Journal of Dental Research

The load-bearing dentoalveolar fibrous joint is composed of biomechanically active periodontal ligament (PDL), bone, cementum, and the synergistic entheses of PDL-bone and PDL-cementum. Physiologic and pathologic loads on the dentoalveolar fibrous joint prompt natural shifts in strain gradients within mineralized and fibrous tissues and trigger a cascade of biochemical events within the widened and narrowed sites of the periodontal complex. This review highlights data from in situ biomechanical simulations that provide tooth movements relative to the alveolar socket. The methods and subsequent results provide a reasonable approximation of strain-regulated biochemical events resulting in mesial mineral formation and distal resorption events within microanatomical regions at the ligament-tethered/enthesial ends. These biochemical events, including expressions of biglycan, decorin, chondroitin sulfated neuroglial 2, osteopontin, and bone sialoprotein and localization of various hypertrophic progenitors, are observed at the alkaline phosphatase–positive widened site, resulting in mineral formation and osteoid/cementoid layers. On the narrowed side, tartrate-resistant acid phosphatase regions can lead to a sequence of clastic activities resulting in resorption pits in bone and cementum. These strain-regulated biochemical and subsequently biomineralization events in the load-bearing periodontal complex are critical for maintenance of the periodontal space and overall macroscale joint biomechanics.

Breastfeeding is a powerful health-promoting behavior. A 2016 Lancet global collaboration to review the health implications of breastfeeding was among the first to consider oral health outcomes. While a role was suggested for breastfeeding in preventing malocclusion, caries was the only included disease condition unfavorably associated with breastfeeding. The present critical review examines the evidence connecting breastfeeding practices to these outcomes and discusses the methodological challenges inherent in reaching causal conclusions. Published systematic reviews show some evidence of a protective effect of breastfeeding against primary dentition malocclusion but no supportive evidence for mixed dentition and permanent dentition malocclusions. Regarding caries, well-conducted studies report a benefit with breastfeeding up to 12 mo but a positive association between caries and breastfeeding of longer duration, at times that vary between 12 and 24 mo, as well as nocturnal feeding. Future studies would be methodologically stronger if focused on specific malocclusion traits that are plausibly associated with sucking movements rather than using general malocclusion indices. Studies should use detailed and consistent terminology for breastfeeding definition, including frequency, intensity, and timing. Analytical studies should be carried out to distinguish between confounders (e.g., prematurity) and mediators (e.g., use of pacifier). Regarding a link to caries, standard terminology for exposures (e.g., nocturnal feeding) is recommended. Statistical analyses must account for known confounding factors (e.g., socioeconomic conditions) but avoid inappropriate adjustment for variables on a causal path between exposure and outcome or for variables not associated with breastfeeding (e.g., tooth brushing), as can be guided using tools such as direct acyclic graphs. For dental practice, the potential caries risk of long-duration breastfeeding should be part of individual patient counseling that incorporates patient values and circumstances. Given the unquestioned overall health benefits of breastfeeding, the dental community should support World Health Organization guidelines that encourage and promote breastfeeding.

Peri-implant diseases affecting the surrounding structures of endosseous dental implants include peri-implant mucositis and peri-implantitis. The prevalence of peri-implantitis ranges between 15% and 20% after 10 y, highlighting the major challenge in clinical practice in the rehabilitation of dental implant patients. The widespread nature of peri-implant bone loss poses difficulties in the management of biological complications affecting the long-term success of osseointegrated implant reconstructions. Metal and titanium particles have been detected in peri-implant supporting tissues. However, it remains unclear what mechanisms could be responsible for the elicitation of particle and ion release and whether these released implant-associated materials have a local and/or systemic impact on the peri-implant soft and hard tissues. Metal particle release as a potential etiologic factor has been intensively studied in the field of orthopedics and is known to provoke aseptic loosening around arthroplasties and is associated with implant failures. In dental medicine, emerging information about metal/titanium particle release suggests that the potential impact of biomaterials at the abutment or bone interfaces may have an influence on the pathogenesis of peri-implant bone loss. This mini-review highlights current evidence of metal particle release around dental implants and future areas for research.

