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

3D printed porous PLA/nHA composite scaffolds with enhanced osteogenesis and osteoconductivity in vivo for bone regeneration

Repair and regeneration of large bone defects is still a challenge, especially for defects which are the irregular and complex. Three-dimension (3D) printing, as an advanced fabrication technology, has been received considerable attentions due to its high precision, customized geometry and personalization. In this study, 3D porous polylactic acid/nano hydroxyapatite (PLA/nHA) composite scaffolds with enhanced osteogenesis and osteoconductivity were successfully fabricated by desktop fused deposition modeling technology. Morphological, composition and structural analysis revealed that nHA was successfully introduced into the PLA system and homogeneously dispersed in the printed PLA/nHA scaffolds. In vitro antibacterial experiment confirmed that the printed porous PLA/nHA scaffolds have good ability for loading and releasing vancomycin and levofloxacin. Meanwhile, MG-63 cells were used to evaluate the cytocompatibility of printed porous PLA/nHA scaffolds by proliferation and cellular morphological analysis. In addition, rabbit model was established to evaluate the osteogenesis and osteoconductivity of printed PLA/nHA scaffolds. All these results suggested that the 3D printed PLA/nHA scaffolds have great potential for repairing and regeneration of large bone defects.

Effects of MgO and SiO2 on Plasma-Sprayed Hydroxyapatite Coating: An in Vivo Study in Rat Distal Femoral Defects

Plasma-sprayed hydroxyapatite (HA)-coated titanium implants have been widely used in orthopedic applications due to their inheritance of an excellent mechanical property from titanium and great osteoconductivity from HA. However, the lack of osteoinductivity limits their further applications. In this study, 1 wt % MgO and 0.5 wt % SiO₂ were mixed with HA for making plasma-sprayed coatings on titanium implants. Plasma-sprayed HA- and MgO/SiO₂-HA-coated titanium implants showed adhesive bond strengths of 25.73 ± 1.92 and 23.44 ± 2.89 MPa, respectively. The presence of MgO and SiO₂ significantly increased the osteogenesis, osseointegration, and bone mineralization of HA-coated titanium implants by the evaluation of their histomorphology after 6, 10, and 14 weeks of implantation in rat distal femoral defects. Implant pushout tests also showed a shear modulus of 149.83 ± 3.69 MPa for MgO/SiO₂-HA-coated implants after 14 weeks of implantation, compared to 52.68 ± 10.41 MPa for uncoated implants and 83.92 ± 3.68 MPa for pure HA-coated implants; These are differences in the shear modulus of 96% and 56.4%, respectively. This study assesses for the first time the quality of the bone-implant interface of induction plasma-sprayed MgO and SiO₂ binary-doped HA coatings on load-bearing implants compared to bare titanium and pure HA coatings in a quantitative manner. Relating the osseointegration and interface shear modulus to the quality of implant fixation is critical to the advancement and implementation of HA-coated orthopedic implants.

In Vitro Investigation of Cell Compatibility of Pure β-TCP Granules

In this study, osteoblastic cells were isolated from rat bone marrow and characterized. The cells were cultured on β-TCP granules and the osteoblast/ β-TCP constructs. For this purpose, bone marrow was harvested under sterile conditions. Cell aggregates were broken up by pipetting and a cell suspension was cultured in DMEM/F12. After three days, the cells that adhered to the surface of the flask were cultured in osteoblast medium. When the cells became confluent, they were passaged and cultured in 24-well polystyrene cell culture dishes. Characterization of the osteoblasts, cell proliferation and alkaline phosphatase activity were measured on days 1, 7, 14, 21 and 30. To investigate the cell compatibility of the β-TCP granules, osteoblastic cells were cultured on β-TCP granules and a polystyrene cell culture dish (control group). Cell proliferation and alkaline phosphatase (ALP) activity were measured on days 1, 7, 14, 21 and 30 in both groups. Cell growth significantly increased at each time point, but on day 30 a decrease was observed. The ALP activity also increased at each time point and also decreased on day 30. This study may be regarded as the first step leading to a therapy for various bone defects.

