Fluid dynamic mechanisms which regulate tooth movement

Rational design of viscoelastic hydrogels for periodontal ligament remodeling and repair

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. STATEMENT OF SIGNIFICANCE.

Fluid flow shear stress and tissue remodeling-an orthodontic perspective: evidence synthesis and differential gene expression network analysis

Introduction: This study aimed to identify and analyze in vitro studies investigating the biological effect of fluid-flow shear stress (FSS) on cells found in the periodontal ligament and bone tissue. Method: We followed the PRISMA guideline for systematic reviews. A PubMed search strategy was developed, studies were selected according to predefined eligibility criteria, and the risk of bias was assessed. Relevant data related to cell source, applied FSS, and locus-specific expression were extracted. Based on this evidence synthesis and, as an original part of this work, analysis of differential gene expression using over-representation and network-analysis was performed. Five relevant publicly available gene expression datasets were analyzed using gene set enrichment analysis (GSEA). Result: A total of 6,974 articles were identified. Titles and abstracts were screened, and 218 articles were selected for full-text assessment. Finally, 120 articles were included in this study. Sample size determination and statistical analysis related to methodological quality and the ethical statement item in reporting quality were most frequently identified as high risk of bias. The analyzed studies mostly used custom-made fluid-flow apparatuses (61.7%). FSS was most frequently applied for 0.5 h, 1 h, or 2 h, whereas FSS magnitudes ranged from 6 to 20 dyn/cm2 depending on cell type and flow profile. Fluid-flow frequencies of 1 Hz in human cells and 1 and 5 Hz in mouse cells were mostly applied. FSS upregulated genes/metabolites responsible for tissue formation (AKT1, alkaline phosphatase, BGLAP, BMP2, Ca2+, COL1A1, CTNNB1, GJA1, MAPK1/MAPK3, PDPN, RUNX2, SPP1, TNFRSF11B, VEGFA, WNT3A) and inflammation (nitric oxide, PGE-2, PGI-2, PTGS1, PTGS2). Protein-protein interaction networks were constructed and analyzed using over-representation analysis and GSEA to identify shared signaling pathways. Conclusion: To our knowledge, this is the first review giving a comprehensive overview and discussion of methodological technical details regarding fluid flow application in 2D cell culture in vitro experimental conditions. Therefore, it is not only providing valuable information about cellular molecular events and their quantitative and qualitative analysis, but also confirming the reproducibility of previously published results.

Movement analysis of primate molar teeth under load using synchrotron X-ray microtomography

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.

The Mechanical Effect of the Periodontal Ligament on Bone Strain Regimes in a Validated Finite Element Model of a Macaque Mandible

The primary anatomical function of the periodontal ligament (PDL) is to attach teeth to their sockets. However, theoretical and constitutive mechanical models have proposed that during mastication the PDL redistributes local occlusal loads and reduces the jaw's resistance to torsional deformations. These hypotheses imply that accurately modeling the PDL's material properties and geometry in finite element analysis (FEA) is a prerequisite to obtaining precise strain and deformation data. Yet, many finite element studies of the human and non-human primate masticatory apparatus exclude the PDL or model it with simplicity, in part due to limitations in μCT/CT scan resolution and material property assignment. Previous studies testing the sensitivity of finite element models (FEMs) to the PDL have yielded contradictory results, however a major limitation of these studies is that FEMs were not validated against in vivo bone strain data. Hence, this study uses a validated and subject specific FEM to assess the effect of the PDL on strain and deformation regimes in the lower jaw of a rhesus macaque (Macaca mulatta) during simulated unilateral post-canine chewing. Our findings demonstrate that the presence of the PDL does influence local and global surface strain magnitudes (principal and shear) in the jaw. However, the PDL's effect is limited (diff. ~200-300 με) in areas away from the alveoli. Our results also show that varying the PDL's Young's Modulus within the range of published values (0.07-1750 MPa) has very little effect on global surface strains. These findings suggest that the mechanical importance of the PDL in FEMs of the mandible during chewing is dependent on the scope of the hypotheses being tested. If researchers are comparing strain gradients across species/taxa, the PDL may be excluded with minimal effect on results, but, if researchers are concerned with absolute strain values, sensitivity analysis is required.

