Bone defects require implantable graft substitutes, especially porous and biodegradable biomaterial for tissue regeneration. The aim of this study was to fabricate and assess a 3D-printed biodegradable hydroxyapatite/calcium carbonate scaffold for bone regeneration. Materials and methods:. A 3D-printed biodegradable biomaterial containing calcium phosphate and aragonite (calcium carbonate) was fabricated using a Bioplotter. The physicochemical properties of the material were characterised. The materials were assessed in vitro for cytotoxicity and ostegenic potential and in vivo in rat intercondylar Φ3mm bone defect model for 3 months and Φ5mm of mini pig femoral bone defects for 6 months. The results showed that the materials contained hydroxyapatite and calcium carbonate, with the compression strength of 2.49± 0.2 MPa, pore size of 300.00 ± 41mm, and porosity of 40.±3%. The hydroxyapatite/aragonite was not cytotoxic and it promoted osteogenic differentiation of human umbilical cord matrix mesenchymal stem cells in vitro. After implantation, the bone defects were healed in the treatment group whereas the defect of controlled group with gelatin sponge implantation remained non-union. hydroxyapatite/aragonite fully integrated with host bone tissue and bridged the defects in 2 months, and significant biodegradation was followed by host new bone formation. After implantation into Φ5mm femoral defects in mini pigs hydroxyapatite/aragonite were completed degraded in 6 months and fully replaced by host bone formation, which matched the healing and degradation of porcine allogenic bone graft. In conclusion, hydroxyapatite/aragonite is a suitable new scaffold for bone regeneration. The calcium carbonate in the materials may have played an important role in osteogenesis and material biodegradation

Targeted delivery of drugs is a major challenge in diseases such as infections and tumors. The aim of this study was to demonstrate that hydroxyapatite (HA) particles can act as a recruiting moiety for various bioactive molecules and as a proof-of-concept demonstrate that the affinity of drugs to hydroxyapatite can exert a biological effect. A bisphosphonate, zoledronic acid (ZA), was used as a model drug. Experiment 1 (ZA seeks HA): Calcium sulphate (CaS)/hydroxyapatite (HA) biomaterial pellets (diameter¸=5 mm, height=2 mm) were implanted in the abdominal muscle pouch of rats. After 2-weeks of implantation, a sub-cutaneous injection of 14C-ZA (0.1 mg/kg) was given. 24 h later, the animals were sacrificed and the uptake of ZA determined in the pellets using scintillation counting. Experiment 2 (Systemically administered ZA seeks HA and exerts a biological effect): A fenestrated implant was filled with the CaS/HA biomaterial and inserted in the proximal tibia of rats. 2-weeks post-op, a subcutaneous injection of ZA (0.1 mg/kg) was given. Animals were sacrificed at 6-weeks post-op. Empty implant was used as a control. Peri-implant bone formation was evaluated using different techniques such as micro-CT, mechanical testing and histology. Welch's t-test was used for mechanical testing and Mann-Whitney U test for micro-CT data analysis. Experiment 1: Uptake of radioactive ZA in the CaS/HA biomaterial was confirmed. Almost no ZA was present in the surrounding muscle. These results show high specific binding between systemically administered ZA and synthetic particulate HA. Experiment 2: Significantly higher peri-implant bone was measured using micro-CT in the group wherein the implant contained the CaS/HA biomaterial and ZA was administered systemically (This study presents a method for biomodulating HA in situ by different bioactive molecules. The approach of implanting a biomaterial capable of recruiting systemically given drugs and thereby activate the material is novel and may present a possibility to treat bone infections or tumors

