Effects of Single-Dose Versus Hypofractionated Focused Radiation on Vertebral Body Structure and Biomechanical Integrity: Development of a Rabbit Radiation-Induced Vertebral Compression Fracture Model.

PURPOSE: Vertebral compression fracture is a common complication of spinal stereotactic body radiation therapy. Development of an in vivo model is crucial to fully understand how focal radiation treatment affects vertebral integrity and biology at various dose fractionation regimens. We present a clinically relevant animal model to analyze the effects of localized, high-dose radiation on vertebral microstructure and mechanical integrity. Using this model, we test the hypothesis that fractionation of radiation dosing can reduce focused radiation therapy's harmful effects on the spine. METHODS AND MATERIALS: The L5 vertebra of New Zealand white rabbits was treated with either a 24-Gy single dose of focused radiation or 3 fractionated 8-Gy doses over 3 consecutive days via the Small Animal Radiation Research Platform. Nonirradiated rabbits were used as controls. Rabbits were euthanized 6 months after irradiation, and their lumbar vertebrae were harvested for radiologic, histologic, and biomechanical testing. RESULTS: Localized single-dose radiation led to decreased vertebral bone volume and trabecular number and a subsequent increase in trabecular spacing and thickness at L5. Hypofractionation of the radiation dose similarly led to reduced trabecular number and increased trabecular spacing and thickness, yet it preserved normalized bone volume. Single-dose irradiated vertebrae displayed lower fracture loads and stiffness compared with those receiving hypofractionated irradiation and with controls. The hypofractionated and control groups exhibited similar fracture load and stiffness. For all vertebral samples, bone volume, trabecular number, and trabecular spacing were correlated with fracture loads and Young's modulus (P < .05). Hypocellularity was observed in the bone marrow of both irradiated groups, but osteogenic features were conserved in only the hypofractionated group. CONCLUSIONS: Single-dose focal irradiation showed greater detrimental effects than hypofractionation on the microarchitectural, cellular, and biomechanical characteristics of irradiated vertebral bodies. Correlation between radiologic measurements and biomechanical properties supported the reliability of this animal model of radiation-induced vertebral compression fracture, a finding that can be applied to future studies of preventative measures.

A mouse model for the study of transplanted bone marrow mesenchymal stem cell survival and proliferation in lumbar spinal fusion.

PURPOSE: Bone marrow aspirate has been successfully used alongside a variety of grafting materials to clinically augment spinal fusion. However, little is known about the fate of these transplanted cells. Herein, we develop a novel murine model for the in vivo monitoring of implanted bone marrow cells (BMCs) following spinal fusion. METHODS: A clinical-grade scaffold was implanted into immune-intact mice undergoing spinal fusion with or without freshly isolated BMCs from either transgenic mice which constitutively express the firefly luciferase gene or syngeneic controls. The in vivo survival, distribution and proliferation of these luciferase-expressing cells was monitored via bioluminescence imaging over a period of 8 weeks and confirmed via immunohistochemistry. MicroCT imaging was performed 8 weeks to assess fusion. RESULTS: Bioluminescence imaging indicated transplanted cell survival and proliferation over the first 2 weeks, followed by a decrease in cell numbers, with transplanted cell survival still evident at the end of the study. New bone formation and increased fusion mass volume were observed in mice implanted with cell-seeded scaffolds. CONCLUSIONS: By enabling the tracking of transplanted bone marrow-derived cells during spinal fusion in vivo, this mouse model will be integral to developing a deeper understanding of the biological processes underlying spinal fusion in future studies. These slides can be retrieved under Electronic Supplementary Material.

Systematic Approach for Anterior Corpectomy through a Transthoracic Exposure.

While several approaches have been suggested for performing a transthoracic corpectomy of the thoracolumbar spine, the steep learning curve and high risk of operative morbidity often dissuades its use clinically. As a result, posterior approaches have been more frequently used, though transthoracic approaches often allow for improved access for more complete decompression of the spinal cord. Additionally, the transthoracic approach allows for more complete removal of vertebral body lesions along with better access for reconstruction of the anterior column. Here we describe a distinct nine-step thoracolumbar corpectomy technique via a transthoracic approach. We include a description of one patient with one-year follow-up to further illustrate the surgical approach and relevant surgical anatomy. We have used this technique for maximal bi-pedicular decompression afforded by the lateral approach. We discuss key anatomical considerations, perioperative techniques and potential complications of the operation. The presented nine-step transthoracic corpectomy technique is a safe method for providing adequate neurological decompression and vertebral body tumor removal.

Prognostic factors associated with pain palliation after spine stereotactic body radiation therapy.

