Effect of hydroxyapatite impregnation on skeletal bonding of porous coated implants

The ultrastructural architecture of the tissue/hard-tissue replacement interface

This investigation examined the tissue response and interfacial bonding between bone and hard-tissue replacement (HTR) using scanning (SEM) and transmission (TEM) electron microscopy. Twenty adult male Sprague-Dawley rats were anesthetized and a hole (1.0 mm deep by 2.0 mm wide) was drilled in the calvarium. Subsequently, HTR was implanted and the wound closed. The implants and surrounding tissues were removed at 7, 14, 28, and 56 days and prepared for examination by SEM or TEM. Scanning electron microscopic analysis revealed a typical inflammatory response that subsided by day 14. At that time, a fine layer of collagen fibrils (fibrous envelope) was observed covering the polymeric surface. Energy dispersive x-ray analysis (EDXA) showed no sign of mineralization. Ultrastructural analysis demonstrated that the fibrous envelope was bilaminar; it consisted of a relatively undifferentiated cellular layer adjacent to the polymer and an outer fibrous region. Scanning electron microscopic analysis of 28-day implants showed that osteoblasts had migrated onto the outer surface of the fibrous envelope and that calcification had been initiated as judged by EDXA. Electron microscopic examination confirmed previous observations of an undifferentiated cellular layer along the interfacial boundary, but also showed both macrophages and foreign-body giant cells. At 56 days, bone was observed to contact and cover the fibrous envelope surrounding the polymeric bead; however, EDXA showed that the fibrous envelope remained noncalcified. Transmission electron microscopic analysis revealed that the inner cellular layer was beginning to mature, as indicated by the presence of numerous cellular organelles. This maturation was accompanied by an increased incidence of macrophages as well as foreign-body giant cells. Within the time constraints of the experimental design, it is apparent that a bilaminar layer of cells and fibers remains between the HTR and the bone. Additional studies will be necessary, over extended time periods, to determine whether the bilaminar layer remains a constant feature between the HTR and the surrounding bone or whether this region is gradually supplanted by the ingrowing bone.

Tissue ingrowth into titanium and hydroxyapatite-coated implants during stable and unstable mechanical conditions

Lack of initial mechanical stability of cementless prostheses may be responsible for fibrous tissue fixation of prosthetic components to bone. To study the influence of micromovements on bony ingrowth into titanium alloy (Ti) and hydroxyapatite (HA)-coated implants, a loaded unstable device producing movements of 500 microns during each gait cycle was developed. Mechanically stable implants served as controls. The implants were inserted into the weight-bearing regions of all four femoral condyles in each of seven mature dogs. Histological analysis after 4 weeks of implantation showed a fibrous tissue membrane surrounding both Ti and HA-coated implants subjected to micromovements, whereas variable amounts of bony ingrowth were obtained in mechanically stable implants. The pushout test showed that the shear strength of unstable Ti and HA implants was significantly reduced as compared with the corresponding mechanically stable implants (p less than 0.01). However, shear strength values of unstable HA-coated implants were significantly greater than those of unstable Ti implants (p less than 0.01) and comparable to those of stable Ti implants. The greatest shear strength was obtained with stable HA-coated implants, which was threefold stronger as compared with the stable Ti implants (p less than 0.001). Quantitative determination of bony ingrowth agreed with the mechanical test except for the stronger anchorage of unstable HA implants as compared with unstable Ti implants, where no difference in bony ingrowth was found. Unstable HA-coated implants were surrounded by a fibrous membrane containing islands of fibrocartilage with higher collagen concentration, whereas fibrous connective tissue with lower collagen concentration was predominant around unstable Ti implants. In conclusion, micromovements between bone and implant inhibited bony ingrowth and led to the development of a fibrous membrane. The presence of fibrocartilage and a higher collagen concentration in the fibrous membrane may be responsible for the increased shear strength of unstable HA implants. Mechanically stable implants with HA coating had the strongest anchorage and the greatest amount of bony ingrowth.

