Periodontal ligament and peri innervation in humans: a basis for physiological integration of oral implants

Field: Dentistry (Health Sciences)

This thesis is composed of 2 literature reviews and 6 scientific studies focusing on the mechanosensory function related to periodontal ligament (PDL) and periimplant region. The scientific research first started with the exploration of PDL nerve fibre distribution in humans (1) and the investigation of special PDL structures, namely the epithelial rests of Malassez (ERM) (2). This was followed by the histological search for mechanoreceptors in the peri-implant tissue of humans (3). Besides these histological approaches, more clinically accessible tools were investigated as possible indication for physiological changes of bone, especially dental radiographs which are daily used in clinic. Thus, morphological aspects of periimplant bone were assessed by cone beam computed tomography (CBCT) and intraoral (IO) radiography, using histological imaging as gold-standard (4). Finally, the influence of implant treatment on mandibular anatomy and innervation was investigated using 3D CBCT images. For this, the variability of neurovascular canals in the mandibular bone had first to be addressed (5, 6). PDL innervation and mechanoreceptors have been extensively described according to their morphology, neurophysiological aspects, spatial arrangement and functional significance (chapter 2). Yet, researches exploring the 3D reconstruction of the PDL and mechanoreception function at cellular and molecular levels are expected to further our understanding of mechanosensory function in teeth. Three-dimensional volume reconstruction from 2D histological slices showed some potential in visualising the complex PDL anatomy, spatial arrangement and interrelationship among the different PDL structures (chapter 2). Regarding nerve fibre distribution in human PDL, bundles of nerve fibres were mostly found at the alveolar related part of the PDL and in the vicinity of blood vessels (chapter 3). The highest number of fibres was found at the buccal and mesial region as well as at the root apex. The diameter of PDL fibres ranged between 2-15µm, and those that were myelinated and in the range of 5-6µm were most frequently seen in the human PDL. Overall, the lingual region showed higher concentration of nerve fibres of larger diameter (8-9µm). The highest concentration of isolated fibres was found at the intermediate region between apex and tooth fulcrum, and this in the cemental part of the PDL. Other PDL special structures such as ERM and cementicles have been described in the literature, however their role in the PDL function is not fully understood (chapter 4). An altered ERM morphology after tooth autotransplantation suggested that this structure is related to PDL regeneration. Additional studies are needed to confirm this finding and to research the likely influence of this finding in PDL regeneration treatments. For the first time in humans, myelinated and unmyelinated nerve fibres were shown in the peri-implant bone mostly localized in the Haversian canals close to the bone-implant interface (chapter 5). However in this study, no structure even resembling a mechanoreceptor was observed in the peri-implant region, which does not explain why some PDL mechanoreceptor functions are partially restored after implant treatment. Therefore, the exact location and mechanism of the structures that would be responsible for those functions remains mostly unknown in fully implant rehabilitated patients. Regarding peri-implant bone tissue estimations, significant correlations could be observed between bone levels histologically assessed and bone levels measured on IO radiographs and CBCT images (chapter 6). Tissue parameters as measured on IO radiographs correlate significantly with some histomorphometric parameters. However, such correlation could not be established for CBCT images. An increased bone loss (>2mm) seemed more likely to occur at low density bones (<5mmAleq). No reliable information about geometrical arrangement of trabecular bone could be obtained from radiographic images since IO and CBCT fractal analysis did not correlate to histological fractal analysis. Using 3D CBCT scans, the anatomical variability of neurovascular canals of the mandible was addressed, not only between modern humans from different timeperiods and different geographical regions, but also between mandibles of human and non-human primates. This contributed to an elaborate overview about neurovascular canal anatomy and the relation with adjacent tooth roots (chapter 7). Geographically, anatomical features which characterize some populations could be related to potential surgical and pathological risks. Furthermore, the incisive canal is suggested to be a unique feature of human mandibles (chapter 7). Considering some study limitations, this thesis suggested that neurovascular canals do not change significantly after tooth extraction and that the resorption rate of mandibular bone after implant treatment in the first years after tooth extraction seemed to be about 50% less than the rate reported in literature (chapter 8). To conclude, PDL and peri-implant tissue were assessed to understand the underlying mechanisms of osseoperception influencing the oral implant rehabilitation. A special focus was also given to the innervation of those tissues, their functional relation and spatial arrangement with other adjacent structures.