Rescuing failed oral implants via Wnt activation

Signaling pathways of dental implants' osseointegration: a narrative review on two of the most relevant; NF-κB and Wnt pathways

INTRODUCTION

Cell signaling pathways are the biological reactions that control cell functions and fate. They also directly affect the body reactions to implanted biomaterials. It is well-known that dental implants success depends on a successful integration with the alveolar bone: "osseointegration" which events comprise early and later responses to the implanted biomaterials. The early events are mainly immune-inflammatory responses to the implant considered by its microenvironment as a foreign body. Later reactions are osteogenic aiming to regulate bone formation and remodeling. All these events are controlled by the cell signaling pathways in an incredible harmonious coordination.

AIM

The number of pathways having a role in osseointegration is so big to be reviewed in a single article. So the aim of this review was to study only two of the most relevant ones: the inflammatory Nuclear Factor Kappa B (NF-κB) pathway regulating the early osseointegration events and the osteogenic Wnt pathway regulating later events.

METHODS

We conducted a literature review using key databases to provide an overview about the NF-κB and Wnt cell signaling pathways and their mutual relationship with dental implants. A simplified narrative approach was conducted to explain these cell signaling pathways, their mode of activation and how they are related to the cellular events of osseointegration.

RESULTS AND CONCLUSION

NF-κB and Wnt cell signaling pathways are important cross-talking pathways that are affected by the implant's material and surface characteristics. The presence of the implant itself in the bone alters the intracellular events of both pathways in the adjacent implant's cellular microenvironment. Both pathways have a great role in the success or failure of osseointegration. Such knowledge can offer a new hope to treat failed implants and enhance osseointegration in difficult cases. This is consistent with advances in Omics technologies that can change the paradigm of dental implant therapy.

Emerging factors affecting peri-implant bone metabolism

Implant dentistry has evolved to the point that standard implant osseointegration is predictable. This is attributed in part to the advancements in material sciences that have led toward improvements in implant surface technology and characteristics. Nonetheless, there remain several cases where implant therapy fails (specifically at early time points), most commonly attributed to factors affecting bone metabolism. Among these patients, smokers are known to have impaired bone metabolism and thus be subject to higher risks of early implant failure and/or late complications related to the stability of the peri-implant bone and mucosal tissues. Notably, however, emerging data have unveiled other critical factors affecting osseointegration, namely, those related to the metabolism of bone tissues. The aim of this review is to shed light on the effects of implant-related factors, like implant surface or titanium particle release; surgical-related factors, like osseodensification or implanted biomaterials; various drugs, like selective serotonin reuptake inhibitors, proton pump inhibitors, anti-hypertensives, nonsteroidal anti-inflammatory medication, and statins, and host-related factors, like smoking, diet, and metabolic syndrome on bone metabolism, and aseptic peri-implant bone loss. Despite the infectious nature of peri-implant biological complications, these factors must be surveyed for the effective prevention and management of peri-implantitis.

Topography-mediated immunomodulation in osseointegration; Ally or Enemy

Osteoimmunology is at full display during endosseous implant osseointegration. Bone formation, maintenance and resorption at the implant surface is a result of bidirectional and dynamic reciprocal communication between the bone and immune cells that extends beyond the well-defined osteoblast-osteoclast signaling. Implant surface topography informs adherent progenitor and immune cell function and their cross-talk to modulate the process of bone accrual. Integrating titanium surface engineering with the principles of immunology is utilized to harness the power of immune system to improve osseointegration in healthy and diseased microenvironments. This review summarizes current information regarding immune cell-titanium implant surface interactions and places these events in the context of surface-mediated immunomodulation and bone regeneration. A mechanistic approach is directed in demonstrating the central role of osteoimmunology in the process of osseointegration and exploring how regulation of immune cell function at the implant-bone interface may be used in future control of clinical therapies. The process of peri-implant bone loss is also informed by immunomodulation at the implant surface. How surface topography is exploited to prevent osteoclastogenesis is considered herein with respect to peri-implant inflammation, osteoclastic precursor-surface interactions, and the upstream/downstream effects of surface topography on immune and progenitor cell function.

