Induced Experimental Peri-Implantitis and Periodontitis: What Are the Differences in the Inflammatory Response?

This preliminary study investigates the differences between experimental periodontitis and peri-implantitis in a dog model, with a focus on the histopathology, inflammatory responses, and specific immunoregulatory activities driven by Th1/Th2-positive cells. Twelve dental implants were inserted into the edentulated posterior mandibles of 6 beagle dogs and were given 12 weeks for osseointegration. Experimental peri-implantitis and periodontitis (first mandible molar) were then induced using cotton-floss ligatures. Twelve weeks later, alveolar bones were quantitated by cone beam-computer tomography. Histopathologic analysis of the inflamed gingiva and periodontal tissues was performed by light microscopy, and the Th1/Th2 cell populations were investigated by flow cytometry. Peri-implantitis and periodontitis were both found to be associated with pronounced bone resorption effects, both to a similar degree vertically, but with a differential bone resorption pattern mesio-distally, and with a significantly higher and consistent bone resorption result in peri-implantitis, although with a higher variance of bone resorption in periodontitis. The histologic appearances of the inflammatory tissues were identical. The percentages of Th1/Th2 cells in the inflamed gingival tissues of both experimental peri-implantitis and periodontitis were also found to be similar. Experimental periodontitis and peri-implantitis in the dog model show essentially the same cellular pathology of inflammation. However, bone resorption was found to be significantly higher in peri-implantitis; the histopathologic changes in the periodontal tissues were similar in both groups but showed a higher interindividual variation in periodontitis and appeared more uniform in peri-implantitis. This preliminary study indicates that more focused experimental in vivo inflammation models need to be developed to better simulate the human pathology in the 2 different diseases and to have a valuable tool to investigate more specifically how novel treatments/prevention approaches may heal the differential adverse effects on bone tissue and on periodontium in periodontitis and in periimplantitis.

Human Bone Typing Using Quantitative Cone-Beam Computed Tomography.

INTRODUCTION: Bone typing is crucial to enable the choice of a suitable implant, the surgical technique, and the evaluation of the clinical outcome. Currently, bone typing is assessed subjectively by the surgeon. OBJECTIVE: The aim of this study is to establish an automatic quantification method to determine local bone types by the use of cone-beam computed tomography (CBCT) for an observer-independent approach. METHODS: Six adult human cadaver skulls were used. The 4 generally used bone types in dental implantology and orthodontics were identified, and specific Hounsfield unit (HU) ranges (grey-scale values) were assigned to each bone type for identification by quantitative CBCT (qCBCT). The selected scanned planes were labelled by nonradiolucent markers for reidentification in the backup/cross-check evaluation methods. The selected planes were then physically removed as thick bone tissue sections for in vitro correlation measurements by qCBCT, quantitative micro-computed tomography (micro-CT), and quantitative histomorphometry. RESULTS: Correlation analyses between the different bone tissue quantification methods to identify bone types based on numerical ranges of HU values revealed that the Pearson correlation coefficient of qCBCT with micro-CT and quantitative histomorphometry was R = 0.9 (P = .001) for all 4 bone types . CONCLUSIONS: We found that  qCBCT can reproducibly and objectively assess human bone types at implant sites.

Crystalline Biomimetic Calcium Phosphate Coating on Mini-Pin Implants to Accelerate Osseointegration and Extend Drug Release Duration for an Orthodontic Application.