In aesthetic sites, the integrity of the facial bone wall dimension in the anterior maxilla is jeopardized by physiologic and structural changes postextraction. An effective regenerative protocol is key to reestablish and maintain the hard and soft tissue dimensions over time. The present prospective case series study examined the effectiveness of early implant placement with simultaneous contour augmentation through guided bone regeneration with a 2-layer composite graft in postextraction single-tooth sites over an observation period of 10 y among 20 patients. The median peri-implant bone loss was 0.35 mm between the 1- and 10-y examination. A success rate of 95% was obtained, with pleasing aesthetic outcomes and a high median Pink Esthetic Score (8). Implant crowns (ICs) revealed significant median facial recession between IC_10y and IC_1y (0.17 mm). The facial bone wall dimensions were assessed by preoperative cone beam computed tomography and 2 subsequent scans taken at 6 and 10 y. The median facial bone wall thickness increased significantly from 0 mm at surgery to 1.67 mm at the 10-y examination. The facial vertical bone wall peak (DIC) was located at a median distance of 0.16 mm coronal to the implant shoulder. The facial vertical bone loss of DIC amounted to 0.02 mm between 6 and 10 y. Equivalence testing was performed for the null hypothesis of a difference of >0.2 mm per year between 2 respective time points, showing stable bone conditions. Modulating factors influencing the regenerative outcomes at 10 y were the preoperative proximal crest width and soft tissue thickness. In conclusion, the present study confirmed the long-term effectiveness of early implant placement with simultaneous contour augmentation through guided bone regeneration with a 2-layer composite graft in postextraction single-tooth sites offering stable bone conditions with low risks of mucosal recessions over an observation period of 10 y (ClinicalTrials.gov NCT03252106).

Streptococcus mutans is a key bacterial species in the caries process, which affects >90% of the population worldwide. However, other acidogenic and aciduric/acidophilic species may contribute to disease development. In Sweden, a country with low prevalences of caries and S. mutans, a significant portion of caries-affected adolescents lack detectable levels of S. mutans. The objectives of the present study were 1) to characterize the tooth biofilm and saliva microbiota of adolescents with caries disease, with or without detectable S. mutans, from tooth biofilm and saliva samples and 2) to assess taxa clustering in the tooth biofilm and saliva samples and relate this information to caries status. For 17-y-old participants (N = 154), enamel and dentin caries (the total number of present carious surfaces in the enamel and dentin) and caries experience (the number of decayed and filled tooth surfaces) were recorded, dental biofilm and saliva samples obtained, and information on medical and lifestyle habits collected. Multiplex 16S rDNA (V3-V4) sequencing of bacterial DNA was performed with the Illumina MiSeq platform. The Human Oral Microbiome Database and the ProbeSeq pipeline were used in the HOMINGS procedure. In subjects with caries experience, high levels of S. mutans were associated with a few species and low levels with a panel of saccharolytic species. Present caries was similarly associated with a panel of saccharolytic species in subjects without S. mutans. Furthermore, tooth biofilm microbiota could be used to establish 4 clusters of subjects with different caries experiences. In particular, high levels of S. mutans were associated with the presence of a few influential species in multivariate modeling, including Scardovia wiggsiae. By contrast, a panel of less avid lactic acid–producing species was influential in patients with undetectable or low S. mutans levels in such modeling. These findings support a prominent role of S. mutans in infected adolescents but also the ecologic concept, especially in S. mutans–free subjects.

Bleaching of vital teeth has become common practice in cosmetic dentistry today. Tooth sensitivity and demineralization of the enamel are, however, common side effects associated with hydrogen and carbamide peroxide bleaching. This study investigated if calcium phosphate microspheres, which have remineralizing properties, could be used as an additive without hindering the diffusion of the bleaching agent and if the spheres could be used as a carrier for carbamide peroxide. A remineralizing agent could increase the safety of bleaching and decrease the severity of its side effects. Comparisons between current hydrogen peroxide diffusion studies and previously published work are difficult since many studies include challenging-to-replicate conditions or lack reporting of important parameters. Hence, a diffusion model was designed by Wu Lab (School of Dentistry, University of California, Los Angeles) to measure the diffusion flux and determine the diffusivity of hydrogen peroxide. Physical parameters (e.g., diffusivity) could then be used for direct comparison to the results obtained by future studies. Three whitening gels with increasing amounts of spheres were formulated and tested with 2 commercially available whitening gels. The flux of hydrogen peroxide through 1-mm discs of bovine enamel was measured at steady-state conditions, and the diffusivity was calculated. The results showed that the spheres could be used as a carrier for carbamide peroxide and that the amount of spheres did not affect the diffusivity of peroxide through the enamel discs. Hence, the microspheres are considered promising as an additive to minimize side effects in bleaching gel formulation.

