the connective tissue between teeth and bone termed periodontal ligament ( pdl ) is a highly differentiated ligamentous apparatus designed to convert chewing forces into tensile forces to transmit them to the alveolar bone . the dental root is entirely coated with cementum , which consists of cementocytes , mineralized hard tissue , and collagen fibers . two types of collagen fibers can be found in the cementum : intrinsic fibers ( ebner fibrils ) that run around the root in a circle and extrinsic fibers ( sharpey 's fibers ) that are inserted into the cementum and run to the inner walls of the tooth socket . in the rest position , these fibers are corrugated and allow tooth mobility up to a certain extent ; during physiological mechanical load , however , these fibers straighten . the collagen fiber bundles of the pdl are surrounded by an incompressible matrix composed of glycosaminoglycan , glycoproteins , and glycolipids . the periodontal ligament mainly consists of fibroblasts and fibrocytes but also of osteoblasts , osteoclasts , macrophages , cementoblasts , and progenitor cells [ 2 , 3 ] . loose connective tissue is found in the apical region of the teeth and in the small gaps between the collagen fiber bundles of the pdl . several mechanisms are assumed to have a cushioning effect on ensuring the nutrition of the tooth - pdl - system during physiological mechanical loading . shock - absorbing effects are attributed to the resilience of the dental hard tissue and the periodontal space filled with tissue fluid [ 4 , 5 ] . in the case of minor external impact , damage to the periodontium is prevented by the elongation of collagen fibers of up to 5% of their initial length and the elastic deformation of tooth sockets . these mechanisms are also thought to be responsible for the fact that light - intermitting orthodontic forces do not induce cellular effects . fibroblasts of the periodontal ligament are known to build new collagen fibrils and differentiate into cementoblasts , osteoblasts , or osteoclasts . because of its rich cellular population and good renewal capability , the pdl can quickly adapt to moderate external influences , such as orthodontic forces . the alveolar process of the maxilla consists of a thin cortical bone plate covered with a thin tough membrane called the periosteum . the space between the cortical bone and the tooth socket is filled with cancellous bone that mainly consists of cross - linked bone trabeculae with a rather inhomogeneous structure . bone tissue is fundamentally based on collagen and hardened by the deposition of calcium apatite . osteoblasts , which are responsible for bone formation and calcification , are in balance with bone - resorbing osteoclasts . the functional interaction of these three cell types is essential for bone remodeling during orthodontic tooth movement , and their activity is strictly controlled by molecular mediators [ 811 ] . although our knowledge on the molecular and cellular regulation of bone formation [ 810 , 1216 ] has increased in recent years , many questions remain with regard to the mechanisms facilitating orthodontic tooth movement . malocclusions are treated by attaching orthodontic appliances ( loose or fixed ) to achieve eugnathia . the process of orthodontic tooth movement with continuous forces is divided into three stages , namely , the initial phase , the lag phase , and the postlag phase . during the first days ( initial phase ) , rapid tooth movement can be observed caused by the cushioning effects of the periodontal system described above [ 5 , 17 ] . for example , the periodontal gap in dogs is distinctly compressed after 3 to 6 days of force application with forces up to 30 n . in the case of prolonged and enhanced orthodontic force application , tooth movement often stops for up to 20 days , a condition termed lag phase . during this stage , the periodontal tissue and alveolar bone on the pressure side of the dental root are suffering from circulatory disturbances . affected parts of the pdl and the adjacent alveolar bone show partial necrosis ; histologically , they appear similar to hyalinized tissue . during the lag phase , resorption of tooth sockets only takes place in terms of undermining resorption of the lamina cribriformis , starting in the vital bone . if the undermining resorption is exceeded at a certain stage , the alveolus will collapse , resulting in sudden tooth movement . in contrast , the postlag phase or late phase is coined by direct resorption of the socket , allowing continuous tooth movement [ 14 , 17 ] . therefore , orthodontists will always try to achieve a smooth transition from the initial phase to the postlag phase by skipping the lag phase . by compressing the pdl , orthodontic forces induce cell deformation in alveolar tissues , stimulating mechanosensitive ion channels and receptors in the cell membrane . periodontal cells seem to react to mechanical stimulation by upregulating cellular mediators , such as cyclic amp ( camp ) , that induce proteinkinases to catalyze the phosphorylation of mediator proteins . after several steps , the information about the mechanical stimulation reaches the nucleus , in which two main pathways can be triggered . dna replication leading to cell proliferation can be initiated , and cell differentiation can be induced . the activity of genes related to bone growth and cell reprogramming is modulated by the phosphorylation of the transcription factors , such as c - jun und c - fos . important cellular mediators , such as protein kinase c ( pkc ) , and inflammation mediators , such as prostaglandin e2   ( pge2 ) , are generated and released into the cytoplasm . in the case of inflammation , cyclooxygenase 2 ( cox-2 ) is stimulated to produce prostaglandin h2   ( pgh2 ) , which is followed by the conversion of pgh2 to pge2 by prostaglandin e synthase . by binding to the two main pge2 receptors e2 and e4 , pge2 has been reported to activate adenylate cyclase in human pdl cells and thus seems to stimulate bone resorption . however , the upregulation of cox-2 and the expression of pge2 are known to play an important role in bone resorption [ 8 , 21 ] . de carlos et al . the literature reports that the inhibition of prostaglandin e2 by vitamin k2 may also inhibit osteoclastogenesis . inflammatory processes stimulate mononuclear phagocytic cells , such as macrophages , that secret proinflammatory cytokines , for example , interleukin-1 ( il-1 ) and interleukin-6 ( il-6 ) . interleukins are part of the peptide hormone family and internal body messenger elements for immune cells .   nevertheless , these proinflammatory cytokines are also known to enhance the secretion of prostaglandins [ 12 , 16 ] . during orthodontic force application , these cytokines are assumed to regulate the secretion of the receptor activator of the nuclear factor kappa ligand ( rankl ) by osteoblasts and pdl cells in periodontal tissues [ 16 , 24 ] . rankl is a ligand to the receptor activator of the nuclear factor kappa ( rank ) that is localized on the surface of osteoclast precursor cells as well as on mature osteoclasts [ 25 , 26 ] . if rankl binds to rank , osteoclast precursor cells fuse and differentiate into active multinucleated osteoclasts responsible for bone resorption [ 11 , 16 , 27 ] . as long as rankl is expressed and the equilibrium of rankl and osteoprotegerin ( opg ) is pushed to the side of rankl , rank stays activated and osteoclasts proceed with bone resorption . to stop resorption , osteoblasts increase opg production , which is a competitive receptor for rankl and related to the tumor necrosis factor receptor ( tnfr ) . in addition to the inhibition of rankl , opg has also the ability to block mature osteoclasts or to downregulate osteoclastogenesis . nakao et al .   showed that opg mrna is downregulated and force - dependent as well as time - dependent , when human pdl cell cultures are charged with compressive load . in contrast , the expression of rankl mrna was induced in compressed cell populations and inconspicuous in unstimulated pdl cells . thus , the occurrence of osteoclastogenesis and bone resorption seems to depend on the balance of rankl and opg [ 11 , 25 , 26 ] . hypoxia describes oxygen deficiency in tissue due to oxygen partial pressure reduced beyond the physiologic level . physiologic oxygen partial pressure in tissue depends on the localization of the tissue and the age of the patient . in pulmonary veins , for example , the oxygen partial pressure fluctuates between 80 and 100  mmhg , whereas 40  mmhg can be found in terminal capillary networks . the cells respond to hypoxia by expressing cellular mediators , particularly the hypoxia - inducible factor 1 ( hif-1 ) , a heterodimer composed of hif-1 and hif-1. the formation of hif-1 is limited by the subunit hif-1. although hif-1 is expressed during both normoxia and hypoxia , this subunit is unstable in normoxia . hydroxylated by hif hydroxylases under normoxic conditions , the proteolytic degradation of hif-1 is facilitated by the binding of the von hippel - lindau tumor suppressor protein ( pvhl ) . this protein induces the attachment of a polyubiquitin chain to hif-1 , thus permitting the anchorage of hif-1 to the proteasome and finally leading to the degradation of hif-1. during hypoxia , the stabilized hif-1 aggregates and binds to hif-1 , creating the active transcription factor hif-1 that can promote angiogenesis , stimulate cell proliferation , and is able to prevent cell death . but hif-1 also induces apoptosis to inhibit hypoxia - induced mutations in cells [ 28 , 30 , 31 ] . matrix metalloproteinases ( mmps ) are known to be important for angiogenesis because of their contribution to the degradation of the vascular basal membrane , leading to endothelial cell migration . during tooth movement , periostin is mainly expressed on the pressure side of the pdl   and is upregulated by hypoxia . used for hypoxia simulation in cell culture  for 24  h. the investigators could show that hif-1 was induced and subsequently found the upregulation of periostin but could not clarify if this upregulation was induced directly or indirectly by hif-1. particularly noteworthy was that periostin seems to contribute to the secretion and transcription regulation of mmp-2 .   