Enhanced bioactivity of a poly(propylene fumarate) bone graft substitute by augmentation with nano-hydroxyapatite

The bioactivity of a nano-hydroxyapatite-augmented, bioresorbable bone graft substitute made from the unsaturated polyester, poly(propylene fumarate), was analyzed by evaluating biocompatibility and osteointegration of implants placed into a rat tibial defect. Three groups of eight animals each were evaluated by grouting bone graft substitutes into 3-mm holes that were made into the anteromedial tibial metaphysis of rats. Thus, a total of 24 animals was included in this study. Two different formulations varying as to the type of hydroxyapatite were used: Group 1 - nano-hydroxyapatite, Group 2 - micron-hydroxyapatite, with a Group 3 control defect remaining unfilled. Animals of each of the three groups were sacrificed in groups of eight at postoperative week three. Histologic analysis revealed best superior biocompatibility and osteointegration of bone graft substitutes when nanohydroxyapatite was employed. At three weeks, there was more reactive new bone formation in this group when compared to the micron-hydroxyapatite group. The control group showed incomplete closure of the defect. This study suggested that nano-hydroxyapatite may improve upon the bioactivity of bone implant and repair materials. The model scaffold used in this study, poly(propylene fumarate), appeared to provide an osteoconductive pathway by which bone will grow in faster. Clinical implications of the use potential advantages of nano-hydroxyapatite on bone repair and orthopaedic implant design are discussed.     

Bioresorbable bone graft substitutes of different osteoconductivities: a histologic evaluation of osteointegration of poly(propylene glycol-co-fumaric acid)-based cement implants in rats

Bioresorbable bone graft substitutes may significantly reduce the disadvantages associated with autografts, allografts and other synthetic materials currently used in bone graft procedures. We investigated the biocompatibility and osteointegration of a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene-glycol-co-fumaric acid), or simply poly(propylene fumarate), PPF, which is crosslinked in the presence of soluble and insoluble calcium filler salts. Four sets of animals each having three groups of 8 were evaluated by grouting bone graft substitutes of varying compositions into 3-mm holes that were made into the anteromedial tibial metaphysis of rats. Four different formulations varying as to the type of soluble salt filler employed were used: set 1--calcium acetate, set 2--calcium gluconate, set 3--calcium propionate, and set 4--control with hydroxapatite, HA, only. Animals of each of the three sets were sacrificed in groups of 8 at postoperative week 1, 3, and 7. Histologic analysis revealed that in vivo biocompatibility and osteointegration of bone graft substitutes was optimal when calcium acetate was employed as a soluble salt filler. Other formulations demonstrated implant surface erosion and disintegration which was ultimately accompanied by an inflammatory response. This study suggested that PPF-based bone graft substitutes can be designed to provide an osteoconductive pathway by which bone will grow in faster because of its capacity to develop controlled porosities in vivo. Immediate applicability of this bone graft substitute, the porosity of which can be tailored for the reconstruction of defects of varying size and quality of the recipient bed, is to defects caused by surgical debridement of infections, previous surgery, tumor removal, trauma, implant revisions and joint fusion. Clinical implications of the relation between developing porosity, resulting osteoconduction, and bone repair in vivo are discussed.     

Natural coral exoskeleton as a bone graft substitute: a review

Natural coral graft substitutes are derived from the exoskeleton of marine madreporic corals. Researchers first started evaluating corals as potential bone graft substitutes in the early 1970s in animals and in 1979 in humans. The structure of the commonly used coral, Porites, is similar to that of cancellous bone and its initial mechanical properties resemble those of bone. The exoskeleton of these high content calcium carbonate scaffolds has since been shown to be biocompatible, osteoconductive, and biodegradable at variable rates depending on the exoskeleton porosity, the implantation site and the species. Although not osteoinductive or osteogenic, coral grafts act as an adequate carrier for growth factors and allow cell attachment, growth, spreading and differentiation. When applied appropriately and when selected to match the resorption rate with the bone formation rate of the implantation site, natural coral exoskeletons have been found to be impressive bone graft substitutes. The purpose of this article is to review and summarize all the pertinent work that has been published on natural coral as a bone graft including in vitro, animal and clinical human studies. Preliminary report of our own experiments as well as our recommendations on the use of coral are also included.     

