The mechanisms of spinal injury PDF

Preview

Summary

Current Anaesthesia & Critical Care (2002) 13, 97^102 c 2002 Published by Elsevier Science Ltd. doi:10.1054/cacc.2001.0372, available online at http://www.idealibrary.com on MEDICINE The mechanisms of spinal injury M. Mackintosh and S.Tucker Department of Spinal Surgery, The Royal National Ortho...

Description

Current Anaesthesia & Critical Care (2002) 13, 97^102

c 2002 Published by Elsevier Science Ltd. doi:10.1054/cacc.2001.0372, available online at http://www.idealibrary.com on

MEDICINE

The mechanisms of spinal injury M. Mackintosh and S.Tucker Department of Spinal Surgery, The Royal National Orthopaedic Hospital, Brockley Hill, Stanmore, Middlesex, HA7 4PL, UK

KEYWORDS spinal injuries, fractures, spinal, spine, classi¢cation, anatomy

Summary Appreciation of the mechanism of injury is essential for the complete understanding of spinal trauma. This article details the relevant anatomy and speci¢c injuries in all areas of the spine. The emphasis is on aetiology. The aim is to provide an understanding of how speci¢c injuries occur, and the relationship of neurological c 2002 Published by Elsevier Science Ltd. function to those injuries.

INTRODUCTION

second cervical vertebrae play in neck and head movements. The atlas (C1) (Fig. 2A) has no body; instead it consists of an anterior and a posterior arch, joined by two lateral masses.The axis (C2) (Fig. 2B), has a large vertical projection called the dens or odontoid process (this embryologically represents the body of C1). The transverse ligament divides the large vertebral foramen of the atlas into two parts. The large posterior foramen contains the spinal cord and the dens of the axis ¢ts into the smaller anterior foramen, providing the C1^ C2 complex with stability. The vertebral bodies are bound together by the strong anterior longitudinal ligament and the weaker posterior longitudinal ligament. The ligamentum £avum connects adjacent laminae.Between each vertebral body is a ¢bro cartilaginous disc, consisting of the tough annulus ¢brosis and a soft pulpy central nucleus pulposus. Traditionally, the spine was considered to consist of two columns, anterior and posterior. Denis, in1983, 1proposed a three-column model of the spine (Fig. 3) which is useful when assessing the stability of a spinal injury. In this model, the anterior column consists of the anterior longitudinal ligament, and the anterior two-thirds of the vertebral body and the intervertebral disc. The middle column consists of the posterior third of the vertebral body and the intervertebral disc, and the posterior longitudinal ligament.The posterior column includes all spinal elements posterior to the posterior longitudinal ligament. In general terms, an injury involving two or more columns is considered unstable. The spinal cord can sustain injury by both primary and secondary mechanisms. The primary mechanism relates to the initial injury with resultant fracture or dislocation of the vertebral column. This can be caused by high or low velocity injuries. The secondary mechanisms of spinal cord injury are by oedema, in£ammation and ischaemia.

There is a fundamental relationship between the mechanism of spinal injury and particular injury patterns. Insight into the mechanism of an injury also provides information on the stability of an injured spine, and is essential for the formation of a rational and consistent treatment plan.

ANATOMY There are 7 cervical,12 thoracic and 5 lumbar vertebrae. The sacrum and coccyx consist of 5 and 4 fused segments, respectively. The normal spine is straight when viewed in the coronal plane. In the sagittal plane the spine presents a cervical lordosis, thoracic kyphosis and lumbosacral lordosis. In general terms, each vertebra has the same basic pattern, consisting of a body, whose function is to support weight, and a posterior vertebral arch, which surrounds the spinal cord (or cauda equina) (Fig. 1). The vertebral arch consists of two pedicles and two laminae.The pedicles are notched superiorly and inferiorly, and each adjacent pair contributes the upper and lower margins of an intervertebral foramen, through which a spinal nerve passes. Each vertebra has two transverse processes, and one spinous process, to which muscles attach.There are also four articular processes (2 superior and 2 inferior), which bear the articular facets. There are certain additional features to the vertebrae, which are particular to the various regions of the spine. The proximal cervical vertebrae are particularly specialized, which re£ects the unique role that the ¢rst and Correspondence to: ST. 0953-7112/02/$ - see front matter

98

CURRENT ANAESTHESIA & CRITICAL CARE

Figure 2 (A) The atlas (C1). (B) The axis (C2).

Figure 1 A typical vertebra: (A) superior view; (B) lateral view.

