Engineering of bone using bone marrow stromal cells and a silicon-stabilized tricalcium phosphate bioceramic: evidence for a coupling between bone formation and scaffold resorption

Resorbable porous ceramic constructs, based on silicon-stabilized tricalcium phosphate, were implanted in critical-size defects of sheep tibias, either alone or after seeding with bone marrow stromal cells (BMSC). Only BMSC-loaded ceramics displayed a progressive scaffold resorption, coincident with new bone deposition. To investigate the coupled mechanisms of bone formation and scaffold resorption, X-ray computed microtomography (muCT) with synchrotron radiation was performed on BMSC-seeded ceramic cubes. These were analyzed before and after implantation in immunodeficient mice for 2 or 6 months. With increasing implantation time, scaffold thickness significantly decreased while bone thickness increased. The muCT data evidenced that all scaffolds showed a uniform density distribution before implantation. Areas of different segregated densities were instead observed, in the same scaffolds, once seeded with cells and implanted in vivo. A detailed muX-ray diffraction analysis revealed that only in the contact areas between deposited bone and scaffold, the TCP component of the biomaterial decreased much faster than the HA component. This event did not occur at areas away from the bone surface, highlighting coupling and cell-dependency of the resorption and matrix deposition mechanisms. Moreover, in scaffolds implanted without cells, both the ceramic density and the TCP:HA ratio remained unchanged with respect to the pre-implantation analysis.     

Bone formation process in porous calcium carbonate and hydroxyapatite.

This study determined the bone formation in porous calcium carbonate (CC) and porous hydroxyapatite (HA) in ectopic sites. The bone formation stimulus was derived from bone marrow cells. CC and HA in the shape of disks were implanted with or without rat marrow cells into subcutaneous sites of syngeneic rats. The CC and HA had identical microstructure: pore size was 190-230 microns, porosity was 50-60% and they were fully interconnected. Bone did not form in any implants without marrow cells (disks themselves), whereas bone consistently formed in the pores of all implants with marrow cells after 4 weeks. The bone formation of both CC and HA occurred initially on surface of the pore regions and progressed toward the center of the pore. Scanning electron microscopy and electron-probe microanalysis revealed a continuum of calcium at the interfaces of both bone/CC and bone/HA implants. These results indicate that the bone formation in calcium carbonate derived from marine corals is comparable to the bioactive hydroxyapatite.     

Nanoindentation study of interfaces between calcium phosphate and bone in an animal spinal fusion model.

Intertransverse process spinal fusion is a common surgical procedure for the treatment of spinal disorders. In the present study, a porous hydroxyapatite (HA)/beta-tricalcium phosphate (beta-TCP) ceramic was tested as graft material using a rabbit lumbar transverse process (L5-L6) fusion model. The porous ceramic blocks were implanted onto the dorsal decorticated surface of the lumbar transverse processes. The specimens were harvested at the seventh week after implantation. Histomorphological observation revealed that the integration of HA/beta-TCP with the host bone of the transverse process occurred by both cancellous bone formation and cartilage formation. Scanning electron microscopy-wavelength dispersive X-ray spectrometry examinations showed significant differences in calcium, phosphorus, and sulfur contents in the newly formed tissues and the porous HA/TCP implants. Nanoindentations were used to evaluate the intrinsic mechanical properties of the implants and the newly formed tissues. The Young's moduli of the newly formed cartilage, new cancellous bone, and HA/TCP, were 0.66 +/- 0.02 GPa, 2.36 +/- 0.50 GPa, and 10.2 +/- 1.21 GPa, respectively. Nanoindentation results revealed degradation of the porous ceramics and incomplete calcification of the new cancellous bone at the seventh week after implantation. Nanoindentation appeared to be a useful technique for assessing the mechanical status of spinal fusion in animal models.     

Analysis of primary bone formation in porous alumina: a fluorescence and scanning electron microscopic study of marrow cell induced osteogenesis.

Porous alumina ceramics alone and combined with rat marrow cells were implanted subcutaneously in the back of syngeneic Fischer rats. Fluorochrome-labeling was performed post operatively and the ceramics were harvested 6 and 8 weeks after implantation. Undecalcified sections of the implants were observed under fluorescence microscopy and the de novo bone-ceramic interfacial areas were analyzed by a scanning electron microscope equipped with an electron probe microanalyzer. Alumina ceramics alone did not show any bone formation, while all marrow cell loaded ceramics showed new bone formation 6 and 8 weeks after implantation. Bone formation was first observed in the center of the pores and proceeded in a centrifugal direction, leading to contact with the ceramic. These results suggest that bone marrow cells have inherent osteogenic capacity and in the pore region of alumina ceramics progression of the osteogenesis causes the dissipation of intervening fibrous tissue between the de novo bone and alumina ceramic surface.     

