Regeneration of diaphyseal bone defects using resorbable poly(L/DL-lactide) and poly(D-lactide) membranes in the Yucatan pig model

Local dual delivery therapeutic strategies: Using biomaterials for advanced bone tissue regeneration

Bone development is a complex process involving a vast number of growth factors and chemical substances. These factors include transforming growth factor-beta, platelet-derived growth factor, insulin-like growth factor, and most importantly, the bone morphogenetic protein, which exhibits excellent therapeutic value in bone repair. However, the spatial-temporal relationship in the expression of these factors during bone formation makes the bone repair a more complicated process to address. Thus, using a single therapeutic agent to address bone formation does not seem to provide a clinically effective option. Conversely, a dual delivery approach facilitating the co-delivery of agents has proved to be a dynamic alternative since such a strategy can provide more efficient spatial-temporal action. Such delivery systems can smartly target more than one pathway or differentiation lineage and thus offer more efficient bone regeneration. This review discusses various dual delivery strategies reported in the literature employed to achieve improved bone regeneration. These include concurrent use of different therapeutic agents (including growth factors and drugs), enhancing bone formation and cell recruitment, and improving the efficiency of bone healing.

An overview of de novo bone generation in animal models

Some of the earliest success in de novo tissue generation was in bone tissue, and advances, facilitated by the use of endogenous and exogenous progenitor cells, continue unabated. The concept of one health promotes shared discoveries among medical disciplines to overcome health challenges that afflict numerous species. Carefully selected animal models are vital to development and translation of targeted therapies that improve the health and well-being of humans and animals alike. While inherent differences among species limit direct translation of scientific knowledge between them, rapid progress in ex vivo and in vivo de novo tissue generation is propelling revolutionary innovation to reality among all musculoskeletal specialties. This review contains a comparison of bone deposition among species and descriptions of animal models of bone restoration designed to replicate a multitude of bone injuries and pathology, including impaired osteogenic capacity.

Pre-Clinical Evaluation of Biological Bone Substitute Materials for Application in Highly Loaded Skeletal Sites

The development of bone substitute materials (BSMs) intended for load-bearing bone defects is highly complicated, as biological and mechanical requirements are often contradictory. In recent years, biological BSMs have been developed which allow for a more efficient integration of the material with the surrounding osseous environment and, hence, a higher mechanical stability of the treated defect. However, while these materials are promising, they are still far from ideal. Consequently, extensive preclinical experimentation is still required. The current review provides a comprehensive overview of biomechanical considerations relevant for the design of biological BSMs. Further, the preclinical evaluation of biological BSMs intended for application in highly loaded skeletal sites is discussed. The selected animal models and implantation site should mimic the pathophysiology and biomechanical loading patterns of human bone as closely as possible. In general, sheep are among the most frequently selected animal models for the evaluation of biomaterials intended for highly loaded skeletal sites. Regarding the anatomical sites, segmental bone defects created in the limbs and spinal column are suggested as the most suitable. Furthermore, the outcome measurements used to assess biological BSMs for regeneration of defects in heavily loaded bone should be relevant and straightforward. The quantitative evaluation of bone defect healing through ex vivo biomechanical tests is a valuable addition to conventional in vivo tests, as it determines the functional efficacy of BSM-induced bone healing. Finally, we conclude that further standardization of preclinical studies is essential for reliable evaluation of biological BSMs in highly loaded skeletal sites.

A preclinical large-animal model for the assessment of critical-size load-bearing bone defect reconstruction

Critical-size bone defects, which require large-volume tissue reconstruction, remain a clinical challenge. Bone engineering has the potential to provide new treatment concepts, yet clinical translation requires anatomically and physiologically relevant preclinical models. The ovine critical-size long-bone defect model has been validated in numerous studies as a preclinical tool for evaluating both conventional and novel bone-engineering concepts. With sufficient training and experience in large-animal studies, it is a technically feasible procedure with a high level of reproducibility when appropriate preoperative and postoperative management protocols are followed. The model can be established by following a procedure that includes the following stages: (i) preoperative planning and preparation, (ii) the surgical approach, (iii) postoperative management, and (iv) postmortem analysis. Using this model, full results for peer-reviewed publication can be attained within 2 years. In this protocol, we comprehensively describe how to establish proficiency using the preclinical model for the evaluation of a range of bone defect reconstruction options.

Effects of Polylactide Copolymer Implants and Platelet-Rich Plasma on Bone Regeneration within a Large Calvarial Defect in Sheep

The aim of this study was to verify whether L-lactide/DL-lactide copolymer 80/20 (PLDLLA) and platelet-rich plasma (PRP) trigger bone formation within critical-sized calvarial defects in adult sheep (n = 6). Two craniectomies, each ca. 3 cm in diameter, were created in each animal. The first craniectomy was protected with an inner polylactide membrane, filled with PRP-polylactide granules, and covered with outer polylactide membrane. The second control craniectomy was left untreated. The animals were euthanized at 6, 7, 17, 19, 33, and 34 weeks after surgery, and the quality and the rate of reossification were assessed histomorphometrically and microtomographically. The study demonstrated that application of implants made of PLDLLA 80/20 combined with an osteopromotive substance (e.g., PRP) may promote bone healing in large calvarial defect in sheep. These promising proof-of-concept studies need to be verified in the future on a larger cohort of animals and over a longer period of time in order to draw definitive conclusions.

A Resorbable Biomaterial Shaped as a Tubular Chamber and Containing Stem Cells: A Pilot Study on Artificial Bone Regeneration

In a previous study, we showed how healing of non-union defects in rabbit radii can be achieved by means of a tubular resorbable chamber, in comparison with untreated defects. In the present study, we placed bone marrow stem cells inside the chamber. Bone marrow was obtained by percutaneous aspiration from the iliac crest in 9 adult New Zealand rabbits. Stem cells were separated by the centrifugation technique. In the same animals, a defect of 10 mm was created in both radii. On the left side, the defect was treated with the poly-DL-Lactide chamber, in which a suspension of autologous cells was injected; on the right side, only autologous cells were used. Radiological and histomorphometric data were compared within this study as well as with the results of our previous study. At 3, 6 and 9 months, there was no healing on the right side. On the left side, progressive bone formation with reunion of the stumps was observed in the chamber. We conclude that stem cells can accelerate bone healing when contained in the tubular chamber.

