Nanomedicine, Volume IIA: Biocompatibility
© 2003 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003
18.104.22.168 Sapphire Dental Implants
Sapphire exhibits an elastic modulus 20 times greater than that of cortical bone , though it is prone to fracture [1021, 1029] if sufficient shear forces are imposed. Tooth implants are short, compact, and require mainly compressive strength. This is why many thousands of single-crystal alumina (sapphire) dental implants [989-996] have been performed over the last two decades in extensive animal [997-1006] and human [1010-1021] clinical trials. Commercially-produced alumina dental implant materials such as Bioceram [990, 995, 1018, 1019], BionitR  and Frialit [1021, 1050] are available for all tooth areas. Such materials also may serve as in-bone anchors for bridges and dentures [956, 4761]. In modern dental practice, titanium bone anchors are preferred because pure sapphire is too brittle in the masticatory environment and tends to fracture after ~6 months of normal use [Thomas G. Wilson, personal communication, 2001], and because of poorer long-term results  – most of these devices are made from titanium [5695, 5696, 5700, 5705] and feature altered surface structures or coatings. However, zirconia frameworks  or composites  provide twice the mechanical strength of Ceram alumina , “allowing the restorations to bear the high mastication forces in the molar region.” Tinschert et al  suggest that current CAD-CAM dental fabrication procedures may induce surface and subsurface flaws that adversely affect the structural reliability of an otherwise more reliable  material. Interestingly, the growth rates of cracks in single-crystal sapphire are significantly higher in C Ringer’s solution at 37 oC (a simulated physiological environment) than in humid air at 24 oC, although no true cyclic fatigue effect has yet been found .
During the 1980s and 1990s it was found that sapphire exhibits good hard-tissue and soft-tissue biocompatibility when used in the mouth [998, 1021, 1024-1031, 1033]. Peri-implant mucosa are nearly free from inflammatory cell infiltrations  and when successful are usually free from connective tissue capsules between the implant and the adjacent alveolar bone . Generally there are no serious inflammatory reactions in the surrounding soft tissues – only a minimal inflammatory infiltrate is seen when the implant/abutment interface is located below the gum tissue [1018, 1021]. One investigation of neutrophil number and activity around sapphire dental implants in 19 partially or completely edentulous patients found lower neutrophil activity around sapphire implants in completely toothless patients, but higher neutrophil activity on both teeth and implants in patients with remaining teeth .
Ultrastructural evidence reveals that an attachment complex forms between gingival epithelium and alumina that is analogous to that seen around natural teeth [1003-1009], with a high rate of bone contact on the sapphire surface [1003, 1005], or osseointegration . In one case of an aluminum oxide ceramic implant removed because of fracture of the abutment after a 30-month loading period, it was observed microscopically that the implant was covered by highly mineralized mature compact lamellar bone, with no connective tissue or inflammatory cells present at the interface . Osteocytes were found very close to the bone-implant interface, indicating the good biocompatibility of the implant . In another study , soft tissues surrounding single crystal sapphire implants were studied by conventional light- and transmission electron microscopy and by immunohistochemical markers for cytokeratin, protein S-100, factor VIII and KP1. Histological sections of biopsies obtained from clinically healthy peri-implant mucosa were separated into a keratinized outer implant epithelium and an inner, non-keratinized epithelium, both immunoreactive towards cytokeratin. The inner implant epithelium terminated in a junctional epithelium, apically not a few cell layers thick. The cells adjacent to the implant showed a condensed cytoplasm that resembled hemidesmosomes [1004, 1007, 1030] – i.e., good biocompatibility.
No significant difference in subgingival microflora is observed between dental plaque that accumulates on natural teeth or on single-crystal sapphire dental implants in the mandibular and maxillary edentulous regions in monkeys  and humans . Microbiota in healthy and diseased implant and natural tooth sites are very similar . Peri-implant tissues behave very similarly to periodontal tissues . Investigations of possible prophylactic treatments of sapphire-coated dental implants have been undertaken by SEM . One minor difference that has been observed between natural and implant dentition is that the tactile sensibility threshold can be slightly higher for sapphire implants than for natural teeth, among sensory receptors located in the connective tissues around the implants .
Last updated on 30 April 2004