The effect of hydroxyapatite coating on bone growth into porous titanium alloy implants

Osseointegration of hydroxyapatite porous-coated femoral implants in a canine model

This study evaluated the quality and quantity of osseointegration of two thicknesses of hydroxyapatite coating on press-fit, porous-coated titanium implants in a canine hip model. In 12 dogs, titanium press-fit porous-coated prostheses were implanted. The stems had a 50-microm thickness hydroxyapatite coating, 100-microm thickness hydroxyapatite coating, or no hydroxyapatite coating. The animals were randomized into one of three groups and received one of the three implants. The implants were retrieved and examined 4 months after implantation. Direct juxtaposition of bone to the surface of the hydroxyapatite-coated implants with no intervening fibrous tissue layer was observed. There was no histologic evidence that hydroxyapatite initiated any foreign body reaction, nor was there any irregularity or resorption of the hydroxyapatite coating. There was a statistically significant greater degree of total bone apposition and bone ingrowth in the implants coated with hydroxyapatite at the level of the isthmus and the calcar. No statistical difference was found between the two groups with hydroxyapatite coatings in the degree of bone ingrowth or bone apposition.

Mechanical and histological evaluations of hydroxyapatite-coated and noncoated Ti6Al4V implants in tibia bone

This paper evaluates the behavior of hydroxyapatite (HAP) coated and noncoated Ti6Al4V implants in dog tibia after 3 and 5 months implantation. HPA-coated implants were obtained by plasma spraying. XRD, SEM, and EPMA were employed to estimate the coating characteristics and their behavior in vivo. Investigation of material characteristics showed that the as-received coatings consisted mainly of amorphism and HAP phase. Other phases such as TCP and CaO were identified due to thermal changes of HAP particles in plasma flame. SEM micrographs showed a typical microstructure of plasma-sprayed coating. The as-received coating was formed by well-melted pancake-like splats that lead to a dense coating with a rough surface. Lamellar structure, micropores, and microcracks, observed inside the coating, are characteristic of plasma spraying. Push-out tests revealed that HAP coating had a significant promotion of interfacial shear strength. The shear strength between bone and HAP-coated implants was much higher than that between bone and noncoated implants due to the different bone-implant interfaces formed after implantation. SEM observation revealed a direct attachment between HAP coating and newly formed bone. However, noncoated implants were separated from newly formed bone by fibrous tissues. Ti ions were found to be released into the surrounding environment after long time immersion in body fluid, and thus caused low shear strength. Prolongation of implantation time had different effects on shear strength. It improved the shear strength between HAP-coated implant and newly formed bone. However, it had little effect on that between noncoated implant and surrounding tissues.

In vivo evaluation of bone-bonding of titanium metal chemically treated with a hydrogen peroxide solution containing tantalum chloride

Apatite formation on implants is important in achieving a direct bonding to bone tissue. We recently showed that titanium metal chemically treated with a hydrogen peroxide solution containing tantalum chloride has the ability to form a hydroxyapatite layer in simulated body fluid which had inorganic ion composition similar to human blood plasma. In this study, a pure titanium cylinder (4.0 mm in diameter, 20.0 mm in length) treated with this method was implanted into a hole (4.2 mm in diameter) in a rabbit's tibia. After implantation for predetermined periods up to 16 weeks, the specimens were extracted with bone tissue, and were examined by push-out test to evaluate the shearing force between the implant and bone tissue. The results were compared with those of non-treated pure titanium. Eight weeks after surgery, the shearing force of the treated titanium implanted in the 4.2 mm-hole was significantly higher than that of non-treated titanium, although the surface roughness was not changed after the treatment. Scanning electron microscopic (SEM) observation and energy-dispersive X-ray (EDX) microanalysis showed that the bone comes very close to the surface of the treated titanium. Moreover, the shearing force was higher for the implanted sample in the 4.0 mm-hole than that in the 4.2 mm-hole. Thus, it is confirmed that the treatment with hydrogen peroxide solution containing tantalum chloride provides higher bonding ability on titanium implants in vivo.

Alkali- and heat-treated porous titanium for orthopedic implants

This study was carried out to investigate the effects of the alkali and heat treatments on the bone-bonding behavior of porous titanium implants. Porous titanium implants had a 4.6 mm solid core and a 0.7 mm thick porous outer layer using pure titanium plasma-spray technique. Three types of porous implants were prepared from these pieces: 1.control implant (CL implant) as manufactured 2.AW-glass ceramic bottom-coated implant (AW implant) in which AW-glass ceramic was coated on only the bottom of the pore of the implant 3.alkali- and heat-treated implant (AH implant), where implants were immersed in 5 mol/L NaOH solution at 60 degrees C for 24 h and subsequently heated at 600 degrees C for 1 h. The implants were inserted into bilateral femora of six dogs hemi-transcortically in a randomized manner. At 4 weeks, push-out tests revealed that the mean shear strengths of the CL, AW, and AH implants were about 10.8, 12.7, and 15.0 MPa, respectively. At 12 weeks there was no significant difference between the bonding strengths of the three types of the porous implants (16.0-16.7 MPa). Histologically and histomorphologically, direct bone contact with the implant surface was significantly higher in the AH implants than the CL and AW implants both at 4 and 12 weeks. Thus, the higher bonding strength between bone and alkali- and heat-treated titanium implants was attributed to the direct bonding between bone and titanium surface. In conclusion, alkali and heat treatments can provide porous titanium implants with earlier stable fixation.

Alkali- and heat-treated porous titanium for orthopedic implants

This study was carried out to investigate the effects of the alkali and heat treatments on the bone-bonding behavior of porous titanium implants. Porous titanium implants had a 4.6 mm solid core and a 0.7 mm thick porous outer layer using pure titanium plasma-spray technique. Three types of porous implants were prepared from these pieces: 1.control implant (CL implant) as manufactured 2.AW-glass ceramic bottom-coated implant (AW implant) in which AW-glass ceramic was coated on only the bottom of the pore of the implant 3.alkali- and heat-treated implant (AH implant), where implants were immersed in 5 mol/L NaOH solution at 60 degrees C for 24 h and subsequently heated at 600 degrees C for 1 h. The implants were inserted into bilateral femora of six dogs hemi-transcortically in a randomized manner. At 4 weeks, push-out tests revealed that the mean shear strengths of the CL, AW, and AH implants were about 10.8, 12.7, and 15.0 MPa, respectively. At 12 weeks there was no significant difference between the bonding strengths of the three types of the porous implants (16.0-16.7 MPa). Histologically and histomorphologically, direct bone contact with the implant surface was significantly higher in the AH implants than the CL and AW implants both at 4 and 12 weeks. Thus, the higher bonding strength between bone and alkali- and heat-treated titanium implants was attributed to the direct bonding between bone and titanium surface. In conclusion, alkali and heat treatments can provide porous titanium implants with earlier stable fixation.

Hydroxyapatite granules interposed at bone-cement interface in total hip replacements: histological study of retrieved specimens

The effect of hydroxyapatite (HA) granules interposed between bone and polymethylmethacrylate (PMMA) bone cement in total hip replacement was histologically evaluated. The technique consisted of smearing 2-5 g of HA granules (straight phi = 100-300 microm) onto the bone surface just before cementing. Four specimens containing well-fixed bone-cement interface were retrieved at 1, 2, 6, and 10 years postoperatively and examined with back-scattered electron microscopy and light microscopy. The majority of HA granules were incorporated into remodeled trabeculae, and highly convoluted bone-cement interface was maintained up to 10 years. The presence of active remodeling in the adjacent bone was observed. There were no significant inflammatory or foreign body reactions against interposed HA granules. In one specimen retrieved from a patient with rheumatoid arthritis, bone formation around HA granules was limited after 1 year. These results have provided histological evidence for the significantly reduced incidence of radiolucent lines in total hip replacement with this cementing technique, reported elsewhere.