* Engineering Membranes for Bone Regeneration

This review is focused on the use of membranes for the specific application of bone regeneration. The first section focuses on the relevance of membranes in this context and what are the specifications that they should possess to improve the regeneration of bone. Afterward, several techniques to engineer bone membranes by using "bulk"-like methods are discussed, where different parameters to induce bone formation are disclosed in a way to have desirable structural and functional properties. Subsequently, the production of nanostructured membranes using a bottom-up approach is discussed by highlighting the main advances in the field of bone regeneration. Primordial importance is given to the promotion of osteoconductive and osteoinductive capability during the membrane design. Whenever possible, the films prepared using different techniques are compared in terms of handability, bone guiding ability, osteoinductivity, adequate mechanical properties, or biodegradability. A last chapter contemplates membranes only composed by cells, disclosing their potential to regenerate bone.

Tubular open-porous β-tricalcium phosphate polycaprolactone scaffolds as guiding structure for segmental bone defect regeneration in a novel sheep model

Large segmental bone defect reconstruction with sufficient functional restoration is one of the most demanding challenges in orthopaedic surgery. Available regenerative treatment options, as the vascularized bone graft transfer, the Masquelet technique or the Ilizarov distraction osteogenesis, are associated with specific indications and distinct limitations. As an alternative, a hollow cylindrical ceramic-polymer composite scaffold (β-tricalcium phosphate and poly-lactid co-ε- caprolactone), facilitating a strong surface guiding effect for tissue ingrowth (group 1; n = 6) was investigated here. In combination with an additional autologous, cancellous bone graft filling, the scaffold's ability to work as an open-porous membrane to improve the defect healing process was analysed (group 2; n = 6). A novel model of a critical size (40 mm) tibia osteotomy defect stabilized with an external hybrid-ring fixator, was established in sheep. Segmental defect regeneration and tissue organization in relation to the scaffold were analysed radiologically, (immune-) histologically, and with second-harmonic generation imaging 12 weeks after surgery. The scaffold's tubular shape and open-porous structure controlled the collagen fibre orientation within the bone defect and guided the following mineralization process along the scaffold surface. In combination with the osteoinductive stimulus, a unilateral bony bridging of the critically sized defect was achieved in one third of the animals. The external hybrid-ring fixator was appropriate for large segmental defect stabilization in sheep.

Establishment of a Segmental Femoral Critical-size Defect Model in Mice Stabilized by Plate Osteosynthesis

The use of tissue-engineered bone constructs is an appealing strategy to overcome drawbacks of autografts for the treatment of massive bone defects. As a model organism, the mouse has already been widely used in bone-related research. Large diaphyseal bone defect models in mice, however, are sparse and often use bone fixation which fills the bone marrow cavity and does not provide optimal mechanical stability. The objectives of the current study were to develop a critical-size, segmental, femoral defect in nude mice. A 3.5-mm mid-diaphyseal femoral ostectomy (approximately 25% of the femur length) was performed using a dedicated jig, and was stabilized with an anterior located locking plate and 4 locking screws. The bone defect was subsequently either left empty or filled with a bone substitute (syngenic bone graft or coralline scaffold). Bone healing was monitored noninvasively using radiography and in vivo micro-computed-tomography and was subsequently assessed by ex vivo micro-computed-tomography and undecalcified histology after animal sacrifice, 10 weeks postoperatively. The recovery of all mice was excellent, a full-weight-bearing was observed within one day following the surgical procedure. Furthermore, stable bone fixation and consistent fixation of the implanted materials were achieved in all animals tested throughout the study. When the bone defects were left empty, non-union was consistently obtained. In contrast, when the bone defects were filled with syngenic bone grafts, bone union was always observed. When the bone defects were filled with coralline scaffolds, newly-formed bone was observed in the interface between bone resection edges and the scaffold, as well as within a short distance within the scaffold.

The present model describes a reproducible critical-size femoral defect stabilized by plate osteosynthesis with low morbidity in mice. The new load-bearing segmental bone defect model could be useful for studying the underlying mechanisms in bone regeneration pertinent to orthopaedic applications.

Influence of structural load-bearing scaffolds on mechanical load- and BMP-2-mediated bone regeneration

A common design constraint in functional tissue engineering is that scaffolds intended for use in load-bearing sites possess similar mechanical properties to the replaced tissue. Here, we tested the hypothesis that in vivo loading would enhance bone morphogenetic protein-2 (BMP-2)-mediated bone regeneration in the presence of a load-bearing PLDL scaffold, whose pores and central core were filled with BMP-2-releasing alginate hydrogel. First, we evaluated the effects of in vivo mechanical loading on bone regeneration in the structural scaffolds. Second, we compared scaffold-mediated bone regeneration, independent of mechanical loading, with alginate hydrogel constructs, without the structural scaffold, that have been shown previously to facilitate in vivo mechanical stimulation of bone formation. Contrary to our hypothesis, mechanical loading had no effect on bone formation, distribution, or biomechanical properties in structural scaffolds. Independent of loading, the structural scaffolds reduced bone formation compared to non-structural alginate, particularly in regions in which the scaffold was concentrated, resulting in impaired functional regeneration. This is attributable to a combination of stress shielding by the scaffold and inhibition of cellular infiltration and tissue ingrowth. Collectively, these data question the necessity of scaffold similarity to mature tissue at the time of implantation and emphasize development of an environment conducive to cellular activation of matrix production and ultimate functional regeneration.

