Proceedings of the NASS 20th Annual Meeting / The Spine Journal 5 (2005) 1S–189S 127S

STUDY DESIGN/SETTING: The operation logbooks of the spinal unitwere reviewed, showing that 242 patients had undergone the procedureduring the study period.PATIENT SAMPLE: An effort was made to recontact all patients todetermine whether they had had additional lumbar surgery, and whetherthe implant had been. The results were submitted to Kaplan-Meier actuarialanalysis of implant survival. At long-term follow-up as long as 16 years afterimplantation responses were obtained concerning 142 subjects (58.7% ofthe entire cohort).OUTCOME MEASURES: Implant survival.METHODS: Patient interview.RESULTS: Sixty-two percent of the patients had an implant at one level,19% at two levels, 15% at three levels and 4% at four levels. Of the 212implants, only 1% were used at L1-L2, while 57% involved the L4-L5level. The principal indications were isolated canal stenosis (44%) andrecurrent herniated disc (24%). The actuarial survival analysis predicted that82.7% of patients would still have the implant in place after 14 years witha 95% confidence interval of �6.6%. There was no significant differencein survival between the single-level implants and the multiple-level im-plants. Only three revision procedures were performed because of eventsdirectly related to the implant (one loose band, one implant breakage, andone spinous process fracture). No revision was related to long-term pain ordiscomfort caused by the implant. No allergic-type reactions or rejectionswere found. Among the142 patients contacted, thenumber ofevents resultingin removal of the implant decreased with. At most, only nine of the twenty-four revision procedures may have resulted from adjacent-level degenera-tion. There were no procedure-related deaths.CONCLUSIONS: This long-term retrospective study provides indirectevidence of the safety of this interspinous stabilization system. It is theonly interspinous motion preserving device in the world which has thisdegree of long-term data possible for long-term analysis. Long-term radio-graphic studies suggest that the system may contribute to long-term protec-tion of both initially symptomatic intervertebral segments and the segmentsadjacent to them. A study of the patients with long-term matched MRIappearance is underway to determine their long-term clinical outcome andto try to understand the prevalence of restoration of the hydration signalon MRI—does this represent healing or revascularization of the interverte-bral disc?DISCLOSURES: No disclosures.CONFLICT OF INTEREST: Author (PM) Consultant: Abbott Spine;Author (PM) Grant research support: Abbott Spine.

doi: 10.1016/j.spinee.2005.05.251P38. An electron microscopic study of nerves within the humanvertebral bodyMichael Heggeness, MD, PhD1, Phillip Kravitz, MD2,Joiner Cartwright, Jr., PhD1; 1Baylor College of Medicine, Houston,TX, USA; 2Houston, TX, USA

BACKGROUND CONTEXT: Recent reports have demonstrated an abun-dant network of intraosseous nerves within human vertebral bodies. Themajority of these newves enter the vertebral body via the large posteriorvascular foramen, and are now called the basivertebral nerves. These nerveshave been shown to contain Substance P. It has been proposed that thesenerves may play a role in some forms of clinical back pain. Althoughreports on the nerve supply to the soft tissue elements of the spine arenumerous, until recently there has been very little information available aboutthe nerves found within the bone of the vertebral body.PURPOSE: Nerve fiber types are described.STUDY DESIGN/SETTING: The present study describes the harvest ofintraosseous nerves from within human vertebral bodies, which were thenstudied by transmission electron microscopy.PATIENT SAMPLE: Not a patient outcome study.OUTCOME MEASURES: Not a patient outcome study.METHODS: Sixteen vertebral bodies including 5 cervical, 4 thoracic, and7 lumbar specimens wereharvested fromsixdifferent freshly obtainedhumancadavers. The posterior elements were removed and the posterior longitudi-nal ligament exposed. The ligament was then carefully reflected from the

P37. Biomechanical analysis of adjacent vertebra behavior followingvertebroplastyRuth Wilcox; University of Leeds, Leeds, England, United Kingdom

BACKGROUND CONTEXT: There has been a rapid increase in the useof vertebroplasty (VP) for the treatment of osteoporotic vertebral fractures.Although short-term clinical results have been positive, recent studies ofthe medium-term effects suggest that the procedure may increase the riskof adjacent vertebra fracture. Experimental studies also indicate a potentialrisk, but high levels of specimen variability make conclusive results difficultto attain.PURPOSE: To use computational methods to determine if VP is likelyto accelerate adjacent vertebral failure.STUDY DESIGN/SETTING: Two finite element (FE) models of fracturedspinal segments, representing (a) a VP-treated and (b) an untreated case,were compared over a number of loading cycles.PATIENT SAMPLE: NA.OUTCOME MEASURES: NA.METHODS: An initial FE model of a functional spinal unit, based on CTscans of human lumbar vertebrae, was generated using custom-writtensoftware. The model comprised two complete vertebrae including posteriorelements, intervertebral disc, facet joints and ligaments. The material prop-erties were set to represent a highly osteoporotic specimen. Verification

of the model was undertaken by comparison with published data fromexperimental tests. The initial fracture was simulated by applying an axialcompressive load to the model until failure occurred in approximately halfthe trabecular elements. A reduced modulus method, based on the localplastic strain, was then used to simulate damage to the trabecular bone onan element-by-element basis. Two copies of the fractured model werethen used. In one, the VP procedure was simulated by replacing the mostdamaged elements in the caudal vertebra with elements representing atypical VP cement, whilst the other model was left untreated. Four cyclesof axial compressive loading were then applied to each model, using thedamage-reduction method to recalculate the trabecular properties after eachload application. The stiffness, end plate bulge and adjacent vertebral strainswere monitored throughout.RESULTS: After the first reloading cycle, the treated model was stiffer thanthe untreated model (0.39, 0.34 kN/mm respectively), but neither restored thestiffness to the initial unfractured level. Following further loading cycles,the untreated model stiffness stayed relatively constant whereas the treatedmodel stiffness dropped below that of the untreated model (0.32 kN/mmafter 4 cycles). There was also greater end plate deformation into the adjacentvertebra in the treated model (1.58 mm) than the untreated model (0.44mm) causing higher strains in the adjacent vertebra (mean 5.2% treated,3.0% untreated) and failure over a larger region of the cancellous core.CONCLUSIONS: These results indicate that over a number of loadingcycles, there is far greater cumulative damage to the adjacent vertebrafollowing VP than there is if the fracture were left untreated. These findingsagree with trends observed in previous experimental studies. Additionally,this study indicates the mechanism by which this failure occurs: the mis-match in properties between the cement and the surrounding bone causes agreater proportion of the load to be transmitted through the cemented regionof the vertebral body; this, in turn, causes greater end plate deflection andhigher strains in the cancellous region of the adjacent vertebra. Careshould therefore be exercised in using standard PMMA cements forthe augmentation of highly osteoporotic vertebrae. Further research is nowrequired to determine more suitable materials for the treatment of suchfractures.DISCLOSURES: No disclosures.CONFLICT OF INTEREST: Author (RW) Grant Research Support:funded by Royal Academy of Engineering Post-doctoral Fellowship.

doi: 10.1016/j.spinee.2005.05.252