Degradation of hydroxylapatite, fluorapatite, and fluorhydroxyapatite coatings of dental implants in dogs

Calcium phosphate coatings on dental implants enhance integration of the material. Resorption of the ceramic coatings has raised some concern about the behavior of the bone-implant interfaces after the coating disappearance. Substitution of the OH- ions by fluoride in the hydroxylapatite (HA) lattice makes the calcium phosphate more stable. We investigated the degradation rate of dental implants with 50- and 100-microm coatings of HA, fluorapatite (FA), or fluorhydroxylapatite (FHA). The implants were inserted in dog jaws and retrieved for histological analysis after 3, 6, and 12 months. The thickness of the calcium phosphate coatings was evaluated using an image analysis device. A relative resorption index and its standard deviation were studied. HA and FA coatings (even at 100-microm thickness) were almost totally degraded within the implantation period. In contrast, the FHA coatings did not show significant degradation during the same period. The standard deviation showed that the resorption process for FHA with thicknesses of 50 or 100 microm was the same. Such a difference was not observed between the 50- and 100-microm thick coatings of FA and HA. In conclusion, the FHA coatings showed good integration in the bone tissue and lasted much longer than classic calcium phosphate coatings.

Stem cell technology and bioceramics: from cell to gene engineering

Mesenchymal stem cells reside in bone marrow and, when these cells are incorporated into porous ceramics, the composites exhibit osteo-chondrogenic phenotypic expression in ectopic (subcutaneous and intramuscular) or orthotopic sites. The expressional cascade is dependent upon the material properties of the delivery vehicle. Bioactive ceramics provide a suitable substrate for the attachment of the cells. This is followed by osteogenic differentiation directly on the surface of the ceramic, which results in bone bonding. Nonbioactive materials show neither surface-dependent cell differentiation nor bone bonding. The number of mesenchymal stem cells in fresh adult bone marrow is small, about one per one-hundred-thousand nucleated cells, and decreases with donor age. In vitro cell culture technology can be used to mitotically expand these cells without the loss of their developmental potency regardless of donor age. The implanted composite of porous ceramic and culture-expanded mesenchymal stem cells exhibits in vivo osteo-chondrogenic differentiation. In certain culture conditions, these stem cells differentiate into osteoblasts, which make bone matrix on the ceramic surface. Such in vitro prefabricated bone within the ceramic provides immediate new bone-forming capability after in vivo implantation. Prior to loading of the cultured, marrow-derived mesenchymal stem cells into the porous ceramics, exogenous genes can be introduced into these cells in culture. Combining in vitro manipulated mesenchymal stem cells with porous ceramics can be expected to effect sufficient new bone-forming capability, which can thereby provide tissue engineering approaches to patients with skeletal defects in order to regenerate skeletal tissues.

Effect of mechanical surface pretreatment on metal ion release

The degree of metal ion dissolution from Ti-6Al-4V alloy hip replacement stems subjected to various mechanical and chemical surface pretreatments was analysed in vitro. High-dissolution rates were observed for nitric acid passivated samples that had been mechanically surface treated to increase the implant surface area. Significantly lower ion release levels were observed for mechanically treated samples which had been aged in de-ionised water. The application of an hydroxyapatite coating decreased the metal ion release from the nitric acid passivated samples (compared to the uncoated sample) and increased the metal ion dissolution from the aged samples. The dissolution behaviour of the samples is explained in terms of the diffusion processes occurring at the stem/solution interface and the morphological and chemical characteristics of the surface treated stems.

Organoapatite growth on an orthopedic alloy surface

We report here a method to coat orthopedic metals with the artificial bone material organoapatite. The growth of organoapatite on titanium alloy surfaces of foils and porous cylinders involves sequential preadsorption of poly(L-lysine) and poly(L-glutamic acid) on metal, followed by exposure to organoapatite-precipitating solutions. The organoapatite characterization of the coating was carried out by transmission electron microscopy, electron diffraction, scanning electron microscopy, energy-dispersive X-ray scattering, powder X-ray diffraction, FT-IR, and elemental analysis. The preadsorbed poly(amino acids) in the form of a self-assembled bilayer of oppositely charged macromolecules can lead to a surface coverage of titanium alloy in the range of 70-90%. The deposition mechanisms could involve the surface capture of embryonic crystals and the nucleation of apatite on the bilayer. Bioabsorbable organoapatite could serve as a tissue-engineering scaffold for bone regeneration into porous implants.

Bioactive ceramics: the effect of surface reactivity on bone formation and bone cell function

Surface reactivity is one of the common characteristics of bone bioactive ceramics. It contributes to their bone bonding ability and their enhancing effect on bone tissue formation. During implantation, reactions occur at the material-tissue interface that lead to time-dependent changes in the surface characteristics of the implant material and the tissues at the interface. This review describes some of the current concepts regarding the surface reactivity of bone bioactive materials and its effect on attachment, proliferation, differentiation and mineralization of bone cells.

Behavior of the femoral stem in the Bihapro hip prosthesis

This study evaluated the hydroxyapatite-coated femoral stem of the Bihapro hip prosthesis (Biomet Ltd, Bridgend, United Kingdom) using radiography, TC-99 scintigraphy, and bone densitometry. Thirty stems with >2 years of follow-up (mean: 31 months) were evaluated. No loosening or changes in the position of the implant were seen, and the mean subsidence was 2.2 mm. Radiography revealed a densification in the metaphyseal zone with reconstruction of the trabecular lines in 21 cases, while in 22 patients, radiolucent lines in the distal area of the femoral component were observed. Scintigraphy showed a diaphyseal normo-captation in 19 cases and a hypercaptation <1.4 with a mean index of 1.1 in 10 patients. Metaphyseal hypercaptation >1.4 was seen in 24 patients. Bone densitometry revealed increased density in the metaphyseal zones in 29 patients with a periprosthetic/normal bone quotient of 1.4.

The influence of hydroxyapatite particles on osteoclast cell activities

Aseptic loosening after total joint arthroplasty is a major problem in orthopedic surgery. Small particles from material wear have been reported as the main cause of implant failure. For this reason, investigation into possible wear particles from the materials used in the implant may lead to longevity after arthroplasty. Hydroxyapatite (HA) has been extensively investigated and reported as an excellent biomaterial with excellent biocompatibility. In this study, we used an in vitro osteoblast/osteoclast model to test the biocompatibility of various-sized HA particles. Primary osteoclasts/osteoblasts were co-cultured with different-sized HA particles (0.5-3.0 microm, 37-53 microm, 177-205 microm, and 420-841 microm) for 3 h, 1 day, 3 days, and 7 days. Cellular responses to the HA particles were evaluated by changes in cell counts and the secretion of transforming growth factor (TGF-beta1), alkaline phosphatase (ALP), tumor necrosis factor (TNF-alpha), prostaglandin (PGE2), and lactate dehydrogenase (LDH) in the supernatant of the culture media. The results showed that osteoblasts/osteoclasts co-cultured with HA particles smaller than 53 microm undergo the most significant changes. Cellular counts significantly decreased, and the changes were more obvious in the osteoblast population. There also was a significant decrease in TGF-beta1 concentration and a significant increase in PGE2 and LDH concentration, but there were no changes in the TNF-alpha or ALP titer. It can be concluded that larger HA particles may be quite compatible with bone cells while smaller-sized HA particles can both activate the osteoclasts and decrease the cell population of the osteoblasts. Justification for the additional expense incurred with the use of hydroxyapatite in primary total hip arthroplasty should be further evaluated.