Progressing innovation in biomaterials. From the bench to the bed of patients

A Translational Research Symposium was organized at the 2014 annual meeting of the European society for biomaterials. This brought together leading Tier one companies in clinical biomaterials and medical device markets, small and medium enterprises and entrepreneurial academics who shared their experiences on taking biomaterials technologies to commercial endpoints, in the clinics. The symposium focused on "Progressing Innovation in Biomaterials. From the Bench to the Bed of Patients". The aim of the present document is to illustrate the content of the symposium and to highlight the key lessons from selected lectures.

A resorbable antibiotic eluting bone void filler for periprosthetic joint infection prevention

Periprosthetic joint infection (PJI) following total knee arthroplasty is a globally increasing procedural complication. These infections are difficult to treat and typically require revision surgery. Antibiotic-loaded bone cement is frequently utilized to deliver antibiotics to the site of infection; however, bone cement is a nondegrading foreign body and known to leach its antibiotic load, after an initial burst release, at subtherapeutic concentrations for months. This work characterized a resorbable, antibiotic-eluting bone void filler designed to restore bone volume and prevent PJI. Three device formulations were fabricated, consisting of different combinations of synthetic inorganic bone graft material, degradable polymer matrices, salt porogens, and antibiotic tobramycin. These formulations were examined to determine the antibiotic's elution kinetics and bactericidal potential, the device's degradation in vitro, as well as osteoconductivity and device resorption in vivo using a pilot rabbit bone implant model. Kirby-Bauer antibiotic susceptibility tests assessed bactericidal activity. Liquid chromatography with tandem mass spectrometry measured antibiotic elution kinetics, and scanning electron microscopy was used to qualitatively assess degradation. Results indicated sustained antibiotic release from all three formulations above the Staphylococcus aureus minimum inhibitory concentration for a period of 5 to 8 weeks. Extensive degradation was observed with the Group 3 formulation after 90 days in phosphate-buffered saline, with a lesser degree of degradation observed in the other two formulations. Results from the pilot rabbit study showed the Group 3 device to be biocompatible, with minimal inflammatory response and no fibrous encapsulation in bone. The device was also highly osteoconductive-exhibiting an accelerated mineral apposition rate. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1632-1642, 2016.

Scaffold-based regeneration of skeletal tissues to meet clinical challenges

The management and reconstruction of damaged or diseased skeletal tissues have remained a significant global healthcare challenge. The limited efficacy of conventional treatment strategies for large bone, cartilage and osteochondral defects has inspired the development of scaffold-based tissue engineering solutions, with the aim of achieving complete biological and functional restoration of the affected tissue in the presence of a supporting matrix. Nevertheless, significant regulatory hurdles have rendered the clinical translation of novel scaffold designs to be an inefficient process, mainly due to the difficulties of arriving at a simple, reproducible and effective solution that does not rely on the incorporation of cells and/or bioactive molecules. In the context of the current clinical situation and recent research advances, this review will discuss scaffold-based strategies for the regeneration of skeletal tissues, with focus on the contribution of bioactive ceramic scaffolds and silk fibroin, and combinations thereof, towards the development of clinically viable solutions.

Preconditioned 70S30C bioactive glass foams promote osteogenesis in vivo

Bioactive glass scaffolds (70S30C; 70% SiO2 and 30% CaO) produced by a sol-gel foaming process are thought to be suitable matrices for bone tissue regeneration. Previous in vitro data showed bone matrix production and active remodelling in the presence of osteogenic cells. Here we report their ability to act as scaffolds for in vivo bone regeneration in a rat tibial defect model, but only when preconditioned. Pretreatment methods (dry, pre-wetted or preconditioned without blood) for the 70S30C scaffolds were compared against commercial synthetic bone grafts (NovaBone® and Actifuse®). Poor bone ingrowth was found for both dry and wetted sol-gel foams, associated with rapid increase in pH within the scaffolds. Bone ingrowth was quantified through histology and novel micro-CT image analysis. The percentage bone ingrowth into dry, wetted and preconditioned 70S30C scaffolds at 11 weeks were 10±1%, 21±2% and 39±4%, respectively. Only the preconditioned sample showed above 60% material-bone contact, which was similar to that in NovaBone and Actifuse. Unlike the commercial products, preconditioned 70S30C scaffolds degraded and were replaced with new bone. The results suggest that bioactive glass compositions should be redesigned if sol-gel scaffolds are to be used without preconditioning to avoid excess calcium release.

A novel murine femoral segmental critical-sized defect model stabilized by plate osteosynthesis for bone tissue engineering purposes

Mouse models are invaluable tools for mechanistic and efficacy studies of the healing process of large bone defects resulting in atrophic nonunions, a severe medical problem and a financial health-care-related burden. Models of atrophic nonunions are usually achieved by providing a highly stable biomechanical environment. For this purpose, external fixators have been investigated, but plate osteosynthesis, despite its high clinical relevance, has not yet been considered in mice. We hereby proposed and investigated the use of an internal osteosynthesis for stabilizing large bone defects. To this aim, a 3.5-mm-long segmental bone defect was induced in the mid-shaft of the femur using a Gigli saw and a jig. Bone fixation was performed using a titanium microlocking plate with four locking screws. The bone defect was either left empty or filled with a syngenic bone graft or filled with a coralline scaffold. Healing was monitored using radiographs. The healing process was further assessed using microcomputed tomography and histology 10 weeks after surgery. With the exception of one mouse that died during the surgical procedure, no complications were observed. A stable and reproducible bone fixation as well as a reproducible fixation of the implanted materials with full weight bearing was obtained in all animals tested. Nonunion was consistently observed in the group in which the defects were left empty. Bone union was obtained with the syngenic bone grafts, providing evidence that, although such defects were of critical size, bone healing was possible when the gold-standard material was used to fill the defect. Although new bone formation was greater in the coralline scaffold group than in the left-empty animal group, it remained limited and localized close to the bony edges, a consequence of the critical size of such bone defect. Our study established a reproducible, clinically relevant, femoral, atrophic nonunion, critical-sized defect, low morbidity mouse model. The present study was successful in designing and testing in a small animal model, a novel surgical method for the assessment of bone repair; this model has the potential to facilitate investigations of the molecular and cellular events involved in bone regeneration in load-bearing, segmental-bone defects.