Miniscrew implants (MSIs) have been widely used as temporary anchorage devices in orthodontic clinics. However, one of their major limitations is the relatively high failure rate. We hypothesize that a biomimetic calcium phosphate (BioCaP) coating layer on mini-pin implants might be able to accelerate the osseointegration, and can be a carrier for biological agents. A novel mini-pin implant to mimic the MSIs was used. BioCaP (amorphous or crystalline) coatings with or without the presence of bovine serum albumin (BSA) were applied on such implants and inserted in the metaphyseal tibia in rats. The percentage of bone to implant contact (BIC) in histomorphometric analysis was used to evaluate the osteoconductivity of such implants from six different groups (n=6 rats per group): (1) no coating no BSA group, (2) no coating BSA adsorption group, (3) amorphous BioCaP coating group, (4) amorphous BioCaP coating-incorporated BSA group, (5) crystalline BioCaP coating group, and (6) crystalline BioCaP coating-incorporated BSA group. Samples were retrieved 3 days, 1 week, 2 weeks, and 4 weeks post-surgery. The results showed that the crystalline BioCaP coating served as a drug carrier with a sustained release profile. Furthermore, the significant increase in BIC occurred at week 1 in the crystalline coating group, but at week 2 or week 4 in other groups. These findings indicate that the crystalline BioCaP coating can be a promising surface modification to facilitate early osseointegration and increase the success rate of miniscrew implants in orthodontic clinics.

Anticytokine Activity Enhances Osteogenesis of Bioactive Implants.

In dental clinical practice, systemic steroids are often applied at the end of implant surgeries to reduce postsurgical inflammation (tissue swelling, etc.) and to reduce patient discomfort. However, the use of systemic steroids is associated with generalized catabolic effects and with a temporarily reduced immunological competence. We hypothesize that by applying locally anticytokine antibodies (antitumor necrosis factor alpha and anti-interleukin-1 beta) together with a bioactive osteogenic implant at the time of the surgical intervention for the placement of a construct, we will be able to achieve the same beneficial effects as those using systemic steroids but are able to avoid the generalized antianabolic effects and the reduced immunocompetence effects, associated with the systemic use of steroids. In an adult rat model, a collagen sponge, soaked with the osteogenic agent bone morphogenetic protein-2, was used as an example for a bioactive implant material and was surgically placed subcutaneously. In the acute inflammatory phase after implantation (2 days after surgery) we investigated the local inflammatory tissue response, and 18 days postsurgically the efficiency of local osteogenesis (to assess possible antianabolic effects). We found that the negative control groups, treated postsurgically with systemic steroids, showed a significant suppression of both the inflammatory response and the osteogenetic activity, that is, they were associated with significant general antianabolic effects, even when steroids were used only at a low dose level. The local anticytokine treatment, however, was able to significantly enhance new bone formation activity, that is, the anabolic activity, over positive control values with BMP-2 only. However, the anticytokine treatment was unable to reduce the local inflammatory and swelling responses.

Mechanical insertion properties of calcium-phosphate implant coatings.

OBJECTIVES: To investigate the influence of protein incorporation on the resistance of biomimetic calcium-phosphate coatings to the shear forces that are generated during implant insertion. MATERIALS AND METHODS: Thirty-eight standard (5 × 13 mm) Osseotite® implants were coated biomimetically with a layer of calcium phosphate, which either lacked or bore a co-precipitated (incorporated) depot of the model protein bovine serum albumin (BSA). The coated implants were inserted into either artificial bone (n=18) or the explanted mandibles of adult pigs (n=12). The former set-up was established for the measurement of torque and of coating losses during the insertion process. The latter set-up was established for the histological and histomorphometric analysis of the fate of the coatings after implantation. RESULTS: BSA-bearing coatings had higher mean torque values than did those that bore no protein depot. During the insertion process, less material was lost from the former than from the latter type of coating. The histological and histomorphometric analysis revealed fragments of material to be sheared off from both types of coating at vulnerable points, namely, at the tips of the threads. The sheared-off fragments were retained within the peri-implant space. CONCLUSION: The incorporation of a protein into a biomimetically prepared calcium-phosphate coating increases its resistance to the shear forces that are generated during implant insertion. In a clinical setting, the incorporated protein would be an osteogenic agent, whose osteoinductive potential would not be compromised by the shearing off of coating material, and the osteoconductivity of an exposed implant surface would not be less than that of a coated one.