Improved osteoconduction of cortical bone grafts by biodegradable foam coating

Alteration of the geometrical surface configuration of cortical bone allografts may improve incorporation into host bone. A porous biodegradable coating that would maintain immediate structural recovery and subsequently allow normal graft healing and remodeling by promoting bony ingrowth could provide an osteoconductive surface scaffold. We investigated the feasibility of augmenting cortical bone grafts with osteoconductive biodegradable polymeric scaffold coatings. Three types of bone grafts were prepared: Type I--cortical bone without coating (control), Type II--cortical bone coated with PLGA-foam, Type III--cortical bone coated with PPF-foam. The grafts were implanted into the rat tibial metaphysis (16 animals for each type of bone graft). Post-operatively the animals were sacrificed at 2 weeks and 4 weeks (8 animals for each type of bone graft at each time point). Histologic and histomorphometric analysis of grafts showed that the amount of new bone forming around the foam-coated grafts was significantly higher than in the control group (uncoated; p < 0.02). Although both foam formulations were initially equally osteoconductive, PLGA-based foam coatings appeared to have degraded at two weeks postoperatively, whereas PPF-based foam coatings were still present at 4 weeks postoperatively. While significant resorption was present in control allografts with little accompanying reactive new bone formation, PLGA-coated bone grafts showed evidence of bone resorption and subsequent bony ingrowth earlier than those coated with PPF-based foams suggesting that PPF-coated cortical bone grafts were longer protected against host reactions resulting in bone resorption.     

The use of coral as a bone graft substitute

Experiments have been performed to investigate the use of coral skeletons as bone graft substitutes. Skeletal fragments of different coral genera were implanted into cortical and spongy bone defects and used to bridge transcortical resections in the femur. The implant site was monitored for up to 18 months. Radiographically, both cortical and spongy bone defects were at least partially filled by new bone after 8 weeks while the implants underwent continuous resorption. Coral resorption and replacement by new tissue was also observed in the transcortical resections. The process of resorption was attributed to the enzymatic attack, especially carboanhydrase. This was confirmed by experiments in which dogs were implanted with coral in transcortical resections and treated daily with acetazolamide, a carboanhydrase inhibitor; the absorption appeared delayed and the resections failed to heal.     

Art of replacing craniofacial bone defects

In the history of medicine, many surgeons have been tried to reconstruct lost tissue and correct deformity, attempts to use implant materials have probably paralleled those involving autogenous tissue. Recently there has been an acceleration in the understanding of the requirements and potentials of implant materials caused by collaboration between material scientists, biomaterials engineers, clinicians, and clinical investigators. Alloplastic materials have become an essential part of reconstructing the function and contour of the craniofacial skeleton. Bone is a specialized form of connective tissue, which provides support, and protects vital and delicate organs. Bone is embryologically derived from mesenchymal tissue through membranous and endochondral ossification. In the clinical field, the need for bone graft has been increased due to trauma, tumor, craniosynostosis, and pure esthetic bone surgery. Various types of bone grafts have been used to repair craniofacial bone defects over many years, but the autogenous graft has many disadvantages, such as, limited donor sites, donor morbidity, pain, growth deformity and resorption. Many surgeons working in a number of centers around the world have created substitutes and simpler methods for bone replacement. As the alloplatic bone substitute has been advanced, many synthetic substitutes are replaced by bone in vivo over time. The ideal material should be cost effective, non-toxic, non-antigenic, non-carcinogenic, and inert in the body fluids, be easily shaped at the operating table, and maintain its desired form and consistency in situ. This article reviews several of the more commonly used materials for craniofacial reconstruction and summarizes their mechanical properties and clinical aspects.     