Can anodised zirconium implants stimulate bone formation? Preliminary study in rat model

The mechanical properties and good biocompatibility of zirconium and some of its alloys make these materials good candidates for biomedical applications. The attractive in vivo performance of zirconium is mainly due to the presence of a protective oxide layer. In this preliminary study, the surface of pure zirconium modified by anodisation in acidic media at low potentials to enhance its barrier protection given by the oxides and osseointegration. Bare, commercially pure zirconium cylinders were compared to samples anodised at 30 V through electrochemical tests and scanning electron microscopy (SEM). For both conditions, in vivo tests were performed in a rat tibial osteotomy model. The histological features and fluorochrome-labelling changes of newly bone formed around the implants were evaluated on the non-decalcified sections 63 days after surgery. Electrochemical tests and SEM images show that the anodisation treatment increases the barrier effect over the material and the in vivo tests show continuous newly formed bone around the implant with a different amount of osteocytes in their lacunae depending on the region. There was no significant change in bone thickness around either kind of implant but the anodised samples had a significantly higher mineral apposition, suggesting that the anodisation treatment stimulates and assists the osseointegration process. We conclude that anodisation treatment at 30 V can stimulate the implant fixation in a rat model, making zirconium a strong candidate material for permanent implants.

Application of calcium phosphate materials in dentistry

Calcium phosphate materials are similar to bone in composition and in having bioactive and osteoconductive properties. Calcium phosphate materials in different forms, as cements, composites, and coatings, are used in many medical and dental applications. This paper reviews the applications of these materials in dentistry. It presents a brief history, dental applications, and methods for improving their mechanical properties. Notable research is highlighted regarding (1) application of calcium phosphate into various fields in dentistry; (2) improving mechanical properties of calcium phosphate; (3) biomimetic process and functionally graded materials. This paper deals with most common types of the calcium phosphate materials such as hydroxyapatite and tricalcium phosphate which are currently used in dental and medical fields.

Impact of vertical loading on the implant-bone interface

OBJECTIVES

The main aim of this study was to evaluate the impact of vertical loading occurring during removal of cemented restorations on the implant-bone interface.

METHODS

Thirty-six titanium implants (Camlog 4.3 × 9 mm) were placed 1 mm supraosseous in the frontal skull of four minipigs. After a 13 week healing period the implants were exposed and the implant stability was measured. Three implants per minipig were vertically loaded using 20 or 100 impulses, respectively with an 18 Ns impulse imitating a crown removal. Three implants were left unloaded as control. The animals were sacrificed after 13 or 18 weeks. The harvested specimens were analyzed using scanning electron microscopy (SEM), light and fluorescence microscopy.

RESULTS

No post operative complications or deaths of the minipigs occurred. All implants osseointegrated. The average bone-implant contact area (BIC) was 78 ± 5.1%. No statistically significant difference could be found when comparing the BIC areas of the control and the experimental groups between the sacrificed animals at 13 weeks and 18 weeks (P > 0.05). Therefore, the results of each subgroup were pooled. No significant differences regarding the BIC area could be detected between the control and the experimental groups (P > 0.05). Except one failing implant no cracks due to vertical loading could be evaluated in the SEM. Fluorescence microscopy revealed a significantly higher bone remodeling activity in the vertically loaded groups.

CONCLUSIONS

Removal of cemented implant restorations seems not to have an impact on the mechanical implant stability, but seems to increase bone remodeling activity.

Preparation and analysis of chemically gradient functional bioceramic coating formed by pulsed laser deposition

Bioactive ceramic coatings based on calcium phosphates yield better functionality in the human body for a variety of metallic implant devices including orthopaedic and dental prostheses. In the present study chemically and hence functionally gradient bioceramic coating was obtained by pulsed laser deposition method. Calcium phosphate bioactive ceramic coatings based on hydroxyapatite (HA) and tricalcium phosphate (TCP) were deposited over titanium substrate to produce gradation in physico-chemical characteristics and in vitro dissolution behaviour. Sintered targets of HA and α-TCP were deposited in a multi target laser deposition system. The obtained deposits were characterized by X-ray diffraction, fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive X-ray analysis. Inductively coupled plasma spectroscopy was used to estimate the in vitro dissolution behaviour of coatings. The variation in mechanical property of the gradient layer was evaluated through scratch test and micro-indentation hardness. The bioactivity was examined in vitro with respect to the ability of HA layer to form on the surface as a result of contact with simulated body fluid. It could be inferred that chemically gradient functional bioceramic coating can be produced by laser deposition of multiple sintered targets with variable chemical composition.