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.

Enhanced ingrowth of porous-coated CoCr implants plasma-sprayed with tricalcium phosphate

Tricalcium phosphate (TCP) is an osteo-conductive bioceramic which, when applied to a porous-coated prosthesis, may enhance osseous ingrowth and mechanical stability. TCP plasma-sprayed and unsprayed porous-coated tibial intramedullary rods were bilaterally implanted in seven adult rabbits. All rabbits were killed at 12 weeks. Pull-out tests were performed on 4 rabbits while all were evaluated histologically for osseous response and adverse tissue reaction. TCP-sprayed implants showed significantly greater osseous ingrowth in comparison to unsprayed implants. Neither implant type exhibited adverse tissue reactions. Average pull-out strengths were 69 lb for treated rods and 72 lb for controls (p greater than 0.05); quality of fit for all pull-out specimens except one was deemed poor. We conclude that plasma-sprayed TCP enhances osseous ingrowth into porous-coated devices. However, our data further suggest that enhanced ingrowth may not always lead to enhanced fixation.

Crystallographic changes in calcium phosphates during plasma-spraying

Coating hydroxyapatite (HA) onto metal implant surfaces using the plasma-spraying technique has been investigated in several laboratories as a means of improving the mechanical properties of the bulk ceramic. This study describes crystallographic changes which can occur during the plasma-spraying of calcium phosphate powders. A precipitated calcium-deficient apatite and a high temperature near-stoichiometric HA were each sprayed onto metal substrate in an argon plasma using several hydrogen gas flow conditions at various temperatures. The surfaces were examined by X-ray diffraction and scanning electron microscopy. The plasma-sprayed products were identified as a mixture of calcium phosphates including HA, beta-tricalcium phosphate (beta-TCP) and calcium oxide. Stoichiometric HA when plasma-sprayed showed the least (5%) degradation. Since beta-TCP is more resorbable than HA in vivo, varying the HA/beta-TCP ratio on the plasma-sprayed surface may provide a method to control surface dissolution of the coating.

Bone tissue growth enhancement by calcium phosphate coatings on porous titanium alloys: the effect of shielding metal dissolution product

The possible mechanism of minimization of prosthesis-derived bone growth inhibitors by shielding of the metal and the reduction, if not elimination, of the associated metal dissolution was investigated. Titanium, aluminium and vanadium release rates were determined in vitro for Ti alloy specimens both with and without a calcium phosphate coating. Ti orderly oriented wire mesh (OOWM) porous coatings on Ti-6Al-4V substrates were used as the metal specimens. Half of the specimens were coated with a 75 microns calcium phosphate ceramic (CPC coating). Seven reference (OOWM) and seven coated (OOWM-CPC) specimens were immersed and placed along with seven control solutions for various periods in an incubator maintained at 37 degrees C and 5% CO2 - air atmosphere. Whereas the reference solutions showed a Ti release increasing as a function of time, the solutions that had the CPC-coated specimens contained no measurable amounts of titanium. The Al in solution around the CPC-coated specimens was significantly greater than the concentration around non-coated specimens. The Al, however, did not increase significantly with time, at least up to 4 wk immersion. The ceramic coating had a small beneficial effect on V concentration. In the absence of a significant adverse effect of Ti on local bone tissue formation, we focus on the Al data of our study. The possible adverse effect of this element is well documented. The calcium phosphate coating produced a significant increase of biological fixation, yet at the same time a greater Al release into solution, calling into question the significance of CPC coating in shielding adverse metal passive dissolution to explain enhanced bone growth [corrected].

