The effects of internal fixation on calcium carbonate. Ceramic anterior spinal fusion in dogs

Spinal fixation after laminectomy in pigs prevents postoperative spinal cord injury

Background

A safe, effective, and ethically sound animal model is essential for preclinical research to investigate spinal medical devices. We report the initial failure of a porcine spinal survival model and a potential solution by fixating the spine.

Methods

Eleven female Dutch Landrace pigs underwent a spinal lumbar interlaminar decompression with durotomy and were randomized for implantation of a medical device or control group. Magnetic resonance imaging (MRI) was performed before termination.

Results

Neurological deficits were observed in 6 out of the first 8 animals. Three of these animals were terminated prematurely because they reached the predefined humane endpoint. Spinal cord compression and myelopathy was observed on postoperative MRI imaging. We hypothesized postoperative spinal instability with epidural hematoma, inherent to the biology of the model, and subsequent spinal cord injury as a potential cause. In the subsequent 3 animals, we fixated the spine with Lubra plates. All these animals recovered without neurological deficits. The extent of spinal cord compression on MRI was variable across animals and did not seem to correspond well with neurological outcome.

Conclusion

This study shows that in a porcine in vivo model of interlaminar decompression and durotomy, fixation of the spine after lumbar interlaminar decompression is feasible and may improve neurological outcomes. Additional research is necessary to evaluate this hypothesis.

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.

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.

Animal models for spinal fusion

Animal models for spinal fusion are essential for preclinical testing of new fusion methods and adjuncts. They allow for control of individual variables and quantification of outcome measures. Model characteristics are considered. Preclinical experiments to evaluate proof of concept, feasibility, and efficacy are generally studied in an orderly progression from smaller to larger animal models with an evolving cascade of evidence which has become known as the "burden of proof". Methods of fusion analysis include manual palpation, radiographs, computed tomography, histology, biomechanical testing, and molecular analysis. Models which have been developed in specific species are reviewed. This sets the stage for the interpretation of studies evaluating bone graft materials such as allograft, demineralized bone matrices, bone morphogenetic proteins, ceramics, and others with consideration of the variables affecting their success. As evidence accumulates, clinical trials and applications are defined.

Primary stability of anterior lumbar stabilization: interdependence of implant type and endplate retention or removal

This is a comparative in vitro biomechanical study of the primary stability of an anterior lumbar interbody stabilization. The objective was to compare the stability of a interbody stabilizing titanium cage with and without the retention of the bordering vertebral endplates, as well as to compare the titanium cage with a tricalcium phosphate block when the endplates are removed. An adequate blood supply is critical for interbody fusion, which suggests surgical treatment of the bordering endplates. On the other hand, primary stability is improved by the retention of the endplates. Furthermore, bone substitute materials are finding more frequent use due to complications associated with autologous bone grafts. Ten bovine lumbar spine motion segments (average age 6 months) were investigated. Pure bending loadings as well as eccentric axial compression loadings were applied. A titanium cage and tricalcium phosphate block, were tested in conjunction with an anterior augmentation (MACS). Range of motion, neutral zone (NZ) and bending stiffness were measured under pure bending to 10 Nm, and bending stiffness under axial loads of up to 1,500 N. Range of motion of both implants in flexion-extension was significantly smaller than physiologic (cage without endplates 4.3 degrees , cage with 2.8 degrees , block without 3.4 degrees , and physiologic 6.6 degrees , all p<0.001). The cage with endplates and the block without endplates were both significantly stiffer than physiologic in all directions except left lateral bending. The block without endplates and the cage with endplates were both stiffer than the cage without endplates. The results suggest that the use of the bone substitute block provides better stability than the cage when the endplates are removed.

Pedicle screw fixation enhances anterior lumbar interbody fusion with porous tantalum cages: an experimental study in pigs

STUDY DESIGN

A porous tantalum implant (Hedrocel, Implex Corp., Allendale, NJ), designed to assist interbody lumbar fusion, was tested biologically in an experimental model. A total of 11 female Danish landrace pigs received 3 levels of anterior lumbar interbody fusion at L2-L3, L4-L5, and L6-L7. Each level was randomly assigned one of the procedures: (1) implantation of PT-ring with pedicle screw fixation (PSF), (2) implantation of a porous tantalum ring (PT-ring) stabilized anteriorly with staples, or (3) implantation of carbon fiber cage (CF-cage) stabilized anteriorly with 2 staples. Each implant was filled with autogenous iliac crest bone graft.