Effects of masticatory loading on bone remodeling around teeth vs. implants: insights from a preclinical model

OBJECTIVES

Teeth connect to bone via a periodontal ligament whereas implants connect to bone directly. Consequently, masticatory loads are distributed differently to periodontal versus peri-implant bone. Our objective was to determine how masticatory loading of an implant versus a tooth affected peri-implant versus periodontal bone remodeling. Our hypothesis was that strains produced by functional loading of an implant would be elevated compared to the strains around teeth, and that this would stimulate a greater degree of bone turnover around implants versus in periodontal bone.

MATERIALS AND METHODS

Sixty skeletally mature mice were divided into two groups. In the Implant group, maxillary first molars (mxM1) were extracted, and after socket healing, titanium alloy implants were positioned sub-occlusally. After osseointegration, implants were exposed, resin crowns were placed, and masticatory loading was initiated. In a Control group the dentition was left intact. Responses of peri-implant and periodontal bone were measured using micro-CT, histology, bone remodeling assays, and quantitative histomorphometry while bone strains were estimated using finite element (FE) analyses.

CONCLUSIONS

When a submerged osseointegrated implant is exposed to masticatory forces peri-implant strains are elevated, and peri-implant bone undergoes significant remodeling that culminates in new bone accrual. The accumulation of new bone functions to reduce both peri-implant strains and bone remodeling activities, equivalent to those observed around the intact dentition.

Bone formation around unstable implants is enhanced by a WNT protein therapeutic in a preclinical in vivo model

OBJECTIVES

Our objective was to test the hypothesis that local delivery of a WNT protein therapeutic would support osseointegration of an unstable implant placed into an oversized osteotomy and subjected to functional loading.

MATERIALS AND METHODS

Using a split-mouth design in an ovariectomized (OVX) rat model, 50 titanium implants were placed in oversized osteotomies. Implants were subjected to functional loading. One-half of the implants were treated with a liposomal formulation of WNT3A protein (L-WNT3A); the other half received an identical liposomal formulation containing phosphate-buffered saline (PBS). Finite element modeling estimated peri-implant strains caused by functional loading. Histological, molecular, cellular, and quantitative micro-computed tomographic (µCT) imaging analyses were performed on samples from post-implant days (PID) 3, 7, and 14. Lateral implant stability was quantified at PID 7 and 14.

RESULTS

Finite element analyses predicted levels of peri-implant strains incompatible with new bone formation. Micro-CT imaging, histological, and quantitative immunohistochemical (IHC) analyses confirmed that PBS-treated implants underwent fibrous encapsulation. In those cases where the peri-implant environment was treated with L-WNT3A, µCT imaging, histological, and quantitative IHC analyses demonstrated a significant increase in expression of proliferative (PCNA) and osteogenic (Runx2, Osterix) markers. One week after L-WNT3A treatment, new bone formation was evident, and two weeks later, L-WNT3A-treated gaps had a stiffer interface compared to PBS-treated gaps.

CONCLUSION

In a rat model, unstable implants undergo fibrous encapsulation. If the same unstable implants are treated with L-WNT3A at the time of placement, then it results in significantly more peri-implant bone and greater interfacial stiffness.

Effects of condensation and compressive strain on implant primary stability: A longitudinal, in vivo, multiscale study in mice

Aims

Surgeons and most engineers believe that bone compaction improves implant primary stability without causing undue damage to the bone itself. In this study, we developed a murine distal femoral implant model and tested this dogma.

Methods

Each mouse received two femoral implants, one placed into a site prepared by drilling and the other into the contralateral site prepared by drilling followed by stepwise condensation.

Results

Condensation significantly increased peri-implant bone density but it also produced higher strains at the interface between the bone and implant, which led to significantly more bone microdamage. Despite increased peri-implant bone density, condensation did not improve implant primary stability as measured by an in vivo lateral stability test. Ultimately, the condensed bone underwent resorption, which delayed the onset of new bone formation around the implant.

Conclusion

Collectively, these multiscale analyses demonstrate that condensation does not positively contribute to implant stability or to new peri-implant bone formation.

Cite this article:Bone Joint Res. 2020;9(2):60–70.

H-type blood vessels participate in alveolar bone remodeling during murine tooth extraction healing

OBJECTIVES

We aimed to investigate whether skeletal-specific H-type blood vessels exist in alveolar bone and how they function in alveolar bone remodeling.