The role of barrier membranes for guided bone regeneration and restoration of large bone defects: current experimental and clinical evidence

Treatment of large bone defects represents a great challenge in orthopedic and craniomaxillofacial surgery. Although there are several methods for bone reconstruction, they all have specific indications and limitations. The concept of using barrier membranes for restoration of bone defects has been developed in an effort to simplify their treatment by offering a single-staged procedure. Research on this field of bone regeneration is ongoing, with evidence being mainly attained from preclinical studies. The purpose of this review is to summarize the current experimental and clinical evidence on the use of barrier membranes for restoration of bone defects in maxillofacial and orthopedic surgery. Although there are a few promising preliminary human studies, before clinical applications can be recommended, future research should aim to establish the 'ideal' barrier membrane and delineate the need for additional bone grafting materials aiming to 'mimic' or even accelerate the normal process of bone formation. Reproducible results and long-term observations with barrier membranes in animal studies, and particularly in large animal models, are required as well as well-designed clinical studies to evaluate their safety, efficacy and cost-effectiveness.

Development and long-term in vivo evaluation of a biodegradable urethane-doped polyester elastomer

We have recently reported upon the development of crosslinked urethane-doped polyester (CUPE) network elastomers, which was motivated by the desire to overcome the drawbacks presented by crosslinked network polyesters and biodegradable polyurethanes for soft tissue engineering applications. Although the effect of the isocyanate content and post-polymerization conditions on the material structure-property relationship was examined in detail, the ability of the diol component to modulate the material properties was only studied briefly. Herein, we present a detailed report on the development of CUPE polymers synthesized using diols 4, 6, 8, 10, or 12 methylene units in length in order to investigate what role the diol component plays on the resulting material's physical properties, and assess their long-term biological performance in vivo. An increase in the diol length was shown to affect the physical properties of the CUPE polymers primarily through lowered polymeric crosslinking densities and elevated material hydrophobicity. The use of longer chain diols resulted in CUPE polymers with increased molecular weights resulting in higher tensile strength and elasticity, while also increasing the material hydrophobicity to lower bulk swelling and prolong the polymer degradation rates. Although the number of methylene units largely affected the physical properties of CUPE, the choice of diol did not affect the overall polymer cell/tissue-compatibility both in vitro and in vivo. In conclusion, we have established the diol component as an important parameter in controlling the structure-property relationship of the polymer in addition to diisocyanate concentration and post-polymerization conditions. Expanding the family of CUPE polymers increases the choices of biodegradable elastomers for tissue engineering applications.

An alginate-based hybrid system for growth factor delivery in the functional repair of large bone defects

The treatment of challenging fractures and large osseous defects presents a formidable problem for orthopaedic surgeons. Tissue engineering/regenerative medicine approaches seek to solve this problem by delivering osteogenic signals within scaffolding biomaterials. In this study, we introduce a hybrid growth factor delivery system that consists of an electrospun nanofiber mesh tube for guiding bone regeneration combined with peptide-modified alginate hydrogel injected inside the tube for sustained growth factor release. We tested the ability of this system to deliver recombinant bone morphogenetic protein-2 (rhBMP-2) for the repair of critically-sized segmental bone defects in a rat model. Longitudinal μ-CT analysis and torsional testing provided quantitative assessment of bone regeneration. Our results indicate that the hybrid delivery system resulted in consistent bony bridging of the challenging bone defects. However, in the absence of rhBMP-2, the use of nanofiber mesh tube and alginate did not result in substantial bone formation. Perforations in the nanofiber mesh accelerated the rhBMP-2 mediated bone repair, and resulted in functional restoration of the regenerated bone. μ-CT based angiography indicated that perforations did not significantly affect the revascularization of defects, suggesting that some other interaction with the tissue surrounding the defect such as improved infiltration of osteoprogenitor cells contributed to the observed differences in repair. Overall, our results indicate that the hybrid alginate/nanofiber mesh system is a promising growth factor delivery strategy for the repair of challenging bone injuries.

Clinical use of resorbable polymeric membranes in the treatment of bone defects

The reconstruction of large bone defects remains a clinically challenging condition. Although many treatment approaches exist, they all have limitations. Recently, bioresorbable polylactide membranes have become commercially available. These membranes, when applied to bone defects, enhance bone healing by direct osteoconduction, exclusion of nonosseous tissues, and enhancing the osteogenic environment for autologous grafts. When combined with appropriate internal fixation and autologous bone graft, bioresorbable polylactide membranes allow for single-step reconstruction of large bone defects.

The challenge of establishing preclinical models for segmental bone defect research

A considerable number of international research groups as well as commercial entities work on the development of new bone grafting materials, carriers, growth factors and specifically tissue-engineered constructs for bone regeneration. They are strongly interested in evaluating their concepts in highly reproducible large segmental defects in preclinical and large animal models. To allow comparison between different studies and their outcomes, it is essential that animal models, fixation devices, surgical procedures and methods of taking measurements are well standardized to produce reliable data pools and act as a base for further directions to orthopaedic and tissue engineering developments, specifically translation into the clinic. In this leading opinion paper, we aim to review and critically discuss the different large animal bone defect models reported in the literature. We conclude that most publications provide only rudimentary information on how to establish relevant preclinical segmental bone defects in large animals. Hence, we express our opinion on methodologies to establish preclinical critically sized, segmental bone defect models used in past research with reference to surgical techniques, fixation methods and postoperative management focusing on tibial fracture and segmental defect models.

