Influence of local environment on incorporation of ceramic for lumbar fusion. Comparison of laminar and intertransverse sites in a canine model

Contribution of the xenograft bone plate-screw system in lumbar transpedicular stabilization: An in vivo study in dogs

OBJECTIVES

Xenograft bone plate-screws (XBPSs) can be alternative tools in lumbar transpedicular stabilization (TS). The aim of this study was to show biomechanical and histopathological contribution of the XBPSs system in lumbar TS.

MATERIALS AND METHODS

Fifteen (n = 15) hybrid dog and ten (n = 10) L_2-4 cadaveric specimens were included in the study. The dogs were separated according to surgical techniques: L₃ laminectomy and bilateral facetectomy (LBF) in Group I (experimental group [EG I] (n = 5), L₃ LBF plus TS with metal plate-screws (MPSs) in Group II (EG II) (n = 5), and L₃ LBF plus TS with XBPSs in Group III (EG III) (n = 5). The cadaveric specimens were separated to L_2-4 intact in Group I (CG I), (n = 5), and L₃ LBF in Group II (CG II), (n = 5). The dogs were sacrificed at the end of 3^rd month, and their L_2-4 spinal segments were en bloc removed and prepared as in control groups. Flexion, extension, left-right bending, rotation, and compression tests were applied to all segments. Stiffness values were calculated and analyzed statistically. All dog segments were evaluated histopathologically.

RESULTS

XBPS system showed a higher average stiffness values for left bending, extension, flexion, and compression compared to MPS, but these differences were not statistically meaningful. XBPS system had superiority to the fusion formation, as well.

CONCLUSIONS

XBPSs provide stability and help the fusion formation, but this system does not have a biomechanical advantage over MPS system in TS.

Triphasic ceramic scaffold in paediatric and adolescent bone defects

We evaluated novel triphasic hydroxyapatite tricalcium phosphate calcium silicate scaffold (HASi) in the management of paediatric bone defects. Their main advantage is considered to be adequate strength and stimulation of bone formation without resorting to autograft. A total of 42 children younger than 16 years of age were recruited over a period of 1 year and were treated with this synthetic bone substitute as a stand-alone graft for pelvic, femur, calcaneal and ulnar osteotomies, cystic bone lesions, subtalar arthrodesis and segmental bone defects. Forty children, 22 boys and 18 girls, mean age 8.3 years and a mean follow-up of 18.51 months, were available for evaluation. Analysis showed that younger age, cancellous defects and no internal fixation were associated with significantly faster healing. Partial incorporation was observed in 22.5% and complete incorporation in 77.5% of cases at 18 months of follow-up. Sex, type of defect, BMI and the shape of the ceramic graft did not significantly affect the rate of healing. Complications attributable to HASi included four nonunions, three of which were diaphyseal. HASi was found to be safe in children with cancellous or benign cavitatory defects. It is not suitable for diaphyseal and segmental bone defects as a stand-alone graft.

Allogeneic mesenchymal progenitor cells for posterolateral lumbar spine fusion in sheep

BACKGROUND CONTEXT

Osteoconductive porous ceramic bone graft materials supplemented with mesenchymal precursor cells (MPC) derived from autologous bone marrow aspirates have been shown to stimulate successful interbody and posterolateral spine fusion in preclinical models. Recent advances in immunomagnetic cell sorting have enabled purification and isolation of pluripotent stem cells from marrow aspirates and have expanded stem cell technology to allogeneic cell sources. Allogeneic MPC technology combined with appropriate synthetic biomaterial carriers could provide both the osteogenic and osteoconductive components needed for successful posterolateral spine fusion without the need for autologous bone harvest or expensive recombinant protein technology.

PURPOSE

To determine the safety and efficacy of a hydroxyapatite:tricalcium phosphate graft material supplemented with allogeneic mesenchymal precursor cells in posterolateral lumbar spine fusion using an ovine model.

