30 Vertebral Augmentation in Instrumented Spinal Fusions
Summary
Vertebral augmentation is a minimally invasive treatment known to reduce pain and disability and improve the quality of life in patients with painful vertebral fractures. In addition to its applications across a wide range of etiologies, it offers promise in select patients when used in combination with instrumented fixation to help alleviate spinal instability. These patients often have elevated risks of hardware failure, collapse of constructs, and junctional fractures. Supplemental vertebral augmentation of pedicle screw fixation may mitigate these risks. Biomechanical studies demonstrate improved pullout strength of augmented screws placed into osteoporotic bone.
30.1 Introduction
The utility of vertebral augmentation is quite broad with applications ranging from the treatment of complications of benign disease such as osteoporosis to primary malignancies and metastatic disease. An important application is its use in combination with instrumented fixation in patients with spinal instability. These patients are a heterogeneous population but they have in common the need for bony vertebral fusion. They often have elevated risks of hardware failure, collapse of constructs, and junctional fractures. Supplemental vertebral augmentation of pedicle screw fixation offers the spine surgeon intraoperative options to mitigate these risks.
30.2 Instrumentation in Spinal Fusion
Roy-Camille is credited with the first description of posterior plates with screws positioned sagittally through the pedicles and articular processes with a system that he and Judet had been using since 1963. 1 This became the foundation for pedicle screw fixation. Harrington and Tullos described the first transpedicular screw placement in the United States in 1969. Subsequent pedicle screw systems and modifications came from Louis, Steffee, Magerl, Luque, Wiltse, and others. Design modifications continue today in an effort to improve purchase, strength, and fusion outcomes. 2 – 5
In 1991, Lorenz et al prospectively evaluated 68 patients with at least 6 months of disabling back pain who had failed conservative care to compare single-level posterolateral lumbar fusions (PLF) with and without pedicle screws. Twenty-nine patients were fused without hardware and 39 were fused with variable screw placement (VSP) fixation. An improved fusion rate was reported for patients undergoing pedicle screw fixation with pseudarthrosis seen in 58.6% of the non-instrumented group and no pseudarthrosis in instrumented patients. Pain improvement was better in the instrumented group (76.9 vs. 41.4%) and the rate of return to work was higher in the instrumented patients (72 vs. 31%) compared to the non-instrumented patients. 6
In a report to the contrary, Thomsen et al suggested that pedicle screw fixation did not affect functional improvement or fusion rates long-term. 7 Despite this controversy, pedicle screw fixation has become routine in posterolateral fusions. Guidelines for the performance of fusion procedures for degenerative disease of the lumbar spine were published in the Journal of Neurosurgery Spine in 2014. An association, but no direct correlation, was reported between increased fusion rates and pedicle screw and rod fixation as assessed with dynamic radiographs. Despite the routine nature of this practice, the guidelines recommend “the use of pedicle screw fixation as a supplement to PLF (posterolateral fusion) be reserved for those patients in whom there is an increased risk of nonunion when treated with only PLF.” 8
Although an in-depth analysis of the addition of pedicle screw fixation to fusion constructs is not the focus of this chapter, one must understand that attempts to further stabilize the pedicle screw may be inextricably linked to data correlating pedicle screw fixation to improve the stability of the fusion and therefore the functional and radiographic outcomes. As seen in the examples above, results are not definitive.
30.3 Vertebral Augmentation in Spinal Fusion
Data supporting vertebral augmentation and its benefit to spinal fusion date back over 30 years. In 1986, Zindrick et al 18 assessed various biomechanical performances of sacral screw fixation, including fixation augmented with polymethyl methacrylate (PMMA). The authors reported that placement of PMMA around a loosened screw restored fixation and doubled its pullout force. This was reproduced in osteoporotic models of the lumbar spine by Soshi et al 9 who measured the pullout force of a 7-mm pedicle screw on normal cadaveric lumbar vertebrae compared to those with mild and severely osteoporotic bone. A pullout force of 1,056.4 N was required in the normal, non-osteoporotic group, 495.6 N in the mildly osteoporotic group, and 269.5 N in the severely osteoporotic group. The use of bone cement for augmentation reduced the risk of screw pullout in the osteoporotic vertebrae by a twofold increase in pullout strength. 9
Mermelstein et al further categorized the bending moments of pedicle screws augmented with calcium phosphate (CP) cement. The bending moment was reduced by 59% in flexion and 38% in extension, and the mean stiffness was increased by 40%. 10 Sarzier et al reported in 2002 that augmentation of osteoporotic vertebrae increased pedicle screw pullout forces. The maximum attainable force was approximately twice the pullout force of the non-augmented pedicle screws for each osteoporotic (Jikei scale) grade. The mean increase in pullout force in osteoporotic spines was 181% for Grade I, 206% for Grade II, and 213% for Grade III. 11
30.4 Indications
Vertebral augmentation is used to treat vertebral compression fractures (VCFs) across a wide range of etiologies. Augmentation of spinal instrumentation may be indicated for a select patient population, including osteoporotic patients, those requiring revision surgery, especially those high-risk patients who may not tolerate lengthy general anesthesia and surgery, and adults with spinal deformity. In osteoporotic patients, augmentation may reduce the risk of screw pullout. In addition, patients who require revision of their instrumented spinal fusion should benefit from augmentation as it increases the stiffness and pullout strength of the replacement screws. Augmentation in high-risk patients can be used to provide construct stability and prolonged pain relief. Finally, augmentation has been shown to decrease the need for revisions and reduce the risk of proximal junctional fractures (PJFs) following instrumentation in adults with spinal deformities. 12 , 13 See ▶Fig. 30.1.