Hydroxyapatite (HAp) is a well-known synthetic biomaterial that has been extensively employed in orthopedic fields as bone grafts or coating of metallic implants. During recent years, ion doping or ionic substitution has been used to improve the performance of bioceramics. Owing to the benefits of a bioactive element such as boron (B) in bone health, and reported impaired bone growth or abnormal development of bone in case of boron deficiency, it was expected that doping of boron could make a positive effect on physicochemical and biological properties of HAp. In this study, boron-doped hydroxyapatite (BHAp) was synthesized successfully through utilizing microwaved assisted wet precipitation route. X-ray diffraction, scanning electron microscopy, and inductively coupled plasma mass spectrometry were used to characterize the phase purity, lattice parameters, degree of crystallinity, particle size and elemental composition of synthesized BHAp powders. Substitution of borate (BO. 3. 3-. ) ion with the phosphate (PO. 4. 3-. ) in HAp crystal caused lattice distortion due to radius difference between the dopant and the replaced element, which also led to smaller crystalline size and lower crystallinity degree in doped samples (∼ 91 % in 0.5 mol doped BHAp compared to 95 % of pure HAp). In vitro results revealed that although there was no significant difference in biodegradability of doped BHAp, after submerging samples in simulated body fluid for 14 days, intense growth of apatite particles (Ca/P ratio of 1.74) was observed on the surface of BHAp pellets, especially in samples with 0.25 and 0.5 mole B. Observed higher bioactivity was expected due to lower crystallinity degree of BHAp samples. Due to the results of this study, incorporation of B into the structure of HAp could be considered as a positive step to improve the bioactivity and biological performance of these biomaterials in orthopedic applications

Abstract. INTRODUCTION. Polyetheretherketone (PEEK) is a high-performance thermoplastic polymer which has found increasing application in orthopaedic implant devices and has a lot of promise for ‘made-to-measure’ implants produced through additive manufacturing [1]. However, a key limitation of PEEK is that it is bioinert and there is a requirement to functionalise its surface to make the material osteoconductive to ensure a more rapid, improved and stable fixation, in vivo. One approach to solving this issue is to modify PEEK with bioactive materials, such as hydroxyapatite (HA). OBJECTIVE. To 3D PEEK/HA composite materials using a Fused Filament Fabrication (FFF) approach to enhance the properties of the PEEK matrix. METHODS. PEEK/HA composites (0–30% w/w HA/PEEK) were 3D printed using a modified Ultimaker 2+ 3D printer. The mechanical, thermal, physical, chemical and in vitro properties of the 3D printed samples were all studied as part of this work. RESULTS. The CT images of both the filament and the 3D printed samples showed that the HA material was evenly dispersed throughout the bulk all the samples. SEM/EDX measurements highlighted that HA was homogenously distributed across the surface. As the HA content of the samples increases, so does the tensile modulus, ranging from 4.2 GPa (PEEK) to 6.1 GPa (30% HA/PEEK) and are significantly higher than datasheet information of injected molded PEEK samples. All materials supported the growth of osteoblast cells on their surface. CONCLUSIONS. The results clearly show that we can successfully and easily 3D print HA/PEEK composite materials up to 30% w/w HA/PEEK. The samples produced have a homogeneous distribution of HA in both the bulk and surface of all the samples, and their mechanical performance of the PEEK is enhanced by the addition of HA