Enhanced biocompatibility and improved osteogenesis of coralline hydroxyapatite modified by bone morphogenetic protein 2 incorporated into a biomimetic coating.

OBJECTIVES: (1) To determine whether the biocompatibility of coralline hydroxyapatite (CHA) granules could be improved by using an octacalcium phosphate (OCP) coating layer, and/or functionalized with bone morphogenetic protein 2 (BMP-2), and (2) to investigate if BMP-2 incorporated into this coating is able to enhance its osteoinductive efficiency, in comparison to its surface-adsorbed delivery mode. METHODS: CHA granules (0.25 g per sample) bearing a coating-incorporated depot of BMP-2 (20 µg/sample) together with the controls (CHA bearing an adsorbed depot of BMP-2; CHA granules with an OCP coating without BMP-2; pure CHA granules) were implanted subcutaneously in rats (n = 6 animals per group). Five weeks later, the implants were retrieved for histomorphometric analysis to quantify the volume of newly generated bone, bone marrow, fibrous tissue and foreign body giant cells (FBGCs). The osteoinductive efficiency of BMP-2 and the rates of CHA degradation were also determined. RESULTS: The group with an OCP coating-incorporated depot of BMP-2 showed the highest volume and quality or bone, and the highest osteoinductive efficacy. OCP coating was able to reduce inflammatory responses (improve biocompatibility), and also simple adsorption of BMP-2 to CHA achieved this. CONCLUSIONS: The biocompatibility of CHA granules (reduction of inflammation) was significantly improved by coating with a layer of OCP. Pure surface adsorption of BMP-2 to CHA also reduced inflammation. Incorporation of BMP-2 into the OCP coatings was associated with the highest volume and quality of bone, and the highest biocompatibility degree of the CHA granules. CLINICAL SIGNIFICANCE: Higher osteoinductivity and improved biocompatibility of CHA can be obtained when a layer of BMP-2 functionalized OCP is deposited on the surfaces of CHA granules.

Polymeric mesh and insulin-like growth factor 1 delivery enhance cell homing and graft-cartilage integration.

Cartilage injury, such as full-thickness lesions, predisposes patients to the premature development of osteoarthritis, a degenerative joint disease. While surgical management of cartilage lesions has improved, long-term clinical efficacy has stagnated, owing to the lack of hyaline cartilage regeneration and inadequate graft-host integration. This study tests the hypothesis that integration of cartilage grafts with native cartilage can be improved by enhancing the migration of chondrocytes across the graft-host interface via the release of chemotactic factor from a degradable polymeric mesh. To this end, a polylactide-co-glycolide/poly-ε-caprolactone mesh was designed to localize the delivery of insulin-like growth factor 1 (IGF-1), a well-established chondrocyte attractant. The release of IGF-1 (100 ng/mg) enhanced cell migration from cartilage explants, and the mesh served as critical structural support for cell adhesion, growth, and production of a cartilaginous matrix in vitro, which resulted in increased integration strength compared with mesh-free repair. Further, this neocartilage matrix was structurally contiguous with native and grafted cartilage when tested in an osteochondral explant model in vivo. These results demonstrate that this combined approach of a cell homing factor and supportive matrix will promote cell-mediated integrative cartilage repair and improve clinical outcomes of cartilage grafts in the treatment of osteoarthritis.

A Novel Experimental Dental Implant Permits Quantitative Grading of Surface-Property Effects on Osseointegration.

