Peri-implant tissue response to three implants engaging retained roots: a histologic report in the pig mandible

PURPOSE: The biological response of dental tissues to implants placed in contact with retained roots is poorly documented. The aim of this report is to describe the histological findings of three implants that inadvertently engaged inflammation-free retained root fragments during placement and were retrieved after 10 and 12 weeks of healing in the pig mandible. MATERIALS AND METHODS: Three implants were retrieved from a preclinical pig model in which fractured root remnants were inadvertently left in situ following tooth extraction; implants were placed after a three-month healing period and were harvested after 10 and 12 weeks. Histological evaluation was carried out using optical microscopy, polarized light microscopy, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy to characterize the tissue responses at the implant-root interface. Three configurations of implant-root contact were identified: (1) an implant fully embedded within a retained root composed of dentin and osteodentin; (2) an implant presenting apical contact with a retained root; and (3) an implant apex engaging fractured-root debris. RESULTS: The implant fully embedded within the retained root exhibited uninterrupted contact with dentin and osteodentin, with no intervening soft-tissue layer. Areas of tight implant-dentin apposition were devoid of cellular activity; conversely, in regions where microgaps or interfacial space were present, osteodentin showed active modeling and remodeling along the implant surface. In the second and third samples, osseointegration was observed along the implant body in contact with bone despite the presence of apical contact with residual root tissues. Newly formed osteodentin was intimately adapted to the implant apex, whereas the periodontal ligament that persisted did not extend appreciably onto the implant surface. CONCLUSIONS: This preclinical report describes the histological response to three implants in contact with non-infected retained roots. In two specimens with partial root contact, bone was present at the interface and osseointegration was observed. In all three specimens, newly formed osteodentin was present at the implant interface, with evidence of early modeling and remodeling. Further controlled investigations are required to confirm these observations and to assess their reproducibility and broader biological relevance.