STUDY DESIGN

Skeletally mature ewes underwent single-level instrumented posterolateral lumbar spine fusion using either autograft (AG), hydroxyapatite:tricalcium phosphate carrier (CP), or CP supplemented with allogeneic mesenchymal progenitor cells (MPCs). Three doses of MPCs were evaluated: 25 × 10⁶ cells (low dose, LD), 75 × 10⁶ cells (mid dose, MD), and 225 × 10⁶ cell (high dose, HD). Animals survived for either 4 or 9 months.

METHODS

Plain radiographs were acquired and scored for bridging bone at regular intervals during healing to monitor fusion development. Hematology, coagulation, and serum chemistry were monitored at regular intervals throughout the study to monitor animal health. After necropsy, computed tomography, high-resolution radiography, biomechanical testing, organ pathology, bone histopathology, and bone histomorphometry were conducted to monitor the safety and ascertain the efficacy of MPC treatment.

RESULTS

MPC treatment in this spine fusion model resulted in no observed adverse systemic or local tissue responses. Radiographically, fusion scores for MPC-treated animals were uniformly higher compared with those treated with carrier alone (CP) after 3 months and continued the same trend throughout 9 month of healing. Quantitative computed tomography confirmed better connectivity of the fusion for MPC treatment groups compared with CP. Biomechanical analyses were not able to differentiate between treatment groups. Histomorphometry results confirmed radiographic and quantitative computed tomography results; cell-supplemented treatment groups and autograft had equivalent amounts of bone within the fusion mass and less bony fusion tissue was found within the fusion mass in specimens from the CP treatment group. No conclusive effects of cell dose of fusion efficacy were noted.

CONCLUSIONS

Adult allogeneic mesenchymal precursor cells delivered via a hydroxyapatite:tricalcium phosphate carrier were both safe and efficacious in this ovine spine fusion model. Results from this preclinical study support that allogeneic mesenchymal precursor cells produced fusion efficacy similar to that achieved using iliac crest autograft, thereby providing a safe and viable option to achieve successful posterolateral spine fusion.

Posterolateral inter-transverse lumbar fusion in a mouse model

BACKGROUND

Spinal fusion is a common orthopaedic procedure that has been previously modeled using canine, lapine, and rodent subjects. Despite the increasing availability of genetically modified mouse strains, murine models have only been infrequently described.

PURPOSE

To present an efficient and minimally traumatic procedure for achieving spinal fusion in a mouse model and determine the optimal rhBMP-2 dose to achieve sufficient fusion mass.

METHOD

MicroCT reconstructions of the unfused mouse spine and human spine were compared to design a surgical approach. In phase 1, posterolateral lumbar spine fusion in the mouse was evaluated using 18 animals allocated to three experimental groups. Group 1 received decortication only (n=3), Group 2 received 10 μg rhBMP-2 in a collagen sponge bilaterally (n=6), and Group 3 received 10 μg rhBMP-2 + decortication (n=9). The surgical technique was assessed for intra-operative safety, efficacy, access and reproducibility. Spines were harvested for analysis at 3 weeks (Groups 1, 2) and 1, 2, and 3 weeks (Group 3). In phase 2, a dose response study was carried out in an additional 18 animals with C57BL6 mice receiving sponges containing 0, 0.5, 1, 2.5, 5 μg of rhBMP-2 per sponge bilaterally.

RESULTS

The operative procedure via midline access was rapid and reproducible, and fusion of the murine articular processes was found to be analogous to the human procedure. Unlike reports from other species, decortication alone (Group 1) yielded no new bone formation. Addition of rhBMP-2 (Groups 2 and 3) yielded a significant bone mass that bridged the L4-L6 vertebrae. The subsequent dose response experiment revealed that 0.5 μg rhBMP-2 per sponge was sufficient to create a fusion mass.

CONCLUSION

We describe a new approach for mouse lumbar spine fusion that is safe, efficient, and highly reproducible. The technique we employed is analogous to the human midline procedure and may be highly suitable for genetically modified mouse models.

Involvement of calcitonin gene-related peptide innervation in the promoting effect of low-intensity pulsed ultrasound on spinal fusion without decortication

STUDY DESIGN

A prospective left-right comparison designed experiment using a rabbit posterolateral intertransverse process fusion model.