MATERIALS AND METHODS

H-type vessels with high expression of CD31 and Endomucin (CD31^hi Emcn^hi ) were immunostained in alveolar bone. Abundance and age-related changes in CD31^hi Emcn^hi endothelial cells (H-ECs) were detected by flow cytometry. Osteoprogenitors association with H-type vessels and bone mass were detected in tooth extraction model of alveolar bone remodeling by immunohistofluorescence and micro-CT, respectively. Transcription and expression of H-EC feature genes during in vitro Notch inhibition were measured by RT-qPCR and immunocytofluorescence.

RESULTS

We verified that H-type vessels existed in alveolar bone, the abundance of which was highest at infancy age, then decreased but maintained a constant level during aging. In tooth extraction model, H-ECs significantly increased with concomitant perivascular accumulation of Runx2⁺ osteoprogenitors and gradually augmentation of bone mass. Notch inhibition of in vitro cultured H-ECs resulted in decreased expression levels of Emcn and hes1, but not Pecam1 or Kdr genes, with decreased expression levels of H-EC numbers, accordingly.

CONCLUSIONS

The present study suggests that H-type vessels promote osteogenesis during alveolar bone remodeling. Notch signaling pathway regulates expression of Emcn and possibly determines fate and functions of alveolar H-ECs.

System for application of controlled forces on dental implants in rat maxillae: Influence of the number of load cycles on bone healing

Experimental studies on the effect of micromotion on bone healing around implants are frequently conducted in long bones. In order to more closely reflect the anatomical and clinical environments around dental implants, and eventually be able to experimentally address load‐management issues, we have developed a system that allows initial stabilization, protection from external forces, and controlled axial loading of implants. Screw‐shaped implants were placed on the edentulous ridge in rat maxillae. Three loading regimens were applied to validate the system; case A no loading (unloaded implant) for 14 days, case B no loading in the first 7 days followed by 7 days of a single, daily loading session (60 cycles of an axial force of 1.5 N/cycle), and case C no loading in the first 7 days followed by 7 days of two such daily loading sessions. Finite element modeling of the peri‐implant compressive and tensile strains plus histological and immunohistochemical analyses revealed that in case B any tissue damage resulting from the applied force (and related interfacial strains) did not per se disturb bone healing, however, in case C, the accumulation of damage resulting from the doubling of loading sessions severely disrupted the process. These proof‐of‐principle results validate the applicability of our system for controlled loading, and provide new evidence on the importance of the number of load cycles applied on healing of maxillary bone.

Enhancing osseointegration of titanium implants through large-grit sandblasting combined with micro-arc oxidation surface modification

PURPOSE

The demand for titanium dental implants has risen sharply. However, the clinical success rate of implant surgery needs to be improved. In this paper, we report a novel surface modification strategy, large-grit sandblasting combined with micro-arc oxidation (SL-MAO), aiming to promote peri-implant bone formation and osseointegration of titanium implants.

MATERIALS AND METHODS

Modified titanium samples were prepared by large-grit sandblasting and acid etching (SLA), micro-arc oxidation (MAO), and SL-MAO. The resulting topographical changes and chemical composition of the samples were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), respectively, and the biocompatibility and bioactivity were analyzed by bone-marrow mesenchymal stem cells (BMMSC) adhesion tests. Modified titanium implants were also inserted into the femurs of beagle dogs, and their competence of osseointegration was appraised by quantitative histomorphometry and micro-computed-tomography (micro-CT) analyses.

RESULTS

Compared to SLA and MAO techniques, SL-MAO surface modification further enhanced titanium surfaces by creating a topographic morphology characterized by both micron-sized craters and sub-micron-scale pits, and resulted in superior chemical composition, which promoted cell adhesion, proliferation, and osteogenic differentiation. SL-MAO-modified titanium implants osseointegrated more efficiently than SLA or MAO controls, with significantly higher bone-area (BA) ratio and bone-implant contact (BIC) in the peri-implant region.

CONCLUSIONS

The SL-MAO surface modification technique optimized the surface properties of titanium implants and enhanced peri-implant bone formation and osseointegration.

