archives of oral biology 57 (2012) 1575–1584 Available online at www.sciencedirect.com journal homepage: http://www.elsevier.com/locate/aob Review Tooth–PDL–bone complex: Response to compressive loads encountered during mastication – A review Gili R.S. Naveh a,*, Netta Lev-Tov Chattah a, Paul Zaslansky b, Ron Shahar c, Steve Weiner a a Department of Structural Biology, Weizmann Institute of Science, 76100 Rehovot, Israel Berlin-Brandenburg Center for Regenerative Therapies, Julius Wolff Institut (JWI), Charité – Universitätsmedizin, Berlin, Germany c Koret School of Veterinary Medicine Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Israel b article info abstract Article history: The components of the tooth–periodontal ligament (PDL)–alveolar bone complex act in a Accepted 10 July 2012 synergistic manner to dissipate the loads incurred during mastication. The complex incorporates a diverse array of structural features for this purpose. These include the non-mineralized Keywords: and hence soft PDL that absorbs much of the initial loads. The internal structure of the tooth Tooth loading also includes soft interphases that essentially surround the dentine core. These interphases, Periodontal ligament although stiffer than the PDL, still are more compliant than the dentine core, and are thus key Interphases components that allow the tooth itself to deform and hence help dissipate the compressive Micro-CT loads. There is also direct evidence that even under moderate compressive loads, when the Mastication tooth moves in the alveolar bone socket, this movement is guided by specific locations where Espi the tooth comes into contact with the bone surface. The combination of all these responses to Alveolar bone load is that each tooth type appears to move and deform in a specific manner when loaded. Much, however, still remains to be learned about these three-dimensional responses to load and the factors that control them. Such an understanding will have major implications for dentistry, that include a better understanding of phenomena such as abfraction, the manner in which tooth implants function even in the absence of a PDL-like tissue and the implications to bone remodelling of the movements imposed during orthodontic interventions. # 2012 Elsevier Ltd. All rights reserved. Contents 1. 2. 3. 4. 5. 6. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bottom-up approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Top down approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. The tooth in the mandible compared to the tooth embedded in Epoxy . 3.2. Direct observations of the tooth root–PDL–alveolar bone complex under Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Implications for dentistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding comment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . * Corresponding author. Tel.: +972 8 934 2547; fax: +972 8 934 4136. E-mail address: Gili.Naveh@weizmann.ac.il (Gili R.S. Naveh). 0003–9969/$ – see front matter # 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.archoralbio.2012.07.006 ........... ........... ........... ........... compression ........... ........... ........... ........... ........... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1576 1577 1578 1580 1580 1580 1582 1582 1583 1583 1576 1. archives of oral biology 57 (2012) 1575–1584 Introduction Teeth residing in the jaw socket incorporate many structural features and a range of materials. These all function in a synergistic manner not only to fulfil the key function of food particle breakdown (comminution), but also to prevent or minimize damage to the tooth.1 Teeth may incur mechanical damage by overload, by abrading each other and by wear due to interactions with hard food particles.1 Some of this irreversible micro-damage occurs when tooth deformation exceeds the elastic limit. One strategy to minimize or prevent damage is the built-in capability of teeth to move and deform in response to load, such that the deformation remains within the reversible elastic range. This is achieved by the incorporation of relatively soft components that can act as shock absorbers.2 By moving and deforming, teeth dissipate the loads encountered in a way that reduces stress concentration and minimizes the chance of irreversible damage.3 This review focuses on ways in which the tooth-periodontal ligament bone complex responds to short term loads encountered during routine mastication. Mastication in humans involves the cyclic loading of teeth by forces of usually tens of Newtons, or occasionally even as much as hundreds of Newtons.4–6 The thrust time during mastication can be less than a second.2,7 Teeth also encounter forces due to clenching that can even last for minutes,5 and in orthodontic treatments for days and weeks.8 During each chewing cycle, teeth may move as much as tens of microns.2,7 The upward movement of the mandible and the resistance to this movement by the maxillary teeth while crushing intervening food particles, results in teeth being