Locally delivered rhTGF-beta2 enhances bone ingrowth and bone regeneration at local and remote sites of skeletal injury

Production of Recombinant Active Human TGFβ1 in Nicotiana benthamiana

The production of recombinant proteins in plant systems is receiving wider attention. Indeed, various plant-produced pharmaceuticals have been shown to be biologically active. However, the production of human growth factors and cytokines in heterologous systems is still challenging because they often act as complex forms, such as homo- or hetero-dimers, and their production is tightly regulated in vivo. In this study, we demonstrated that the mature form of human TGFβ1 produced and purified from Nicotiana benthamiana shows biological activity in animal cells. To produce the mature form of TGFβ1, various recombinant genes containing the mature form of TGFβ1 were generated and produced in N. benthamiana. Of these, a recombinant construct, BiP:M:CBM3:LAP[C33S]:EK:TGFβ1, was expressed at a high level in N. benthamiana. Recombinant proteins were one-step purified using cellulose-binding module 3 (CBM3) as an affinity tag and microcrystalline cellulose (MCC) beads as a matrix. The TGFβ1 recombinant protein bound on MCC beads was proteolytically processed with enterokinase to separate mature TGFβ1. The mature TGFβ1 still associated with Latency Associated Protein, [LAP(C33S)] that had been immobilized on MCC beads was released by HCl treatment. Purified TGFβ1 activated TGFβ1-mediated signaling in the A549 cell line, thereby inducing phosphorylation of SMAD-2, the expression of ZEB-2 and SNAIL1, and the formation of a filopodia-like structure. Based on these results, we propose that active mature TGFβ1, one of the most challenging growth factors to produce in heterologous systems, can be produced from plants at a high degree of purity via a few steps.

BMP and TGFβ use and release in bone regeneration

A fracture that does not unite in nine months is defined as nonunion. Nonunion is common in fragmented fractures and large bone defects where vascularization is impaired. The distal third of the tibia, the scaphoid bone or the talus fractures are furthermore prone to nonunion. Open fractures and spinal fusion cases also need special monitoring for healing. Bone tissue regeneration can be attained by autografts, allografts, xenografts and synthetic materials, however their limited availability and the increased surgical time as well as the donor site morbidity of autograft use, and lower probability of success, increased costs and disease transmission and immunological reaction probability of allografts oblige us to find better solutions and new grafts to overcome the cons. A proper biomaterial for regeneration should be osteoinductive, osteoconductive, biocompatible and mechanically suitable. Cytokine therapy, where growth factors are introduced either exogenously or triggered endogenously, is one of the commonly used method in bone tissue engineering. Transforming growth factor β (TGFβ) superfamily, which can be divided structurally into two groups as bone morphogenetic proteins (BMPs), growth differentiation factors (GDFs) and TGFβ, activin, Nodal branch, Mullerian hormone, are known to be produced by osteoblasts and other bone cells and present already in bone matrix abundantly, to take roles in bone homeostasis. BMP family, as the biggest subfamily of TGFβ superfamily, is also reported to be the most effective growth factors in bone and development, which makes them one of the most popular cytokines used in bone regeneration. Complications depending on the excess use of growth factors, and pleiotropic functions of BMPs are however the main reasons of why they should be approached with care. In this review, the Smad dependent signaling pathways of TGFβ and BMP families and their relations and the applications in preclinical and clinical studies will be briefly summarized.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

Postnatal intramembranous bone regeneration plays an important role during a wide variety of musculoskeletal regeneration processes such as fracture healing, joint replacement and dental implant surgery, distraction osteogenesis, stress fracture healing, and repair of skeletal defects caused by trauma or resection of tumors. The molecular basis of intramembranous bone regeneration has been interrogated using rodent models of most of these conditions. These studies reveal that signaling pathways such as Wnt, TGFβ/BMP, FGF, VEGF, and Notch are invoked, reminiscent of embryonic development of membranous bone. Discoveries of several skeletal stem cell/progenitor populations using mouse genetic models also reveal the potential sources of postnatal intramembranous bone regeneration. The purpose of this review is to compare the underlying molecular signals and progenitor cells that characterize embryonic development of membranous bone and postnatal intramembranous bone regeneration.

Acid bone lysate activates TGFβ signalling in human oral fibroblasts

Demineralized bone matrix is a widely used allograft from which not only the inorganic mineral but also embedded growth factors are removed by hydrochloric acid (HCl). The cellular response to the growth factors released during the preparation of demineralized bone matrix, however, has not been studied. Here we investigated the in vitro impact of acid bone lysate (ABL) prepared from porcine cortical bone chips on oral fibroblasts. Proteomic analysis of ABL revealed a large spectrum of bone-derived proteins including TGF-β1. Whole genome microarrays and RT-PCR together with the pharmacologic blocking of TGF-β receptor type I kinase with SB431542 showed that ABL activates the TGF-β target genes interleukin 11, proteoglycan 4, and NADPH oxidase 4. Interleukin 11 expression was confirmed at the protein level by ELISA. Immunofluorescence and Western blot showed the nuclear localization of Smad2/3 and increased phosphorylation of Smad3 with ABL, respectively. This effect was independent of whether ABL was prepared from mandible, calvaria or tibia. These results demonstrate that TGF-β is a major growth factor that is removed upon the preparation of demineralized bone matrix.

Bone-conditioned medium contributes to initiation and progression of osteogenesis by exhibiting synergistic TGF-β1/BMP-2 activity

Guided bone regeneration (GBR) often utilizes a combination of autologous bone grafts, deproteinized bovine bone mineral (DBBM), and collagen membranes. DBBM and collagen membranes pre-coated with bone-conditioned medium (BCM) extracted from locally harvested autologous bone chips have shown great regenerative potential in GBR. However, the underlying molecular mechanism remains largely unknown. Here, we investigated the composition of BCM and its activity on the osteogenic potential of mesenchymal stromal cells. We detected a fast and significant (P < 0.001) release of transforming growth factor-β1 (TGF-β1) from autologous bone within 10 min versus a delayed bone morphogenetic protein-2 (BMP-2) release from 40 min onwards. BCMs harvested within short time periods (10, 20, or 40 min), corresponding to the time of a typical surgical procedure, significantly increased the proliferative activity and collagen matrix production of BCM-treated cells. Long-term (1, 3, or 6 days)-extracted BCMs promoted the later stages of osteoblast differentiation and maturation. Short-term-extracted BCMs, in which TGF-β1 but no BMP-2 was detected, reduced the expression of the late differentiation marker osteocalcin. However, when both growth factors were present simultaneously in the BCM, no inhibitory effects on osteoblast differentiation were observed, suggesting a synergistic TGF-β1/BMP-2 activity. Consequently, in cells that were co-stimulated with recombinant TGF-β1 and BMP-2, we showed a significant stimulatory and dose-dependent effect of TGF-β1 on BMP-2-induced osteoblast differentiation due to prolonged BMP signaling and reduced expression of the BMP-2 antagonist noggin. Altogether, our data provide new insights into the molecular mechanisms underlying the favorable outcome from GBR procedures using BCM, derived from autologous bone grafts.