by coculturing human primary peripheral blood mononuclear cells ( pbmncs ) with and without osteoblasts ( obs ) under normoxic ( 21.0% ) and .   showed that pbmnc can differentiate into functional osteoclasts ( ocs ) when triggered by hypoxia . after the induction of a 2.5-fold increase of hif-1 , the authors found a steep slope of the vascular endothelial growth factor ( vegf ) between 24  h and 72  h of exposure , and the upregulation of rankl directly correlated with the hif-1 level . vegf is known as one of the most important mitogen that induces angiogenesis . by adhering to receptors of endothelial cells , vegf activates signal cascades , resulting in a broad variety of cellular and vascular reactions . via the expression of nitric oxide ( no ) no is not only a fundamental biological messenger but also a radical gas . as a signaling molecule , no plays a crucial role in a variety of biological processes , for instance , the regulation of vasodilation , blood flow , and inflammation . thus , orthodontic tooth movement not only has cellular effects , such as the above - mentioned activation of osteoclasts or osteoblasts , but also induces vascular changes [ 9 , 37 ] . khouw and goldhaber   investigated vascular changes in the pdl after 1 , 3 , and 7 days by applying tipping forces onto the teeth of rhesus monkeys and german shepherd dogs . the authors observed similar changes in both species for all test conditions and therefore did not discuss the results for each species separately . after 24  h of force application , vessels on the tension side of the root were widened , whereas blood vessels on the pressure side , particularly above the rotation center of the root , showed partial or complete occlusion . results for the tension side in the group of 72  h force application appeared similar to those after 24  h , but osteoblasts were accompanied by new bone formation . vascularisation of the pdl was still suppressed in the areas of pressure , and no active bone resorption could be observed . after seven days of force application , the authors found new bone formation throughout all tension areas . although all blood vessels were dilated , no angiogenesis could be observed at the tension site . the alveolar bone showed surface resorption as well as undermining resorption in the areas of pressure , and increased numbers of new blood vessels were found near the areas affected by resorption . these findings correspond to the fact that the occlusion of blood vessels naturally decreases blood supply , resulting in hypoxia in areas of pressure [ 9 , 37 ] . subsequently , hypoxia induces the formation of the active transcription factor hif-1 and activates genes encoding vegf . in cells with chronic or extreme hypoxia , interestingly , two pathways have been discovered by which hif-1 itself seems to induce apoptosis . on the one hand , hypoxia triggers the expression of the proapoptotic nineteen kd interacting protein-3 ( nip3 ) , which seems to be strongly related to the presence of hif-1. on the other hand , hypoxia stabilizes the p53 tumor suppressor protein . this transcription factor is able to activate genes , such as the apoptosis - regulator bax , that initiate cell death or stop proliferation ( figure 1 ) . according to nip3 , p53 also relies on the presence of hif-1 , since none of the two transcription factors can be found in cells without hif-1 . pdl cells suffering hypoxia release biochemical mediators , such as the cc chemokine ligand 2 ( ccl2 ) . this mediator can be particularly found in the resorption site after the application of orthodontic forces . ccl2 activates the cc chemokine receptor 2 ( ccr2 ) that is mainly localized on leucocytes , monocytes or macrophages , and lymphocytes . chemokines are assumed to induce leucocyte chemotaxis and cellular activation , leading to cell proliferation and angiogenesis . but ccl2 is also known to trigger protective effects against cell death , particularly in the case of mild hypoxia . kitase et al .   