Characteristics of calcium sulfate/gelatin composite biomaterials for bone repair

A novel hybrid biomaterial composed of calcium sulfate (CS) and gelatin (GEL) was prepared with the potential of being used as bone filler or scaffold owing to its osteoconduction. Such composite biomaterial, cross-linked or un-cross-linked, could provide a suitable absorbing rate and prevent the CS crystals migrating from the implant for tissue engineering. The structure of the composite was analyzed with infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicated that the crystal pattern of CS was affected by the addition of GEL. The GEL part affected the development of the CS dihydrate (CSD) crystal by slowing the conversion from CS hemihydrate (CSH) to CSD; thus, the composite actually contained CSD, CSH and GEL. The compressive strength of the CS/CLGEL composite was also investigated. The compressive strength was correlated to the weight proportions of CS in the CS/cross-linked GEL (CS/CLGEL) composite, and the highest compressive strength of 82 MPa was obtained for the composite containing 40 wt% CS. The in vitro absorption test and the SEM results showed that a porous scaffold was formed in situ with the absorption of CS in the CS/CLGEL composite in a certain time. Therefore, the CS/CLGEL composite material can be used as an in situ porous scaffold with a high initial mechanical strength, and the remaining porous GEL scaffold will enable further in-growth of cells. Human osteoblasts were cultured in contact with the CS/CLGEL composite and the primary results suggested that human osteoblasts could attach and spread on the surface of CS/CLGEL films. The preliminary animal model experiment was operated for assessing the potential of the CS/CLGEL composite as a biodegradable bone substitute. The primary results showed that the CS/CLGEL composite filler could promote new bone in-growth, which will stimulate further study.

Prosthetic devices shaped as tubular chambers for the treatment of large diaphyseal defects by guided bone regeneration

Guided tissue regeneration is based on the hypothesis that the different tissues have unequal abilities to penetrate a wounded area during the healing process. The use of a device acting as a chamber allows the growth of a particular tissue and prevents the ingrowth of other tissues which impair the healing process. At the same time the chamber protects and maintains in situ the intrinsic growth factors so that they may perform their specific activity. Guided tissue regeneration currently plays a well-recognized role mostly in dentistry and peripheral nerve surgery but interesting perspectives have also opened up in orthopedics. Considering the possibility of using guided bone regeneration in the repair of diaphyseal bone defects, this updated survey highlights some critical points and pathways related to the state-of-the-art of this promising procedure, focusing particularly on the properties of the material to make the tubular chamber, the use of osteopromotive factors and the most appropriate animal model to be used for the experimental evaluation.

Grafting of massive tibial subchondral bone defects in a caprine model using beta-tricalcium phosphate versus autograft

OBJECTIVE

This study evaluated the ability of beta-tricalcium phosphate particles (beta-TCP) and autograft (AUTO) to maintain joint surface morphology when used to supplement massive subchondral bone defects in a caprine model.

DESIGN

This was a prospective, parallel arm study with 2 experimental arms and a control group.

METHODS

Unilateral, 11 mm diameter, 25 mm deep cylindrical defects were created in tibial subchondral bone of anesthetized goats (n = 16) and filled with autograft or beta-tricalcium phosphate particles. The contralateral limbs served as internal controls. Goats were killed at 3 months and both tibiae harvested. Molds made of the tibial plateau surface were used to create positive casts from which medial and lateral tibial plateau surfaces of both experimental (beta-tricalcium phosphate particles, autograft) and control limbs were digitized in 3 dimensions. Mirror images of the medial condyle surface contours from the controls were superimposed onto the experimental surfaces and deviations were compared using a Student t test (alpha = 0.05). Tibiae were then cut sagittally into medial (biomechanics) and lateral (histology) halves. Compressive modulus within the defect area was assessed by indentation to 2.0 mm at 0.2 mm per second using a 6-mm diameter pin. Specimens from the lateral tibial plateau were processed for undecalcified histology and the area of bone within the defect region measured. The articular surface of 86% of the autograft and 0% of the beta-tricalcium phosphate particles group had degenerative changes, with 29% of autograft goats exhibiting large-scale plateau collapse. Mean surface deviation for autograft was significantly greater than for beta-tricalcium phosphate particles (2.19 +/- 1.49 mm versus 0.78 +/- 0.19 mm), as was maximum surface deviation (11.19 +/- 8.02 mm versus 4.39 +/- 1.33 mm) (P < 0.05). The compressive modulus within the defect area for control animals was significantly higher than the experimental groups (P < 0.05). Significantly more bone was regenerated within beta-tricalcium phosphate particle-grafted defects compared to autograft (P < 0.05). These results indicated that beta-tricalcium phosphate particles might be a useful graft material for local repair of load bearing skeletal sites such as depressed tibial plateau fractures.

Fabrication of calcium sulfate/PLLA composite for bone repair

The bone-repairing composite material CS/PLLA was fabricated by mixing poly-L-lactic acid (PLLA) and calcium sulfate hemihydrate (CSH). The structure of the composite was analyzed with Infrared spectroscope, X-ray diffraction, and scanning electron microscope. The results indicated that the crystal pattern of calcium sulfate was affected by the addition of PLLA. PLLA part impacted the development of calcium sulfate dihydrate (CSD) crystal by slowing the conversion from CSH to CSD, so the composites are actually composed of CSH, CSD, and PLLA. The absorbing test in vitro showed that CS/PLLA composite absorbed more slowly than pure CS, suggesting the addition of PLLA can adjust the absorption rate of CS to meet different requirements. The pH value changes of the media had similar trends for different composites during the absorbing test of CS/PLLA samples in aqueous medium, which was connected to the absorption of calcium sulfate. The absorption of calcium sulfate in a certain time left a porous PLLA scaffold that will enable cells to further grow in. The surface of CS/PLLA pellets was inoculated with human osteoblasts, and the primary results showed that the osteoblasts could attach and spread on the surface, which will stimulate our desire for further study.