In vitro stability predictions for the bone/hydroxyapatite composite system

Electroacoustic measurements of the zeta (zeta) potential as a function of pH were collected and used to probe the nature of the ionic contributions to the bond formed between synthetic hydroxyapatite (HA) and bone. HA powder and wet bone powder were dispersed into an electrolyte solution comprised of physiologic saline (0.154M NaCl), electroacoustic measurements collected, and the zeta potential calculated as a function of pH. The zeta potential and particle size then were used to calculate the stability of the composite dispersion, where stability is the ability of a particulate suspension to remain unagglomerated. The stability was used to predict the homo- (HA to HA and bone to bone) versus heterocoagulation (HA to bone) behaviors for the HA/bone system. Although single component bone and HA demonstrated stability against agglomeration, the HA/bone interaction was determined to be unstable for all pH levels tested, including pH 7.4, the normal in vivo pH. These results establish one factor responsible for the observed physicochemical bonding between bone and HA noted by many in the orthopedic community.

Differences in microstructural characteristics of dense HA and HA coating

Two implant types of hydroxyapatite (HA) currently are available for dental implants: dense HA-cemented titanium (Ti) and HA-coated. It has been shown in previous reports that there are differences in the chemical and mechanical stabilities between the dense HA and HA coated. The differences are thought to be due to structural differences between the two ceramic types. The aim of this study was to investigate the differences in microstructural characteristics of currently available dense HA and HA coated implants before implantation and at periods of 3 weeks and 10 months after implantation in canine bone. X-ray diffractometry, infrared analysis, transmission electron microscopy, and energy dispersive X-ray analysis were used. The dense HA is composed of crystal grains, with a well crystallized structure of HA, closely bound to each other and approximately 0.4-0.6 micron in size. Implantation did not change the original sintered structure of the dense HA. The HA coating was composed of an amorphous phase with a Ca/P ratio of 1.46 and a crystal phase consisting of oxyhydroxyapatite, tricalcium phosphate, tetracalcium phosphate, and CaO, with a Ca/P ratio of 1.57. In the amorphous phase, compared to other portions in the amorphous phase, there were some layers with lower atomic density and with no significant difference in Ca/P ratio. After implantation, the crystallization of super fine crystals of approximately 4-5 nm in thickness occurred in the amorphous phase, and with time it progressed and spread from the surface to the deeper portion of the HA coating. A Ca/P ratio of 1.58 in the crystallized portion was close to the ratio (1.60) in the dense HA, suggesting that the super fine crystals were HA. This crystallization cannot significantly decrease the solubility of the amorphous phase portion and poses risks of stress accumulation within the coating and a decrease of binding strength between the HA coating and the substrate.

The process of physical weakening and dissolution of the HA-coated implant in bone and soft tissue

Hydroxyapatite (HA)-coated implants were developed to promote osseointegration of titanium implants and to overcome the mechanical drawbacks of solid HA implants. Although many clinical reports on the prognosis of HA-coated implants have reported high success rates, the risks of dissolution and weakening of the coating have been noted. We hypothesized that the chemical and mechanical stability of HA coating are affected by its microstructural characteristics. The present study investigates differences in the microstructures of available HA-coated implants, before and after implantation into the coxal bones of dogs for periods ranging from 3 weeks to 10 months and under the coxal periosteum of dogs for 10 months. The results of transmission electron microscopy and energy-dispersive x-ray analysis revealed that crystallization of super-fine HA crystals occurred in the amorphous phase of the HA coating and progressed over time. This crystallization weakens HA-coated implants by making the amorphous phase brittle, causing stress accumulation within the coating, and causing a decrease in the binding strength between the coating and the substrate. Furthermore, the HA coating dissolved in soft tissue. Dissolution started with the super-fine HA crystals in the crystallized portion that was originally part of the amorphous phase.

Osteoblastic phenotype expression on the surface of hydroxyapatite ceramics

To analyze the bone-bonding property of hydroxyapatite ceramics (HA), composites of rat marrow cells and porous HA were implanted subcutaneously and harvested at 3 to 4 weeks postimplantation. De novo bone formation was observed primarily on the HA surface without fibrous tissue interposition. The HA/tissue interface was analyzed by the observations of thin undecalcified histological sections and fractured surfaces of the implants. The observations were done with a light microscope and a scanning electron microscope (SEM) connected to an energy dispersive spectrometer. The interfacial analyses showed the appearance of osteoblastic cells on the HA surface and that the cells had initiated partially mineralized bone (osteoid) formation directly onto the surface. The osteoid matured into fully mineralized bone, resulting in firm bone bonding to the HA surface. Characterization of osteoblastic cells on the surface was done by determining levels of protein and gene expression of bone Gla protein (BGP, a.k.a. Osteocalcin), i.e., immunohistochemistry and in situ hybridization, respectively. The existence of BGP and mRNA in the cytoplasmic area of the cells confirmed that active osteoblast apposition fabricated primary bone on the HA surface. All of these results indicate the importance of the HA surface in supporting osteoblastic differentiation of marrow stromal stem cells, which leads to firm bone bonding.

Osteoblastic phenotype expression on the surface of hydroxyapatite ceramics

To analyze the bone-bonding property of hydroxyapatite ceramics (HA), composites of rat marrow cells and porous HA were implanted subcutaneously and harvested at 3 to 4 weeks postimplantation. De novo bone formation was observed primarily on the HA surface without fibrous tissue interposition. The HA/tissue interface was analyzed by the observations of thin undecalcified histological sections and fractured surfaces of the implants. The observations were done with a light microscope and a scanning electron microscope (SEM) connected to an energy dispersive spectrometer. The interfacial analyses showed the appearance of osteoblastic cells on the HA surface and that the cells had initiated partially mineralized bone (osteoid) formation directly onto the surface. The osteoid matured into fully mineralized bone, resulting in firm bone bonding to the HA surface. Characterization of osteoblastic cells on the surface was done by determining levels of protein and gene expression of bone Gla protein (BGP, a.k.a. Osteocalcin), i.e., immunohistochemistry and in situ hybridization, respectively. The existence of BGP and mRNA in the cytoplasmic area of the cells confirmed that active osteoblast apposition fabricated primary bone on the HA surface. All of these results indicate the importance of the HA surface in supporting osteoblastic differentiation of marrow stromal stem cells, which leads to firm bone bonding.

