Tissue reaction to orthodontic tooth movement-a new paradigm

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Tissue reaction to orthodontic tooth movement-a new paradigm

  1. 1. European 23 Journal Orthodontics of (2001) 671–681 ♥ 2001 European Orthodontic SocietyTissue reaction to orthodontic tooth movement—a new paradigm*Birte MelsenDepartment of Orthodontics, Royal Dental College, Aarhus University, DenmarkSUMMARY Direct or indirect resorption are both perceived as a reaction to an applied force.This is in contrast to orthopaedic surgeons who describe apposition as ‘the reaction to loadingof bone’. The article reviews the literature on intrusion of teeth with periodontal breakdown,and on the basis of clinical and experimental studies. The conclusion is reached thatintrusion can lead to an improved attachment level, and that forces have to be to low andcontinuous. The tissue reaction to a force system generating translation of premolars and molars inthe five Macaca fascicularis monkeys is described. Three force levels, 100, 200, and 300 cNwere applied for a period of 11 weeks. Undecalcified serial sections were cut parallel to theocclusal plane and a grid consisting of three concentric outlines of the root intersected bysix radii was placed on each section so that areas anticipated to be subject to differingstress/strain distributions were isolated. A posteriori tests were utilized in order toseparate areas that differed with regard to parameters reflecting bone turnover. Based on these results and a finite element model simulating the loading, a newhypothesis regarding tissue reaction to change in the stress strain distribution generated byorthodontic forces is suggested. The direct resorption could be perceived as a result of lower-ing of the normal strain from the functioning periodontal ligament (PDL) and as such as astart of remodelling, in the bone biological sense of the word. Indirect remodelling could beperceived as sterile inflammation attempting to remove ischaemic bone under the hyalinizedtissue. At a distance from the alveolus, dense woven bone was observed as a sign of aregional acceleratory phenomena (RAP). The results of the intrusion could, according to thenew hypothesis, be perceived as bending of the alveolar wall produced by the pull fromSharpey’s fibres.In t r o d u c t i o n A possible explanation for the controversies regarding tooth movement may be that theThe orthodontic literature is characterized by design of the clinical or experimental studiesmany controversies in relation to tooth movement. varies, resulting in conclusions that cannot beThis article will suggest possible solutions, compared. It is important to differentiate betweensupported by the results of studies carried out what is possible in an adult and in a growingon intrusion of periodontally damaged teeth. In individual. It is likewise important to note thataddition, the biological reaction of alveolar bone the local environment has a major influence;to orthodontic tooth movement will be analysed the healthy and inflammed periodontium reactsin the light of the strain distribution of the differently. The force magnitude is often the onlyperiodontal tissues and, based on this, a new parameter considered by investigators. Differentperception of the tissue reaction to orthodontic*Sheldon Friel Memorial Lecture. types of tooth movements do, however, generateforces will be suggested. different force distributions. Since it is most likely that tissue reaction is a result of changes in the stress/strain distribution in the periodontium,
  2. 2. information on the moment to force ratio of theforce system, on the anatomy of the tooth, andon the constraints determined by the perio-dontium is required.672 B. M E L S E N Another explanation for the controversies in overbite caused by an over-eruption of thethe orthodontic literature could be the lack of incisors. If they are to be treated orthodonticallyuniformity and definition. An example of the use with a normal periodontium it can be anticipatedof similar words for different events is the term that the height of the alveolar process will be‘intrusion’, which has been used to describe reduced as a result of this intrusion. However,the reaction following both an occlusal onlay, this is not desirable in patients with a reducedthe displacement of a tooth after tipping, and the periodontium who, in addition to a deep over-result of vertical forces developed by different bite, suffer from increased clinical crown length.orthodontic appliances. Compression of the If intrusion would result in root resorption andmarginal periodontium on the side toward which a a reduced height of the alveolar process astooth is tipped as described by Polson etal.