Abstract. Objectives. The objective of this study is to investigate the effect of solvents and rheological properties of PCL/Hydroxyapatite ink on the shape fidelity of the 3D printed scaffolds for bone tissue engineering. Methods. A series of inks were made consisting of 50% (w/v) of polycaprolactone (PCL) filled with 0%, 3.5% and 12.5% (w/V) of hydroxyapatite (HA) in dichloromethane (DCM) and chloroform (CHF). Steady and oscillatory shear rheological tests were performed on a rheometer (Discovery HR-3). Solvent-cast direct ink writing was performed with a custom-made 3D printer for the fabrication of PCL/HA scaffold structures with 2–8 layers. Optical microscope and scanning electron microscopy (SEM) were used to assess the shape fidelity. Results. Shape fidelity of the inks was quantitively assessed on the 3D printed scaffold structures allowing subjective comparisons. The addition of HA particles increased zero-shear viscosity by up to 900%. For oscillatory tests, plateau of storage modulus was observed in the low-frequency region which is attributed to good dispersion of the HA particles inside the matrix that leads to the formation of filler networks, resulting in pseudo-solid behavior and shape fidelity improvement. As the HA concentration increases, the plateau becomes more pronounced and the shape fidelity increases. With the same concentration, all DCM inks also show higher viscosity (from 10% to 200%) and better shape fidelity than CHF inks. As DCM has a lower boiling point (39.6 °C) than CHF (61.2°C), DCM evaporates quicker reducing the fusion and diffusion of deposited ink filaments before solidification which is observed in SEM images. Conclusions. This study reveals insights into using rheological characterizations as a tool for evaluation of shape fidelity of solvent-based DIW inks and also provides fundamental information on the influence of different solvents on the fidelity of 3D printed scaffolds. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Carbonate-substituted hydroxyapatite (CHA) is more osteoconductive and more resorbable than hydroxyapatite (HA), but the underlying mode of its action is unclear. We hypothesised that increased resorption of the ceramic by osteoclasts might subsequently upregulate osteoblasts by a coupling mechanism, and sought to test this in a large animal model. Defects were created in both the lateral femoral condyles of 12 adult sheep. Six were implanted with CHA granules bilaterally, and six with HA. Six of the animals in each group received the bisphosphonate zoledronate (0.05 mg/kg), which inhibits the function of osteoclasts, intra-operatively. After six weeks bony ingrowth was greater in the CHA implants than in HA, but not in the animals given zoledronate. Functional osteoclasts are necessary for the enhanced osteoconduction seen in CHA compared with HA

Objectives. We have observed clinical cases where bone is formed in the overlaying muscle covering surgically created bone defects treated with a hydroxyapatite/calcium sulphate biomaterial. Our objective was to investigate the osteoinductive potential of the biomaterial and to determine if growth factors secreted from local bone cells induce osteoblastic differentiation of muscle cells. Materials and Methods. We seeded mouse skeletal muscle cells C2C12 on the hydroxyapatite/calcium sulphate biomaterial and the phenotype of the cells was analysed. To mimic surgical conditions with leakage of extra cellular matrix (ECM) proteins and growth factors, we cultured rat bone cells ROS 17/2.8 in a bioreactor and harvested the secreted proteins. The secretome was added to rat muscle cells L6. The phenotype of the muscle cells after treatment with the media was assessed using immunostaining and light microscopy. Results. C2C12 cells differentiated into osteoblast-like cells expressing prominent bone markers after seeding on the biomaterial. The conditioned media of the ROS 17/2.8 contained bone morphogenetic protein-2 (BMP-2 8.4 ng/mg, standard deviation (. sd. ) 0.8) and BMP-7 (50.6 ng/mg, . sd. 2.2). In vitro, this secretome induced differentiation of skeletal muscle cells L6 towards an osteogenic lineage. Conclusion. Extra cellular matrix proteins and growth factors leaking from a bone cavity, along with a ceramic biomaterial, can synergistically enhance the process of ectopic ossification. The overlaying muscle acts as an osteoinductive niche, and provides the required cells for bone formation. Cite this article: D. B. Raina, A. Gupta, M. M. Petersen, W. Hettwer, M. McNally, M. Tägil, M-H. Zheng, A. Kumar, L. Lidgren. Muscle as an osteoinductive niche for local bone formation with the use of a biphasic calcium sulphate/hydroxyapatite biomaterial. Bone Joint Res 2016;5:500–511. DOI: 10.1302/2046-3758.510.BJR-2016-0133.R1