An ideal ceramic restorative material should possess excellent aesthetic and mechanical properties. We hypothesize that the high translucency and strength of polycrystalline ceramics can be achieved through microstructural tailoring. The aim of this study is to demonstrate the superior optical and mechanical properties of a new class of submicron grain-sized alumina ceramics relative to the current state-of-the-art dental ceramic materials. The translucency, the in-line transmission (T_IT) in particular, of these submicron alumina ceramics has been examined with the Rayleigh-Gans-Debye light-scattering model. The theoretical predictions related very well with the measured T_IT values. The translucency parameter (TP) and contrast ratio (CR) of the newly developed aluminas were measured with a reflectance spectrophotometer on a black-and-white background. For comparison, the T_IT, TP, and CR values for a variety of dental ceramics, mostly measured in-house but also cited from the literature, were included. The flexural strength of the aluminas was determined with the 4-point bending test. Our findings have shown that for polycrystalline alumina ceramics, an average grain size <1 µm coupled with a porosity level <0.7% could yield translucency values (T_IT, TP, CR) similar to those of the commercial high-translucency porcelains. These values are far superior to the high-translucency lithium disilicate glass-ceramic and zirconias, including the most translucent cubic-containing zirconias. The strength of these submicron grain-sized aluminas was significantly higher than that of the cubic-containing zirconia (e.g., Zpex Smile) and lithia-based glass-ceramics (e.g., IPS e.max CAD HT). A coarse-grained alumina could also reach a translucency level comparable to that of dental porcelain. However, the relatively low strength of this material has limited its clinical indications to structurally less demanding applications, such as orthodontic brackets. With a combined high strength and translucency, the newly developed submicron grain-sized alumina may be considered a suitable material for dental restorations.

The effects of pH cycling immersion on the corrosion of glass-based ceramic materials were investigated by examining the silicon release level in the immersion solution and the surface morphology of the ceramic after immersion. The hypothesis that pH cycling causes more surface degradation than constant immersion was tested. An inductively coupled plasma atomic emission spectrometer was used for Si ion concentration determination and scanning electron microscopy for surface morphology analyses. Two pH cycling sequences (pH 2, 7, 10 and pH 10, 2, 7) were employed in this study. Glass-ceramic disks were immersed in each pH solution for 3 d, then cycled for 27 d. The silicon release levels during the pH cycling were significantly higher than those in the constant pH immersion. The silicon levels for both cycling sequences were around 47 and 2 times higher than that in constant pH conditions for 2 and 10, respectively. The morphology of the ceramic treated with cycling was also significantly degraded as compared with the ceramic immersed in the constant pH solution. Thus, the severity of glass-ceramic degradation depends not only on the pH of the immersed solution but also on the pH of the previous solution. Since the pH of the oral environment can vary depending on the diet and buffering capacity of saliva, materials testing in constant pH immersion might underestimate the in vivo corrosion. New mechanisms were proposed to account for the effect of pH cycling on glass-ceramic corrosion.

Periodontal regeneration is still a challenge for periodontists and tissue engineers, as it requires the simultaneous restoration of different tissues—namely, cementum, gingiva, bone, and periodontal ligament (PDL). Here, we synthetized a chitosan (CH)–based trilayer porous scaffold to achieve periodontal regeneration driven by multitissue simultaneous healing. We produced 2 porous compartments for bone and gingiva regeneration by cross-linking with genipin either medium molecular weight (MMW) or low molecular weight (LMW) CH and freeze-drying the resulting scaffolds. We synthetized a third compartment for PDL regeneration by CH electrochemical deposition; this allowed us to produce highly oriented microchannels of about 450-µm diameter intended to drive PDL fiber growth toward the dental root. In vitro characterization showed rapid equilibrium water content for MMW-CH and LMW-CH compartments (equilibrium water content after 5 min >85%). The MMW-CH compartment degraded more slowly and provided significantly more resistance to compression (28% ± 1% of weight loss at 4 wk; compression modulus H_A = 18 ± 6 kPa) than the LMW-CH compartment (34% ± 1%; 7.7 ± 0.8 kPa) as required to match the physiologic healing rates of bone and gingiva and their mechanical properties. More than 90% of all human primary periodontal cell populations tested on the corresponding compartment survived during cytocompatibility tests, showing active cell metabolism in the alkaline phosphatase and collagen deposition assays. In vivo tests showed high biocompatibility in wild-type mice, tissue ingrowth, and vascularization within the scaffold. Using the periodontal ectopic model in nude mice, we preseeded scaffold compartments with human gingival fibroblasts, osteoblasts, and PDL fibroblasts and found a dense mineralized matrix within the MMW-CH region, with weakly mineralized deposits at the dentin interface. Together, these results support this resorbable trilayer scaffold as a promising candidate for periodontal regeneration.

