Titanium and bioactive glass-ceramic coated titanium as materials for keratoprosthesis.

The current problem with keratoprosthesis is the ingrowth of corneal or conjunctival epithelium into the anterior chamber. This may lead to infections and extrusion of the prosthesis as well as to the development of retroprosthetic membrane and secondary glaucoma. Glass-ceramic coated and uncoated titanium has been tested as material for the keratoprosthesis to prevent epithelial ingrowth. Twenty-two Supra-Descemet's membrane keratoprostheses were inserted in the eyes of 22 rabbits for 1, 2, 4, 8, or 12 months. The prosthesis had an optic part made of polymethylmetacrylate (PMMA). The support for the optic part and the flange of the prosthesis were made of titanium. Eleven of the prostheses were coated with glass-ceramic. The histological sections of the enucleated eyes were prepared through the central part of the cornea and the prosthesis using a cutting-grinding method. The histological analysis was made on both halves of the implants separately giving two analysis areas in each eye. All 11 titanium prostheses were retained for the time period planned. Two glass-ceramic coated prostheses were lost at 2 and 4 weeks, respectively. This was caused by difficulties at surgery due to a thick coating. These eyes were excluded from the histological analysis. No significant ingrowth of epithelium was seen in 15/18 (83%) and in 16/22 (73%) of the analysed areas of the glass-ceramic coated and titanium prostheses, respectively. Titanium appears to be a suitable material for the keratoprosthesis. The ingrowth of the epithelium may be hindered further by coating the titanium with bioactive glass-ceramic.

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.

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.

Effect of glass bioactivity on new bone development induced by demineralized bone matrix in a rat extraskeletal site.

The effects of two kinds of bioactive glass and two kinds of phosphate-free glass on new bone development induced by demineralized bone matrix (DBM) were studied in the rat abdominal muscle pouch model. After 8 weeks' implantation histomorphometric analysis revealed that the amount of new bone in DBM combined with bioactive glass was comparable to DBM without bioactive glass. DBM grafts combined with phosphate-free glass showed significantly less new bone formation. Scanning electron microscopic examination confirmed that new bone bonded to the surface of bioactive glass. The release of ions from the glass seemed to slow down after new bone had bonded to it. Exclusion of phosphate from a bioactive glass resulted in loss of ability to develop the Ca,P-rich surface layer needed for bone bonding.

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.

Dissolution, leaching, and Al2O3 enrichment at the surface of bioactive glasses studied by solution analysis.

Five glass compositions in or near the bioactive region in the system SiO2-Na2O-CaO-P2O5-Al2O3-B2O3 were studied in vitro by immersion in Tris buffer. The Si concentration can be taken as a measure of the amount of dissolved glass, whereas the Na concentration can be used to estimate the thickness of the Si-rich (Si gel) layer. Upon immersing a bioactive glass into Tris buffer, a surface layer of a few micrometer thickness is dissolved during the first 8 h. During the first few hours of immersion, the rate of dissolution of the glass network is equal to or exceeds that of the growth of the Si-rich layer. If the glass contains Al2O3, most of the aluminum that would be released due to dissolution of the silica network is enriched in the Si-rich surface layer that forms due to leaching. Al2O3 is not only bonded by the Si gel but also interferes with formation of calcium phosphate.

Induction of new bone by allogeneic demineralized bone matrix combined to bioactive glass composite in the rat.

Allogeneic diaphyseal demineralized bone matrix (DBM) cylinders containing bioactive glass rods were implanted for 4 and 8 weeks in the abdominal muscle wall of rats. DBM without glass served as control. The results suggest that new bone induction by DBM was accelerated by the presence of bioactive glass implants. However, the bone formation induced by DBM on the glass surface was relatively small. The biocompatibility of the glass was verified by the absence of adverse cellular reactions in the interface region between glass and bone. The method used provides a simple and fast means of exploring the characteristics of potential bone substitutes.

Effect of bovine bone morphogenetic protein and bioactive glass on demineralized bone matrix grafts in the rat muscular pouch.

New bone formation induced by allogeneic demineralized bone matrix (DBM) and bone morphogenetic protein (BMP) combined with bioactive glass (BG) was studied in a rat abdominal muscle pouch model. At four weeks the amount of new bone was not influenced by DBM combined with BMP and/or bioactive glass. The mean proportional areas of new bone varied among different DBM test groups from 8.6% to 13.4%. New bone was induced in inactivated DBM samples containing BG, while no bone formation was seen in DBM samples without BG. The results indicate that bioactive glass favours bone induction in inactivated allogeneic bone matrix.

Calcium phosphate formation at the surface of bioactive glass in vitro.

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.