Efficacité et performance des substituts osseux pour remplacer les allogreffes et autogreffes

Although bone tissue possesses the capacity for regenerative growth, the bone repair process is impaired in many clinical and pathological situations. For example massive bone loss caused by trauma and tumor resection as well as deformities require reconstructive surgery. In this context, there was a critical need to develop implant technologies to promote bone healing. Cortical and cancellous bone grafts are the materials of choice for bone filling or reconstruction, but their clinical use involves some difficulties. Septic complications, viral transmission and unavailability of native bone have therefore led to the development of synthetic bone substitutes. Allograft bone, or tissue harvested from a cadaver, while more readily available, may carry with it the risk of disease transmission and is also difficult to shape [1–3]. A significant additional limitation of allograft bone is the delayed remodeling by the host. In the case of very large defects, the allograft may remain in the implant site throughout the patient's life, creating an area more prone to fracture or infection. The development of calcium phosphate ceramics and other related biomaterials for bone graft involved a better control of the process of biomaterials resorption and bone substitution. Synthetic bone graft materials available as alternatives to autogeneous bone for repair, substitution or augmentation, in particular synthetic biomaterials include, special glass ceramics described as bioactive glasses; calcium phosphates (calcium hydroxyapatite, HA; tricalcium phosphate, TCP; and biphasic calcium phosphate, BCP). These materials differ in composition and physical properties from each other and from bone; and must be take in consideration for more efficient bone ingrowth at the expense of the biomaterials and to adapt to new development of dedicated biomaterials. In the last decade synthetic calcium phosphate materials, principally calcium hydroxyapatite (HA) ceramics, was commercially used. However the concept of bioactivity (release of ions of biological interest) well described for glass ceramic was not particularly take in account for HA and other related biomaterials(ACP Amorphous Calcium Phosphate, CdA Calcium Phosphate deficient Apatite). HA until recently was considered to be non able to be resorbed. Calcium phosphate biomaterials differ in their solubility or extent of dissolution: ACP > > α-TCP > > β-TCP > CdA > > ACP. These ceramics are osteoconductive (act as a support for new bone formation requiring the presence of porosity) and able to be resorbed (degradable through chemical and cellular processes). They are also biocompatible (do not induce adverse local tissue reaction, immunogenicity or systematically toxicity); and more recently, some papers report osteoinductive properties associated to the chemical nature (biphasic Ca P) and the microstruture. Past decade, these bioceramics have been marketed and approved for use in humans as bone substitutes. Various presentations are currently used in orthopaedic and maxillo-facial surgery such as wedges, blocks or granules. Owing to their bone substitution properties, CaP ceramics have naturally been considered as a potential matrix for tissue engineering and the development of a bioactive drug delivery system (DDS) in bone sites. The paper presents the current knowledge on Calcium phosphate bioceramics, Bone tissue engineering and Calcium Phosphate Drug Delivery.     

The effect of sterilization methods on the osteoconductivity of allograft bone in a critical-sized bilateral tibial defect model in rabbits

Clinically, allogeneic bone graft is used extensively because it avoids the donor site morbidity associated with autograft. However, there are concerns over the optimal sterilization method to eliminate immunological risks whilst maintaining the biological efficacy of the graft. This study compared the effect of Supercritical fluid (SCF) treatment and gamma irradiation at 25 kGy on the osteoconductivity of allograft bone in a bilateral critical sized defect rabbit model. Osteoconductivity was evaluated at 2 and 4 weeks using X-ray, CT, histology (qualitative and quantitative) and immunohistochemistry (Alkaline Phosphatase and Cathepsin-K). Both grafts were well tolerated and osteoconductive. At 2 weeks, there was decreased bone volume and density in the gamma irradiated graft compared to the SCF treated graft, corresponding with a greater inflammatory response histologically and increased Cathepsin-K expression. Catabolic activity predominated at 4 weeks, with both grafts undergoing significant resorption and remodeling inside the defect. Alkaline Phosphatase expression was greater in the SCF group at both time points indicative of a more anabolic response. Allograft bone sterilized with either gamma irradiation or SCF treatment was osteoconductive and capable of healing a critical sized tibial defect in a rabbit. Gamma irradiated allografts elicited an acute inflammatory reaction when implanted which may increase the amount of graft resorption compared to the SCF treated bone.     