A glass ionomer for reconstructive surgery. Ionogran--an ionomeric micro implant. A biological evaluation

A porous glass-ionomer (Ionogran) was evaluated and compared to hydroxyapatite Interpore 200. In an in-vivo screening test procedure, microfilter diffusion chambers embedded on the tibial surfaces of adult baboons were used to evaluate cell survival, cell regeneration and cell differentiation. In a bulk testing procedure a standardized bone core was removed from the tibia through the knee joint and the material exposed to the defense mechanisms of the system. Both materials were found to be biocompatible, non-toxic and did not inhibit cell proliferation in the enclosed environment of diffusion chambers. Histological response and cell arrangement within the chambers containing Ionogran were similar to that of the controls with cancellous bone only. Highly specialized haemopoietic tissues were in direct contact with the Ionogran. In the presence of hydroxyapatite new bone was actively formed. Haemopoietic elements were never found in contact with the hydroxyapatite. In the bulk testing procedure, the general histological appearance was that of a reactive tissue response to a low grade persistent stimulation coupled to surface degradation products. The Ionogran was found to be stable, osteo-conductive and promoted osteoid formation when in contact with bone. From a clinical point of view the material could serve as a permanent scaffold, maintaining form, and will be of value in bone reconstructive procedures.

Physico-chemical considerations of titanium as a biomaterial

Physico-chemical properties of titanium are discussed. Special attention is paid to those of amorphous TiO 2 that contact tissues in vivo. In aqueous environments TiO 2. (aq) has low ion-formation tendency and low reactivity with macromolecules. This is accompanied by low toxicity. Titanium does not facilitate reactive oxygen radical generation during inflammatory conditions as observed in in-vitro experiments. The outermost layers of the oxide are in the Ti(IV) oxidation state, although using electron spin resonance (ESR) techniques, formation of Ti(III) is observed at atmospheric conditions. The impact of similarities between water and TiO 2 is speculated upon, and the physico-chemical properties of titanium are tentatively linked to some in-vivo consequences.

Fixation of titanium and hydroxyapatite-coated implants in arthritic osteopenic bone

Retrieval studies of porous-coated prostheses have demonstrated deficient bony ingrowth in high percentages. Possible reasons for this are lack of initial mechanical stability and the presence of osteopenia. The authors studied ingrowth of osteopenic bone into titanium alloy (Ti) porous-coated implants with and without hydroxyapatite (HA) coating in an experimental dog model. Unilateral osteopenia of the knee with a 20% reduced bone density as judged by computed tomography (CT) scanning (P less than .001) was induced in 12 mature dogs by weekly intraarticular injections of Carragheenin into the right knee for 12 weeks, with the left knee serving as control. Ti porous-coated cylinders were inserted in press-fit bilaterally in the lateral femoral condyles in six dogs. HA-coated titanium plugs were implanted similarly in another sex-, age-, and weight-matched group of six dogs. Bony ingrowth after 4 weeks was significantly reduced for Ti implants in osteopenic bone compared to control bone, but HA-coated implants were covered by equal amounts of bone tissue. Bone-implant shear strength of Ti implants also was reduced in osteopenic bone compared to control bone. In control bone, the anchorage of Ti implants was stronger than HA-coated implants, whereas the fixation of Ti and HA-coated implants was equal in the osteopenic bone. The results demonstrate that the bony fixation of Ti porous-coated implants is weakened by the presence of experimentally induced osteopenia. However, the fixation of HA-coated implants was not affected by the osteopenic condition in the surrounding bone. The fixation of Ti and HA-coated implants was equal in osteopenic bone, whereas the fixation of Ti porous-coated implants was superior to that of HA-coated implants in control bone.

The effect of partial coating with hydroxyapatite on bone remodeling in relation to porous-coated titanium-alloy dental implants in the dog

For inhibition of crestal bone resorption due to stress shielding and disuse atrophy, an hydroxyapatite (HA) plasma coating was added to the coronal portion of partially porous-coated endosseous dental implants. These implants, as well as control non-HA-coated implants were placed in healed mandibular premolar extraction sites in dogs for a 72-week period of function. Histological examination showed that both implant designs became securely fixed by bone ingrowth into the porous-coated apical region of the implants. The plasma-sprayed HA coating resulted in significantly greater bone height formation and maintenance next to the coronal portion of the implant compared with non-HA-coated implants of similar design. In addition, significant resorption of the 20-to-50-microns-thick plasma-sprayed HA coating occurred over the 18-month period of function.