Bone Response of Loaded Periodontal Ligament

The tooth-periodontal ligament-alveolar bone complex acts symbiotically to dissipate the mechanical loads incurred during mastication and/or orthodontic tooth movement. The periodontal ligament functions both in the tension and compression. At the molecular and celleular levels, the loads in the periodontal ligament trigger mechanobiological events in the alveolar bone, which leads to bone modeling and remodeling. The current review focuses on the bone response to mechanical loading of the periodontal ligament on the tension and pressure sides. Understanding the bone response has major implications for dentistry, including a better understanding of the different types of orthodontic tooth movement.

Role of polycystin-1 in bone remodeling: orthodontic tooth movement study in mutant mice

OBJECTIVE

To test the hypothesis that polycystin-1 (PC1) is involved in orthodontic tooth movement as a mechanical sensor.

MATERIALS AND METHODS

The response to force application was compared between three mutant and four wild-type 7-week-old mice. The mutant mice were PC1/Wnt1-cre, lacking PC1 in the craniofacial region. An orthodontic closed coil spring was bonded between the incisor and the left first molar, applying 20 g of force for 4 days. Micro-computed tomography, hematoxylin and eosin staining, and tartrate-resistent acid phosphatase (TRAP) staining were used to study the differences in tooth movement among the groups.

RESULTS

In the wild-type mice the bonded molar moved mesially, and the periodontal ligament (PDL) was compressed in the compression side. The compression side showed a hyalinized zone, and osteoclasts were identified there using TRAP staining. In the mutant mice, the molar did not move, the incisor tipped palatally, and there was slight widening of the PDL in the tension area. Osteoclasts were not seen on the bone surface or on the compression side. Osteoclasts were only observed on the other side of the bone-in the bone marrow.

CONCLUSIONS

These results suggest a difference in tooth movement and osteoclast activity between PC1 mutant mice and wild-type mice in response to orthodontic force. The impaired tooth movement and the lack of osteoclasts on the bone surface in the mutant working side may be related to lack of signal from the PDL due to PC1 deficiency.

The mechanical function of the periodontal ligament in the macaque mandible: a validation and sensitivity study using finite element analysis

Whilst the periodontal ligament (PDL) acts as an attachment tissue between bone and tooth, hypotheses regarding the role of the PDL as a hydrodynamic damping mechanism during intraoral food processing have highlighted its potential importance in finite element (FE) analysis. Although experimental and constitutive models have correlated the mechanical function of the PDL tissue with its anisotropic, heterogeneous, viscoelastic and non-linear elastic nature, in many FE simulations the PDL is either present or absent, and when present is variably modelled. In addition, the small space the PDL occupies and the inability to visualize the PDL tissue using μCT scans poses issues during FE model construction and so protocols for the PDL thickness also vary. In this paper we initially test and validate the sensitivity of an FE model of a macaque mandible to variations in the Young's modulus and the thickness of the PDL tissue. We then tested the validity of the FE models by carrying out experimental strain measurements on the same mandible in the laboratory using laser speckle interferometry. These strain measurements matched the FE predictions very closely, providing confidence that material properties and PDL thickness were suitably defined. The FE strain results across the mandible are generally insensitive to the absence and variably modelled PDL tissue. Differences are only found in the alveolar region adjacent to the socket of the loaded tooth. The results indicate that the effect of the PDL on strain distribution and/or absorption is restricted locally to the alveolar bone surrounding the teeth and does not affect other regions of the mandible.