(1984) described above, such treatment would seem tomay be comparable to the compression of the be unwise in these patients.periodontal ligament (PDL) described by Reitan The major concern is therefore to find an(1964) after occlusally directed force application. answer to the following question: consecutively A prospective clinical study of 30 How does The evaluation of the amount of intrusion is intrusion affect the clinical crown where patients treated subjects was conducted, length and thealso controversial. The amount of true intrusion Su b j e c t s an d marginal a l s level in patients with deep over- with e r i bone m a ta reduced periodontium and a a reduceddepends on the type of appliance used (Sander periodontium? bite with an identified treatment aim to intrudeetal.,1996; Melsen etal.,1997; Nanda, 1997).Weiland etal.(1996) demonstrated that intrusion the upper incisors were selected (Melsen etal.,occurred only when applying the segmented arch 1989). All patients underwent a prophylaxisapproach, whereas this is not the case when a programme including motivation and instruction.continuous arch is used. The predictive value Once the oral hygiene was satisfactory, a modifiedof treatment time, force level, and amount of Widman flap surgery was performed and theintrusion vary between studies, and differences gingiva repositioned apically, so that no pocketsin appliance design renders it impossible to above 3 mm were present. Following healing,compare the results. The different results can, on impressions were made and study casts produced.the other hand, be explained by the difference A lateral head film and four periapical radio-in force level and in the load deflection rate used graphs were taken, the latter by means of a filmin the two approaches. holder allowing for reproducible exposure of the A third controversy regarding intrusion is its individual teeth. The orthodontic treatment wasrelationship to root resorption. Reitan (1964) then initiated with an appliance comprising threedemonstrated that intrusion caused both severe passive segments of full wire size stainless steel:root resorption and reduction in the height of two lateral segments connected transpalatallythe alveolar process. Dellinger (1967), however, and an anterior segment consolidating thewhen intruding primate premolars, emphasized four upper incisors. The intrusion of the fourthe importance of the force level for root upper incisors was undertaken using a 0.017 ⋅resorption. Stenvik and Mjör (1970) analysed 0.025-inch TMA base arch adjusted for 50 g ofhuman premolars following intrusion and intrusive force corresponding to 12.5 g per tooth.confirmed this. The increased risk of apical root The point of force application and the line ofresorption seen in relation to intrusion has been action of the force were monitored accordingreported in several studies (McFadden etal.,1989; to the need of the patient, so that either anCostpoulos and Nanda, 1996; Baumrind etal., intrusion in combination with a proclination or a retroclination was obtained. The appliance
  3. 3. clinical crown height and increase the area of the alveolus. As a consequence of these results another question arose: How does intrusion of teeth with 673 a reduced periodontium affect the attachmentwas adjusted monthly until sufficient clinical level?movement was obtained. The treatment time To answer this question an experiment wasvaried between 8 and 18 months. At the end of carried out on six adult Macaca cularis fascitreatment a further set of records was obtained. monkeys. Periodontal breakdown was produced The amount of intrusion was measured with by the placement of orthodontic elastics aroundrespect to a co-ordinate system, where the X-axis the premolars and the upper incisors, which wererepresented the palatal plane and the Y-axis replaced with new elastics until pockets of 4 mma perpendicular through the pterygomaxillary were obtained. Periapical radiographs verified apoint. The displacement of a best-fit template breakdown of the marginal bone correspondingtransferred from the first to the last cephalogram to a minimum of 2 mm. At this point, reversewas used in order to minimize the error of the bevel flap surgery was performed and amethod. Intrusion of the apex, the centroid, and reference notch corresponding to the most apicalthe incisal edge (incision) was recorded. The part of the epithelial junction was produced withclinical crown height was measured on the study a round burr (Figure 2). Following surgery thecasts with an electronic calliper to within 0.01 mm. right side of the jaw was kept meticulously cleanFor each of the four upper incisors the distance with brushing and chlorhexidine rinsing twice abetween the incisal edge, and the highest point of week, while the other side served as the control.the gingival margin was measured at the begin- One week following surgery an orthodonticning and end of