Histological and biomechanical analysis of bone and interface reactions around hydroxyapatite-coated intramedullary implants of different stiffness: a pilot study on the goat

We hypothesized that reduced stem stiffness of orthopaedic implants contributes to a high risk of loosening, since interface stresses and relative motions may exceed a tolerable range. To study this hypothesis, three types of load-bearing implant with different stiffnesses were inserted into the tibia of the goat. Histological analysis was performed of bone repair after insertion of the implant, bone ingrowth, interface disruption and loosening. A finite element model of the configuration provided the quantitative range of interface stresses and relative motions for the present experiment. The implants were made out of stainless steel, hollow titanium and a thin titanium core covered with a polyacetal coating. The stiffness ratios of these implants were approximately 10:4:1, respectively. All implants were coated with a layer of hydroxyapatite (HA) in order to minimize the possible biological effects of the different implant materials. Irrespective of the type of implant, there was a repair phase that lasted 6-12 weeks. The stiff implants functioned well. Large areas of bone bonding to the HA layer were found after the repair phase at 12 weeks postoperatively. After 24 weeks, some signs of loosening were observed. More loosening occurred with the hollow titanium and polyacetal implants, mainly during the repair phase. Three hollow titanium and three polyacetal coated implants survived this period, and were killed after 24 weeks. The integrity of the HA layer at the bone-implant interface of the titanium implants was good. In the polyacetal implants, the repair reaction of the cortical bone was incomplete. Bone ingrowth into HA was largely lacking. In conclusion, we found significant differences in the repair and interface reactions around implants of different stiffness. Stiff implants showed favourable initial interface conditions for bone ingrowth. Intermediate and flexible implants provoked unfavourable interface conditions for initial bone ingrowth. The finite element study showed that the flexible stems produce larger micromotions and higher interface stresses at the bone-prosthesis interface than the stiff stems, indicating an explanation for the histological findings.

Apatite layer-coated titanium for use as bone bonding implants

For development of thin bioactive coatings on metal implants, a dense and uniform apatite layer was coated onto titanium (Ti) implants in situ by using a new biomimetic method, which is composed of apatite nucleation and growth steps in simulated body fluid (SBF). Analysis of the coatings by thin film X-ray diffraction and scanning electron microscopy-energy dispersive X-ray microanalysis (SEM-EMPA) before implantation showed that its characteristics were very similar to those of natural bone. The coated and uncoated rectangular plates were bilaterally implanted into the tibial proximal metaphyses of rabbits. After 6, 10 and 25 weeks post-implantation, the bone bonding and bone formation at the bone-implant interfaces were evaluated by a detachment test and undecalcified histological examination. Mechanical testing in tension showed that the failure load of apatite layer-coated Ti implants was significantly higher than that of uncoated control at each time period (all P < 0.001). Histologically, it was shown that bone was deposited directly onto the apatite coating without any intervening soft tissue, while in the paired controls, interpositional soft tissue was seen at the bone-implant interface. By SEM-EPMA, a uniform calcium- and phosphorus-rich layer was detected between the coated implants and bone, but not in uncoated controls at either earlier or later time periods. The results indicate that the apatite layer deposited on Ti in situ may significantly increase the bone bonding strength by providing a bioactive surface, which allows for an early bone apposition to the implant. In addition, the apatite layer-coated Ti produced by the biomimetic process may fulfil the requirements of favourable thin coatings and strong adhesion at the metal-coating interface.

Intramedullary implant of plasma-sprayed hydroxyapatite coating: an interface study

An intramedullary implant model in the canine femora was developed to evaluate the mechanical and histological responses between cancellous bone and plasma-sprayed hydroxyapatite coatings (HACs) on ti-6A1-4V implants, with 12- and 24-week follow-ups. HACs of different thicknesses were investigated. Results of the mechanical testings revealed that after 24 weeks of implantation, the mean shear strength (2.49 +/- 0.12 MPa) of the 50 microns HACs was significantly higher (p < 0.05) than that of the 200 microns HACs (1.44 +/- 0.19 MPa). However, using backscattered electron images (BEIs) throughout all the implant periods, no substantial histological variations in the extent of new bone apposition between the two HACs were observed. Occasionally, solution-mediated disintegration of the 50 microns HAC was found 24 weeks postimplantation. Histomorphometric studies from the BEIs demonstrated that for both HACs the percentage of the direct HAC-cancellous bone contact was approximately 50% at 12 weeks and 75% at 24 weeks. After the mechanical tests, the 200 microns HACs had fracture sites either inside the coating layers or at the HAC-titanium interfaces, which might explain why the mechanical performance of the 200 microns HACs was inferior to that of the 50 microns HACs even though both HACs had the same histological behaviors.

Hydroxyapatite-coating on titanium arc sprayed titanium implants

We developed a new titanium spray technique using an inert gas shielded arc spray (titanium arc spray). Hydroxyapatite (HA)-coating can be applied to the implant without any surface pore obstruction after the rough surface is made by this technique. Scanning electron microscopy (SEM) of various porous implant surfaces after HA-coating revealed that the bead and fiber metal-coated implants had either a pore obstruction or an uneven HA-coating. On the other hand, the titanium arc sprayed implant demonstrated an even HA-coating all the way to the bottom of the surface pore. In the first set of animal experiments (Exp. 1), the interfacial shear strength to bone of four kinds of cylindrical Ti-6A1-4V (Ti) implants were compared using a canine transcortical push-out model 4 and 12 weeks after implantation. The implant surfaces were roughened by titanium arc spray (group A-C) and sand blasting (group D) to four different degrees (roughness average, Ra = group A: 56.1, B: 44.9, C: 28.3, D: 3.7 microns). The interfacial shear strength increased in a surface roughness-dependent manner at both time periods. However, the roughest implants (group A) showed some failed regions in the sprayed layers after pushout test. In the second set of animal experiments (Exp. 2), four kinds of Ti implants; HA-coated smooth Ti (sHA) with Ra of 3.4 microns, bead-coated Ti (Beads), titanium arc sprayed Ti (Ti-spray) with Ra of 38.1 microns and HA-coated Ti-spray (HA + Ti-spray) with Ra of 28.3 microns were compared using the same model as that in Exp. 1. The interfacial shear strength of HA + Ti-spray was significantly greater than that of sHA and Beads at both time periods, and that of Ti-spray at 4 weeks. Although a histological examination revealed that HA-coating enhanced bone ingrowth, sHA showed the lowest shear strength at both time periods. SEM after pushout test showed that sHA consistently demonstrated some regional failure at the HA-implant substrate interface. HA + Ti-spray had many failed regions either at the HA-bone interface or within the bone tissue rather than at the HA-implant substrate interface. These results suggested that the HA-coated smooth surfaced implants had a mechanical weakness at the HA-substrate interface. Therefore, HA should be coated on the rough surfaced implants to avoid a detachment of the HA-coating layer from the substrate and thus obtain a mechanical anchoring strength to bone. HA-coating on this new type of surface morphology may thus lead to a solution to the problems of conventional HA-coated and porous-coated implants.

Histological and biomechanical evaluation of calcium phosphate coatings applied through surface-induced mineralization to porous titanium implants

The purpose of this pilot study was to evaluate surface-induced mineralization (SIM) as a potential technique to apply ceramic coatings to metal orthopaedic implants. Cylindrical titanium porous-coated implants were either coated by SIM or plasma-spray (PLS) techniques with calcium phosphate, or left uncoated (CTL). The implants were bilaterally implanted into the intramedullary canal of the proximal femur of 24 adult New Zealand white rabbits segregated into the following groups: PLS/CTL, SIM/CTL, and SIM/PLS. After 6 weeks in vivo, biomechanical and histologic evaluations were completed. Biomechanically, SIM had consistently greater mechanical interlock than PLS implants. However, CTL implants had greater mechanical interlock than both PLS and SIM. The small sample size prevented statistical evaluation and definitive biomechanical conclusions. Histologically, SIM and PLS had significantly greater ingrowth than CTL implants (p < 0.05). The SIM coating technique produced similar ingrowth characteristics as standard PLS coatings, yet may prevent osteolysis by providing a stronger, more reliable, covalent bond between the ceramic and metal.