30.5 Outcomes and Variables
Many variables affect outcomes following instrumented fusion, with or without augmentation. Here, we review outcomes of fusions with vertebral augmentation as they relate to etiology, method of delivery, instrumentation, cement composition, volume, and timing.
30.5.1 Osteoporosis
Significant risk of screw pullout and fractures exist in osteoporotic patients as coercive attempts at deformity correction are made with instrumentation. The risk of hardware failure is proportional to corrective forces applied. In osteoporotic patients, bone quality becomes most important in determining outcomes and the axiom, “bone holds metal; metal doesn’t hold bone” applies. Pedicle screw augmentation with PMMA improves the initial fixation strength and fatigue strength of instrumentation in osteoporotic vertebrae. Numerous studies of the lumbar and thoracic spine, ilium, and sacrum demonstrate a 1.5- to 2-fold increase in pullout strength of augmented screws compared to non-augmented screws. 9 , 11 , 14 – 16 This benefit may be realized only in low-quality bone of osteopenic and osteoporotic patients. 17
30.5.2 Revision Surgery
Similar to results in osteoporotic patients, PMMA or other cements can salvage screw fixation and increase the force required for pullout twofold. 18 The pullout strength of augmented replacement screws in thoracolumbar vertebrae returned to baseline or increased above baseline in these revision cases. 19 , 20 Initial and final stiffness of the larger diameter screws was also increased with the addition of the cement augmentation. 21
30.5.3 Select High-Risk Patients
Comorbid disease or poor general health may preclude internal fixation and fusion surgery. In these select patients, vertebral augmentation may provide an alternative. For example, Puri and Erdem 22 described two patients with multiple myeloma who had failed posterior spinal interbody fusions and had significant pain, but were felt to be at high risk of complications from general anesthesia due to their multiple comorbidities. Rather than lengthy construct revisions under general anesthesia, unilateral transpedicular vertebroplasties were performed under intravenous conscious sedation.
The first patient had previously undergone thoracic vertebrectomy with interbody device placement and dorsal internal fixation from T6 to T10. Six months later, when recurrent pain prompted MR and CT imaging, construct failure and apparent loosening of the interbody device was identified. Believing that constant micromotion by the cage might be causing the pain, the authors performed transpedicular vertebroplasty with a diamond-tipped needle placed in the anterior third of the T9 vertebral body. With real-time fluoroscopic guidance, PMMA cement was deliberately directed upward toward the cage at T6–T8. The cement crossed several disk spaces and surrounded the cage anteriorly with some minor filling of the cage.
The second patient had undergone partial corpectomy at L3 with placement of a left paracentral interbody device and dorsal fixation from L2 and L4. Pain recurred approximately 7 months after surgery. MR and CT imaging demonstrated a new compression fracture at L1 and lucencies around the L2–L4 construct. After vertebroplasty was performed at L1, attention was turned to L3. A right parapedicular injection was used to deliver PMMA cement across the midline to the left side anterior to the cage, then a transpedicular injection delivered additional cement to the right side of remaining L3 vertebral body, also anterior to the cage.
Both patients had significant reductions in pain and at 18 months, the constructs demonstrated stability and patients had continued pain relief. 22
30.5.4 Adult Spinal Deformity
Adult patients with spinal deformity and long-segment (>5 levels) instrumented fusions are at high risk of proximal junctional kyphosis (PJK), PJFs, and hardware failure (▶Fig. 30.2). This is particularly true in patients with deformity in the sagittal plane. While the specific etiology of these junctional problems is often multifactorial, they are generally due to the stress riser that naturally occurs at the junction of the instrumented and uninstrumented spine.
This is particularly true at the upper instrumented vertebra (UIV) and the vertebra immediately cephalad to the construct (UIV+1). Junctional kyphosis is differentiated into PJK and PJF. PJK is defined as kyphosis of greater than 10 degrees in the segment above a long construct compared with preoperative measurements, while PJF is defined as a structural failure of the spinal column with vertebral body fracture, failure of the posterior ligamentous complex, screw pullout, and vertebral subluxation. Vertebral augmentation is used to prevent both PJK and PJF and has been shown to be effective. 23 Hart et al 24 revealed a 15% reduction in the incidence of PJF following kyphoplasty cranial to the UIV. Martin et al 25 reported that only 5% of patients who underwent vertebral augmentation at UIV and UIV+1 had PJF, much lower than the historic rate without vertebral augmentation. Theologis and Burch 12 went further to report that patients with UIV and UIV+1 augmentation had fewer revisions due to fractures (0 vs. 19), better functional outcomes, and significantly less disability. Patients without augmentation were 9.2 times more likely to undergo revision surgery than those with prophylactic UIV and UIV+1 augmentation. Ghobrial et al 13 reported a decreased incidence (23.7 vs. 36%) and magnitude (5.65 vs. 9.36 degrees) of PJK in patients with UIV and UIV+1 kyphoplasties. In addition to the early clinical studies examining the effectiveness of vertebral augmentation in the prevention of junctional kyphosis, biomechanical studies have been done as well. Cadaveric analyses of augmentation at UIV and UIV+1 (▶Fig. 30.1) demonstrated fewer PJFs (17%) than those with only a single-level vertebroplasty (that was not at the UIV or UIV+1 level) (67%) or spines without cement (100%) 26 (▶Fig. 30.3).
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