Morphology of neural endings in the human periodontal ligament: an electron microscopic study

In vivo measurement of strain in the periodontal space of pig (Sus scrofa) incisors using in-fiber Bragg sensors

The incisor teeth in pigs, Sus scrofa, function in association with a disc-shaped snout to explore the environment for potential food. Understanding how mechanical loading applied to the tooth deforms the periodontal ligament (PDL) is important to determining the role of periodontal mechanoreceptors during food exploration and feeding. The objective of this study was to use fiber Bragg (FBG) sensors to measure strain in vivo within the PDL space of pig incisors. The central mandibular incisors of pigs underwent spring loaded lingual tipping during FBG strain recording within the labial periodontal space. FBG sensors were placed within the periodontal space of the central mandibular incisors of ~2-3-month-old farm pigs. The magnitude and orientation of spring loads are expected to mimic incisor contact with food. During incisor tipping with load calibrated springs, FBG strains in vitro (N = 6) and in vivo (N = 6) recorded at comparable load levels overlapped in range (-10-20 με). Linear regressions between peak FBG strains, that is, the highest recorded strain value, and baseline strains, that is, strain without applied spring load, were significant across all in vivo experiments (peak strain at 200 g vs. baseline, p = .04; peak strain at 2000 g vs. baseline p = .03; peak strain at 2000 g vs. 200 g, p = .004). These linear relationships indicate that on a per experiment basis, the maximum measured strain at different spring loads showed predictable differences. A Friedman test of the absolute value of peak strain confirmed the significant increase in strain between baseline, 200 g, and 2000 g spring activation (p = .02). Mainly compressive strains were recorded in the labial PDL space and increases in spring load applied in vivo generated increases in FBG strain measurements. These results demonstrate the capacity for FBG sensors to be used in vivo to assess transmission of occlusal loads through the periodontium. PDL strain is associated with mechanoreceptor stimulation and is expected to affect the functional morphology of the incisors. The overall low levels of strain observed may correspond with the robust functional morphology of pig incisors and the tendency for pigs to encounter diverse foods and substrates during food exploration.

Nociceptor-Macrophage Interactions in Apical Periodontitis: How Biomolecules Link Inflammation with Pain

Periradicular tissues have a rich supply of peripheral afferent neurons, also known as nociceptive neurons, originating from the trigeminal nerve. While their primary function is to relay pain signals to the brain, these are known to be involved in modulating innate and adaptive immunity by initiating neurogenic inflammation (NI). Studies have investigated neuroanatomy and measured the levels of biomolecules such as cytokines and neuropeptides in human saliva, gingival crevicular fluid, or blood/serum samples in apical periodontitis (AP) to validate the possible role of trigeminal nociceptors in inflammation and tissue regeneration. However, the contributions of nociceptors and the mechanisms involved in the neuro-immune interactions in AP are not fully understood. This narrative review addresses the complex biomolecular interactions of trigeminal nociceptors with macrophages, the effector cells of the innate immune system, in the clinical manifestations of AP.

The mechanism for regulating the isometric contraction of masseter muscles is involved in determining the vertical dimension of occlusion

We previously demonstrated that accurate regulation of isometric contraction (IC) of jaw-closing muscles to counteract the ramp load applied to the jaw in the jaw-opening direction is achieved through the calibration between the two sensations arising from muscle spindles (MSs) and periodontal mechanoreceptors (PMRs). However, it remains unclear whether this calibration mechanism accurately works at any jaw positions, i.e., any vertical dimensions of occlusion (VDO). In the present study, we examined the effects of altering VDO on the IC of the masseter muscles in complete dentulous and edentulous subjects. At a VDO higher than the original VDO (O-VDO), the root mean square (RMS) of masseter EMG activity increased more steeply with a load increase, resulting in an over-counteraction. The regression coefficient of the load-RMS relationship significantly increased as the VDO was increased, suggesting that the overestimation became more pronounced with the VDO increases. Consistently also in the edentulous subjects, at a higher VDO than the O-VDO, a steeper increase in the RMS emerged with a delay in response to the same ramp load whereas a similar steeper increase was seen surprisingly even at a lower VDO. Thus, the edentulous subjects displayed a delayed overestimation of the ramp load presumably due to less and slowly sensitive mucous membrane mechanoreceptor (MMR) in alveolar ridge compared with the PMR. Taken together, the accurate calibration between the two sensations arising from MSs and PMRs/MMRs can be done only at the O-VDO, suggesting that the O-VDO is the best calibration point for performing accurate IC.NEW & NOTEWORTHY Since 1934, the vertical dimension of occlusion (VDO) in edentulous individuals has been anatomically determined mostly by referring to the resting jaw position. However, such a static method is not always accurate. Considering the dynamic nature of clenching/mastication, it is desirable to determine VDO dynamically. We demonstrate that VDO can be accurately determined by measuring masseter EMG during the voluntary isometric contraction of jaw-closing muscles exerted against the ramp load in the jaw-opening direction.