In situ Mineralization of Hydroxyapatite for a Molecular Control of Mechanical Responses in Hydroxyapatite-Polymer Composites for Bone Replacement

In situ mineralization of hydroxyapatite (HAP) and the role of organics in initial nucleation and growth of HAP is critical for the resulting nano and microstructure of HAP. In situ mineralization of hydroxyapatite (HAP) in the presence of Ca binding polymers such as polyacrylic acid has shown some promise towards improvement of mechanical response of uniaxial compressed HAP/polymer composites to loading. This work represents fundamental studies on the nature of in situ HAP precipitation on resulting microstructure of the composite and bulk mechanical properties. Specifically, an experimental study, evaluating the role of initial stage mineralization of HAP on bulk mechanical responses is conducted. Fourier transform infrared (FT-IR) spectroscopic (with micro attenuated total reflectance) techniques are utilized to evaluate the association of polymer (polyacrylic acid) with HAP during mineralization of HAP. In situ HAP exhibits a faster mineralization as compared to the ex situ mineralization samples, This improved kinetics is responsible for altering the resulting micro and nanostructure of the HAP/polymer composite. Small spectral changes are detected in the absorbance spectra of in situ HAP as compared to ex situ samples. Changes in mechanical response to loading included improvement in strain-to-failure and resulting toughness characteristics of the in situ composite. The control and development of molecular-level associations of polymer with HAP is suggested to be critical for the resulting macro properties. Our results may have significant implications for design of nanocomposites for biomedical applications.

Tetracalcium phosphate: Synthesis, properties and biomedical applications

Monoclinic tetracalcium phosphate (TTCP, Ca(4)(PO(4))(2)O), also known by the mineral name hilgenstockite, is formed in the (CaO-P(2)O(5)) system at temperatures>1300 degrees C. TTCP is the only calcium phosphate with a Ca/P ratio greater than hydroxyapatite (HA). It appears as a by-product in plasma-sprayed HA coatings and shows moderate reactivity and concurrent solubility when combined with acidic calcium phosphates such as dicalcium phosphate anhydrous (DCPA, monetite) or dicalcium phosphate dihydrate (DCPD, brushite). Therefore it is widely used in self-setting calcium phosphate bone cements, which form HA under physiological conditions. This paper aims to review the synthesis and properties of TTCP in biomaterials applications such as cements, sintered ceramics and coatings on implant metals.

Biological Responses to New Advanced Surface Modifications of Endosseous Medical Implants

Implantable medical devices are increasingly important in the practice of modern medicine. However, patients with severely poor bone quality and quantity require highest implant osseointegration for the long-term success. A number of newly-developed advanced surface modifications of medical implants have recently been introduced to the medical implant system. Understanding the mechanisms by which osteogenic cells respond to such materials is therefore of major importance in developing the most effective materials to promote functional osseointegration. Diverse studies using materials with a wide range of new surface modification techniques have demonstrated the pivotal role of surface treatments in cell adhesion, proliferation and lineage specific differentiation. These events underlie the tissue responses required for bone healing following implant placement, with the interaction between adsorbed proteins on the implant surface and surrounding cells eliciting body responses to the treated implant surface. This review illustrates tissue responses to the implant material following implant placement and highlights cellular responses to new advanced implant surface modifications. Such information is of utmost importance to further develop several new advanced surface modifications to be used in the new era medical implantable devices.

A novel murine model of orthopaedic wear‐debris associated osteolysis

OBJECTIVE

To develop a mouse model of bone resorption to quantitatively evaluate wear-debris induced osteolysis.

METHODS

Air pouches were established on the back of BALB/c mice, followed by the surgical introduction of a section of femur or calvaria from a syngeneic mouse donor. One group of bone-implanted pouches was stimulated with ultra-high molecular weight polyethylene (UHMWPE) debris, and the remaining bone-implanted pouches received saline alone as controls. The tissues were harvested at 2, 7, and 14 days after bone implantation for molecular and histological analyses.

RESULTS

Marked inflammatory responses (thicker membrane and increased cellular infiltration) were observed in UHMWPE-stimulated pouches, compared with the saline control. Intensive tartrate-resistant acid phosphatase (TRAP) staining was identified in the UHMWPE-stimulated pouches, especially at the attachment site of inflammatory tissue with implanted bone, where active osteolysis occurred. Image analysis showed that the bone collagen loss was closely related to the amount of UHMWPE within the tissue, and was most prevalent at the contact site of bone with inflammatory tissue. UHMWPE stimulation also significantly increased the release of free calcium into the pouch fluids.

CONCLUSION

This model demonstrates a sensitive, rapid, and reproducible method for studying wear-debris induced osteolysis seen in patients with aseptic loosening.