Developing porosity of poly(propylene glycol-co-fumaric acid) bone graft substitutes and the effect on osteointegration: a preliminary histology study in rats

Bioresorbable bone graft substitutes could eliminate disadvantages associated with the use of autografts, allografts and other synthetic materials. We investigated a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene fumarate) which is crosslinked in the presence of soluble and insoluble calcium filler salts. This compact bone graft substitute material develops porosity in vivo by leaching of the soluble filler salts. In attempt to develop materials whose in vivo porosity can be designed such that implant degradation would occur at a rate that remains supportive of the overall structural integrity of the repairing defect site, we studied the early tissue response upon implantation in a bony defect. Three grout formulations of varying solubilities using slightly soluble hydroxyapatite (HA) and soluble calcium acetate (CA) were evaluated in 3 mm holes made in the anteromedial tibial metaphysis of 200 g Sprague Dawley rats (n = 16 per formulation for a total of 48 animals). Grout formulations cured in situ. Animals from each formulation were sacrificed in groups of 8 at 4 days and 3 weeks postoperatively. Histologic analysis of the healing process revealed improved in vivo osteointegration of bone graft substitutes when a higher loading of calcium acetate was employed. All formulations maintained implant integrity and did not provoke sustained inflammatory responses. This study suggested that the presence of a soluble salt permits in vivo development of porosity of a poly(propylene fumarate) based bone graft substitute material.     

Porous poly(propylene fumarate) foam coating of orthotopic cortical bone grafts for improved osteoconduction

A porous biodegradable scaffold coating for perforated and demineralized cortical bone allografts could maintain immediate structural recovery and subsequently allow normal healing and remodeling by promoting bony ingrowth and avoiding accelerated graft resorption. This new type of osteoconductive surface modification should improve allograft incorporation by promoting new bone growth throughout the biodegradable scaffold, hence encasing the graft with the recipient's own bone. We investigated the feasibility of augmenting orthotopically transplanted cortical bone grafts with osteoconductive biodegradable polymeric scaffold coatings. Five types of bone grafts were prepared: type I, untreated fresh-frozen cortical bone grafts (negative control); type II, perforated and partially demineralized cortical bone grafts without additional coating (positive control); type III, perforated and partially demineralized cortical bone coated with a low-porosity poly(propylene fumarate) (PPF) foam; type IV, perforated and partially demineralized cortical bone coated with a medium-porosity PPF foam; and type V, perforated and partially demineralized cortical bone coated with a high-porosity PPF foam. Grafts were implanted into the rat tibial diaphysis. Fixation was achieved with an intramedullary threaded K-wire. Two sets of animals were operated on. Animals were killed in groups of eight with one set being killed 12 weeks, and the other 16 weeks, postoperatively. Radiographic, histologic, and histomorphometric analyses of grafts showed that the amount of new bone forming around the foam-coated grafts was significantly higher than that in the type I control group (uncoated) or that in type II group (perforated and partially demineralized cortical bone grafts). Although all foam formulations appeared initially equally osteoconductive, histologic evaluation of medium-porosity PPF foam-based coatings appeared to result in a sustained response 16 weeks postoperatively. Significant resorption was present in perforated and partially demineralized cortical bone graft allografts, with some accompanying new bone formation occurring primarily within the laser holes. Therefore, PPF foam-coated cortical bone grafts appeared to be better protected from excessive bone resorption, as frequently seen with invasion of fibrovascular tissue. Biomechanical analysis of the PPF foam-coated grafts corroborated findings of the morphometric analysis in that the failure strength at the allograft-host bone junction sites of all PPF-coated cortical bone grafts was higher than in the uncoated controls.     

