Fractures of L4 and L5 (Low Lumbar Fractures).

Peter A Robertson, MD FRACS

INTRODUCTION

Fractures of L4 and L5 differ from those at the thoracolumbar junction. The differences involve anatomy, biomechanics, treatment options and classification. The rarity of these injuries is evident from their limited discussion in the literature. Treatment must be individualised and the recommendations for thoracolumbar trauma management cannot necessarily been transferred to low lumbar fractures.

CLASSIFICATION

The AO classification and nomenclature for thoracolumbar fractures cannot usefully be applied to L4 and L5 fractures. This classification system would exclude some common fracture types and include rare sub groups. The compression and burst fracture (Type A) occurs in the low lumbar spine. Type B fractures (Chance etc) are exceptionally rare (Khare et al 1989). Type C fractures (rotationally unstable fracture dislocations) differ from those seen at the thoracolumbar junction and warrant their own classification system. Any classification system for low lumbar fracture should include process fractures (transverse or spinous), fractures associated with sacral and pelvic trauma (Leone 1997) and fracture dislocations of L5 (also considered as traumatic spondylolistheses) (Aihara 1998).

A useful Classification of Low Lumbar Fractures should include

• Isolated process fractures (spinous or transverse process fractures)
• Type A (compression and burst) fractures.
• Fracture dislocations (traumatic spondylolisthesis). (Aihara 1998)
• Lumbosacral junction injury associated with pelvic fractures. (Leone 1997)
• Mixed injuries.

ANATOMY

The L4 and L5 vertebra and associated discs contribute to 50% of the lumbar lordosis. Compression of the trapezoidal body of L5 can significantly reduce this and alter the biomechanics at L4/5 and L5/S1. A narrow or trefoil spinal canal will expose traversing and exiting nerve roots to trauma and the potential for isolated root injury in burst fractures or fracture dislocation. The seating of the lumbosacral junction within the pelvis, the ilio–lumbar ligaments and the major muscle support groups require high level energy transfer to result in major injury to the low lumbar spine.

The posterior approach to the spine is well know to all surgeons, but the anterior approach to L4 and L5 can be difficult with the great vessels adherent to the bony structures at these levels. While anterior access to the L4/5 and L5/S1 disc is frequently performed, access to the body is more difficult. Anterior stabilising devices that are bulky cannot be used in this region because of the anterior vascular anatomy (Acromed Publications).

BIOMECHANICS

In comparison to the thoracolumbar junction the low lumbar spine is protected by the pelvis and the strong ligamentous and muscular attachment. Injuries in the low lumbar spine involve the transfer of high amounts of energy. Falls, motor vehicle accidents or major crush injuries occur. As noted flexion distraction injuries (AO Type B) are rare.

The anterior weightbearing structures are frequently compromised in such injuries. Type A fractures will result in varying degrees of vertebral body injury. Fracture dislocation with displacement results in significant disc discruption and loss of load bearing capacity. These anterior column defects make management options more difficult. Anterior column deficiency in the acute stage has implications for sagittal plane deformity, failure of posterior instrumentation systems, and altered posterior elements loading with accelerated spinal stenosis. Any coronal plane deformity will also result in asymmetrical facet loading with likely accelerated degenerative change. The sloping superior dome of the sacrum results in translational deformities at the lumbosacral junction.

When instrumentation placement is planned the surgeon should be aware that distal attachment sites of the sacrum are mechanically weak in comparison to pedicle fixation within the proximal lumbar spine. The distal fixation sites may be further exposed to failure within increasing anterior column deficit. The site of low lumbar fracture adjacent to the sacropelvic complex has implications for bracing. Biomechanical evidence demonstrates increased forces transferred through the lumbosacral junction when TLSO braces are used. Bracing to immobilise the lumbosacral junction requires pelvic immobilisation with the inclusion of a single thigh in the cast or brace.

INCIDENCE

These injuries are rare and there is little evidence that any single unit has great experience. One multi–centre review noted 31 burst fractures (L4 and L5 only) collected from three centres over 16 years (Seybold 1995). Other small series often include mixed cases, mixed treatment strategies that have evolved over long time periods, case reports or small numbers of L4 and L5 fractures in other expanded groupings (An 91, An 92, Andreychic 96, Court–Brown 87, Finn 1992, Fredrickson 82, Huang 94, Mick 93, Van Savage 92).

The experience in our own Trauma Unit serving approximately one million people in the Auckland region of New Zealand, over five years, is probably representative. The Trauma Unit audit revealed 7,041 admissions with a total of 824 spinal injuries (351 cervical spine, 218 thoracic spine, 255 lumbar spine). Of the 255 lumbar spine fractures or fracture dislocations, only 63 included the L4 and L5 vertebral levels. This group included 37 process fractures (mainly transverse processes) and of these 21 cases were associated with major pelvic trauma. There were 14 compression fractures, six burst fractures and three fracture dislocations. One pedicle fracture occurred and in two cases the fracture was undefined. Clearly the incidence of L4 and L5 fractures with potential for neurological injury or major biomechanical instability (burst fractures or fracture dislocations) is low, representing only 1.1% of spinal fractures in this series.