Attachment, growth, and activity of rat osteoblasts on polylactide membranes treated with various low-temperature radiofrequency plasmas

Nonporous and porous membranes from poly(L/DL-lactide) 80/20% were treated with low-temperature oxygen, ammonia, or sulphur dioxide-hydrogen plasmas and the late effects of plasma treatment on physicochemical characteristics of the membranes' surface were analyzed. The plasma treatment resulted in the permanent attachment of sulphur and nitrogen functionalities to the membrane's surface, and increased the surface concentration of oxygen, thereby increasing the surface wettability. To assess whether the plasma treatment affects the cellular response, primary rat osteoblasts were cultured on nontreated and plasma-treated nonporous and microporous membranes, and attachment, growth, and activity of cells were investigated. It was found that attachment and growth of osteoblasts on all the plasma-treated membranes were greater compared with nontreated controls. The treatment with ammonia plasma was most efficacious. The beneficial effects of plasma treatment on cells were most pronounced for microporous polylactide membranes irrespective of the plasma used. The results of the study suggest that the treatment of porous polylactide structures with plasma can be an effective means of enhancing their suitability for tissue engineering. Plasma exposure may also have an advantageous effect on bone healing when polylactide membranes are used to treat bone defects.

A technique for creating critical-size defects in the metatarsus of sheep for use in investigation of healing of long-bone defects

OBJECTIVE

To develop a technique for use in investigation of healing of long-bone defects by creation of a critical-size defect in the left metarsal III and IV bone (metatarsus) of sheep.

ANIMALS

18 healthy adult sheep.

PROCEDURE

Sheep were allocated to 4 groups (3, 3, 5, and 7 sheep in groups 1 to 4, respectively). An ostectomy with various segmental length-to-diaphyseal diameter ratios (0.5, 1.0, 2.0, and 2.0 for groups 1 to 4, respectively) was performed on the left metatarsus of each sheep. The defect was left empty in sheep of groups 1, 2, and 3, whereas the defect was filled with a massive corticocancellous bone autograft in sheep of group 4.

RESULTS

All sheep tolerated the surgical procedure well and were able to use the affected limb the day after surgery. Radiographic and histologic examinations conducted 16 weeks after surgery revealed nonunion in all sheep of groups 1, 2, and 3, whereas consistent bone healing with abundant bone formation was observed in all sheep of group 4.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of these findings suggests that the sheep metatarsal model is a critical-size defect model with low morbidity. It should allow the assessment of new technologies for bone regeneration in conditions closely mimicking the clinical setting.

IMPACT FOR HUMAN MEDICINE

Use of this technique in sheep should be of benefit for the preclinical study of osteoconductive, osteoinductive, or osteogenic biomaterials for use in humans.

Repair of rabbit segmental defects with the thrombin peptide, TP508

The synthetic peptide, TP508 (Chrysalin), was delivered to rabbit segmental bone defects in biodegradable controlled-release PLGA microspheres to determine its potential efficacy for enhancing healing of non-critically and critically sized segmental defects. Non-critically sized radial defects were created in the forelimbs of New Zealand White rabbits, which were randomized into three treatment groups receiving 10, 50 and 100 microg doses of TP508 in the right radius and control microspheres (without TP508) in the left radius. Torsional testing of the radii at six weeks showed a significant increase in ultimate torque, failure torque, ultimate energy, failure energy, and stiffness when treated with TP508 compared to controls (p<0.01 for all measures). Thus, TP508 appeared to enhance or accelerate bone growth in these defects. In a second set of experiments, critically sized ulnar defects were created in the forelimbs of New Zealand White rabbits, which were randomized into two groups with each rabbit receiving microspheres with 100 or 200 microg of TP508 into the right ulnar defect and control microspheres (without TP508) alone into the left ulnar defect. Bone healing was evaluated with plain radiographs, synchrotron-based microtomography, and mechanical testing. Radiographs of the rabbit limbs scored by three blinded, independent reviewers demonstrated a significantly higher degree of healing when treated with TP508 than their untreated control limbs (p<0.05). Three-dimensional synchrotron tomography of a limited number of samples showed that the new bone in TP508-treated samples had a less porous surface appearance and open marrow spaces, suggesting progression of bone remodeling. Torsional testing of the ulnae at nine weeks showed a significant increase in maximum torque and failure energy when treated with TP508 compared to controls (p<0.01 for both measures). These results suggest that TP508 in a controlled release delivery vehicle has the potential to enhance healing of segmental defects in both critically and non-critically sized defects.

Protein adsorption, attachment, growth and activity of primary rat osteoblasts on polylactide membranes with defined surface characteristics

The adsorption of proteins and growth and activity of primary rat osteoblasts cultured for 1, 2 and 3 weeks on nonporous and porous resorbable poly(L/DL-lactide) 80/20% membranes with defined surface characteristics were investigated. The growth and activity of cells were estimated from the measurements of DNA, alkaline phosphatase activity and the total amount of protein in the cell lysate. The cell morphology was assessed from scanning electron microscopy and rhodamine staining. The protein adsorption to the membrane surface was assessed from the amide I peak at 1640-1660 cm(-1) and the amide II peak at 1540-1560 cm(-1) in the attenuated total reflection infrared spectra. The relative amount of proteins adsorbed on the nonporous and porous membranes was comparable. The cells growing on the nonporous and porous membranes maintained the phenotype and revealed morphology typical for osteoblasts. The mineralized noduli were larger in size on the porous membranes. The number of cells, the amount of DNA, the alkaline phosphatase activity, and the total amount of protein increased with time of the experiment and were higher for the porous membranes than for the nonporous ones.