treatment, and the difference appliance for the intrusion of upper incisorsrecorded. Based on the periapical radiographs, and premolars was inserted in five monkeysdrawings swere made, and the area of the alveolusRes ult whereas one served as the control. The appliancefor the individual teeth was calculated andThe results revealed that the intrusion of the comprised passive anchorage units of cast stain-corrected for shortening due to root resorptionindividual teeth and the single reference points less steel caps for the molars, passive steel wire(Melsen etal.,1988).varied according to the type of movement consolidating the incisors, and active U-shapedperformed (Table 1 and Figure 1A). The clinical intrusion wires (0.018-inch TMA) extendingcrown height was reduced in 28 subjects and the from the molar splint and loading the occlusalarea of the alveolus was increased in 25 (Tables 2 surfaces of the premolar with 10 g per tooth.and 3; Figure 1B,C). Only two patients without Intrusion of the incisors was undertaken withreduction of the clinical crown height had the same force level by means of a base archpockets above 3 mm at the end of treatment. (0.018-inch TMA) extending from the molar Based on these results it was concluded that splints. The observation time was 3 and 12–16clinical intrusion with low continuous forces of weeks. At the end of the observation period, theperiodontally healthy teeth is recommended, animals were killed under Ketalar anaesthesiaTa bl e 1True intrusion-range in mm.and that this type of treatment may reduce the with perfusion of buffered formalin. The jaws were excised and the tissue blocks decalcified Min Max in EDTA and double embedded in paraffinApex –2 5 celloidin. Eight-µm thick serial sections were cutCR 0 4 parallel to the mesio-distal axis, and stainedIss –2 6 alternatively with hematoxylin and eosin, and van Giesen connective tissue stain. The following parameters were evaluated
  4. 4. B. M E L S E N Fi g u r e 1 (A) Examples of tooth movements performed in an individual patient. (B) Immediately following periodontal surgery. Note that the clinical crown length is increased, but that there has been previous shortening of the incisors by grinding the incisal edge once the teeth had started to elongate. (C) Following orthodontic intrusion. Note the improved gingival level and aesthetic appearance. Reproduced with kind permission from Melsen etal.(1989).Ta bl e 2 Change in bone support area of bony Ta bl e 3 Reduction in crown length (mm).alveolus (expressed as a percentage).x x Min Max Min Max676 –15 22 –1.1 2.1 –4Twenty-five out 30 had an increased area. Twenty-eight out 30 had a reduction.
  5. 5. 675Fi g u r e 2 (A) Photograph of the teeth of a monkey during surgery. An important breakdown of the periodontium has takenplace. (B) Reference points used for the evaluation of the changes occurring during treatment. (C) Clinical photograph ofan intruded tooth (c), where no attention has been given to the periodontal health. The tooth is intruded almost to the levelof the neighbouring tooth (d). (D) Microphotograph of the same situation. Note that the attachment level (a) is still at thenotch and that there is a clear osteoclast activity reducing the height of the alveolar ridge (b). The intruded tooth (c) hasbeen intruded almost to the level of the apex of the neighbouring tooth (d). (E) Composite of a periapical radiograph andthe histological image of the same area. Note that the intruded tooth (a) has kept the same bone level (b) as the non-intruded tooth and the intrusion has thus been ‘into’ bone. (F) Micrograph illustrating the new attachment generated duringintrusion. Note that the notch (n) is significantly below the bone level, and the collagen fibres are inserting into the cellularcementum coronally to the previous attachment level (a, the apical level of the notch). The new epithelial junction (e) hasbeen displaced coronally. Reproduced with kind permission from Melsen etal.(1988).
  6. 6. B. M E L S E Nthe bone level (BL), the gingival level (GM) Figures 2E,F). The width of the PDL was narrowed, the collagen fibres were stretched inand the level of the junctional epithelial (EJ) an apical direction and the cell density in the vicinity of the alveolar wall was increased(Figure 2B). In addition the width of the PDL, (Table 5). Ischaemic areas were avoided and thethe angulation between the collagen fibres of cells were stimulated to an increased DNA synthesis, as reflected by the shift of the cellthe PDL and the alveolar wall were measured. population to a larger component with a large volume of the nucleolus. This study led to theThe cellular density close to the alveolar wall was conclusion that, in the case of a healthy perio- dontium, intrusion improved the periodontalestimated and finally stereometric measurements