Material-dependent bone induction by calcium phosphate ceramics: a 2.5-year study in dog

Bone induction by different calcium phosphate biomaterials has been reported previously. With regard to (1) whether the induced bone would disappear with time due to the absence of mechanical stresses and (2) whether this heterotopically formed bone would give rise to uncontrolled growth, a long-time investigation of porous hydroxyapatite ceramic (HA), porous biphasic calcium phosphate ceramic (TCP/HA, BCP), porous alpha-tricalcium phosphate ceramic (alpha-TCP) and porous beta-tricalcium phosphate ceramic (beta-TCP) was performed in dorsal muscles of dog, for 2.5 years. Histological observation, backscattered scanning electron microscopy observation and histomorphometric analysis were made on thin un-decalcified sections of retrieved samples. Normal compact bone with bone marrow was found in all HA implants (n = 4) and in all BCP implants (n = 4), 48 +/- 4% pore area was filled with bone in HA implants and 41 +/- 2% in BCP implants. Bone-like tissue, which was a mineralised bone matrix with osteocytes but lacked osteoblasts and bone marrow, was found in all beta-TCP implants (n = 4) and in one of the four alpha-TCP implants. Both normal bone and bone-like tissues were confined inside the pores of the implants. The results show that calcium phosphate ceramics are osteoinductive in muscles of dogs. Although the quality and quantity varied among different ceramics, the induced bone in both HA and BCP ceramics did neither disappear nor grow uncontrollably during the period as long as 2.5 years.     

Viable osteogenic cells are obligatory for tissue-engineered ectopic bone formation in goats

In this study we investigated the bone-forming capacity of tissue-engineered (TE) constructs implanted ectopically in goats. As cell survival is questionable in large animal models, we investigated the significance of vitality, and thus whether living cells instead of only the potentially osteoinductive extracellular matrix are required to achieve bone formation. Vital TE constructs of porous hydroxyapatite (HA) covered with differentiated bone marrow stromal cells (BMSCs) within an extracellular matrix (ECM) were compared with identical constructs that were devitalized before implantation. The devitalized implants did contain the potentially osteoinductive ECM. Furthermore, we evaluated HA impregnated with fresh bone marrow and HA only. Two different types of HA granules with a volume of approximately 40 microm were investigated: HA70/800, a microporous HA with 70% interconnected macroporosity and an average pore size of 800 microm, and HA60/400, a smooth HA with 60% interconnected macropores and an average size of 400 microm. Two granules of each type were combined and then treated as a single unit for cell seeding, implantation, and histology. The tissue-engineered samples were obtained by seeding culture-expanded goat BMSCs on the HA and subsequently culturing these constructs for 6 days to allow cell differentiation and ECM formation. To devitalize, TE constructs were frozen in liquid nitrogen according to a validated protocol. Fresh bone marrow impregnation was performed perioperatively (4 mL per implant unit). All study groups were implanted in bilateral paraspinal muscles. Fluorochromes were administered at three time points to monitor bone mineralization. After 12 weeks the units were explanted and analyzed by histology of nondecalcified sections. Bone formation was present in all vital tissue-engineered implants. None of the other groups showed any bone formation. Histomorphometry indicated that microporous HA70/800 yielded more bone than did HA60/400. Within the newly formed bone, the fluorescent labels showed that mineralization had occurred before 5 weeks of implantation and was directed from the HA surface toward the center of the pores. In conclusion, tissue-engineered bone formation in goats can be achieved only with viable constructs of an appropriate scaffold and sufficient BMSCs.     