OBJECTIVES

To evaluate the effects of PSF on the incorporation of autologous bone in a porous tantalum interbody device and to compare healing in PT-rings to that in CF-cages of autologous bone.

SUMMARY OF BACKGROUND DATA

Despite the promising results that early clinical trials have shown, interbody fusion cage technology is still under debate because of uncertainties that include indications for surgery, criteria for fusion, material, cage design, cage subsidence, and the effect of immediate stabilization.

METHODS

Pigs were euthanized 6 months after surgery. Fusion segments were evaluated by plain radiography, conventional radiograph tomography, and histology.

RESULTS

Fusion segments with PSF had significantly fewer radiolucencies than the other 2 levels (P = 0.002). Improved interface healing and fusion rate were observed in PT-rings when supplemented with PSF (P = 0.03). His tomorphometric results showed that the percentage of bone and bone marrow space in the center of a PT-ring was not significantly different from that of its adjacent vertebral body, but PSF increased bone marrow and decreased fibrous tissue formation in a tantalum cage. However, a CF-cage had higher bone volume and lower bone marrow space inside the cage compared with its adjacent vertebral body (P < 0.001). Fibrous tissue formation inside and around a CF-cage was more than that of a PT-ring (P < 0.05).

CONCLUSION

Interbody fusion using a PT-ring cage packed with autologous bone achieved higher interface healing and more reliable fusion when fixated with supplementary pedicle screws than did fixated anteriorly with 2 staples. A lesser amount of bone graft was required, and bone remodeling was enhanced in the PT-ring when compared to the CF-cage.

An investigational study on the healing process of anterior spinal arthrodesis using a bioactive ceramic spacer and the change in load-sharing of spinal instrumentation

STUDY DESIGN

Ceramic anterior lumbar interbody arthrodesis was performed using an in vivo sheep model. Observations of fusion status and the load-sharing of spinal instrumentation were studied at sequential intervals for 1 year after surgery.

OBJECTIVES

To elucidate the healing process of spinal arthrodesis performed with a bioactive ceramic spacer and the change in load-sharing of anterior spinal instrumentation.

SUMMARY OF BACKGROUND DATA

With the improved development of spinal instrumentation, anterior spinal arthrodesis has become a standard spinal reconstruction technique; however, the mechanistic basis underlying the healing process is not well documented. Moreover, it remains unclear how load-distribution through the fusion mass and spinal instrumentation change throughout the healing process.

METHODS

Using 24 sheep, a two-level anterior lumbar interbody fusion (L2-L3, L4-L5) was performed using a smooth surface and a porous surface-modified bioactive ceramic, with each segment instrumented using a one-rod anterior spinal instrumentation system. Four animals each were killed at 2, 4, 8, 12, 24, and 52 weeks after surgery. Postmortem analysis included quantification of anterior rod strain under multidirectional flexibility testing and radiographic and histologic analyses of the arthrodesed segments.

RESULTS

From 0 to 8 weeks after surgery, the bending strain of the rod gradually decreased despite no obvious bone formation. From 8 to 24 weeks after surgery, the rod strain markedly decreased with the development of bridging trabeculated bone formation between vertebral bodies. After 24 weeks after surgery, minimal changes were observed in rod strain; however, the fusion mass volumetrically increased with corresponding facet joint atrophy. The porous surface-modification of ceramic did not influence the histologic healing process, despite the improvement of interface osseous union rate.

CONCLUSIONS

In anterior spinal arthrodesis, spinal instrumentation is mainly exposed to bending stress, with decreased load-sharing with corresponding development of the spinal fusion. Continuous bone remodeling of the anterior fusion mass results in concurrent decreases in spinal instrumentation and posterior spinal element load-transmission. The principal healing mechanism of ceramic anterior interbody spinal fusion is not an osseous union between the ceramic and vertebral body, but bridging bone formation around the ceramic, which directly connects the vertebral bodies above and below the disc.