The aseptic loss of bone after hip replacement is a serious problem leading to implant instability. Hydroxyapatite coating of joint replacement components produces a bond with bone and helps to reduce loosening. However, over time bone remodeling at the implant interface leads to loss of hydroxyapatite. One possible solution would be to develop a coating that reduces hydroxyapatite and bone loss. Hydroxyapatite can be chemically modified through the substitution of ions to alter the biological response. Zinc is an essential trace element that has been found to inhibit osteoclast-like cell formation and decrease bone resorption. It was hoped that by substituting zinc into the hydroxyapatite lattice, the resultant zinc-substituted hydroxyapatite (ZnHA) would inhibit ceramic resorption and the resorption of bone. The aim of this work was to investigate the effect of ZnHA on the number and activity of osteoclasts. Discs of phase pure hydroxyapatite (PPHA), 0.37wt% ZnHA and 0.58wt% ZnHA were produced, sintered at 1100 degrees Celsius and ground with 1200 grit silicon carbide paper. They were cultured in medium containing macrophage colony stimulating factor and receptor activator of nuclear factor kappa B ligand (RANKL) for 11 and 21 days. A control disc of PPHA cultured in medium containing no RANKL was also used. On the required dates the discs were removed and the cells stained for actin with phalloidin-TRITC and the cell nuclei with 4',6-Diamidino-2-phenylindole dihydrochloride. Cells with 3 or more nuclei were classed as osteoclasts and counted using ImageJ. On day 21 after the cells had been counted, the cells were removed and the discs coated in platinum before viewing with a scanning electron microscope. Resorption areas were then measured using ImageJ. The addition of zinc was observed to significantly decrease the number of differentiated osteoclasts after 21 days (p<0.005 for 0.58wt% ZnHA compared to PPHA and p<0.01 for 0.37wt% ZnHA compared to PPHA). The area of resorption was also significantly decreased with the addition of zinc (p<0.005 for the comparison of 0.58wt% ZnHA with PPHA). The work found that zinc substituted hydroxyapatite reduced the number and subsequent activity of osteoclasts

The feasibility of bone transport with bone substitute and the factors which are essential for a successful bone transport are unknown. We studied six groups of 12 Japanese white rabbits. Groups A to D received cylindrical autologous bone segments and groups E and F hydroxyapatite prostheses. The periosteum was preserved in group A so that its segments had a blood supply, cells, proteins and scaffold. Group B had no blood supply. Group C had proteins and scaffold and group D had only scaffold. Group E received hydroxyapatite loaded with recombinant human bone morphogenetic protein-2 and group F had hydroxyapatite alone. Distraction osteogenesis occurred in groups A to C and E which had osteo-conductive transport segments loaded with osteo-inductive proteins. We conclude that scaffold and proteins are essential for successful bone transport, and that bone substitute can be used to regenerate bone

The aim of this study was to evaluate the trochlear bone and cartilaginous regeneration of rabbits using a composite based on platelet rich plasma (PRP), chitosan and hydroxyapatite. The study was approved by the ethics committee of the Federal University of Campina Grande under number 72/2017. Surgical holes measuring four millimetres in diameter were performed in rabbit trochleae, one surgical hole in each animal remained empty and another one was filled with the composite. Clinical-orthopaedic and radiographic evaluations were carried out for 60 days, after which the animals were euthanized for histomorphometric evaluations. Clinical-evaluations exhibited lameness of two members of the treatment (T) group and one member of control (C) group. The radiographic evaluation of T group exhibited absence of subchondral bone reaction (33%); nonetheless, presence of moderate subchondral bone reaction was more frequently reported in group C with 67%. Microscopic evaluation revealed the presence of tissue neoformation, composed of dense connective tissue. Microscopic findings were similar in both groups, with a difference in the amount of neoformed tissue, which was confirmed after the morphometric analysis, revealing a significant difference in the quantity of newly formed tissue at the bone / cartilage / implant interface in the T group. The results indicate that the composite based on chitosan, hydroxyapatite and PRP enhanced bone and cartilage healing