Comparative study of biphasic calcium phosphate ceramics impregnated with rhBMP-2 as bone substitutes

We investigated pellet-shaped implants prepared from biphasic calcium phosphate (BCP) ceramics with five different ratios of hydroxyapatite (HAP) to beta-tricalcium phosphate (beta-TCP) to evaluate these ceramics as bone substitutes. BCP ceramics impregnated with different doses of recombinant human bone morphogenetic protein 2 (rhBMP-2) (1, 5, and 10 microg) were used for experimental purposes and ceramics without rhBMP-2 were used for control. The pellets were implanted under the pericranium in adult Wistar male rats and were harvested 8 weeks after implantation. The retrieved pellets were then examined radiologically, histologically, and histomorphometrically. The results revealed that the pellets treated with rhBMP-2 exhibited new bone and bone marrow, whereas control pellets produced fibrous connective tissues. The formation of new bone induced by rhBMP-2 was dose dependent. The extent of bone and bone marrow formation and the degree of resorption of the ceramic particles were significantly higher in the pellets composed of 25% HAP-75% TCP. In this study, bioresorption of the ceramic produced favorable conditions for rhBMP-2-induced bone formation.     

Bone tissue engineering on amorphous carbonated apatite and crystalline octacalcium phosphate-coated titanium discs

Poor fixation of bone replacement implants, e.g. the artificial hip, in implantation sites with inferior bone quality and quantity may be overcome by the use of implants coated with a cultured living bone equivalent. In this study, we tested, respectively, amorphous carbonated apatite (CA)- and crystalline octacalcium phosphate (OCP)-coated discs for their use in bone tissue engineering. Subcultured rat bone marrow cells were seeded on the substrates and after 7 days of culture, the implants were subcutaneously implanted in nude mice for 4 weeks. After 7 days of culture, the cells had formed a continuous multi-layer that covered the entire surface of the substrates. The amount of cells was visually higher on the crystalline OCP-coated discs compared to the amorphous CA-coated discs. Furthermore, the amorphous CA-coated discs exhibited a visually higher amount of mineralized extracellular matrix compared to the crystalline OCP-coated discs. After 4 weeks of implantation, clear de novo bone formation was observed on all discs with cultured cells. The newly formed bone on the crystalline OCP-coated discs was more organized and revealed a significantly higher volume compared to the amorphous CA-coated discs. The percentage of bone contact with the discs was also significantly higher on the OCP-coated discs. Overall, the results suggest that a crystalline OCP coating is more suitable for bone tissue engineering than an amorphous CA coating.     

Comparison of plasma-sprayed hydroxyapatite coatings and hydroxyapatite/tricalcium phosphate composite coatings: in vivo study

This study aimed to compare biological properties, including osteoconduction, osseointegration, and shear strength, between plasma-sprayed hydroxyapatite (HA) and HA/tricalcium phosphate (TCP) coatings, using a transcortical implant model in the femora of canines. After 3 and 12 weeks of implantation, the implants with surrounding bone were assessed histologically in undecalcified sections in backscattered electron images (BEIs) under a scanning electron microscope (SEM). After short-term (3 week) follow-up, both coatings conducted new bone formation and revealed direct bone-to-coating contact. The HA/TCP coating could not enhance early host-to-coating responses. At 12 weeks, serious dissolution of the HA/TCP coatings evidently occurred. By the new bone healing index (NBHI) and apposition index (AI), we found no significant difference between HA/TCP-coated implants and HA-coated implants throughout all implant periods. At 12 weeks of implantation, some particles dissociated from the HA/TCP coating were found within the remodeling canal. After push-out measurements, the shear strength and failure mode of HA/TCP-coated implants were similar to those of HA-coated implants, and no statistical differences were found between either coating. Consequently, this study indicates that HA/TCP coatings have excellent biological response and may be considered suitable bioactive ceramic coatings for short-term clinical use.     

Ectopic osteogenesis with biphasic ceramics of hydroxyapatite and tricalcium phosphate in rabbits.

Porous calcium phosphate ceramics consisting of hydroxyapatite (HA) and tricalcium phosphate (TCP) with different HA to TCP ratio were implanted intramuscularly in rabbits for six months in order to carry out a comparative study on osteogenic activity of the ceramics. Bone formation was detected only in HT73 (HA to TCP ratio, 7-3) specimens. Other implants, HT28 (2-8) and HT010 (0-10), could not induce bone. After a six-month period of implantation, HT28 and HT010 implants showed obvious degradation of the implants changing their shape and size macro and microscopically. Microscopically, they showed aggregates of fine particles and appearance of multinucleated cells. However, HT73 implants was less degraded and could maintain their original structure macro and microscopically. This study showed that HT73 ceramics can induce bone in rabbit muscle tissue and it is considered that maintenance of porous structure, that is, degradation rate of the materials may be one of the affecting factors in ceramic-induced osteogenesis.     