‘Bone-conditioned medium’ could improve oral bone regeneration therapy by promoting the proliferation and maturation of bone-forming cells. Building on recent research demonstrating the benefits of using cell culture medium prepared with bone chips (BCM) in such treatments, researchers led by Maria Asparuhova of the University of Bern, Switzerland, set out to elucidate the medium’s mechanisms. The team found that BCM incubated with bone chips for short periods—as little as ten minutes—contained heightened levels of signaling protein TGF-β1, which enhanced mouse bone marrow cell proliferation while downregulating maturation. BCM incubated for longer periods also generated increased levels of another protein, BMP-2, which boosted the maturation of bone-forming cells. This study reveals a sequential role of these two factors in oral bone development, and the potential physiological actions of BCM when used in regenerative therapies.

Healing in peri-implant gap with BMP-2 and systemic bisphosphonate is dependent on BMP-2 dose-A canine study

The bone-implant interface of cementless orthopedic implants can be described as a series of uneven sized gaps with discontinuous areas of direct bone-implant contact. Bridging these voids and crevices by addition of an anabolic stimulus to increase new bone formation can potentially improve osseointegration of implants. Bone morphogenetic protein 2 (BMP-2) stimulates osteoblast formation to increase new bone formation but also indirectly stimulates osteoclast activity. In this experiment, we investigate the hypothesis that osseointegration, defined as mechanical push-out and histomorphometry, depends on the dose of BMP-2 when delivered as an anabolic agent with systemic administration of the anti-resorptive agent zoledronate to curb an increase in osteoclast activity. Four porous coated titanium implants (one with each of three doses of surface-applied BMP-2 (15 µg; 60 µg; 240 µg) and untreated) surrounded by a 0.75 mm empty gap, were inserted into the distal femurs of each of twelve canines. Zoledronate IV (0.1 mg/kg) was administered 10 days into the observation period of 4 weeks. Bone-implant specimens were evaluated by mechanical push-out test and histomorphometry. The 15 µg implants had the best fixation on all mechanical parameters and largest surface area covered with new bone compared to the untreated, 60 and 240 µg implants, as well as the highest volume of new bone in the implant gap compared to 60 and 240 µg implants. The results in a canine implant model demonstrated that a narrow range of BMP-2 doses have opposite effects in bridging an empty peri-implant gap with bone, when combined with systemic zoledronate. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1406-1414, 2018.

Bone-conditioned medium modulates the osteoconductive properties of collagen membranes in a rat calvaria defect model

OBJECTIVES

Collagen membranes are not limited to be occlusive barriers as they actively support bone regeneration. However, the impact of bone-derived growth factors on their osteoconductive competence has not been examined.

METHODS

Twenty adult Sprague Dawley rats were included in the study. Calvaria defects with a diameter of five millimeter were created. The defect was covered with one layer of a collagen membrane previously soaked in conditioned medium of porcine bone chips or in culture medium alone. After 4 weeks, microcomputed tomography was performed. Undecalcified thin-ground sections were subjected to light and scanning electron microscopy. Primary outcome parameter was the bone volume in the defect. Unit of analysis was the bone-conditioned medium (BCM).

RESULTS

In the central defect area of the control and the BCM group, median new bone connected to the host bone was 0.54 and 0.32 mm³, respectively (p = .10). In the ectocranial defect area, the control group showed significantly more bone than the BCM group (0.90 and 0.26 mm³; p = .02). Based on an exploratory interpretation, the control group had smaller bony islands than the BCM group. Scanning electron microscopy and histology indicate the formation of bone but also the collagen membrane to be mineralized in the defect site.

CONCLUSIONS

These results demonstrate that the commercial collagen membrane holds an osteoconductive competence in a rat calvaria defect model. Soaking collagen membranes with BCM shifts bone formation toward the formation of bony islands rather than new bone connected to the host bone.

Implant-delivered Alendronate Causes a Dose-dependent Response on Net Bone Formation Around Porous Titanium Implants in Canines

BACKGROUND

Bony fixation of cementless orthopaedic implants is not always achieved, particularly in challenging scenarios such as revision surgery, trauma, and tumor reconstruction. An adjunct therapy for improving porous implant fixation could improve the reliability and durability of these reconstructive procedures.

QUESTIONS/PURPOSES

In this study, we asked whether there is a positive and dose-dependent effect of the local release of the bisphosphonate alendronate from (1) alendronate/hydroxyapatite (HA) porous-coated titanium implants compared with bare metal porous controls; and (2) alendronate/HA on porous-coated titanium implants compared with HA-coated porous controls with respect to extent of bone ingrowth, bone apposition, and periimplant bone formation in a canine model?

METHODS

Three-dimensional printed porous-coated cylindrical implants coated with three different doses (0.02, 0.06, and 0.18 mg/cm(2)) of alendronate were inserted bilaterally in the intramedullary canal of the proximal femora of 15 adult mongrel dogs (age range, 3-9 years; mean, 5 years) weighing between 36 kg and 60 kg (mean, 43 kg). In each dog, an implant coated with HA and one of three different doses of alendronate was inserted on one side while the contralateral femur had a bare metal porous control implant and an identical control implant with a coating of HA. The dose effect of locally released alendronate on the extent of bone ingrowth, bone apposition, and periimplant bone was assessed by backscattered electron microscopy of three pairs of cross-sections taken from each implant at 12 weeks after surgery. A linear mixed model was used to perform the statistical analyses to account for the correlation in the data resulting from the multiple measures performed on each dog.

RESULTS

Compared with paired bare metal controls, periimplant bone increased by 92% (p = 0.007), and 114% (p < 0.001) in the femora with the alendronate implants with a dose of 0.06 mg/cm(2), or 0.18 mg/cm(2), respectively. At a dose of 0.02 mg/cm(2), there was no difference (46% change; p = 0.184, with the numbers available). The comparison of the alendronate-dosed implants with their HA-coated controls showed that the intermediate dose of 0.06 mg/cm(2) alendronate had the greatest effect on net bone formation. Bone apposition was enhanced with the 0.06-mg/cm(2) alendronate femoral implants (82%; p = 0.008), although there was no change in bone ingrowth (37% change; p = 0.902, with the numbers available). When compared with the HA-coated control implants, the greatest effect of the alendronate-dosed implants was the increased amount of periimplant bone at the intermediate dose of 0.06-mg/cm(2) (108%, p = 0.009). There was no effect of the low (0.02-mg/cm(2)) and high (0.18-mg/cm(2)) alendronate-dosed implants (4%, and 6%, respectively; p = 0.321, p = 0.502). Overall, all three alendronate-dosed implants revealed little to no effect on bone ingrowth compared with the HA-coated control implants.

