A. M. Radder

Application of porous PEO/PBT copolymers for bone replacement

A range of polyethylene oxide/polybutylene terephthalate (PEO/PBT) copolymers (70-30% PEO) was investigated for nonloadbearing bone replacement application. Porous PEO/PBT cylinders (d = 5 mm, h = 7 mm) were implanted transcortically in the diaphyseal femur of 10 goats, and the animals were sacrificed at 3, 6, 9, 12, and 26 weeks. Qualitative evaluation was performed using light and fluorescence microscopy, scanning electron microscopy, and back-scatter electron imaging with an attached X-ray microanalysis system. The percentage of bone ingrowth and the percentage of bone contact in the pore region were quantitatively assessed using undecalcified histological sections. The hydrogel properties of the PEO/PBT copolymers provided a rapid closure of the defect upon press-fit implantation, due to postoperative water uptake and subsequent swelling behavior of the materials. Bridging of the defect by bone and the occurrence of bone bonding were observed 6 weeks postoperatively for the material with the highest PEO content (70/30). For the 60/40 and 55/45 PEO/PBT proportions, union of the defect and bone bonding were observed at 9 and 12 weeks, respectively. The stiffer 40/60 and 30/70 PEO/PBT implants showed bridging of the 5-mm gap after 12 weeks, but did not reveal bone bonding up to 26 weeks. Peripheral fragmentation, mainly in the marrow cavity, was found for the 70/30 material at 12 weeks and had increased at 26 weeks. Degradation was not seen for the other materials. The histomorphometrical data confirmed the microscopical observations and demonstrated a direct relation between PEO content in the PEO/PBT co-polymers, the rate of bone ingrowth, and the amount of bone contact. Porous PEO/PBT copolymers are degradable, bone-bonding elastomeric substrates with favorable handling properties and a high percentage of bone ingrowth (69-78 at 26 weeks). It was therefore concluded that PEO/ PBT copolymers are highly promising materials for bone-replacement surgery.

In vitro biocompatibility testing of polylactides Part I Proliferation of different cell types

Four polylactides, P-L-LAs 100 KD, 240 KD, 500 KD and a P-DL-LA 400 KD, were testedin vitro by using five cell types. Middle ear, ear canal and nasal septum epithelial cells were used as well as fibroblasts and osteosarcoma cells. The proliferation of cells was studied by culturing on polylactide films and by culturing with media based on artificially aged PLA. The fibroblasts and the osteosarcoma cells were also cultured with media containingL- orD-monomers in different concentrations. Significant differences in cell numbers of polylactide cultures and/or controls were observed. These differences varied per cell type and experimental setting. In general the biocompatibility of the PLAs was satisfactory. Some inhibitory effect on the proliferation of high (10 mg ml−1) monomer concentrations in culture media was seen.

Bone-bonding behaviour of poly(ethylene oxide)-polybutylene terephthalate copolymer coatings and bulk implants: a comparative study

A range of poly(ethylene oxide)-polybutylene terephthalate (PEO-PBT) copolymers (70-30% PEO), both as coating on titanium alloy as well as bulk cylinders, was press-fit implanted in the diaphyseal femur of 16 goats. At early survival times (4 wk), a high degree of cortical bone contact was observed for bulk implants using light microscopy and this was confirmed by backscatter electron microscopy. This was attributed to the swelling behaviour of PEO-PBT copolymers. At this stage, bone contact was also revealed for coated implants, but to a lesser extent. At a later stage (12 wk), bone bonding was demonstrated both morphologically and by X-ray microanalysis, at the interface of 70:30 PEO-PBT bulk as well as 70:30 PEO-PBT-coated implants. Bone bonding increased with time (26 and 52 wk) for this PEO-PBT proportion and was also observed for 60:40 and 55:45 implants, although less frequently. For 40:60 and 30:70 PEO-PBT proportions, bone bonding was not shown. Based on these qualitative data, it was not possible to differentiate between coated and bulk implants with respect to bone bonding. This study demonstrated that the application of PEO-PBT elastomers as coatings does not alter the bone-bonding properties. It was therefore concluded that PEO-PBT coatings are beneficial over the bone-bonding but brittle ceramic coatings, due to their flexibility. In addition, the bone-bonding capacities of these PEO-PBT coatings surpass the non-bonding behaviour of currently available flexible coatings.