The combined bone forming capacity of human periosteal derived cells and calcium phosphates

Current knowledge suggests that the periosteum, a fibrous tissue which covers the surface of all bones, contains a population of progenitor cells which mediate the repair of bone defects. In an effort to optimise the utilisation of this source of cells for bone engineering, herein we describe the rational selection of calcium phosphate (CaP) containing materials, based on biomaterial properties, and evaluation of their combined bone forming capacity. Five different commercially available orthopaedic 3D matrices composed of CaP particles in an open collagen network (NuOss?, CopiOs?, Bio-Oss(?), Collagraft? and Vitoss(?)) were evaluated in vitro and in vivo with human periosteal derived cells (hPDCs). It was found that the cell-material combinations behaved quite differently in vivo, despite apparent in vitro similarities in gene expression profiles. Bone formation was highest within the NuOss?/hPDC implant at 13.03%, which also contained the highest incidence of bone marrow formation. The bone formed in this implant was chimeric with approximately 65% originating from implanted cells. Upon analysis of human specific gene expression, although it was found that predominantly osteogenic differentiation was observed within NuOss?/hPDC implants, a lesser induction of chondrogenic genes was also observed. The formation of a cartilage intermediate was confirmed by histology. Additionally the NuOss?/hPDC implant integrated into the mouse environment with apparent active scaffold resorption. This study demonstrates the importance of matching a cell support/biological matrix with a cell type and subsequently has outlined parameters which can be used for the rational selection of biomaterials for bone engineering.     

Mathematical modelling of the distribution of newly formed bone in bone tissue engineering

New bone formation in bone substitutes is usually investigated by histomorphometric global analysis. This study provides a novel mathematical modelling approach of new bone formation in the use of osteoinductive and functionalized biomaterials for bone tissue engineering. We discuss here the repartition and the probability to get new bone formation inside Biphasic Calcium Phosphate (BCP) loaded with autologous osteogenic cells, functionalized with a cyclo RGD peptide, after implantation in rabbits for 2 and 4 weeks. This local analysis allowed us to complement classical global findings and to demonstrate that after 2 weeks of implantation, the probability of new bone formation was significantly higher in RGD-grafted BCP and that new formed bone was largely distributed from the edge to the centre of the implant. While no significant differences were obtained after 4 weeks of implantation between RGD-grafted and non-grafted materials, distribution of new bone formation inside RGD-grafted materials was significantly more homogeneous as demonstrated by our mathematical modelling approach. In conclusion, local analysis of new bone formation inside macroporous substitutes coupled with mathematical modelling constitutes a potential quantitative approach for the evaluation of the osteoconductive and osteoinductive characteristics of such biomaterials.     

Rabbit pilot study on the resorbability of three-dimensional bioactive glass fibre scaffolds

Bioactive glass composed of Na2O-K2O-MgO-CaO-B2O3-P2O5-SiO2 is used in this study to manufacture three-dimensional glass fibre scaffolds for a synthetic bone filler material for the treatment of bone defects. The glass is characterized by a large working range, which is the temperature interval at which forming of glass can take place. A preliminary in vivo study on New Zealand skeletally mature rabbit's tibia is reported here. Bone defects were prepared in the medial surfaces of the diaphyses of the tibia. For the first time melt derived three-dimensional bioactive glass fibre constructs were used to fill the cavities. The different implants investigated here were a scaffold with a porosity of 45-50%, scaffold with a porosity of 55-60% and morsels with a porosity of 55-60%. The implanted bone substitutes were dissected after 6 months and evaluated by histological and synchrotron radiation micro tomography analysis. PerioGlas and empty defects were used as positive and negative controls, respectively. The result was that the surgically created tibial defects were healed and new bone formation was found in the medullary cavities. Despite the intrinsic limitations of a pilot study, the preliminary results indicate that in 6 months the glass fibre scaffolds are completely resorbed and that the osteoconductive properties of the filling material are strictly correlated with the structural and morphological characteristics of the bone substitute.     