CONCLUSIONS

The local release of alendronate from a three-dimensional printed porous-coated implant from the three doses studied showed an overall improvement in bone apposition and periimplant bone at the intermediate dose compared with bare metal or with HA-coated controls, although the effect was more pronounced compared with bare metal. Long-term studies to show the effects of localized alendronate delivery and mechanical fixation would be the next step for future studies.

CLINICAL RELEVANCE

Local release of alendronate from a three-dimensional printed porous-coated implant may improve the reliability of cementless fixation of currently available porous-coated bare metal implants.

Bioactive coatings for orthopaedic implants-recent trends in development of implant coatings

Joint replacement is a major orthopaedic procedure used to treat joint osteoarthritis. Aseptic loosening and infection are the two most significant causes of prosthetic implant failure. The ideal implant should be able to promote osteointegration, deter bacterial adhesion and minimize prosthetic infection. Recent developments in material science and cell biology have seen the development of new orthopaedic implant coatings to address these issues. Coatings consisting of bioceramics, extracellular matrix proteins, biological peptides or growth factors impart bioactivity and biocompatibility to the metallic surface of conventional orthopaedic prosthesis that promote bone ingrowth and differentiation of stem cells into osteoblasts leading to enhanced osteointegration of the implant. Furthermore, coatings such as silver, nitric oxide, antibiotics, antiseptics and antimicrobial peptides with anti-microbial properties have also been developed, which show promise in reducing bacterial adhesion and prosthetic infections. This review summarizes some of the recent developments in coatings for orthopaedic implants.

Regulation of postnatal bone homeostasis by TGFβ

Perhaps more so than any other tissue, bone has pivotal mechanical and biological functions. Underlying the ability of bone to execute these functions, whether providing structural support or preserving mineral homeostasis, is the dynamic remodeling of bone matrix. Cells within bone integrate multiple stimuli to balance the deposition and resorption of bone matrix. Transforming growth factor-β (TGFβ) uniquely coordinates bone cell activity to maintain bone homeostasis. TGFβ regulates the differentiation and function of both osteoblasts and osteoclasts, from lineage recruitment to terminal differentiation, to balance bone formation and resorption. TGFβ calibrates the synthesis and material quality of bone matrix and bone's responsiveness to applied mechanical loads. Therefore, by coupling the activity of bone forming and resorbing cells, and by sensing, responding to and defining physical cues, TGFβ integrates physical and biochemical stimuli to maintain bone homeostasis. Disruption of TGFβ signaling has significant consequences on bone mass and quality. Alternatively, TGFβ is a powerful lever that has the potential to yield therapeutic benefit in cases where bone homeostasis needs to be recalibrated.

Role of TGF-β in Survival of Phagocytizing Microglia: Autocrine Suppression of TNF-α Production and Oxidative Stress

Microglia are recognized as residential macrophageal cells in the brain. Activated microglia play a critical role in removal of dead or damaged cells through phagocytosis activity. During phagocytosis, however, microglia should survive under the harmful condition of self-producing ROS and pro-inflammatory mediators. TGF-β has been known as a classic anti-inflammatory cytokine and controls both initiation and resolution of inflammation by counter-acting inflammatory cytokines. In the present study, to understand the self-protective mechanism, we studied time-dependent change of TNF-α and TGF-β production in microglia phagocytizing opsonized-beads (i.e., polystyrene microspheres). We found that microglia phagocytized opsonized-bead in a time-dependent manner and simultaneously produced both TNF-α and TGF-β. However, while TNF-α production gradually decreased after 6 h, TGF-β production remained at increased level. Microglial cells pre-treated with lipopolysaccharides (a strong immunostimulant, LPS) synergistically increased the production of TNF-α and TGF-β both. However, LPS-pretreated microglia produced TNF-α in a more sustained manner and became more vulnerable, probably due to the marked and sustained production of TNF-α and reduced TGF-β. Intracellular oxidative stress appears to change in parallel with the microglial production of TNF-α. These results indicate TGF-β contributes for the survival of phagocytizing microglia through autocrine suppression of TNF-α production and oxidative stress.

Controlling the biological function of calcium phosphate bone substitutes with drugs

There is a growing interest in bone tissue engineering for bone repair after traumatic, surgical or pathological injury, such as osteolytic tumor or osteoporosis. In this regard, calcium phosphate (CaP) bone substitutes have been used extensively as bone-targeting drug-delivery systems. This localized approach improves the osteogenic potential of bone substitutes by delivering bone growth factors, thus extending their biofunctionality to any pathological context, including infection, irradiation, tumor and osteoporosis. This review briefly describes the physical and chemical processes implicated in the preparation of drug-delivering CaPs. It also describes the impact of these processes on the intrinsic properties of CaPs, especially in terms of the drug-release profile. In addition, this review focuses on the potential influence of drugs on the resorption rate of CaPs. Interestingly, by modulating the resorption parameters of CaP biomaterials, it should be possible to control the release of bone-stimulating ions, such as inorganic phosphate, in the vicinity of bone cells. Finally, recent in vitro and in vivo evaluations are extensively reported.

Sclerostin antibody increases bone volume and enhances implant fixation in a rat model

BACKGROUND

Previous studies have demonstrated that sclerostin blockade is anabolic for bone. This study examined whether systemic administration of sclerostin antibody would increase implant fixation and peri-implant bone volume in a rat model.

METHODS

Titanium cylinders were placed in the femoral medullary canal of ninety male Sprague-Dawley rats. One-half of the rats (n=45) received murine sclerostin antibody (Scl-Ab, 25 mg/kg, twice weekly) and the other one-half (n=45) received saline solution. Equal numbers of rats from both groups were sacrificed at two, four, or eight weeks after the implant surgery and the femora were examined by microcomputed tomography, mechanical pull-out testing, and histology.

RESULTS

Fixation strength in the two groups was similar at two weeks but was 1.9-fold greater at four weeks (p=0.024) and 2.2-fold greater at eight weeks (p<0.001) in the rats treated with sclerostin antibody. At two weeks, antibody treatment led to increased cortical area, with later increases in cortical thickness and total cross-sectional area. Significant differences in peri-implant trabecular bone were not evident until eight weeks but included increased bone volume per total volume, bone structure that was more plate-like, and increased trabecular thickness and number. Changes in bone architecture in the intact contralateral femur tended to precede the peri-implant changes. The peri-implant bone properties accounted for 61% of the variance in implant fixation strength, 32% of the variance in stiffness, and 63% of the variance in energy to failure. The implant fixation strength at four weeks was approximately equivalent to the strength in the control group at eight weeks.