status, since the attachment level was displacedwere applied for the determination of the in an apical direction in all intruded teeth. The discrepancy in the literature regardingnuclear volume of the cells in the vicinity of the intrusion, particularly of periodontally-involved teeth, can be explained by difference in perio-alveolar wall (Melsen and Kragskov, 1992). dontal health (Ericsson and Thilander, 1978;Ta bl e 4 Variations in apical limit of the notch (AM), the bone leveletal.,1984).gingival level (GM),pockets, Polson (BL), the The existence of and thelevel of the junctionaldemonstrated that the tissue orthodontic intrusionclinically healthy, may still present The results clearly epithelium (EJ) with and without even if kept of the teeth.reaction to intrusionWithperiodontally-damaged of intrusion a risk factor. The tissue reaction generated by Without intrusion + Hygiene – Hygiene + Hygiene – Hygieneteeth depended on the periodontal status of x x x x SD SD SD SDthe teeth. Where inflammation occurred, aAM–BL 1.3 0.29 0.3 0.28 0.4 0.21 0.1 0.5breakdown of the alveolar bone 0.99 observedAM–GM 3.7 was 2.8 0.94 2.1 –3.7 1.7 0.48AM–EJ 1.5 0.76 0.9 0.75 0.3 0.32 –0.1 0.67(Figure 2C,D). In the case of a good oral hygieneT a bconstant forces passing close to the long axis periodontal ligament of orthodontically intruded teeth,low l e 5 Variation in histological parameters of theteeth with mechanical periodontal support and teeth without periodontal disease.of the teeth resulted in a coronal displacement of Intruded teeth Teeth with reduced Teeth with normal support supportthe attachment level in all teeth varying from 0.7 x x x SD SD SDto 2.4 mm, with an average of 1.3 mm (Table 4,The width of the 138.53 9.62 247.44 24.09 188.53 9.75 periodontal ligament 41.27 3.90 69.55 3.26 57.99 5.57Angulation of the collagen fibres to the root surface 59.16 3.32 44.02 4.43 42.41 3.26The relative extension of the 235.30 44.61 182.03 45.08 177.36 50.02 root surface covered by cellsNuclear volume
  7. 7. an orthodontic force system resembles thatof inflammation and, if not kept sterile, willresult in an uncoupling of tissue resorption andformation, leading to a negative balance, i.e. 677more breakdowns will occur. Waerhaug occurrence of hyalinization of the PDL following(1978) demonstrated that, in the case of a pocket high force application. Bone biologists, on thedeeper than 4 mm, it is practically impossible to other hand, relate loading to formation (Frost,obtain satisfactory cleansing by means of scaling 1986). The question to be asked was thus: ‘Whywithout surgical exposure. Therefore, it can do orthodontists relate loading to resorption,be concluded that intrusion of teeth with a while bone biologists relate it to formation’?reduced periodontium should only be carried One of the major theories that relatesout in patients with a healthy periodontium mechanical load to biological reaction is thewithout pathologically increased pockets. ‘mechanostat theory’ developed by Frost (1987), Differences in the type of force delivery may who described a relationship between variousalso contribute to the different results reported strain values and the balance between modellingin the literature (Polson etal.,1984; Melsen, and remodelling of bone (Figure 3). In the case1986; Goerigk etal.,1992; Wilson etal.,1994; of low strain values, a net loss of bone occursCostopoulos and Nanda, 1996; Baumrind etal., as a consequence of increased remodelling. With1996; Parker and Harris, 1998). Intrusion should increasing strain, modelling is initiated and abe completed with light and constant forces positive balance is achieved. Where the strain(5–15 g per root) so that the visco-elasticity of curve crosses the neutral line resorption andthe PDL is taken into consideration. High forces apposition are in balance, and the newly-formedwill result in a trauma to the PDL followed by bone consists of lamellar bone. In contrast,repair, which starts a breakdown of the alveolus. woven bone is formed as a result of even largerForces above a certain magnitude result in strains. Still higher strains will result in a negativeischaemia of the compressed PDL with initially balance, since repair cannot keep up with theno true intrusion occurring. This reaction is occurrence of micro-fractures (Burr etal.,1985).easily distinguished from the stretching of the The borderline between a noxe, i.e. a traumaticPDL fibres seen when the tooth is intruded with stimulus, and mechanical stimulus resulting inlight forces along its long axis. With light forces an increased bone mass, has not yet beenno compression of the marginal part of the established. It is likely that the present modelalveolus occurs; on the contrary a stretching of evokes strain values perceived as trauma, as wellthe fibres and increased cellular activity is as