Bone in-growth induced by biphasic calcium phosphate ceramic in femoral defect of dogs

Biphasic calcium phosphate (BCP) ceramics consisting of hydroxyapatite (HA) and tricalcium phosphate (TCP) has been used as a bone graft material during the last decade. In this paper, we report the bone in-growth induced by BCP ceramic in the experimentally created circular defects in the femur of dogs. This BCP ceramic consists of 55% hydroxyapatite (HA) and 45% b-tricalcium phosphate (TCP) prepared in situ by the microwave method. The defects were created as 4-mm holes on the lateral aspect of the femur of dogs and the holes were packed with the implant material. The defective sites were radiographed at a period of 4, 8, and 12 weeks postoperatively. The radiographical results showed that the process of ossification started after 4 weeks and the defect was completely filled with new woven bone after 12 weeks. Histological examination of the tissue showed the formation of osteoblast inducing the osteogenesis in the defect. The collageneous fibrous matrix and the complete Haversian system were observed after 12 weeks. The blood serum was collected postoperatively and biochemical assays for alkaline phosphatase activity were carried out. The measurement of alkaline phosphatase activity levels also correlated with the formation of osteoblast-like cells. This microwave-prepared BCP ceramic has proved to be a good biocompatible implant as well as osteoconductive and osteoinductive materials to fill bone defects.     

Simultaneous in vivo comparison of bone substitutes in a guided bone regeneration model

A direct, simultaneous comparison of bone substitutes is hampered by the limited number of samples that can be tested simultaneously. The goal of this study was to establish a preclinical model for guided bone regeneration that offers testing of different bone substitutes in a one-wall defect situation. We show here that up to eight titanium hemispheres can be placed on the calvaria of minipigs. To establish our model, titanium hemispheres were filled with and without Bio-Oss, a deproteinized bovine bone mineral, Ostim, an aqueous paste of synthetic nanoparticular hydroxyapatite, and Osteoinductal, an oily calcium hydroxide suspension, before being positioned on the calvaria. After 6 and 12 weeks, titanium hemispheres were subjected to histological and histomorphometric analysis. We show here that bone filled approximately one-tenth of the area below the hemispheres which were left empty, indicating a critical size model for guided bone regeneration. In accordance with the documented osteoconductive properties of Bio-Oss and Ostim, titanium hemispheres were almost completely filled with bone. Moreover, the expected degradation profile of Bio-Oss and Ostim could be confirmed by histologic and histomorphometric analysis. Under the same conditions, Osteoinductal failed to exert osteoconductive properties, rather a progressive resorption of the host bone was observed. These results demonstrate that the preclinical model presented here is suitable to simultaneously compare bone substitutes with different material properties. Our model based on the titanium hemispheres allows evaluation of graft consolidation under standardized conditions thereby avoiding intra-individual variations.     

