S. C. Hesseling

Effect of implantation site on phagocyte/polymer interaction and fibrous capsule formation

Implants of Silastic, Estane, polypropylene oxide and an HPOE/PBT segmented polyether polyester copolymer were qualitatively and quantitatively evaluated, with respect to interaction with mononuclear and multinucleated phagocytes as well as fibrous capsule formation, after implantation at three sites in the rat middle ear. The volume of the phagocyte exudate surrounding the implants, the degree of implant degradation and fragmentation and the thickness of the fibrous capsules were found to be correlated with the implantation site. From these findings, it can be concluded that it is important to assess the biological performance of a biomaterial at carefully chosen implantation sites.

Bone growth in biomimetic apatite coated porous Polyactive® 1000PEGT70PBT30 implants

We recently, developed a simple one-day one-step incubation method to obtain bone-like apatite coating on flexible and biodegradable Polyactive 1000PEGT70PBT30. The present study reports a preliminary biological evaluation on the coated polymer after implantation in rabbit femurs. The porous cylindrical implants were produced from a block fabricated by injection molding and salt leaching. This technique provided the block necessary mechanical integrity to make small cylinders (diameter 3.5 x 5 mm2) that were suitable for implantation in rabbits. The coating continuously covered the surface of the polymer, preserving the porous architecture of outer contour of the cylinders. Two defects with a diameter of 3.5 or 4 mm were drilled in the proximal and distal part of femur diaphysis. The implants were inserted as press-fit or undersized into the cortex as well as in the marrow cavity. The polymer swelled after implantation due to hydration, leading to a tight contact with the surrounding bone in both defects. The adherence of the coating on the polymer proved to be sufficient to endure a steam sterilization process as well as the 15% swelling of the polymer in vivo. The coated Polyactive 1000PEGT70PBT30 has a good osteoconductive property, as manifested by abundant bone growth into marrow cavity along the implant surface during 4-week implantation. A favorable bioactive effect of the coating with an intimate bone contact and extensive bone bonding with this polymer was qualitatively confirmed. Concerning the bone ingrowth into the porous implant in the defect of 4 mm diameter, only marginal bone formation was observed up to 8 weeks with a maximal penetration depth of about 1 mm. The pore interconnectivity is important not only for producing a coating inside the porous structure but also for bone ingrowth into this biodegradable material. This preliminary study provided promising evidence for a further study using a bigger animal model.

Biocompatibility and degradation of poly(ether–ester) microspheres: in vitro and in vivo evaluation

Microspheres of a hydrophobic and a hydrophilic poly(ether-ester) copolymer were evaluated for their in vitro and in vivo biocompatibility and degradation. The microspheres prior to and after sterilization were tested for in vitro cytotoxicity. The in vivo biocompatibility of the poly(ethylene glycol) terephthalate and poly(butylene terephthalate) (PEGT/PBT) microspheres was evaluated subcutaneously and intramuscularly for 24 weeks in rabbits. The in vivo degradation of the microspheres was studied microscopically and compared to the in vitro degradation. The in vitro and in vivo studies showed the biocompatibility of the microspheres of both the hydrophobic and the hydrophilic PEGT/PBT copolymer. Extracts of these microspheres showed no cytotoxic reactivity in the in vitro cytotoxicity test. Sterilization of the microspheres by gamma irradiation did not affect the cytotoxicity. PEGT/PBT microspheres injected subcutaneously and intramuscularly in rabbits showed a mild tissue response in vivo, in terms of the inflammatory response, the foreign body reaction and the granulation tissue response. Although an in vitro degradation experiment showed a decrease in molecular weight due to hydrolysis, the in vivo degradation of the microspheres was slower than previously published.

Reactions of cells at implant surfaces

The surface of implants is an important parameter in host-implant integration. Several strategies can be used to obtain integration, such as the application of grooves or pores at the implant surface. Most of these surface alterations, however, will lead to an increase of total implant surface area which might influence the inflammatory response to an implant. As far as integration with bone is concerned several biomaterials have been successful in mimicking this material, by having similar crystals at their surface (calcium phosphate ceramics) or by containing a certain amount of calcium and phosphorus. Polyactive, a poly(ethylene oxide)-poly(butylene terephthalate) segmented copolymer, also possesses favourable integration properties with bone, but initially lacks calcium and phosphorus. It is proposed that the application of hydrogels as biomaterial may add a new dimension to integration capacity.