Worldwide 500,000 cases of maxillofacial cancer are diagnosed each year. After surgery, the reconstruction of large bone defect is often required. The induced membrane approach (Masquelet, 2000) is one of the strategies, but exhibits limitations in an oncological context (use of autografts with or without autologous cells and Bone Morphogenetic Proteins). The objectives of this work are to develop an injectable osteoinductive and osteoconductive composite matrix composed of doped strontium (Sr) hydroxyapatite (HA) particles dispersed within a polysaccharide scaffold, to evaluate in vitro their ability to stimulate osteoblastic differentiation of human mesenchymal stem cells (hMSC) and to stimulate in vivo bone tissue regeneration. HA particles were synthesized with different ratios of Sr. X-ray diffraction (XRD), Inductively Coupled Plasma (ICP), and particle size analysis (Nanosizer™) were used to characterize these particles. HA and Sr-doped HA were dispersed at different ratios within a pullulan-dextran based matrices (Autissier, 2010), Electronic scanning microscopy Back Scattering Electron microscopy (ESEM-BSE) and ICP were used to characterize the composite scaffolds. In vitro assays were performed using hMSC (cell viability using Live/Dead assay, expression of osteoblastic markers by quantitative Polymerase Chain Reaction). Matrices containing these different particles were implanted subcutaneously in mice and analyzed by Micro-Computed Tomography (micro-CT) and histologically (Masson's trichrome staining) after 2 and 4 weeks of implantation. XRD analysis was compatible with a carbonated hydroxyapatite and patterns of Sr-doped HA are consistent of Sr substitution on HA particles. Morphological evaluation (TEM and Nanosizer™) showed that HA and Sr-doped HA particles form agglomerates (150 nm to 4 µm). Matrices composed with different ratios of HA or Sr-doped-HA, exhibit a homogenous distribution of the particles (ESEM-BSE), whatever the conditions of substitution. In vitro studies revealed that Sr-doped HA particles within the matrix stimulates the expression of osteoblastic markers, compared to non-doped HA matrices. Subcutaneous implantation of the matrices demonstrated the formation of a mineralized tissue. Quantitative analyses show that the mineralization of the implants is dependent of the amount of HA particles dispersed, with an optimal ratio of 5% of particles. Histological analysis revealed osteoid tissue in contact to the matrix. In conclusion, the ability of this injectable composite scaffold to promote ectopically tissue mineralization is promising for bone tissue engineering. Osseous implantation in a femoral bone defect in rats is now in progress. 5% of doped HA particles were implanted within the induced membranes in a context of radiotherapy procedure. Micro-CT analyses are ongoing. This new matrix could represent an alternative to the autografts for the regeneration of large bone defects in an oncological context

The success of long-term transcutaneous implants depends on dermal attachment to prevent downgrowth of the epithelium and infection. Hydroxyapatite (HA) coatings and fibronectin (Fn) have independently been shown to regulate fibroblast activity and improve attachment. In an attempt to enhance this phenomenon we adsorbed Fn onto HA-coated substrates. Our study was designed to test the hypothesis that adsorption of Fn onto HA produces a surface that will increase the attachment of dermal fibroblasts better than HA alone or titanium alloy controls. . Iodinated Fn was used to investigate the durability of the protein coating and a bioassay using human dermal fibroblasts was performed to assess the effects of the coating on cell attachment. Cell attachment data were compared with those for HA alone and titanium alloy controls at one, four and 24 hours. Protein attachment peaked within one hour of incubation and the maximum binding efficiency was achieved with an initial droplet of 1000 ng. We showed that after 24 hours one-fifth of the initial Fn coating remained on the substrates, and this resulted in a significant, three-, four-, and sevenfold increase in dermal fibroblast attachment strength compared to uncoated controls at one, four and 24 hours, respectively

Impacted morsellised allografts have been used successfully to address the problem of poor bone stock in revision surgery. However, there are concerns about the transmission of pathogens, the high cost and the shortage of supply of donor bone. Bone-graft extenders, such as tricalcium phosphate (TCP) and hydroxyapatite (HA), have been developed to minimise the use of donor bone. In a human cadaver model we have evaluated the surgical and mechanical feasibility of a TCP/HA bone-graft extender during impaction grafting revision surgery. A TCP/HA allograft mix increased the risk of producing a fissure in the femur during the impaction procedure, but provided a higher initial mechanical stability when compared with bone graft alone. The implications of the use of this type of graft extender in impaction grafting revision surgery are discussed