OBJECTIVE

To investigate the involvement of calcitonin gene-related peptide (CGRP) innervation in the promoting effect of low intensity pulsed ultrasound stimulation (LIPUS) on spinal fusion without decortication.

SUMMARY OF BACKGROUND DATA

Sensory neuropeptide CGRP is involved in bone repair and ectopic ossification. Comparison of CGRP innervations in ectopic bone between sham LIPUS and LIPUS sides can help us to understand the relationship between sensory nerve innervation and LIPUS.

METHODS

A total of 27 New Zealand white rabbits underwent bilateral posterolateral intertransverse process fusion with implantation of porous poly-D,L-lactic acid blocks loaded with 1.25 μg recombinant human bone morphogenetic protein-4 solution. One side was provided LIPUS daily whereas the other side served as control. Animals were killed and the operated lumbar vertebrae were harvested for histomorphologic evaluation at 3 days (n = 3), 1 week (n = 6), 3 weeks (n = 6), 7 weeks (n = 6), and 12 weeks (n = 6) following surgery, respectively.

RESULTS

LIPUS accelerated the invasion of CGRP-positive nerve fibers during ectopic ossification spatially and temporally. Spatially, CGRP-positive nerve fibers were also observed in the new formed cartilage and bone tissues on LIPUS side, whereas they were only detected in the fibrous tissue and bone marrow on sham LIPUS side. Temporally, the density of CGRP-positive nerve fibers was significantly higher on the LIPUS side when compared with the sham LIPUS side.

CONCLUSION

LIPUS promoted the invasion of CGRP sensory nerve in ectopic bone, which may in turn contribute to the promoting effect of LIPUS on ectopic ossification.

Can iliac crest reconstruction reduce donor site morbidity?: a study using degradable hydroxyapatite-bioactive glass ceramic composite

STUDY DESIGN

Prospective study.

OBJECTIVE

To prospectively validate the hypothesis that iliac crest donor site morbidity may be a structural issue and by reconstructing the crest its incidence might be reduced. The study also evaluates the efficacy of Chitra hydroxyapatite-bioactive glass ceramic composite (Chitra-HABG) as a material for reconstructing the iliac crest.

SUMMARY OF BACKGROUND DATA

Tricortical iliac crest bone graft harvesting is associated with significant donor site morbidity, varying from 3% to 61%. Reconstruction of the defect has been shown to reduce this morbidity, but the only materials which have been shown to be useful and readily available are bioactive apatite-wollastonite glass ceramic and morcellized beta-tricalcium phosphate.

METHODS

Twenty-six patients in whom tricortical graft was harvested from the iliac crest and defect reconstructed with an indigenously developed and tested graft substitute-Chitra HABG-were followed up to duration of 1 year. Outcome measures were donor site morbidity as assessed clinically and radiologic assessment for ceramic incorporation, dissolution, fragmentation, and migration.

RESULTS

At the end of 1 year from surgery, 25 of the 26 patients (96.15%) had no pain at the donor site, which had been reconstructed. Radiologic evaluation showed that in 21 cases the ceramic incorporation was complete, partial in 3, and absent in 2. Partial dissolution of ceramic was noticed in 3 patients and migration in 1.

CONCLUSIONS

This study validates our hypothesis that the donor site morbidity after tricortical iliac crest graft harvesting is probably a structural issue and it can be reduced by reconstruction of the defect. It also highlights the fact that the Chitra-HABG block is an excellent material for reconstruction of the iliac crest defect, as it gets incorporated into the surrounding bone without adverse effects.

Hydroxyapatite-bioactive glass ceramic composite as stand-alone graft substitute for posterolateral fusion of lumbar spine: a prospective, matched, and controlled study

STUDY DESIGN

Prospective, matched, and controlled study.

OBJECTIVE

To evaluate the efficacy of hydroxyapatite-bioactive glass ceramic composite (Chitra-HABg) as a stand-alone graft substitute in promoting posterolateral fusion in the lumbar spine as compared with autologous bone.