Coralline hydroxyapatite reinforced with polylactide fibres in lumbar interbody implantation

Porous hydroxyapatite HA blocks reinforced with poly-l/dl-lactide fibres were used to maintain the lumbar disc space and to start to create intercorporeal fusion in 23 growing pigs. In four pigs two emptied non adjacent disc spaces were left open. After 3, 6, 12 and 16 weeks the implanted disc blocks were studied radiologically, histologically, histomorphometrically, microradiographically, and with oxytetracycline fluorescence. In plain films slight to moderate ossification of the implanted disc spaces was detected at 12 and 16 weeks. Resorption of the implants was seen radiologically from 3 weeks and fragmentation from 12 weeks onwards. In microradiographs disintegration of the coralline inner structure started at 3 weeks. Histologically, connective tissue ingrowth was seen inside the porous structure from three weeks onwards. Small amounts of new bone were visible and connective tissue inside the implant increased from a mean of 65.6% at 3 weeks to a mean of 79.4% at 16 weeks histomorphometrically. The bone ingrowth varied from 0.7 to 1.7%. A loss of height in the implanted disc spaces was seen (p < 0.05, linear regression analysis). In control pigs the emptied disc spaces lost their height similarly. The implants used were not strong enough to maintain the lumbar disc height.

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.

Morphometry of the thoracic spine in German shepherd dog: a computed tomographic study

Computed tomographic images of the thoracic spine of 13 German shepherd dogs were examined in order to determine the thoracic spine morphometry. Examinations were carried out in the transverse plane both intervertebral and mid-vertebral levels of the each thoracic vertebrae. The dorsoventral and interpedicular diameters of the spinal canal, the dorsoventral and transverse diameters of the vertebral body, the dorsoventral and transverse diameters of the spinal cord and also the cross-section area of the spinal canal were measured. The maximum values were found to be at the level of C7-T1. The shapes of the spinal canal and cord were circular in middle part, the shape became transverse oval in the cranial and caudal parts of the thoracic spine. The most significant correlation between the diameters was found to be in male dogs, except between dorsoventral diameters of the spinal canal and that of the vertebral body and between dorsoventral diameters of the spinal canal and transverse diameters of the vertebral body.

Successful transpedicular lumbar interbody fusion by means of a composite of osteogenic protein-1 (rhBMP-7) and hydroxyapatite carrier: a comparison with autograft and hydroxyapatite in the sheep spine

STUDY DESIGN

Transpedicular lumbar interbody fusion (TLIF) was performed in a sheep model comparing three treatment groups: a composite of osteogenic protein (OP)-1 and hydroxyapatite carrier (HA), HA without OP-1, and autograft.

OBJECTIVE

To evaluate the efficacy of the composite of OP-1 and HA (HA-OP-1) in achieving reliable TLIF.

SUMMARY OF BACKGROUND DATA

Anterior fusion techniques directly address disc-related problems and achieve primary axial stability. However, they are characterized by high morbidity. Alternatively, the theoretically advantageous posterior TLIF technique using autograft fails clinically because it lacks compressive stability.

METHODS

In 36 sheep, lumbar vertebrae L4 to L6 were instrumented posteriorly. Endoscopically assisted TLIF of L4 to L5 was performed. In 12 sheep, the defect was filled with injectable HA-OP-1. Another 12 sheep were treated with HA and another 12 with autograft. Animals were killed at 8 weeks and evaluated by radiologic, histologic, and histomorphometric analysis and by fluorochrome labeling.

RESULTS

Only 10 autograft sheep were available for evaluation. Radiologically and histologically, TLIF with HA-OP-1 led to a fusion rate of 10 in 12 compared with autograft (one in 10 fused) and HA (two in 12 fused) ( = 0.0016). Semiquantitative radiologic and histologic scoring also revealed significant differences with superiority of HA-OP-1 ( = 0.0011). Compared with HA, HA-OP-1 presented significantly more ossification at the bone-cement interface ( = 0.0003) and less cement resorption ( = 0.0209). In four of 12 HA sheep, excessive resorption was responsible for local aseptic inflammation.

CONCLUSIONS

Biointegration of the osteoconductive HA does not occur, because shear forces cause early HA fracture, subsequent fragmentation, and gross resorption (initiating severe inflammation in four of 12 sheep). In contrast, osteoinductive effects of HA-OP-1 enable bio-integration, resulting in full osseous composite sheathing and solid fusion. By use of this composite, TLIF is successfully applied in sheep. Harvesting autograft and the anterior approach are avoided.