investigated the gene expression in pdl cells and exposed primary pdl cells to hypoxia by reducing the total oxygen concentration to less than 1% for 96  h. the investigators also studied the protective effect of recombinant human ccl2 . after incubating human pdl cells for 12  h with less than 1% oxygen concentration , kitase et al .   identified 11 hypoxia - responsive genes . particularly the insulin - like growth factor - binding protein 3 ( igfbp3 ) and ccr2 the authors were able to prove that the negative effects of hypoxia in pdl cells can be prevented by adding external ccl2 . they also reported a 55.4% increase in nonviable cells due to hypoxia , whereas the addition of 10  ng / ml of ccl2 significantly decreased nonviable cells to 9.8% . also proposed that igfbp3 is one of the main mediators responsible for cell death under hypoxia . .   reported that light hypoxic conditions ( 10% o2 ) seemed to increase the proliferation rate of osteoblast - enriched cultures in vitro . in contrast , hyperoxia ( 90% o2 ) even showed a reversed effect , because changing oxygen tension from low to high immediately results in a drastic decrease in the proliferation rate . in summary , hypoxia seems to fundamentally contribute to bone remodeling processes [ 9 , 34 , 39 ] . resorption of permanent teeth is always regarded as a pathologic process , and the etiology of root resorption is not yet completely understood . in general , root resorption can be located in three different areas of the dental root and is classified as lateral resorption , apical resorption , and internal resorption . after replantation , lateral root resorption can be seen in avulsed teeth , particularly after damage to the periodontal tissue . root resorption can also occur in the case of neoplasia , such as pulp polyps or tumors . a special case is the undermining resorption of permanent teeth , which can occur in impacted teeth or teeth with abnormal eruption paths . depending on the individual predisposition , idiopathic resorption may even occur without any prior trauma [ 40 , 41 ] . but root resorption  apical as well as lateral  is also an undesirable risk of orthodontic treatment [ 4043 ] . some orthodontic treatment factors are especially in discussion for being related to root resorption , such as force level , tipping forces , intrusion or extrusion of teeth . but also anatomical factors such as tooth root morphology might be relevant [ 44 , 45 ] . by applying a continuous force of 50  cn , respectively , 200  cn for the duration of seven weeks on human premolars , owman - moll et al . observed a mean tooth displacement of 3.5  mm for 50  cn and 5.1  mm for the group with four times higher force levels . however , no significant difference could be found comparing both force levels regarding the occurrence and severity of root resorption . instead they found all test teeth suffering root resorption independently from the applied force but with large differences between individuals . in order to investigate whether intermitting or continuous forces are harmful to the dental root , another group applied tipping forces of 100  g on human premolars for 12 and 24 hours a day . after nine weeks , the teeth were extracted and prepared for scanning electron microscopy ( sem ) . the authors could show that continuous force application resulted in a higher percentage of root resorption , whether in the palatal , buccal , or apical region . even though they also found distinct individual differences in the severity of root resorption , the authors verified root resorption on all test teeth . as intrusion of teeth is considered as a high risk treatment , often resulting in root resorptions , harris et al .   conducted a study in which 54 human premolars were intruded for 28 days using either low ( 25  g ) or heavy ( 225  g ) forces . the analysation of x - ray microtomography images of the extracted test teeth unveiled that the resorption crater dimension increased according to the increase of the intrusive force . in contrast to that , another group directly compared the effect of intrusive and extrusive forces on the dental root . using a continuous force of 100  cn on 18 maxillary first premolars in nine patients , they randomly intruded one first premolar and extruded the contralateral premolar in each patient . subsequently they investigated the root surface of every specimen via sem images and could show that there was a significant difference between extruded and intruded teeth . while the extruded specimens did not show a significant difference in root resorption compared to the control group , all of the intruded teeth showed obvious signs of resorption in the apical part of the root . it is well known that maxillary incisors with pipette - like or blunt - ended roots seem to be more frequently affected by orthodontically induced root resorption ( oirr ) than others , particularly in the apical region . .   