PURPOSE: To test the hypothesis if a novel single-chamber experimental dental implant allows in vivo the quantitative assessment of osseointegration over time and as a function of different surface properties (physical, chemical, geometric, biologic [osteoconductive or osteoinductive]) in a biologically unfavorable environment (local osteoporosis). MATERIALS AND METHODS: Three prototypes of a novel experimental implant with different chamber sizes (small, medium, and large) were compared with each other to find out the minimum size of bone chambers needed to allow a discriminative quantification of osseointegration over time. For the comparison of low and high surface osteoconductivity properties, conventional sandblasted, acid-etched chamber surfaces (low surface osteoconductivity) were compared with biomimetically (calcium phosphate) coated ones (high surface osteoconductivity). The implants (4 implants per animal; 88 implants per time point) were inserted into the edentulous maxillae of a total of 66 adult goats with a physiologically osteoporotic masticatory apparatus. Two, 4, and 8 weeks later, they were excised and prepared for a histomorphometric analysis of the volume of neoformed bone within the chamber space and of the bone-to-implant contact (BIC) area. RESULTS: The implants with small chambers did not show significant differences in bone coverage (BIC) nor bone volume (relative and absolute volume), neither as a function of time nor as of implant surface property (low versus high surface osteoconductivity). However, medium and large chambers revealed significant differences respecting both of these parameters over the 8-week postoperative time period. CONCLUSION: The new implant model permits a discriminative quantification of osseointegration in vivo in an osteoporotic bone environment for implants with medium-sized and large-sized chambers. Quantitative assessment of osseointegration is possible, both over time and as a function of low and high surface osteoconductivity properties.

The kinetics and mechanism of bone morphogenetic protein 2 release from calcium phosphate-based implant-coatings.

Biomimetically deposited calcium phosphate-based coatings of prostheses can serve as a vehicle for the targeted delivery of growth factors to the local implant environment. Based on indirect evidence in previous studies we hypothesize that such agents are liberated gradually from the coating via a cell-mediated degradation. In the present study, we tested this hypothesis by investigating the release mechanism and its kinetics by use of a radiolabeled osteogenic agent (131 I-BMP-2) under conditions in which native cell populations with a coating-degradative potential were either absent or present. The release of 131 I-BMP-2 was monitored for 5 weeks, either in vitro or after implantation at an ectopic (subcutaneous) site in rats in vivo. Only from implants that bore a coating-incorporated depot of bone morphogenetic protein 2 (BMP-2) was the agent released slowly and steadily over 5 weeks, that is, 50% of the loaded dose was liberated in vivo (5 to 10% weekly), as against 14.6% in vitro (less than 1% weekly). The coatings bearing an incorporated depot of BMP-2 underwent significant cell-mediated degradation, whereas under cell-free conditions no degradation occurred, and the spontaneous release of BMP-2 was negligible. Our findings confirm this carrier system to be a suitable vehicle for the sustained and cell-mediated delivery of BMP-2. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A:2363-2371, 2018.

The influence of BMP-2 and its mode of delivery on the osteoconductivity of implant surfaces during the early phase of osseointegration.

Osteogenic agents, such as bone morphogenetic protein-2 (BMP-2), can stimulate the degradation as well as the formation of bone. Hence, they could impair the osteoconductivity of functionalized implant surfaces. We assessed the effects of BMP-2 and its mode of delivery on the osteoconductivity of dental implants with either a naked titanium surface or a calcium-phosphate-coated one. The naked titanium surface bore adsorbed BMP-2, whilst the coated one bore incorporated, adsorbed, or incorporated and adsorbed BMP-2. The implants were inserted into the maxillae of adult miniature pigs. The volume of bone deposited within a defined "osteoconductive" (peri-implant) space, and bone coverage of the implant surface delimiting this space, were estimated morphometrically 1-3 weeks later. After 3 weeks, the volume of bone deposited within the osteoconductive space was highest for coated and uncoated implants bearing no BMP-2, followed by coated implants bearing incorporated BMP-2; it was lowest for coated implants bearing only adsorbed BMP-2. Bone-interface coverage was highest for coated implants bearing no BMP-2, followed by coated implants bearing either incorporated, or incorporated and adsorbed BMP-2; it was lowest for uncoated implants bearing adsorbed BMP-2. Hence, the osteoconductivity of implant surfaces can be significantly modulated by BMP-2 and its mode of delivery.

