A systematic review of effects related to patient, screw, surgery, and loading on the stability of miniscrews was conducted.
Reports of clinical trials published before September 2007 with at least 30 miniscrews were reviewed. Parameters examined were patient sex and age, location and method of screw placement, screw length and diameter, time, and amount of loading.
Fourteen clinical trials included 452 patients and 1519 screws. The mean overall success rate was 83.8% + or - 7.4%. Patient sex showed no significant differences. In terms of age, 1 of 5 studies with patients over 30 years of age showed a significant difference (P <0.05). Screw diameters of 1 to 1.1 mm yielded significantly lower success rates than those of 1.5 to 2.3 mm. One study reported significantly lower success rates for 6-mm vs 8-mm long miniscrews (72% vs 90%). Screw placement with or without a surgical flap showed contradictory results between studies. Three studies showed significantly higher success rates for maxillary than for mandibular screws. Loading and healing period were not significant in the miniscrews' success rates.
All 14 articles described success rates sufficient for orthodontic treatment. Placement protocols varied markedly. Screws under 8 mm in length and 1.2 mm in diameter should be avoided. Immediate or early loading up to 200 cN was adequate and showed no significant influence on screw stability.
"With the exception of five TSADs (two failed between 0 and 4 weeks and three TSADs became loose between 4 and 8 weeks), the remaining twenty were stable and well integrated with the surrounding alveolar bone during the 8 week experimental period. Our success rate of 92% at 4 weeks and 80% at 8 weeks was comparable to a success rate of 83.6% reported in recent meta-analyses [18, 19]. Various factors affect TSAD stability during insertion and following loading and can be broadly divided into factors affecting primary and secondary stability. "
[Show abstract][Hide abstract] ABSTRACT: The aim of this animal study was to develop a model of orthodontic tooth movement using a microimplant as a TSAD in rodents. A finite element model of the TSAD in alveolar bone was built using μCT images of rat maxilla to determine the von Mises stresses and displacement in the alveolar bone surrounding the TSAD. For in vivo validation of the FE model, Sprague-Dawley rats (n = 25) were used and a Stryker 1.2 × 3 mm microimplant was inserted in the right maxilla and used to protract the right first permanent molar using a NiTi closed coil spring. Tooth movement measurements were taken at baseline, 4 and 8 weeks. At 8 weeks, animals were euthanized and tissues were analyzed by histology and EPMA. FE modeling showed maximum von Mises stress of 45 Mpa near the apex of TSAD but the average von Mises stress was under 25 Mpa. Appreciable tooth movement of 0.62 ± 0.04 mm at 4 weeks and 1.99 ± 0.14 mm at 8 weeks was obtained. Histological and EPMA results demonstrated no active bone remodeling around the TSAD at 8 weeks depicting good secondary stability. This study provided evidence that protracted tooth movement is achieved in small animals using TSADs.
International Journal of Dentistry 09/2014; 2014:917535. DOI:10.1155/2014/917535
[Show abstract][Hide abstract] ABSTRACT: A growing number of studies have reported that mini-implants do not remain in exactly the same position during treatment, although they remain stable. The aim of this review was to collect data regarding primary displacement immediately straight after loading and secondary displacement over time. A systematic review was performed to investigate primary and secondary displacement. The amount and type of displacement were recorded. A total of 27 studies were included. Sixteen in vitro studies or studies using finite element analysis addressed primary displacement, and nine clinical studies and two animal studies addressed secondary displacement. Significant primary displacement was detected (6.4-24.4 µm) for relevant orthodontic forces (0.5-2.5 N). The mean secondary displacement ranged from 0 to 2.7 mm for entire mini-implants. The maximum values for each clinical study ranged from 1.0 to 4.1 mm for the head, 1.0 to 1.5 for the body and 1.0 to 1.92 mm for the tail part. The most frequent type of movement was controlled tipping or bodily movement. Primary displacement did not reach a clinically significant level. However, clinicians can expect relevant secondary displacement in the direction of force. Consequently, decentralized insertion within the inter-radicular space, away from force direction, might be favourable. More evidence is needed to provide quantitative recommendations.International Journal of Oral Science (2013) 5, doi:10.1038/ijos.2013.92; published online 13 December 2013.
International Journal of Oral Science 12/2013; 6(1). DOI:10.1038/ijos.2013.92 · 2.53 Impact Factor
"A high thread depth to outer diameter ratio providing a thinner shank is more prone to TF (Fig. 3, No 1) and less appropriate for clinical use. Immediate loading of small OMIs up to 20 N is possible (18). "
[Show abstract][Hide abstract] ABSTRACT: Objectives: Orthodontic mini-implants (OMIs) are increasingly used in orthodontics but can fail for various reasons. This study investigates the effects of OMI design characteristics on the mechanical properties in artificial bone.
Material and Methods: Twelve self-drilling OMIs (2 small, 6 medium, 4 large) from 8 manufacturers were tested for their primary stability in simulated medium-high cancellous bone and the risk to fracture in high-density methacrylate blocks. For the assessments of the maximum insertion torque (IT) and torsional fracture (TF) 5 of each OMI were used and for the pull-out strength (POS) 10. The OMIs were inserted with a torque screwdriver (12 sec/360°) until the bottom at 8 mm depth was reached. OMI designs were analyzed with a scan electron microscope (SEM).
Results: SEM images revealed a great variation in product refinement. In the whole sample, a cylindrical OMI shape was associated with higher POS (p<0.001) but lower IT (p=0.002) values. The outer and inner OMI diameters were design characteristics well correlated with POS, IT and TF values (ranging from 0.601 to 0.961). Greater thread depth was related to greater POS values (r= 0.628), although OMIs with similar POS values may have different IT values. Thread depth and pitch had some impact on POS. TF depended mainly on the OMI inner (r= 0.961) and outer diameters (r=0.892). A thread depth to outer diameter ratio close to 40% increased TF risk.
Conclusions: Although at the same insertion depth the OMI outer and inner diameters are the most important factors for primary stability, other OMI design characteristics (cylindrical vs. conical, thread design) may significantly affect primary stability and torsional fracture. This needs to be considered when selecting the appropriate OMI for the desired orthodontic procedures.
Key words:Orthodontic mini-implants, primary stability, insertion torque, pullout strength, torsional fracture.
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