OBJECT The number of patients with spinal tumors is rapidly increasing; spinal metastases develop in more than 30% of cancer patients during the course of their illness. Such lesions can significantly decrease quality of life, often necessitating treatment. Stereotactic radiosurgery has effectively achieved local control and symptomatic relief for these patients. The authors determined prognostic factors that predicted pain palliation and report overall institutional outcomes after spine stereotactic body radiation therapy (SBRT). METHODS Records of patients who had undergone treatment with SBRT for either primary spinal tumors or spinal metastases from June 2008 through June 2013 were retrospectively reviewed. Data were collected at the initial visit just before treatment and at 1-, 3-, 6-, and 12-month follow-up visits. Collected clinical data included Karnofsky Performance Scale scores, pain status, presence of neurological deficits, and prior radiation exposure at the level of interest. Radiation treatment plan parameters (dose, fractionation, and target coverage) were recorded. To determine the initial extent of epidural spinal cord compression (ESCC), the authors retrospectively reviewed MR images, assessed spinal instability according to the Bilsky scale, and evaluated lesion progression after treatment. RESULTS The study included 99 patients (mean age 60.4 years). The median survival time was 9.1 months (95% CI 6.9-17.2 months). Significant decreases in the proportion of patients reporting pain were observed at 3 months (p < 0.0001), 6 months (p = 0.0002), and 12 months (p = 0.0019) after treatment. Significant decreases in the number of patients reporting pain were also observed at the last follow-up visit (p = 0.00020) (median follow-up time 6.1 months, range 1.0-56.6 months). Univariate analyses revealed that significant predictors of persistent pain after intervention were initial ESCC grade, stratified by a Bilsky grade of 1c (p = 0.0058); initial American Spinal Injury Association grade of D (p = 0.011); initial Karnofsky Performance Scale score, stratified by a score of 80 (p = 0.002); the presence of multiple treated lesions (p = 0.044); and prior radiation at the site of interest (p < 0.0001). However, when multivariate analyses were performed on all variables with p values less than 0.05, the only predictor of pain at last follow-up visit was a prior history of radiation at the site of interest (p = 0.0038), although initial ESCC grade trended toward significance (p = 0.073). Using pain outcomes at 3 months, at this follow-up time point, pain could be predicted by receipt of radiation above a threshold biologically effective dose of 66.7 Gy. CONCLUSIONS Pain palliation occurs as early as 3 months after treatment; significant differences in pain reporting are also observed at 6 and 12 months. Pain palliation is limited for patients with spinal tumors with epidural extension that deforms the cord and for patients who have previously received radiation to the same site. Further investigation into the optimal dose and fractionation schedule are needed, but improved outcomes were observed in patients who received radiation at a biologically effective dose (with an a/b of 3.0) of 66.7 Gy or higher.

Analgesic therapy for major spine surgery.

Pain following spine surgery is often difficult to control and can persist. Reduction of this pain requires a multidisciplinary approach that depends on contributions of both surgeons and anesthesiologists. The spine surgeon's role involves limiting manipulation of structures contributing to pain sensation in the spine, which requires an in-depth understanding of the specific anatomic etiologies of pain originating along the spinal axis. Anesthesiologists, on the other hand, must focus on preemptive, multimodal analgesic treatment regimens. In this review, we first discuss anatomic sources of pain within the spine, before delving into a specific literature-supported pain management protocol intended for use with spinal surgery.

Automatic localization of target vertebrae in spine surgery: clinical evaluation of the LevelCheck registration algorithm.

STUDY DESIGN: A 3-dimensional-2-dimensional (3D-2D) image registration algorithm, "LevelCheck," was used to automatically label vertebrae in intraoperative mobile radiographs obtained during spine surgery. Accuracy, computation time, and potential failure modes were evaluated in a retrospective study of 20 patients. OBJECTIVE: To measure the performance of the LevelCheck algorithm using clinical images acquired during spine surgery. SUMMARY OF BACKGROUND DATA: In spine surgery, the potential for wrong level surgery is significant due to the difficulty of localizing target vertebrae based solely on visual impression, palpation, and fluoroscopy. To remedy this difficulty and reduce the risk of wrong-level surgery, our team introduced a program (dubbed LevelCheck) to automatically localize target vertebrae in mobile radiographs using robust 3D-2D image registration to preoperative computed tomographic (CT) scan. METHODS: Twenty consecutive patients undergoing thoracolumbar spine surgery, for whom both a preoperative CT scan and an intraoperative mobile radiograph were available, were retrospectively analyzed. A board-certified neuroradiologist determined the "true" vertebra levels in each radiograph. Registration of the preoperative CT scan to the intraoperative radiograph was calculated via LevelCheck, and projection distance errors were analyzed. Five hundred random initializations were performed for each patient, and algorithm settings (viz, the number of robust multistarts, ranging 50-200) were varied to evaluate the trade-off between registration error and computation time. Failure mode analysis was performed by individually analyzing unsuccessful registrations (>5 mm distance error) observed with 50 multistarts. RESULTS: At 200 robust multistarts (computation time of ∼26 s), the registration accuracy was 100% across all 10,000 trials. As the number of multistarts (and computation time) decreased, the registration remained fairly robust, down to 99.3% registration accuracy at 50 multistarts (computation time ∼7 s). CONCLUSION: The LevelCheck algorithm correctly identified target vertebrae in intraoperative mobile radiographs of the thoracolumbar spine, demonstrating acceptable computation time, compatibility with routinely obtained preoperative CT scans, and warranting investigation in prospective studies. LEVEL OF EVIDENCE: N/A.

