Evaluation of HealosMP52 osteoinductive bone graft for instrumented lumbar intertransverse process fusion in sheep

Histopathological Evaluation of Orthopedic Medical Devices: The State-of-the-art in Animal Models, Imaging, and Histomorphometry Techniques

Orthopedic medical devices are continuously evolving for the latest clinical indications in craniomaxillofacial, spine, trauma, joint arthroplasty, sports medicine, and soft tissue regeneration fields, with a variety of materials from new metallic alloys and ceramics to composite polymers, bioresorbables, or surface-treated implants. There is great need for qualified medical device pathologists to evaluate these next generation biomaterials, with improved biocompatibility and bioactivity for orthopedic applications, and a broad range of knowledge is required to stay abreast of this ever-changing field. Orthopedic implants require specialized imaging and processing techniques to fully evaluate the bone-implant interface, and the pathologist plays an important role in determining the proper combination of histologic processing and staining for quality slide production based on research and development trials and validation. Additionally, histomorphometry is an essential part of the analysis to quantify tissue integration and residual biomaterials. In this article, an overview of orthopedic implants and animal models, as well as pertinent insights for tissue collection, imaging, processing, and slide generation will be provided with a special focus on histopathology and histomorphometry evaluation.

GDF5 significantly augments the bone formation induced by an injectable, PLGA fiber-reinforced, brushite-forming cement in a sheep defect model of lumbar osteopenia

BACKGROUND CONTEXT

Biodegradable calcium phosphate cement (CPC) represents a promising option for the surgical treatment of osteoporotic vertebral fractures. Because of augmented local bone catabolism, however, additional targeted delivery of bone morphogenetic proteins with the CPC may be needed to promote rapid and complete bone regeneration.

PURPOSE

In the present study, an injectable, poly(l-lactide-co-glycolide) acid (PLGA) fiber-reinforced, brushite-forming cement (CPC) containing the bone morphogenetic protein GDF5 was tested in a sheep lumbar osteopenia model.

STUDY DESIGN/SETTING

This is a prospective experimental animal study.

METHODS

Defined bone defects (diameter 5 mm) were placed in aged, osteopenic female sheep. Defects were treated with fiber-reinforced CPC alone (L4; CPC+fibers) or with CPC containing different dosages of GDF5 (L5; CPC+fibers+GDF5; 1, 5, 100, and 500 µg GDF5; n=5 or 6 each). The results were compared with those of untouched controls (L1). Three and 9 months postoperation, structural and functional effects of the CPC (±GDF5) were assessed ex vivo by measuring (1) bone mineral density (BMD); (2) bone structure, that is, bone volume/total volume (assessed by micro-computed tomography and histomorphometry), trabecular thickness, and trabecular number; (3) bone formation, that is, osteoid volume/bone volume, osteoid surface/bone surface, osteoid thickness, mineralized surface/bone surface, mineral apposition rate, and bone formation rate/bone surface; (4) bone resorption, that is, eroded surface/bone surface; and (5) compressive strength.

RESULTS

Compared with untouched controls (L1), both CPC+fibers (L4) and CPC+fibers+GDF5 (L5) numerically or significantly improved all parameters of bone formation, bone resorption, and bone structure. These significant effects were observed both at 3 and 9 months, but for some parameters they were less pronounced at 9 months. Compared with CPC without GDF5, additional significant effects of CPC with GDF5 were demonstrated for BMD and parameters of bone formation and structure (bone volume/total volume, trabecular thickness, and trabecular number, as well as mineralized surface/bone surface). The GDF5 effects were dose-dependent (predominantly in the 5-100 µg range) at 3 and 9 months.

CONCLUSIONS

GDF5 significantly enhanced the bone formation induced by a PLGA fiber-reinforced CPC in sheep lumbar osteopenia. The results indicated that a local dose as low as ≤100 µg GDF5 may be sufficient to augment middle to long-term bone formation. The novel CPC+GDF5 combination may thus qualify as an alternative to the bioinert, supraphysiologically stiff poly(methyl methacrylate) cement currently applied for vertebroplasty/kyphoplasty of osteoporotic vertebral fractures.

Radiographic, biomechanical, and histological evaluation of rhBMP-2 in a 3-level intertransverse process spine fusion: an ovine study