Laser processing of bioactive tricalcium phosphate coating on titanium for load-bearing implants

Laser-engineered net shaping (LENS), a commercial rapid prototyping (RP) process, was used to coat titanium with tricalcium phosphate (TCP) ceramics to improve bone cell-materials interactions. During LENS coating process, the Nd:YAG laser melts the top surface of Ti substrate in which calcium phosphate powder is fed to create a TCP-Ti composite layer. It was found that an increase in laser power and/or powder feed rate increases the thickness of the coating. However, coating thickness decreased with increasing laser scan speed. TCP coating showed columnar titanium grains at the substrate side of the coating and transitioned to equiaxed titanium grains at the outside. When the scan speed was reduced from 15 to 10mms(-1), coating hardness increased from 882+/-67 to 1049+/-112Hv due to an increase in the volume fraction of TCP in the coating. Coated surfaces showed uniformly distributed TCP particles and X-ray diffraction data confirmed the absence of any undesirable phases, while maintaining a high level of crystallinity. The effect of TCP coating on cell-material interaction was examined by culturing osteoprecursor cells (OPC1) on coated surfaces. The results indicated that TCP coating had good biocompatibility where OPC1 cells attached and proliferated on the coating surface. The coating also initiated cell differentiation, ECM formation and biomineralization.

Soaking morselized allograft in bisphosphonate can impair implant fixation

The use of impacted, morselized allograft is a well-established way to provide initial stability of revision joint replacements. We investigated whether rinsing morselized allograft in bisphosphonate and subsequently impacting it around experimental titanium-coated implants would further facilitate biomechanical implant fixation and graft incorporation. In 10 dogs, a pair of titanium implants surrounded by a 2.5-mm gap was inserted into the proximal part of each humerus during two separate surgeries to allow two observation periods. The gap was filled with impacted, morselized allograft soaked in either bisphosphonate (alendronate, 2 mg/mL) or saline (control). Unbound alendronate was not rinsed away. During the first surgery, one pair of implants (alendronate and control) was inserted into one humerus. Eight weeks later, a second pair of implants was inserted into the contralateral humerus. The first pair of implants was observed for 12 weeks and the second pair for 4 weeks. Implants were evaluated by histomorphometry and biomechanical pushout test. We found substantially decreased biomechanical implant fixation for all implants surrounded by impacted, morselized allograft that had been soaked in alendronate. Furthermore, the alendronate treatment blocked formation of new bone and inhibited resorption of the graft material. Although limited by the specific dose of alendronate used and the omission of rinsing away excess bisphosphonate, this study warrants caution and calls for further experimental research before impacting alendronate-soaked morselized allograft around clinical joint replacements.

Murine model of prosthesis failure for the long-term study of aseptic loosening

We examined a novel mouse model of wear debris-induced prosthesis instability and osteolysis, and its application for the evaluation of therapy. A stainless steel or titanium-alloy pin was implanted into the proximal tibia to form a contiguous surface with the articular cartilage. In some mice, titanium particles were injected into the tibial canal during the surgery, followed by monthly intraarticular injection. MicroCT scans revealed that the implants without particle challenge were stable without bone mineral density changes for 6 months. Histological analysis showed new bone formation around the implant at 6 weeks postsurgery. Periprosthetic soft tissue with inflammatory cells was a ubiquitous finding at the interface between the implant and surrounding bone in samples exposed to titanium particles, and expression of IL-1beta, TNFalpha, and CD68 was common in these joints. Pullout tests indicated that an average 5N load was required to pull out stable implants from surrounding bone. However, particle stimulation dramatically reduced the pullout force to less than 0.4 N. The feasibility of in vivo gene transfer on this model was confirmed by X-gal staining of synovial membrane and periprosthetic tissue after injection of AAV-LacZ in the prosthetic joint. This murine model of weight-bearing knee prosthesis provides an economical, reproducible, and easily obtained means to study joint arthroplasty failure. The ability to evaluate the biomechanical properties of the prosthetic joint, in addition to histological and biochemical examination, results in a useful model to investigate many of the properties of prosthetic joint components during the response to debris-associated osteolysis.

Effect of topical alendronate treatment on fixation of implants inserted with bone compaction

Bone compaction has been shown to enhance the critical initial implant stability that is important for secure long-term fixation. We investigated whether topical bisphosphonate treatment improves fixation of implants inserted with bone compaction. Porous-coated titanium implants were inserted with bone compaction into the knees of eight dogs. In the right knee, topical bisphosphonate treatment was applied before bone compaction. Saline was used as a control in the left knee. The knees were observed for 4 weeks. We found an increase in total bone-to-implant contact and total bone density around the implants in the bisphosphonate-treated group. These were results of increased nonvital bone-to-implant contact and increased nonvital periimplant bone density. No change in biomechanical fixation was found. Studies with a longer followup are needed to investigate whether the preservative effect of alendronate on nonvital bone might enhance implant fixation by osteoconduction.

