Immediate postextraction implant placement in sheep's mandibles: a pilot study

In this study, sheep were examined as a potential animal model for immediate implant placement in fresh extraction sockets using experimental photopolymerisable bioabsorbable polymers.

A total of 22 cylindrical implants were placed in fresh mandibular premolar extraction sockets of 7 sheep. Residual bone-implant voids were filled with a biocompatible composite of poly-methyl-methacrylate and poly-hydroxyl-ethyl-methacrylate (Bioplant 24). Photopolymerisation of a viscose mixture of experimental prepolymers and Bioplant 24 applied to the neck of the implants provided additional support before gingival closure. Clinical and radiographic controls were performed 30, 90, and 180 days after surgery. At 180 days postoperatively, the sheep were sacrificed and the mandibular segments were isolated for histological processing.

High cumulative implant failure rates of 45.5%, 63.6%, and 77.3% at respectively 30, 90 and 180 days were recorded. Significantly more implants were lost when the position of the neck was located above the level of the alveolar crest (P < 0.05). Clinical and histological observations demonstrated poor implant osseointegration characterized by ingrowth of soft tissue into the extraction sockets. Bone substitutes were lost in all cases.

Sheep have many practical advantages compared with other animal models. However, their specific oral biomechanics inherent to their constant ruminant activity accounted for a high degree of the reported implant failures. Important adaptations to the implantation technique and postoperative management will be necessary to use sheep as an animal model for future oral implant related experiments.