loaded in compression both on the crown and on the root. In addition, the mandible can also move horizontally and may cause a lateral movement of teeth. As a result, an individual tooth may be loaded in a complex, three-dimensional manner. To facilitate withstanding loads without failure, each tooth incorporates several structural features, known as interphases. These are three-dimensional interfaces between the dentine and enamel in the tooth crown9–11 and between dentine and cementum in the tooth root12 (Fig. 1a). Both crown and root interphases are composed of a relatively soft layer that acts as a cushion between two harder materials in the tooth itself.11,13,14 The presence of this soft layer with a relatively low elastic modulus as compared to that of the materials flanking it,15 reduces the stiffness of the whole tooth and minimizes irreversible damage during mastication.11 There is however another much softer component in the complex, namely the periodontal ligament (PDL). The PDL is unmineralized and hence it is at least an order of magnitude Fig. 1 – (A) Schematic illustration of a mesial-distal longitudinal section of a rat mandibular first molar (M1) showing the different components of the tooth–PDL–alveolar bone complex. Note that the light yellow band (the sub-DEJ soft zone) in the crown and the dark yellow band (the cementum–dentine junction) in the root form an almost continuous soft interphase over the entire dentine core. (B) Schematic illustration, drawn approximately to scale, showing the relative extents to which the various components of the tooth–PDL–bone complex will contract (strain) under an axial stress of 100 N/mm2 based on the elastic moduli cited in the text. Note that the PDL compresses a lot, the enamel very little, and the interphases somewhere in between. archives of oral biology 57 (2012) 1575–1584 softer in compression than even the softest component of the tooth interphases. Therefore when the entire tooth complex is loaded in compression, and since the tooth, PDL and bone are essentially loaded in series, it is the PDL that initially deforms the most. The other stiffer structures, including the different interphases, enamel, dentine and alveolar bone, also deform, but to a much lesser extent due to their much higher stiffness. This follows the rule of multiple springs in series whereby the overall stiffness of the system (the so-called equivalent spring constant keq) is: 1 1 1 ¼ þ keq k1 k2 (1) where k1 and k2 are the individual spring constants of the different springs. This expression for two springs in series, means that in cases where k1  k2, keq is close to k2, implying that when compressed the system will deform mostly in the weak spring. However, since the two springs are in series, both are subjected to the same load and both will deform. As a result, when loading the tooth–PDL–bone complex in compression, the PDL will initially deform much more than the other, much stiffer components (Fig. 1). It is therefore helpful to classify all the components of the tooth–PDL–bone complex according to their stiffnesses or elastic moduli. The PDL elastic modulus in compression is on the order of tens to hundreds of MPa,16 whereas the moduli of the different components of the tooth and bone are at least an order of magnitude stiffer: 3–10 GPa for the soft zone beneath the DEJ,15 2–4 GPa for the cementum – dentine interphase,17 3– 15 GPa for cementum,13 10–20 GPa for bone in general18 and intertubular dentine 19, around 30 GPa for peritubular dentine20 and 75–100 GPa for enamel.21,22 The little that is known about the elastic modulus of the alveolar bone in the proximity of the tooth socket, shows that it ranges from 0.2 to 10 GPa (reduced modulus).23 As these components are arranged in series, the softest component (the PDL) will deform much more than the others under compressive load. This is schematically illustrated in Fig. 1b. This complex response to load is exemplified in the forcedisplacement curve obtained when teeth are loaded while still in the intact mandible (Fig. 2). Mühlemann24 was probably the first to produce such a force-displacement curve for a tooth. He even carried out the measurements in vivo. In an insightful experiment he compared a re-implanted incisor to the adjacent original incisor of a young human male, and clearly demonstrated that the first part of the curve (where low loads are generated in response to large displacements) is absent in the re-implanted tooth. He thus convincingly demonstrated that this part of the curve must be due to the presence of a viable PDL. Basically the same curves are obtained from different teeth, provided the loading time spans seconds and minutes (the masticatory regime).7,25,26 The specific objective of this review is to shed light on the structural components of the tooth–PDL–bone complex that are responsible for this loading curve. This can be approached in two ways. 2. Fig. 2 – Representative force displacement curve obtained from a rat 1st molar compressed while still present in the intact