A comparison of lithium-substituted phosphate and borate bioactive glasses for mineralised tissue repair

OBJECTIVES

Wnt/β-catenin signalling plays important roles in regeneration, particularly in hard tissues such as bone and teeth, and can be regulated by small molecule antagonists of glycogen synthase kinase 3 (GSK3); however, small molecules can be difficult to deliver clinically. Lithium (Li) is also a GSK3 antagonist and can be incorporated into bioactive glasses (BG), which can be used clinically in dental and bone repair applications and tuned to quickly release their constituent ions.

METHODS

Here, we created phosphate (P)- and borate (B)-based BG that also contained Li (LiPBG and LiBBG) and examined their ion release kinetics and the toxicity of their dissolution ions on mouse 17IA4 dental pulp cells.

RESULTS

We found that although LiPBG and LiBBG can both quickly release Li at concentrations known to regulate Wnt/β-catenin signalling, the P and B ions they concomitantly release are highly toxic to cells. Only when relatively low concentrations of LiPBG and LiBBG were placed in cell culture medium were their dissolution products non-toxic. However, at these concentrations, LiPBG and LiBBG's ability to regulate Wnt/β-catenin signalling was limited.

SIGNIFICANCE

These data suggest that identifying a BG composition that can both quickly deliver high concentrations of Li and is non-toxic remains a challenge.

Mechanical and Biological Advantages of a Tri-Oval Implant Design

Of all geometric shapes, a tri-oval one may be the strongest because of its capacity to bear large loads with neither rotation nor deformation. Here, we modified the external shape of a dental implant from circular to tri-oval, aiming to create a combination of high strain and low strain peri-implant environment that would ensure both primary implant stability and rapid osseointegration, respectively. Using in vivo mouse models, we tested the effects of this geometric alteration on implant survival and osseointegration over time. The maxima regions of tri-oval implants provided superior primary stability without increasing insertion torque. The minima regions of tri-oval implants presented low compressive strain and significantly less osteocyte apoptosis, which led to minimal bone resorption compared to the round implants. The rate of new bone accrual was also faster around the tri-oval implants. We further subjected both round and tri-oval implants to occlusal loading immediately after placement. In contrast to the round implants that exhibited a significant dip in stability that eventually led to their failure, the tri-oval implants maintained their stability throughout the osseointegration period. Collectively, these multiscale biomechanical analyses demonstrated the superior in vivo performance of the tri-oval implant design.

A Novel Osteotomy Preparation Technique to Preserve Implant Site Viability and Enhance Osteogenesis

The preservation of bone viability at an osteotomy site is a critical variable for subsequent implant osseointegration. Recent biomechanical studies evaluating the consequences of site preparation led us to rethink the design of bone-cutting drills, especially those intended for implant site preparation. We present here a novel drill design that is designed to efficiently cut bone at a very low rotational velocity, obviating the need for irrigation as a coolant. The low-speed cutting produces little heat and, consequently, osteocyte viability is maintained. The lack of irrigation, coupled with the unique design of the cutting flutes, channels into the osteotomy autologous bone chips and osseous coagulum that have inherent osteogenic potential. Collectively, these features result in robust, new bone formation at rates significantly faster than those observed with conventional drilling protocols. These preclinical data have practical implications for the clinical preparation of osteotomies and alveolar bone reconstructive surgeries.

Development of a quantitative preclinical screening model for implant osseointegration in rat tail vertebra

Objectives

Functional tooth replacement and bone regeneration are parts of the daily practice in modern dentistry, but well-reproducible and relatively inexpensive experimental models are still missing. We aimed to develop a new small animal model to monitor osseointegration utilizing the combination of multiple evaluation protocols.

Material and methods

After cutting the tail between the C4 and C5 vertebrae in Wistar rats, costume made, parallel walled, non-threaded implants were placed into the center of the tail parallel with its longitudinal axis using a surgical guide. Osseointegration of the titanium implants was followed between 4 and 16 weeks after surgery applying axial extraction force, and resonance frequency analysis as functional tests, and histomorphometry and micro-CT as structural evaluations.

Results

In functional tests, we observed that both methods are suitable for the detection of the time-dependent increase in osseointegration, but the sensitivity of the pull-out technique (an approximately five times increase with rather low standard error) was much higher than that of the resonance frequency analysis. In structural evaluations, changes in the detected bone implant contact values measured by histomorphometry (yielding 1.5 times increase, with low variations of data) were more reliable than micro-CT based evaluations to screen the developments of contact between bone and implant.

