title: Effect of 8-hour intermittent orthodontic force on osteoclasts and root resorption.
abstract: BACKGROUND
  The duration of force application is an important factor in optimizing orthodontic tooth movement with less root resorption. It has previously been shown that the amount of tooth movement achieved by 8-hour intermittent force application exceeds what would be the expected by the duration. The purpose of this study was to compare osteoclast recruitment and the extent of root resorption in response to an 8-hour intermittent force regimen with those from a continuous force.
  
  
  METHODS
  Rat molars (n = 110) were assigned to 3 groups: 8-hour intermittent force, continuous force, and control. A coil spring that exerted 40 cN of force was suspended between the maxillary first molars and the incisors. At 1, 3, 5, 7, and 14 days, osteoclast numbers, osteoclast surfaces, and root resorption were quantified histomorphometrically in the undecalcified sections and statistically analyzed.
  
  
  RESULTS
  At the mesial sites, osteoclast numbers in the intermittent force group were 100.5% of the continuous force group, and osteoclast surfaces in the intermittent force group were 68.2% of the continuous force group. At the mesial sites, root resorption of the intermittent group was approximately 30.0% of the continuous group (P <0.01).
  
  
  CONCLUSIONS
  Our results show that an 8-hour intermittent force efficiently recruits osteoclasts while causing minimal root resorption.
authors.affiliation: Department of Growth and Development of Functions, Division of Orofacial Functions and Orthodontics, Kyushu Dental College, Kitakyushu, Japan.
authors.forename: Tomoko
authors.initials: T
authors.lastname: Kumasako-Haga
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Konoo
authors.forename: Kazunori
authors.initials: K
authors.lastname: Yamaguchi
authors.forename: Hidetetsu
authors.initials: H
authors.lastname: Hayashi
mesh_terms: D000539:Alveolar Process / Q000473:pathology; D000818:Animals; D002452:Cell Count; D048430:Cell Shape; D008297:Male; D009969:Orthodontic Wires; D010010:Osteoclasts / Q000473:pathology*; D010513:Periodontal Ligament / Q000473:pathology; D011897:Random Allocation; D051381:Rats; D017207:Rats, Sprague-Dawley; D012391:Root Resorption / Q000209:etiology* / Q000473:pathology; D013314:Stress, Mechanical; D013997:Time Factors; D019227:Tooth Apex / Q000473:pathology; D019228:Tooth Crown / Q000473:pathology; D014087:Tooth Movement Techniques / Q000295:instrumentation*; D014092:Tooth Root / Q000473:pathology