Effect of hydroxyapatite/tricalcium-phosphate coating on osseointegration of plasma-sprayed titanium alloy implants

This study determined the effects of a plasma-sprayed hydroxyapatite/tricalcium phosphate (HA/TCP) coating on osseointegration of plasma-sprayed titanium alloy implants in a lapine, distal femoral intramedullary model. The effects of the HA/TCP coating were assessed at 1, 3, and 6 months after implant placement. The HA/TCP coating significantly increased new bone apposition onto the implant surfaces at all time points. The ceramic coating also stimulated intramedullary bone formation at the middle and distal levels of the implants. Fluorescent bone labeling indicated that new bone formation occurred primarily during the first 3 months after implantation, with comparatively little activity detected in the latter stages of the study. There was no associated increase in pullout strength at either 3 or 6 months; however, post-pullout evaluation of the implants indicated that the HA/TCP coating itself was not the primary site of construct failure. Rather, failure was most commonly observed through the periprosthetic osseous struts that bridged the medullary cavity. The demonstrated osteoconductive activity of HA/TCP coating on plasma-sprayed titanium alloy implant surfaces may have considerable clinical relevance to early host-implant interactions, by accelerating the establishment of a stable prosthesis-bone interface.     

Intramembranous bone tissue response to biodegradable octacalcium phosphate implant

Previous studies showed that synthetic octacalcium phosphate (OCP) enhances bone formation coupled with its own osteoclastic biodegradation more than non-biodegradable hydroxyapatite (HA), including sintered HA ceramic, when implanted in animal bone defects. The present study was designed to investigate whether synthetic OCP in granule form has biodegradable characteristics when implanted in the subperiosteal area of mouse calvaria in comparison with non-sintered stoichiometric HA, especially in relatively short periods after implantation. OCP crystals exhibited plate-like morphology, whereas HA crystals had a sphere-like structure. Both crystals had large pore volumes >75% in total, with micropores within the granules. Direct bonding of newly formed bone was discernible in HA until 35 days after implantation by element analysis for calcium and phosphorus. However, histomorphometric analysis demonstrated that bone formation was facilitated on OCP surfaces with greater alkaline phosphatase activity than on HA up to 21 days. The surfaces attacked by tartrate-resistant acid phosphatase positive osteoclast-like cells were significantly greater than those of HA. OCP became encapsulated and replaced with new bone with prolonged implantation periods up to 180 days. The results suggest that the enhanced bone formation in mouse calvaria could be associated with the biodegradable nature of OCP, and that OCP could be used in augmenting intramembranous bone volume.     

Recombinant growth/differentiation factor-5 (GDF-5) stimulates osteogenic differentiation of marrow mesenchymal stem cells in porous hydroxyapatite ceramic

To evaluate the growth/differentiation factor-5 (GDF-5) in the in vivo osteogenic potential of bone marrow mesenchymal stem cells (MSCs), we subcutaneously implanted five different kinds of hydroxyapatite (HA) ceramic implants: HA alone, GDF-5/HA composites (GDF/HA), MSCs/HA composites, the MSCs/HA composites supplemented with GDF-5 (GDF/MSCs/HA), and recombinant bone morphogenetic protein-2 (BMP/MSCs/HA). Neither the HA alone nor the GDF/HA composites exhibited any bone formation at any time after implantation. At 4 weeks, the MSCs/HA composites exhibited a certain amount of bone formation in some pore areas. In contrast, at 2 weeks, the GDF/MSCs/HA composites exhibited histologically obvious de novo bone formation together with active osteoblasts in many pore areas and additional bone formation at 4 weeks. In the de novo formed bone, neither chondrocytes nor endochondral bone was detected. The GDF/MSCs/HA composites also showed high alkaline phosphatase (ALP) and osteocalcin expression determined at both the protein and gene levels and the high level of expression was well maintained even at 4 weeks. Compared with GDF/MSCs/HA, the BMP/MSCs/HA composites exhibited excellent osteogenesis with relatively early osteoblastic phenotype expression. The results indicate that GDF-5 synergistically enhances de novo bone formation capability of MSCs/HA composite and suggest that tissue-engineered GDF/MSCs/HA composites could be used as bone graft substitutes.     