strain values perceived as a mechanical usageobserved (Melsen and Kragskov, 1992). The provoking a structural adaptation to mechanicalincreased cellular activity could also explainthe coronal displacement of the attachment level. Another controversy in relation to tissuereaction to orthodontic forces is found betweenthe orthodontic and the orthopaedic literature Fi g u r e 3 Graphical illustration of bone biological reaction to variation in strain values. In the case of low strain value(Rubin, 1998). Orthodontic forces are ‘known’ (1) remodelling is turned on and a negative balance will beto generate a compression and tension zone. the result. With higher strain values (2), modelling is turnedOrthodontists have traditionally looked upon on and a formation of lamellar bone is occurring. Furtherthe resorption zone as a result of compression increase in strain (3) results in the formation of wovenand apposition as a result of tension. Related to bone. Severe overloading (4) results in a negative balance as a result of the repair process related to the micro-compression, either direct or indirect resorption fractures occurring at this strain level.can be observed. Direct resorption of the
  8. 8. usage [SATMU] (Frost, 1986). Frost (1992) listed fraction recorded as resorption or apposition.the threshold strain values for lamellar bone Bone density was additionally evaluated in thefibrous tissue. A typical minimum effective strain areas mesial and distal to the third molars and(MES) is reported to be 1500–3000 ∝µ for678 second premolars (Gundersen etal.,1988). B. M E L S E Nlamellar bone to start modelling, and if the strain The type of tooth movement registered variedis below 100–300 ∝ remodelling is activated as a from translation to controlled tipping aroundresult of inactivity. Based on the loading of bone a centre localized at varying distances abovetransferred from implants to trabecular bone, the apices of the teeth. The quantity of tooththe values recorded for trabecular bone seem movement varied between 0.21 and 0.9 mmto be slightly higher (Melsen etal.,1996). A per month, measured at the bone margin.large body of evidence concerning structural No relationship between force magnitude, theadaptation to changes in strain has been type of tooth movement, and the amount ofgenerated experimentally and confirmed Frost’s displacement could be verified. All parameters(1992) hypothesis (Goodship etal.,1979; Lanyon measured in areas surrounding the loaded teethetal.,1979; Rubin and Lanyon, 1984). It is logical deviated markedly from the correspondingto ask whether the biological reactions to variables from the unloaded control teeth. Thereorthodontic tooth movement could be explained was an increase in the relative extension ofby the mechanostat theory. In this perspective, resorption from 3–5 per cent observed in theresorption observed in the direction of the force specimens from the control teeth to 7–13 percould be either a result of under- or over-loading. cent of the total cancellous surfaces surroundingThe main question that arose from these the loaded teeth. There was likewise an increasespeculations was thus: ‘How is the stress strain in the extension of appositional surfaces fromdistribution in the periodontium related to the 15–20 per cent in the control, to 35–49 per centbiological reaction of the supporting tissues?’ around the loaded teeth. In order to answer this question an inves- At the time of observation the alveolar wall intigation combining strain levels and biological the direction of the tooth movement was com-reactions was needed. The tissue reaction was pletely resorbed, while woven bone formationstudied in six Macaca cularis fasci monkeys in was seen in the alveolar bone ahead of thewhich the first and the second molars had been direction of the tooth movement. Relative to theextracted, and an appliance was inserted for control teeth, the density in the direction ofapproximation of the second premolar and the tooth movement was increased by a factor ofthird molar by translation. (Melsen, 1999). After ⋅2 3. The alveolar surface opposite to the tooth11 weeks the monkeys were killed under Ketalar movement was clearly appositional (Figure 4).anaesthesia by perfusion with neutral formalin. In order to identify the range of strain levels inThe alveolar process was subsequently excised, the alveolar bone during orthodontic toothembedded in methylmethacrylate, and prepared movement, a three-dimensional finite elementfor the production of 15–20-µm parallel model was constructed representing a conically-horizontal sections between the marginal bone shaped root and the surrounding supportinglevel and the apex of the teeth cut with a structures. Young’s modulus of the root wasdiamond saw. The sections were then stained 20 GPa, while that of the alveolar bone variedwith fast green and analysed by histomorpho- between 0.5 and 2 GPa. The 0.2-mm thick PDLmetric measurements. The fractional resorption was modelled by non-linear spring elements. Insurface [Sfract (f)) ∝µ2/µm2: the extent of compression, their stiffness corresponded to aresorption lacunae as a fraction of the total Young’s modulus of 0.1 MPa, while in tensiontrabecular bone surface] and fractional formation this stiffness steadily increased from 0.1 MPa atsurfaces [Sfract (f)) ∝µ2/µm2 the extent of osteoid 0 mm deformation to 1000 MPa at maximum