Mechanical strength and viscoelastic response of the periodontal ligament in relation to structure

The mechanical strength of the periodontal ligament (PDL) was first measured as force required to extract a tooth from its socket using human specimens. Thereafter, tooth-PDL-bone preparations have extensively been used for measurement of the mechanical response of the PDL. In vitro treatments of such specimens with specific enzymes allowed one to investigate into the roles of the structural components in the mechanical support of the PDL. The viscoelastic responses of the PDL may be examined by analysis of the stress-relaxation. Video polarised microscopy suggested that the collagen molecules and fibrils in the stretched fibre bundles progressively align along the deformation direction during the relaxation. The stress-relaxation process of the PDL can be well expressed by a function with three exponential decay terms. Analysis after in vitro digestion of the collagen fibres by collagenase revealed that the collagen fibre components may play an important role in the long-term relaxation component of the stress-relaxation process of the PDL. The dynamic measurements of the viscoelastic properties of the PDL have recently suggested that the PDL can absorb more energy in compression than in shear and tension. These viscoelastic mechanisms of the PDL tissue could reduce the risk of injury to the PDL.

Tooth-root form and function in platyrrhine seed-eaters

Research into the functional and adaptive basis of tooth crown form has provided a useful framework for the inference of diet in extinct primates. However, our understanding of variation in tooth-root form is limited. Studies within the clinical literature emphasize the influence of tooth-root surface area on stress resistance, but it is not known if root form has diversified during primate evolution in relation to dietary specialization. This hypothesis was tested by quantifying maxillary canine and postcanine tooth-root surface areas in four platyrrhine species that differ in the material properties of their diet: Cebus apella, Cebus albifrons, Chiropotes satanas, and Pithecia pithecia. Pairwise comparisons between closely related taxa support predictions based on dietary differences. Taxa that regularly consume resistant seeds (Cebus apella and Chiropotes satanas) exhibit significantly larger relative surface area values for those teeth used in seed processing than closely related taxa that consume resistant foods less often (Cebus albifrons and Pithecia pithecia). Additionally, relative molar-root surface area appears to be greater in Pithecia than in Chiropotes, as predicted from the more folivorous diet of Pithecia. Tooth-root surface area was also found to vary along the tooth row and should therefore have a significant influence on antero-posterior bite-force gradients. The results of this study suggest a close relationship between tooth-root form and patterns of occlusal loading. Further elucidation of this relationship could improve our inferences of diet in extinct taxa, and augment research into the mechanics and evolution of feeding.

Experimental tooth movement under light orthodontic forces: rates of tooth movement and changes of the periodontium

AIM

To investigate light forces for experimental tooth movement.

METHOD

Light orthodontic forces of 1.2, 3.6, 6.5, and 10 g force (gf) were applied for 14 days to move rat molars, and the effects of the forces on the rate of tooth movement and changes of the periodontium were examined.

RESULTS

In the early period, despite the different levels of force used in each group, there were no significant differences in tooth displacement. From hour 56 to day 14, the tooth displacement in the 1.2 gf group was significantly smaller than that in the other groups and the rate was nearly constant. The rates of tooth displacement in the 3.6, 6.5, and 10 gf groups fluctuated repeatedly, while the orthodontic forces gradually decreased.

CONCLUSION

Experimental tooth movement in rats, tipping without friction under light forces, were either constant or fluctuated in cycles of several days' duration. This is in contradiction to the three-phases-theory of tooth movement described in previous investigations using heavy forces.