Importance of Osseoperception and Tactile Sensibility during Masticatory Function in Different Prosthetic Rehabilitations: A Review

Background and Objectives: Tactile sensibility is an important characteristic for evaluating the masticatory efficiency in different occlusal situations. When a tooth is extracted, relevant proprioceptors from the periodontal ligament get lost; and after the rehabilitation of this abscess by means of oral prosthesis, this sensibility decreases influencing masticatory function. Osseoperception is a sensitive phenomenon associated with dental implants that allows an increased tactile sensibility to those wearing implant prostheses. The purpose of this study was to determine the difference in tactile sensibility values between implant prosthesis, complete dentures, and natural teeth through a review of the available literature. Materials and Methods. In order to dissect the information, 24 articles from 2004 to 2021 were analyzed from MEDLINE, PubMed Central, and Web of Science databases. These articles were directly related to measuring tactile sensibility in different situations and demonstrating the influence of osseoperception in an improved masticatory function. Results: Tactile sensibility in implant prosthesis is slightly reduced compared with natural dentition but presents improved values with regard to complete dentures. Conclusions: Implant prosthesis are more effective during masticatory function than complete dentures, as they present an increased tactile sensibility, very similar to that present in natural dentition. This enhanced sensibility in implants is due to the osseoperception phenomenon.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Immunohistochemical Analysis of Nerve Distribution in Mandible of Rats

After review of the literature, there appears to be no report on the histology of the mandibular nerve fiber distribution. Therefore, using a Wistar rat model, immunohistochemical staining with protein gene product (PGP) and calcitonin gene-related peptide (CGRP) antibody for all nerves and only the pain-sensitive nerves, respectively, was performed. We also statistically compared the nerve distribution density by mandibular region. The section of the mandible from the alveolar crest to the mandibular canal was compartmentalized to several regions. Subsequently, nerve distribution density by region was measured microscopically in both the PGP- and CGRP-positive nerves. Furthermore, the ratio of CGRP- to PGP-positive nerves was measured in each region and statistically compared. In both the PGP- and CGRP-positive nerves, the nerve distribution density significantly increased vertically toward the mandibular canal from the alveolar crest and horizontally toward the periodontal ligament from the periosteum. From the CGRP- to PGP-positive nerve ratio, the pain-sensitive nerve accounted for approximately >70% in each region. Pain would therefore be more likely to develop when surgical invasiveness deepens toward the mandibular canal or periodontal ligament. Therefore, sufficient local anesthetic infiltration and/or combined use of conduction anesthesia or periodontal ligament injection may be required. These results may aid in the development of more effective surgical and anesthetic techniques for mandibular surgery.

Effect of platelet-rich and platelet-poor plasma on peri-implant innervation in dog mandibles

Background

Autologous plasma fractions, such as platelet-rich plasma (PRP) and platelet-poor plasma (PPP), contain growth factors that can enhance neural cell survival and are therefore likely to have the ability to promote nerve regeneration. The present study compared the effect of PRP and PPP application on myelinated nerve density and diameter in the peri-implant bone region. In addition, the effect of healing time on nerve regeneration was assessed.

Materials and methods

Nine beagle dogs randomly received 54 dental implants in the bilateral mandible according to a split-mouth design. Each implant was randomly assigned to one of three implant protocols: delayed implant placement with delayed loading (DIP + DL) with local application of PRP, DIP + DL with local application of PPP and DIP + DL without any plasma additive. The animals were euthanized at 1, 3, and 6 months after loading (3 dogs per time point). Block biopsies were prepared for histomorphometry in the peri-implant bone within 500 μm around the implants.