mandible. A linear actuator programmed to move 150 mm and the generated forces resisting the tooth axial (apical) movement were recorded. Note that the x-axis shows the movement of the anvil. This however does not reflect the actual movement of the tooth, as some of the displacement is taken up by the load cell and the mounting medium. Forces used were well within the range normally encountered during mastication in rat incisors57 and probably molars as well.58 The loading rate was between 0.5–4 mm/s, the change in rate had no effect on the curve shape. The first and third parts of the curve are more or less linear. Note that initially the system is extremely compliant, manifesting significant displacement with very little increase in load (1st part). After a non-linear transition period (2nd part), the system once again behaves linearly, but in a much stiffer manner. A detailed description of the loading system can be found in Naveh et al.56 1577 Bottom-up approach The first approach is a ‘‘bottom-up’’ approach based on determining the precise geometry and materials properties of the different components of the tooth–PDL–bone complex. With this information it is possible to create a numerical computer model that introduces these materials properties into the 3D geometry of the internal structure of the tooth–PDL–bone complex. The complex is then loaded in silico, and the model may be used to calculate how the entire complex deforms under load. This so-called finite element analysis (FEA) is a widely used tool for predicting how isolated teeth and parts of teeth behave under load. 27–29 The validity of such predictions depends on the precision of the geometric representation and the precision and accuracy of the assigned material properties. Modern highresolution imaging methods allow the model of a tooth–PDL– bone complex to be geometrically very precise. However, the accuracy and precision of the mechanical properties of the tissues – PDL, cementum, dentine and the various interphases are much more problematic. This stems from the fact that these biological tissues are anisotropic, graded and viscoelastic. Thus their properties are extremely dependent on location, direction and time. The mechanical properties of the PDL have been the focus of many studies, since the PDL is the most compliant 1578 archives of oral biology 57 (2012) 1575–1584 component of the system. The PDL thus dominates the tooth movements induced by low loads. 9,25 One of the first studies of the materials properties of one of the components of the PDL, was that of Synge. Synge considered the PDL to be an incompressible membrane and speculated that the resistance to load on the tooth is due to hydrostatic pressure.30 Measurements of the ‘elastic modulus’ of the PDL in different studies range from 0.07 to 1750 MPa (reviewed in Rees and Jacobsen16). This large range reflects the complexity of the tissue properties that are also sensitive to the methods used for measuring and making the calculation, the non-linear response of the PDL to loads, as well as species type, age of the individual and tooth functional differences.31–33 The PDL response to load is now generally thought to involve two systems. The first is a hydrostatic response, which is due to the presence of blood vessels, extracellular matrix incorporating proteoglycans, glycoproteins and bound water.34,35 The second is an elastic system mainly in the form of collagen fibre bundles that connect the tooth root surface (cementum) to the alveolar bone surface of the socket.35 One view is that the PDL alone is responsible for the tooth movements under small loads, and the tooth itself behaves essentially as a rigid body.9,25 In this case the load displacement curve, especially for low loads, reflects the complex interplay between the collagen fibre system and the hydrostatic/blood vessel system. The inherent problem with the FE approach to understanding how whole teeth respond to load, is that the key structural elements of the tooth and the PDL are inhomogeneous, graded, anisotropic and some (such as the PDL) highly viscoelastic.36–39 This means that there is no single value of the elastic modulus that can be inserted into the FE model for a particular component of the system. These different properties make the creation of a reliable FE model virtually impossible as the elastic properties vary in location and orientation at each point, and are also dependent on the rate of loading.40 Furthermore, the structural elements within the tooth that deform the most under load are the interphases, namely the soft zone that stretches from the dentine to enamel junction (DEJ) about 50–200 mm into the crown dentine (depending on the species),11,41 and the cementum–dentine junction (CDJ) of the root13,42 (Fig. 1). These interphases include zones with relatively low moduli where the strain is highest when a tooth is loaded. Huo managed to incorporate the soft zone interphase into an FE model of the crown and to effectively simulate the strain pattern of a tooth slice