BACKGROUND

The use of ceramics as stand-alone graft substitutes in posterolateral fusion remains controversial. The Chitra-HABg is a new composite that has undergone clinical trials in various orthopedic applications with excellent clinical and radiologic outcomes.

METHODS

Twenty-four patients underwent instrumented posterolateral fusion, with Chitra-HABg laid on the left intertransverse bed and autogenous graft on the right side. The primary outcome measure was radiologic consolidation of the graft, and secondary outcome measures were the work status and the Modified Oswestry Disability index. The McNamara and Student chi test were applied for statistical analysis.

RESULTS

Although the study was prematurely terminated owing to the high incidence of resorption of Chitra-HABg, 22 of the 24 subjects were followed-up for a minimum of 1 year. At the end of 1-year, excellent radiologic outcome was seen on the right side (autogenous graft) in all the cases, whereas 95% (21/22) of the cases had poor consolidation on the left side (Chitra-HABg). The clinical outcome was rated as good in 16/22 (73%) patients, fair in 5, and poor in only 1 patient, but this had no statistically significant association with the consolidation of the fusion mass.

CONCLUSIONS

This study clearly demonstrates that hydroxyapatite-bioactive glass ceramic composites (Chitra-HABg) has no role as stand-alone bone graft substitutes in posterolateral fusion of the lumbar spine, as the composite undergoes resorption without the formation of bridging callus.

LEVEL OF EVIDENCE

Level 1.

The use of hydroxyapatite for arthrodesis in dogs and cats: a clinical study

Twenty-five arthrodeses were performed in four cats and 17 dogs using synthetic hydroxyapatite as fresh autogenous graft cancellous bone substitute. Arthrodesis was performed in the carpal joint in eight cases, in the tarsal joint in 10, in the elbow joint in six, and in the knee joint in one case. The mean radiographic follow-up time was 30 days in one animal, 45 days in another animal and 60 days in the 19 remaining cases. Bone union was observed in 24 arthrodeses. Non-union of one elbow arthrodesis was due to failure of stabilization. Restoration of limb functionality was classified as good to excellent in 22 cases. Hydroxyapatite was able to promote bone growth and is suitable for using in routine surgical procedures for small animals.

Efficacy of silicated calcium phosphate graft in posterolateral lumbar fusion in sheep

BACKGROUND CONTEXT

Conditions requiring posterior lumbar spinal fusion remain a clinical challenge. Achieving arthrodesis using autogenous bone graft is inconsistent when rigid internal fixation such as transpedicular instrumentation is applied. Synthetic materials, particularly calcium phosphate-based ceramics, have shown promise for spine fusion applications, especially when combined with autograft. Silicate substitution has been shown to enhance the bioactivity of calcium phosphates and may obviate the need for autologous supplementation.

PURPOSE

Determine efficacy of silicated calcium phosphate (Si-CaP) compared with autograft to generate solid lumbar fusion.

STUDY DESIGN

Comparison of healing of instrumented posterolateral lumbar fusion in ewes at 2 and 6 months using Si-CaP or iliac crest autograft.

METHODS

Eighteen skeletally mature ewes underwent implantation of either autograft or Si-CaP in the space spanning the L4-L5 transverse process. In vivo quantitative computed tomography (CT) scans were made at 2-month intervals and after euthanasia. Harvested spine segments were radiographed and biomechanically tested in bending at 6 months. Histological assessments were made at 2 and 6 months.

RESULTS

Animals receiving Si-CaP graft were biomechanically and radiographically equivalent to those receiving autograft. Fusion mass density and volume were higher for the Si-CaP group throughout the healing period. Si-CaP regenerated normal bone tissue morphology, cellularity, and maturation with no inflammatory responses despite the fact that no autograft, bone marrow aspirate, or blood was mixed with the material. Histomorphometrically, fusion mass was higher for Si-CaP and bony bridging was equivalent when compared with autograft treatment.

CONCLUSIONS

Si-CaP was biomechanically, radiographically, and histologically equivalent to autograft in generating a solid, bony, intertransverse process fusion in an ovine model. Both treatment groups achieved 100% bridging fusion after 6 months of healing.