Results

Myelinated nerve fibers were identified in the trabecular bone and in the osteons near the implants surface. The nerve fibers in the PRP group (median ± IQR; 2.88 ± 1.55 μm) had a significantly (p < 0.05) greater diameter compared to the PPP (2.40 ± 0.91 μm) and control (2.11 ± 1.16 μm) group. The nerve diameter after 6 months healing (3.18 ± 1.58 μm) was significantly (p < 0.05) greater compared to 1 (2.08 ± 0.89 μm) and 3 (2.49 ± 1.22 μm) months. No significant difference was found for myelinated nerve density between groups and healing time.

Conclusions

The present study showed that the healing time significantly influenced the diameter of the myelinated nerve fibers in peri-implant bone. PRP exerted a significant effect on the diameter of the myelinated nerve fibers as compared to PPP. Large-scale animal studies and longer follow-up periods are needed to confirm these findings and to verify whether platelet plasma can facilitate nerve regeneration process.

Occlusal sensitivity in individuals with different frequencies of oral parafunction

STATEMENT OF PROBLEM

Oral parafunctional behaviors are characterized by an overuse of the masticatory muscles, thus applying aberrant occlusal force to the teeth for prolonged periods. However, whether increased periodontal receptor activity in individuals with a high degree of oral parafunction alters the occlusal sensitivity is unclear.

PURPOSE

The purpose of this clinical study was to evaluate occlusal sensitivity in adults reporting a high or low frequency of awake oral parafunctional behaviors, as assessed with the short version of the oral behavior checklist (OBC-It 6).

MATERIAL AND METHODS

Self-reporting questionnaires (N=212) were distributed among dental and medical students. Individuals with an OBC-It 6 score below the 20th percentile (low-frequency parafunction (LFP) group: 13 men and 15 women) and above the 80th percentile (high-frequency parafunction [HFP] group: 11 men and 17 women) were selected for the occlusal sensitivity assessment. The occlusal sensitivity was tested with 10 different thicknesses: 9 aluminum foils ranging from 8 μm to 72 μm with a constant increment of 8 μm and 1 sham test without any foil. The testing foils were presented 10 times in random order (100 tests in total). The participants were instructed to close their mouth only once and to report whether they felt the aluminum foil between their teeth. A between-group comparison (HFP versus LFP) was performed for each testing thickness (Student t test for unpaired data, Bonferroni correction) (α=.005).

RESULTS

For the sham test and for the testing thicknesses between 8 μm and 48 μm, no statistically significant differences were found between the 2 groups. The thicknesses 0.56 mm, 0.64 mm, and 0.72 mm were significantly better perceived in the HFP group than in the LFP group (P<.005, P<.001, and P<.001).

CONCLUSIONS

Individuals with a high frequency of self-reported awake oral parafunction presented higher occlusal sensitivity.

Single-channel Recordings of TREK-1 K+ Channels in Periodontal Ligament Fibroblasts

The periodontal ligament (PDL) works as a suspensory ligament when external mechanical stress is placed on the teeth. PDL fibroblasts, the principal cells in the PDL, are responsible for many PDL functions. We hypothesized that mechanosensitive ion channels are present in human PDL fibroblasts, which are capable of responding to mechanical stress during normal function of the tissue. Using patch-clamp techniques, we detected mechanosensitive TREK-1 K+ channels (a member of the two-pore-domain K+ channel family), whose single-channel conductance was 104 pS in symmetrical K+-rich solutions. The open probability of the channel was low in the quiescent state, but it was strongly increased by the induction of membrane stretch. Arachidonic acid also enhanced the channel activity. RT-PCR and immunocytochemical observations showed the expression of TREK-1 K+ channels in PDL fibroblasts. The results suggest that the activation of TREK-1 K+ channels by masticatory stress contributes to the hyperpolarization of PDL fibroblasts.

Osseoperception in Dental Implants: A Systematic Review

PURPOSE

Replacement of lost teeth has significant functional and psychosocial effects. The capability of osseointegrated dental implants to transmit a certain amount of sensibility is still unclear. The phenomenon of developing a certain amount of tactile sensibility through osseointegrated dental implants is called osseoperception. The aim of this article is to evaluate the available literature to find osseoperception associated with dental implants.