CONCLUSIONS

Sclerostin antibody treatment accelerated and enhanced mechanical fixation of medullary implants in a rat model by increasing both cortical and trabecular bone volume.

Intra-operatively customized implant coating strategies for local and controlled drug delivery to bone

Bone is one of the few tissues in the human body with high endogenous healing capacity. However, failure of the healing process presents a significant clinical challenge; it is a tremendous burden for the individual and has related health and economic consequences. To overcome such healing deficits, various concepts for a local drug delivery to bone have been developed during the last decades. However, in many cases these concepts do not meet the specific requirements of either surgeons who must use these strategies or individual patients who might benefit from them. We describe currently available methods for local drug delivery and their limitations in therapy. Various solutions for drug delivery to bone focusing on clinical applications and intra-operative constraints are discussed and drug delivery by implant coating is highlighted. Finally, a new set of design and performance requirements for intra-operatively customized implant coatings for controlled drug delivery is proposed. In the future, these requirements may improve approaches for local and intra-operative treatment of patients.

Onlay and inlay bone grafts with platelet-rich plasma: histologic evaluations from human biopsies

PURPOSE

The aim of the present study was to histomorphometrically evaluate biopsies from 1) sinus inlay bone grafts prepared with or without platelet-rich plasma (PRP) and 2) onlay bone grafts in the anterior maxilla with particulate bone and PRP or block bone without PRP after 3 months.

MATERIALS AND METHODS

Biopsies were retrieved with a trephine drill, and samples were processed in laboratories to result in nondecalcified cut and ground sections. Light microscopic measurements of total bone area, new bone, number of vessels, and vessel area were performed at 3 regions in each sample. Calculations of ratios of new bone to total bone and vessel area to total area were performed.

RESULTS

There was a tendency toward higher mean values for total bone area percentage and area of newly formed bone in the PRP-treated inlay biopsies. However, there was no measurable effect on vessel formation in the PRP-treated inlay samples compared with untreated inlay samples. Particulate and PRP-treated onlay bone biopsies showed higher mean values in all measured parameters compared with block bone samples; however, no differences were statistically significant.

CONCLUSION

PRP treatment may favor bone formation in grafted bone, and further research is needed to describe the outcome of PRP treatment and grafting techniques in the maxilla.

Osteogenic protein-1 delivered by hydroxyapatite-coated implants improves bone ingrowth in extracortical bone bridging

BACKGROUND

Extracortical bone bridging for treatment of massive bone loss can improve stability and longevity of massive endoprostheses. Osteogenic protein-1 (OP-1), when used with allograft bone, reportedly improves extracortical bone bridging and bone ingrowth.

QUESTIONS/PURPOSES

We asked whether OP-1 delivered by hydroxyapatite (HA) without bone grafting could improve bone ingrowth and bone formation in the context of extracortical bone bridging.

METHODS

We implanted unilateral segmental femoral diaphyseal replacement prostheses in 18 dogs (three groups of six dogs). The groups consisted of an HA-coated group augmented with OP-1, an HA-coated group, and a plain porous group. Bone grafting techniques were not used to augment bone formation. The implants were retrieved at 12 weeks for histologic assessment.

RESULTS

After removing one specimen owing to a complication, 17 femora were analyzed (six HA-coated augmented with OP-1, five HA-coated, and six plain). We observed better bone ingrowth in the HA-coated OP-1 group than in the plain porous and HA-coated groups, with no difference between the latter two groups. There also was better bone apposition and callus height in the HA-coated OP-1 group than in the plain group but no differences between the HA-coated OP-1 and HA-coated groups or between the HA-coated and plain groups.

CONCLUSIONS

OP-1 (2.9 mg) delivered by HA-coated segmental replacement prostheses in this canine extracortical bone bridging model revealed improved bone ingrowth over HA-coated implants without OP-1 or plain porous-coated prostheses.

Osseointegration by bone morphogenetic protein-2 and transforming growth factor beta2 coated titanium implants in femora of New Zealand white rabbits

BACKGROUND

Intramembranous bone formation is essential in uncemented joint replacement to provide a mechanical anchorage of the implant. Since the discovery of bone morphogenic proteins (BMPs) by Urist in 1965, many studies have been conducted to show the influence of growth factors on implant ingrowth. In this study, the influence of bone morphogenetic protein-2 (rhBMP-2) and transforming growth factor β2 (TGF-β2) on implant osseointegration was investigated.

MATERIALS AND METHODS

Thirty-two titanium cylinders were implanted into the femoral condyles of both hind legs of New Zealand White Rabbits. Four experimental groups were investigated: controls without coating, a macromolecular copolymer + covalently bound BMP-2, adsorbed BMP-2, and absorbed BMP-2+TGF-β2. All samples were analyzed by ex vivo high-resolution micro-computed-tomography after 28 days of healing. Bone volume per total volume (BV/TV) was recorded around each implant. Afterward, all samples were biomechanically tested in a pull-out setup.

RESULTS

The highest BV/TV ratio was seen in the BMP-2 group, followed by the BMP-2+TGF-β2 group in high-resolution micro-computed-tomography. These groups were significantly different compared to the control group (P < 0.05). Copolymer+BMP-2 showed no significant difference in comparison to controls. In the pull-out setup, all groups showed higher fixation strength compared to the control group; these differences were not significant.

CONCLUSIONS

No differences between BMP-2 alone and a combination of BMP-2+TGF-β2 could be seen in the present study. However, the results of this study confirm the results of other studies that a coating with growth factors is able to enhance bone implant ingrowth. This may be of importance in defect situations during revision surgery to support the implant ingrowth and implant anchorage.

