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tooth movement by orthodontic force application is characterized by remodeling changes in periodontal tissues , including , periodontal ligament ( pdl ) , alveolar bone , and gingiva .
the response to applied orthodontic forces is bone formation ( due to osteoblasts ) on the tension side and bone resorption on the compression side ( due to osteoclasts ) of the lamina dura [ 25 ] .
however , this represents an oversimplification of the three dimensional changes that appear in the microarchitecture of the periodontium .
several studies used histological staining and light microscopy , scanning electron microscopy , finite element models , genetic modifications   in order to demonstrate the complexity of periodontal transformations .
these methods need sophisticated procedures and animal sacrifice , and can not be used to evaluate longitudinal changes of periodontium due to orthodontic forces . a new technique aiming at observing real time , in vivo modifications induced by otm in the morphology of pdl , alveolar bone and cementum would be beneficial in order to better conduct the treatment plan and to evaluate the tissue response to orthodontic forces .
several studies   demonstrated that ultrasonography could be successfully used in assessing the cortical bone , sulcular depth , periodontal space , length of the anatomical crown and the characteristics of the gingiva .
the aim of the present study was to assess whether changes that appear during orthodontic tooth movement in the anatomical structures of the periodontium can be monitored using ultrasonography .
to the best of our knowledge , this is the first attempt to understand the modifications that appear during otm using ultrasonic measurements of periodontal tissues , especially the gingival characteristics .
the null hypothesis of the present study was that by using ultrasonic measurements , anatomic features of periodontal structures ( gingival sulcus depth ; free gingival thickness ; distance between marginal gingiva and alveolar crest ; and width of the periodontal space in most coronal position ) of teeth subjected to otm will indicate the same results , overtime .
maxillary canines were subjected to orthodontic retraction , as part of the orthodontic treatment plan ; 8 teeth were measured as a result - all with typical dental anatomy ( average root sizes , bone insertion , and shape ) .
the research protocol was approved by the institutional ethical committee , and an informed consent was obtained from the subjects .
all patients were fitted with alexander s prescription preadjusted edgewise appliance of .018 slot ( minimaster american orthodontics ) .
upper first premolars were extracted due to severe crowding ( 8 upper bicuspids - 5 first right premolars and 3 first left premolars ) ( figure 1 ) . after leveling and aligning ,
the canines were distalized on 016 ss archwire , using elastomeric memory chain ( ao ) with a net force of approximately 100cn . the elastomeric chain delivering the force
the average time for canine retraction was 5 months , which varied depending on the size of the initial space and bone density . a commercially available ultrasound scanner ( ultrasonix sonotouch ) with a linear 1.5 cm footprint ,
us scans were performed in three distinct areas of the canine buccal surface : mesial , middle and distal , with a percutaneous transgenial approach .
the reference point was the bracket , placed in the center of the canine , which appeared hyperechoic on the us scan .
the images were obtained by positioning the transducer in a longitudinal plane in the lateral area of the maxillary alveolar process ( figure 2 ) .
the us scan revealed the cortical bone , the buccal surface of the tooth with the bracket placed in the center , the gingival sulcus and the periodontal space in the most coronal position ( figure 3 ) . on the 40 mhz image ,
the following distances were measured at a micrometric level : d1 - gingival sulcus depth ; d2 - free gingival thickness ; d3 - distance between marginal gingiva and alveolar crest ; d4 - width of the periodontal space in the most coronal position .
all scans were performed by a single trained radiologist , at different moments of the orthodontic treatment .
the us scans were performed at three different moments of otm : before distalization ( moment 1 ) , after 2 days of 100 cn force delivery , when tooth displacement was expected ( moment 2 ) and after 30 days to 60 days or the postlag stage ( moment 3 ) . paired sample t - test and pearson s correlation test
all ultrasound measurements for d1 , d2 , d3 and d4 ( mm ) are presented in table i. an increase of d1 ( sulcus depth ) was observed in all three areas of the periodontium , immediately after force delivery , whereas at the end of tooth movement the distance slightly decreased . however , significant differences were observed only for the mesial ( p=0.031 ) and middle areas ( p=0.048 ) .
a strong correlation was also noted for d1 before and immediately after force delivery ( r=0.945 , p<0.05 ) .
d2 ( free gingival thickness ) was the most stable measurement during otm , the free gingival thickness slightly decreased from the initial moment , yet no statistically significant difference was observed between the measurements ( p>0.05 ) .
the distance between marginal gingiva and alveolar crest ( d3 ) was strongly correlated before and immediately after force delivery only for the mesial area ( r=0.828 , p<0.05 ) .
furthermore , a very strong inverse correlation was observed for the middle area between moment 2 and 3 ( r=0.998 , p<0.05 ) .
the width of the periodontal space ( d4 ) increased immediately after force delivery on the mesial , middle and distal area . in the last stage ,
nonetheless , no significant difference was found between the measurements , in none of the three moments evaluated ( p>0.05 ) .