found a significant difference between root - filled teeth and vital teeth with regard to their susceptibility for resorption . the authors investigated 43 patients who had received endodontic treatment on one or more anterior teeth prior to orthodontic therapy with a multi - bracket appliance . for each patient , radiographs of the root - filled teeth and the contralateral vital teeth were taken before and after orthodontic therapy . the findings showed a higher number as well as more severe cases of root resorption for vital teeth than for root - filled control teeth . therefore , the authors suggested that , in one way or the other , the dental pulp might contribute to apical root resorption . the dental pulp is divided into two major portions , the crown 's pulp and the root . the pulp consists of glycosaminoglycans as a basic element , in which the pulpal cells , such as pulp fibroblasts , odontoblasts , and pulpal stem cells , are embedded . a dense network of arteries , veins , arterioles , venules , and capillaries ensures the high vascularization of the pulp tissue . however , almost all blood vessels enter a tooth via the apical constriction ( figure 2 ) , making it a trouble spot for pulpal blood supply [ 52 , 53 ] . pulpal blood vessels are usually accompanied by other functional structures , such as nerves or lymph vessels [ 54 , 55 ] . the nociceptive innervation of the pulp is mainly based on a--fibers , a--fibers , and c - fibers , whereas the vasomotoric nerve fibers of the vegetative nervous system control the muscular tonus of pulpal arterioles and therefore contribute to the regulation of pulpal blood flow . venules of the dental pulp are known to have very thin walls that tend to collapse in case of high pulpal pressure . in this context , it is also interesting that vasodilation induced by inflammation mediators , such as pge2 , seems to have different effects in the dental pulp as in other tissues . by increasing pulpal pressure and therefore hydraulically inducing secondary vasoconstriction , vasodilation might halt the spread of infections , if induced in a very localized area ; however , vasodilation may also facilitate necrosis in cases of generalization . induced severe root resorption by applying intermitting tensile loads of 50  g onto 15 first molars in rats for up to 30 days to investigate inflammatory patterns in the dental pulp and the pdl . the investigators found typical signs of inflammation in the compressed pdl , such as immigration of macrophages , monocytes , and dendritic cells . increased angiogenesis could also be observed in root areas affected by resorption . nevertheless , no new formation of nerve structures as usually seen in inflamed periodontal and pulpal tissues could be observed . orthodontic tooth movement is a process that is based on a variety of mechanisms , such as cell deformation or inflammatory processes . however , some accompanying factors of orthodontic tooth movement , such as force level , tipping forces , intrusion , or extrusion of teeth , but also special anatomical structures might induce circulatory disturbances in the pulpal tissues . data in the literature shows a possible correlation between orthodontically induced hypoxia in the dental pulp and root resorption . yet , cellular reactions accompanying orthodontically induced root resorption ( oirr ) significantly differ from those seen in inflamed periodontal tissues , indicating that there might be a different pathway of activation . the finding that root - filled teeth seem to be less vulnerable to oirr than vital teeth requires further investigations into the role of the dental pulp . a full understanding of the mechanism of cellular activation underlying oirr may facilitate measures to prevent tooth resorption during orthodontic therapy .
orthodontic forces are known to have various effects on the alveolar process , such as cell deformation , inflammation , and circulatory disturbances . 
 each of these conditions affecting cell differentiation , cell repair , and cell migration , is driven by numerous molecular and inflammatory mediators . as a result , 
 bone remodeling is induced , facilitating orthodontic tooth movement . 
 however , orthodontic forces not only have cellular effects but also induce vascular changes . 
 orthodontic forces are known to occlude periodontal ligament vessels on the pressure side of the dental root , decreasing the blood perfusion of the tissue . 
 this condition is accompanied by hypoxia , which is known to either affect cell proliferation or induce apoptosis , depending on the oxygen gradient . because upregulated tissue proliferation rates are often accompanied by angiogenesis 
 , hypoxia may be assumed to fundamentally contribute to bone remodeling processes during orthodontic treatment .