THORACO-LUMBAR INJURIES The lumbar spine is a mobile segment of the spine, situated between the relatively sti¡ thoracic spine and the sacrum. The thoraco-lumbar junction is therefore very susceptible to injury, and accounts for 60% of all spinal column injuries. Minor injuries, such as fractures of the transverse and spinous processes will not be discussed, other than to emphasize that their presence should always raise the index of suspicion for a more signi¢cant spinal injury, than the radiographs might suggest. Major injuries may be divided into compression fractures, burst fractures, seatbelt type injuries and fracture dislocations.

Compression fractures The mechanism of injury in these common fractures is axial load, in combination with a £exion or lateral bending movement. In the healthy young patient, a signi¢cant force is necessary to produce such a fracture whereas in the elderly patient only minor trauma may be required. The anterior column is compressed and the middle column remains intact. Compression fractures are inherently stable.

Figure 3 The three-column model ofthe spine redrawn after Denis,1983.1

Such fractures are usually treated conservatively, with or without a hyperextension cast/brace. If there is greater than 50% loss of height of the vertebral body, and or greater than 301 of kyphosis, a posterior osteosynthesis can be performed, whereby the vertebral body height is restored by inserting pedicle screws into the vertebra above and below the fractured level, and lordosing the two connecting rods. The vertebral body height is restored indirectly by traction through the intact disc

THE MECHANISMS OF SPINAL INJURY

spaces. Such fractures are not accompanied by neurological injury.

Burst fractures The burst fracture results from failure of a vertebra under axial load. Both the anterior and middle columns fail. There may be ligamentous or bony injury of the posterior column. Concurrent trauma is an associated feature, as they usually result from high energy forces. Radiologically, these fractures display loss of posterior vertebral body height, retropulsion of the posterior vertebral wall (middle column) and an increase of the interpedicular distance. It should be noted that the displacement at the time of injury may be up to 90% greater compared to that seen on CTscan. Neurological injury results from direct impaction of the retropulsed vertebral body into the spinal canal and can include both complete and incomplete injuries. Neurological deterioration can occur by secondary mechanisms. The presence of a palpable step, tenderness over the spinous processes or an increased interspinous distance on antero-posterior radiographs implies posterior column involvement and gross instability. Our institution treats burst fractures aggressively, with anterior spinal canal decompression via corpectomy and reconstruction using a meshed titanium cage ¢lled with bone graft.

99

These injuries are extremely unstable and are typically associated with profound neurological de¢cit. Occasionally, an injury may spontaneously reduce and therefore be di⁄cult to recognize on initial radiographs (although a magnetic resonance scan would be grossly abnormal). Associated features, such as transverse and spinous process fractures, multiple rib fractures, unilateral articular process fractures, laminar fractures, and sternal fractures must raise the index of suspicion.

CERVICAL INJURIES On normal lateral radiological examination of the cervical spine, there should be ¢ve smooth convex lines, which assess the alignment of the vertebrae and are important in the evaluation of cervical spine trauma (Fig. 4). Soft tissue swelling must also be assessed. There should be less than a vertebral body’s width of prevertebral soft

Seat-belt type injuries This group of fractures has many di¡erent names in the literature including £exion-distraction and Chance-type fractures.They are often sustained in a motor vehicle accident, when a person is restrained only by a lap belt. Clinical features include a tender palpable gap over the spinous processes and associated abdominal injury. The mechanism of seat-belt-type injuries is a £exion-distractive force, with failure of the posterior and middle columns.The anterior part of the anterior column may fail, but retains its role as a fulcrum. The Chance fracture is a speci¢c injury within this group. It is a transverse injury with its fulcrum anterior to the spinal column, and the fracture line passes through the bony structures. With an atypical Chance injury, the fracture line passes through the intervertebral disc and ligamentous structures. These fractures can easily be missed as radiological changes can be subtle.

Fracture-dislocations The key feature of fracture-dislocations is the failure of all three columns, with the translation of one part of the spinal column relative to another. The mechanism of injury may be compression, tension, rotation or shear.

Figure 4 Lateral view of a normal cervical spine. There are ¢ve smooth convex lines which are important in the evaluation of cervical spine trauma.1.Tips ofthe spinous processes; 2. spinolaminar line; 3. posterior facet joints; 4. posterior vertebral body; 5. anterior vertebral body. There should be less than 6 mm of prevertebral shadow anterior to C2 and less than 2 cm anterior to C6 as shown.