Biological reaction to debris in relation to joint prostheses

Bone loss induced by the inflammatory response to wear particles is a major cause of long-term failure of total joint replacement. This review describes the cellular reaction occurring in response to these particles and what is currently known about the inflammatory mechanisms contributing to bone resorption.

Histological and biochemical evaluation of osteogenic response in porous hydroxyapatite coated alumina ceramics

We compared the osteogenic response in porous alumni (Al) and hydroxyapatite coated porous alumni ceramic (HA/Al) by grafting rat bone marrow cells with these implants subcutaneously in the back of syngeneic rats. The ceramics did not show any bone formation without marrow, but did when combined with marrow cells. The osteogenesis in HA/Al began directly on the ceramic surface, but in Al began away from the ceramic surface, and fibrous tissue interposition was seen between the de novo bone and its surface. Alkaline phosphatase activity and bone specific bone gla protein content in HA/Al with marrow were both about three times higher than in Al with marrow. These results indicate that HA/Al has excellent osteoconductive properties and that a composite of marrow and HA/Al is clinically applicable osteogenic biomaterial.

Biomechanical and histological evaluation of a hydroxyapatite-coated titanium femoral stem fixed with an intramedullary morsellized bone grafting technique: an animal experiment on goats

To reconstruct femoral intramedullary bone-stock loss in revision surgery of failed total hip arthro-plasties, morsellized trabecular bone grafts can be used. In 14 goats a noncemented hydroxyapatite-coated titanium stem was fixed within a circumferential construction of bone allografts. After 6 or 12 wk, four goats were used for mechanical tests and three for histology. The stability of the stems relative to the bone was determined in a loading experiment with Roentgenstereo-Photogrammatic Analysis (RSA). Owing to two loosenings and two fractures, only one 6-wk specimen and three 12-wk specimens were available for mechanical testing. The prostheses were very stable at 12 wk. The most important movements were axial rotation (maximal 0.17 degrees at 800 N) and subsidence (maximal 0.036 mm at 800 N). After unloading, there was 40-60% elastic recovery. Histological examination showed revascularization and remodelling of the graft in all the specimens investigated. At the graft site, bone apposition and bone resorption had resulted in a mixture of graft and new bone. Bone incorporation was mainly seen in the proximal areas. Graft lysis was evident in the midshaft region and at distal levels around the prostheses.

Hydroxyapatite-coated femoral stems. A matched-pair analysis of coated and uncoated implants

Fifty-two pairs of patients who had had a total hip arthroplasty with a porous-coated femoral implant were studied in a retrospective, matched-pair analysis. Half of the patients had received a femoral component coated with hydroxyapatite and the other half (the controls), an identical component but without hydroxyapatite. The patients were matched for age, sex, weight, diagnosis, Charnley class, operative approach, and duration of follow-up. Identical uncoated hemispherical acetabular implants were used in both groups. At the time of follow-up, at a mean of 2.2 years (range, two to 3.4 years) after the operation, the mean Charnley scores for pain, function, and motion were 5.6, 5.5, and 5.6 points, respectively, in the group that had received a hydroxyapatite-coated femoral component and 5.6, 5.6, and 5.6 points, respectively, in the group that had received a non-coated component; none of these differences were significant (p = 0.86, 0.89,, and 0.80, respectively). There were no revisions in either group. Radiographs indicated stable fixation in both groups and no differences in the radiographic parameters of loosening between the two groups. Within the relatively short time-frame of this study, there appeared to be no clinical or radiographic advantage to the use of hydroxyapatite in primary total hip arthroplasties. However, these results should be considered as preliminary. Longer follow-up may reveal unrecognized advantages or disadvantages.

Effect of hydroxyapatite thickness on metal ion release from Ti6Al4V substrates

The electrochemical dissolution behaviour of Ti6Al4V alloy coated with hydroxyapatite (HA) by plasma spraying was studied in Hank's balanced salt solution (HBSS) and compared with that of polished and grit-blasted passivated surfaces. Two different nominal thicknesses of HA (50 and 200 micro m) were used. Taking a polished passivated surface as reference, grit blasting of the substrate increased the electrical charge used in the oxidation of Ti6Al4V alloy at constant potential, as a result of increased surface area. However, only HA coatings with a thickness of 200 micro m were capable of reducing the charge to values lower than those measured for polished surfaces. Electrochemical impedance spectroscopy has also shown that only 200 micro m thick coatings are effective in reducing the oxidation rate of the substrate. Furthermore, in potentiostatic experiments the 50 micro m thick coating detached from the substrate, which did not occur with the 200 micro m thick coating. However, after 6 months immersion in HBSS, detachment occurred in some regions of both coatings. No titanium, aluminium or vanadium were detected in solution by electrothermal atomic absorption spectroscopy. These data indicate that HA is an effective barrier to metal ion release, even for the thinner coatings, due to formation of metal phosphates or to incorporation of metal ions in the HA structure.

Effects of hydroxyapatite tricalcium phosphate coating and intracancellous placement on bone ingrowth in titanium fibermetal implants

The purpose of this study was to compare the host-bone response to hydroxyapatite/tricalcium phosphate (HA/TCP)-coated and noncoated titanium fibermetal implants placed in a load-sharing cancellous bone environment of the distal femurs of rabbits. The influence of implantation site was also investigated by comparing these intracancellous implants with intramedullary implants evaluated in a previous study. Three parameters were measured: percentage implant perimeter surface length in contact with new bone, percentage internal fibermetal surface length in contact with ingrown bone, and percentage of available pore space filled with bone. The HA/TCP coating significantly accelerated and increased bone ongrowth, new bone formation on the perimeter and internal surface of the implants. This effect was evident as early as 2 weeks after implantation. In contrast, there was no difference between HA/TCP-coated and noncoated implants in the bone ingrowth parameter, percentage of available pore space filled with bone, or pull-out strength. Scanning electron microscopy in the backscatter mode demonstrated that new bone formed directly onto the HA/TCP-coated fibers and did not usually form directly on noncoated fibers. Analysis of fluorochrome labeling revealed that bone formation in weeks 1 through 4 was primarily woven and thereafter lamellar. Compared with intramedullary placement, intracancellous placement significantly accelerated the apposition of bone to the perimeter and internal surface of HA/TCP-coated implants and both accelerated and increased bone ingrowth as a percentage of available pore volume. These data show that the host response to titanium fibermetal implants is influenced both by HA/TCP coating and by the implantation site.

Changes in phases and crystallinity of plasma-sprayed hydroxyapatite coatings under heat treatment: a quantitative study

With three kinds of plasma-sprayed hydroxyapatite coatings (HACs) prepared, the objectives of this study were (1) to establish the calibration methods for quantitatively measuring the concentration of impurity phases and the degree of crystallinity of the HACs, and (2) to explore the effects of postheat treatments at various temperatures in vacuo on the changes of phases and crystallinity of the HACs. By the internal standard method used, the concentrations of impurity phases, such as alpha-TCP, beta-TCP, and TP, of the assprayed HACs were significantly higher than those measured by the direct intensity-ratio method, and the CaO phase was lower than the direct intensity-ratio method. When the HACs were heat treated in the temperature interval 630-850 degrees C, the concentrations of impurity phases obviously decreased, and the coating crystallinity apparently increased. After annealing at 850 degrees C, an HAC consisting of at least 95% crystallinity with few impurity phases was obtained. As the annealing temperatures in the interval 850-1000 degrees C were applied, however, the HA phase seriously decomposed, resulting in the appearance of a large number (higher than 20 wt%) of impurity phases in the HACs. This work suggests that the optimum heat treatment conditions in vacuo for maximizing crystallinity and minimizing impurity phases of the HACs do not occur at the same temperature.