Subcutaneous evaluation of RF magnetron-sputtered calcium pyrophosphate and hydroxylapatite-coated Ti implants

The in vivo behavior of infrared-heated, RF magnetron-sputtered hydroxylapatite (HA) and calcium pyrophosphate (DCPP) coated titanium discs was investigated. The discs were implanted subcutaneously in the back of six goats for 2, 4, 8 and 12 weeks. At the end of the study, coated discs were removed and examined on their physicochemical properties by X-ray diffraction (XRD) and scanning electron microscopy (SEM), including energy dispersive spectroscopy (EDS). Also, implants were prepared for light microscopical evaluation of the tissue response. The results showed that heat-treated HA coatings showed a stable behavior, i.e. no changes in the XRD pattern occurred during implantation. Also, no dissolution of the coating was observed by SEM. EDS revealed that the Ca/P ratio of the HA coatings remained stable during implantation. In contrast, heat-treated DCPP coatings showed a compositional change into apatite and tricalcium phosphate (TCP) during implantation. This was confirmed by the SEM and EDS analysis. The Ca/P ratio of the DCPP coatings changed from 0.8 to 1.52 during implantation. Finally, histology showed that both heat-treated HA and DCPP coatings showed no adverse tissue response, as characterized by the presence of thin, dense fibrous tissue capsule. Consequently, it can be concluded that 2 mum thick heat-treated, RF magnetron-sputtered HA and DCPP coatings are of sufficient thickness to withstand dissolution during 12 weeks of implantation in a subcutaneous location in goats. In addition, both coatings showed a biocompatible tissue behavior. Further, heat-treated DCPP coatings revealed a gradual compositional change into apatite and TCP.

Poly(D,L-lactide) coating is capable of enhancing osseous integration of Schanz screws in the absence of infection

Pin loosening is a major complication in external fixation. Biological and mechanical conditions play an important role in the maintenance and enhancement of the implant-bone interface in fracture fixation. It is thought that biodegradable coatings may be capable of preventing pin track infection and pin loosening. The goal of this study was therefore to analyze the influence of a biodegradeable coating on the osseous integration of Schanz' screws during fracture treatment. Standardized osteotomies (3-mm fracture gap) of the right tibiae were performed on 16 sheep and stabilized by an AO mono-lateral external fixator. Additional, mechanically less loaded Schanz' screws were also mounted. All screws were randomly coated with biodegradable poly(D,L-lactide). The sheep were sacrificed after 9 weeks. All screws were removed and rolled on blood agar plates for microbiological analysis. Histological sections of the pin tracks were histochemically and morphometrically analyzed. Clinically, no signs of severe infection were visible. Microbiological analysis revealed 14.8% colonization by Staphylococcus aureus in the coated and 29% in the uncoated screws. Histomorphometry of the bone surrounding the Schanz' screws revealed that significantly more osseous integration had occurred on poly(D,L-lactide)-coated screws in the absence of bacterial colonization. Significantly more bone remodeling and a higher osteoclastic activity was seen near the screw-bone interface in the uncoated screw group. Up to a threefold increase in new bone formation and more severe remodeling was observed around the screw entry compared to the pin exit in all groups. Loaded screws showed significantly more callus formation around the exit sites than their less loaded counterparts. In the present study, poly(D,L-lactide) coating of Schanz' screws was found to enhance osseous integration in the absence of bacterial colonization in sheep by causing less cortical remodeling and less osteoclastic activity in the cortices compared to uncoated screws. Additionally, the coating appeared to reduce the instances of pin track infections. Mechanical loading showed an adverse effect on bone formation and remodeling. It has been shown that both biological and mechanical factors play an important role in the maintenance of osseous integrity of the pin-bone interface. Poly(D,L-lactide) coating of Schanz' screws does not prevent osseous destruction and severe bacterial colonization along the pin tracts, but can improve osseous integration of Schanz' screws in the absence of infection.