Bone morphogenetic protein 2 incorporated into biomimetic coatings retains its biological activity.

We have previously shown that proteins can be incorporated into the latticework of calcium phosphate layers when biomimetically coprecipitated with the inorganic components, upon the surfaces of titanium-alloy implants. In the present study, we wished to ascertain whether recombinant human bone morphogenetic protein 2 (rhBMP-2) thus incorporated retained its bioactivity as an osteoinductive agent. Titanium alloy implants were coated biomimetically with a layer of calcium phosphate in the presence of different concentrations of rhBMP-2 (0.1-10 microg/mL). rhBMP-2 was successfully incorporated into the crystal latticework, as revealed by protein blot staining. rhBMP-2 was taken up by the calcium phosphate coatings in a dose-dependent manner, as determined by ELISA. Rat bone marrow stromal cells were grown directly on these coatings for 8 days. Their osteogenicity was then assessed quantitatively by monitoring alkaline phosphatase activity. This parameter increased as a function of rhBMP-2 concentrations within the coating medium. rhBMP-2 incorporated into calcium phosphate coatings was more potent in stimulating the alkaline phosphatase activity of the adhering cell layer than was the freely suspended drug in stimulating that of cell layers grown on a plastic substratum. This system may be of osteoinductive value in orthopedic and dental implant surgery.

Bovine primary chondrocyte culture in synthetic matrix metalloproteinase-sensitive poly(ethylene glycol)-based hydrogels as a scaffold for cartilage repair.

A poly(ethylene glycol) (PEG)-based hydrogel was used as a scaffold for chondrocyte culture. Branched PEG-vinylsulfone macromers were end-linked with thiol-bearing matrix metalloproteinase (MMP)-sensitive peptides (GCRDGPQGIWGQDRCG) to form a three-dimensional network in situ under physiologic conditions. Both four- and eight-armed PEG macromer building blocks were examined. Increasing the number of PEG arms increased the elastic modulus of the hydrogels from 4.5 to 13.5 kPa. PEG-dithiol was used to prepare hydrogels that were not sensitive to degradation by cell-derived MMPs. Primary bovine calf chondrocytes were cultured in both MMP-sensitive and MMP-insensitive hydrogels, formed from either four- or eight-armed PEG. Most (>90%) of the cells inside the gels were viable after 1 month of culture and formed cell clusters. Gel matrices with lower elastic modulus and sensitivity to MMP-based matrix remodeling demonstrated larger clusters and more diffuse, less cell surface-constrained cell-derived matrix in the chondron, as determined by light and electron microscopy. Gene expression experiments by real-time RT-PCR showed that the expression of type II collagen and aggrecan was increased in the MMP-sensitive hydrogels, whereas the expression level of MMP-13 was increased in the MMP-insensitive hydrogels. These results indicate that cellular activity can be modulated by the composition of the hydrogel. This study represents one of the first examples of chondrocyte culture in a bioactive synthetic material that can be remodeled by cellular protease activity.

Modulation of human osteoblasts by metal surface chemistry.

The use of metal implants in dental and orthopedic surgery is continuously expanding and highly successful. While today longevity and load-bearing capacity of the implants fulfill the expectations of the patients, acceleration of osseointegration would be of particular benefit to shorten the period of convalescence. To further clarify the options to accelerate the kinetics of osseointegration, within this study, the osteogenic properties of structurally identical surfaces with different metal coatings were investigated. To assess the development and function of primary human osteoblasts on metal surfaces, cell viability, differentiation, and gene expression were determined. Titanium surfaces were used as positive, and surfaces coated with gold were used as negative controls. Little differences in the cellular parameters tested for were found when the cells were grown on titanium discs sputter coated with titanium, zirconium, niobium, tantalum, gold, and chromium. Cell number, activity of cell layer-associated alkaline phosphatase (ALP), and levels of transcripts encoding COL1A1 and BGLAP did not vary significantly in dependence of the surface chemistry. Treatment of the cell cultures with 1,25(OH)2 D3 /Dex, however, significantly increased ALP activity and BGLAP messenger RNA levels. The data demonstrate that the metal layer coated onto the titanium discs exerted little modulatory effects on cell behavior. It is suggested that the microenvironment regulated by the peri-implant tissues is more effective in regulating the tissue response than is the material of the implant itself.