MATERIALS AND METHODS

To identify suitable literature, an electronic search was performed using Medline and PubMed database. Articles published in English and articles whose abstract is available in English were included. The articles included in the review were based on osseoperception, tactile sensation, and neurophysiological mechanoreceptors in relation to dental implants. Articles on peri-implantitis and infection-related sensitivity were not included. Review articles without the original data were excluded, although references to potentially pertinent articles were noted for further follow-up. The phenomenon of osseoperception remains a matter of debate, so the search strategy mainly focused on articles on osseoperception and tactile sensibility of dental implants. This review presents the histological, neurophysiological, and psychophysical evidence of osseoperception and also the role of mechanoreceptors in osseoperception.

RESULTS

The literature on osseoperception in dental implants is very scarce. The initial literature search resulted in 90 articles, of which 81 articles that fulfilled the inclusion criteria were included in this systematic review.

CONCLUSION

Patients restored with implant-supported prostheses reported improved tactile and motor function when compared with patients wearing complete dentures.

A systematic review on the innervation of peri-implant tissues with special emphasis on the influence of implant placement and loading protocols

OBJECTIVES

To systematically review the available literature on the influence of dental implant placement and loading protocols on peri-implant innervation.

MATERIAL AND METHODS

The database MEDLINE, Cochrane, EMBASE, Web of Science, LILACS, OpenGrey and hand searching were used to identify the studies published up to July 2013, with a populations, exposures and outcomes (PEO) search strategy using MeSH keywords, focusing on the question: Is there, and if so, what is the effect of time between tooth extraction and implant placement or implant loading on neural fibre content in the peri-implant hard and soft tissues?

RESULTS

Of 683 titles retrieved based on the standardized search strategy, only 10 articles fulfilled the inclusion criteria, five evaluating the innervation of peri-implant epithelium, five elucidating the sensory function in peri-implant bone. Three included studies were considered having a methodology of medium quality and the rest were at low quality. All those papers reported a sensory innervation around osseointegrated implants, either in the bone-implant interface or peri-implant epithelium, which expressed a particular innervation pattern. Compared to unloaded implants or extraction sites without implantation, a significant higher density of nerve fibres around loaded dental implants was confirmed.

CONCLUSIONS

To date, the published literature describes peri-implant innervation with a distinct pattern in hard and soft tissues. Implant loading seems to increase the density of nerve fibres in peri-implant tissues, with insufficient evidence to distinguish between the innervation patterns following immediate and delayed implant placement and loading protocols. Variability in study design and loading protocols across the literature and a high risk of bias in the studies included may contribute to this inconsistency, revealing the need for more uniformity in reporting, randomized controlled trials, longer observation periods and standardization of protocols.

Changes in the Distribution of Periodontal Nerve Fibers during Dentition Transition in the Cat

The periodontal ligament has a rich sensory nerve supply which originates from the trigeminal ganglion and trigeminal mesencephalic nucleus. Although various types of mechanoreceptors have been reported in the periodontal ligament, the Ruffini ending is an essential one. It is unknown whether the distribution of periodontal nerve fibers in deciduous teeth is identical to that in permanent teeth or not. Moreover, morphological changes in the distribution of periodontal nerve fibers during resorption of deciduous teeth and eruption of successional permanent teeth in diphyodont animals have not been reported in detail. Therefore, in this study, we examined changes in the distribution of periodontal nerve fibers in the cat during changes in dentition (i.e., deciduous, mixed and permanent dentition) by immunohistochemistry of protein gene product 9.5. During deciduous dentition, periodontal nerve fibers were concentrated at the apical portion, and sparsely distributed in the periodontal ligament of deciduous molars. During mixed dentition, the periodontal nerve fibers of deciduous molars showed degenerative profiles during resorption. In permanent dentition, the periodontal nerve fibers of permanent premolars, the successors of deciduous molars, increased in number. Similar to permanent premolars, the periodontal nerve fibers of permanent molars, having no predecessors, increased in number, and were densely present in the apical portion. The present results indicate that the distribution of periodontal nerve fibers in deciduous dentition is almost identical to that in permanent dentition although the number of periodontal nerve fibers in deciduous dentition was low. The sparse distribution of periodontal nerve fibers in deciduous dentition agrees with clinical evidence that children are less sensitive to tooth stimulation than adults.