100

tissue shadow anterior to C5 and less than half the vertebral body’s width of soft tissue shadow below this level. It is essential that the C7/T1 level be clearly identi¢ed on the lateral view.Failure to achieve adequate imaging of this level results in a signi¢cant, and unacceptable, incidence of missed cervico-dorsal dislocation in trauma patients. If despite the use of a swimmer’s view or shoulder traction, this level has not been clearly visualized, a CTor MRI scan should be performed.

C1^C2 injuries Injuries in the C1^ C2 region have unique injury patterns, which sets them apart from the rest of the cervical spine fractures. In patients who survive, there is a relatively low incidence of neurological injury, because the spinal cord proportionally occupies a smaller proportion of the canal at this level. Steele’s Rule of Thirds2 states that one-third of the canal is occupied by the odontoid, one-third by the spinal cord, and the remainder provides space for the cord during movements of the head and neck. The mechanism of injury also di¡ers from that in the rest of the cervical spine, as the injurious force is usually applied via the base of the skull onto the adjacent vertebra, and the position of the skull at the point of injury determines the injury pattern. This also results in a high incidence of multiple injuries within the C1^ C2 complex.

CURRENT ANAESTHESIA & CRITICAL CARE

lateral views (4.5 mm in children 70.5 mm on £exion/extension). Atlanto-axial subluxation of up to 9 mm, in neutral rotation, usually results in no cord compression. However, any associated rotation will result in spinal cord compromise with atlanto-axial subluxation of much less than 9 mm.3 Posterior subluxation, with or without fracture, is rare. Rotary subluxation occurs secondary to excessive rotary force upon the head. The patient presents with the head tilted to one side and rotated to the other (Cock^Robin position).

C2-Hangman’s fracture Hangman’s fracture (Fig. 5) is a traumatic spondylolisthesis of C2 on C3, with bilateral fractures of the pars interarticularis (the area between the superior and inferior facets). It is most frequently caused by sudden extension forces as in judicial hanging, and in sudden deceleration motor vehicle accidents where the forehead strikes the dashboard or the steering wheel. Neurological injury is not common, as the diameter of the spinal canal is actually increased with this type of injury.

Atlas fractures (C1) These fractures are caused by an axial load, and depending on the position of the head on impact, there are three di¡erent patterns of injury, involving either the anterior arch, the posterior arch, or both. Je¡erson fractures consist of both anterior and posterior fractures of the C1 bony ring, and/or lateral mass fractures. Such a fracture is caused by an axial load alone. Neurological injury is uncommon, as the spinal canal is not compromised. Such fractures are usually treated conservatively, with immobilization in a halo-vest. Open reduction and internal ¢xation is reserved for severely displaced fractures.

Atlanto-axial subluxation/dislocation Anterior C1^ C2 subluxation occurs due to a rupture of the transverse ligament, either alone or in association with a fracture of the atlas or the dens e.g. secondary to a fall and subsequent impact upon the occiput. Radiological signs include retropharangeal swelling and an increase in the atlanto-dens interval on lateral radiographs. The atlanto-dens interval is measured from the posterior cortex of the anterior arch of C1 to the anterior border of the odontoid process. It should be less than 3 mm in adults, with no change on £exion/extension

Figure 5 Hangman’s fractureFa traumatic spondylolisthesis of C2 on C3 with a fracture ofthe parsinterarticularis of C2 and disruption of the C2.C3 disc.

THE MECHANISMS OF SPINAL INJURY

Odontoid fractures (C2) Flexion is the most common mechanism of injury. These fractures may be identi¢ed on the open-mouth (odontoid) view and on lateral radiographs. An increased retropharayngeal soft tissue shadow may often be the only abnormal radiological sign. Anderson and D’Alonzo4 classi¢ed dens fractures into three types. Type I fractures are of the tip of the dens; Type II occur at the junction of the dens and body of C2; Type III fractures extend into the body of C2.Type I fractures are of little clinical signi¢cance. Type III fractures generally heal well with immobolization of the head in a halo-vest. Type II fractures have a high incidence of nonunion when treated conservatively.This is explained embryologically. Because the dens is embryologically the body of C2, such fractures correspond to a watershed in the vascular supply of this area. Thus type II fractures are frequently treated by screw ¢xation, either primarily or following the diagnosis of non-union (failed conservative treatment).

Atlanto-occipital dislocation A rare and usually fatal injury. The mechanism is distraction, extension and/or £exion.