Correlative radiological, self-assessment and clinical analysis of evolution in instrumented dorsal and lateral fusion for degenerative lumbar spine disease. Autograft versus coralline hydroxyapatite

This prospective longitudinal randomized clinical and radiological study compared the evolution of instrumented posterolateral lumbar and lumbosacral fusion using either coralline hydroxyapatite (CH), or iliac bone graft (IBG) or both in three comparable groups, A, B and C, which included 19, 18 and 20 patients, respectively, who suffered from symptomatic degenerative lumbar spinal stenosis and underwent decompression and fusion. The patients were divided randomly according to the graft used and the side that it was applied. The spines of group A received autologous IBG bilaterally; group B, IBG on the left side and hydroxyapatite mixed with local bone and bone marrow on the right side; group C, hydroxyapatite mixed with local bone and bone marrow bilaterally. The age of the patients in the groups A, B and C was 61+/-11 years, 64+/-8 years and 58+/-8 years, respectively. The SF-36, Oswestry Disability Index (ODI), and Roland-Morris (R-M) surveys were used for subjective evaluation of the result of the surgery and the Visual Analogue Scale (VAS) for pain severity. Plain roentgenograms including anteroposterior, lateral and oblique views, and lateral plus frontal bending views of the instrumented spine and CT scan were used to evaluate the evolution of the posterolateral fusion in all groups and sides. Two independent senior orthopaedic radiologists were asked to evaluate first the evolution of the dorsolateral bony fusion 3-48 months postoperatively with the Christiansen's radiologic method, and secondly the hydroxyapatite resorption course in the spines of groups B and C. The diagnosis of solid spinal fusion was definitively confirmed with the addition of the bending views, CT scans and self-assessment scores. The intraobserver and interobserver agreement (r) for radiological fusion was 0.71 and 0.69, respectively, and 0.83 and 0.76 for evaluation of CH resorption. T(12)-S(1) lordosis and segmental angulation did not change postoperatively. There was no radiological evidence for non-union on the plain roentgenograms and CT scans. Radiological fusion was achieved 1 year postoperatively and was observed in all groups and vertebral segments. Six months postoperatively there was an obvious resorption of hydroxyapatite granules at the intertransverse intersegmental spaces in the right side of the spines of group B and both sides of group C. The resorption of hydroxyapatite was completed 1 year postoperatively. Bone bridging started in the third month postoperatively in all instrumented spines and all levels posteriorly as well as between the transverse processes in the spines of the group A and on the left side of the spines of group B where IBG was applied. SF-36, ODI, and R-M score improved postoperatively in a similar way in all groups. There was one pedicle screw breakage at the lowermost instrumented level in group A and two in group C without radiologically visible pseudarthrosis, which were considered as having non-union. Operative time and blood loss were less in the patients of group C, while donor site complaints were observed in the patients of the groups A and B only. This study showed that autologous IBG remains the "gold standard" for achieving solid posterior instrumented lumbar fusion, to which each new graft should be compared. The incorporation of coralline hydroxyapatite mixed with local bone and bone marrow needs adequate bleeding bone surface. Subsequently, hydroxyapatite was proven in this series to not be appropriate for intertransverse posterolateral fusion, because the host bone in this area is little. However, the use of hydroxyapatite over the decorticated laminae that represents a wide host area was followed by solid dorsal fusion within the expected time.

Anterior lumbar interbody fusion with carbon fiber cage loaded with bioceramics and platelet-rich plasma. An experimental study on pigs

