A Study On The Stability Of Miniscrew Anchorage

Anchorage control, a major challenge faced by orthodontists at clinical practice, decides the success of orthodontic treatment. There have been many attempts to devise suitable anchorage methods, including intraoral and extraoral appliances. All intraoral appliances, however, show some lose of anchorage. Extraoral appliances do not provide reliable anchorage without patient compliance. While traditional means of anchorage control depends much on patients'cooperation, the treatment effect was influenced by how well patients cooperated when being treated. When using skeletal anchorage such as osseous dental implants, onplants miniplates, miniscrews, clicians can expect reliable anchorage without patient compliance. Compared with bone-borne anchorage like dental implants, miniplates, and onplants, miniscrews show their great distinction in the following aspects:small size, easy placement and removel, low cost, good access to various to various placement sites, minimal discomfort, and immediate loading with the mechanical interdigitation at the bone-minicrew interface. Miniscrews are used in anterior retraction, molar intrusion to correct an open-bite, controlling vertical dimension, molar distalization, molar protraction, and canting correction, or even moving the entire dentition in the same direction. Because they provide an excellent alternative to traditional compliance-dependent, miniscrews have expended the scope of the traditional orthodontic treatment. According to the literature, the success rates of the miniscrews under orthodontic loading vary between 70%-91%, the aims of this study were to evaluate the factors that affect the stability of miniscrews.This study was divided into three parts, in the first part of this study, Forty-eight malocclusional patients (18 males,30 females, mean age was 23.2±6.5 years old) with a total 152 miniscrews were examined. The miniscrew's success rates during a 10-month period of force application were determined according to 11 clinical variables. The purpose of this study was to evaluate the success rates and find factors affecting the clinical success of miniscrew used as orthodontic anchorage.The conclusions were as follows:To improve the success rate of miniscrews, the thickness of buccal cortical bone in subjects with a high mandibuar plane angle should be examined carefully, good oral hygiene should be maintained, inflammation around the miniscrews must be controlled.In the second part of this study,32 nonorthodontic adults with nomal occlusion (16 males and 16 females, and their age ranged from 21 to 44 years with 30.1 years as the mean age) were used to investigate the cortical bone thickness by CBCT. Cortical bone thicknesses were measured at various sites in the buccal and palatal aspects of maxillary alveolar bone, in the buccal aspect of mandible alveolar bone, and in the hard palate. The purpose of this study was to provide guidelines for miniscrew placement to enhance the success rate of miniscrew. The conclusions were drawn as follo wings:1. This study found out that cortical bone thickness showed no significant difference between males and females and between the left and right sides in both maxilla and mandible, cortical bone thickness in the mandible is thicker than in the maxilla, buccal cortical bone thickness in the maxilla is thicker than cortical bone thickness in the palate side.2. The thickest site at buccal cortical bone in the mandible is mesial to the second molar. The thickest site of the buccal cortical bone in the maxilla is mesial to the first molar. The thickest site at palatal cortical bone in the maxilla is the site mesial to the second premolar.3. The bone thickness of the hard palate was found have significant differences between males and females at midsaggital suture and its proximity. It showed decreased gradually from anterior part to the posterior part.4. The thicknest bone of the hard palate was located at midsaggital suture and lateral border of the hard palate.In the three part of this study, the three dimensional element models of bone and miniscrew (1.6×8mm) were build up, they were designed with different cortical bone thickness from 0.5mm to 3mm and different insert angle from 30°to 90°(to the long axis of the tooth). The purpose of this study was to analyze the impact of cortical bone thickness and insert angle on primary stability of orthodontic miniscrews. The conclusions were as follows:To achieve the best primary stability, the miniscrew should place in an area with cortical bone thickness>lmm, an insertion angle 70°is advisable. Very oblique insertion angle (<30°) will result in reduced primary stability.