Mineralization and calcium fixation within a porous apatitic ceramic material after implantation in the femur of rabbits

Cylinders (0.8 cm long, 1.0 cm of diameter and with an axial hole), constituted, after firing, of a ceramic mixture of hydroxylapatite (HA) and beta-tricalciumphosphate (beta-TCP) in a 10:1 ratio, were implanted into mid-diaphyseal defects of one femur of 20 rabbits and stabilized with intramedullary rods. The implantation sites were checked radiographically every month, and after 3 months (3 animals) and 6 months (17 animals) the rabbits were sacrificed and the implants with the surrounding tissue were embedded in methylmethacrylate, cut to thick sections and analyzed by scanning electron microscopy (SEM). Porosimetric and x-rays diffraction analyses were carried out before and after implantation of the cylinders, and the state of mineralization at the bone-implant interface was determined by EDAX microprobe analysis. Bony callus formation started at 1 month at the osteotomy sites, as judged by radiography, but after 3 months a not-mineralized zone had still been demonstrated between bone and the implants. At 6 months, 13 implants showed themselves firmly fixed in their implantations beds, while 4 implants were only incorporated at their proximal ends. In bone contact zones, an enrichment of Ca2+ was displayed by microanalytical techniques in the outer zone of the implanted samples which may be explained by an apparent additional phase transformation of HA into TCP, thanks to the change of the Ca/P ratio, that takes place in vivo.     

HA/TCP双相陶瓷肌内长期植入诱导成骨的实验观察

通过肌内长期植入研究了Ca-P生物材料骨诱导性及诱生新骨的生长状况。HA／TCP多孔陶瓷植入狗背部肌内15月后，植入体为一层很薄的纤维结缔组织膜包裹。3种形态的组织，即间充质组织，骨和骨髓在植入体内部孔中有规律地分布：植入体内部所有的孔中都有骨组织存在；但中部内孔中的骨为较成熟的板状骨，有明显的骨陷窝，已形成髓腔和骨髓，边缘外孔中的骨组织为未成熟的编织骨，与间充质组织相连，无髓腔和骨髓形成，这表明HA／TCP具有骨诱导性，在非骨环境中确实能够诱导成骨，而且，各种组织有规律地分布在植入材料孔中，表明植入体内部骨组织形态的发生和发展遵循特定的时空顺序。     

Pre-culture period of mesenchymal stem cells in osteogenic media influences their in vivo bone forming potential

The objective of this study was to investigate if the in vitro pre-culture period in osteogenic media of rat mesenchymal stem cells (MSCs), influences their ability to regenerate bone when implanted in a critical size cranial defect. MSCs were harvested from the bone marrow of 6-8 weeks old male Fisher rats and expanded in vitro in osteogenic media for different time periods (4, 10, and 16 days) in tissue culture plates (TCP), seeded on sintered titanium fiber meshes without the extracellular matrix (ECM) generated in vitro, and implanted in the rat cranium after 12 h. Thirty two adult Fisher rats received the implants, divided in four groups. Three groups were implanted with cells cultured for 4, 10, or 16 days in osteogenic media and at that time their alkaline phosphatase activity and mineral deposition denoted that they were at different stages of their osteoblastic maturation (undifferentiated MSC, committed, and mature Osteoblasts, respectively). MSCs cultured without osteogenic media for 6 days were used as controls. The constructs were retrieved 4 weeks later and processed for histomorphometric analysis. Implants seeded with cells that have been cultured with osteogenic media for only 4 days revealed the highest bone formation. The lowest bone formation was obtained with the implants seeded with MSCs cultured for 16 days in the presence of osteogenic media. The results of this study suggested that the in vitro pre-culture period of MSCs is a critical factor for their ability to regenerate bone when implanted to an orthotopic site.     

Enhancement of the in vivo osteogenic potential of marrow/hydroxyapatite composites by bovine bone morphogenetic protein

A composite of marrow mesenchymal stem cells and porous hydroxyapatite (HA) has in vivo osteogenic potential. To investigate factors enhancing the osteogenic potential of marrow/HA composites, we prepared a bone morphogenetic protein (BMP) fraction from the 4M guanidine extract of bovine bone by heparin-sepharose affinity chromatography. Marrow/HA composites or composites containing marrow mesenchymal stem cells, BMP, and HA (marrow/BMP/HA composites) were implanted subcutaneously in 7-week-old male Fischer rats. BMP/HA composites and HA alone were also implanted. The implants were harvested after 2, 4, or 8 weeks and were prepared for histological and biochemical studies. Histological examination showed obvious de novo bone formation together with active osteoblasts at 2 weeks, as well as more extensive bone formation at 4 and 8 weeks in many pores of the marrow/BMP/HA composites. The marrow/HA composites did not induce bone formation at 2 weeks, but there was moderate bone formation at 4 weeks. At 2 weeks, only marrow/BMP/HA composites resulted in intensive osteogenic activity, judging from alkaline phosphatase and osteocalcin expression at both the protein and gene levels. These results indicate that the combination of marrow mesenchymal stem cells, porous HA, and BMP synergistically enhances osteogenic potential, and may provide a rational basis for their clinical application, although further in vivo experiment is needed.     