Evaluation of an experimental periodontal ligament for dental implants

To improve the devices used in dental implantology, a new experimental supporting element has been developed. The device, made of titanium, poly(methylmethacrylate) and Dacron tissue, has been tested for the osteointegrability of its outer Dacron coat by implants in mandibular bone of rabbits. The Dacron filamentous tissue became incorporated by the bone at 3 month post-implantation. This might provide a reliable mechanical anchorage of the device and a barrier against epithelial proliferation and microbial contamination. Preliminary results relating to in vitro evaluation of the poly(methyl methacrylate)-Dacron fixation strength predict favourable mechanical behaviour at this interface and in the complete implanted device.

Compositional variations in the surface and interface of calcium phosphate ceramic coatings on Ti and Ti-6Al-4V due to sintering and immersion

The compositions of the surface and the interface of calcium phosphate ceramic (CPC) coatings electrophoretically deposited and sintered on titanium or its alloy, were determined by scanning Auger electron spectroscopy before and after 4 wk of immersion in a simulated physiological solution. In the CPC coating-metal interfaces, the phosphorus diffused beyond the titanium oxide layer. The phosphorus concentration in the interface followed a Gaussian distribution for both unalloyed and alloyed titanium. The diffusion depleted P in the ceramic adjacent to the metal. The surface of the ceramic, however, was substantially unchanged. A major change in the compositional depth profiles was induced by immersion: thick and uniform titanium phosphide layers of constant composition were observed on the Ti-based metal substrates.

Orthopaedic applications of hydroxyapatite

Hydroxyapatite (HA) has been used for more than six years. More recently we have used HA composites in clinical orthopaedics for spacing or filling bone defects. In this application the advantages of HA include lack of immuno-reaction and absence of postoperative morphological change or volume decrease. HA implants fixed with cement avoid problems from high density polyethylene wear particles. Other applications of HA include femoral plugs in total hip replacement and HA coating on metal components for cementless fixation. For rapid and strong cementless fixation porous metal surfaces are used; HA coating of porous metal gives improved results. We have also developed a technique for bioactive interfacial bone cementation by interposing fine HA granules between the bone and PMMA cement.

Effect of calcium phosphate coating characteristics on early post-operative bone tissue ingrowth

The synthesis of model porous metal-CPC materials, and their use in one-parametric studies of bone tissue ingrowth enhancement were considered. By using the same starting calcium-deficient hydroxyapatite powder, three different coatings, CAP1, CAP2 and CAP3, were obtained of thicknesses 50 +/- 5, 75 +/- 5 and 75 +/- 5 microns, respectively. CAP1 and 2 were either the starting powder mixed in a 3:1 ratio CPC: poly(lactic acid) or the powder by itself. The CAP3 coating was the result of a thermal treatment producing a mixture of oxyhydroxyapatite, alpha- and beta-tricalcium phosphate. Orderly oriented wire mesh porous coated specimens were implanted, along with the same specimens lined with CAP 1, 2 or 3. Subsequently, the total of 156 specimens was retrieved at 2, 4 or 6 wk, and tested mechanically and processed for histomorphometry. The data produced considerable evidence for the CPC-dependent enhancement of bone tissue ingrowth in porous metals immediately after implantation. They prove that the materials processing of CPC coatings influences the resulting biological behaviour substantially. Furthermore, they support the hypothesis that ceramic dissolution is a causative factor on the bone tissue growth enhancement mechanisms.

