Pedicle Screw Fixation in the Aging Spine

Published on 11/04/2015 by admin

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57 Pedicle Screw Fixation in the Aging Spine

Hajeer Sabet, Frank M. Phillips

KEY POINTS

• When performing reconstructive spinal surgery in the elderly patient, the surgeon must consider the fragility of osteoporotic bone, the stability of the spine, and the potential failure mechanisms of any applied instrumentation.

• Increasing pedicle screw length, diameter, or both can be the first line in improving pedicle screw construct rigidity.

• Undertapping pedicle screws can achieve increased insertional torque and screw pullout strength.

• Triangulation of screws increases the overall pullout strength of the construct and provides higher resistance against loads perpendicular to the pedicle screws.

• A twofold to threefold increase in screw pullout can be achieved with the use of polymethylmethacrylate injected into the vertebral body around the screws.

Introduction

The number of people with osteoporosis is expected to rise with the increasing longevity of the population, so spine surgeons must appreciate the impact of osteoporosis on the management of spinal disorders in the elderly. Older patients desire to remain active and are reluctant to accept disability and deformity as an inevitable consequence of aging. These patient expectations coupled with advances in spinal surgical techniques have resulted in more spinal procedures being performed on the elderly. The spinal surgeon may be required to treat direct sequelae of osteoporosis in the form of painful spinal fractures or resultant deformity, or may be required to consider osteoporosis as it relates to spinal reconstruction in the older patient. Regardless of any surgical decisions in the osteoporotic patient, the spine surgeon must ensure that the patient is being appropriately medically treated for osteoporosis.

As larger reconstructive spine surgeries are performed on older patients, the ability of the osteoporotic spine to support spinal implants must be considered. The selection of spinal instrumentation must take into account the fragility of osteoporotic bone, the stability of the spine, and the likely failure mechanisms of any applied instrumentation. The preoperative workup should include evaluation for the severity of osteoporosis, which might impact the surgeon’s choice of reconstruction techniques.

Posterior instrumentation is most commonly applied to the osteoporotic spine in an effort to stabilize the spine and promote fusion after decompression of neural elements. In this situation, the anterior column is typically intact and no frank instability exists, so that posterior instrumentation alone is often adequate. Surgery primarily for deformity correction in the elderly is challenging and infrequently indicated. Posterior instrumentation may be used to correct spinal deformity; however, if the deforming forces exceed the stability of the implant–bone interface, posterior construct failure will occur.

In current clinical practice, the large majority of posterior instrumentation spinal surgeries involve pedicle screw instrumentation. In the osteoporotic spine, the weak link in the instrumentation construct is the implant–bone interface. The majority of instrumentation failures involve screw loosening and pull-out, which may lead to failure of fusion or the development of recurrent or de novo deformity. Posterior thoracolumbar instrumentation failure has been shown to correlate with bone mineral density (BMD).¹^–³ Screw pull-out and also cutout through the adjacent endplate with cyclical flexion–extension loading are directly related to BMD and may occur even at physiologic loads in the osteoporotic spine.¹^–³ In a biomechanical study, Soshi and colleagues² concluded that pedicle screw fixation should be avoided in patients with a BMD less than 0.3 g/cm².

At the time of pedicle screw insertion, the surgeon may recognize poor screw purchase in osteoporotic bone because of the low insertion torque required to advance the screw. Insertion torque not only correlates with BMD and screw pull-out, but also predicts early screw failure.⁴^–⁶ If poor screw purchase is recognized intraoperatively, the surgeon should attempt to salvage the situation rather than rely on inadequate fixation to achieve the goals of instrumentation.

Pedicle Screws in the Osteoporotic Spine

Screw Placement

The surgeon may consider increasing the length or diameter of the pedicle screw in an attempt to improve the screw purchase in bone (Table 57-1). Increasing screw length does increase screw pull-out strength, although this effect may be less pronounced in osteoporotic bone.^7,⁸ Use of bicortical screws in the lumbar spine is limited because of risk of vascular injury. However, in the sacrum, a bicortical screw can be placed safely and improves pull-out strength.^9,¹⁰ The inability to accurately gauge the anterior vertebral body cortex intraoperatively may affect the surgeon’s ability to safely place longer screws, since screws extending beyond the anterior vertebral body may predispose to vascular injury. At the sacrum, bicortical purchase may be safely accomplished with medially directed pedicle screws with a low risk of vascular injury. Increasing screw diameter will also increase pull-out strength^7,¹¹^–¹³; however, the dimensions of the pedicle being cannulated may limit the screw diameter. In the osteoporotic spine, when the screw diameter exceeds 70% of the pedicle diameter, a risk of pedicle fracture is created.¹⁴

TABLE 57-1 Pedicle Screw Size Relationship

Screw Size	6.0	5.0
Screw outer diameter (mm)	6.0	5.0
Screw minor diameter (mm)	4.8	3.8
Tap minor diameter (mm)	4.75	3.75

Directing pedicle screws toward the stronger subchondral bone adjacent to the vertebral body endplate will improve pull-out resistance.^15,¹⁶ In the sacrum, optimal screw purchase is achieved by directing the screws toward the disc space anteriorly or through the sacral promontory.¹⁷^–¹⁹

Another strategy to improve stability of the pedicle screw construct in osteoporotic bone is to distribute forces by increasing the number of fixation points to the spine by including additional levels in the construct. The advantages of this approach must be weighed against the risks and morbidity associated with the additional level surgery as well as the potential long-term consequences of a fusion spanning additional levels. The surgeon may also augment the pedicle screw construct with offset sublaminar hooks, which are well suited for use in the osteoporotic spine because they rely on the relatively unaffected cortical laminar bone for fixation.^1,²⁰ Biomechanical studies have supported the ability of supplemental sublaminar hooks to increase the rigidity and pull-out strength of pedicle screw constructs.^21,²²

Convergence of pedicle screws with a triangulation effect can substantially increase the overall pull-out strength of the construct and provides higher resistance against loads perpendicular to the pedicle screw (Figure 57-1).²³ Triangulation of pedicle screws increased pullout strength by 143% over single pedicle screws.²³ Bilateral triangulated pedicle screws allow the screws to, in effect, hold all of the bone between the screws rather than just the bone within the threads of the individual screws. Ruland and colleagues²³

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