Osteonecrosis of the jaws (ONJ) is a rare but severe complication of antiresorptive medications, such as bisphosphonates, used in the treatment of bone malignancy or osteoporosis. Tooth extraction and dental disease have been strongly associated with ONJ development. Here, we investigated molecular and cellular markers of socket healing after extraction of healthy or teeth with experimental periodontitis (EP) in Wistar-Han rats treated with zoledronic acid (ZA). We included 4 experimental groups: vehicle-treated animals with extraction of healthy teeth or teeth with ligature-induced EP and ZA-treated animals with extraction of healthy teeth or teeth with EP. Animals were pretreated with vehicle or ZA for a week, and EP was induced. Four weeks later, the second maxillary molars were extracted; sockets were allowed to heal for 4 wk; animals were euthanized; and maxillae were isolated. Radiographically, extraction sockets in groups 1, 2, and 3 demonstrated normal healing. Contrary incomplete socket healing was noted after extraction of teeth with EP in ZA-treated rats of group 4. Histologically, persistent inflammation and extensive osteonecrosis were seen in group 4. Disorganization of the collagen network, collagen type III predominance, and lack of collagen fiber insertion in the necrotic bone were associated with impaired socket healing. Cells positive for MMP-9, MMP-13, and α-SMA expression were present at the areas of epithelial invagination and adjacent to osteonecrotic bone. Importantly, human biopsies from patients with ONJ showed similar findings. Our data emphasize the importance of dental disease and tooth extraction in ONJ pathogenesis and help delineate an altered profile in wound-healing markers during ONJ development.

Temporomandibular joint (TMJ) disorders are often associated with development of osteoarthritis-like changes in the mandibular condyle. Discoidin domain receptor 2 (DDR2), a collagen receptor preferentially activated by type I and III collagen found in the TMJ and other fibrocartilages, has been associated with TMJ degeneration, but its role in normal joint development has not been previously examined. Using Ddr2 LacZ-tagged mice and immunohistochemistry, we found that DDR2 is preferentially expressed and activated in the articular zone of TMJs but not knee joints. To assess the requirement for Ddr2 in TMJ development, studies were undertaken to compare wild-type and smallie (slie) mice, which contain a spontaneous deletion in Ddr2 to produce an effective null allele. Analysis of TMJs from newborn Ddr2^slie/slie mice revealed a developmental delay in condyle mineralization, as measured by micro–computed tomography and histologic analysis. In marked contrast, knee joints of Ddr2^slie/slie mice were normal. Analysis of older Ddr2^slie/slie mice (3 and 10 mo) revealed that the early developmental delay led to a dramatic and progressive loss of TMJ articular integrity and osteoarthritis-like changes. Mutant condyles had a rough and flattened bone surface, accompanied by a dramatic loss of bone mineral density. Mankin scores showed significantly greater degenerative changes in the TMJs of 3- and 10-mo-old Ddr2^slie/slie mice as compared with wild-type controls. No DDR2-dependent degenerative changes were seen in knees. Analysis of primary cultures of TMJ articular chondrocytes from wild-type and Ddr2^slie/slie mice showed defects in chondrocyte maturation and mineralization in the absence of Ddr2. These studies demonstrate that DDR2 is necessary for normal TMJ condyle development and homeostasis and that these DDR2 functions are restricted to TMJ fibrocartilage and not seen in the hyaline cartilage of the knee.