Onlay silicone and hydroxyapatite-tricalciumphosphate composite (HAP-TCP) blocks interfere with nasal bone growth in rabbits

We investigated how a new type of synthetic porous hydroxyapatite ceramic (HAP-TCP) acts when it is implanted in growing membranous bone. Seventy-six New Zealand White infant male rabbits (4-week-olds) were used. Rabbits received HAP-TCP block or silicone block implantation in their right nasal bone. The left nasal bone was used for a sham. Serial or cross-sectional examinations by morphometry, radiology, blood biochemistry, and histology were carried out. Both the HAP-TCP and silicone groups exhibited no systemic growth disturbance in terms of morphometry and blood biochemistry. Dual energy x-ray absorptiometry (DEXA) revealed, however, a decrease in the bone mineral content (BMC) of the right nasal bone in the silicone group. Histology revealed a superior affinity of HAP-TCP to bone tissue than that of silicone. When a HAP-TCP block was implanted under the periosteum it bonded directly to bone tissue. However, sinking of the implants into the bone tissue were noted both in the HAP-TCP and the silicone groups in longitudinal observation. These results suggest that although HAP-TCP has superior affinity to bone tissue, this by itself is not enough sufficient reason to believe that HAP-TCP can be effectively applied during the growth period.

Tissue reactions around a hydroxyapatite-coated hip prosthesis. Case report of a retrieved specimen

Bone reactions were studied around a titanium, hydroxyapatite (HA)-coated Osteonics (Allendale, NJ) bipolar hip prosthesis, which was revised for severe midthigh pain 4 years after implantation. Inspection of the retrieved prosthesis using a dissecting microscope revealed scarce remnants of a coating-like material on the surface of the prosthesis; however, histology of this layer and histology of the bony side of the bone--HA interface failed to reveal any remnants of an HA coating. The interface was covered predominantly by trabecular bone, which closely followed the contour of the prosthesis, and was partly woven nonmineralized bone. At locations where mineralized bone faced the prosthesis, many small dark titanium wear particles were found. Similar particles were found in macrophages in the intertrabecular medullary space. Polyethylene wear particles were specifically located in macrophages in a soft tissue interface at more distal levels along the stem of the prosthesis. Although the observations presented in this case cannot be generalized, it clearly shows that the HA coating layer had completely disappeared after 4 years. More detailed retrieval studies and longer clinical follow-up studies are needed before a final evaluation of the behavior of HA coatings and long-term fixation of HA-coated prostheses can be made.

Hydroxyapatite clay for gap filling and adequate bone ingrowth

In uncemented total hip arthroplasty, a complete filling of the gap between femoral prosthesis and the host bone is difficult and defects would remain, because the anatomy of the reamed intramedullary canal cannot fit the prosthesis. Therefore, it seems practical to fill the gap with a clay containing hydroxyapatite (HA), which has an osteoconductive character. The clay (HA clay) is made by mixing HA granules (size 0.1 mm or more) having a homogeneous pore distribution and a porosity of 35-48 vol%, and a viscous substance such as a saline solution of sodium alginate (SSSA). In the first experiment, the ratio of HA granules and sodium alginate in SSSA is set for the same handling properties of HA clay and polymethylmethacrylate bone cement (standard viscosity) before hardening. As a result, the ratio is set for 55 wt% of HA in the clay and 12.5 wt% of sodium alginate in SSSA (i.e., HA:sodium alginate:saline solution = 9.8:1:7). In the second study, the gap between the femoral stem and bone model is completely filled with HA clay. However, the gap is not filled only with HA granules or HA granules mixed with saline solution. In the third animal experiment, using an unloaded model, histology shows that HA clay has an osteoconductive property bridging the gap between the implant and the cortical bone without any adverse reaction. HA clay is considered a useful biomaterial to fill the gap with adequate bone ingrowth.

Histological and mechanical comparison of hydroxyapatite-coated cobalt-chrome and titanium implants in the rabbit femur

The purpose of this study was to compare hydroxyapatite (HA)-coated titanium (Ti) and HA-coated cobalt-chrome (CoCr) implants in the distal femur of the rabbit by evaluating bone apposition and interfacial shear strength. Bilateral cylindrical implants with a plasma sprayed 50-microns thick HA coating were press-fit into the metaphyseal cancellous bone of the lateral femoral condyles in a transverse fashion, and the animals were sacrificed at 2, 4, and 8 weeks postimplantation. Mechanical strength of the interface between HA and bone was measured using the pushout method. For histologic analysis, the fractional linear extent of bone apposition was quantitated. No differences were found in the interfacial shear strength between the Ti and CoCr at any time period. The amount of bone apposition increased significantly at each time interval for both substrate metals, but there were no significant differences between the two substrates at any of the time periods studied. The HA-coated CoCr implants performed in a similar manner to the HA-coated Ti implants, both mechanically and histologically, suggesting that HA-coated CoCr implants deserve further study as a viable alternative to Ti for the biological fixation of total joint components in orthopaedic surgery.

Histomorphometry of hydroxyapatite coated and uncoated porous titanium bone implants

Two different groups of hydroxyapatite (HA) coated and uncoated porous titanium implants, 250-350 microns and 500-700 microns diameter beads, were press-fitted into femoral canine cancellous bone. After 12 wks, the dogs were killed and histomorphometric backscattered electron microscopy studies were carried out. Comparing HA coated versus uncoated implants in the 250-350 microns bead diameter group, the percentage of bone (P = 0.01) and bone index (P = 0.01) were higher in the HA coated implants. Comparing HA coated versus uncoated implants in the 500-700 microns bead diameter group, bone ingrowth (P = 0.01) and bone depth penetration (P = 0.008) were higher in HA coated samples. It can be concluded that the HA coating was an effective method for improving bone formation and ingrowth in the porous implants.

Bone response to hydroxyapatite-coated and commercially pure titanium implants in the human arthritic knee

Rough and smooth commercially pure (c.p.) titanium implants and hydroxyapatite-coated (HA-C) implants were inserted in arthritic human knees and left in situ in order to compare the response of bone to these three implants. Radiographic examination alone could not determine if bone apposition had occurred. After 3 and 6 months, histomorphometric analyses of undecalcified sections, 10 microns thick, revealed a statistical significance in the amount of bone apposition to rough-surfaced and HA-C implants as compared with smooth uncoated implants. Most of the rough-surfaced c.p. titanium implants and the HA-C implants had achieved bone apposition on the order of 50%. No statistically significant differences in apposition were seen between the rough-surfaced and HA-C implants. The smooth c.p. titanium implants were mostly encapsulated in fibrous tissue.