Conclusion

Our results provide evidence that the caudal vertebrae osseointegration model is useful for the preclinical evaluation of implant integration into the bone.

Clinical relevance

The combination of the biomechanical and structural tests offers a well-reproducible small animal system that can be suitable for studying the integration of various implant materials and surface treatments.

Biomechanics of Immediate Postextraction Implant Osseointegration

The aim of this study was to gain insights into the biology and mechanics of immediate postextraction implant osseointegration. To mimic clinical practice, murine first molar extraction was followed by osteotomy site preparation, specifically in the palatal root socket. The osteotomy was positioned such that it removed periodontal ligament (PDL) only on the palatal aspect of the socket, leaving the buccal aspect undisturbed. This strategy created 2 distinct peri-implant environments: on the palatal aspect, the implant was in direct contact with bone, while on the buccal aspect, a PDL-filled gap existed between the implant and bone. Finite element modeling showed high strains on the palatal aspect, where bone was compressed by the implant. Osteocyte death and bone resorption predominated on the palatal aspect, leading to the loss of peri-implant bone. On the buccal aspect, where finite element modeling revealed low strains, there was minimal osteocyte death and robust peri-implant bone formation. Initially, the buccal aspect was filled with PDL remnants, which we found directly provided Wnt-responsive cells that were responsible for new bone formation and osseointegration. On the palatal aspect, which was devoid of PDL and Wnt-responsive cells, adding exogenous liposomal WNT3A created an osteogenic environment for rapid peri-implant bone formation. Thus, we conclude that low strain and high Wnt signaling favor osseointegration of immediate postextraction implants. The PDL harbors Wnt-responsive cells that are inherently osteogenic, and if the PDL tissue is healthy, it is reasonable to preserve this tissue during immediate implant placement.

Lithium enhances alveolar bone formation during orthodontic retention in rats

OBJECTIVE

To investigate the effect of lithium on alveolar bone formation during orthodontic retention in rats.

MATERIALS AND METHODS

After 2 weeks of orthodontic tooth movement (OTM), 42 8-week-old male Wistar rats were randomly divided into two orthodontic retention groups: one without (control) and the other with LiCl treatment (LiCl group). Samples were collected on days 0, 3, 7 and 14 during the retention period. We evaluated the bone volume/total volume (BV/TV) ratio and new bone formation in the region of interests (ie, the root, the periodontal ligament and the adjacent alveolar bone around the distal buccal surface of the distal root of the maxillary first molar). We performed quantitative analyses, including histology, histomorphometry and immunohistochemistry to identify Runx2 and Osterix expression.

RESULTS

The density of trabecular bone, the quantity of osteoblasts and the expression of osteogenic markers, Runx2 and Osterix, were significantly higher in the LiCl group than in the control group during the orthodontic retention period.

CONCLUSION

LiCl enhances alveolar bone formation during orthodontic retention in rats.

Biophysical regulation of osteotomy healing: An animal study

BACKGROUND

Osteotomies have been performed for centuries yet there remains a remarkable lack of consensus on optimal methods for cutting bone. There is universal agreement, however, that preserving cell viability is critical.

PURPOSE

To identify mechanobiological parameters influencing bone formation after osteotomy site preparation.

MATERIALS AND METHODS

A murine maxillary osteotomy model was used to evaluate healing. Computational modeling characterized stress and strain distributions in the osteotomy, as well as the magnitude and distribution of heat generated by drilling. The impact of osteocyte death and bone composition were assessed using molecular and cellular assays.

RESULTS

The phases of osteotomy healing in mice align closely with results in large animals; in addition, molecular analyses extended our understanding of osteoprogenitor cell proliferation, differentiation, and mineralization. Computational analyses provided insights into temperature changes caused by drilling and the mechanobiological state in the healing osteotomies, while concomitant cellular assays correlate drill speed with osteocyte apoptosis and bone resorption. Even when drilling was controlled, trabeculated, spongy (Type III) bone healed faster than densely lamellar (Type I) bone because of the abundance of Wnt responsive osteoprogenitor cells in the former.

