Characterization of the peri-implant epithelium in hamster palatine mucosa: Behavior of Merkel cells and nerve endings

Distribution of mature and newly regenerated nerve fibers after tooth extraction and dental implant placement: an immunohistological study

BACKGROUND

The time-dependent peri-implant innervation needs to be elucidated in detail.

OBJECTIVES

To examine the distribution of mature and newly regenerated nerves around the implant with immunofluorescence during 28-days follow-up after implantation.

METHODS

35 male Sprague-Dawley rats were grouped into non-operated(n=5), extraction(n=5), and implant(n=25) groups. For rats in the extraction and implant groups, three right maxillary molars were extracted. One month later, a titanium implant was placed into the healed alveolar ridge in the implant group. The implant group was further divided into 5 subgroups according to day 1, 3, 7, 14, or 28 after implantation, on which day serial histological sections were prepared for immunohistochemistry. On day 28, the serial sections were also prepared in the non-operated and extraction groups. Soluble protein-100 and growth-associated protein-43 were used to immunolabel mature and newly regenerated nerve fibers respectively.

RESULTS

In the peri-implant soft tissues, the number of both mature and newly regenerated nerves showed an increasing trend in 28 days. In the bone tissues, the number of mature or newly regenerated nerves in both areas at less than 100 μm and 100-200 μm from the implant surface on day 28 grew significantly compared with that on day 1 or 3. In addition, the closest distance from mature nerves to the implant surface decreased evidently.

CONCLUSION

The number of peri-implant nerves increased in 28 days since implantation. The innervation in the soft tissue took place faster than in the bone tissue. The mature nerves in the bone tissue approached the implant gradually.

Osseoperception following dental implant treatment: a systematic review

OBJECTIVES

Osseoperception is defined as the ability to perceive tactile sensation via mechanoreception in the peri-implant environment. The objective was to systematically review the available literature on the osseoperception phenomenon following dental implant placement and to explore potential factors which might improve the perception capacity.

MATERIAL AND METHODS

A literature search was performed using PubMed, Cochrane, Embase, and Web of Science databases up to June 2021. Both human and animal studies assessing sensory capacity or innervation following implant placement were included in this review. Two reviewers independently performed the study selection, data extraction and quality assessment of the included studies. The methodological quality of the included papers was assessed using Cochrane risk of bias for human studies and SYRCLE's risk of bias tool for animal studies.

RESULTS

The electronic search of databases yielded 1667 results. Following removal of duplicates, title and abstract screening and full-text reading, 22 publications were eligible to be included in the review. Psychophysical evidence from 14 studies indicated a lower tactile function of implants, where active and passive threshold level were found to be 5 and 50 times higher for implants compared to natural teeth, respectively. The neurophysiological evidence from 3 studies indicated cortical plasticity following dental implant placement, measured via trigeminal nerve evoked potentials and functional magnetic resonance imaging. Histological evidence from 5 studies demonstrated the presence of myelinated nerve fibres in the peri-implant tissues. Additionally, immediate implant placement and loading showed beneficial effect on peri-implant tissue (re)innervation, however, no other biological or physiological factors could be identified influencing osseoperception.

CONCLUSIONS

The reported evidence supported the existence of so-called osseoperception phenomenon for restoring the sensory feedback pathway following oral implant rehabilitation. Further studies are required to identify factors that might further assist physiological integration of the oral implants in the human body as such to approach natural oral function.

Peri-implant myelinated nerve fibers: Histological findings in dogs

BACKGROUND AND OBJECTIVE

While osseointegration following various dental implant placement protocols has been extensively investigated, the neurohistological integration has received little attention. The primary aim of this study was to compare the myelinated nerve fibers density in peri-implant bone tissue following various implant placement protocols. The secondary aim assessed the effect of follow-up on peri-implant nerve fibers density.

METHODS

Ten beagle dogs randomly received 68 commercially pure titanium implants in the mandibular premolar or molar region bilaterally following extraction utilizing one of the six treatment protocols: (a) immediate implant placement (IIP) and immediate loading (IL); (b) IIP and delayed loading (DL); (c) IIP and left unloaded (UL); (d) delayed implant placement (DIP) and IL; (e) DIP and DL; and (f) DIP and UL. Histomorphometric analysis of the peri-implant myelinated nerve fibers was performed in a 300 μm peri-implant zone at the cervical, middle, and apical level following implant placement. The follow-up assessment involved longitudinal observation at 3 months following each implant treatment protocol and at 6 months for IIP+IL and IIP+DL protocols.