under compression.43 This significant achievement does not however take into account that measurements of the elastic moduli on the distal and mesial sides of the soft zone of a human premolar crown show that the elastic modulus varies from around 3.5 GPa on the buccal side to around 9.7 GPa on the lingual side.15 So even this interphase is graded laterally (and its real 3D spatial variation is likely to be much more complex)! A second difficulty with the FE approach is that the model obtained has to be validated against actual independent experimental measurements. Huo for example used strain maps generated by Moiré interferometry.43 Ideally the data used for verification should be the manner in which the whole tooth–PDL–bone complex responds to load. One experimental approach for obtaining the validation data could be to load the tooth–bone complex and map the surface displacements; in essence a ‘‘top-down’’ approach (see below). Such a displacement map can then be used to partially validate an FE model. This FE ‘‘reversed engineering’’ approach has been effectively used to study the crowns of isolated teeth embedded in Epoxy.44 This study showed, for example, that the surface displacements caused by axially loading the tooth crown are not much affected by drastic variations in the elastic modulus of the bulk dentine, as long as the ‘soft zone’ was modelled with its characteristic low stiffness. This in turn is consistent with the notion that much of the strain incurred during compression of the isolated tooth is taken up by the soft zone interphase between the enamel and the bulk dentine. 3. Top down approach Although it is impossible to accurately simulate the in vivo environment in which teeth function, any attempt to learn about a tooth response to load that is relevant to the in vivo environment should take the following into account. (1) The normal range of forces that teeth are subjected to during physiologic mastication is up to hundreds of Newtons in humans.45,46 (2) The tooth displacements during mastication that can be expected for physiologic forces range from microns to a few tens of microns.7,41,47 (3) The displacements are three-dimensional. (4) The tooth–PDL–bone complex operates in a wet environment. (5) There are enzymes in the PDL tissue that degrade the connective tissues after removal from the animal, and this limits the time during which data can be collected. These are very stringent requirements for any ex vivo system. Surprisingly, the first experiments to use this top-down approach were carried out in vivo, despite all the inherent difficulties. These experiments involved ingenious in-house built devices that were capable of measuring micron-scale displacements. One of the first such experiments was that of Mühlemann who produced a simple version of the characteristic force–displacement curve for human maxillary incisors.48 Movements of the teeth were recorded at force levels of 1 N, 5 N and 15 N. He showed that in this force range tooth movement can be divided into three linear phases: initial, intermediate and terminal. Besides demonstrating that the initial part of the curve is due to the PDL response to load, he also inferred that the high load part of the curve is due to interactions between the tooth root surface and the alveolar bone. Parfitt developed a more sophisticated instrument that could measure force and displacement continuously in vivo26. The force applied was up to 10 N and the response was attributed to the PDL collagen fibre system as well as the blood vessels in the PDL. Picton made in vivo measurements of tooth movements, as well as the associated labial bone due to loads of up to 10 N.49 This study clearly showed that the bone also deformed as a result of the load applied to the tooth, but the magnitudes of the deformation of the alveolar bone were much less than those of the tooth. In many respects these ‘‘first generation’’ in vivo experiments set the stage for further studies. The major deficiencies of these in vivo studies were that movements were measured in only one dimension, whereas the force application and the archives of oral biology 57 (2012) 1575–1584 tooth movements are in fact 3 dimensional. There were also problems with the precision and accuracy of the measurements which were relative to other moving object (such as neighbouring teeth and cortical plates), and the experimental apparatus was invasive and influenced the tooth movements. The focus then shifted to using non-invasive optical metrology methods either in vivo or on model teeth,50 including models composed of photoelastic materials where stress distributions due to loads can be directly measured.51,52 Although no radically new insights appear to have been obtained from these early optical studies, they highlighted the potential of optical metrology for carrying out dynamic studies of whole tooth displacements as a function of increasing load. The basic idea is to superimpose images containing 2 or 3 dimensional information, such as holograms or Moiré