A new canine model to evaluate the biological response of intramedullary bone to implant materials and surfaces

A new canine model utilizing an implantable chamber with multiple bone ingrowth channels has been used to study the response of intramedullary bone to various implant materials and surfaces. The first group of dogs received implants containing channels lined by smooth-surfaced coupons of titanium, titanium alloy, sputter-hydroxyapatite-coated (HA-coated) titanium alloy, and polyethylene. A pattern of early initial bone ingrowth by 2 weeks, becoming maximal at 6 to 12 weeks with remodeling to a more mature lamellar bone, and later resorption by 24 weeks was seen for all test groups, with fibrous tissue interfaces covering the smooth test coupons at all time points. Significantly increased bone ingrowth in the sputter-HA coated group was found only at 6 weeks. The second group of dogs received implants with channels lined by surface-roughened coupons of either titanium or plasma-HA-coated titanium, half of which were also packed with a crystalline-HA grouting at the time of surgery. At both 6 and 12 weeks, bone ingrowth was greatly enhanced by the presence of the plasma-HA coating or the crystalline-HA grouting as compared to the uncoated titanium channels. Histologically, bone was seen to bond directly to the plasma-HA coating and the crystalline-HA grouting. A thin fibrous tissue layer was noted between bone and the titanium in most areas, but evidence of direct bone contact to the metal surface was seen. Mechanical testing in tension of intact coupon-bone-coupon units revealed significant strength of the bone-plasma-HA bond, with failure initiating at the metal-HA interface with forces of 15.3 N at 6 weeks, increasing to 44.8 N at 12 weeks. Plasma-HA-lined channels with crystalline-HA packing required similar forces for failure. No significant adhesion strength was noted for the titanium channels at 6 weeks, and only the crystalline-HA-filled channels displayed measurable strength of the bone-titanium interface at 12 weeks, with a force of 9 N needed for failure.

Hydroxyapatite coating enhances fixation of porous coated implants. A comparison in dogs between press fit and noninterference fit

Intimate contact at the bone-porous surface interface is not always achievable in noncemented prosthetic implantation. We investigated the effect of hydroxyapatite (HA) coating on skeletal attachment in noninterference fit 4 weeks after implantation in 6 mature dogs. The push-out test of HA-coated implants surrounded by a 1-mm gap showed a twofold increased shear strength and fivefold increased shear stiffness compared with titanium alloy (Ti) coated implants. The fixation of Ti implants was reduced by two thirds when inserted in a gap as compared with press fit, whereas HA-coated implants in gap showed anchorage close to implants in press fit. Only minor differences were found between HA and Ti implants in press fit. Histomorphometric analysis showed a significant increase in bone in direct contact to HA-coated implant as compared with Ti implants inserted both in gap and press fit. The study indicates that tightness of surgical fit is an important factor for sufficient fixation of the implant. However, our results demonstrate that hydroxyapatite coating almost eliminates the negative influence of noninterference fit between bone and unloaded implant.

Calcium phosphate ceramic coatings on porous titanium: effect of structure and composition on electrophoretic deposition, vacuum sintering and in vitro dissolution

Bioactive calcium phosphate ceramics (CPC) guide bone formation along their surface. This property is conceptually attractive from the viewpoint of enhancing early bone tissue formation in porous metal coatings. The various studies conducted to exploit this idea, however, reveal a considerable variability of the effect. This suggests material- and processing-induced parametric influences. Thus this study focuses on the formulation of model porous metal-CPC materials for use in one-parametric analyses of material factors. Easily reproducible, porous metals with a uniform porous structure and CPC coating are made with orderly oriented wire mesh (OOWM) porous metal coatings and electrophoretically deposited CPC films. The deposition of the ceramic can be hampered by adsorbed water. Subsequent vacuum sintering leads to several phase transformations: hydroxyapatite is transformed to a mixture of oxyhydroxyapatite and tetracalcium phosphate; the underlying titanium promotes the beta- to alpha-tricalcium phosphate transformation; and Ca-deficient hydroxyapatite is transformed to a mixture containing oxyhydroxyapatite and alpha- and beta-tricalcium phosphate. These phase transformations provoke a considerable increase of in vitro dissolution in 0.05 M tris buffered physiological solution.