Time-dependent mechanical behaviour of the periodontal ligament

The process of tooth displacement in response to orthodontic forces is thought to be induced by the stresses and strains in the periodontium. The mechanical force on the tooth is transmitted to the alveolar bone through a layer of soft connective tissue, the periodontal ligament. Stress and/or strain distribution in this layer must be derived from mathematical models, such as the finite element method, because it cannot be measured directly in a non-destructive way. The material behaviour of the constituent tissues is required as an input for such a model. The purpose of this study was to determine the time-dependent mechanical behaviour of the periodontal ligament due to orthodontic loading of a tooth. Therefore, in vivo experiments were performed on beagle dogs. The experimental configuration was simulated in a finite element model to estimate the poroelastic material properties for the periodontal ligament. The experiments showed a two-step response: an instantaneous displacement of 14.10 +/- 3.21 microns within 4 s and a more gradual (creep) displacement reaching a maximum of 60.00 +/- 9.92 microns after 5 h. This response fitted excellently in the finite element model when 21 per cent of the ligament volume was assigned a permeability of 1.0 x 10(-14) m4/N s, the remaining 97 per cent was assigned a permeability of 2.5 x 10(-17) m4/N s. A tissue elastic modulus of 0.015 +/- 0.001 MPa was estimated. Our results indicate that fluid compartments within the periodontal ligament play an important role in the transmission and damping of forces acting on teeth.

A comparative FEM-study of tooth mobility using isotropic and anisotropic models of the periodontal ligament. Finite Element Method

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.

Blood volume in human bicuspid periodontal ligament determined by electron microscopy

The microvascular volume of periodontal ligament is reported to range from 1.63 to 3.5% in man, whereas that of animals varies from 7.5 to 11.5%. This transmission electron-microscopic investigation was undertaken to determine stereologically the volume in human periodontal ligament. The hypothesis tested was that the ligament blood volume in man is similar to that in animals. Left and right segments of mandible containing first and second premolars came from an adult burns' victim who underwent jaw reconstruction. The segments were immersion-fixed in 2.5% glutaraldehyde, demineralized at 4 degrees C in 0.1 M EDTA and processed for microscopy. Segments of distal periodontal ligament were sectioned at 150-micron intervals from the alveolar crest to the root apex and random tissue quadrats recorded for point counting and data analysis using a generalized linear-regression statistical model. Mean adjusted microvascular luminal volume was 9.52 +/- 2.28% (SEM) and the abluminal volume 12.91 +/- 2.76%; the wall volume was 3.39%. Significant differences existed between the luminal and abluminal volumes of the different vessel type (p < 0.05) and their distribution across the circumferential thirds of the ligament (p < 0.05). Total length density of the blood vessels was 149.84 x 10(3) cm/cm3 and the surface density 330.19 cm2/cm3. Postcapillary-sized venules held 69.1% of the total blood volume and provided 49.3% of the luminal surface area. Venous capillaries were the most common vessel, comprising 48.5%, and they contributed 71.5% of the overall length density. This study confirmed the hypothesis for the blood volume in the periodontal ligament in man. Blood volumes do not reflect the configurations of microvascular beds.

A rationale for a simplified occlusal design in restorative dentistry: historical review and clinical guidelines

An occlusal contact pattern in which the number of occlusal contacts has been substantially reduced as compared with traditional schemes is described. Concepts that may have had a justification in balanced occlusions have been needlessly transferred to anterior disclusion mechanics. No natural dentition presents occlusal contacts as described in many texts and yet stability is established. The temporomandibular joint does present structural changes that should be accounted for when an occlusal anatomy is designed. The force vectors that are active on teeth are not directed along the longitudinal axes of the roots only, and thus occlusal contact locations will not determine the direction of functional forces. The stability of the teeth on the arch depends primarily on the forces of eruption from the periodontium and the balance between the resting pressures of the muscles of the cheeks and the tongue. The mechanics of the stomatognathic system are not as accurate as their counterpart on an articulator. The variability of the guiding surfaces inherent to the temporomandibular joints should be incorporated into an occlusal design. Occlusal contacts that do not fulfill a justifiable purpose may be eliminated, and the number of contacts may be reduced to one per tooth.