CONCLUSIONS

These data provide a mechanobiological framework for evaluating tools and technologies designed to improve osteotomy site preparation.

Contribution of the PDL to Osteotomy Repair and Implant Osseointegration

Our objective was to clarify the fate of the periodontal ligament (PDL) retained in the socket after tooth extraction, then determine if this tissue contributed to the osseointegration of "immediate" implants placed in these fresh extraction sockets. Mice underwent maxillary first molar extraction, the residual PDL was removed by an osteotomy, and titanium implants were placed. The osteotomy was created in such a way that the palatal surface was devoid of PDL remnants while the buccal, mesial, and distal surfaces retained PDL fibers. At multiple time points after surgery, tissues were analyzed using a battery of molecular, cellular, and histomorphometrical assays. We found that PDL remnants mineralized and directly contributed to new bone formation in the extraction site. Compared with regions of an extraction site where the PDL was removed by osteotomy, regions that retained PDL fibers had produced significantly more new bone. Around immediate implants, the retained PDL remnants directly contributed to new bone formation and osseointegration. Thus, we conclude that PDL remnants are inherently osteogenic, and if the tissue is healthy, it is reasonable to conclude that curetting out an extraction socket prior to immediate implant placement should be avoided. This recommendation aligns with contemporary trends toward minimally invasive surgical manipulations of the extraction socket prior to immediate implant placement.

Relationships among Bone Quality, Implant Osseointegration, and Wnt Signaling

A variety of clinical classification schemes have been proposed as a means to identify sites in the oral cavity where implant osseointegration is likely to be successful. Most schemes are based on structural characteristics of the bone, for example, the relative proportion of densely compact, homogenous (type I) bone versus more trabeculated, cancellous (type III) bone. None of these schemes, however, consider potential biological characteristics of the bone. Here, we employed multiscale analyses to identify and characterize type I and type III bones in murine jaws. We then combined these analytical tools with in vivo models of osteotomy healing and implant osseointegration to determine if one type of bone healed faster and supported osseointegration better than another. Collectively, these studies revealed a strong positive correlation between bone remodeling rates, mitotic activity, and osteotomy site healing in type III bone and high endogenous Wnt signaling. This positive correlation was strengthened by observations showing that the osteoid matrix that is responsible for implant osseointegration originates from Wnt-responsive cells and their progeny. The potential application of this knowledge to clinical practice is discussed, along with a theory unifying the role that biology and mechanics play in implant osseointegration.

Promotion of natural tooth repair by small molecule GSK3 antagonists

The restoration of dentine lost in deep caries lesions in teeth is a routine and common treatment that involves the use of inorganic cements based on calcium or silicon-based mineral aggregates. Such cements remain in the tooth and fail to degrade and thus normal mineral volume is never completely restored. Here we describe a novel, biological approach to dentine restoration that stimulates the natural formation of reparative dentine via the mobilisation of resident stem cells in the tooth pulp. Biodegradable, clinically-approved collagen sponges are used to deliver low doses of small molecule glycogen synthase kinase (GSK-3) antagonists that promote the natural processes of reparative dentine formation to completely restore dentine. Since the carrier sponge is degraded over time, dentine replaces the degraded sponge leading to a complete, effective natural repair. This simple, rapid natural tooth repair process could thus potentially provide a new approach to clinical tooth restoration.

Effects of Condensation on Peri-implant Bone Density and Remodeling

Bone condensation is thought to densify interfacial bone and thus improve implant primary stability, but scant data substantiate either claim. We developed a murine oral implant model to test these hypotheses. Osteotomies were created in healed maxillary extraction sites 1) by drilling or 2) by drilling followed by stepwise condensation with tapered osteotomes. Condensation increased interfacial bone density, as measured by a significant change in bone volume/total volume and trabecular spacing, but it simultaneously damaged the bone. On postimplant day 1, the condensed bone interface exhibited microfractures and osteoclast activity. Finite element modeling, mechanical testing, and immunohistochemical analyses at multiple time points throughout the osseointegration period demonstrated that condensation caused very high interfacial strains, marginal bone resorption, and no improvement in implant stability. Collectively, these multiscale analyses demonstrate that condensation does not positively contribute to implant stability.