Functionalization of deproteinized bovine bone with a coating-incorporated depot of BMP-2 renders the material efficiently osteoinductive and suppresses foreign-body reactivity.

The repair of critical-sized bony defects remains a challenge in the fields of implantology, maxillofacial surgery and orthopaedics. As an alternative bone-defect filler to autologous bone grafts, deproteinized bovine bone (DBB) is highly osteoconductive and clinically now widely used. However, this product suffers from the disadvantage of not being intrinsically osteoinductive. In the present study, this property was conferred by coating DBB with a layer of calcium phosphate into which bone morphogenetic protein 2 (BMP-2) was incorporated. Granules of DBB bearing a coating-incorporated depot of BMP-2--together with the appropriate controls (DBB bearing a coating but no BMP-2; uncoated DBB bearing adsorbed BMP-2; uncoated DBB bearing no BMP-2)--were implanted subcutaneously in rats. Five weeks later, the implants were withdrawn for a histomorphometric analysis of the volume densities of (i) bone, (ii) bone marrow, (iii) foreign-body giant cells and (iv) fibrous capsular tissue. Parameters (i) and (ii) were highest, whilst parameters (iii) and (iv) were lowest in association with DBB bearing a coating-incorporated depot of BMP-2. Hence, this mode of functionalization not only confers DBB with the property of osteoinductivity but also improves its biocompatibility--thus dually enhancing its clinical potential in the repair of bony defects.

Biomimetic coating of organic polymers with a protein-functionalized layer of calcium phosphate: the surface properties of the carrier influence neither the coating characteristics nor the incorporation mechanism or release kinetics of the protein.

Polymers that are used in clinical practice as bone-defect-filling materials possess many essential qualities, such as moldability, mechanical strength and biodegradability, but they are neither osteoconductive nor osteoinductive. Osteoconductivity can be conferred by coating the material with a layer of calcium phosphate, which can be rendered osteoinductive by functionalizing it with an osteogenic agent. We wished to ascertain whether the morphological and physicochemical characteristics of unfunctionalized and bovine-serum-albumin (BSA)-functionalized calcium-phosphate coatings were influenced by the surface properties of polymeric carriers. The release kinetics of the protein were also investigated. Two sponge-like materials (Helistat® and Polyactive®) and two fibrous ones (Ethisorb™ and poly[lactic-co-glycolic acid]) were tested. The coating characteristics were evaluated using state-of-the-art methodologies. The release kinetics of BSA were monitored spectrophotometrically. The characteristics of the amorphous and the crystalline phases of the coatings were not influenced by either the surface chemistry or the surface geometry of the underlying polymer. The mechanism whereby BSA was incorporated into the crystalline layer and the rate of release of the truly incorporated depot were likewise unaffected by the nature of the polymeric carrier. Our biomimetic coating technique could be applied to either spongy or fibrous bone-defect-filling organic polymers, with a view to rendering them osteoconductive and osteoinductive.

Long-term cell-mediated protein release from calcium phosphate ceramics.