patterns, before and after the application of a load, and determine the displacements based on the interference pattern. Our approach to this subject started with the question of how the whole tooth responded to load after being extracted from the mandible and embedded in a stiff polymeric matrix. Using Moiré interferometry we showed that upon compression of a slice of a human premolar, most of the contraction (strain) was taken up in a zone or interphase some 200 microns wide just below the DEJ.11 This soft zone has a distinctly different structure than the bulk dentine15 and is less mineralized.11 Earlier studies of hardness variations on tooth sections had clearly identified this zone,14 but its important role in load response was not appreciated. Morita et al. also conducted deformation mapping of a tooth slice, but still in the mandibular bone.53 A porcine molar tooth was loaded apically. By comparing images before and after deformation using the digital image correlation (DIC) technique displacements were measured with a resolution of several microns. These 2D movements under wet conditions showed that PDL contraction was first detected, followed by movement of the alveolar bone. No contraction within the tooth was observed.53 Clearly however, extrapolation from 1579 the study of a tooth slice to how a whole tooth responds to load, is problematic. Measurements of whole tooth deformation may be performed with Electronic Speckle Pattern Interferometry (ESPI). This is an optical non-destructive metrology method that maps displacements at the sub-micron level even on irregular surfaces. ESPI measures the changes in intensity of interference patterns between superimposed reflected laser beams from images captured before and after loading. The loading changes the light path length and hence changes are seen in the intensity of the different interference patterns across the observed surface. The method is capable of detecting surface displacements of about 1/30 of the laser wavelength (around 30 nm), and by a combination of X, Y and Z interferometers, these displacements can be measured along 3 orthogonal directions. Liu et al. used ESPI to measure whole human incisor displacements and from this combined with an FE model they deduced the elastic modulus of the PDL.54 These experiments were not however carried out under wet conditions. Zaslansky et al. used ESPI to measure displacements of the surfaces of human premolars extracted from the mandible and embedded in Epoxy. The measurements were made under water.55 The maps of the human premolars showed that initially little deformation occurred when the load was applied to the larger cusp. Then the enamel cap both deformed in an off-axis direction and rotated. Interestingly an exact replica of the same tooth composed of acrylic, deformed in the same way, but the displacement magnitudes were very different as the material used for replication was much softer. This showed that morphology is a key component in determining how a tooth responds to load, over and above tooth internal structure. There were however interesting differences between the tooth and the replica. One is that the location of minimal surface displacements of the tooth crown is close to the contact area with the neighbouring tooth. This ‘‘design’’ feature reduces potential damage to the neighbouring teeth during movement. Fig. 3 – Vertical displacements (in microns) of a mini-pig M1 crown loaded in the axial direction and measured by ESPI. In situ: tooth inside the mandible, isolated: the tooth was extracted and embedded in a stiff polymer. The short thick arrows show the loading direction, and the long thin arrows the crown movement directions. x and y axes are the rows and columns respectively of the CCD pixel array. Colour bar: the displacement range in microns. The rectangles show the displacement average and standard deviation of an area in the tooth (in mm). The figure is adapted from Lev-Tov Chattah et al.41 (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.) 1580 archives of oral biology 57 (2012) 1575–1584 3.1. The tooth in the mandible compared to the tooth embedded in Epoxy Clearly, the study of an isolated tooth embedded in Epoxy provides only partial information on the manner in which the whole tooth–PDL–bone complex functions under load. We therefore used the ESPI to study teeth while still in the mandible (Fig. 3). When the M1 of a mini-pig was loaded (up to 75 N) while still in the mandible, the tooth responded such that the loaded cusp immediately started moving down in the loading direction, and the opposing cusp moved up and vice versa. Thus the predominant movement of the tooth crown while still in the mandible and subjected to point-contacts on either of the cusps resembles a see-saw. A small tilt in the lingual direction was also observed. The tooth after extraction and embedding in Epoxy was loaded in the same way and surprisingly the tooth crown movement responses were the same as when the tooth was loaded in the mandible, but the magnitudes of the