Temporal gene expression profiling during rat femoral marrow ablation-induced intramembranous bone regeneration

Enhanced understanding of differential gene expression and biological pathways associated with distinct phases of intramembranous bone regeneration following femoral marrow ablation surgery will improve future advancements regarding osseointegration of joint replacement implants, biomaterials design, and bone tissue engineering. A rat femoral marrow ablation model was performed and genome-wide microarray data were obtained from samples at 1, 3, 5, 7, 10, 14, 28, and 56 days post-ablation, with intact bones serving as controls at Day 0. Bayesian model-based clustering produced eight distinct groups amongst 9,062 significant gene probe sets based on similar temporal expression profiles, which were further categorized into three major temporal classes of increased, variable, and decreased expression. Osteoblastic- and osteoclastic-associated genes were found to be significantly expressed within the increased expression groups. Chondrogenesis was not detected histologically. Adipogenic marker genes were found within variable/decreased expression groups, emphasizing that adipogenesis was inhibited during osteogenesis. Differential biological processes and pathways associated with each major temporal group were identified, and significantly expressed genes involved were visually represented by heat maps. It was determined that the increased expression group exclusively contains genes involved in pathways for matrix metalloproteinases (MMPs), Wnt signaling, TGF-β signaling, and inflammatory pathways. Only the variable expression group contains genes associated with glycolysis and gluconeogenesis, the notch signaling pathway, natural killer cell mediated cytotoxicity, and the B cell receptor signaling pathway. The decreased group exclusively consists of genes involved in heme biosynthesis, the p53 signaling pathway, and the hematopoietic cell lineage. Significant biological pathways and transcription factors expressed at each time point post-ablation were also identified. These data present the first temporal gene expression profiling analysis of the rat genome during intramembranous bone regeneration induced by femoral marrow ablation.

Calcium Phosphate Ceramics as Bone Drug-Combined Devices

The use of bone grafts is constantly increasing, their employ is principally linked to bone trauma, prosthesis revision surgery, and arthrodesis applications. In the case of biological bone grafts and depending on the origin of the graft, these grafts are classified as autografts, allografts, or xenografts. The autograft is the most commonly used and corresponds to a fresh bone graft harvesting taken from a second operating site, i.e. iliac crest, parietal bone, tibial plateaux or the fibula. The autograft has many advantages in terms of biotolerance and osteogenic potential, which justify its widespread utilization in reconstructive surgery[1]. From a practical point of view, sampling and grafting take place during the same surgical session. However, the longer exposure to the anesthetic and the surgical operation per se increases the risk of complications. For example, this procedure results in sever post-operation pain, iliac hernias, or even haemorrhages[2]. Furthermore, the volume of the bone graft taken is generally limited to 20 cm3. In the case of allografts, it generally leads to an acute inflammatory reaction which participates to the resorption/substitution process. Xenografts are less used since it involves a donor and a recipient from different species.

Effect of recombinant human transforming growth factor-beta2 dose on bone formation in rat femur titanium implant model

Previously, we reported that application of 10 microg recombinant human TGF-beta2 (rhTGF-beta2) enhanced peri-implant bone formation and bone-implant contact in a rat model. To further investigate the dose effect, the present experiment evaluated doses of rhTGF-beta2 bracketed around 10 microg (5, 10, 20 microg) using the same model. Four groups (including buffer-only control) received femoral implantation of hydroxyapatite/tricalcium phosphate-coated titanium implants. Four weeks post-surgery, all femurs were collected and analyzed by micro computed tomography followed by a mechanical test or histology. Compared with control, all rhTGF-beta2-treated groups had significantly higher bone volume. Bone-implant contact was not different between the control, 5, and 10 microg groups; however, the 20 microg group had less contact than the control. There were significant decreases in the strength of fixation in all rhTGF-beta2 treated groups compared with the control. In particular, while rhTGF-beta2 was able to enhance bone formation in the vicinity of the implant, the relative lack of bone-implant contact in the 20 microg group depressed the strength of fixation, suggesting that the location as well as the amount of new bone formed is important for implant fixation.

Distraction osteogenesis enhances remodeling of remote bones of the skeleton: a pilot study

Bone injuries have a systemic influence on the remodeling of bone. This effect has not been examined concerning its extent and duration. We measured the systemic effect of distraction osteogenesis on the remodeling of bones of the axial skeleton by means of the mineral apposition rate and bone formation rate in an animal experiment. Distraction osteogenesis was performed on the tibiae of 24 mature Yucatan minipigs. After a 4-day latency period, the tibiae were distracted 2 mm/day for 10 days. The ensuing consolidation phase lasted 10 days. Three fluorescent labeling substances were applied intravenously: calcein green at the second postoperative day, tetracycline 1 day after the end of the distraction phase, and xylene orange 2 days before sacrifice. We prepared ground sections from the ninth right ribs. The mineral apposition rate and bone formation rate were measured histomorphometrically on labeled osteons. The median mineral apposition rate during distraction was 2.39 microm/day (2.12-2.62 microm/day), which was higher than the rate during consolidation (median, 1.62 microm/day; 1.54-1.84 microm/day). The median bone formation rate confirmed this result and was 840.51 microm(2)/day (744.20-1148.26 microm(2)/day) during distraction and 384.25 microm(2)/day (330.84-467.71 microm(2)/day) during consolidation. Thus, a short period of distraction osteogenesis appears to have an anabolic effect on the mineral apposition rate of remote cortical bone.

Controlled release of bioactive transforming growth factor beta-1 from fibrin gels in vitro

This study analyzed the ability of fibrin gels to deliver added recombinant transforming growth factor beta-1 (TGF-beta1) in a controlled manner and biologically active form. First, the effects of the amount of TGF-beta1 on the release kinetics were analyzed using a single fibrin gel formulation (fibrinogen complex (FC) at 25 mg/mL, thrombin at 2 IU/mL). Then, the effects of FC and thrombin concentrations were analyzed. Finally, to test the biological activity of the released TGF-beta1 from the gels, medium supernatants taken from gels at day 3 were used as culture medium for human mesenchymal stem cell (HMSC) monolayers. Cell proliferation was analyzed after staining with calcein dye, and changes in cell morphology were observed under fluorescence microscopy at days 1, 4, and 7. At day 7, HMSC chondrogenic differentiation was assessed by Alcian Blue staining and osteogenic differentiation by alkaline phosphatase activity and Alizarin Red staining. Results showed that TGF-beta1 added to fibrin gels was gradually released from the gels and increased with the amount of TGF-beta1 initially seeded, with a total of approximately 50% of the initial amount released by day 10 (with gels containing 25 mg/mL of FC and 2 IU/mL of thrombin). The release was lower with increasing FC concentrations, suggesting a binding affinity of TGF-beta1 with the FC component. Varying the thrombin concentration had a lesser effect. HMSC monolayers cultured with medium supernatants collected from gels at day 3 and containing released TGF-beta1 showed a change in morphology (squared to polygonal), lower cell proliferation, positive Alcian Blue staining but low levels of osteogenic differentiation markers. These results demonstrated that released TGF-beta1 was still bioactive and tended to induce mainly chondrogenic differentiation of the HMSC. Overall, the present study demonstrated that fibrin gels could be used as a carrier matrix for controlled release of bioactive TGF-beta1 by adjusting the concentrations of FC and thrombin in the gels.

