Örjan H. Andersson

Calcium phosphate formation at the surface of bioactive glass in vivo

The calcium phosphate formation at the surface of bioactive glass was studied in vitro. Glass rods and grains were immersed in different aqueous solutions and studied by means of scanning electron microscopy and energy dispersive x-ray analysis. Surface morphological changes and weight loss of corroded grains were monitored. In-depth compositional profiles were determined for rods immersed in the different solutions. The solutions used were tris-buffer (tris-hydroxymethylaminomethane + HCl), tris-buffer prepared using citric acid (tris-hydroxymethylaminomethane + C6H8O7.H2O), and a simulated body fluid, SBF, containing inorganic ions close in concentration to those in human blood plasma. It was found that the calcium phosphate formation at the surface of bioactive glass in vitro proceeds in two stages. When immersing the glass in tris or in SBF a Ca,P-rich surface layer forms. This accumulation takes place within the silica structure. Later, apatite crystals forming spherulites appear on the surface. The Ca/P-ratio of initially formed calcium phosphate was found to be about unity. It is proposed that this is due to bonding of phosphate to a silica gel. The surface is stabilized, i.e., leaching is retarded, by the rapid Ca,P-accumulation within the silica structure before apatite crystals are observed on the surface. It is proposed that the initially formed calcium phosphate is initiated within the silica gel. The crystallizing surface provides nucleation sites for extensive apatite formation on the glass surface. In the presence of citrate no Ca,P-accumulation occur at the glass surface, but soluble Ca-citrate complexes form. By comparing the weight loss during corrosion in tris with that in the calcium and phosphate containing SBF, it is possible to establish whether the glass can induce apatite formation at its surface or not.

On the bioactivity of silicate glass

Abstract In order to improve the properties of bioactive glasses, one must be able to control the bioactivity. This requires a good description of the bone bonding mechanism or, preferably, a good understanding of it. In this paper the bioactivity is approached both phenomenologically and deterministically. The phenomenological description of the property-composition relationship provides a tool for the formulation of bioactive glass compositions. The complexation of phosphate by the initially formed silica gel is proposed as the mechanism responsible for the bioactivity.

Analysis of the in vivo reactions of a bioactive glass in soft and hard tissue

A bioactive glass, S53P4, was implanted as granules subcutaneously in muscles and connective tissue of rabbits, as well as in the mandibular bone of a sheep. After the implantation period of 2-3 months, cross-sections were prepared and studied by scanning electron microscopy and energy dispersive X-ray analysis. The glass reacted essentially in the same way in all types of tissue. The granules consisted of an unreacted core and a reacted layer with a silica-rich and calcium phosphate-rich zone. Large hydroxyapatite crystals were occasionally found on top of the calcium phosphate surface of the granules implanted in soft tissue. On the basis of elemental analysis of the reaction layers it was found that the release of calcium from inside the glass is sufficient to account for the formation of the calcium phosphate surface layer, whereas the release of phosphate from the glass is not sufficient.

Bioactive glass versus hydroxylapatite in reconstruction of osteochondral defects in the rabbit

We studied osseointegration of a bioactive glass (BG) and hydroxylapatite (HA) in rabbit femur epiphyseal and metaphyseal regions. 17 BG and 24 HA cones implanted in defects through arthrotomy were analyzed. The holes for implants were drilled through distal femur joint surfaces. The cartilage wound repaired generally by fibrous tissue. Histomorphometry showed that 61, 78, and 79 percent of BG surface was covered by bone at 3, 6, and 12 weeks, respectively. The corresponding figures for HA were 47, 67, and 78 percent. Chemical bonding between bone and implants of both types was confirmed by scanning electron microscopy (SEM) and energy-dispersive x-ray analysis (EDXA). Formation of a calcium phosphate-rich layer on the surface BG implant was demonstrated by EDXA. Our results indicate that the osseointegration rate of bioactive glass does not differ from that of hydroxylapatite.

Bioactive glass and glass-ceramic-coated hip endoprosthesis: experimental study in rabbit

Co−Cr−Mo endoprostheses with a dual bioactive glass (BG) coating and titanium implants coated with a bioactive glass-ceramic (BGC) were studied under lead-bearing conditions in the rabbit hip. The dual BG coating contained an inner layer of high durability and an outer bioactive layer. Each type of coating improved the stabilization of prosthesis during the experiment period of 8 weeks compared to non-coated control implants. EDXA analysis confirmed the ability of BG and BGC coatings to bond chemically to bone. The BGC coating on titanium alloy proved superior to the dual BG coating on Co−Cr−Mo prosthesis with regard to bone formation on the surface of the implant. The bioactive top layer of the dual BG coating showed resorption, especially in the areas without direct bone contact. This is explained by partial crystallization of the glass during firing. Thermal discrepancy between BGC coating and titanium core caused cracking of the coating, which remains a major obstacle to its use as a bioactive coating.