  9. 9. 679 Fi g u r e 5 Stress profile of the tissue surrounding anFi g u r e 4 Micrograph of a molar that has been submitted orthodontically loaded tooth moving to the right. Note thatto 11 weeks of orthodontic force in the direction of the the stress of the alveolar wall in the direction of the force isarrow. Note that the alveolar wall in the direction of the very low, whereas it reaches a high value at the marginalforce is resorbing and that the bone in the direction of bone level corresponding to the insertion of the stretchedfrom the level of the of densecrest, to a little overthe movement consists apical woven bone (W). The woven bone formation seen ahead of periodontal fibres.Reproduced with kind permission from Melsen (1999).half the length of the root with a horizontal the alveolus in the direction of the displacementmovement of the root of 25 ∝µ at the level of the could be interpreted as an expression of apoint of force application, the maximal tensile so-called regional acceleratory phenomena (RAP)stresses in the alveolar bone were found to (Figure 3, region 3). According to Frost (1986)occur when the point of force application was any regional noxious stimulus of sufficient magni-located at one-quarter of the root’s length. The tude can evoke a RAP. The extension ofmagnitude of these stresses was then 0.2 MPa, the affected region and the intensity of thewhile on the ‘compression side’ the compressive response varies directly with the magnitude andstresses reached a peak value of 0.04 MPa. the nature of the stimulus. The indirect resorptionThe magnitude of these stresses appeared to takes place in the PDL when ischaemia andbe almost independent of the stiffness of the hyalinization occurs. This is probably due to thebone. The corresponding strain levels in the bone disappearance of the lining cells necessary for thewere 400 ∝στραιν, on the tension and 80 ∝στραιν communication of the osteocytes. The hyalinizedon the compression side. When taken into tissue of the PDL is removed by non-clast cellsconsideration that bone is inhomogeneous and the underlying bone is simultaneouslyand porous, the strain values corresponded to resorbed by osteoclasts, as a repair response to the2.000 ∝στραιν in tension and 400 ∝στραιν in com- damaged tissues (Figure 3, region 4).pression (Figure 5). In the studies reported, the nature of the The strain values found in the direction of the stimulus was a controlled orthodontic force. Thedisplacement were below the MES. In the light strain developed, however, varied in magnitudeof the mechanostat theory this would release both due to the difference in force, and tounderload remodelling, thus explaining direct biological variations related to root size andresorption in the so-called compression side (Figure structure of the bone surrounding the teeth3, region 1). On the other hand, the stretching of (Melsen etal.,1996). In that investigation,the PDL fibres on the opposite side, generated a the most conspicuous result was the increase instrain level corresponding to modelling, thus both activation level and density of the boneexplaining new bone formation on the so-called subjected to compression in the direction oftension side (Figure 3, region 2). tooth movement.
  10. 10. Con clu si o n sThe studies regarding tissue reaction to ortho-dontic forces were initiated with the purposeof finding an explanation of the controversies680 B. M E L S E Nin the literature. The intrusion of periodontallydamaged teeth was chosen as this movement is of the occurrence of microcracks in the case ofparticularly controversial. It was found that indirect resorption will contribute to the validationthe biological reaction was dependent on the of this paradigm.force level and the stress/strain distribution. Ad d r e s s forHistological examination demonstrated that the corr e s p o n d e n c ePDL fibres were stretched and formation activity Professor Birte Melsenwas found along the major part of the alveolus. Department of OrthodonticsOnly apical fibres were not observed to stretch. Royal Dental CollegeInterpreting these findings retrospectively in Aarhus Universitythe light of the mechanostat theory, it can be Vennelyst Boulevard 9concluded that stretched fibres will most likely DK-8000 Aarhus Clead to