displacements were about half of those observed when in the mandible41 (Fig. 3). This experiment shows that when forces of around 75 N are applied, the mechanisms responsible for movement of the tooth while still in the mandible involve the PDL, as well as the tooth internal structures and morphology, whereas after extraction the movements can only involve the tooth internal structure and the morphology. Despite this, the overall movements are similar, implying that all the components of the system act in a synergistic manner. A similar experiment was carried out on the incisors of a macaque monkey.47 The macaque incisors responded to load quite differently from the mini-pig molars. When still in the mandible, the incisors barely moved downward (apically) in the direction of the load, but mainly moved laterally (buccolingually). The overall movement in three dimensions was similar to the movement of a cantilever. Again, once extracted and embedded in Epoxy, the cantilever-like movement was maintained, but the magnitudes of the displacements were reduced. A key question is thus how do the structural features of the tooth–PDL–bone complex work together in order to enable the different movements of each tooth type? Direct observations of the tooth root–PDL–alveolar 3.2. bone complex under compression Micro-CT can provide direct 3D views of the whole root–PDL– bone complex. A high-resolution image of an uncompressed tooth clearly shows that the thickness of the PDL varies substantially in different locations (Fig. 4). From this it can be surmised that if the tooth mainly moves apically into the socket during axial loading, the tooth will eventually contact the bone at specific locations and not in a uniform manner over most of the cementum–bone surfaces.23 This expectation was directly demonstrated by loading an M1 tooth in a freshly extracted rat hemi-mandible and monitoring the tooth movement as a function of load using a micro-CT (4 mm pixel size resolution).56 The loads used are within the physiological range measured for rat incisors57 and, by extrapolation from data on human teeth58 taking into account the size difference, probably molars as well. The loading experiment was repeated on the same specimen several times with the same results, indicating that the PDL and other soft and hard tissues had not significantly degraded during Fig. 4 – A high-resolution micro-CT 2D image showing the variable thickness of the PDL of the rat M1 (coronal section). Note too that the soft tissues of the PDL are visible even in the unstained state (Naveh et al., in preparation). The image was obtained using a microXCT 400 (XRadia). Arrows indicate the thickness variations of the PDL. Scale bar: 500 mm. the experiment. The fact that the experiment was carried out ex vivo implies that any contribution of blood pressure to the first part of the load-displacement curve,59–61 would be absent (Fig. 2). See Naveh et al.56 for more details. Contact with the alveolar bone was observed at around 10 N, and 3 specific contact areas were identified – at the furcation between the 4 roots, and one contact area in each of the buccal and lingual roots. During axial loading, no contacts were observed in the distal and mesial roots. Fig. 5 shows the high-resolution (around 1 mm) detailed structure of the furcation area before and after compression. A contact area can be seen between the cementum and the alveolar bone at the furcation. As the pixel size is about one micron, it cannot be excluded that some highly compressed PDL tissue is present between the bone and the root cementum surface. The identification of these three contact points can also account for the see-saw like motion observed for mini-pig molars using ESPI. The furcation acts as a fulcrum and the buccal and lingual contacts essentially restrict much of the movement in the buccal–lingual direction. The lack of contacts and the relatively thick PDL in the mesial and distal roots allow for unrestricted movement in this direction. The result would be a rocking or see-saw like motion. (Fig. 6). 4. Discussion The direct observations of the tooth under load using the micro-CT leave little doubt that the alveolar bone is involved in archives of oral biology 57 (2012) 1575–1584 1581 Fig. 5 – Micro-CT 2D images of a reconstructed rat M1 (a) before and (b) after compression with an applied load of 20 N. A contact between the tooth and the bone is clearly visible in the furcation area (arrows). For more information on the loading method and results, see Naveh et al.56 Scale bar: 150 mm. resisting short term responses even under loads of a few tens of Newtons. This confirms Mühlemann’s [24] conclusion and various direct measurements of strain in alveolar bone during mastication.24,48,49,51,52 We would therefore interpret the structural basis for the force-displacement curve shown in Fig. 2, as involving mainly, but not only the PDL at low loads (first part of the curve in Fig. 2), and mainly the alveolar bone, Fig. 6 – 3D view of the rat M1 in the alveolar bone