Mechanical and chemical bonding of artificial joints

Biomaterials which create chemical and mechanical bonds with tissue, i.e. (1) non-porous materials with or without a hydroxyapatite coating, (2) porous titanium alloy (beads) with or without a hydroxyapatite coating, (3) alpha-tricalcium phosphate bioactive bone cement and PMMA cement, and (4) interface bioactive bone cement made by interposing hydroxyapatite granules between polymethylmethacrylate cement and the bone, were used in animal experiments and clinical applications. The common problem with cementless fixation is that some patients complain of slight pain on weight-bearing, because a complete initial fixation is not obtained and micro-movement of the component may occur. Porous metal with hydroxyapatite coating is found to be better than that without coating for producing earlier and stronger fixation, and problems with fatigue and peeling of hydroxyapatite from the base metal are eliminated when the beads are coated with hydroxyapatite. As hydroxyapatite bonds chemically to the bone, pain on weight-bearing due to micromovement should never occur. In order to obtain long-term and stable fixation for severe bony atrophy, bioactive bone cement or interface bioactive bone cement (interposing hydroxyapatite at the bone interface) is desirable.

Ionos bone cement (glass-ionomer): an experimental and clinical evaluation in joint replacement

Ionos bone cement (glass-ionomer) was evaluated in total joint replacement procedures, initially in a baboon model, and finally in a highly selected group of patients in whom polymethyl methacrylate cement was contra-indicated. It was found that the cement has no inhibitory effect on bone tissue development. It is effectively incorporated into the bone, both structurally and functionally without the interposition of fibrous tissue. Under the functional stress situation in baboon hip and knee arthroplasties, the bone and marrow in direct contact with the cement responded favourably. A direct bone bond was demonstrated. The clinical results achieved were satisfactory, notwithstanding the complexity of the clinical problems included in the series. The continued clinical evaluation on a controlled basis has been shown to be well-justified.

Tissue response to facial contour augmentation with dense and porous hydroxylapatite in rhesus monkeys

Using extraoral incisions, subperiosteal pockets were created bilaterally over the zygomatic and mandibular areas in six rhesus monkeys. One side of each anatomic site received a dense block of hydroxylapatite (HA) implant, and the contralateral side received an equivalent-sized block of porous HA implant. Monkeys were killed at 3, 6, and 12 months, and the implants were manually tested for mobility. The implants were then retrieved in block specimens. Half of each specimen was decalcified, embedded in paraffin, and stained. The other half was embedded in plastic, and sections were stained or carbon coated for scanning electron microscope histometry. The dense HA implants showed complete fibrous encapsulation and they popped out when cut in half. The porous HA implants were attached to the underlying cortex by bone ingrowth. The porous implant volume sampled within 2.5 mm of underlying cortex contained 33.0% HA matrix and 35.7% bone. The surface area of the porous HA matrix (4.8 mm2/mm3) was 54.3% covered by bone ingrowth. It is hypothesized that micromotion may have accounted for the lack of osseointegration of the dense HA specimens. In contrast, the early ingrowth of fibrous tissue into the porous HA block might be responsible for reducing micromotion to levels acceptable for ingrowth of bone. The contrasting biologic response of implant sites to two variants of the same implant material supports the value of comparative studies to permit informed surgical selection decisions.

Tissues at the surface of the new composite material titanium/glass-ceramic for replacement of bone and teeth

A new composite implant material titanium/glass-ceramic was tested in rabbits using light microscopy, histomorphometry, and biomechanical testing methods. Two rabbit implant models were used. The first premolar tooth was replaced and cylinders inserted into the trabecular bone of the distal femur below the patella sliding plane. There was bone bonding to the glass-ceramic component and additional mechanical interlocking, due to bone ingrowth between the titanium matrix into secondary pores. This was proved by measuring the tensile strength at the interface of the new composite material which was in the same range as compared to pure glass-ceramic implants. In tooth replacement there was a tight attachment of gingival epithelium and stroma to composite titanium/glass-ceramic. These results are of particular clinical interest: physicochemical bone bonding and additional mechanical interlocking result in a resistance of the implant material against shear and tensile loads at the interface. Therefore this new composite material should be suitable for further load-bearing applications.