Relative motion of hip stems under load. An in vitro study of symmetrical, asymmetrical, and custom asymmetrical designs

Symmetrical, asymmetrical, and custom asymmetrical stems without collars were manufactured and inserted without cement, in that order, into each of eight femora. Displacement transducers were mounted at different locations to measure relative motion between the stem and the bone. A load was applied to simulate the forces acting on a hip joint in a single-limb stance. A force of 1000 newtons was applied three times in order to seat the prosthesis, and then a cyclical force of 1000 newtons was applied to the head of the prosthesis; readings of relative motion were taken every 500 cycles, until 2500 cycles had been reached. The relative motion diminished as the number of cycles increased, with most of the reduction occurring within the first 500 cycles. The symmetrical stem had the least relative axial motion, with an average of six micrometers, but the other stems also had low values: eleven micrometers for the custom stem, and nineteen micrometers for the asymmetrical stem. In terms of axial rotation, however, the symmetrical stem had several times more motion than did the other two stems. Motion of the stem tip toward or away from the canal in the frontal plane and in the sagittal plane also was higher for the symmetrical stem. Axial migration (permanent sinkage after 2500 cycles) was very similar between all three types of stem. In a torsional loading test, the symmetrical stem showed about ten times more relative rotational motion than did the other two stems. Over-all, the custom stem and the asymmetrical stem had similar results.(ABSTRACT TRUNCATED AT 250 WORDS)

The in-vivo effects of an intramedullary implant evoking a constant radial stress to bone. An animal study in the tibia of the goat

Initial stability is essential for successful bone ingrowth into non-cemented prostheses. An entire new concept to increase the initial stability directly after implantation of intramedullary stems was developed (the tension rod prosthesis). The concept is based on a tension rod made out of memory metal that pulls a proximal stem of a prosthesis towards a distal anchor with a constant force. The stress generated along the long axis of the bone produces a radial stress around the prosthesis in the proximal femur. The main goal of this design is to increase the primary stability of the prosthesis during the ingrowth phase and to prevent stress shielding and bone resorption, as realized by the radial force applied to the proximal endosteum of the bone. To assess the efficacy of this concept and to collect data for the anchor design, an implant was developed for implantation into the tibia of the goat. Analyses of push-out strength and bone reactions were performed postoperatively. After 48 weeks the push-out strength of this implant was increased and the histological evaluation showed almost complete osseousintegration. Histomorphometrical analysis showed pronounced, permanent periosteal reactions, located around the anchor of the implant, which generates the radial stress. These first results showed that the bone can withstand the radial stress provoked by the anchor of the tension rod. It is concluded that the concept of a tension rod prosthesis is viable.

Enhancement of bone growth into metal screws implanted in the medullary canal of the femur in rats

The kinetics of growth of bone into control (nontreated) and heat-treated screws made of stainless steel (type 316L) and Ti-6Al-4V, implanted in the medullary canal of the femur in rats, were studied by mechanical, histological, and biochemical methods. A progressive and significant increase in the ingrowth of bone, as reflected by interfacial shear strengths of the screws, was measured with time after implantation. At all time intervals for as long as 35 days after implantation, the shear strength of the heat-treated Ti-6Al-4V and stainless-steel screws was significantly higher than (1.6-3.4 times) that of the control screws. The specific activity of alkaline phosphatase in extracts of tissue from around the implanted screws peaked 6 days after insertion, with significantly higher values at 5, 6, and 7 days postoperatively for the heat-treated screws than for the controls. The extent of calcification also was higher at all time intervals for the heat-treated screws than for the controls. The histological evaluation of formation of bone between the ridges of the implanted screw corroborated the mechanical and biochemical measurements. At each time interval, a more mature bone was noted around the heat-treated screws than around the controls. It was concluded that the heat treatment of metal implants before insertion can result in augmentation of osseous ingrowth 1.6-5.3 times that into control implants in an in vivo experimental model.

Bone growth to metal implants is regulated by their surface chemical properties

Bony ingrowth to control (non-treated) and heat-treated stainless steel and Ti-6Al-4V implants into the medullary canal of the femur in rats was studied by mechanical, chemical and Auger electron spectroscopic methods. At all time intervals up to 35 d post-implantation, the shear strengths of the heat-treated Ti-6Al-4V and stainless steel implants were significantly higher (1.6-fold to 3.4-fold) than in control implants. Using Auger electron spectroscopy depth profiling methods, it was found that the heat treatment modified the implant surface composition significantly, resulting in a thicker oxide layer and other chemical changes. It is concluded that heat treatment of metal implants prior to their insertion alters their chemical surface properties and augments bony ingrowth to them.

Particle microelectrophoresis of calcium-deficient hydroxyapatite: solution composition and kinetic effects

Concurrent work demonstrates that the zeta potential of bone is multivalued and systematically alterable by changes in sample preparation, steeping fluid composition, and steeping time. Since bone mineral is a mixture of carbonated calcium-deficient hydroxyapatites, and since the zeta potential of calcium-deficient hydroxyapatite (CDHA) is altered by pH and time in HNO3-KOH solutions, the zeta potential of CDHA in physiologic Neuman's fluid (NF) compared with that seen in bone could reveal important information on the contribution of the mineral phase to the zeta potential of bone. In addition, such information may be valuable in designing and evaluating calcium-phosphate ceramics for increased bone ingrowth. Results demonstrate that the zeta potential of CDHA in NF is negative. With increasing calcium in NF, the zeta potential magnitude of CDHA decreases and inverts to positive values given sufficient calcium concentration and steeping time. This result is opposite to that seen in bone, suggesting that exposed CDHA is not the predominant bone microsurface and implicating a bone surface protein component. With increasing phosphate in NF, the zeta potential magnitude increases to more negative values. While low concentrations of fluoride showed no effect, the possibility of an effect with higher concentrations is still to be determined.

Mechanical and histological evaluation of amorphous calcium phosphate and poorly crystallized hydroxyapatite coatings on titanium implants

The effect of amorphous calcium phosphate (Ca/P) and poorly crystallized (60% crystalline) hydroxyapatite (HA) coatings on bone fixation to "smooth" and "rough" (Ti-6A1-4V powder sprayed) titanium-6Al-4V (Ti) implants was investigated. Implants were evaluated histologically, mechanically, and by scanning electron microscopy (SEM) after 4 and 12 weeks of implantation in a rabbit transcortical femoral model. Histological evaluation of amorphous vs. poorly crystallized HA coatings showed significant differences in bone apposition (for rough-coated implants only) and coating resorption (for smooth- and rough-coated implants) that were increased within cortical compared to cancellous bone. The poorly crystallized HA coatings showed most degradation and least bone apposition. Mechanical evaluation, however, showed no significant differences in push-out shear strengths between the two types of coatings evaluated. Differences between 4 and 12 weeks were significant for coating resorption and push-out shear strength but not for bone apposition. Significant enhancement in interfacial shear strengths for bioceramic coated as compared to uncoated implants were seen for smooth-surfaced implants (3.5-5 times greater) but not for rough-surfaced implants at 4 and 12 weeks. Rough implants showed greater mean interfacial strengths than uncoated smooth implants at 4 and 12 weeks (seven times greater) and to coated smooth implants at 12 weeks only (two times greater). Mechanical failure of the bone/coating/implant interface consistently occurred within the bone, even in the case of the poorly crystallized HA coatings, which had almost completely resorbed on rough implants. These results suggest that once early osteointegration is achieved biodegradation of a bioactive coating should not be detrimental to the bone/coating/implant fixation.