socket. The molar and surrounding bone were volume rendered using ‘‘Avizo’’ software, and superimposed on a 2D slice. The bone is virtually ‘‘cut’’ in the distal–mesial plane that includes the furcation to expose the internal structure, and reveals the relatively thick PDL of the mesial and distal roots. The arrows indicate the rocking or see-saw like motion observed when the tooth is loaded on either the mesial or distal cusps.41 Scale bar: 500 mm. but not only the alveolar bone at high loads (third part of the curve in Fig. 2). The smooth transition from the low load part of the curve to the high load part of the curve (second part of the curve in Fig. 2) represents the increasing contribution of the tooth interacting with the bone as the compressed PDL becomes stiffer. The factors responsible for this smooth transition are probably a complex interplay between the manner in which the tooth root surface contacts the alveolar bone at specific points, the response of the interphases within the tooth to the load and possibly other factors such as deformation of the alveolar bone. Clearly much more needs to be known about this crucial part of the load–displacement curve and the state of the PDL at the contact points. In the tooth isolated from the mandible, much of the deformation within the tooth crown caused by compressive loading is in the interphase below the DEJ, as the interphase contains the softest component in the tooth crown. Thus the simplest interpretation of the see-saw like motion observed for the molar crown embedded in Epoxy, is that as the tooth was loaded on one side, the interphase beneath the load contracts and the interphase on the other side of the crown is essentially unaffected. When the single rooted human premolar embedded in Epoxy was loaded more or less across its whole occlusal surface, the crown at some point underwent an asymmetric, rotational deformation.55 This observation leads us to assume that some asymmetry in the interphase elastic properties must exist. Zaslansky et al.15 indeed showed (using ESPI to measure strain on rectangular shaped samples cut out of human premolars to include the enamel–dentine boundary) that the lingual side of the interphase is about 3 times stiffer than the buccal side of these teeth. Such a difference in stiffness must lead to an asymmetric deformation and motion of the entire crown. We clearly need to know much more about the graded properties of this sub-DEJ interphase in three dimensions, as well as the root interphase between the dentine and the cementum, in order to understand the basis of tooth crown differential contractions even when embedded in Epoxy. 1582 archives of oral biology 57 (2012) 1575–1584 The markedly different responses to load in the axial direction of the mini-pig molar and the macaque incisor, raise many interesting questions. Are the observed responses representative of all incisors and all molars in different species? We do have preliminary observations that suggest that isolated and embedded human molars also respond to load in the see-saw like manner. If indeed incisors and molars in general respond in a characteristic manner, do other tooth types also have their characteristic movement response modes? An early in vivo study by Picton showed that different tooth types move differently in the socket when relating to their lateral and vertical mobility, despite the measurement difficulties involved with such experiments.62 It would appear that surface structure of the bony socket plays a crucial role in guiding tooth movements under compression. It has been reported that there is a unique bone structure in this region where the collagen fibres are incorporated into the bone. This is called bundle bone.23,63 It has also been observed that there are hyper-mineralized areas in the alveolar bone close to the socket surface.23 It would be very interesting to determine if these unique structures are associated with the contact regions between the tooth surface and the alveolar bone. A close examination of the furcation area in the molar alveolus will also be of much interest. Curiously in the rat M1, the furcation area is composed of relatively porous bone, and not a solid compact bone-like structure (Fig. 5). Could this porous structure provide the bone with more resilience and be yet another shock absorbing structural feature? The number of roots that a tooth has will influence the type of movement that loading can cause. A single root, such as in the macaque incisor, will allow simple downward (apically) movement along the tooth axis until the tooth surface encounters the alveolar bone. This is unlikely to be at the root apex as this has to remain open. It is more likely to be somewhere along the root. It is difficult to a priori conceptualize how 2, 3 and 4 root teeth will move without knowing more about the relative thicknesses of the PDL in three dimensions for each root. For example, in the rat M1 the PDL thicknesses of the buccal and lingual roots are in general much thinner than the mesial and distal roots.56 This is in turn responsible for the major movement being in the mesial – distal plane, and not the mere fact that this is a 4 root tooth. Only more experiments involving direct observation of tooth root movement under load can shed light on this issue. Loading the tooth from different directions will also be of much interest. 