Summary Statement. Parathytorid hormone-related protein (107–111) loaded onto biopolymer-coated nanocrystalline hydroxyapatite (HA. Glu. ) improves the bone repair in a cavitary defect in rat tibiae. Introduction. Biopolymer-coated nanocrystalline hydroxyapatite (HA. Glu. ) made as macroporous foams are promising candidates as scaffolds for bone tissue engineering applications. They exhibit optimal features, promoting internalization, proliferation and differentiation of osteoprogenitors, with an adequate cell colonization over the entire scaffold surface. Parathyroid hormone-related protein (PTHrP) is an important modulator of bone formation. Its 107–111 epitope (osteostatin) exhibits osteogenic properties at least in part by directly acting on osteoblasts. The main aim of this study was to evaluate whether osteostatin loading into HA. Glu. scaffolds might improve their bone regeneration capacity. Materials and Methods. HA. Glu. scaffolds were prepared as previously described (Sánchez-Salcedo S et al. J. Mater. Chem. 2010; 20:6956-61). Osteostatin was adsorbed onto HA. Glu. material by dipping into a solution containing this peptide at 100 nM (in phosphate-buffered saline, pH 7.4), following a standard protocol. We performed a cavitary defect (2 mm in diameter and 3 mm in depth) in both distal tibial metaphysis using a drill under general anesthesia in male Wistar rats (n=8) of 6 months of age. Unloaded HA. Glu. material (7 mg) was implanted into left tibial defects, whereas rigth tibial defects received the osteostatin-loaded material. Animals were sacrificed after 4 weeks for histological, μ-computerised tomography and gene expression analysis of the callus. Our protocol was approved by the Institutional Animal Care and Use Committee at the IIS-FJD. Mouse osteoblastic MC3T3-E1 cells were grown in differentiation medium (α-MEM with 10% fetal bovine serum, 50 µg/ml ascorbic acid, and 10 mM β-glycerolphosphate), in the presence or absence of HA. Glu. material with or without osteostatin. Cell viability (assessed by trypan blue staining), alkaline phosphatase (ALP) activity and mineralization (alizarin red) were analyzed at different culture times. Results. The mean uptake of osteostatin by HA. Glu. scaffolds was about 60 % (representing 0.7 μg/implanted scaffold) after 24 h of loading, and they released a mean of 80 % of loaded peptide to the surrounding medium within 1–24 h. At 4 weeks, this osteostatin-containing HA. Glu. material significantly increased the bone volumen fraction and trabecular thickness of regenerating bone in the tibial methaphysis, compared to those observed with unloaded HAGlu scaffolds. In addition, osteostatin-coated HA. Glu. scaffolds increased (2-fold) the gene expression of osteocalcin and vascular cell adhesion molecule 1, but decreased (2-fold) that of the Wnt inhibitors, SOST and Dickkopf homolog 1 (DKK-1) in the fracture callus. In MC3T3-E1 cell cultures, osteostatin-loaded HA. Glu. material increased cell viability and ALP activity (each by 30%), and matrix mineralization (by 50%) at days 4 and 10, respectively. Conclusions. These results indicate that osteostatin loading improves the bone regeneration capacity of HA. Glu. scaffolds. Our findings suggest that these scaffolds might be promising implants in orthopaedic applications. This work has been supported by a grant from Comunidad Autónoma de Madrid (S-2009/MAT/1472)