异种煅烧骨材料修复骨缺损：特点、优势与问题

文题释义： 拔牙位点保存：当由于各种原因拔除牙齿后,其周围的牙槽骨在前6个月将会发生水平向和垂直向上的严重吸收,导致种植手术时骨量不足,严重影响种植修复的成功率。许多学者建议在彻底拔除牙齿的同时,于拔牙窝内放置骨粉和骨膜进行保护和修复性干预,将会显著减少周围牙槽嵴生理或病理性的骨吸收、阻断周围龈缘和龈乳头的退缩,保存牙槽窝周围软硬组织的高度和形态,并改善新生软硬组织的质量,为后期的种植美学修复创造良好条件。 爬行替代：在具有良好血供的骨损伤区域植入骨移植材料后,将会激活骨再生程序：破骨细胞开始活动,并在植骨材料上进行骨吸收,形成一条条“沟”状缺损,引导骨原细胞进入,随之分化为成骨细胞,成骨细胞分泌大量的类骨质,钙化成骨基质,成熟为骨单位。即在骨移植材料逐渐吸收的部位新生骨组织逐渐形成,充填骨吸收的凹陷。 背景：牙种植区域的牙槽骨骨量不足增加了种植手术难度,植骨材料的出现成功解决了此类难题。在所有单一植骨材料中,异种煅烧骨被广泛用于治疗骨缺损,但单独使用煅烧骨时仍然存在一些缺点。 目的：结合最新的相关研究动态,对异种煅烧骨材料修复骨缺损的相关研究进展作以综述。 方法：第一作者以“calcined bone,xenogeneic bone,bone augmentation,bone substitute”为英文检索词,以“煅烧骨、异种骨、骨增量、骨替代材料”为中文检索词,应用计算机检索 PubMed、维普、万方、知网数据库中1986年1月至2019年11月已发表的相关文献,并进行筛选、归纳与总结,最终纳入69篇相关文献进行综述。 结果与结论：在所有骨移植材料中,异种煅烧骨的结构与人类骨骼相似,具有良好的互连孔性,便于血管因子和骨原细胞进入,为新骨形成提供了通道和支架作用,同时具有良好的生物相容性及一定的降解作用,受到临床医生们的青睐。但其本身仍然存在溶解度较低、吸收缓慢、机械性能较差等不足,这就促使研究者们研发出将煅烧骨与陶瓷、聚合物、骨诱导因子及金属离子等其他材料复合,发挥出所参与材料的优点,从而使复合材料成为骨增量材料的首选。 ORCID: 0000-0002-0313-7462(李芳) 中国组织工程研究杂志出版内容重点：生物材料;骨生物材料; 口腔生物材料; 纳米材料; 缓释材料; 材料相容性;组织工程     

Developing porosity of poly(propylene glycol-co-fumaric acid) bone graft substitutes and the effect on osteointegration: A preliminary histology study in rats

Abstract -Bioresorbable bone graft substitutes could eliminate disadvantages associated with the use of autografts, allografts and other synthetic materials. We investigated a bioresorbable bone graft substitute made from the unsaturated polyester poly(propylene fumarate) which is crosslinked in the presence of soluble and insoluble calcium filler salts. This compact bone graft substitute material develops porosity in vivo by leaching of the soluble filler salts. In attempt to develop materials whose in vivo porosity can be designed such that implant degradation would occur at a rate that remains supportive of the overall structural integrity of the repairing defect site, we studied the early tissue response upon implantation in a bony defect. Three grout formulations of varying solubilities using slightly soluble hydroxyapatite (HA) and soluble calcium acetate (CA) were evaluated in 3 mm holes made in the anteromedial tibial metaphysis of 200 g Sprague Dawley rats (n = 16 per formulation for a total of 48 animals). Grout formulations cured in situ. Animals from each formulation were sacrificed in groups of 8 at 4 days and 3 weeks postoperatively. Histologic analysis of the healing process revealed improved in vivo osteointegration of bone graft substitutes when a higher loading of calcium acetate was employed. All formulations maintained implant integrity and did not provoke sustained inflammatory responses. This study suggested that the presence of a soluble salt permits in vivo development of porosity of a poly(propylene fumarate) based bone graft substitute material.     