Platelet-rich plasma (PRP) is an autogenous source of growth factor and has been shown to enhance bone healing both in clinical and experimental studies. PRP in combination with porous hydroxyapatite has been shown to increase the bone ingrowth in a bone chamber rat model. The present study investigated whether the combination of beta tricalcium phosphate (beta-TCP) and PRP may enhance spinal fusion in a controlled animal study. Ten Danish Landrace pigs were used as a spinal fusion model. Immediately prior to the surgery, 55 ml blood was collected from each pig for processing PRP. Three-level anterior lumbar interbody fusion was performed with carbon fiber cages and staples on each pig. Autogenous bone graft, beta-TCP, and beta-TCP loaded with PRP were randomly assigned to each level. Pigs were killed at the end of the third month. Fusion was evaluated by radiographs, CT scanning, and histomorphometric analysis. All ten pigs survived the surgery. Platelet concentration increased 4.4-fold after processing. Radiograph examination showed 70% (7/10) fusion rate in the autograft level. All the levels with beta-TCP+PRP showed partial fusion, while beta-TCP alone levels had six partial fusions and four non-fusions ( P=0.08). CT evaluation of fusion rate demonstrated fusion in 50% (5/10) of the autograft levels. Only partial fusion was seen at beta-TCP levels and beta-TCP+PRP levels. Histomorphometric evaluation found no difference between beta-TCP and beta-TCP+PRP levels on new bone volume, remaining beta-TCP particles, and bone marrow and fibrous tissue volume, while the same parameters differ significantly when compared with autogenous bone graft levels. We concluded from our results in pigs that the PRP of the concentration we used did not improve the bone-forming capacity of beta-TCP biomaterial in anterior spine fusion. Both beta-TCP and beta-TCP+PRP had poorer radiological and histological outcomes than that of autograft after 3 months.

Small-animal models for testing macroporous ceramic bone substitutes

The aim of this study was to compare the bone colonization of a macroporous biphasic calcium phosphate (MBCP) ceramic in different sites (femur, tibia, and calvaria) in two animal species (rats and rabbits). A critical size defect model was used in all cases with implantation for 21 days. Bone colonization in the empty and MBCP-filled defects was measured with the use of backscattered electron microscopy (BSEM). In the empty cavities, bone healing remained on the edges, and did not bridge the critical size defects. Bone growth was observed in all the implantation sites in rats (approximately 13.6-36.6% of the total defect area, with ceramic ranging from 46.1 to 51.9%). The bone colonization appeared statistically higher in the femur of rabbits (48.5%) than in the tibia (12.6%) and calvaria (22.9%) sites. This slightly higher degree of bone healing was related to differences in the bone architecture of the implantation sites. Concerning the comparison between animal species, bone colonization appeared greater in rabbits than in rats for the femoral site (48.5% vs. 29.6%). For the other two sites (the tibia and calvaria), there was no statistically significant difference. The increased bone ingrowth observed in rabbit femurs might be due to the large bone surface area in contact with the MBCP ceramics. The femoral epiphysis of rabbits is therefore a favorable model for testing the bone-bonding capacity of materials, but a comparison with other implantation sites is subject to bias. This study shows that well-conducted and fully validated models with the use of small animals are essential in the development of new bone substitutes.

Bone Tissue Engineering for Spine Fusion: An Experimental Study on Ectopic and Orthotopic Implants in Rats

Alternatives to the use of autologous bone as a bone graft in spine surgery are needed. The purpose of this study was to examine tissue-engineered bone constructs in comparison with control scaffolds without cells in a posterior spinal implantation model in rats. Syngeneic bone marrow cells were cultured in the presence of bone differentiation factors and seeded on porous hydroxyapatite particles. Seven rats underwent a posterior surgical approach, in which scaffolds with (five rats) or without cells (two rats) were placed on both sides of the lumbar spine. In addition, separate scaffolds were inserted intramuscularly and subcutaneously during the surgical procedure. After 4 weeks, all rats were killed and examined radiographically, by manual palpation of the excised spine and histologically for signs of bone formation or spine fusion. All rats that received cell-seeded scaffolds showed newly formed bone in all three locations, whereas none of the locations in the control rats showed bone formation. The results of this study support the concept of developing tissue-engineering techniques in posterior spine fusion as an alternative to autologous bone.