OBJECTIVE The objective of this study was to evaluate bone grafts consisting of rhBMP-2 on an absorbable collagen sponge with a ceramic composite bulking agent, rhBMP-2, directly on a ceramic-collagen sponge carrier or iliac crest bone graft (ICBG) in combination with local bone graft to effect fusion in a multisegmental instrumented ovine lumbar intertransverse process fusion model. METHODS Thirty-six sheep had a single treatment at 3 spinal levels in both the right and left intertransverse process spaces. Group 1 sheep were treated with 7.5 cm³ of autograft consisting of ICBG plus local bone for each intertransverse process space. For Groups 2-4, 4 cm³ of local bone was placed within the intertransverse process space followed by 4.5-5 cm³ of the rhBMP-2 graft material. Group 2 animals received 1.5 mg/cm³ rhBMP-2 on an absorbable collagen sponge with a commercial bone void filler consisting of Type I lyophilized collagen with a biphasic hydroxyapatite/β-tricalcium phosphate ceramic with local bone. Group 3 animals received 0.75 mg/m cm³ of rhBMP-2 on a collagen ceramic sponge carrier with local bone. Group 4 animals received 1.35 mg/cm³ of rhBMP-2 on the same collagen ceramic sponge carrier with local bone. Sheep were euthanized 6 months postoperatively. Manual palpation, biomechanical testing, CT, radiography, and undecalcified histology were performed to assess the presence of fusion associated with the treatments. RESULTS All animals in Groups 2-4 that received grafts containing rhBMP-2 achieved radiographic and CT fusion at all 3 levels. In Group 1 (bone autograft alone), only 19% of the levels demonstrated radiographic fusion, 14% resulted in possible radiographic fusion, and 67% of the levels demonstrated radiographic nonfusion. Biomechanical testing showed that Groups 2-4 demonstrated similar stiffness of the L2-5 segment in all 6 loading directions, with each of the 3 groups having significantly greater stiffness than the autograft-only group. In Group 1, only 2 of 18 levels were rated as achieving bilateral histological fusion, with an additional 3 levels showing a unilateral fusion. The majority of the treated levels (13/18) in Group 1 were scored as histological nonfusions. There were no histological nonfusions in Groups 2 through 4. All 18 levels in Group 2 were rated as bilateral histological fusions. A majority (34/36) of the levels in Group 3 were rated as bilateral histological fusions, with 2 levels showing a unilateral fusion. A majority (35/36) of the levels in Group 4 were rated as bilateral histological fusions, with 1 level showing a unilateral fusion. CONCLUSIONS In the ovine multilevel instrumented intertransverse process fusion model, rhBMP-2 was able to consistently achieve CT, radiographic, biomechanical, and histological fusion. Compared with ICBG, the gold standard for bone grafting, rhBMP-2 was statistically superior at achieving radiographic and histological fusion.

Ceramic granules enhanced with B2A peptide for lumbar interbody spine fusion: an experimental study using an instrumented model in sheep

OBJECT

New generations of devices for spinal interbody fusion are expected to arise from the combined use of bioactive peptides and porous implants. The purpose of this dose-ranging study was to evaluate the fusion characteristics of porous ceramic granules (CGs) coated with the bioactive peptide B2A2-K-NS (B2A) by using a model of instrumented lumbar interbody spinal fusion in sheep.

METHODS

Instrumented spinal arthrodesis was performed in 40 operative sites in 20 adult sheep. In each animal, posterior instrumentation (pedicle screw and rod) and a polyetheretherketone cage were placed in 2 single-level procedures (L2-3 and L4-5). All cages were packed with graft material prior to implantation. The graft materials were prepared by mixing (1:1 vol/vol) CGs with or without a B2A coating and morselized autograft. Ceramic granules were coated with B2A at 50, 100, 300, and 600 microg/ml granules (50-B2A/CG, 100-B2A/CG, 300-B2A/CG, and 600-B2A/CG, respectively), resulting in 4 B2A-coated groups plus a control group (uncoated CGs). Graft material from each of these groups was implanted in 8 operative sites. Four months after arthrodesis, interbody fusion status was assessed with CT, and the interbody site was further evaluated with quantitative histomorphometry.

RESULTS

All B2A/CG groups had higher CT-confirmed interbody fusion rates compared with those in controls (CGs only). Seven of 8 sites were fused in the 50-B2A/CG, 100-B2A/CG, and 300-B2A/CG groups, whereas 5 of 8 sites were fused in the group that had received uncoated CGs. New woven and lamellar bone spanned the fusion sites with excellent osseointegration. There was no heterotopic ossification or other untoward events attributed to the use of B2A/CG in any group. Each B2A/CG treatment produced more new bone than that in the CG group.

CONCLUSIONS

Bioactive treatment with B2A effectively enhanced the fusion capacity of porous CGs. These findings suggest that B2A/CG may well represent a new generation of biomaterials for lumbar interbody fusion and indicate that additional studies are warranted.

The effects of magnesium particles in posterolateral spinal fusion: an experimental in vivo study in a sheep model

Object

Magnesium has recently become a material of interest as a biocompatible and biodegradable implant metal. Authors of several reports have noted the potential bone-cell activating or bone-healing effect of high Mg ion concentrations. The classic method for achieving intertransverse process fusion involves using an autologous iliac crest bone graft. Several studies have been performed to investigate enhancement of this type of autograft fusion. To the authors’ knowledge, no research has been conducted in which the efficacy of pure Mg particles in posterolateral spinal fusion has been investigated. The objective of this study was to determine whether Mg particles enhance the effectiveness of intertransverse process lumbar fusion in a sheep model.

