Fluid Dynamic Mechanisms Which Regulate Tooth Movement

The periodontal ligament (PDL) is a distinctive yet critical connective tissue vital for maintaining the integrity and functionality of tooth-supporting structures. However, PDL repair poses significant challenges due to the complexity of its mechanical microenvironment encompassing hard-soft-hard tissues, with the viscoelastic properties of the PDL being of particular interest. This review delves into the significant role of viscoelastic hydrogels in PDL regeneration, underscoring their utility in simulating biomimetic three-dimensional microenvironments. We review the intricate relationship between PDL and viscoelastic mechanical properties, emphasizing the role of tissue viscoelasticity in maintaining mechanical functionality. Moreover, we summarize the techniques for characterizing PDL's viscoelastic behavior. From a chemical bonding perspective, we explore various crosslinking methods and characteristics of viscoelastic hydrogels, along with engineering strategies to construct viscoelastic cell microenvironments. We present a detailed analysis of the influence of the viscoelastic microenvironment on cellular mechanobiological behavior and fate. Furthermore, we review the applications of diverse viscoelastic hydrogels in PDL repair and address current challenges in the field of viscoelastic tissue repair. Lastly, we propose future directions for the development of innovative hydrogels that will facilitate not only PDL but also systemic ligament tissue repair.

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  An overview of the construction and application of viscoelastic hydrogels in periodontal ligament repair and regeneration from a materials perspective.

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  The first summary of the methods and results of measuring the viscoelasticity of the periodontal ligament.

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  Viscoelastic materials have the potential for periodontal ligaments bionic repair.

Mammalian teeth have to sustain repetitive and high chewing loads without failure. Key to this capability is the periodontal ligament (PDL), a connective tissue containing a collagenous fibre network which connects the tooth roots to the alveolar bone socket and which allows the teeth to move when loaded. It has been suggested that rodent molars under load experience a screw-like downward motion but it remains unclear whether this movement also occurs in primates. Here we use synchroton micro-computed tomography paired with an axial loading setup to investigate the form-function relationship between tooth movement and the morphology of the PDL space in a non-human primate, the mouse lemur (Microcebus murinus). The loading behavior of both mandibular and maxillary molars showed a three-dimensional movement with translational and rotational components, which pushes the tooth into the alveolar socket. Moreover, we found a non-uniform PDL thickness distribution and a gradual increase in volumetric proportion of the periodontal vasculature from cervical to apical. Our results suggest that the PDL morphology may optimize the three-dimensional tooth movement to avoid high stresses under loading.

Although exhaustively studied, the mechanism responsible for tooth support and the mechanical properties of the periodontal ligament (PDL) remain a subject of considerable controversy. In the past, various experimental techniques and theoretical analyses have been employed to tackle this intricate problem. The aim of this study was to investigate the viscoelastic behaviors of the PDL using three-dimensional finite element analysis.

Three dentoalveolar complex models were established to simulate the tissue behaviors of the PDL: (1) deviatoric viscoelastic model; (2) volumetric viscoelastic model; and (3) tension-compression volumetric viscoelastic model. These modified models took into consideration the presence of tension and compression along the PDL during both loading and unloading. The inverse parameter identification process was developed to determine the mechanical properties of the PDL from the results of previously reported in vitro and in vivo experiments.

The results suggest that the tension-compression volumetric viscoelastic model is a good approximation of normal PDL behavior during the loading-unloading process, and the deviatoric viscoelastic model is a good representation of how a damaged PDL behaves under loading conditions. Moreover, fluid appears to be the main creep source in the PDL.

We believe that the biomechanical properties of the PDL established via retrograde calculation in this study can lead to the construction of more accurate extra-oral models and a comprehensive understanding of the biomechanical behavior of the PDL.

This review is intended to highlight and discuss discrepancies in the literature of the periodontal ligament's (PDL) mechanical properties and the various analytical models, approaches and assumptions used in simulating its behaviour. The present study then offers to propose a model development that allows for a better phenomenological description of PDL behaviour under static, near clinical, orthodontic loading conditions.

Searches were performed on biomechanical and orthodontic publications (in databases: Compendex, EMBASE, MEDLINE, PubMed, ScienceDirect and Scopus).

The review revealed that significant variations exist, some on the order of six orders of magnitude, in the PDL's elastic constants and mechanical properties. Possible explanations may be attributable to different modelling approaches and behavioural assumptions.

The discrepancies highlight the need for further research into determining what the key factors that contribute to tooth movement are, their correlations and their degree of impact. Despite the PDL's definitive role in orthodontic tooth movement, proposed models of the PDL's mechanical behaviour thus far have been unsatisfactorily inadequate. Hence, there is a need to develop a robust PDL model that more accurately simulates the PDL's biomechanical response to orthodontic loads. Better understanding of the PDL's biomechanical behaviour under physiologic and traumatic loading conditions might enhance the understanding of the PDL's biologic reaction in health and disease. Providing a greater insight into the response of the PDL would be instrumental to orthodontists and engineers for designing more predictable, and therefore more efficacious, orthodontic appliances.

Orthodontic tooth movement is usually characterized by two centres: the centre of resistance and the centre of rotation. A literature survey shows that both centres vary to a significant extent in both clinical and computational experiments. This paper reports on studies upon five different hypothetical mechanical representations of the periodontal ligament (PDL) which plays the most significant role in tooth mobility. The first model considers the PDL as an isotropic and linear-elastic continuum without fibres; it also discusses some preliminary visco-elastic aspects. The next three models assume a nonlinear and anisotropic material composed of fibres only that are arranged in three different orientations, two hypothetical that have appeared previously in the literature and one more consistent with actual morphological data. The fifth model considers the PDL as an orthotropic material consisting of both a continuum and of fibres. Results were obtained by applying the Finite Element Method (FEM) on a maxillary central incisor. It was found that the isotropic linear-elastic PDL leads to occlusal positions of both centres in comparison with those obtained through the well-known Burstone's theoretical formula, while histological anisotropic fibres locate them apically and eccentrically.

External apical root resorption (EARR) is the most common iatrogenic consequence of orthodontics, and orthodontics is the most common cause of EARR. Localized root resorption is a normal and constant remodeling process, a response to oral microtraumas throughout life. Roots do not shorten naturally with age unless forces (eg, bruxism, tongue thrusting) overcompress the periodontal ligament. Appositional repair normally corrects resorptive defects. Irreversible root shortening occurs with excessive forces or decreased resistance to normal forces. Orthodontically induced root resorption starts adjacent to hyalinized zones and occurs during and after elimination of hyalinized tissue. Incisors are most susceptible to EARR, probably because of their roots' spindly apex and because incisors typically are moved farther than other teeth during correction. Intrusion is probably the most detrimental direction of tooth movement, although simply the distance the apex is moved is often correlated with the degree of root shortening. The strongest single association with EARR seems to be a person's genotype. Familial studies show that a person's genotype accounts for about two-thirds of the variation in the extent of periapical resorption. In most instances, this absolves the orthodontist from blame that treatment markedly influenced the extent of resorption, and it also means that a test can be developed that will flag individuals at particular risk of developing EARR. In any event, all patients' root status should be monitored periodically. Rapid resorption can be diminished with slow, intermittent forces with pauses of 2 to 3 months to allow repair of the eroded cementum.