Aluminium release from glass ionomer cements during early water exposure in vitro

Aluminium is a major constituent of glass ionomer cements. During mixing and setting aluminium is released from the glass into the polyalkeonic acid solution. Part of this aluminium may not combine with the polyalkeonic acid, but may be released from the cement. The aluminium release from auto-cured and light-cured glass ionomer cements during early water exposure was studied. The former cements released more aluminium than the latter. Scanning electron microscopy (SEM) showed extensive loss of polymer matrix for the cements with the highest aluminium release. Insufficient curing of light-cured cements also resulted in loss of matrix. It is suggested that the considerable release of aluminium from glass ionomer cements during early water exposure may explain the reported lack of mineralization of predentin in the pulp beneath glass ionomer cements. This would correspond to the inhibiting effect of aluminium on bone mineralization.

Solubility and film formation of phosphate and alumina containing silicate glasses

Dissolution, surface film formation and the stabilising effect of alumina was studied for glasses in or near the bioactive composition region. Tris(hydroxylmethyl-aminomethane) buffers at pH 7.4, which contained chloride or citrate were used as test solutions. Solution analysis showed that the silicon-release was higher in the chloride than in the citrate solution. Analysis of glass surfaces with X-ray photoemission spectroscopy (XPS) showed that the citrate inhibited calcium phosphate film formation, but allowed aluminosilicate formation. Thus, by adding citrate to the test solution the stabilizing effect of calcium phosphate film formation is excluded and the effect of alumina is consequently more easily observed. The stabilising effect of alumina was due to both stabilization of the glass structure and to aluminosilicate film formation. Analysis showed that glasses with calcium phosphate film formation in Tris, also accumulated borate at their surfaces. This can be explained by partial substitution of the phosphate by borate in the developing hydroxylapatite surface layer.

Glass-ceramic-coated titanium hip endoprosthesis

Titanium alloy hip endoprostheses coated with a bioactive glass ceramic (BGC) were followed in rabbits. All test endoprostheses remained stable, and image analysis showed an average of 78% bonding of the BGC-coated implants to bone at 52 weeks. The uncoated Ti-alloy controls demonstrated an average of 37% bone coverage after 52 weeks. By scanning electron microscopy the thickness of the BGC reaction layer was found to stabilize at 60 μm after bioactive bone bonding. The results indicate that the BGC coating must be thicker than the reaction layer to prevent detachment from the core metal.

Short-term Reaction Kinetics of Bioactive Glass in Simulated Body Fluid and in Subcutaneous Tissue

ABSTRACT Reaction kinetics of a bioactive glass (S53P4) were compared for up to 72 hours in simulated body fluid (SBF) and in subcutaneous tissue. The samples were studied by scanning electron microscopy and energy dispersive X-ray analysis. Glass tested subcutaneously or in SBF at SA/V of 0.4 cm−1 induced calcium phosphate formation more rapidly than glass tested in SBF at a SA/V of 0.1 cm−1. However, a pure apatite surface layer developed more rapidly at the low than at the high SA/V ratio or subcutaneously. The calcium phosphate accumulation started later but continued more rapidly at the low than at the high SA/V ratio. Attachment of fibers, most likely collagen, and their mineralisation at the glass surface indicates ability of the glass to bond to soft tissue.

Bioactive double glass coatings for Co-Cr-Mo alloy

Glass compositions for double coatings for a Co-Cr-Mo alloy were developed. The glass compositions were chosen to fulfil such requirements as matching thermal expansion, low glass transition temperature and moderate solubility. For the ground coat a fairly high durability is required, whereas the cover coat must be bioactive, i.e. become attached to living bone by a chemical bond. Two compositions of each type were developed by computer-aided optimization. The glasses were chosen in the Na2O−CaO−B2O3−Al2O3−SiO2−P2O5 system. The bioactivity was tested in vitro by immersion in a simulated body fluid. The double coatings on Co−Cr−Mo alloy released hexavalent chromium into the solution as detected by yellow colouration and spectrophotometry. This colouration was strong at the margin between coated and uncoated metal and may be explained by oxidation of trivalent chromium of the alloy in the presence of glass. The released chromium did not have any notable effect on the calcium phosphate formation. After replensihing the solution no coloration was observed. This suggests that the chromate is easily dissolved and that it may be possible to wash it out prior to implantation.