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A comparative study of flexible (Polyactive) versus rigid (hydroxylapatite) permucosal dental implants. II. Histological aspects.

Of the many materials that are applied in dental implantology, most have a high Young's modulus. In the concept of osseointegration, which is generally preferred, chewing forces are therefore directly transmitted from rigid implant materials to the relatively flexible surrounding bone. Polyactive is an elastomeric polyethylene-oxide polybutylene-terephthalate (PEO:PBT) copolymer, with a low modulus of elasticity, that exhibits bone-bonding characteristics. Previous finite-element analysis emphasized the benefit of the application of flexible implant materials. To assess the validity of this finite element model, one dense and two porous types of flexible Polyactive permucosal dental implants and one rigid hydroxylapatite (HA) implant were clinically tested in a beagle dog study and were compared during 30 weeks of loading. In an earlier report, it was concluded that dense Polyactive implants function clinically adequately and resemble the mobility of natural teeth. In the current study, the amount of bone contact was quantitatively assessed. Polyactive implants showed a statistically significantly higher bone contact, as compared to the HA implants, probably due to a significant water uptake and a subsequent increase in the volume of the denser Polyactive. Morphological analysis, based on back-scatter electron microscopy and light microscopy, revealed little remodelling activity of the bone surrounding the Polyactive implants. Few remodelling lacunae were seen and de novo bone formation was rarely observed. At some locations, a continuity between calcification zones with the polymer surface and the surrounding interfacial bone was observed, indicating the occurrence of bone-bonding. Also, the bone surrounding the HA implants showed little remodelling activity, although large resorption lacunae were observed, in which HA particles were present. Based on the observations of this study, flexible bone bonding implants might be more capable of transferring stresses to the surrounding bone and are therefore promising alternatives to "routine' rigid implants.