MORPHOLOGICAL AND DIMENSIONAL CHANGES AFTER BARRIER REMOVAL IN BONE FORMED BEYOND THE SKELETAL BORDERS AT TITANIUM IMPLANTS - A KINETIC-STUDY IN THE RABBIT TIBIA

The aim of the present experimental investigation was to study the mor phological and dimensional changes of bone, augmented at titanium impl ants by a membrane technique, taking place after membrane removal. In 12 rabbits, screw-shaped titanium implants were inserted in the tibial metaphyses in such a way that 5 threads became uncovered with bone. S urgery was performed on 2 occasions in order to retrieve specimens wit h different followup times. An e-PTFE barrier and a titanium device we re used to provide space for bone formation. In 1 tibia of each rabbit , the membranes and spacers were removed after 8 weeks of healing, and the implants followed for 16 more weeks. Impressions were taken at da y 0 and after 8 and 24 weeks of healing and plaster models were produc ed. In the contralateral tibiae, implants were inserted either 16 or 8 weeks prior to sacrifice. Measurements were made on the plaster model s in 3 dimensions at 35 points around each implant in a coordinate mea suring machine. Specimens taken 8, 16 and 24 weeks after insertion wer e analysed by means of light microscopical morphometry, The coordinate measurements showed that, in mean, 1.92 mm of bone had been formed du ring the first 8 weeks. A statistically significant loss of the height of the newly formed bone (0.70 mm) and thereby reduction of bone volu me was found 24 weeks postoperatively. The volume decrease of the newl y formed bone was more pronounced beside the implants than over the im plant body. The histology showed that woven bone had been formed at th e implants after 8 weeks. Further bone formation and remodelling and a net increase of mineralized bone were seen. The degree of bone-implan t contact and bone area in the threads increased with time. The presen t study showed that coordinate measurements on plaster models, obtaine d from the experimental areas, in combination with histology, form a u seful technique to study long-term changes of augmented bone. It was f ound that bone formed by a barrier membrane technique, decreased in vo lume during a 16-week follow-up period after barrier removal. Less dim ensional changes were observed for the bone formed over the implant bo dy, indicating that a solid surface may have a stabilizing effect on t he augmented bone.