Differentiation, growth and activity of rat bone marrow stromal cells on resorbable poly(l/dl-lactide) membranes

Nonporous and porous membranes produced from poly(L/DL-lactide) 80/20% were characterized using profilometry, contact-angle measurements, infra-red spectroscopy, X-ray photoemission spectroscopy and scanning electron microscopy, and used to culture bone marrow stromal cells isolated from the rat femora. The cells were cultured for 5, 10, 15 and 20 days. Cell growth and activity was estimated from the amounts of DNA, alkaline phosphatase activity and total protein amount present in the cell lysate and cell differentiation was assessed histochemically. Cell morphology was estimated from scanning electron microscopy. The cells fully expressed osteoblastic phenotype, revealed spindle-shaped, ellipsoidal morphology, developed podia, produced an abundant fibrillar extracellular matrix and mineral noduli. The number of cells on the membranes increased with time of culturing and was higher for the porous membranes than the nonporous membranes. Osteoblastic differentiation was most significant between 5 and 10 days of culture. The total amounts of DNA, alkaline phosphatase and proteins increased with time of culturing. The surface characteristics of the porous membranes were superior to the nonporous membranes.

Enhanced guided bone regeneration with a resorbable chamber containing demineralized bone matrix

BACKGROUND

The effectiveness of a nonporous poly-DL-lactide tubular chamber in guiding bone regeneration through a long bone defect had already been assessed in an experimental model using the rabbit radius. The injection of bone marrow stem cells into the chamber had proven to enhance bone regeneration.

METHODS

The present study reports on the development of the above research project in a subsequent stage. Demineralized bone matrix (DBM) obtained by milling New Zealand rabbit femoral and tibial diaphyses was placed into a tubular chamber. A 10-mm defect was bilaterally created in the radii of 10 rabbits. On the left side (chamber side) the defect was treated by means of a poly-DL-lactide chamber filled with DBM, whereas DBM alone was used on the right side (control).

RESULTS

Controls were performed at 3 and 6 months by radiographs and histomorphometry and demonstrated better bone growth on the chamber side versus the control side. A comparison with the results previously obtained by stem cell injection into the chamber revealed significant acceleration of bone regrowth in the first 3 months because of the addition of DBM to the chamber. However, no significant difference was found between the two sides after 6 months.

CONCLUSION

These results have confirmed the effectiveness of the chamber as a container for the factors promoting bone regeneration, probably because the osteogenetic activity is maintained in situ.

Development of a 'mechano-active' scaffold for tissue engineering

In this study. we investigate the potential for manipulating bone cell mechanotransducers in tissue engineering. Membrane ion channels such as voltage operated calcium channels (VOCC) have been shown to be a critical component of the bone cell transduction pathway with agonists and inhibitors of this pathway having profound effects on the load signal. By encapsulating a calcium channel agonist with slow release within a poly(L-lactide) (PLLA) scaffold, we can generate a 'mechano-active' scaffold for use in skeletal tissue engineering. PLLA scaffolds with and without a calcium channel agonist, BAY K8644, were seeded with primary human bone cells or the human MG63 bone cell line and cultured for 13 weeks followed by mechanical stimulation with a four-point bending model. Our results show that addition of the agonist for slow release is sufficient to enhance the load-related responses in bone cells within the scaffolds. Specifically, collagen type I expression and the ratio of alkaline phosphatase to protein are elevated in response to cyclical mechanical stimulation of approximately 1000 microstr which is then further enhanced in the mechano-active' scaffolds. As the agonists only act when the calcium channels are open by attenuating the calcium flux, the stimulation is specifically targeted to scaffolds subjected to load either in vitro or ultimately in vivo. Our results suggest that manipulating the VOCC and attenuating the opening of the calcium channels may be an effective technique to amplify matrix production via mechanical stimulation which may be applied to bone tissue engineering and potentially engineering of other load-bearing connective tissues.

Bone regeneration in segmental defects with resorbable polymeric membranes: IV. Does the polymer chemical composition affect the healing process?

Diaphyseal segmental defects 10 mm in length in the radii of 36 skeletally mature rabbits were covered with tubular microporous membranes prepared from poly(L/D-lactide) (18 rabbits) and poly(L/DL-lactide) (18 rabbits) to determine whether chemical composition of the membrane affected the bone healing in the defect. The results of a previous study in which similar defects of the rabbits radii were not covered with membranes or covered with poly(L-lactide) membranes were used as controls. The control defects were rapidly filled with overlying muscle and soft tissues, producing a radio-ulnar synostosis. The osseous activity of control defects was limited to the bone ends. The defects covered with membranes were progressively filled with new bone. At 1 year, complete bone regeneration in the defects covered with the poly(L/D-lactide) membrane was found in 16 cases, no regeneration in 1 animal and pseudoarthrosis in 1 animal. For the poly(L/DL-lactide) membrane there was complete bone regeneration in 17 cases (1 animal died during surgery). The quality of the interface between the new bone and the membrane seemed to be affected by the chemical structure of the polylactides used for membranes preparation. For poly(L/D-lactide), the connective tissue layer entirely separated the new bone from the polymeric membrane. This has been observed before for poly(L-lactide) membranes. In the case of poly(L/DL-lactide) the new bone was formed in some places in direct contact with the membrane and the membrane fragments were osteointegrated. The differences in chemical composition of the polylactide membranes did not have an evident effect on the bone regeneration process in segmental defects of the rabbit radii.

Biomaterial developments for bone tissue engineering

The development of bone tissue engineering is directly related to changes in materials technology. While the inclusion of materials requirements is standard in the design process of engineered bone substitutes, it is also critical to incorporate clinical requirements in order to engineer a clinically relevant device. This review presents the clinical need for bone tissue-engineered alternatives to the present materials used in bone grafting techniques, a status report on clinically available bone tissue-engineering devices, and recent advances in biomaterials research. The discussion of ongoing research includes the current state of osseoactive factors and the delivery of these factors using bioceramics and absorbable biopolymers. Suggestions are also presented as to the desirable design features that would make an engineered device clinically effective.