Matrices and scaffolds for delivery of bioactive molecules in bone and cartilage tissue engineering

Regeneration of bone and cartilage defects can be accelerated by localized delivery of appropriate growth factors incorporated within biodegradable carriers. The carrier essentially allows the impregnated growth factor to release at a desirable rate and concentration, and to linger at injury sites for a sufficient time to recruit progenitors and stimulate tissue healing processes. In addition, the carrier can be formulated to have particular structure to facilitate cellular infiltration and growth. In this review, we present a summary of growth factor delivery carrier systems for bone and cartilage tissue engineering. Firstly, we describe a list of growth factors implicated in repair and regeneration of bone and cartilage by addressing their biological effects at different stages of the healing process. General requirements for localized growth factor delivery carriers are then discussed. We also provide selective examples of material types (natural and synthetic polymers, inorganic materials, and their composites) and fabricated forms of the carrier (porous scaffolds, microparticles, and hydrogels), highlighting the dose-dependent efficacy, release kinetics, animal models, and restored tissue types. Extensive discussion on issues involving currently investigated carriers for bone and cartilage tissue engineering approaches may illustrate future paths toward the development of an ideal growth factor delivery system.

In vivo study on the healing of bone defects treated with bone marrow stromal cells, platelet-rich plasma, and freeze-dried bone allografts, alone and in combination

The repair of confined trabecular bone defects in rabbits treated by autologous bone marrow stromal cells (BMSC), platelet-rich plasma (PRP), freeze-dried bone allografts (FDBA) alone and in combination (BMSC + PRP; FDBA + BMSC; FDBA + PRP; FDBA + PRP + BMSC) was compared. A critical size defect was created in the distal part of the femurs of 48 adult rabbits. Histology and histomorphometry were used in the evaluation of healing at 2, 4, and 12 weeks after surgery. The healing rate (%) was calculated by measuring the residual bone defect area. Architecture of the newly formed bone was compared with that of bone at the same distal femur area of healthy rabbits. The defect healing rate was higher in PRP + BMSC, FDBA + PRP, FDBA + BMSC, and FDBA + PRP + BMSC treatments, while lower values were achieved with PRP treatment at all experimental times. The highest bone-healing rate at 2 weeks was achieved with FDBA + PRP + BMSC treatment, which resulted significantly different from PRP (p < 0.05) and BMSC (p < 0.05) treatments. At 4 weeks, the bone-healing rate increased except for PRP treatment. Finally, the bone-healing rate of FDBA + PRP, FDBA + BMSC, and FDBA + PRP + BMSC was significantly higher than that of PRP at 12 weeks (p < 0.05). At 12 weeks, significant differences still existed between PRP, BMSC, and FDBA groups and normal bone (p < 0.05). These results showed that the combination of FDBA, BMSC and PRP permitted an acceleration in bone healing and bone remodeling processes.

Spatial control of protein within biomimetically nucleated mineral

An ideal approach for bone tissue engineering allows for osteoconductivity, osteoinductivity, and cell transplantation. In this study, we examined coprecipitation and surface adsorption schemes with respect to their abilities to control the spatial quantity and localization of a model protein, bovine serum albumin (BSA), that is incorporated into a biomimetic apatite layer nucleated onto polylactic-co-glycolic acid (PLGA) films. Protein incorporation was characterized by determining protein: presence, quantity loaded, retention, effects on mineral morphology, and localization. FT-IR confirmed the presence of protein in all coprecipitation samples with stronger peaks in the coprecipitated samples compared to the surface adsorbed samples. Coprecipitation resulted in higher loading capacities and higher protein retention versus adsorption. Protein incorporation via coprecipitation changed the mineral morphology from sharp plate-like structures to more rounded structures, whereas, surface adsorption did not change mineral structure. By using confocal microscopy to examine the incorporation of fluorescently labeled proteins, spatial control over protein localization was exhibited. By controlling the loading quantity and localization of the model protein through the mineral thickness, a desired release profile can be achieved. A desired and effective delivery system of biological agents utilizing coprecipitation for bone regeneration can therefore be designed.

The Otto Aufranc Award: bone augmentation around and within porous implants by local bisphosphonate elution

The bisphosphonate zoledronic acid chemically and physically was bound to hydroxyapatite-coated porous tantalum implants. The zoledronic acid elution characteristics in saline were determined as a function of time and the in vivo effects of elution were quantified at 12 weeks in a canine ulnar implant model. Intramedullary implants surgically were implanted bilaterally into the ulnae of a control group of five dogs and a zoledronic acid-dosed (0.05 mg zoledronic acid) group of four dogs. Computerized image analysis of undecalcified histologic sections was used to quantify the amount of peri-implant bone within the intramedullary canal, the percentage of available pore space filled with new bone, and the number and size of the individual bone islands within the implant pores. The data were analyzed using a hierarchical analysis of variance with 95% confidence intervals. The peri-implant bone occupied a mean of 13.8% of the canal space in controls and 32.2% of the canal space in zoledronic acid-dosed dogs, a relative difference of 134% (2.34-fold) that was significant. The mean extent of bone ingrowth was 12.5% for the control implants and 19.8% for the zoledronic acid-dosed dogs, a relative difference of 58% that was statistically significant. Individual islands of new bone formation with the implant pores were similar in number in both implant groups but were 71% larger on average in the ZA-dosed group. We are the first authors to show that local elution of a bisphosphonate can cause substantial bone augmentation around and within porous orthopaedic implants. The concept represents a potential tool for restoration of bone stock and enhancement of implant fixation in primary and revision cementless joint arthroplasty surgeries in the face of compromised or deficient bone.

Restoration of bone defect and enhancement of bone ingrowth using partially demineralized bone matrix and marrow stromal cells

PURPOSE

This study aimed to investigate the capability of combining marrow stromal cells (MSC) and partially demineralized bone matrix (PDBM) to fill bone defect and enhance bone ingrowth using a canine non-weight-bearing gap model.

METHODS

Custom-made implants with 3mm gap between the porous surface and the host bone were used. The implants were inserted into the distal femurs of 25 mongrel dogs and the gaps were randomly assigned to be filled with culture-expanded autologous MSC-loaded PDBM, autograft, fresh-frozen allograft, PDBM alone, or nothing as controls. Histomorphometry using backscattered scanning electron microscopic examination, and mechanical push-out test were performed at 6 months after surgery.

RESULTS

Histomorphometry showed that amounts of bone regeneration in the gap and bone ingrowth into the porous-coated surface in the MSC-loaded PDBM-treated group were comparable to those of autograft-treated group and were significantly greater than those of allograft-treated, PDBM-treated, or non-grafted groups. Mechanical test showed the same differences.

CONCLUSION

The results of this study showed that combining PDBM and autologous culture-expanded MSC restored bone stock and enhanced bone ingrowth into the porous-coated area in a canine non-weight-bearing gap model. This combination may provide an option for reconstructing bone defect when we perform a cementless revision arthroplasty.