RESULTS

The influence of different treatment protocols, including the fixed effects of implant groups (IIP+IL, IIP+DL, IIP+UL, DIP+IL, DIP+DL, DIP+UL) and regions (cervical, middle, apical), was examined via a linear mixed model. The IIP+IL group showed a significantly higher myelinated nerve density compared to the IIP+UL and DIP+UL group. Peri-implant nerve re-innervation was significantly higher (P = .002) in the apical region compared to the cervical region. After immediate implant placement, the IL group showed a significantly (P = .03) higher density of myelinated nerve fibers compared to DL. No significant (P = .19) effect of follow-up on nerve density was observed.

CONCLUSION

The immediate implant placement and loading protocol showed most beneficial effect on peri-implant innervation with highest myelinated nerve density in the apical region. A longer loading time had no influence on the peri-implant nerve density.

Loss of Aβ-nerve endings associated with the Merkel cell-neurite complex in the lesional oral mucosa epithelium of lichen planus and hyperkeratosis

The Merkel cell-neurite complex initiates the perception of touch and mediates Aβ slowly adapting type I responses. Lichen planus is a chronic inflammatory autoimmune disease with T-cell-mediated inflammation, whereas hyperkeratosis is characterized with or without epithelial dysplasia in the oral mucosa. To determine the effects of lichen planus and hyperkeratosis on the Merkel cell-neurite complex, healthy oral mucosal epithelium and lesional oral mucosal epithelium of lichen planus and hyperkeratosis patients were stained by immunohistochemistry (the avidin-biotin-peroxidase complex and double immunofluorescence methods) using pan cytokeratin, cytokeratin 20 (K20, a Merkel cell marker), and neurofilament 200 (NF200, a myelinated Aβ- and Aδ-nerve fibre marker) antibodies. NF200-immunoreactive (ir) nerve fibres in healthy tissues and in the lesional oral mucosa epithelium of lichen planus and hyperkeratosis were counted and statistically analysed. In the healthy oral mucosa, K20-positive Merkel cells with and without close association to the intraepithelial NF200-ir nerve fibres were detected. In the lesional oral mucosa of lichen planus and hyperkeratosis patients, extremely rare NF200-ir nerve fibres were detected only in the lamina propria. Compared with healthy tissues, lichen planus and hyperkeratosis tissues had significantly decreased numbers of NF200-ir nerve fibres in the oral mucosal epithelium. Lichen planus and hyperkeratosis were associated with the absence of Aβ-nerve endings in the oral mucosal epithelium. Thus, we conclude that mechanosensation mediated by the Merkel cell-neurite complex in the oral mucosal epithelium is impaired in lichen planus and hyperkeratosis.

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.

Sensory innervation around immediately vs. delayed loaded implants: a pilot study

Although neurophysiological and psychophysical proof of osseoperception is accumulating, histomorphometric evidence for the neural mechanisms of functional compensation following immediate and delayed implant loading is still lacking. For this randomized split-mouth study, six mongrel dogs randomly received one of four treatment protocols at 36 implant-recipient sites over 16 weeks (third maxillary incisor, third and fourth mandibular premolar): immediate implant placement and immediate loading (IIP+IL); delayed implant placement and delayed loading (DIP+DL); delayed implant placement and immediate loading (DIP+IL); and natural extraction socket healing (control). Histomorphometry was performed in the peri-implant bone and soft tissues within 300 µm around the implants. Immunocytochemistry and transmission electron microscopy were used to confirm the presence of neural structures and to reveal their ultrastructural characteristics, respectively. Myelinated nerve fibres densely populated the peri-implant crestal gingival and apical regions, although they were also identified in the woven bone and in the osteons near the implant threads. Compared with the control group in the mandible, the group that received IIP+IL showed a higher innervation (in N⋅mm⁻², 5.94±1.12 vs. 3.15±0.63, P<0.001) and smaller fibre diameter (in µm, 1.37±0.05 vs. 1.64±0.13, P=0.016), smaller axon diameter (in µm, 0.89±0.05 vs. 1.24±0.10, P=0.009) and g-ratio (0.64±0.04 vs. 0.76±0.05, P<0.001) in the middle region around the implants. Compared with DIP+IL in the mandible, IIP+IL had a higher nerve density (in N⋅mm⁻², 13.23±2.54 vs. 9.64±1.86, P=0.027), greater fibre diameter (in µm, 1.32±0.02 vs. 1.20±0.04, P=0.021), greater axon diameter (in µm, 0.92±0.01 vs. 0.89±0.03, P=0.035) and lower g-ratio (0.69±0.01 vs. 0.74±0.01, P=0.033) in the apical region around the implants. It may be assumed that the treatment protocol with IIP+IL is the preferred method to allow optimized peri-implant re-innervation, but further functional measurements are still required.