The effect of plasma-sprayed calcium phosphate ceramic coatings on the metal ion release from porous titanium and cobalt-chromium alloys

Bone tissue ingrowth in porous materials is enhanced by the deposition of bioactive calcium phosphate ceramic linings onto the pore walls. These bioactive coatings can be deposited using several methods which yield a variety of coating efficiencies and thereby influence the mechanisms and kinetics of ion release from the metal. We analyzed the effect of plasma-spraying hydroxyapatite onto titanium and cobalt-chromium alloys by measuring the release of Ti, Al, V, Co, and Cr in vitro. Plasma-sprayed coatings significantly reduced the Ti and Al release from titanium-based alloy specimens. The tendencies of release from the cobalt-based specimens are less pronounced. The data substantiate that neither localized enhanced passive dissolution of metal ions nor ceramic shielding of the metal occurs. The Scanning Auger Electron Microprobe Spectroscopic data suggest that the dissipation of thermal and kinetic energy of the ceramic particle at the time of impact can produce compositional and structural changes in the metal surfaces. The resulting effects are significant for the titanium alloy but less significant for the Co-Cr alloy system.

Designing to counteract the effects of initial device instability: materials and engineering

Implants for hard tissue replacement have evolved over the last few decades, but a critical assessment of their design reveals that most dental implants and most orthopedic appliances for any given indication are basically similar in design to their commercial competitors. Some unique features are contained in the outer 0.7 mm, but shadow pictures of the devices could almost be superimposed upon one another. Near-optimal designs have evolved for the materials systems commonly in use. The fight to minimize initial instability of implants, which leads to failure, has caused certain attachment mechanisms to emerge as acceptable, based upon research that indicates firm fixation in bone has resulted in longer average implant lifetime. The problem of initial stabilization is one of materials and design, both of which are necessary for a successful implant system. The nonmetallic calcium phosphate glass and ceramics technology available today provides materials that may counteract the effects of initial device instability by not relying on mechanical means of retention, alone, but chemical as well.

Calcium phosphate-coated porous titanium implants for enhanced skeletal fixation

Porous titanium fiber implants for cementless skeletal fixation by bone ingrowth were treated with a calcium phosphate coating applied by a plasma flame-spray technique. In a paired experiment, treated and control implants were inserted in the humeri and olecranons of 36 adult dogs for periods of 1, 2, 4, and 6 weeks. After the animals were sacrificed, a biomechanical evaluation of the strength of skeletal fixation of the implants and a histologic evaluation of bone ingrowth was done. The mean shear strength of skeletal fixation at four weeks for the calcium phosphate-coated implants was 24% greater (P less than .01) than for paired controls. No difference in strength of fixation between treated and control implants was present at other time periods. The osteoconductive properties of the ceramic coating were demonstrated by bone forming in direct contact with the calcium phosphate coating on the metal fibers of the treated implants. No significant increase for the volume of bone ingrowth was established for treated implants compared to paired controls at any time period.

Historical review of porous-coated implants

The implementation of cementless joint replacement prostheses is proving to be a significant advance in orthopedic surgery. The porous-coated implant, stabilized by the biologic fixation resulting from bone ingrowth, is one of several types of cementless devices. A historical review of porous materials reveals that the concepts underlying biologic fixation date back decades. It was the initial concerns about the long-term performance of bone cement that, in the early 1970s, stimulated several groups more actively to pursue the development of porous systems. As a result of their efforts, there are currently many different porous coatings and prosthetic designs undergoing clinical investigation, with generally encouraging results. However, it is becoming clear that the importance of porous-coated prosthetic devices is not that they will serve to replace cemented prostheses entirely, but rather that they offer viable alternatives for the treatment of certain populations of patients. An understanding of the evolution of porous-coated implants can help to identify gaps in our knowledge and areas that require additional study, to direct future design modifications of implants, and to foster effective implementation of the prostheses.