Biological performance in goats of a porous titanium alloy-biphasic calcium phosphate composite

In this study, porous 3D fiber deposition titanium (3DFT) and 3DFT combined with porous biphasic calcium phosphate ceramic (3DFT+BCP) implants, both bare and 1 week cultured with autologous bone marrow stromal cells (BMSCs), were implanted intramuscularly and orthotopically in 10 goats. To assess the dynamics of bone formation over time, fluorochrome markers were administered at 3, 6 and 9 weeks and the animals were sacrificed at 12 weeks after implantation. New bone in the implants was investigated by histology and histomorphometry of non-decalcified sections. Intramuscularly, no bone formation was found in any of the 3DFT implants, while a very limited amount of bone was observed in 2 BMSC 3DFT implants. 3DFT+BCP and BMSC 3DFT+BCP implants showed ectopic bone formation, in 8 and 10 animals, respectively. The amount of formed bone was significantly higher in BMSC 3DFT+BCP as compared to 3DFT+BCP implants. Implantation on transverse processes resulted in significantly more bone formation in composite structure as compared to titanium alloy alone, both with and without cells. Unlike intramuscularly, the presence of BMSC did not have a significant effect on the amount of new bone either in metallic or in composite structure. Although the 3DFT is inferior to BCP for bone growth, the reinforcement of the brittle BCP with a 3DFT cage did not negatively influence osteogenesis, osteoinduction and osteoconduction as previously shown for the BCP alone. The positive effect of BMSCs was observed ectopically, while it was not significant orthotopically.     

Bonding osteogenesis in coralline hydroxyapatite combined with bone marrow cells

Coralline hydroxyapatite ceramics alone (control) and the ceramics combined with rat marrow cells were implanted subcutaneously in the backs of syngeneic Fischer rats and harvested at 1,2,3,4,6,8 and 24 wk after surgery. None of the control ceramics (without marrow) showed bone formation. However, ceramics combined with marrow cells showed consistent new bone formation in the pore regions. Histometrical results revealed increased new bone formation over time. Undecalcified sections of the ceramics studied by fluorochrome labelling showed that the osteogenesis began directly on the surface of the ceramic and proceeded centripetally towards the centre of the pores (bonding osteogenesis). SEM-EPMA analysis of the bone-ceramic interface also revealed direct bonding of bone to the ceramic surface.     

Improved bone anchorage of hydroxypatite coated implants compared with tricalcium-phosphate coated implants in trabecular bone in dogs

Tricalcium phosphate (TCP) and hydroxyapatite (HA) ceramic coatings are bioactive coatings that have been shown to stimulate bone apposition onto ceramic-coated implants. TCP and HA ceramics have well-documented differences in physical properties, but both types of ceramics are used for stimulation of bone ongrowth to cementless endo-prosthetic components clinically. However, little is known about the difference in osteoconductive properties between these coatings when inserted into trabecular bone in a controlled experimental situation. Unloaded cylindrical gritblasted titanium (Ti-6A1-4V) implants (6 x 10 mm) coated with either hydroxyapatite (HA) or tricalcium phosphate (TCP) ceramic were inserted into the proximal humerus of 20 skeletally mature dogs. The implants were initially surrounded by a 2 mm gap. Each animal received one HA-coated implant and one TCP-coated implant. All dogs were sacrificed 6 weeks after surgery. Results were evaluated by histomorphometry and mechanical push-out test. Push-out tests demonstrated that HA-coated implants were 10-fold stronger fixated in comparison to TCP-coated implant. Bone ongrowth was significantly higher for HA-coated implants compared to TCP-coated implants. Bone volume in the gap showed a tendency to less bone volume around HA-coated implants compared to TCP-coated implants but this difference was insignificant. As expected almost all of the TCP coating were resorbed after 6 weeks and almost none of the HA coating. HA-coated implants with a grit-blasted surface provide a favorable early mechanical implant anchorage most likely due to superior ceramic stability compared to TCP-coated implants.