In a rabbit model we investigated the efficacy of a silk fibroin/hydroxyapatite (SF/HA) composite on the repair of a segmental bone defect. Four types of porous SF/HA composites (SF/HA-1, SF/HA-2, SF/HA-3, SF/HA-4) with different material ratios, pore sizes, porosity and additives were implanted subcutaneously into Sprague-Dawley rats to observe biodegradation. SF/HA-3, which had characteristics more suitable for a bone substitite based on strength and resorption was selected as a scaffold and co-cultured with rabbit bone-marrow stromal cells (BMSCs). A segmental bone defect was created in the rabbit radius. The animals were randomised into group 1 (SF/HA-3 combined with BMSCs implanted into the bone defect), group 2 (SF/HA implanted alone) and group 3 (nothing implanted). They were killed at four, eight and 12 weeks for visual, radiological and histological study. The bone defects had complete union for group 1 and partial union in group 2, 12 weeks after operation. There was no formation of new bone in group 3. We conclude that SF/HA-3 combined with BMSCs supports bone healing and offers potential as a bone-graft substitute

The clinical use of hydroxyapatite (HA) coating is controversial especially in regard to the long-term performance of the coating and the effects of resorption. In each of 15 consenting patients we inserted two implants, coated with either HA or fluorapatite (FA) into the iliac crest. They were harvested at a mean of 13.6 ± 0.6 months after surgery. Histological examination showed that bone ongrowth on the HA-coated implants was significantly greater (29%) than that on the FA-coated implants. When bone was present on the coating surface the HA coating was significantly thicker than the FA coating. When bone marrow was present, the HA coating was significantly thinner than the FA coating. The reduction in coating thickness when covered by bone or bone marrow was 23.1 ± 9.7 μm for HA and 5.1 ± 1.7 μm for FA (p < 0.01) suggesting that FA is more stable than HA against resorption by bone marrow. The findings suggest that in man the osteoconductive properties of HA coating are superior to those of FA. Resorption rates for both coatings were approximately 20% of the coating thickness per year. Bone ongrowth appears to protect against resorption whereas bone marrow seems to accelerate resorption. No adverse reaction was seen in the surrounding bone

We inserted two hydroxyapatite (HA)-coated implants with crystallinities of either 50% (HA-50%) or 75% (HA-75%) bilaterally into the medial femoral condyles of the knees of 16 dogs. The implants were allocated to two groups with implantation periods of 16 and 32 weeks. They were weight-bearing and subjected to controlled micromovement of 250 μm during each gait cycle. After 16 weeks, mechanical fixation of the HA-50% implants was increased threefold as compared with the HA-75% implants. After 32 weeks there was no difference between HA-50% and HA-75%. Fixation of HA-75% increased from 16 to 32 weeks whereas that of HA-50% was unchanged. HA-50% implants had 100% more bone ingrowth than HA-75% implants after 16 weeks. More HA coating was removed on HA-50% implants compared with HA-75% implants after both 16 and 32 weeks. No further loss of the HA coating was shown from 16 to 32 weeks. Our study suggests that the crystallinity of the HA coating is an important factor in its bioactivity and resorption during weight-bearing conditions. Our findings suggest two phases of coating resorption, an initial rapid loss, followed by a slow loss. Resorbed HA coating was partly replaced by bone ingrowth, suggesting that implant fixation will be durable

We measured the insertion and extraction torque forces in a randomised study of 76 external fixation screws in 19 patients treated by hemicallotasis for osteoarthritis of the medial side of the knee. The patients were randomised to have either standard tapered screws (Orthofix 6/5 mm) or the same screws with hydroxyapatite (HA) coating. One patient had two standard and two HA-coated screws. All patients had an anterior external fixator (Orthofix T-garche), with two screws in the proximal tibial metaphysis parallel to and about 2 cm below the joint surface and two in the tibial diaphysis. The mean torque forces for insertion of the standard screws were 260 Ncm for the proximal to medial screw, 208 for the proximal to lateral screw and 498 and 546 Ncm for the diaphyseal pins. The corresponding forces for the HA-coated pins were not significantly different. The torque forces for the extraction of the standard pins were 2 Ncm for the proximal pins, 277 and 249 Ncm for the distal pins and 482, 478, 585 and 620 Ncm, respectively (p < 0.005) for the HA-coated pins. All 18 of the metaphyseal standard screws were loose at extraction (extraction force < 20 Ncm), but only one of the HA screws in the metaphysis was loose. In the diaphysis the standard screws lost about 40% of their fixation in contrast to the HA-coated screws which retained full fixation strength