Methods

Sixteen skeletally mature female sheep were subjected to intertransverse process spinal fusions with pedicle screw fixation at L2–3 and L5–6. Each animal was given a 5-cm3 bone autograft at one fusion level, and a combined 5-cm3 bone autograft with the addition of 1 cm3 Mg at the other level. Six months after surgery, bone formation was evaluated by gross inspection and palpation, and by radiological, histological, scanning electron microscopic, and x-ray diffraction analyses. Radiological results were graded from 0 to 4 according to the status of the bridging bone, which was determined by evaluating both x-ray films and computed tomography scans. The quality of the spinal fusion was assigned a histological score of 0 to 7, in which a score of 0 represented an empty cleft and a score of 7 represented complete bridging of bone between the transverse processes. The trabecular bone formation at each fusion level and the Ca hydroxyapatite crystalline structure in core biopsy specimens were evaluated using scanning electron microscopy and x-ray diffraction analyses, respectively.

The rate of rigid bone fusion, according to both palpation and radiological assessment, in the combined Mg and autologous bone treatment group was higher (81.25%) than in the autograft bone treatment group (62.5%), but this difference was not statistically significant. The quality of bone fusion, according to the histological grading system and scanning electron microscopy inspection, was higher in the bone fusion segments of the Mg and autologous graft combined group than in the group with autograft-only arthrodesis, and this difference was statistically significant. The x-ray diffraction analyses further confirmed the effect of Mg in promoting the formation of the crystalline portion of the bone (hydroxyapatite).

Conclusions

Based on the results of this study, adding Mg particles to autologous corticocancellous bone in a posterolateral intertransverse process fusion enhances the quality of bone formation. However, radiological findings did not reveal a statistically significant effect of Mg on the rate of solid bone fusion formation between the two transverse processes.

Healos/recombinant human growth and differentiation factor-5 induces posterolateral lumbar fusion in a New Zealand white rabbit model

STUDY DESIGN

Posterolateral lumbar spine fusions in New Zealand white rabbits.

OBJECTIVE

To evaluate the efficacy of recombinant human growth and differentiation factor-5 (rhGDF-5) lyophilized to a Healos carrier (cross-linked type I collagen with hydroxyapatite coating; DePuy Spine, Inc., Raynham, MA) in inducing fusion.

SUMMARY OF BACKGROUND DATA

Bone graft substitutes have become an area of considerable interest. rhGDF-5 is one such product. Limited lumbar preclinical studies have been performed with this product.

METHODS

Single-level, intertransverse process fusions were performed in 67 rabbits using iliac crest autograft (n = 13), Healos alone (n = 13), or 0.5, 1.0, or 1.5 mg/cc rhGDF-5 lyophilized to Healos (n = 13 per group). At 8 weeks, the rabbits were euthanized. Fusion masses were assessed.

RESULTS

There were 2 animals (3%) lost to complication. Manual palpation revealed fusion rates for autograft of 38% (5/13), Healos alone of 0% (0/13), and each of the Healos/rhGDF-5 groups of 100% (13/13). Histologic analyses were 95% sensitive and 95% specific for confirming fusion. Histologic differences were found among the treatment groups.

CONCLUSIONS

In this rabbit fusion model, Healos/rhGDF-5 induced fusion in 100% of the rabbits studied. This rate was significantly higher than the fusion rate induced by autograft (38%). Overall, these results support continued research of Healos/rhGDF-5 as a potential bone graft alternative.

Surgical treatment of the painful motion segment: matching technology with indications

STUDY DESIGN

Literature review of current bone graft technology and bone graft substitutes used in spinal fusion applications.

OBJECTIVE

We reviewed current bone graft technologies and identified the range of bioactive properties that each possesses, such as osteoconductivity, osteoinductivity, and structural, load-bearing capabilities that can be used to either augment or replace autogenous bone graft in spinal fusions.

SUMMARY OF BACKGROUND DATA

Bioactive materials are used in spinal fusion applications to encourage bone formation across an intervertebral motion segment. To be an effective replacement for autogenous bone grafts, a bioactive material must possess the properties of osteoinduction and osteoconduction.

METHODS

Literature review.

RESULTS

Platelet gel concentrates deliver nonspecific cytokines that influence local cells at the implantation site. Demineralized bone matrix acts as a bone graft enhancer that excludes fibrous tissue of muscle interposition in a fusion mass and is a mildly osteoinductive material. Bone morphogenetic proteins are an integral part of natural bone formation response. They function as differentiation factors that act on mesenchymal stem cells to induce bone formation.

CONCLUSIONS

Patient-derived therapies such as platelet gel concentrates contain cytokines that play a role in bone formation; however, none of them is capable of inducing the entire bone formation cascade. Clinical use of these concentrates could possibly interfere with new bone formation. The use of bone marrow aspiration and concentration techniques has not been convincingly studied in spinal fusions in lower order animal or human clinical studies. Demineralized bone matrix contains small and variable amounts of naturally occurring bone morphogenetic proteins. These products can only function as bone graft extenders. Recombinant bone morphogenetic protein products contain much more highly concentrated and focused amounts of bone morphogenetic proteins, and some have been shown to be clinically effective bone graft replacements.