Osteointegration of femoral stem prostheses with a bilayered calcium phosphate coating

Our purpose was to evaluate the osteointegration of bilayered calcium phosphate (CaP)-coated femoral hip stems in a canine model. A first layer of hydroxyapatite (HA) 20 microm thick and a superficial layer of Biphasic Calcium Phosphate (BCP) 30 microm thick were plasma-sprayed on to the proximal region of sandblasted Ti6Al4V prostheses. Bilayered CaP-coated and non-coated canine femoral stems were implanted bilaterally under general anesthesia in 6 adult female Beagle dogs. After 6 and 12 months, a significant degradation of the bilayered coating occurred with a remainder of 33.1+/-12.4 and 23.6+/-9.2 microm in thickness, respectively. Lamellar bone apposition was observed on bilayered coated implants while fibrous tissue encapsulation was observed on non-coated femoral stems. The bone-implant contacts (BIC) were 91+/-3% and 81+/-8% for coated and 7+/-8% and 8+/-12% for non-coated implants, at 6 and 12 months, respectively. Our study supports the concept of a direct relationship between the biodegradation of CaP coating and the enhanced osteointegration of titanium prostheses. A bilayered CaP coating might therefore enhance bone apposition in the early stages because of the superior bioactivity of the BCP layer while the more stable HA layer might sustain bone bonding over long periods.

Histological findings in titanium implants coated with calcium phosphate ceramics installed in rabbit's tibias

Oral reconstruction using osteointegrated implants are widely indicated nowadays. The implant bone anchorage is very important for its functional stability. Thus, ceramic biomaterials are widely used as coatings of the implant surfaces to accelerate local osteogenesis. The purpose of this study is to assess the biocompatibility and the osteoconduction of two types of calcium phosphate ceramics used as titanium dental implant coatings. These implants were installed in rabbit tibia during an 8-week healing period. The light and fluorescent microscopy observations showed that the materials are biocompatible and that they have osteoconductive activities.

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.

Influence of fluorapatite on the properties of thermally sprayed hydroxyapatite coatings

Thermally sprayed hydroxyapatite has been the widely used on orthopaedic prosthesis to induce bone growth and facilitate bone attachment. However, hydroxyapatite has a greater affinity for the formation of an amorphous phase in the thermally sprayed coating that results in the release of excessive amount of mineral ions from the implant coating leading to a saturated environment in the immediate vicinity of the bone cells. Fluorapatite however is highly crystalline and offers the potential for lower mineral ion release by dissolution. Thus study investigates the influence of fluorapatite in a thermally sprayed hydroxyapatite coating. Mechanical blends of fluorapatite with hydroxyapatite were thermally sprayed, characterized with X-ray diffraction, SEM, FTIR, optical microscopy for microstructure, roughness and tested for solubility. Cathodoluminescence microscopy was used to examine the resorbed coating surface. Fluorapatite coatings crystallized more readily and produce a greater coating roughness. The roughness in fluorapatite coatings arises from less flattened droplets that show a tendency for finger formation. Addition of fluorapatite increases coating crystallinity. The use of slower resorbing fluorapatite produces less particle release which favors improved osseointegration. Less change in the surface topography during resorption can be used to an advantage to control the coating surface presented to cells and extra cellular matrix proteins.

A new electrochemically graded hydroxyapatite coating for osteosynthetic implants promotes implant osteointegration in a rat model

Hydroxyapatite (HAP) is widely used as an osteoconductive coating for orthopedic implants. So far standard coating methods like plasma spraying produce a relatively thick coating layer (>30 microm). In addition, the chemical structure of the HAP may be altered because of the heating throughout the coating process. This may have negative effects on the coating stability, implant fixation, and induction of bone formation. The relatively thick layer may detach from the implant with the risk of wear debris. In the present study the potential of a newly developed HAP coating of implants on osteointegration was investigated in a rat model. The coating method, based on an electrochemical process, is applied in a graded manner and results in a biodegradable HAP coating with a thickness of approximately 2 mum. Coated versus uncoated titanium Kirschner wires (1.4-mm diameter) were inserted into the medullary cavity of the right femora of 5-month old female Sprague Dawley rats (n=36) in a retrograde fashion. Throughout an experimental period of 2 months the osteointegration was traced radiologically. After this time the animals were sacrificed and the implant integration was tested biomechanically with the use of a push-out test. To analyze the bone-implant interface, histological sections (80 mum) were investigated with an image analyzing system. The biomechanical testing revealed a significantly higher implant fixation in the group treated with the HAP-coated implant (shear strength: 27.8 +/- 6.7 MPa) compared to control (shear strength: 8.08 +/- 3.4 MPa). The histological analyses demonstrated a better ingrowth of the implants in the HAP group with significantly more direct bone-implant contacts compared to the control group. The results demonstrate that the HAP coating promotes implant osteointegration in a rat model.