Analysis of stress-strain curves at fast and slow velocities of loading in vitro in the transverse section of the rat incisor periodontal ligament following the administration of beta-aminopropionitrile

The in vitro mechanical properties of this ligament were examined by analysing the stress-strain curve obtained from a transverse section of the mandible. Mechanical measures were compared between normal rats and lathyritic rats given drinking water containing 0.2% of beta-aminopropionitrile (BAPN) for 20 days, and between the velocity of loading at 10(4) and 1 mm/24 h. The daily dose of BAPN decreased gradually because the body weight increased gradually. At the velocity of 10(4) mm/24 h, the maximum shear stress, elastic stiffness and failure strain energy density in the experimental subgroup fell to 43-50% of the control values, and at 1 mm/24 h to 71-80%. The maximum strains were not significantly different between the control and experimental subgroups either at 10(4) or at 1 mm/24 h. In the control subgroups, the maximum shear stress, elastic stiffness and failure strain energy density at 1 mm/24 h fell to 0.04-0.30% of those at 10(4) mm/24 h, and in the experimental subgroups to 0.08-0.43%. The maximum strains at 1 mm/24 h were 1.7-1.8 times greater than those at 10(4) mm/24 h in both the control and experimental subgroups. It is assumed that changes in the mechanical properties of the periodontal ligament were caused by inhibition of maturation of the periodontal collagen fibres. Assuming that the periodontal ligament is viscoelastic in nature, it is suggested that the main component reacting at 10(4) mm/24 h was an elastic one and that both components, with emphasis on the viscous one, interact at 1 mm/24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Tooth movement

This article reviews the evolution of concepts regarding the biological foundation of force-induced tooth movement. Nineteenth century hypotheses proposed two mechanisms: application of pressure and tension to the periodontal ligament (PDL), and bending of the alveolar bone. Histologic investigations in the early and middle years of the 20th century revealed that both phenomena actually occur concomitantly, and that cells, as well as extracellular components of the PDL and alveolar bone, participate in the response to applied mechanical forces, which ultimately results in remodeling activities. Experiments with isolated cells in culture demonstrated that shape distortion might lead to cellular activation, either by opening plasma membrane ion channels, or by crystallizing cytoskeletal filaments. Mechanical distortion of collagenous matrices, mineralized or non-mineralized, may, on the other hand, evoke the development of bioelectric phenomena (stress-generated potentials and streaming potentials) that are capable of stimulating cells by altering the electric charge on their membrane or their fluid envelope. In intact animals, mechanical perturbations on the order of about 1 min/d are apparently sufficient to cause profound osteogenic responses, perhaps due to matrix proteoglycan-related "strain memory". Enzymatically isolated human PDL cells respond biochemically to mechanical and chemical signals. The latter include endocrines, autocrines, and paracrines. Histochemical and immunohistochemical studies showed that during the early places of tooth movement, PDL fluids are shifted, and cells and matrix are distorted. Vasoactive neurotransmitters are released from periodontal nerve terminals, causing leukocytes to migrate out of adjacent capillaries. Cytokines and growth factors are secreted by these cells, stimulating PDL cells and alveolar bone lining cells to remodel their related matrices. This remodeling activity facilitates movement of teeth into areas in which bone had been resorbed. This emerging information suggests that in the living mammal, many cell types are involved in the biological response to applied mechanical stress to teeth, and thereby to bone. Essentially, cells of the nervous, immune, and endocrine systems become involved in the activation and response of PDL and alveolar bone cells to applied stresses. This fact implies that research in the area of the biological response to force application to teeth should be sufficiently broad to include explorations of possible associations between physical, cellular, and molecular phenomena. The goals of this investigative field should continue to expound on fundamental principles, particularly on extrapolating new findings to the clinical environment, where millions of patients are subjected annually to applications of mechanical forces to their teeth for long periods of time in an effort to improve their position in the oral cavity.(ABSTRACT TRUNCATED AT 400 WORDS)

In-vitro mechanics of intrusive loading in porcine cheek teeth with intact and perforated root apices