Transforming growth factor-beta1 to the bone

TGF-beta1 is a ubiquitous growth factor that is implicated in the control of proliferation, migration, differentiation, and survival of many different cell types. It influences such diverse processes as embryogenesis, angiogenesis, inflammation, and wound healing. In skeletal tissue, TGF-beta1 plays a major role in development and maintenance, affecting both cartilage and bone metabolism, the latter being the subject of this review. Because it affects both cells of the osteoblast and osteoclast lineage, TGF-beta1 is one of the most important factors in the bone environment, helping to retain the balance between the dynamic processes of bone resorption and bone formation. Many seemingly contradictory reports have been published on the exact functioning of TGF-beta1 in the bone milieu. This review provides an overall picture of the bone-specific actions of TGF-beta1 and reconciles experimental discrepancies that have been reported for this multifunctional cytokine.

Local application of rhTGF-beta2 enhances peri-implant bone volume and bone-implant contact in a rat model

Orthopedic and dental implant fixation depends upon bone regeneration. Growth factors such as transforming growth factor-beta (TGF-beta) have been shown to enhance bone repair and strengthen the mechanical connection between implant and host skeleton in canine models. To provide a platform for studying molecular mechanisms of growth factor stimulated bone regeneration and implant fixation, the present study examined peri-implant bone volume as a response to TGF-beta treatment in a rodent model. The rat femoral ablation model in which an implant is placed in the medullary cavity of the femur was used to examine the dose response to TGF-beta2 applied to the implant (0, 0.1, 1.0, or 10 microg). The study included a total of 40 rats (10 per dose) examined at 28 days. Peri-implant bone volume and bone-implant contact were assessed through microcomputed tomography and implant fixation strength was determined by a mechanical pullout test. Treatment of the implant with 10 microg TGF-beta2 led to a 2-fold increase in bone volume (P<0.001) and a 1.5-fold increase in bone-implant contact (P<0.01) with a trend of increasing fixation strength (non-significant increase of 1.4-fold). TGF-beta2 treatment with 10 microg led to uniform peri-implant bone volume and bone-implant contact along the length of the implant, whereas the other groups had less bone at the mid-point compared to the proximal and distal aspects of the implant. About 50% of the variance in implant fixation strength was explained by a regression model involving both bone-implant contact and peri-implant bone volume.

Imparting Mineral Affinity to Fetuin by Bisphosphonate Conjugation: A Comparison of Three Bisphosphonate Conjugation Schemes

Protein conjugation to bisphosphonic acids (BPs), such as 1-amino-1,1-diphosphonate methane (aminoBP) and 3,5-di(ethylamino-2,2-bisphosphono)benzoic acid (diBP), was proposed as a foundation for bone-specific delivery of protein therapeutics. This study was performed to directly compare the mineral affinity of protein-BP conjugates prepared by three different approaches. Fetuin, serving as a model protein, was derivatized with BPs by the following approaches: (i) by attaching the aminoBPs onto protein lysines using succinimidyl-4-(N-maleimidomethyl)-cyclohexane-1-carboxylate (SMCC); (ii) by attaching the aminoBPs onto protein carbohydrates using 4-(maleimidomethyl)-cyclohexane-1-carboxyl-hydrazide (MMCCH), and (iii) by conjugating diBP to protein lysines using the carbodiimide chemistry. The results indicated that conjugation of aminoBP and diBP to fetuin by all three means unequivocally enhanced the protein's affinity for hydroxyapatite in vitro. Similarly, conjugation of aminoBP and diBP onto fetuin increased the protein's retention in a mineral-containing matrix (Pro-Osteon) when the proteins were implanted in a rat subcutaneous model. Upon parenteral administration, however, no discernible differences were found between the SMCC- or MMCCH-linked conjugates and unmodified fetuin to target to bony tissues. DiBP-fetuin conjugates, however, led to successful bone targeting after intravenous injection in rats. We conclude that all three conjugation schemes were equally effective in imparting an affinity to the proteins toward mineral-containing matrices. Bone targeting, however, was achieved only with diBP conjugation to fetuin, supportive of the superior ability of this BP with a higher density of bisphosphonic acid groups.

Depressed bone mineralization following high dose TGF-beta1 application in an orthopedic implant model

Several previous studies of bone repair have shown 2- to 4-fold increases in bone formation following local delivery of exogenous transforming growth factor-beta (TGF-beta). Here, we use quantitative backscatter electron microscopy to test the effect of TGF-beta1 on mineralization of regenerated bone by examining tissue samples from a previously published canine study in which we found increased bone formation. In the experiment, the proximal humeri of 10 male canines were implanted bilaterally for 28 days with porous-coated implants in the presence of a 3 mm gap between the surface of the implant and the host bone. Implants placed in the left humeri were treated with TGF-beta1 at a dose of either 120 microg (n = 5) or 335 microg (n = 5), and the implants placed in the contralateral humeri served as untreated controls. Quantitative backscatter scanning electron microscopy was used to assess the volume fraction of bone and its degree of mineralization in the 3 mm gaps. The calibrated grayscale mean and median values were depressed compared to the controls in the high dose group (p = 0.048 and p = 0.041, respectively), suggesting that high dose TGF-beta delayed or inhibited mineralization of newly formed osteoid.

Zoledronic acid causes enhancement of bone growth into porous implants

The effect of zoledronic acid on bone ingrowth was examined in an animal model in which porous tantalum implants were placed bilaterally within the ulnae of seven dogs. Zoledronic acid in saline was administered via a single post-operative intravenous injection at a dose of 0.1 mg/kg. The ulnae were harvested six weeks after surgery. Undecalcified transverse histological sections of the implant-bone interfaces were imaged with backscattered scanning electron microscopy and the percentage of available pore space that was filled with new bone was calculated. The mean extent of bone ingrowth was 6.6% for the control implants and 12.2% for the zoledronic acid-treated implants, an absolute difference of 5.6% (95% confidence interval, 1.2 to 10.1) and a relative difference of 85% which was statistically significant. Individual islands of new bone formation within the implant pores were similar in number in both groups but were 69% larger in the zoledronic acid-treated group. The bisphosphonate zoledronic acid should be further investigated for use in accelerating or enhancing the biological fixation of implants to bone.

Protein- and gene-based tissue engineering in bone repair

A tissue engineering approach to bone regeneration includes the use of a scaffold, cells and bioactive factors alone or in various combinations. Several investigators have demonstrated enhanced bone formation when the tissue-engineered construct possesses traits inherent to autogenic bone grafts, namely osteoconductivity, osteoinductivity and osteogenicity. Use of the biodegradable polymer poly(lactide-co-glycolide) in combination with bone morphogenetic protein or primary cells genetically modified to release osteogenic protein have demonstrated the ability to induce osteogenic differentiation of, and subsequent mineralization by, muscle-derived cells and mesenchymal stem cells in both in vitro and in vivo applications.