Tympanic membrane structure during a Staphylococcus aureus-induced middle ear infection. A study in the rat middle ear.

In response to a Staphylococcus aureus-induced middle ear infection the tympanic membrane showed infiltration of polymorphonuclear granulocytes, lymphocytes, and macrophages and increased areas covered by ciliary and secretory epithelium. These reactions, which were comparable to the cellular and mucociliary responses seen in the middle ear mucosa during infection, were restricted to the pars flaccida and to predominantly the annular and manubrial regions of the pars tensa. This showed that the greater part of the tympanic membrane, where the lamina propria is composed of collagenous bundles and only very thin layers of loose connective tissue, is hardly affected by or barely responds to the inflammatory stimulus.

Cellular reaction on the intraperitoneal injection of four types of polylactide particulates

Four types of polylactide particulates, P-L-LA 100, 250, 550 KD and a P-DL-LA 400 KD were injected into the peritoneal cavity of mice. The inflammatory reaction showed an increase in cell number (mainly neutrophilic granulocytes) up to 48 h after which the cell numbers decreased below the control (phosphate-buffered saline). All four polylactide particulates aggregated and intermingled with inflammatory cells. The aggregates remained throughout the investigation period of 6 months. Quantitative measurements showed that standardization of the particle form and size is essential. From this study and other experiments in which calcium phosphates and asbestos were injected intraperitoneally, it is concluded that the inflammatory response observed in the peritoneal cavity is related to the type of material injected and probably to form and size of the individual particles, but not to molecular weight.

Cytokeratin Patterns of Tissues Related to Cholesteatoma Pathogenesis

Specimens of cholesteatoma matrix, meatal epidermis, and middle ear epithelium were removed during surgery, and immunohistochemical techniques were used to investigate cytokeratin expression. The use of five chain-specific anticytokeratin monoclonal antibodies and one broad specific anticytokeratin monoclonal antibody showed the divergent behavior of middle ear epithelium compared with the cytokeratin expression of the other two types of epithelium. Middle ear epithelium was characterized by the presence of cytokeratins 4, 8, 18, and 19, whereas in both cholesteatoma and meatal epidermis cytokeratin 10 predominated. Furthermore, cholesteatoma showed an infrequent focal presence of cytokeratins 4, 18, and 19. The similarity between cholesteatoma and meatal epidermis with respect to morphology, and the presence of cytokeratin 10 support an epidermal origin of cholesteatoma. However, a metaplastic origin cannot be excluded, because of the infrequent occurrence of a small amount of cytokeratins 4, 18, and 19 in cholesteatoma matrix that was not found in meatal epidermis but was a component of the cytokeratin pattern of middle ear epithelium.

Effect of HA-1A monoclonal IgM antibody on endotoxin-induced proliferation of cultured rat middle ear epithelium.

The effect of human monoclonal antibody HA-1A (Centoxin) on the effect of endotoxin on cultured rat middle ear epithelium was investigated. The addition of endotoxin to the standard culture medium revealed a concentration-related proliferative effect on cultured rat middle ear epithelium, leading to cobblestone cells, cell tracks, and stratification of epithelium, whereas rat middle ear epithelium cultured in standard medium grew as a monolayer composed of flat polygonal cells. Addition of HA-1A to standard medium supplemented with endotoxin gave rise to a statistically significant suppression of the proliferative effects of endotoxin on these cells. The morphology of rat middle ear epithelium cultured in the presence of HA-1A and endotoxin showed that these cells still had a tendency to form cobblestone-like cells and cell tracks, but to a substantially lower degree. The present results support the hypothesis that HA-1A suppresses the proliferative and morphological effects of endotoxin on rat middle ear epithelium and may play an important role in the pathogenesis of otitis media.

Polyactive: A Bone-Bonding Polymer

Polyactive a 55/45 poly(ethylene oxide)/poly(butylene terephthalate) copolyether ester, was investigated as regards its general in vitro and in vivo biocompatibility. Special attention was directed to the interactions of the polymer with bone.

Biodegradation and Phagocyte/Polymer Interaction

The degree of implant degradation was found to be correlated with both the implantation site and the properties of the implant materials under study. Implant degradation was found to be associated with interactions between the implants and macrophages or foreign-body giant cells: the numbers of these phagocytes varied with the implantation site in patterns corresponding with those accompanying implant degradation, and the morphology of the phagocytes also varied with the degree of implant degradation.