A comparison of ProOsteon, DBX, and collagraft in a rabbit model

Many bone graft substitutes (BGSs) have been developed and are commercially available. These products differ in the tailoring of their properties, including size, form, osteoconductivity, osteoinductivity, and resorption kinetics. Differential enhancement of these properties may optimize the performance of these materials for varying applications. BGSs offer an opportunity to lessen morbidity of harvesting and use of autogenous and/or allograft bone. The purpose of this study is to quantitatively compare the magnitude of bony ingrowth and biodegradation of different commercially available BGS materials in a rabbit femoral defect model. BGSs from each of three classes (ceramic (ProOsteon), demineralized bone matrix (DBX), and composite (Collagraft)) were implanted in cylindrical defects in bilateral femoral condyles of 12 adult New Zealand White rabbits. Each of the three BGS materials and the empty controls were compared. The specimens were harvested at 3 months postimplantation for radiographic and histologic evaluation. Histomorphometry yielded resorption of graft material remaining in the index defect. Magnitude of bony ingrowth was assessed based on an 8-bit 256 densitometry model. Histomorphometric analysis of the data demonstrated statistical differences in the resorption and magnitude of bony ingrowth of the three BGS materials. The three BGS were significantly different for ingrowth (p = 0.046) when using the Wilcoxon Test. The ceramic graft material averaged 47% bony ingrowth. Rabbit-based DBX material showed extensive osseous ingrowth (35%) and the composite graft material demonstrated significant bony ingrowth (56%). The control, as anticipated, showed the least amount of bony ingrowth (29%). Fisher's Exact Test yielded statistical differences (p = 0.0003) when comparisons for resorption were conducted. An ideal BGS material should be biocompatible, be able to withstand the local load environment for a given application, degrade in concert with bony replacement, and be both osteoinductive and osteoconductive. This in-vivo, head-to-head comparison of three commercially available BGS materials in an animal model compares these characteristics and demonstrates differences between them, which may act as a guide in the use of these products in human applications.     

Osteointegration of orthopaedic devices

The different properties of bone must be considered in order to understand the relation between orthopaedic devices and bone. The epi-/metaphyseal areas are defined by their rigidity, their high vascularity and their quick remodelling process. In contrast, the diaphyses of bone are rather elastic and built of dense, scarcely vascularised bone presenting slow remodelling. Implants can integrate by pure mechanical contact without real affinity to bone or, alternatively, they can favour ongrowth of bone, provided that they are osteoconductive. Amongst different bone substitutes, only some of them are absorbable. Only derivates of bone may present the property of osteoinduction, which is the power to create new bone in any region of the body. Orthopaedic devices are characterised by their shape, their stiffness or elasticity and by the characteristic properties of material. They may be osteoconductive such as titanium alloys and some ceramics, allowing integration in bone. Alternatively, other materials such as steel, CoCr alloys and PMMA cements remain separated from bone by a tiny layer of collagen. The surface structure influences the quality of integration. The integration of implants depends on the mutual interaction of the material with the tissue on the implantation site. All implants undergo fatiguing which can lead to fracture of the implant. All implant–bone contacts are threatened by granulation tissue mainly formed because of wear products, infection and other reasons.     

Extensive H(+) release by bone substitutes affects biocompatibility in vitro testing

Bone substitutes are widespread in orthopedic and trauma surgery to restore critical bony defects and/or promote local bone healing. Cell culture systems have been used for many years to screen biomaterials for their toxicity and biocompatibility. This study applies a human bone marrow cell culture system to evaluate the toxic in vitro effects of soluble components of different bone substitutes, which are already in clinical use. Different specimens of tricalcium phosphates (TCP) (Vitoss, Cerasorb), nondecalcified bovine bone (Lubboc), demineralized human bone matrices (DBM) (Grafton Flex/Putty), and collagen I/III matrix (ACI-Maix) were tested in Dulbecco's modified Eagle's medium (DMEM) and MesenCult culture solution and compared with a biomaterial-free cell culture. Biocompatibility parameters were cell viability evaluated by phase-contrast microscopy and laser flow cytometry, morphology, and the local H(+) release by bone substitutes. There were significant differences (p < 0.05) between the tested biomaterials and culture solutions. Collagen I/III, non-demineralized bovine bone, and TCP materials showed advantages for cell survival over other tested biomaterials (average values of vital cells/mL MesenCult/DMEM: Collagen I/III: 1090/1083; Vitoss: 893/483; Cerasorb: 471/523; Lubboc: 815/410; Grafton Putty: 61/44; Grafton Flex: 149/57). Especially the DBM materials lead to a significant decrease of pH, which is considered to be a major factor for cell death. DMEM culture solution supports cell survival for those bone substitutes that induce an alkaline reaction, whereas MesenCult media promotes cell vitality in biomaterials, which leads to an acidification of culture solution.