Locally delivered rhBMP-2 enhances bone ingrowth and gap healing in a canine model

The purpose of the present study was to determine if recombinant human bone morphogenetic protein-2 (rhBMP-2) enhances bone ingrowth into porous-coated implants and gap healing around the implants. In the presence of a 3-mm gap between the implant and host bone, porous-coated implants were placed bilaterally for four weeks in the proximal humeri of skeletally mature, adult male dogs. In three treatment groups, the test implant was treated with HA/TCP and rhBMP-2 in buffer at a dose of 100 microg/implant (n=5), 400 microg/implant (n=6), or 800 microg/implant (n=5) and placed in the left humerus. In these same animals, an internal control implant was treated only with HA/TCP and buffer and placed in the right humerus. These groups were compared with a previously reported external control group of seven animals in which no growth factor was delivered [J. Orthop. Res. 19 (2001) 85]. The BMP treated implants in the two lower dose groups had significantly more bone ingrowth than the external controls with the greatest effect in the 100 g/implant group (a 3.5-fold increase over the external control, p=0.008). All three dose groups had significantly more bone formation in the 3-mm gap surrounding the BMP treated implants than the external controls with the greatest effect in the 800 microg group (2.9-fold increase, p<0.001). Thus, application of rhBMP-2 to a porous-coated implant stimulated local bone ingrowth and gap healing. The enhancement of bone formation within the implant (bone ingrowth) was inversely related to dose.

The effects of sex and estrogen therapy on bone ingrowth into porous coated implant

Amounts of bone ingrowth into porous cobalt-chromium plugs were compared between male and female dogs, and among sham-operated and ovariectomized female dogs, with or without estrogen treatment, to investigate the effect of gender and estrogen therapy on biologic fixation. Each group consisted of eight skeletally mature dogs. Plugs were implanted bilaterally in the distal femur at 6 months after ovariectomy or sham operation. Estrogen treatment group received estradiol 20 microg/kg/day subcutaneous injection. Three months after implantation, histological examination showed significantly more bone ingrowth in areas with cortical bone contact than in areas with cancellous bone contact (P<0.001 for all groups). Bone ingrowth was essentially the same in male and female control dogs. Ovariectomized dogs showed less overall bone ingrowth than male and female controls (P=0.007). Bone ingrowth in areas with cortical bone contact did not decrease significantly, whereas bone ingrowth in areas with cancellous bone contact was significantly impaired (P<0.001) in ovariectomized dogs compared with female controls. Short-term, high-dose estradiol treatment did not increase bone ingrowth volume fraction. Mechanical tests did not show any statistical differences among groups.

CONCLUSION

Type of bone contact is the key factor affecting the amount and pattern of bone ingrowth into the porous surface. Ovariectomy results in decreased bone ingrowth in areas with cancellous bone contact, but does not compromise bone ingrowth in areas with cortical bone contact. Short-term, high-dose estradiol treatment does not enhance bone ingrowth into the porous surface. Extensively coated or full-coated porous prostheses are recommended to achieve enough cortical bone contact and ingrowth for post-menopausal patients.

Collagen-hydroxyapatite/tricalcium phosphate microspheres as a delivery system for recombinant human transforming growth factor-beta 1

The purpose of this study is to evaluate the carrier capability of collagen-hydroxyapatite/tricalcium phosphate (Col-HA/TCP) microspheres to the rhTGF-beta 1 (recombinant human transforming growth factor-beta 1). After anesthesia, a bone defect (7.0 mm in diameter and 10.0 mm in depth) was created at the distal femoral condyles of New Zealand white rabbits. These defects were then completely filled with the implant materials. After 5, 7, 9, 11, 13, and 15 weeks, the animals were sacrificed and histological evaluations were performed. The results showed that when the defects were treated with Col-HA/TCP microspheres without rhTGF-beta 1, there was only spotty new bone formation during the 15 week experimental period and most of the defect was filled with fibrous tissue and inflammatory cells, whereas active bone formation with mature marrow tissue formation was evident in the defect treated with Col-HA/TCP microspheres containing rhTGF-beta 1. Collagen-hydroxyapatite/tricalcium phosphate microspheres were expected to be replaced by the regenerated bone structure as the bone reconstruction and bone-remodeling process occurred. It was apparent that bone regeneration was influenced by the addition of rhTGF-beta 1. Collagen-hydroxyapatite/tricalcium phosphate microspheres were a good carrier for rhTGF-beta 1.

Aging does not lessen the effectiveness of TGFbeta2-enhanced bone regeneration

Controversy exists over the potency of bone healing in the aged skeleton, and there is concern that enhancement of bone regeneration after use of bone-stimulating growth factors may not be effective in the aged. In this study, 30 skeletally mature beagles (1-2 or 10-12 years old) had titanium implants placed bilaterally in the proximal humerus for a period of 4 weeks in a model of intramembranous bone regeneration. A bony defect made at the time of surgery created a 3-mm gap between the implant surface and the host bone. Some of the implants were treated with recombinant human TGFbeta2 (rhTGFbeta2) at various does (0.32-35 microg per implant), and some served as paired controls. The dose response was similar in young and old animals. The most effective dose, 35 microg, led to a 3-fold increase in the volume fraction of new bone within the gap in both the young (p = 0.001) and old (p = 0.002) animals. At this dose, there was a 5-fold increase in osteoblast surface. While age did not significantly affect the quantity of new bone formed as assessed by backscatter scanning electron microscopy, the older animals had thinner regenerated trabeculae that tended to be spaced more closely than the younger animals. Coupled with the finding that the increase in osteoid was greater in the old animals compared with the young animals, these qualitative differences suggest that there may have been a slight delay in the rate or a defect of mineralization in the old animals.

Medical applications of transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) proteins and their antagonists have entered clinical trials. These multi-functional regulators of cell growth and differentiation induce extracellular matrix proteins and suppress the immune system making TGF-betas useful in treatment of wounds with impaired healing, mucositis, fractures, ischemia-reperfusion injuries, and autoimmune disease. In diseases such as keloids, glomerulonephritis and pulmonary fibrosis, excessive expression of TGF-beta has been implicated as being responsible for accumulation of detrimental scar tissue. In these conditions, agents that block TGF-beta have prevented or reversed disease. Similarly, in carcinogenesis, blocking TGF-beta activity may be valuable in stimulating an immune response towards metastasis. As these blocking agents receive approval, we will likely have new therapies for previously recalcitrant diseases.