CGRP-alpha application: a potential treatment to improve osseoperception of endosseous dental implants

Dental implants have been used to restore missing teeth for several decades. However, the capacity of implants to feel the mechanical stimuli and transmit neural signals remains lower than that of natural teeth. The poor osseoperception of dental implants is due to the absence of periodontal ligaments and Ruffini-like endings as well as the secondary injury during the implant surgery and then the insufficient regeneration of damaged peripheral nerve fibers around the implants. It is a hot topic to improve the quantity and density of peripheral nerve fibers or mechanoreceptors around endosseous dental implants. Calcitonin gene-related peptide-alpha (αCGRP), a neuropeptide widely distributed throughout the central and peripheral nervous systems, is found to be upregulated in regenerating axons within injury zones and be capable of promoting local Schwann cells proliferation, which is critical for partnering during peripheral nerve regeneration. Moreover, researches show that αCGRP is a potent vasodilator and a physiologic activator of bone formation. Thus, we hypothesize that local application of αCGRP may promote peripheral nerve fibers regeneration during the bone healing progress after dental implant surgery, thus improve the osseoperception of dental implants.

Histomorphological study of myelinated nerve fibres in the periodontal ligament of human canine

OBJECTIVE

This study aims to describe the human periodontal ligament (PDL) using serial sections, with a focus on mechanoreceptor distribution and morphology.

MATERIALS AND METHODS

One permanent lower canine with surrounding PDL and alveolar bone tissues was retrieved from a human cadaver. After being embedded into paraffin block, the canine was horizontally cut in 6 μm thick serial sections. At root levels of 0.3, 1.5, 3, 4.5 and 6 mm from the apex, five slices each level were evaluated. Immunocytochemisty was performed on the same serial sections, enabling a more reliable description of neural structures.

RESULTS

The distribution of myelinated fibres varied from apical to coronal level, with a total number of 38 at 0.3 mm from the apex, 25 at 1.5 mm, 25 at 3 mm, 31 at 4.5 mm and 32 at 6 mm. At all times, mesial and buccal regions were typically more densely innervated (p < 0.01) except at the 3 mm level. The average density of myelinated nerve fibres increased by arriving closer to the apex. However, the average diameter did not show any significant differences amongst quadrants or root levels (p > 0.05). The average diameter of myelinated fibres varied between 5.3-7.8 μm. Grouped myelinated axons were twice as common as isolated ones, with the innervation being rather close to the alveolar bone. Isolated myelinated axons showed a tendency to group around large blood vessels.

CONCLUSION

The present results add to the understanding of human PDL innervation, indicating dense innervations by myelinated nerve fibres in close proximity to collagen fibres and alveolar bone. It also reveals that apical as well as mesial and buccal sites of the human canine are more densely innervated.

Peripheral nerve may regulate the jaw bone resorption after tooth extraction

A various amount of alveolar ridge resorption is likely to occur after tooth extraction, making it difficult to restore the missing teeth with either endosseous dental implants or prosthodontics approaches. It is commonly thought that the bone resorption is due to the absence of the mechanical stimulation from the occlusal force. However, regulation of the bone mass is a complex homeostatic system involved in hormonal, paracrine/autocrine, mechanical and neuronal nature. Studies have shown that the central and peripheral nervous system play an important role in bone remodeling. The hypothalamus integrates peripheral and central signals, and sends efferent hormonal and neuronal signals in response to stimulation. Numerous neuropeptides detected in the bone marrow have effect on the osteoblast and osteoclast. After tooth extraction, great loss of the axons is observed in the edentulous bone. So we speculate that innervation in the alveolar bone regulates the bone resorption in edentulous area. Methods to promote the nerve regeneration are expected to prevent the jaw bone resorption. The hypothesis also implies that after the placement of the oral implant the abundant nerves in the alveolar bone can increase the bone healing ability and long term survival rate of the implant.

The presence of arachidonic acid-activated K+ channel, TREK-1, in human periodontal ligament fibroblasts

Human periodontal ligament (PDL) fibroblasts expressed following two-pore-domain K(+) channels, TWIK-2 > TREK-1 > TWIK-1 >> TASK-1 > TRAAK > TASK-2. TREK-2 message was not detectable. We found the presence of arachidonic acid-activated and mechanical stress-sensitive K(+) channel, TREK-1, in the PDL fibroblasts by patch-clamp technique. It was also found the significant increase of intracellular concentration of arachidonic acid upon the application of cyclic stretch. Therefore, we suppose that the mechanical stretch due to the mastication activates phospholipase A(2) to release arachidonic acid (AA) from membrane, then, the released AA activates TREK-1. Thus, TREK-1 K(+) channels may play a protective role to maintain the negative membrane potential of PDL fibroblasts against the environmental stimuli.

