On surgical techniques to increase bone density and volume. Studies in the rat and the rabbit

Abstract

BACKGROUND. Various techniques of surgical intervention have been suggested to promote bone densityand volume; however, the predictability of such techniques is not always sufficiently supported by preclinicalevaluations.AIM. The general aim of the present thesis was to evaluate some surgical techniques claimed to increasebone volume and density and thereby improve implant retention and survival. In particular, the aims were toinvestigate the effect of cortical perforations of tibial, maxillary, and calvarial bone alone or in conjunctionwith a) titanium implants, b) guided bone augmentation (GBA) barriers, or c) GBA barriers combined withgrafting materials. In addition, the histologic characteristics of untreated calvarial bone were studied withspecial emphasis on vessel topography.MATERIALS AND METHODS. In study I, the effect of cortical perforations in the tibia and the maxilla wasevaluated in the rabbit, and the healing response in the two bones was compared. In study II, the effect ofsurgical intervention in the rabbit maxilla 4 weeks prior to implant placement was evaluated and comparedwith the effect of the implant placement per se. In study III, a rat calvarial model was used to study the effectof bovine bone mineral (BBM) placement as an adjunct to a silicone GBA barrier. In study IV, the impact ofcortical perforations in the rabbit calvaria contiguous to a titanium GBA barrier was evaluated. In study V,placement of autogeneic bone or BBM combined with a titanium GBA barrier was studied in a rabbit calvarialmodel. In study VI, vessel topography and bone density were studied in untreated rabbit calvaria.RESULTS. As observed 8 weeks postoperatively, cortical perforation resulted in significant bone densityincrease in the rabbit maxilla while no significant changes were found in the tibia. Implant placementsignificantly increased bone density while surgical intervention 4 weeks prior to implant placement did notfurther increase bone density or bone-implant contact. Cortical perforations were not found to promote boneaugmentation in the rabbit GBA model. Similar amounts of bone tissue and mineralized bone were found withthe adjunctive placement of autogeneic bone or BBM in the rabbit GBA model. In both the rat and the rabbitGBA models, the adjunctive placement of BBM significantly increased augmented bone tissue volumecompared to placement of barriers only. In the rabbit model, the addition of BBM was found to promotesignificantly more mineralized bone than placement of a barrier only. In the rat model, on the contrary,placement of the silicone barrier alone promoted more mineralized bone than with the addition of BBM. Theproportions of cortical, trabecular, and marrow areas found in the right and left untreated parietal bones weresimilar, as were the number and width of natural hollow connections in the cortical plates and the distributionof vessels. A similar number of these hollow connections was found in untreated and GBA-treated parietalbone.CONCLUSIONS. The different bone response to cortical perforations in the tibia and maxilla emphasizesthe importance of choice of experimental sites in bone/implant studies. Pre-implant surgical intervention doesnot seem to promote titanium implant incorporation in rabbit jawbone. The reason why cortical perforations didnot promote augmentation or bone density in the calvarial GBA model might be related to the presence of thenatural hollow connections in the cortical plates. Placement of particulated autogeneic bone or BBMsignificantly promoted bone augmentation in GBA. BBM promoted as much bone as autogeneic bone inconjunction with a stable, titanium GBA barrier. The lack of promotive effect of BBM on the formation ofmineralized bone in the rat GBA model might be because of ingrowth of soft tissue from the sagittal sutureinterfering with the GBA-site. Since the symmetry of the large vessel topography and the histomorphometricparameters assessed was high between the left and right parietal bones, the bilateral use of the parietal bones issuggested to be reliable in experimental GBA models regarding blood supply and bone quality.