Plasma membrane stretch activates transient receptor potential vanilloid and ankyrin channels in Merkel cells from hamster buccal mucosa

Merkel cells (MCs) have been proposed to form a part of the MC-neurite complex with sensory neurons. Many transient receptor potential (TRP) channels have been identified in mammals; however, the activation properties of these channels in oral mucosal MCs remain to be clarified. We investigated the biophysical and pharmacological properties of TRP vanilloid (TRPV)-1, TRPV2, TRPV4, TRP ankyrin (TRPA)-1, and TRP melastatin (TRPM)-8 channels, which are sensitive to osmotic and mechanical stimuli by measurement of intracellular free Ca(2+) concentration ([Ca(2+)]i) using fura-2. We also analyzed their localization patterns through immunofluorescence. MCs showed immunoreaction for TRPV1, TRPV2, TRPV4, TRPA1, and TRPM8 channels. In the presence of extracellular Ca(2+), the hypotonic test solution evoked Ca(2+) influx. The [Ca(2+)]i increases were inhibited by TRPV1, TRPV2, TRPV4, or TRPA1 channel antagonists, but not by the TRPM8 channel antagonist. Application of TRPV1, TRPV2, TRPV4, TRPA1, or TRPM8 channel selective agonists elicited transient increases in [Ca(2+)]i only in the presence of extracellular Ca(2+). The results indicate that membrane stretching in MCs activates TRPV1, TRPV2, TRPV4, and TRPA1 channels, that it may be involved in synaptic transmission to sensory neurons, and that MCs could contribute to the mechanosensory transduction sequence.

Perceptual changes in the peri-implant soft tissues assessed by directional cutaneous kinaesthesia and graphaesthesia: a prospective study

BACKGROUND

The innervation of skin and oral mucosa plays a major physiological role in exteroception. This innervation is also clinically relevant as sensory changes occur after neurosurgical procedures.

PURPOSE

The goal of this study was to compare the perception of mechanical stimuli applied to the buccal mucosa in the vicinity of osseointegrated oral implants with that in the controlateral dentate side. The role of the previously reported increased innervation in the peri-implant soft tissues in the oral sensorimotor function was thus examined.

MATERIALS AND METHODS

Seventeen subjects with 20 implants were tested. Directional cutaneous kinaesthesia (DCK) and graphesthesia (G) were performed on the buccal side of the alveolar mucosa before and at planned intervals after implant placement. The observation was pursued until 6 months after the prosthetic rehabilitation. In each subject, the contralateral mucosa served as a control to the implant sites. Average percentages of correct responses in a four-choice task for DCK and a three-choice task for G were calculated.

RESULTS

Despite an intersubject variation in both the DCK and G, high intraindividual correlations were found (p < .005). The implant sites showed a significant difference toward the control sites at the four interval test for both tests. For DCK and G, the average of correct responses decreased after abutment connection (i.e., after the implant uncovering surgery) to increase afterwards to reach a level close to, but still lower than, the control sites 3 to 6 months after the prosthetic rehabilitation.

CONCLUSION

The DCK and G are simple but reliable sensory tests that can be easily applied in the oral region. This prospective study indicates that tooth loss reduces tactile function compared with implant-supported prostheses. The peri-implant soft tissues could be partially involved in the osseoperception function.