Degradation and calcification of a PEO/PBT copolymer series

In this study the influence of polyethylene oxide content (70, 60, 55, 40 and 30%) on degradation and calcification of dense polyethylene oxide (PEO)/polybutyleneterephthalate (PBT) copolymer cylinders was evaluated at subcutaneous sites in goats. Histologically, the 30/70 and 40/60 PEO/PBT implants revealed no signs of calcification and of degradation, although slight surface alterations were evident after 1 year. In contrast, peripheral fragmentation and intermediate calcification were observed for 55/45 and 60/40 PEO/PBT specimens. After subsidence of the inflammatory reaction to the surgical intervention, the tissue reaction to these four PEO/PBT proportions was characterized by a fibrous capsule, although macrophages were frequently seen bordering 55/45 and 60/40 fragments. More extensive degradation and calcification were encountered for 70/30 PEO/PBT samples. Peripheral crack formation occurred from 4 weeks on and at 12 weeks, surface fragmentation was observed for this material. Fragments were usually surrounded by macrophages and, in addition, phagocytes with a foamy appearance were found. Fragmentation and phagocytotic activity increased during the course of the experiment. Using confocal Raman microspectroscopy, it was demonstrated that phagocytes contained 70/30 PEO/PBT fragments in their cytoplasm, as intracellular fragments were chemically composed of PEO and PBT. Systemic effects of the fragmentation were not observed in axial lymph nodes. It can be concluded from the data obtained in this model that a PEO/PBT copolymer series was well toierated. This study showed that PEO content in PEO/PBT copolymers is directly related to degradation and calcification of these materials and that the extent of these parameters may vary under different experimental conditions. The application of confocal Raman microspectroscopy enables analysis of the chemical composition of intracellular fragments and may therefore be a valuable instrument in the prediction of long-term implant performances.

Abundant postoperative calcification of an elastomer: matrix calcium phosphate-polymer composite for bone reconstruction

In this experiment the behaviour of an 80/20 PEO/PBT copolymer in bone defeets was assessed. Porous cylinders were press-fit inserted into the diaphyseal femur of goats and evaluated by light and electron microscopy and X-ray microanalysis. The most important finding in this study was that almost complete calcification throughout the implant was displayed after 26 weeks. The 80/20 PEO/PBT copolymer did not contain calcium and phosphorus prior to implantation, however, it apparently has the ability to take up considerable amounts of calcium and phosphate postoperatively, resulting in a calcium phosphate-polymer composite. As a consequence of the high calcification rate of 80/20 cylinders, bone-bonding (a continuum between calcification of the material surface and bone) was encountered as early as 3 weeks after implantation. Union of the 5 mm defect was observed at 6 weeks and ingrowth was frequently centripetal. After 26 weeks bone tissue occupied most of the pore area and was often seen in continuity with the calcified polymer. We conclude that an elastomeric matrix that is capable of abundant postoperative calcification behaves favourably with respect to the repair of bone defects. Porous 80/20 PEO/PBT copolymer is, therefore, a promising alternative for bone-replacement applications.

Gene expression and protein activity in bone-bonding and non-bonding PEO/PBT copolymers

The formation of a reactive carbonate-apatite surface is generally considered a critical step for bone-bonding. However, the contribution of proteins in the establishment of bone-bonding is debatable. In the current study, osteocalcin, osteopontin and β-actin mRNA expression and alkaline phosphatase activity were therefore determined in different porous substrates loaded with marrow after subcutaneous implantation for 2 weeks. Two polyethylene oxide (PEO)/polybutylene terephthalate (PBT) copolymers were used for this purpose. First, an 80/20 PEO/PBT which generates a carbonate-apatite surface and bonds to bone rapidly. Second, a 30/70 PEO/PBT which previously did not show bone-bonding up to 1 year postoperative. Bone had formed within the pores and occasionally at the calcified surface of the 80/20 materials. In contrast, 30/70 materials did not calcify and bone was not found in the pores. Despite these morphological differences, gene expression and protein activity was similar in 80/20 and 30/70 PEO/PBT materials. These results suggested that bone-bonding of PEO/PBT copolymers is more likely to depend on the formation of a carbonate-apatite surface than on a specific influence of a bioactive material on osteogenic cells.