Angiogenesis is a complex, multicellular process that is critical for bone development and generation. Endochondral ossification depends on an avascular cartilage template that completely remodels into vascularized bone and involves a dynamic interplay among chondrocytes, osteoblasts, and endothelial cells. We have discovered fibrocartilage stem cells (FCSCs) derived from the temporomandibular joint (TMJ) mandibular condyle that generates cartilage anlagen, which is subsequently remodeled into vascularized bone using an ectopic transplantation model. Here we explore FCSC and endothelial cell interactions during vascularized bone formation. We found that a single FCSC colony formed transient cartilage and host endothelial cells may participate in bone angiogenesis upon subcutaneous transplantation in a nude mouse. FCSCs produced an abundance of the proangiogenic growth factor vascular endothelial growth factor A and promoted the proliferation of human umbilical vein endothelial cells (HUVECs). Using a fibrinogen gel bead angiogenesis assay experiment, FCSC cell feeder layer induced HUVECs to form significantly shorter and less sprouts than D551 fibroblast controls, suggesting that FCSCs may initially inhibit angiogenesis to allow for avascular cartilage formation. Conversely, direct FCSC-HUVEC contact significantly enhanced the osteogenic differentiation of FCSCs. To corroborate this idea, upon transplantation of FCSCs into a bone defect microenvironment, FCSCs engrafted and regenerated intramembranous bone. Taken together, we demonstrate that the interactions between FCSCs and endothelial cells are essential for FCSC-derived vascularized bone formation. A comprehensive understanding of the environmental cues that regulate FCSC fate decisions may contribute to deciphering the mechanisms underlying the role of FCSCs in regulating bone formation.

The proteome and N-terminome of the human odontoblast cell layer were identified for the first time by shotgun proteomic and terminal amine isotopic labeling of substrates (TAILS) N-terminomic analyses, respectively, and compared with that of human dental pulp stroma from 26 third molar teeth. After reverse-phase liquid chromatography–tandem mass spectrometry, >170,000 spectra from the shotgun and TAILS analyses were matched by 4 search engines to 4,888 and 12,063 peptides in the odontoblast cell layer and pulp stroma, respectively. Within these peptide groups, 1,543 and 5,841 protein N-termini, as well as 895 and 2,423 unique proteins, were identified with a false discovery rate of ≤1%. Thus, the human dental pulp proteome was expanded by 974 proteins not previously identified among the 4,123 proteins in our 2015 dental pulp study. Further, 222 proteins of the odontoblast cell layer were not found in the pulp stroma, suggesting many of these proteins are synthesized only by odontoblasts. When comparing the proteomes of older and younger donors, differences were more apparent in the odontoblast cell layer than in the dental pulp stroma. In the odontoblast cell layer proteome, we found proteomic evidence for dentin sialophosphoprotein, which is cleaved into dentin sialoprotein and dentin phosphoprotein. By exploring the proteome of the odontoblast cell layer and expanding the known dental pulp proteome, we found distinct proteome differences compared with each other and with dentin. Moreover, between 61% and 66% of proteins also occurred as proteoforms commencing with a neo–N-terminus not annotated in UniProt. Hence, TAILS increased proteome coverage and revealed considerable proteolytic processing, by identifying stable proteoforms in these dynamic dental tissues. All mass spectrometry raw data have been deposited to ProteomeXchange with the identifier <PXD006557>, with the accompanying metadata at Mendeley Data (https://data.mendeley.com/datasets/b57zfh6wmy/1).

Schwann cells are glial cells that support axonal development, maintenance, defense, and regeneration in the peripheral nervous system. There is limited knowledge regarding the organization, plasticity, and aging of Schwann cells within the dental pulp in adult permanent teeth. The present study sought to relate changes in the pattern of Schwann cell phenotypes between young and old adult teeth with neuronal, immune, and vascular components of the dental pulp. Schwann cells are shown to form a prominent glial network at the dentin-pulp interface, consisting of nonmyelinating and myelinating phenotypes, forming a multicellular neuroimmune interface in association with nerve fibers and dendritic cells. Schwann cell phenotypes are recognized by the expression of S100, glial fibrillary acidic protein (GFAP), myelin basic protein (MBP), Sox10, GAP43, and p75NTR markers. In young adult teeth, a dense population of nonmyelinating Schwann cells projects processes in close association with sensory nerve terminals through the odontoblast layer, reaching the adjacent predentin/dentin domain. While GAP43 and p75NTR are highly expressed in nonmyelinating Schwann cells from young adult teeth, the presence of these markers declines significantly in old adult teeth. Myelinated axons, identified by MBP expression, are mainly present at the Raschkow plexus and within nerve bundles in the dental pulp, but their density is significantly reduced in old adult versus young adult teeth. These data reveal age-related changes within the glial network of the dental pulp, in association with a reduction of coronal dental pulp innervation in old adult versus young adult teeth. The prominence of Schwann cells as a cellular component at the dentin-pulp interface supports the notion that their association with sensory nerve terminals and immune system components forms part of an integrated multicellular barrier for defense against pathogens and dentin repair.