Thoracolumbar Fracture Fixation in the Osteoporotic Patient

Theodore J. Choma

Introduction

With the continuing aging of the population,1 spine surgeons must consider the unique issues involved with caring for older trauma patients. It has been suggested that an elderly patient who sustains thoracolumbar trauma may be more susceptible to spinal cord injury (SCI) and multiple fracture injuries than is a younger patient.² These elderly thoracolumbar injury patients are certainly more likely to have osteoporosis as well,³ and this needs to be taken into account. This chapter discusses the challenges of thoracolumbar fracture fixation in the osteoporotic patient and considers some technical options to address these challenges.

Mechanism of Failure

Thoracolumbar spinal fixation tends to have higher rates of fixation failure in the elderly, largely due to the higher rates of osteoporosis in this age group.^4–6 Pedicle screw fixation has gained wide favor in treating thoracolumbar fractures, as it can often convey immediate fixation, and it is relatively easy to connect longitudinal rods and maintain fracture reductions. When fixation failure occurs, the mechanism most commonly occurs at the bone–pedicle screw interface. The weaker bone trabeculae begin to microfracture until the screws become loose and either pull out or cause catastrophic fracture of the host vertebra. Routine administration of teriparatide (the best option) or a bisphosphonate (the next best) decreases the risk of subsequent screw loosening in postmenopausal women.⁷

Traditional Approaches

Experienced spine surgeons have recognized the potential for fixation failure in the osteoporotic patient and have in the past simply recommended cast treatment or bed rest for unstable thoracolumbar fractures in the osteoporotic patient. These treatment options carry nontrivial risks of morbidity, such as pneumonia, thromboembolic phenomena, and pressure ulcers. Surgeons who have chosen operative internal fixation have traditionally used additional points of fixation in an attempt to mitigate the risk of catastrophic failure. Further, they have emphasized factors such as screw length, screw diameter, bicortical screw fixation, and optimal screw placement (Fig. 10.1). Others have reported that the addition of laminar hooks to pedicle screws can significantly enhance fixation in the osteoporotic spine.⁸ This technique has not been widely used, however, due to the technical difficulty in connecting both to the rod at the same level. In addition, surgeons have been concerned that application of supralaminar hooks at the top of constructs would potentially destabilize the transition zone from fusion segment to mobile spine.

Fig. 10.1a–e A 68-year-old woman with a history of rheumatoid arthritis and osteoporotic L2 compression fracture presented with continued collapse despite attempted kyphoplasty. (a,b) Radiographs demonstrated degenerative scoliosis as well as spondylolisthesis. (c) Magnetic resonance imaging demonstrated multiple areas of stenosis requiring decompression. The patient underwent staged L1–L5 posterior decompression, L2 subtotal vertebrectomy, and T10–S1 posterior instrumented fusion, utilizing calcium phosphate cement to augment solid-core pedicle screws. (d,e) These radiographs demonstrate well-maintained fixation and improved alignment 4 years after surgery.

Expandable screws are a technology that have held some promise in pedicle screw fixation of the osteoporotic spine for over a decade.9 It has taken some time to see additional clinical reports of this technology, but recently we have seen clinical reports confirming that expandable pedicle screws can reduce the risk of loosening in osteoporotic patients.¹⁰

Theoretical Options

There has been some interest in screw material processes that might enhance pedicle fixation in osteoporotic vertebrae. Fini et al¹¹ found that hydroxyapatite (HA) coating of titanium pedicle screws enhanced the bone–screw interface in osteopenic sheep. Although HA coating of screws has some promise, one drawback is that it takes substantial time from implantation for the bone to respond and the effect to be manifest. This has been seen as a drawback to widespread adoption of HA coating of screws in clinical practice.

There has also been a theoretical interest in varying rod material for fixation in the osteoporotic spine. Theoretically, the use of a less rigid rod material could enable some strain to be absorbed by the rods in the instrumented segment, affording some protection to the screw–bone interface in the osteoporotic spine. However, there has been a dearth of studies addressing this issue. In one report, it was suggested that an all-polyetheretherketone (PEEK) construct might sacrifice too much rotational stability in a corpectomy model.¹² This will likely be an area for future investigation.

Screw Cement Augmentation

Cement augmentation of pedicle screws is a technique that has received attention from several investigators recently. The basic concept is that adding a cement mantle around a pedicle screw will distribute stresses such that the adjacent trabeculae are less likely to fail (initiating the loosening or pullout process). This effect has been most thoroughly demonstrated with the use of polymethylmethacrylate (PMMA).¹³ This substantial enhancement (two to five times increased pullout strength in osteoporotic vertebrae) has been replicated in many studies and seems to be shared if one uses other bioactive cements based on calcium sulfate or calcium phosphate.¹⁴

As experience with the cement augmentation of pedicle screws has grown, the techniques for doing this have evolved into two versions. In one version, a pilot hole is prepared, and then cement is injected into the pilot hole, and a solid-core traditional screw is placed into the pedicle (Fig. 10.2). In the second method, a cannulated screw is placed into the pedicle over a guidewire. Then cement is injected through the screw, extravasating out through fenestrations on the sides to create a cement mantle (Fig. 10.3). It appears that both methods effectively increase holding strength in osteoporotic bone,¹⁵ although it may be safer to inject cement through cannulated screws to mitigate the risk of inadvertent cement extravasation into the spinal canal. Because injecting cement through cannulated pedicle screws is not currently approved by the United States Food and Drug Administration, American surgeons are restricted to the first method described (Fig. 10.1).

There are other technical aspects to placing pedicle screws that may enhance fixation in osteoporotic vertebrae. In one investigation, angling the screw tips toward the superior end plate, augmenting with cement, and pre-tapping the screw holes prior to cement injection increased the fixation effect.¹⁶ In addition, concentrating cement around the tips of the screws seems to maximize this enhancement effect.¹⁷

There are some theoretical risks when employing cement augmentation of pedicle screws. They include cement extravasation into the venous system with risk of embolism,¹⁸ extravasation into the spinal canal with risk of neurologic injury, and extravasation into an adjacent disk.¹⁹ There have been no large clinical trials of screw augmentation with cement to adequately characterize the overall risk of this technique, but in my experience this risk profile is manageable.