Intercostal aneurysm causing spinal cord compression in an NF1 patient.

PURPOSE: The authors illustrate a case where an intercostal aneurysm was observed in a patient with type 1 neurofibromatosis. METHODS: A 32-year-old man with NF1 presented with thoracic back pain. The patient's symptoms progressed to include myelopathic symptoms, including difficulty urinating, numbness in the lower extremities, and increased weakness. Imaging revealed what appeared to be a neurofibroma at the T4-T5 level and a plan to resect the mass was formulated. Upon initial limited hemilaminotomy, significant arterial blood was encountered. The patient was then taken to the interventional suite and angiography was performed, revealing a left T4 intercostal aneurysm. The aneurysm was coil-embolized with no residual filling. RESULTS: By 6 months post-surgery, the patient had regained full strength and sensation in his lower extremities and no longer had difficulty urinating. There has been no recurrence of symptoms 3 years postoperatively. CONCLUSIONS: Intercostal artery lesions must be considered as a possible diagnosis in NF1.

Techniques and accuracy of thoracolumbar pedicle screw placement.

Pedicle screw instrumentation has been used to stabilize the thoracolumbar spine for several decades. Although pedicle screws were originally placed via a free-hand technique, there has been a movement in favor of pedicle screw placement with the aid of imaging. Such assistive techniques include fluoroscopy guidance and stereotactic navigation. Imaging has the benefit of increased visualization of a pedicle's trajectory, but can result in increased morbidity associated with radiation exposure, increased time expenditure, and possible workflow interruption. Many institutions have reported high accuracies with each of these three core techniques. However, due to differing definitions of accuracy and varying radiographic analyses, it is extremely difficult to compare studies side-by-side to determine which techniques are superior. From the literature, it can be concluded that pedicles of vertebrae within the mid-thoracic spine and vertebrae that have altered morphology due to scoliosis or other deformities are the most difficult to cannulate. Thus, spine surgeons would benefit the most from using assistive technologies in these circumstances. All other pedicles in the thoracolumbar spine should theoretically be cannulated with ease via a free-hand technique, given appropriate training and experience. Despite these global recommendations, appropriate techniques must be chosen at the surgeon's discretion. Such determinations should be based on the surgeon's experience and the specific pathology that will be treated.

Incidence and clinical significance of vascular encroachment resulting from freehand placement of pedicle screws in the thoracic and lumbar spine: analysis of 6816 consecutive screws.

STUDY DESIGN: Retrospective case series. OBJECTIVE: Evaluate the incidence and clinical significance of vascular encroachment resulting from freehand placement of pedicle screws in the thoracic and lumbosacral spine. SUMMARY OF BACKGROUND DATA: Pedicle screws are routinely used to effectively stabilize all 3 columns of the spine but can be technically demanding to place in the setting of variable anatomy. There is a paucity of data regarding iatrogenic major vascular injuries during posterior instrumentation procedures. METHODS: We retrospectively reviewed the records of all patients undergoing freehand pedicle screw placement without image guidance in the thoracic or lumbar spine during a 7-year period. The incidence and extent of vascular encroachment by a pedicle screw was determined by review of routine postoperative computed tomographic scans obtained within 24 hours of all surgical procedures. Vascular encroachment was defined as a pedicle screw that was touching or deforming the wall of a major vessel. RESULTS: A total of 964 patients received 6816 freehand-placed pedicle screws in the thoracolumbar spine. Fifteen (0.22%) screws that encroached a major vascular structure were identified. Ten (0.29%) thoracic pedicle screws encroached on the aorta, 4 (0.14%) lumbar screws on the common iliac vein, and 1 S1 screw (0.19%) on the internal iliac vein. In consultation with vascular surgery, it was determined whether revision surgery and the technique/approach for the revision procedure should be recommended. Two (0.21%) patients required revision surgery to remove the encroaching pedicle screw (T5 and T8) due to concern for vascular injury. Both patients were asymptomatic and recovered without further complications after revision surgery. CONCLUSION: Vascular encroachment of major vessels occurs rarely in the setting of freehand pedicle screw placement in the thoracolumbar spine. Although rare, delayed vascular injury from errant pedicle screw placement has been reported in the literature. The aorta seems to be the vessel at the highest risk of injury. Routine intraoperative or postoperative computed tomographic scanning allows for early identification of pedicle screws encroaching on vascular structures thereby facilitating early revision surgery. LEVEL OF EVIDENCE: 4.