Assessment of resorbable bioactive material for grafting of critical-size cancellous defects

Bioactive glasses form a surface apatite layer in vivo that enhances the formation and attachment of bone. Sol-gel Bioglass graft material provides greater nanoscale porosity than bioactive glass (on the order of 50-200 A), greater particle surface area, and improved resorbability, while maintaining bioactivity. This study histologically and biomechanically evaluated, in a rabbit model, bone formed within critical-sized distal femoral cancellous bone defects filled with 45S5 Bioglass particulates, 77S sol-gel Bioglass, or 58S sol-gel Bioglass and compared the bone in these defects with normal, intact, untreated cancellous bone and with unfilled defects at 4, 8, and 12 weeks. All grafted defects had more bone within the area than did unfilled controls (p < 0.05). The percentage of bone within the defect was significantly greater for the 45S5 material than for the 58S or 77S material at 4 and 8 weeks (p < 0.05), yet by 12 weeks equivalent amounts of bone were observed for all materials. By 12 weeks, all grafted defects were equivalent to the normal untreated bone. The resorption of 77S and 58S particles was significantly greater than that of 45S5 particles (p < 0.05). Mechanically, the grafted defects had compressive stiffness equivalent to that of normal bone at 4 and 8 weeks. At 12 weeks, 45S5-grafted defects had significantly greater stiffness (p < 0.05). At 8 and 12 weeks, all grafted defects had significantly greater stiffness than unfilled control defects (p < 0.05). In general, the 45S5-filled defects exhibited greater early bone ingrowth than did those filled with 58S or 77S. However, by 12 weeks, the bone ingrowth in each defect was equivalent to each other and to normal bone. The 58S and 77S materials resorbed faster than the 45S5 materials. Mechanically, the compressive characteristics of all grafted defects were equivalent or greater than those of normal bone at all time points.

Polylactide bioabsorbable polymers for guided tissue regeneration

OBJECTIVE

Guided tissue regeneration is a procedure to improve tissue repair, which creates an optimal environment for the intrinsic growth ability of tissues.

METHODS

A prerequisite for guided tissue regeneration is the availability of materials with suitable physicochemical and biocompatibility properties for the preparation of the devices. We investigated bone and peripheral nerve guided tissue regeneration, making two conduits from poly[L-lactide-co-6-caprolactone] (PLLC--peripheral nerve) and with poly [DL-lactide] (PDLLA--bone) with different features. After the polymer synthesis and chemical characterization, the conduits were evaluated in vivo in rat sciatic nerve gaps and in rabbit radius defects.

RESULTS

The results demonstrated good biocompatibility of both polymeric conduits. A good axonal regeneration and the restoration of the nerve trunk continuity, similar to that observed with autologous grafts has been obtained with PLLC conduits, that slowly degrade in about 6 months. PDLLA conduits protected the bone defect against the invasion of surrounding soft tissues; an effective bone growth bridging the defect was observed in their lumen.

CONCLUSION

These results confirm the versatility of polylactides as biomaterials and will encourage further investigations on hard and soft tissues.

Regeneration of segmental diaphyseal defects in sheep tibiae using resorbable polymeric membranes: a preliminary study

OBJECTIVE

To investigate whether a long bone cortex of well-defined thickness can be regenerated by using an anatomically designed membranous resorbable "tube-in-tube" implant and to establish the functions of membranes in the healing of segmental diaphyseal bone defects larger than the "critical size."

DESIGN

Bone healing in segmental diaphyseal defects larger than the critical size in the sheep tibiae covered with a single porous tubular membrane or implanted with anatomically shaped porous double tube-in-tube membranes was evaluated. Membranes with different pore structures were applied alone and/or in combination with autogenous bone graft.

BACKGROUND

Healing of segmental diaphyseal bone defects in animals can be enhanced by covering the defects with resorbable polylactide membranes. Based on the results of bone healing in defects ten millimeters long in the rabbit radii, it was suggested that the membrane prevents muscle and soft tissue from invading the defect and maintains osteogenic cells and osteogenic substances within the space covered with membrane, thus promoting new bone formation. The functions of membranes may differ, however, depending on the size and the location of the defect and on the experimental species used. Bone defects larger than the critical size may not heal at all, even if membranes are used. The critical-size defect is defined as the smallest bone defect that does not heal spontaneously when covered with polymeric membranes. To heal such defects, it is mandatory that membranes are used in combination with autogenic bone graft and/or a suitable bone substitute. If bone graft is used to fill the defect, the structure and geometry of the covering membrane will determine whether the graft will be vascularized and/or nourished from the surrounding soft tissue and, in consequence, survive. It can be appreciated that bone healing in areas of good vascularity should be more efficient than bone healing in poorly vascularized areas. The influence of all these factors on healing of bone in segmental diaphyseal defects covered with membranes is not known.

METHODS

Four-centimeter-long diaphyseal segmental defects in the tibiae of six- to seven-year-old Swiss mountain sheep were covered with resorbable membranes from poly(LDL-lactide). In Group 1, a single microporous external membrane was used. In Group 2, one microporous membrane was inserted into the medullary cavity at the cut ends of the tibiae (internal membrane), and the other microporous membrane was placed on the outer surface of the cortex (external membrane). In Group 3, a single microporous external membrane was also laser-perforated to produce openings with a diameter in the range of 800 to 900 micrometers. In Group 4, the defect was filled with autogenous cancellous bone graft and covered with a single perforated membrane. In Group 5, one perforated internal membrane was inserted into the medullary cavity at the cut ends of the tibiae, and the other perforated membrane was placed on the outer surface of the cortex. Group 6 was identical to Group 5, except that cancellous bone graft was placed in the space between these two membranes.

RESULTS

There was no bone healing in Groups 1, 2, 3, and 5. Only in Groups 4 and 6 did the defects heal. In Group 4, new bone was dispersed across the "medullary canal" formed by the membrane. In Group 6, the new bone had grown into the space between the outer and inner membranes, forming the "neocortex."

CONCLUSIONS

The resorbable polymeric implant consisting of two concentric perforated membranes (the tube-in-tube implant) used in combination with cancellous bone graft to treat segmental diaphyseal defects in sheep tibiae allows for the reconstitution of the "neocortex" with well-defined thickness. (ABSTRACT TRUNCATED)