Reflex control of human jaw muscles

The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects.

From osseoperception to implant-mediated sensory-motor interactions and related clinical implications

Osseointegration of implants in the jaw bone has been studied thoroughly, dealing with various aspects such as bone apposition, bone quality, microbiology, biomechanics, aesthetics, etc. A key issue that has received much less attention is the physiologic integration of the implant(s) and the associated prosthesis in the body. The latter aspect is however very important to obtain new insights in oral functioning with implant-supported prostheses. Amputated patients rehabilitated with a lower limb prosthesis anchored to the bone by means of an osseointegrated implant, have reported that they could recognize the type of soil they were walking on. Clinical observations on patients with oral implants, have confirmed a special sensory perception skill. The underlying mechanism of this so-called 'osseoperception' phenomenon remains a matter of debate, because extraction of teeth involves elimination of the extremely sensitive periodontal ligaments while functional reinnervation around implants is still uncertain. Histological, neurophysiological and psychophysical evidence of osseoperception have been collected, making the assumption more likely that a proper peripheral feedback pathway can be restored when using osseointegrated implants. This implant-mediated sensory-motor control may have important clinical implications, because a more natural functioning with implant-supported prostheses can be attempted. It may open doors for global integration in the human body.

The neurophysiology of osseointegrated oral implants. A clinically underestimated aspect

Amputation of a limb or a tooth leads to the loss of a large number of exteroceptors. These play an important role in sensory perception and feedback, which tune the motor control. Even after rehabilitation with a prosthetic device, tactile function remains impaired. This can present a subsequent risk of overloading the prosthesis. The peri-implant interface of oral osseointegrated implants is characterized by the absence of a periodontal ligament, a crucial difference towards the natural dentition, from a biomechanical but also from a neurophysiologic point of view. Patients rehabilitated with osseointegrated implants seem subjectively not much impaired in their masticatory and other oral functions. This observation might be linked to the presence of some peripheral feedback pathway to the sensory cortex. It may be attributed to the activation of receptors in the peri-implant environment (either bone or periosteum). This phenomenon called osseoperception, when it relates to the consciousness of the applied stimuli, has been described for both oral and skeletal implants. In the present review, besides osseoperecption other neurophysiological aspects of oral implants, such their reflex function, will be outlined and their clinical meaning pointed out.

Unilateral Severe Chronic Periodontitis Associated With Ipsilateral Surgical Resection of Cranial Nerves V, VI, and VII

BACKGROUND

The central and peripheral nervous systems participate in several local physiological and pathological processes. There is experimental evidence that the inflammatory, local immune, and wound healing responses of a tissue can be modulated by its innervation. The aim of this clinical report is to present a case of unilateral severe periodontitis associated with ipsilateral surgical resection of the fifth, sixth, and seventh cranial nerves and to discuss the possible contribution of the nervous system to periodontal pathogenesis.

METHODS

A 39-year-old female patient with a history of a cerebrovascular accident caused by a right pontine arteriovenous malformation and destruction of the right fifth, sixth, and seventh cranial nerves was diagnosed with severe chronic periodontitis affecting only the right maxillary and mandibular quadrants. The patient's oral hygiene was similar for right and left sides of the mouth. Percentages of tooth surfaces carrying dental plaque were 41% and 36% for right and left sides, respectively. Non-surgical and surgical periodontal therapy was performed, and the patient was placed on a regular periodontal maintenance schedule.

RESULTS

Healing following initial periodontal therapy and osseous periodontal surgery occurred without complications. Follow-up clinical findings at 1 year revealed stable periodontal health.

CONCLUSIONS

This case report suggests that periodontal innervation may contribute to the regulation of local processes involved in periodontitis pathogenesis. It also suggests that periodontal therapy can be performed successfully at sites and in patients affected by paralysis.

Analysis of strain around endosseous dental implants opposing natural teeth or implants

STATEMENT OF PROBLEM

Natural teeth and implants have different force transmission characteristics to bone.