the delivery of strain values corres- Denmarkponding to modelling. In the apical region, the Ackno wl e d g e m e n tloading is minimal as the PDL fibres seem not M. Dalstra is thanked for finite element analysis.to be stretched. Apical bone resorption can thus Figures 1 and 2 reproduced with kind permissionbe interpreted as a remodelling. The stretching of the American Journal of Orthodontics andof the fibres may induce a slight bending of Dentofacial Orthopedics, and Figure 4 with kindthe alveolar wall. This bending in terms of an permission of The Angle Orthodontist.increase in the curvature of the alveolar wall has Ref e r e n c e spreviously been suggested by Epker and Frost Baumrind S, Korn E L, Boyd R L 1996 Apical root(1965) as the cause of new bone formation, in an resorption in orthodontically treated adults. Americanattempt to harmonize the orthodontic and the Journal of Orthodontics and Dentofacial Orthopedicsorthopaedic perception of the tissue reaction to Burr D311–320 R B, Schaffler M B, Radin E L 1985 Bone 110: B, Martinmechanical load. Based on the above-mentioned remodeling in response to in vivofatigue microdamage.experiments it is suggested that the mechanostat Journal of Biomechanics 18: 189–200 Costopoulos G, Nanda R 1996 An evaluation of roottheory can also be used as a model for under- resorption incident to orthodontic intrusion. Americanstanding orthodontic tissue reaction (Figure 3). Journal of Orthodontics and Dentofacial OrthopedicsDirect resorption will be perceived as a 109: 543–548 Dellinger E L 1967 A histologic and cephalometricremodelling induced by under-loading of the investigation of premolar intrusion in the Macacaalveolar wall and apposition as modelling induced monkey. speciosa American Journal of Orthodontics 53: 325–355by the load exerted by stretched fibres. The Epker B N, Frost H M 1965 Correlation of bone resorptionwoven bone formation observed in the direction and formation with the physical behavior of loaded bone. Journal of Dental Research 44: 33–41of the tooth movement could finally be explained Ericsson I, Thilander B 1978 Orthodontic forces andas a RAP phenomenon developing as a reaction recurrence of periodontal disease. An experimental study in the dog. American Journal of Orthodontics 74: 41–50to overloading (Frost, 1994). The indirect resorption Faltin R M, Arana-Chavez V E, Faltin K, Sander F G,takes place when ischaemia and hyalinization of Wichelhaus A 1998 Root resorptions in upper firstthe PDL is generated by high stress values. premolars after application of continuous intrusive forces. Intra-individual study. Journal of OrofacialDisappearance of the lining cells (Marotti, 1996) Frost H M 1986 Intermediary organization of the skeleton. Orthopedics 59: 208–219necessary for the communication of the osteocytes CRC Press Inc., Boca Rotonprobably leads to pathological reaction of theunderlying overloaded bone, which is probablyaffected by microfractures.
  11. 11. T I S S U E R E AC T I O N A N D O RT H O D O N T I C S 681Frost H M 1987 The mechanostat: a proposed pathogenetic cellular reaction studied in trabecular bone by means of a mechanism of osteoporoses and the bone mass effects finite element analysis. In: Davidovitch Z (ed.) Biological of mechanical and nonmechanical agents. Bone and mechanisms of tooth movement and craniofacial Mineral 2: 73–85Frost H M 1992 The role of changes in mechanical usage set adaption. Harvard Society for the Advancement of points in the pathogenesis of osteoporosis. Journal of Melsen B, Konstantellos USA pp. 213–221 Planert J Orthodontics, Boston, V, Lagoudakis M, Bone Mineral Research 7: 253–361 1997 Combined intrusion and retraction generatedFrost H M 1994 Wolff’s Law and bone’s structural adaptions by cantilevers with helical coils. Journal of Orofacial to mechanical usage: an overview for clinicians. Angle Orthopedics 58: 232–241 Nanda R 1997 Correction of deep overbite in adults. Dental Orthodontist 64: 175–188Goodship A E, Lanyon L E, McFie H 1979 Functional Clinics of North America 41: 67–87adaption of bone to increased stress. An experimental Parker R J, Harris E F 1998 Directions of orthodontic toothstudy. Journal of Bone and Joint Surgery 61: 539–546 movements associated with external apical rootGoerigk B, Diedrich P, Wehrbein H 1992 Intrusion of resorption of the maxillary central incisor. Americanthe anterior teeth with the segmented-arch technic of Journal of Orthodontics and Dentofacial OrthopedicsBurstone—a clinical study. Fortschritte der Kieferortho- 114: 677–683 Polson A, Caton J, Polson A P, Nyman S, Novak J, Reed Bpädie 53: 16–25Gundersen H J G etal.1988 The new stereological tools: 1984 Periodontal response after tooth movementdisector, fractionator and point sampled intercepts into infrabony defects. 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