charts representing the evolution of periodontal structures for d1 ( sulcus depth ) , d2 ( free gingival thickness ) , d3 ( distance between marginal gingiva and alveolar crest ) , d4 ( width of the periodontal space in the most coronal position ) , on the mesial area ( blue line ) , middle area ( red line ) , distal area ( green line ) , during otm are presented in figures 5 to 8 ( x- axis : moments of the us scans : m1- initial phase ; m2-intermediary phase ; m3- final phase ; y - axis : dimensional changes in millimeters ) .
the definition of orthodontic tooth movement by proffit is the result of a biologic response to the interference in the physiological equilibrium of the dentofacial complex by an externally applied force .
two main mechanisms are considered to be responsible for tooth movement  the application of pressure and tension to the pdl , and bending of the alveolar bone .
however , few studies focused on the real time changes induced by otm in the area of marginal periodontium . in this study
, we propose a novel approach for the clinical evaluation of the free gingiva , gingival sulcus , supracrestal fibers and position of the bone crest .
high - resolution ultrasonography ( 40 mhz ) was used , for a better visualization of the superficial periodontium .
the null hypothesis of the study states that ultrasound imaging does not have the capacity to evaluate the changes , which appear during otm . in the present study , a standardized orthodontic force of approximately 100cn
the period of time between the scans were deliberately chosen , according to the lag stages found in the literature : initial stage ( 2448h ) represented by tooth displacement in the periodontal ligament space .
lag stage lasts 2030 days and is characterized by the formation of necrosis and hyalinization . in this lag stage
postlag stage , characterized by tooth movement mediated by bone remodeling through the agency of osteoclasts and osteoblasts on a background of neoangiogenesis .
the null hypothesis was rejected since significant changes in the periodontium were measured by us method after orthodontic tooth movements .
the distance between marginal gingiva and alveolar crest ( d3 ) was strongly correlated before and immediately after force delivery only for the mesial area ( r=0.828 , p<0.05 ) .
furthermore , a very strong inverse correlation was observed for the middle area between moment 2 and 3 ( r=0.998 , p<0.05 ) .
an increase of d1 ( sulcus depth ) observed in all three areas of the periodontium , immediately after force delivery .
however , significant differences were observed only for the mesial ( p=0.031 ) and middle areas ( p=0.048 ) , on the opposite side of that of force delivery . a recent study demonstrated that gingival inflammation and dental plaque increase during orthodontic treatment .
this might be one of the possible explanations for our findings , i.e. the increased sulcus depth measurement .
the thickness of human pdl is reported to be around 0.10.3 mm . in our study ,
measurements of periodontal space in the most coronal position ( d4 ) varied between 0.2 and 0.52 , with an average value of 0.28 in the initial stage .
the width of the periodontal space increased immediately after force delivery on all of the buccal areas : mesial , distal and middle . in the last stage ,
nonetheless , no significant difference was found between the measurements , in none of the three moments evaluated ( p>0.05 ) .
although d2 ( free gingival thickness ) was the most stable measurement during otm , no statistically significant difference was observed between the measurements ( p>0.05 ) .
the prevalence , extent , and severity of gingival recession were correlated with past orthodontic treatment .
future studies will be carried out to see if there is a significant change in the thickness of the gingiva , in the last stage .
the limit of the study was the relative small sample of subjects ( 5 patients- with 8 canines to be distalized ) , one of them was lost during the research .
further assessment of the modifications in the width , thickness and height of the marginal peridontium is needed , on a larger group of patients .
ultrasonographic measurements , with high resolution , detected changes in the anatomic landmark of periodontal tissues during orthodontic tooth movement .
significant changes occurred immediately after force delivery on the middle and mesial area of the canine , for sulcus depth measurement and distance between marginal gingiva and alveolar crest .
background and aimorthodontic tooth movement ( otm ) is a process whereby the application of a force induces bone resorption on the pressure side and bone apposition on the tension side of the lamina dura . 
 however , only limited data are available on the in vivo behavior of the periodontal tissues . 
 the aim of this study was to assess the changes of periodontal tissues , induced by the orthodontic canine retraction , using 40 mhz ultrasonography.methodsultrasonographic evaluation of periodontal tissues was conducted in 5 patients with indication for orthodontic treatment . 
 the upper first premolars were extracted bilaterally due to severe crowding , and the canines were distalized using elastomeric chain with a net force of 100 cn . 
 ultrasonographic scans ( us scans ) were performed before , during and after retraction , in three distinct areas of the canines buccal surface : mesial , middle and distal . 
 the reference point was the bracket , which appeared hyperechoic on the us scan . 
 four different dimensions were obtained : d1 ( depth of the sulcus ) , d2 ( thickness of the gingiva ) , d3 ( length of the supracrestal fibers ) , d4 ( width of periodontal space).resultsan increase of d1 was observed in all three areas of the periodontium , during orthodontic treatment . 
 d3 was strongly correlated before and immediately after force delivery only for the mesial area ( r=0.828 , p<0.05 ) . in total 
 , 228 variables were statistically analyzed using pearson s correlation coefficients , in order to demonstrate the relationship between periodontal findings during orthodontic tooth movement.conclusionhigh-resolution ultrasonography has the capability to obviate changes in periodontal ligament space and free gingiva during orthodontic tooth movement .