Recombinant human bone morphogenetic protein-4 (rhBMP-4) enhanced posterior spinal fusion without decortication

In posterior spinal fusion, insufficient decortication may decrease the number of bone marrow derived ostoprogenitor stem cells and affect the success of bony fusion. The finding of bone formation through interaction between rhBMP-4 and non-marrow derived mesenchymal cells constituted the basis of the current study. The aim is to investigate the possibility of molecular enhancement of posterior spinal fusion by site-specific application of rhBMP-4 with or without surgical decortication. Eighteen adult rabbits underwent single level bilateral posterior intertransverse process spinal fusion at L5-L6. one side with decortication, and the other side without decortication. Two animals underwent sham operation without bone grafts, the other 16 animals were randomly allocated into three groups, using hydroxyapatite-tricalcium phosphate (HA-TCP) ceramic blocks augmented with 0, 125 and 5 micromg [corrected] of rhBMP-4 respectively. Spinal fusion morphology was evaluated with sequential X-ray, microradiography and histomorphology. At week 7, complete bony fusion was achieved in none of the groups without rhBMP-4 irrespective of whether the bony contact surface was decorticated or not. In the groups with low dose rhBMP-4, complete fusion occurred in two of six un-decorticated sites (33%) and in three of six (50%) decorticated sites. 100% complete fusion was found in the high dose rhBMP-4 group independent of surgical decortication. The dorsal cortices of the un-decorticated transverse processes were replaced by newly formed trabecular bone through biological remodeling. This study suggested that rhBMP-4 can induce non-marrow derived mesenchymal cells to differentiate into osteogenic cells and thus enhance the high success rate of pesterior spinal fusion in both the decorticated and un-decorticated model.

In vivo evaluation of calcium sulfate as a bone graft substitute for lumbar spinal fusion

BACKGROUND CONTEXT

Posterolateral fusions of the lumbar spine have nonunion rates as high as 35%. The availability of autologous bone to promote fusion is limited, particularly for multilevel fusions. Bone substitutes have been proposed to augment or replace autologous bone for spinal surgery. Calcium sulfate offers high porosity, osteoconductivity, and high resorption rate. This material has been used successfully for treatment of long bone defects but has not been investigated as a bone graft substitute for spinal fusions.

PURPOSE

To determine whether the use of calcium sulfate granules in conjunction with an implantable electrical stimulator is a safe and effective means of attaining spinal fusion.

STUDY DESIGN/SETTING

A rabbit lumbar fusion model was used to assess a calcium sulfate bone graft substitute in combination with electrical stimulation for spinal fusion.

METHODS

Thirty-six adult New Zealand White female rabbits were divided into three groups. Each group underwent a single-level (L5-L6) fusion, receiving 3.0 cc calcium sulfate granules with bone marrow aspirate from the iliac crest. Group 1 had no electrical stimulator applied. Groups 2 and 3 received a 40-microA (Group 2) or a 100-microA (Group 3) implantable electrical stimulator. The animals were sacrificed at 8 weeks, and the rabbit spines were subjected to radiographic assessment, manual palpation, and mechanical testing.

RESULTS

Two rabbits died postoperatively. The radiographic assessment revealed no fusions occurred at the adjacent nonoperated control levels (L4-L5). There were no fusions observed within Group 1, containing the calcium sulfate and bone marrow aspirate alone. The sites with the implantable stimulators showed a dose-dependent increase in fusion stiffness. However, no fusion mass in Group 2 or 3 was graded as bilaterally complete.

CONCLUSION

This study found that calcium sulfate as a bone graft substitute was unsuccessful in promoting spine fusion in a rabbit model. There was radiographic evidence of rapid resorption of the calcium sulfate within 4 weeks after surgery. The use of electrical stimulation created a dose-dependent increase in mechanical competence of the bony mass. However, the addition of direct current (DC) current did not significantly alter fusion rates with calcium sulfate used as the bone graft substitute in this model.

The biology of posterolateral lumbar spinal fusion

This article reviews the existing knowledge base concerning the biology of spinal fusion, with the understanding that the focus is weighted toward posterolateral lumbar spinal fusion because of a relative paucity of biologic information on healing of other types of fusions. The discussion focuses first on the basic science of spinal fusion healing from the standpoint of animal modeling. Next, the discussion centers on the multitude of local factors that can affect fusion healing. Finally, the numerous systemic factors known to affect fusion healing are discussed.