Clinical applications of bone graft substitutes

Autogenous bone grafting remains the gold standard for osseous reconstruction in clinical practice. It is associated with several limitations. The search for an alternative bone graft substitute with combined osteoinductive, osteoconductive, and osteogenic properties continues. This article highlights the properties of the various bone grafting materials currently available and discusses their efficacy in clinical practice.

Posterolateral and anterior interbody spinal fusion models in the sheep

Posterolateral and anterior interbody spinal arthrodesis is a frequent procedure, but high nonunion rates are reported and harvesting autologous bone graft from the iliac crest significantly increases morbidity. Bone graft substitutes are an alternative, but to date clinical results are not conclusive. Bone substitutes can be organic or inorganic, biologic or synthetic. They can have osteoconductive properties, inductive properties or both. Animal experiments are essential to investigating bone substitutes using biomechanical and histologic methods not available in clinical studies. Few authors reported on instrumented anterior fusion models, but none used the sheep model. In the current study posterolateral and anterior interbody fusion models in sheep are described. Both models used instrumented fusions, applying porous mineral scaffolds, alone or mixed with bone. The surgical techniques are described step-by-step and potential difficulties are highlighted. Preliminary results are reported for the posterolateral fusion model using coralline graft substitutes. The coral granules mixed with locally harvested bone had fusion outcomes similar to pure autologous bone. The graft substitute showed marked resorption between 12 and 20 weeks. All fusions had bone cortex and good trabecular connectivity. Histologic evaluation suggests after 20 weeks nearly the entire surface of the substitute is covered with new bone. Porous mineral bone substitutes mixed with locally harvested autologous bone are thought to be a valid alternative for posterolateral fusions.

Biomechanical issues in bone transplantation

Because the biomechanical competence of the graft is a central issue, many research studies of bone grafts include a biomechanical component. The biomechanical evaluation serves as a bottom-line measure of the experimental outcome in which some measure of mechanical performance is compared among treatment groups. This article considers the biomechanics of grafts in the context of this experimental work and focuses on three issues: (1) the interplay between biology and biomechanics of grafts, (2) the effects of treatment choices on biomechanical properties, and (3) model factors that may influence biomechanical performance.

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.

Biologic enhancement of spinal fusion

Scientific advances in the past decade have generated considerable clinical interest in developing biologic tools that may ultimately enhance spinal fusion. This article reviews the current understanding of each of these and other fusion-enhancing tools with particular attention to the results of in vivo animal experimentation and, where available, objective clinical data.

Ceramic anterior spinal fusion. Biologic and biomechanical comparison in a canine model

STUDY DESIGN

Three types of porous ceramic bone graft substitutes were used for anterior interbody fusion in the canine thoracic spine.

OBJECTIVES

To compare the biomechanical stiffness and histologic appearance of fused spinal segments using ceramic graft substitutes versus autogenous bone graft.

SUMMARY OF BACKGROUND DATA

The relative success or failure of ceramic grafts is influenced by many variables, including the composition of the ceramic, location in the spine, stability, and the animal model used.

METHODS

Four experimental groups were evaluated: autogenous tricortical iliac crest (n = 6); hydroxyapatite ceramic (Interpore-200; n = 6); biphasic (60 : 40) hydroxyapatite/tricalcium phosphate ceramic (Zimmer; n = 4); and calcium carbonate ceramic (Inoteb; n = 4). All dogs were killed 8 weeks after surgery. After postmortem removal of anterior spinal instrumentation, the spinal segments underwent nondestructive biomechanical testing and light microscopic histologic evaluation.

RESULTS

Biomechanical testing showed that spines from the autogenous tricortical iliac crest group were statistically significantly stiffer in flexion, extension, left and right bending, and torsion than all ceramic groups. No differences in stiffnesses were observed among the three ceramic groups. Histologically, the autogenous tricortical iliac crest graft performed best, with osseous union at 10 of 12 interfaces. Of the ceramic grafts, hydroxyapatite/tricalcium phosphate and calcium carbonate demonstrated more consistent junction healing than the hydroxyapatite group, where four of 12 interfaces resulted in a nonunion. In the ceramic groups, a variable amount of revascularization and new bone was observed within the grafts.

CONCLUSIONS

Autogenous iliac crest bone graft provides superior healing in this anterior spine fusion model. Additional investigation is needed before ceramic grafts can be considered satisfactory alternatives to anterior autogenous bone grafts.