PURPOSE

The purpose of this study was to compare strains induced around a natural tooth opposing an implant with strains around occluding implants under static and dynamic loads.

MATERIAL AND METHODS

Occlusion was created between a natural molar tooth and an implant in 1 side, and 2 implants in the contralateral side of acrylic resin models of both jaws. Strain-gauges were bonded around the neck of the natural tooth and implants, and strains were measured under 75 N and 100 N static axial and lateral dynamic loads in separate load situations using a data acquisition system at sample rate of 1000 Hz. The strain data of the natural tooth and implants were compared for each load by Kruskal-Wallis testing followed by the multiple comparison test (alpha=.05).

RESULTS

Compressive strains were induced around natural tooth and implants as a result of static axial loading, whereas combinations of compressive and tensile strains were observed during lateral dynamic loading. Strains around the natural tooth were significantly lower than the opposing implant and occluding implants in the contralateral side for most regions under all loading conditions ( P <.05). There was a general tendency for increased strains around the implant opposing natural tooth under higher loads and particularly under lateral dynamic loading ( P <.05).

CONCLUSION

Under static and dynamic loads, strain magnitudes around a natural tooth were significantly lower than that of an opposing implant and occluding implants in the contralateral side.

Ultrastructures of mechanoreceptors in the oral mucosa

The present review describes the fine structures of lamellated mechanoreceptive corpuscles, Merkel cell-neurite complexes and free nerve endings in the oral mucosae of mammals, with special attention to axon terminals and lamellar cells. The mechanoreceptive nerve endings of the oral mucosa were studied using histochemistry, immunohistochemistry and transmission electron microscopy techniques. The organized mechanoreceptive corpuscles are present in the mucosae of gingiva, cheek, tongue and soft and hard palate. They are elongated or globular in shape, being located in the connective tissue papillae. The capsule is composed of several layers of cytoplasmic extensions of perineural cells. Numerous bundles of collagen fibers are noted at the periphery of the corpuscle. The lamellated corpuscles are surrounded by several layers of superimposed flattened capsular cell processes. The interlamellar spaces are 0.2-0.4 micron in width and filled with thin fibrillar collagen fibers embedded in the amorphous substance. The lamellar cells contain rich microtubules and are characterized by the presence of caveolae on the surface plasma membrane. The terminal axon contains an abundance of mitochondria and small clear vesicles (20-50 nm in diameter). There are neurofilaments in the center of the axon terminal. Intermediate-type junctions are seen between the adjacent lamellar cells and between the axon and adjacent lamellae. The free nerve endings are found in the subepithelial regions, very close to the basal laminae of mucosal epithelium. They are surrounded by a thin cytoplasm of Schwann cells. Sometimes Schwann cell basal larinae become multilayered. Merkel cells are present within the basal layer of mucosal epithelium and contain characteristic electron-dense granules that are located almost exclusively at the side of cytoplasm in contact with axon terminals. Intermediate-type junctions are noted between axon terminals and Merkel cells.

Effects of machined/turned, TiO2-blasted and sandblasted/acid-etched titanium oral implant surfaces on nerve conduction: A study on isolated rat sciatic nerves

The purpose of this study was to explore the clinical relevance of the effects of machined/turned, TiO(2)-blasted and sandblasted/acid-etched titanium oral implant surfaces on nerve conduction. Isolated rat sciatic nerves were placed between two suction electrodes in a pyrex bath containing a tyrode solution. Evoked compound action potentials (cAPs) of the nerves were recorded before and after contact with the implants. The mandibular incisors of randomly selected animals were extracted and changes in cAP amplitudes were used as controls. The differences in final cAP values of Astra Tech implants and rat natural teeth were insignificant (P < 0.05), whereas the differences between other groups were significant (P < 0.05). Machined/turned-surface implants did not cause any change in cAPs. A slight decrease in cAPs was observed for TiO(2)-blasted and sandblasted/acid-etched implants, and the natural teeth. The reductions of cAPs in latter groups were not 50% after an application time of 300 min. The cAP changes of nerves contacting TiO(2)-blasted and sandblasted/acid-etched oral implants fall within physiologic limits in vitro. Machined/turned, TiO(2)-blasted, and sandblasted/acid-etched titanium implant surfaces do not lead to irreversible neurotoxic effects.

