Abstract
This inquiry investigated the suitability of the jaw of domestic sheep as an animal model for dental implantology research. Initially, parameters for osseous healing of critical size defects (CSD) in the sheep mandible were established. Pilot studies were conducted using machined-surface implants and a surgical protocol established for dental implant placement in ovine mandibular sites. Subsequent experiments considered the utility of this animal model for examination of techniques designed to enhance osseointegration. Hydroxyapatite-coated implants were compared with titanium plasma-sprayed (TPS) implants, either alone or combined with autogenous bone grafts or a bone graft/collagen vehicle loaded with transforming growth factor-beta (TGF-B). Immunofluorescent bone labelling gave information on the mineral apposition rate (MAR). Implant survival and "acceptability" (likelihood of clinical success) were major output variables, along with histomorphometric analysis of percent bone-implant contact (%BIC) and percent peri-implant bone density (%density). Naturally-occurring "broken-mouth" periodontitis in sheep was identified as a potential confounder. Subsequent experiments considered implants with different surfaces. The model was also extended from a two-stage surgical protocol to include single-stage implants. The effect of pre-existing ovine peridontitis was also examined. A systematic review and meta-analysis of published animal implant experiments was conducted in order to validate the candidate sheep model.
Major findings were as follows. The size of non-healing sheep mandibular unicortical CSD is > 12mm. Attempts to establish a chronic non-healing CSD were unsuccessful. The sheep diastema proved unsuitable for implant placement. The model was modified to a postextraction protocol. Implant "acceptability" rates after 3 months integration in the sheep mandible (defined as implant survival with %BIC >10%) ranged from 50% - 100% for different implant surface treatments and placement protocols. Histomorphometric analyses revealed that %BIC ranged from 11 ± 17% to 81 ± 29 % for different titanium surfaces and up to 85 ± 11 % for hydroxyapatite surfaces. Implants with TGF-B plus autogenous bone grafts had %BIC of 36 ± 30% compared with 43 ± 30% for implants with grafts alone. Bone per unit area (%density) adjacent to, but outside of the implant threads, ranged from 63 ± 16% to 86 ± 3% and was markedly lower for titanium plasma-sprayed surfaces and for one-stage implants. Within the implant threads, %density varied from 31 ± 33% to 73.4 ± 8.3%, and was markedly lower for machined titanium surfaces. Sheep periodontitis had little effect on the protocols investigated. The sheep mandibular model was found to be comparable to similar models in other species and merits further development.