Cyclic intrusive loads were applied to deciduous cheek teeth and monitored by radiography in dissected mandibles in three successive states: (A) intact, (B) vertically bisected in a bucco-lingual plane, (C) with perforation of the apices by lateral drilling of the mandible subsequent to (B). Loading was in a compression cage in an Instron mechanical testing-machine utilizing cross-head speeds of 0.5 and 5 mm/min, leading to loading rates of approximately 2 and approximately 20 N/s. Peak loads were approximately 100 N, causing an intrusion of approximately 200 microns. Load/recovery curves for each tooth in the three states were recorded and the data treated assuming: (a) a simple shear on the periodontal ligament, (b) tensile loading on the fibres of the collagen fraction of the ligament alone. Treatment (b) gave the best agreement with published mechanical data on isolated mammalian periodontal ligament. The small effect of apicectomy on the mechanical behaviour indicates that a mobile fluid-like support mechanism is unlikely to operate for major loads in vitro and tensile support by the collagen fibres of the periodontal ligament is the most likely dominant mechanism.

Collagen and prostaglandin synthesis in force-stressed periodontal ligament in vitro

A periodontal organ culture system capable of receiving orthodontic type forces was developed. Histological, radioautographical, collagen, and prostaglandin synthetic data demonstrated the vitality of the organ over a 24-hour period of culture. Significant increases in the proportion of type III collagen synthesized during periods of active stress were found, but no alterations in relative levels of prostaglandins synthesized during periods of force application were discernible.

The effects of continuous axially-directed intrusive loads on the erupting rabbit mandibular incisor

A cantilever beam strain-gauge transducer was used to apply continuous light intrusive loads (0.2-0.4 and 2.5 g) to unimpeded mandibular incisors. Tooth position was continuously monitored using a variable capacitance displacement transducer. After 4 days of unimpeded eruption, the animals were divided into three experimental groups: no axial loading (6 rabbits); 0.2-0.4 g of axial loading (3 rabbits); 2.5 g of axial loading (3 rabbits). During a 4-h period, light forces (0.2-0.4 g) slowed eruption to a stop, whereas a heavier force (2.5 g) intruded the teeth. The data support Burn-Murdoch's (1981) (Archs oral Biol. 26, 939-943) contention that the eruptive force of unimpeded continuously-erupting incisors is significantly less than previously thought.

Movements of healthy and periodontally involved teeth measured with laser reflection technique

The laser reflection method of remote measurement was used to register small tooth movements of upper central incisors from a group of subjects with healthy teeth and another group with periodontally involved teeth. The mobility of the healthy maxillary incisors over a period of 24 hours was well within normal limits. Among orthodontically treated and periodontally involved maxillary incisors relapse was larger sagittally than laterally and largest buccally. This tendency was greatest during the first 5 hours and almost reached its maximum after 1 day. These movements were within the limits of mobility but were not always correlated with mobility or the loss of marginal bone. Furthermore, the relapse was not correlated with the magnitude of orthodontic correction. In this respect, the mean buccal relapse was low, however, individual variations were large. The extent of relapse may be related to the varying intensity of parafunctional habits.

Experimental intrusion of rat incisors with continuous loads of varying magnitude

The present investigation deals with the relationship between continuous intrusive loads and the rate of intrusion of rat incisors. A method for the application of constant, defined loads by means of a closed coil spring is described. In seventeen rats the left mandibular incisor was shortened to prevent occlusion. The animals were exposed to direct light, medium, and heavy intrusive loads for a period of 12 days. Light loads (1.5 to 8.0 Gm./cm.2) did not cause active intrusion of the teeth. Medium loads (12.0 to 18.5 Gm./cm.2) initially elicited marked intrusion, followed by a short rest period after which the intrusive movement progressed steadily at a daily rate of about 25 micrometers. Heavy loads (30.5 to 32.0 Gm./cm.2) brought about active intrusion, which commenced only after 8 days of force application. The medium loads, having a magnitude in the range of rat systolic blood pressure, proved to be optimal for the intrusive movement.