TREATMENT OPTIONS

Functional treatment including early active mobilisation seems appropriate for stable compression fractures without significant vertebral body comminution. It would also seem appropriate for isolated process fractures without major pelvic trauma.

For burst type fractures with normal neurology the literature has suggested that conservative care is associated with satisfactory outcome. Conservative care includes bed rest (to allow vertebral body fractures to united without the deforming forces of axial compression) and/or bracing of the low lumbar spine. It seems highly unlikely that bed rest or postural reduction might result in significant vertebral height reconstitution or any improvement in lumbar lordosis after a burst fracture. Bracing should include a TLSO with a thigh extension. It seems probable that early mobilisation in a brace will be associated with further loss of anterior vertebral body height and reduced lordosis. The short term functional outcomes for this form of treatment have been satisfactory. Long term problems include potential for painful degeneration related to disc and endplate injury, and acceleration of degeneration with potential for acquired spinal stenosis

Posterior surgical approaches where there have been fractures with cauda equina damage will allow open reduction of facet fracture dislocations, facetectomy if open reduction cannot be achieved, or decompression where retropulsed burst fragments require impaction away from compressed neural structures. Neurological recovery following compressive injury to the cauda equina and nerve roots is considered to be more favourable than more proximal neurological injury. Decompression is an appropriate therapeutic option for those patients with significant neurological involvement.

Posterior or posterolateral fusion without stabilisation may immobilise the fractured segments once the fusion mass is solid. It is likely that any early mobilisation whilst the fusion is maturing would result in progressive loss of vertebral height and lumbar lordosis after burst fracture.

Internal fixation with older generation implants (Harrington rod systems or segmental sublaminar wire/rod systems) is clearly associated with inferior results and outcomes. The Harrington rod distraction system will further flatten any lumbar lordosis when used to treat a low lumbar fracture. This would be associated with the early development of a junctional syndrome at proximal segments. Segmental fixation with sublaminar wiring usually requires an extra extension of the instrumentation, and long fusions are associated with early development of problems at adjacent unfused levels. Lordosis is also lost if early compressive force were to be applied over the fractured segment despite sublaminar fixation. These systems for stabilisation of low lumbar fractures should only be of historical interest.

Posterior pedicle screw fixation systems require two level stabilisation for single level burst injuries, but single level stabilisation may be adequate for fracture dislocations. Because of the tendency for burst fractures to consolidate, with anterior column height loss, fully constrained rigid systems are required. The choice of pedicle screw implant system requires adequate screw size to resist bending moments, rigid rod attachment to the screws, and adequate rod size to resist bending moments. The patient characteristics to be considered included adequate pedicle and sacral anatomy to take normally positioned screws, and adequate bone density. Surgical factors to be optimised include accurate placement with minimal posterior cortical destruction, 80% pedicle fill without pedicle wall breach so as to optimise mechanical hold of the screw within the pedicle, screw placement to the anterior vertebral body cortex of the lumbar vertebra to maximise hold within the vertebral body, bicortical screw placement at S1, and consideration of both S1 body and alar screws to improve sacral fixation. The surgery should include operative positioning that optimises the lordosis over the segments being instrumented. The patient is best positioned prone with the hips and knees fully extended rather than on a kneeling frame.

Anterior column reconstruction is a more difficult proposition. It is intuitively attractive to consider reconstitution of the vertebral body as is often considered at the thoracolumbar junction after a burst fracture. The absence of satisfactory anterior stabilising devices (ie plate or rod systems that would fit under the great vessels) means that both front and back surgery would be required. The difficulty of anterior access to the L4 and L5 bodies, because of the great vessels, makes this option technically demanding.

In patients with burst fractures with major vertebral body height loss and neurological damage at the low lumbar level, an acceptable alternative includes postural reduction and open posterior decompression and stabilisation with a pedicle screw system and then subsequent supportive care. Posterior transpedicular bone grafting of the vertebral body may also have a role. The supportive care may include bed rest and/or bracing to allow fracture union and this may eventually lower the bending moments applied to the posterior implants and prevent implant failure.

In cases of fracture dislocation at the lumbosacral junction significant translation will result in damage to the disc. This traumatic derangement of the intervertebral disc is probably different to disc height loss with degeneration, and markedly compromises the load bearing capacity of the intervertebral disc. If open reduction and stabilisation from a posterior approach is performed, and disc height is maintained, the bending moments on the implant may result in implant failure. In this situation interbody anterior column structural support should be considered. Options include the use of cage device or a structural bone graft and these can be placed either from an anterior (ALIF) or posterior (PLIF) approach depending on preference.

SUMMARY

Fractures of the low lumbar spine are relatively uncommon and of varied injury pattern. Treatment must be individualised and taken in to account the injury pattern, neurological injury, biomechanical deficiencies and the limitations of surgical implants and anatomical approaches available. Conservative or non operative care has been associated with good outcomes for the neurologically intact patient with a burst fracture. For low lumbar burst fractures, or fracture dislocations of the lumbo–sacral segment, where neurological injury has occurred, posterior surgery is appropriate. This surgery should include decompression, spinal realignment with maintainance of lumbar lordosis, rigid posterior instrumentation over minimal segments, and a period of bed rest and/or bracing to allow bony union and fusion maturation.

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