Behavior of the bone-titanium interface after push-in testing: a morphological study

Fourteen titanium dental implants (Tioblast) were implanted singly in the proximal tibia of New Zealand rabbits for 120 days. A bone defect was surgically produced and filled with Bio-Oss around six of these implants. After the animals were sacrificed and their organs harvested, bone segments were fixed and methacrylate embedded after the push-in test had been performed. Microradiography was performed on longitudinal sections of the implants, whereas scanning electron microscope analysis was performed on the remaining embedded half-implants using secondary electrons only. The results showed that the implants were apically and coronally surrounded by bone, whether Bio-Oss was used or not. Fractures were evident through the newly formed bone and between the pre-existing and newly formed bone. Some fracture lines propagated through the bone and stopped at the implant surface without continuing along the bone-titanium interface. Detachment between the implant and the bone occurred at the coronal extremity of the implants and along its cervical region. These results highlight the fact that the bone-titanium interface has a high resistance to loading. It exhibited greater resistance than the newly formed bone and seems to behave in a manner similar to the cement lines of osteons.

Comparison of hydroxyapatite and hydroxyapatite tricalcium-phosphate coatings

This study compared the effects of hydroxyapatite (HA) coating and biphasic HA/tricalcium-phosphate (HA/TCP) coating on the osseointegration of grit-blasted titanium-alloy implants. Each coated implant was compared with uncoated grit-blasted implants as well. The implants were press-fit into the medullary canal of rabbit femora, and their osseointegration was evaluated 3 to 24 weeks after surgery. The coated implants had significantly (P<.05) greater new bone ongrowth than the uncoated implants (HA, 56.1 +/- 3.1%; HA/TCP, 53.8 +/- 2.6%; uncoated, 32.2 +/- 1.4% of the implant perimeter, 12 weeks). Unmineralized tissue (cartilage and osteoid) was seen on the uncoated implants but never on the coated implants. The coated implants had significantly (P<.05) greater interfacial shear strength than the uncoated implants (HA, 4.1 +/- 0.4 MPa; HA/TCP, 4.8 +/- 0.5 MPa; uncoated, 2.6 +/- 0.2 MPa, 12 weeks). There was no difference between HA and HA/TCP coating in regard to new bone growth or interfacial shear strength. These data show a comparable enhancement effect of HA and HA/TCP coatings on the osseointegration of titanium-alloy implants.

Polyethylene and methyl methacrylate particle-stimulated inflammatory tissue and macrophages up-regulate bone resorption in a murine neonatal calvaria in vitro organ system

There is considerable evidence that orthopaedic wear debris plays a crucial role in the pathology of aseptic loosening of joint prostheses. This study examined the effect of inflammatory membranes stimulated with methyl methacrylate and polyethylene on bone resorption, using the murine air pouch model. The capacity of RAW 264.7 mouse macrophages exposed to polymer particles to produce factors affecting bone metabolism was also studied. Neonatal calvaria bones were co-cultured with either pouch membranes or conditioned media from activated macrophages. Bone resorption was measured by the release of calcium from cultured bones, and the activity of tartrate-resistant acid phosphatase in both bone sections and culture medium was also assayed. Results showed that inflammatory pouch membrane activated by methyl methacrylate and polyethylene enhanced osteoclastic bone resorption. Conditioned media from particles stimulated mouse macrophages also stimulated bone resorption, although this effect was weaker than resorption induced by inflammatory pouch membranes. The addition of the particles directly into the medium of cultured calvaria bones had little effect on bone resorption. Our observations indicate that both inflammatory tissue and macrophages provoked by particles can stimulate bone resorption in cultured mouse neonatal calvaria bones. This simple in vitro bone resorption system allows us to investigate the fundamental cellular and molecular mechanism of wear debris induced bone resorption and to screen potential therapeutic approaches for aseptic loosening.

