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

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].

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.

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.

The effect of hydroxyapatite coating on ingrowth of bone into cavities in an implant

A canine implant study was performed to assess whether hydroxyapatite (HA) could induce bone to close gaps at the bone-prosthesis junction. Titanium alloy (TiAlV) plates with varying grooves cut into them (1, 2, and 3 mm deep) and identical on both sides were sprayed on one side with HA and left as bare metal on the other. These were then implanted into the distal femora of dogs that were killed 4 and 8 weeks afterwards. Specimens were analyzed using optical microscopy and microradiography. Results show that the presence of HA induces bone to grow almost completely down the deepest groove by 8 weeks. In the 1- and 2-mm grooves at 4 weeks the depth of penetration of bone into the grooves was greater on the HA coated side (but the actual volume of bone in the grooves was greater on the TiAlV side because the bone present was more dense). We conclude that the presence of HA induces bone to close relatively large gaps. Although bone does not grow all the way into uncoated defects there can be more bone here in the short term. This feature of HA may be of great value in hip implant fixation.

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.

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.