A histological assessment of the initial healing response adjacent to porous-surfaced, titanium alloy dental implants in dogs

We report here the results of a histological assessment of the initial healing response following implantation into the dog mandible of a porous-surfaced, titanium alloy endosseous dental implant. Two implants were placed in edentulous areas on each side of the mandible of each dog and covered with a full-thickness mucoperiosteal flap. The implant sites on one side of the mandible were allowed to head for four weeks, while those on the other side were allowed to head for eight weeks before the animals were killed. Histological specimens were obtained and assessed both qualitatively and by computer-assisted morphometry. All but one of the 24 implants were well-tolerated and healed with a variable ingrowth of bone into the porous-surface geometry. The histomorphometric measurements revealed that bone ingrowth had reached a plateau by four weeks of initial healing.

Structural analysis of hydroxyapatite coatings on titanium

Hydroxyapatite from two sources was electrophoretically deposited onto flat titanium plate material. Depending upon the deposition conditions various changes in the structure of the ceramic were identified. A well-adhering Ti-P compound was present at the interface. Hydroxyapatite oxygenated to various degrees and tetracalcium phosphate were reproducibly formed in the coating.

Bone remodeling associated with a flexible femoral intramedullary implant

The tissue response and bone remodeling associated with a flexible femoral intramedullary implant was evaluated. The implant consisted of wafers of porous Co-Cr-Mo alloy and microporous LTI pyrolytic carbon. Six wafers of each material were assembled in a stack and held together using a central acrylic rod. Radiography and histology demonstrated that these implants were associated with gross bone remodeling changes in the femoral shaft. The bone remodeling consisted of endosteal surface bone resorption and periosteal surface bone deposition, most likely due to a loss of structural support from the reamed medullary canal. The periosteal surface bone deposition resulted in an increase in femoral shaft diameter of 5% - 140%. The bone-implant interface consisted of a fibrous connective tissue with limited areas of bone ingrowth or bone apposition.

Surface-active biomaterials

Since the discovery in 1969 of a man-made surface-active material that would bond to bone, a range of materials with the same ability has been developed. These include glass, glass-ceramic, and ceramic materials which have a range of reaction rates and from which it should be possible to select a surface-active material for a specific application. The available materials and their similarities, differences, and current clinical applications are reviewed.

In vitro corrosion study of porous metal fibre coatings for bone ingrowth

As a first part of a biocompatibility testing programme, in vitro corrosion tests were carried out on porous stainless steel AISI 316L and titanium compacts made of 100 microns thick fibres. The present porous metal structures are used as coatings on permanent orthopaedic implants; with osseous tissue invading the pores, the implant becomes securely anchored to the surrounding bone. The results show that no inadvertent reactions occur with porous titanium. It can probably be used with no greater risk of localized electrochemical attack than the parent bulk material.

Short term bonding behaviour of bioglass coatings on metal substrate

Bioglass specimens were implanted in 11 dogs for periods ranging between two and twelve weeks in order to study the short term bonding behaviour of bioglass coated on metal. Metal fibre porous coated specimens without bone growth inducive microlayer were implanted as controls. The conclusions reached pertain to the validity of push out test results, bioglass short term bonding and the effect of the dipping procedure to coat bioglass on metal: 1. It is impossible to compare the bonding effectiveness of bonding materials tested under slightly differing circumstances, and a fortiori at different laboratories, 2. there is no sufficient evidence to establish a difference in bone bonding strength with regard to trabecular bone of bioglass and metal fibre porous coated specimens at 2 and 4 weeks; at 12 weeks, however, a higher interfacial failure shear strength is obtained with the control porous specimens, 3. bone bonding may be hampered by compositional differences arising as a result of the processing of a bioglass on metal coating. Technologically it is, however, possible to exclude this problem.