5. Implications for dentistry The perspectives presented on the roles of the PDL, the internal structures of the tooth and the alveolar bone during loading, raise several issues that are relevant to dentistry. Abfraction is a term describing wedge shpaed non-carious loss of tooth material in the cervical region of the tooth crown, which may lead to tooth fracture.64 The aetiology is related to strains that are developed within the tooth due to occlusal forces, that result in localized fractures in the cervical region.65,66 An intriguing fact is that abfractions are most likely to form on the buccal aspect of pre-molar and/or molar teeth, which are in most cases multi-rooted teeth.67 We therefore propose that the formation of abfractions is related to the seesaw-like motion of multi-rooted teeth with the furcation acting as a fulcrum. Furthermore, the fact that the occlusal area and the long axis of the roots are not perpendicular to each other56,62 may also be related to the formation of abfractions. Dental implants function remarkably well without a PDL. The more common implants essentially abut the alveolar bone.6 In this review we emphasize that even in normal tooth response to loads, the alveolar bone plays a significant role by interacting directly with the tooth root surface. This may explain in part why implants that are well integrated into the alveolar bone are more successful than when connective tissue is formed.68 It would be of particular significance to better understand whether alveolar bone has specific characteristics that are adapted to the role of stress absorption during mastication, and then to determine if such structural properties also develop around implants. We also highlight the fact that the internal structures within the tooth, and in particular the interphases, play an important role in stress distribution, even under moderately low loads. This raises the question of whether or not implants might benefit from having their own ‘‘built-in’’ interphases. In fact implants with ‘‘softer’’ inner parts were developed.69 However, success was not achieved. With the advanced understanding of the functionality of the tooth bone complex this interesting initiative could be readdressed. We have observed that single rooted and multi-rooted teeth move differently in the socket under loads. If this proves to be a general phenomenon, then it follows that the combination of different tooth types as abutments for a single dental bridge may not be advisable. This should be further investigated. In orthodontics tooth movements are generally divided into three types; translation, rotation and combined translation and rotation.70 The extent and type of these motions are dependent on the centre of rotation and resistance of the moving tooth. These movement ‘‘centres’’ represent a rather simplistic view of what in reality is a cascade of 3D movements that differ between different tooth types. It is possible that an actual ‘‘centres of rotation and resistance’’ can be identified by monitoring the 3D movements of teeth under load using a micro-CT. These might well reflect the specific tooth-bone contact areas. The state of the PDL in the contact area between cementum and bone is enigmatic. Fig. 5 shows that under compression the thickness of the contact is within one pixel size, which in this case is less than a micron. This raises the questions of whether collagen fibrils which are around 100 nm thick, can be compressed to such an extent, and/or do the fibres move away from the contact area leaving only non-fibrillar macromolecules in the contact area, or possibly even resulting in a direct contact between cementum and bone. See Naveh et al.56 for more discussion. 6. Concluding comment This review highlights the benefits of trying to understand the manner in which whole teeth, while still in the mandible, archives of oral biology 57 (2012) 1575–1584 respond to loads incurred during mastication. Even though the system is very complicated, modern imaging techniques enable deformations and displacements to be measured on whole complexes in three dimensions. The insights gained from this holistic approach highlight many basic issues that need to be better understood, and are beginning to provide novel perspectives on various dental topics. Funding The research was funded by Israel Science Foundation, Grant number 407/10. Competing interests The authors declare there is no conflict of interests. Ethical approval Nothing to declare. Acknowledgements We thank Prof. Robert Druzinsky for critically reading the manuscript. This research was funded by Israel Science Foundation Grant number 407/10. 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