Histomorphometric assessment of the mechanisms for rapid ingrowth of bone to HA/TCP coated implants

The purpose of this work is to use dynamic histomorphometry to evaluate the basic biological mechanisms by which hydroxyapatite/tricalcium phosphate (HA/TCP) implant coatings accelerate bone formation rates. Twenty-five rabbits had an HA/TCP coated cylindrical titanium fiber metal mesh implant surgically placed in the subchondral bone of the proximal tibia and a noncoated implant placed in the contralateral tibia. Twenty-two of these animals had HA/TCP coated cylindrical solid titanium implants placed in the distal femur and an uncoated implant placed in the contralateral femur. The animals were double labeled with vital stains, and sacrificed at 3, 6, 16, or 26 weeks after surgery. Histomorphometric analyses were done of the bone implant interfaces. Both static and dynamic histomorphometric parameters indicate that HA/TCP coatings stimulate faster bone ingrowth to coated fiber metal implants through the early production of woven bone and by subsequent rapid lamellar bone formation rates. Coated fiber metal implants demonstrated significantly more bone ingrowth than noncoated implants through 16 weeks postimplantation, but not by 26 weeks. In solid implants, the differences between coated and noncoated implants are less pronounced and not statistically significant, although there is a trend toward increased bone apposition to the surface of the implants over the first 16 weeks following implantation. The clinical significance of these results is that coated implants may allow earlier return to normal weightbearing.

Hydroxyapatite ceramic coating for bone implant fixation. Mechanical and histological studies in dogs

The success of bone ingrowth into porous coated implants depends on several factors which can be separated into five main groups: implant related factors, such as design of implant, surface structure and pore characteristics. status of host bone bed, such as underlying disease (rheumatoid arthritis, osteoporosis), available bone stock, use of drugs and surgical technique. mechanical stabilization and loading conditions applied on the implant. adjuvant therapies such as bone grafting and HA coating which might enhance the amount of bone ingrowth. remodeling of periprosthetic bone. Once bone ingrowth has occurred, maintenance of bony anchorage depends on bone remodeling at the interface. The present series of studies were performed in order to investigate the effect of some of these factors on bone ingrowth in relation to hydroxyapatite (HA) and titanium alloy (Ti) coating when subjected to pathological and mechanical conditions mimicking the clinical situation. HA- and Ti-coated implants were inserted into the femoral condyles of mature dogs. The observation period ranged from 4 to 16 weeks, and the results were evaluated by mechanical push-out testing, histomorphometric analysis, polarized light microscopy, UV fluorescence microscopy, collagen analysis and transmission electron microscopy (microanalysis). There were no complications related to the operative procedures and all dogs were terminated according to the original time schedule. Host bone related factors were studied in the initial experiments. First, the effect of a gap between bone and implant was studied and compared with press-fit insertion. The HA-coating yielded superior effect on bone ingrowth compared to Ti in situations where the implant was surrounded by a gap and also where the implants were inserted in press-fit. Gaps of 1 mm and 2 mm around the implant were bridged by bone around HA implants whereas significantly less amounts of bone filled the gap around Ti implants. The gap-healing capacity of bone was increased even at a relatively great distance (400 microns) from the HA surface. This finding indicates that the osteoconductive effect of HA is not limited to the bone forming capacity on the surface of the implant. A positive gradient of newly formed bone was found towards the HA-coating, this gradient not being found towards the Ti-coating. In order to investigate the significance of arthritic bone changes (osteopenia) on fixation of porous coated implants we adopted the Carragheenin-induced gonarthritis model resulting in substantial bone loss as determined by CT-scanning.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydroxylapatite coating of porous implants improves bone ingrowth and interface attachment strength

The effect of a plasma-sprayed hydroxylapatite (HA) coating on the degree of bone ingrowth and interface shear attachment strength was investigated using a canine femoral transcortical implant model. Cylindrical implants were fabricated by sintering spherical Co-Cr-Mo particles 500-710 microns in diameter; the nominal implant dimensions were 5.95 +/- 0.05 mm diameter by 18 mm in length. One half of each implant was coated with hydroxylapatite, 25-30 microns in thickness, by a plasma-spray technique. Using strict aseptic technique, the implants were placed through both femoral cortices into defects approximately 0.05 mm undersized. After 2, 4, 6, 8, 12, 18, 26, and 52 weeks, the implants were harvested and subjected to mechanical pullout testing and undecalcified histologic evaluation. The application of the HA coating to porous implants enhanced both the amount of bone ingrowth and the interface attachment strength at all time periods. These differences were statistically significant for the percent of bone ingrowth at the 4-, 6-, 12-, 18-, 26-, and 52-week time periods, and interface shear strength values were significantly different at the 6-, 8-, 12-, 18-, and 26-week time periods. The rate of development of interface strength and bone ingrowth was also more rapid for the HA-coated implants. No evidence of any disruption, mechanical failure, or biologic resorption of the HA coating was observed. The results of the present study--demonstrating a beneficial effect of the HA coating at all time periods--are believed to be due to the use of paired comparisons, which allow assessment of subtle differences that might otherwise have been obscured by normal biological variability.

Mechanical failure of hydroxyapatite- and polysulfone-coated titanium rods in a weight-bearing canine model

Three types of material that have shown potential as coatings for orthopaedic implants were studied. Using a weight-bearing canine model, Ti-6A1-4V femoral intramedullary rods coated with (1) sintered titanium beads, (2) plasma-sprayed hydroxyapatite, and (3) silyl coupled polysulfone beads were evaluated for mechanical strength and bone ingrowth. The model was designed to secure optimal prosthetic stability by obtaining maximal bony ingrowth during an initial non-weight-bearing phase, then stressing the implant during a full-weight-bearing phase. None of the rods coated with titanium beads failed. All 17 polysulfone-coated rods failed, 13 of them at the interface between the polysulfone coating and the titanium core. Of 18 rods coated with hydroxyapatite, 15 suffered implant breakdown at the interface between the hydroxyapatite coating and the titanium core. This may be due to dissolution of the plasma-sprayed hydroxyapatite in vivo. Testing of retrieved specimens from both hydroxyapatite- and polysulfone-coated implants showed that the shear strength at the coating-rod interface had decreased to less than 40% of the shear strength at manufacture. Despite mechanical failure, histologic study showed extensive bone ingrowth or apposition onto both the polysulfone and hydroxyapatite coatings.

The effect of phase differences on the time-dependent variation of the zeta potential of hydroxyapatite

The osteoconductive nature of calcium phosphate ceramics (CPC) follows from several proven effects, such as a direct bone attachment and enhanced bone tissue formation. Mechanisms leading to these phenomena are still largely undiscovered. Specifically, little is known about the CPC surface and cell-driven reactions. These atomic and molecular level events are involved in tissue attachment and enhanced tissue formation. It is hypothesized that the zeta potential of these ceramics is directly related to the surface reactivity governing osteoconductivity. As a first step in our analysis, the zeta potential of stoichiometric and Ca-deficient hydroxyapatite was determined as a function of immersion time. It is concluded that, under the conditions of the experiment, the observations support the hypothesis in a dual way. First, the absolute values of the zeta potential which were measured are related to electrokinetic potentials known to cause substantial effect on the cellular activities and bone tissue formation when applied exogenously. Second, the magnitude and duration of the changes in zeta potential are related to an ion exchange between the hydrated layer around the ceramic and the ceramic surface, and a net precipitation of new material. If these findings could be confirmed in other solutions, i.e., solutions with a substantially equivalent composition as the fluids in developing bone tissue, a basis would be provided to explain the bridging of the ceramic surface with the surrounding developing tissue.