Efficient delivery of growth factors from carrier biomaterials depends critically on the release kinetics of the proteins that constitute the carrier. Immobilizing growth factors to calcium phosphate ceramics has been attempted by direct adsorption and usually resulted in a rapid and passive release of the superficially adherent proteins. The insufficient retention of growth factors limited their bioavailability and their efficacy in the treatment of bone regeneration. In this study, a coprecipitation technique of proteins and calcium phosphate was employed to modify the delivery of proteins from biphasic calcium phosphate (BCP) ceramics. To this end, tritium-labeled bovine serum albumin ([(3)H]BSA) was utilized as a model protein to analyze the coprecipitation efficacy and the release kinetics of the protein from the carrier material. Conventional adsorption of [(3)H]BSA resulted in a rapid and passive release of the protein from BCP ceramics, whereas the coprecipitation technique effectively prevented the burst release of [(3)H]BSA. Further analysis of the in vitro kinetics demonstrated a sustained, cell-mediated release of coprecipitated [(3)H]BSA from BCP ceramics induced by resorbing osteoclasts. The coprecipitation technique described herein, achieved a physiologic-like protein release, by incorporating [(3)H]BSA into its respective carriers, rendering it a promising tool in growth factor delivery for bone healing.

Impact of pore size on the vascularization and osseointegration of ceramic bone substitutes in vivo.

The repair of bone defects with biomaterials depends on a sufficient vascularization of the implantation site. We analyzed the effect of pore size on the vascularization and osseointegration of biphasic calcium phosphate particles, which were implanted into critical-sized cranial defects in Balb/c mice. Dense particles and particles with pore sizes in the ranges 40-70, 70-140, 140-210, and 210-280 microm were tested (n = 6 animals per group). Angiogenesis, vascularization, and leukocyte-endothelium interactions were monitored for 28 days by intravital microscopy. The formation of new bone and the bone-interface contact (BIC) were determined histomorphometrically. Twenty-eight days after implantation, the functional capillary density was significantly higher with ceramic particles whose pore sizes exceeded 140 microm [140-210 microm: 6.6 (+/-0.8) mm/mm(2); 210-280 microm: 7.3 (+/-0.6) mm/mm(2)] than with those whose pore sizes were lesser than 140 microm [40-70 microm: 5.3 (+/-0.4) mm/mm(2); 70-140 microm: 5.6 (+/-0.3) mm/mm(2)] or with dense particles [5.7 (+/-0.8) mm/mm(2)]. The volume of newly-formed bone deposited within the implants increased as the pore size increased [40-70 microm: 0.07 (+/-0.02) mm(3); 70-140 microm: 0.10 (+/-0.06) mm(3); 140-210 microm: 0.13 (+/-0.05) mm(3); 210-280 microm: 0.15 (+/-0.06) mm(3)]. Similar results were observed for the BIC. The data demonstrates pore size to be a critical parameter governing the dynamic processes of vascularization and osseointegration of bone substitutes.

Release kinetics of transforming growth factor-beta1 from fibrin clots.

In any therapeutic model involving a tissue-engineering approach to the repair of partial-thickness articular cartilage defects, a chondrogenic differentiation factor is required to ensure tissue-specific healing. Transforming growth factor-beta1 (TGF-beta1) is known to act in this capacity, but at such high concentrations as to render its direct injection into the joint cavity inadvisable. This situation calls for a delivery system that can be applied directly to the defect site and that will release the drug gradually over a period of some weeks. Liposome encapsulation represents one such system, and has been recently implemented with some success in an animal model for cartilage repair. However, the kinetics of TGF-beta1 release have not been determined, it was the purpose of the present study to characterize these. The liberation of [(125)I]-labeled TGF-beta1 from fibrin matrices containing this agent in either a free or liposome-encapsulated form was monitored by liquid scintillation counting for 25 days in vitro. During the initial 5 days, fibrin clots containing liposome-encapsulated TGF-beta1 released this cytokine at a slower rate (2% to 4% per day) than did those containing the free molecules (10% to 20% per day); thereafter, the release rates were similar. At the end of the incubation period, only 40% of the liposome-encapsulated TGF-beta1 had been released from the fibrin clots, as compared with 68% from those containing the free molecules. Liposome encapsulation thus represents a suitable means of establishing a slow-delivery system in tissue-engineering approaches to articular cartilage repair.