Quantitative study of parathyroid hormone (1-34) and bone morphogenetic protein-2 on spinal fusion outcomes in a rabbit model of lumbar dorsolateral intertransverse process arthrodesis.

STUDY DESIGN: A posterolateral rabbit spinal fusion model was used to evaluate the effects of recombinant human bone morphogenetic protein-2 (rhBMP-2) and teriparatide (PTH [1-34]) used individually and in combination on spinal fusion outcomes. OBJECTIVE: To test the efficacy of parathyroid hormone on improving spinal fusion outcomes when used with BMP-2. SUMMARY OF BACKGROUND DATA: Of the more than 250,000 spinal fusion surgical procedures performed each year, 5% to 35% of these will result in pseudarthrosis. Growing controversy on the efficacy and cost of rhBMP-2 for improving spinal fusion outcomes has presented a challenge for clinicians. Research into PTH as an adjunct therapy to rhBMP-2 for spinal fusion has not yet been investigated. METHODS: Forty-eight male New Zealand white rabbits underwent bilateral posterolateral intertransverse process arthrodesis surgery at the L5-L6 level. Animals were divided into 6 groups. Two groups were treated with autograft alone or autograft and PTH (1-34), whereas the other 4 groups were treated with low-dose rhBMP-2 alone, high-dose rhBMP-2 alone, or either dose combined with PTH (1-34). All animals were euthanized 6 weeks after surgery. The L4-L7 spinal segment was removed and assessed using manual palpation, computed tomography (CT), and biomechanical testing. RESULTS: CT assessments revealed fusion in 50% of autograft controls, 75% of autograft PTH (1-34) animals, 87.5% in the 2 groups treated with low-dose rhBMP-2, and 100% in the 2 groups treated with high-dose rhBMP-2. CT volumetric analysis demonstrated that all groups treated with biologics had fusion masses that were on average significantly larger than those observed in the control group (P < 0.0001). Biomechanical data demonstrated no statistical difference between controls, PTH (1-34), and low-dose rhBMP-2 in any testing orientation. PTH (1-34) did not increase bending stiffness when used adjunctively with either low-dose or high-dose rhBMP-2. CONCLUSION: Although intermittent teriparatide administration results in increased fusion mass volume, it does not improve biomechnical stiffness over use of autograft alone. When delivered concurrently with high- and low-dose rhBMP-2, teriparatide provided no statistically significant improvement in biomechanical stiffness. LEVEL OF EVIDENCE: N/A.

Desmoid Tumor Formation following Posterior Spinal Instrumentation Placement.

Study Design Case report. Objective The objective of the article is to illustrate a case of desmoid tumor (DT) formation after posterior instrumentation of the thoracic spine. Methods A 57-year-old woman presented with lower extremity clumsiness, balance, and ambulation difficulty resulting from spinal cord compression due to an upper thoracic atypical vertebral hemangioma. Ten months after undergoing embolization, resection, and placement of instrumentation for this lesion, the patient developed a growing mass at the rostral end of the incision. Biopsy revealed desmoid fibromatosis. The mass was removed via an en bloc resection. Histology revealed an infiltrative DT above the laminectomy site abutting the instrumentation. Results At 2-year follow-up, there was no evidence of recurrence of the tumor. Conclusion Paraspinal DTs have been reported in the literature to develop after surgical procedures of the spine. Often times, patients attribute swelling or fullness at the site of their surgery to scar tissue formation or instrumentation. One must consider the possibility of a DT in the setting of reported surgical site fullness or mass after spine surgery. It is thought that postoperative inflammation present in the surgical bed may promote formation of DTs. Instrumentation may also contribute to inflammation and increase the likelihood of developing a DT. Generous margins must be taken to prevent recurrence.