Receptive field properties of human periodontal afferents responding to loading of premolar and molar teeth

Impulses in 45 single mechanoreceptive afferents were recorded from the human inferior alveolar nerve with permucosally inserted tungsten microelectrodes. All afferents responded to mechanical stimulation of one or more premolar or molar teeth and most likely innervated their periodontal ligaments. For each afferent, isolated "ramp-and-hold" shaped force profiles of similar magnitudes (252 +/- 24 mN; mean +/- SD) were applied to the lower first premolar, the second premolar, and the first molar on the recording side. The tooth loads were applied in six directions: lingual, facial, mesial, and distal in the horizontal plane and up and down in the vertical direction of the tooth. The afferents response during the static phase of the stimulus was analyzed. All afferents were slowly adapting, discharging continuously in response to static forces in at least one stimulation direction. Twenty-nine afferents (64%) were spontaneously active, exhibiting an ongoing discharge in the absence of external stimulation. Stimulation of a single tooth was found to excite each afferent most strongly. The most sensitive tooth (MST) was the first premolar for 23, the second premolar for 13, and the first molar for 9 afferents. About half of the afferent population also responded to loading of one or two more teeth. The response profiles of these afferents indicated that the multiple-teeth receptive fields were due to mechanical coupling between the teeth rather than branching of single afferents to innervate several teeth. The afferent responses to loading the mesial and distal halves of the first molars were very similar. Thus both intensive and directional aspects of the afferent response when loading one side of the tooth was preserved to a great extent when loading the other side. When loading the MST, the afferents typically showed excitatory responses in two to four of the six stimulation directions, i.e., the afferents were broadly tuned to direction of tooth loading. In the horizontal plane, the afferent populations at the premolar teeth expressed no clear directional preferences. The afferents at the molar, however, showed a strong directional bias in the distal-lingual direction. In the vertical plane, there was a preference for downward-directed forces with a gradually decreasing sensitivity distally along the dental arch. The present results demonstrate that human periodontal afferents supplying anterior and posterior teeth differ in their capacity to signal horizontal and vertical forces, respectively.

Distribution of periodontal afferent input to motoneurons of human masseter

The distribution of the synaptic input from the periodontal mechanoreceptors onto the motoneurons of the human masseter is studied. Periodontal mechanoreceptors were activated using slowly rising force profiles of 2.5 N, which are known to induce predominantly excitatory reflex responses in the surface electromyogram (EMG) of the masseter. The reflex responses of single motor units (SMUs) were recorded to quantify the distribution of the periodontal input onto the masseter motoneurons. The relative sizes of motoneurons were estimated by comparing the peak-to-peak amplitude of the MacroRep (i.e. the representation of the SMU in the Macro EMG record). It was found that the larger SMUs had more excitatory and less inhibitory reflex responses than those of smaller size. This study demonstrates that the inputs from the periodontal mechanoreceptors, activated by slowly rising force profiles, are not distributed equally to the masseteric motoneurons. This may cause recruitment of motoneurons contrary to the size principle under some circumstances.

Reflex responses induced by tooth unloading

The reflex response of the masseter muscle to the rapid unloading of a single maxillary incisor tooth was studied. Unloading of a static force of 2 N in the horizontal direction resulted in a short-latency excitation, inhibition, and long-latency excitation of masseter muscle activity occurring at latencies of approximately 13, 20, and 40 ms, respectively, with a corresponding change in bite force occurring slightly later in each case. Following the blocking of periodontal input by the injection of local anesthetic around the stimulated tooth, inhibitory responses were abolished. Therefore, it is concluded that the observed masseteric inhibition was caused by the unloading of periodontal mechanoreceptors and thus that these receptors may contribute to the jaw unloading reflex.

Role of periodontal ligament receptors in the tactile function of teeth: a review

The tactile function of the human periodontal mechanoreceptors has mostly been studied by psychophysical approaches. It was concluded that periodontal mechanoreceptors play a major role in the tactile function of teeth. It must be noted however that the interocclusal tactile threshold is not solely determined by periodontal mechanoreceptors but also by pulpal, muscular or articular receptors. While temporomandibular joint receptors play a minor role, muscular receptors are important in the discriminatory ability for a mouth opening of 5 mm and more. To discriminate between the contribution of periodontal and other receptors in the oral tactile function, future studies should use appropriate psychophysical methodologies and well-defined stimulus parameters.