Glass reinforced hydroxyapatite for hard tissue surgery--part II: in vitro evaluation of bone cell growth and function

Hydroxyapatite (HA)-based materials are considered to be potentially useful as bone implant materials, particularly those reinforced with glass to improve mechanical strength. However, the precise effects of glass-reinforced HA on the growth and functions of bone cells are still unclear. The present study has therefore examined the response of human osteoblast-like cells to HA and HA reinforced with two different proportions of glass, namely 2.5% and 5%. All materials enabled the cells to attach and proliferate during 7 days in culture and, although the growth was less than on control plastic surfaces, there was no deleterious effect of the 5% glass composite compared with HA alone. Flow cytometry analysis showed that there was no effect on cell size and granularity, but there were marked and highly selective changes in the expression of certain connective tissue proteins. Thus, while bone sialoprotein and osteonectin were down-regulated on HA alone, the expression of these antigens was relatively enhanced on the composite materials, and collagen type I was also up-regulated on the glass-reinforced HA. Thus, modulation of the glass composition of HA materials could be used to produce not only improved mechanical strength, but also enhanced biocompatibility.

Physicochemical study of plasma-sprayed hydroxyapatite-coated implants in humans

This study represents the first report of the physical and chemical changes occurring in coatings of failed hydroxyapatite (HA)-coated titanium implants obtained from a comprehensive, multicenter human dental implant study. A total of 53 retrieved samples were obtained and compared with unimplanted controls with the same manufacturer and similar manufacture dates. Forty-five retrieved implants were examined for surface characteristics and bulk composition. Implants were staged based on implantation history: stage 1 (implants retrieved between surgical placement and surgical uncovering), stage 2 (implants retrieved at surgical uncovering and evaluation), stage 3 (implants retrieved between surgical uncovering evaluation and occlusal loading), and stage 4 (implants retrieved after occlusal loading). Scanning electron microscopy showed progressive coating thinning with implantation time. At later stages, bare Ti metal was detected by energy-dispersive X-ray analysis and electron spectroscopy for chemical analysis. Increases in Ti and Al (2-7.5 atm % each) were detected at the apical ends of all stage 4 samples. In unimplanted coatings, X-ray diffraction analysis demonstrated the presence of amorphous calcium phosphate, beta-tricalcium phosphate, tetracalcium phosphate, and calcium oxide in addition to large hydroxyapatite crystals (c axis size, D002 = 429 +/- 13 A; a axis size, D300 = 402 +/- 11 A, a/c aspect ratio 0.92). The nonapatitic phases disappeared with increased implantation time, although there was a persistence of amorphous calcium phosphate. Bulk coating chemical analysis showed that Ca/P ratios for implant controls (1.81 +/- 0.01) were greater than stoichiometric HA (1.67) and decreased for implant stages 3 and 4 (1.69 +/- 0.09 and 1.67 +/- 0.09, respectively), explained by the dissolution of the non apatitic phases. Crystal sizes also changed with implantation times, being smaller than the control at all but stage 4. Fourier transform infrared analyses agreed with these results, and also indicated the accumulation of bone (protein and carbonate-apatite) in the retrieved coatings. The accumulation of bone was not stage dependent. These findings indicate that there was some biointegration with the surrounding bone, but the greatest changes occurred with the HA coating materials, their loss, and chemical change.

Quantitative small-animal surrogate to evaluate drug efficacy in preventing wear debris-induced osteolysis

Individuals who suffer from severe joint destruction caused by the various arthritidies often undergo total joint arthroplasty. A major limitation of this treatment is the development of aseptic loosening of the prosthesis in as many as 20% of patients. The current paradigm to explain aseptic loosening proposes that wear debris generated from the prosthesis initiates a macrophage-mediated inflammatory response by resident macrophages, leading to osteoclast activation and bone resorption at the implant interface. No therapeutic interventions have been proved to prevent or inhibit aseptic loosening. The development of therapeutic strategies is limited due to the absence of a quantitative surrogate in which drugs can be screened rapidly in large numbers of animals. We have previously described a model in which titanium particles implanted on mouse calvaria induce an inflammatory response with osteolysis similar to that observed in clinical aseptic loosening. Here, we present new methods by which the osteolysis in this model can be quantified. We determined that 6-8-week-old mice in normal health have a sagittal suture area of 50 (+/-6) microm2, which contains approximately five osteoclasts. As a result of the titanium-induced inflammation and osteolysis, the sagittal suture area increases to 197 (+/-27) microm2, with approximately 30 osteoclasts, after 10 days of treatment. The sagittal suture area and the number of osteoclasts in the calvaria of sham-treated mice remained unchanged during the 10 days. We also determined the effects of pentoxifylline, a drug that blocks the responses of tumor necrosis factor-alpha to wear debris, and the osteoclast inhibitor alendronate. We found that both drugs effectively block wear debris-induced osteolysis but not osteoclastogenesis. In conclusion, we found the measurements made with this model to be reproducible and to permit quantitative analysis of agents that are to be screened for their potential to prevent aseptic loosening.