We have studied the beneficial effects of a hydroxyapatite (HA) coating on the prevention of the migration of wear debris along the implant-bone interface. We implanted a loaded HA-coated implant and a non-coated grit-blasted titanium alloy (Ti) implant in each distal femoral condyle of eight Labrador dogs. The test implant was surrounded by a gap communicating with the joint space and allowing access of joint fluid to the implant-bone interface. We injected polyethylene (PE) particles into the right knee three weeks after surgery and repeated this weekly for the following five weeks. The left knee received sham injections. The animals were killed eight weeks after surgery. Specimens from the implant-bone interface were examined under plain and polarised light. Only a few particles were found around HA-coated implants, but around Ti implants there was a large amount of particles. HA-coated implants had approximately 35% bone ingrowth, whereas Ti implants had virtually no bone ingrowth and were surrounded by a fibrous membrane. Our findings suggest that HA coating of implants is able to inhibit peri-implant migration of PE particles by creating a seal of tightly-bonded bone on the surface of the implant

Problems associated with allograft are well known. The addition of hydroxyapatite (HA) to allograft has various mechanical advantages, especially within revision arthroplasty. The mixing of bone and HA results in mechanical properties different from the individual parts. However, at present the changes in material properties the mix have not been fully investigated and the optimum mixing ratio not characterized. A compressive uniaxial chamber was used to investigate the change in mechanical properties occurring with the addition of HA in varying proportions to morcellised bone graft (MBG). Materials and methods. MBG was prepared using femoral heads donated from patients undergoing total hip replacement surgery using a bone mill in a standard manner. Non porous HA (npHA) was prepared using a precipitation method of Calcium Carbonate and Orthophosphoric acid. The porous HA, (pHA) is a 60% macroporosity HA commercially prepared. Chamber. The uniaxial compression chamber was a 30mm diameter, steel chamber. Holes were drilled to allow fluid drainage. Loads were applied using a 10 kN load cell. Specimens were prepared in the volumetric proportions pure HA, pure MBG, 2:1, 1:1, 1:2 ratio of MBG to HA. The samples were subjected to compressive forces of incrementally increasing loads of up to 2 KN for 60 cycles. The sample was then allowed to creep under a stress of 2 kN. MBG was also tested up to forces of 7 kN. The mechanical parameters that were examined were the stiffness of the sample at the 60th cycle, (Ec60), and creep. Statistical analysis. The different bone: HA mixes were compared by Mann Whitney U tests. The parameters analysed were the Ec60 and creep for the different mixes. All samples were also compared to MBG compacted with higher forces. Results. Apparent stiffness. The Ec60 of the MBG increased with increasing axial stress. The Ec60 of the different proportions of MBG and both npHA and pHA lay between the values of the MBG and HA individually and were significantly different from one another (p<0.05). MBG:npHA were significantly stiffer than the comparative mixes of MBG:pHA. (p<0.05). Creep rate. With increasing axial stress the creep rate for pure MBG decreased up to a precycling stress of 4 MPa. Subsequent increases in stress had no significant effect. Pure npHA and all of the mixes of MBG:npHA demonstrated significantly less creep than the comparable mixes of MBG:pHA (p=0.01). Applicability of results. The increase in apparent stiffness associated with an increase in the volume fraction of HA is accompanied by a concomitant decrease in the creep rate. Comparison of the mechanical response of the MBG at the high precycling stresses (4 - 7 MPa) to the mechanical response of the MBG:HA mixes at 3 MPa demonstrated an overlap in the mechanical response of the pure MBG at high stress and both the MBG:npHA and MBG: pHA mixes. Conclusions. Greater proportions of HA in the mix led to improvement in the mechanical response without the need for increased force of impaction. These improvements can be quantified by using a uniaxial compression test. The results raise the interesting possibility of tailoring bone graft mixtures to specific patient requirements dependent upon their weight and the quality of their bone stock