101

body wall involvement.There is typically retropulsion of the middle column into the spinal canal, and therefore neurological injury is common. These fractures arise from vertical axial load, or compression ^£exion forces, and the classical mechanism of injury is a diving accident. Tear drop fractures are considered by some to be a variation of the burst fracture, although they di¡er in their mechanism, which is primarily £exion. Tear drop fractures are also associated with more severe bony destruction and neurological injury.

Facet subluxations/dislocations These ligamentous, hyper£exion injuries present a spectrum from subluxation to dislocation of the facet joints. The incidence of neurological injury increases through the spectrum. Unilateral facet dislocation has a signi¢cant rotatory component and this usually results in anterior translation of the upper vertebra on the lower vertebra of approximately 25% of the AP diameter of the vertebra, as seen on lateral radiograph.There is frequently associated disc injury, and it essential to know the position of the disc before reduction is attempted. Reduction in the pre-

Subaxial cervical spine (C3 ^ C7) Most lower cervical spine injuries result from forces applied to the head or the trunk, which lead indirectly to an increase in axial load on the cervical spine. Superimposed on this there may also be £exion, extension or rotation. Allen et al.5 devised a mechanistic classi¢cation of lower cervical spine injuries. This divides fractures into various stages of compressive £exion, vertical compression, distractive £exion, compressive extension, distractive extension and lateral £exion injuries. We will discuss subaxial cervical spine injuries under three broad categories: compression fractures, burst fractures and facet joint subluxation/dislocation.

Compression fractures Due to £exion forces, these injuries occur most often at the C4 ^ C5 and C5^ C6 levels and neurological injury is rare. A compression injury without posterior element fracture or posterior ligament rupture is a stable injury. Radiologically, if there is more than 50% compression of the vertebral body the injury is considered unstable.

Burst fractures These are comminuted body fractures, most commonly of C5^ C6, which by de¢nition have posterior vertebral

Figure 6 Bilateral facet joint dislocation with anterior subluxation of C5 on C6.

102

CURRENT ANAESTHESIA & CRITICAL CARE

sence of a traumatic disc herniation into the spinal canal can result in paralysis. MRI scanning is therefore advisable before any reduction procedure is attempted. With bilateral dislocation of the facet joints, there is anterior displacement of the dislocated vertebra of at least 50%. Bilateral facet dislocations are unstable, even after reduction, and require stabilization (Fig. 6).

REFERENCES 1. 2. 3.

4.

Denis F.The three column spine and its sign¢cance in the classi¢cation of acute thoracolumbar spinal injuries. Spine 1983; 8; 817^ 831. Steel H H. Anatomical and mechanical considerations of the atlanto-axial articulations. J Bone Joint Surg 1968; 50 -A; 1481^ 1482. Tucker S. K, Taylor B A. Spinal canal capacity in simulated displacements of the atlantoaxial segment. J Bone Joint Surg [Br] 1998; 80 -B; 1073^ 1078. Anderson L D, D’Alonzo R T. Fractures of the odontoid process of the axis. J Bone Joint Surg [Am] 1974; 56; 1663^ 1674.

5.

Allen B L, Ferguson R L, LehmanT R, O’Brien R P. A mechanistic classi¢cation of closed, indirect fractures and dislocations of the lower cervical spine. Spine 1982; 7; 1^ 27.

FURTHER READING Aebi M,Thalgott J S, Webb J K. Ao ASIF Principles in Spine Surgery. Berlin: Springer-Verlag,1998. Clark C R et al. The Cervical Spine/The Cervical Spine Research Society. Editorial 3rd edn.USA: Lippincott-Raven,1998. Cristcitiello A A, Fredrickson B E. Thoracolumbar spine fractures. Orthopaedics 1997; 20; 939^ 944. Dee R, Hurst L C, Gruber M A, Kottmeier SA. Principles of Orthopaedic Practice, 2nd edn. USA: McGraw-Hill,1997. Ferguson R L, Allen J R. A mechanistic classi¢cation of thoracolumbar spine fractures. Clin Orthop Rel Res1984; 189; 77^ 88. Levine A M, Edwards C C.Treatment of injuries in the C1^ C2 complex. Orthop Clin North Am 1986; 17; 31^ 44. Miller M D, Brimker M R. Review of Orthopaedics. 3rd edn. USA: WB Saunders, 2000. Rockwood Jr C A, Green D P. Fractures in Adults, 2nd edn. USA.: JB Lippincott,1984....