the importance of neurologic status and account for differences between cord and nerve root level injuries. The system is also the first to move away from purely mechanistic criteria. For these reasons, it of great historical significance.

AO CLASSIFICATION

The AO classification is a comprehensive system developed for thoracolumbar spine injuries; however, it is often applied to cervical spine injuries (12). This system was developed after reviewing 1,445 thoracolumbar injuries and assessing for the level of the injury, frequency of fracture types/groups, and incidence of neurologic deficits. Categories in this system are based on the mechanism of injury and morphology. The system uses the traditional AO 3-3-3 scheme for grouping injury patterns: three types, A, B, and C, with corresponding subgroups (Table 48.2). The classification takes into account injury severity/stability. Group A (compressive injuries of the vertebral body), group B (distractive disruption of either ventral or dorsal elements), and group C (axial rotation/torque or translational injuries).

TABLE 48.1 White-Panjabi Classification System— Diagnosis of Clinical Instability in the Middle and Lower Cervical Spine

Element			Point Value
Anterior elements destroyed or unable to function			2
Dorsal elements destroyed or unable to function			2
Positive stretch test			2
Radiographic criteria
	A. Flexion-extension radiographs
		1. Sagittal plane translation >3.5 mm or 20°	2
		2. Sagittal plane rotation >20°	2
	Or B. Resting radiographs
		1. Sagittal plane displacement >3.5 mm	2
		2. Relative sagittal plane angulation >11°	2
Developmental narrow spinal canal
	1. Sagittal diameter <13 mm
	2. Pavlov ratio <0.8
Abnormal disk narrowing			1
Spinal cord damage			2
Nerve root damage			1
Dangerous anticipated loading			1
Total score 5 or more points = unstable

TABLE 48.2 AO Classification

A. Compression Injury
	A1: Impaction Fracture
		A1.1 End Plate Impaction
		A1.2 Wedge Impaction
		A1.3 Vertebral Body Collapse
	A2: Split Fracture
		A2.1 Sagittal Split Fracture
		A2.2 Coronal Split Fracture
		A2.3 Pincer Fracture
	A3: Burst Fracture
		A3.1 Incomplete Burst Fracture
		A3.2 Burst-Split Fracture
		A3.3 Complete Burst Fracture
B. Distraction Injury
	B1: Dorsal ligamentous lesion
		B1.1 With Disk Rupture
		B1.2 With Type A Fracture
B2: Dorsal osseous lesion
		B2.1 Transverse Bicolumn
		B2.2 With Disk Rupture
		B2.3 With Type A Fracture
	B3: Ventral disk rupture
		B3.1 With Subluxation
		B3.2 With Spondylolysis
		B3.3 With Dorsal Dislocation
C. Rotation Injury
	C1: Type A with rotation
		C1.1 Rotational Wedge Fracture
		C1.2 Rotational Split Fracture
		C1.3 Rotational Burst Fracture
	C2: Type B with rotation
		C2.1 B1 Lesion with Rotation
		C2.2 B2 Lesion with Rotation
		C2.3 B3 Lesion with Rotation
	C3: Rotational shear
		C3.1 Slice Fracture
		C3.2 Oblique Fracture