Mitsuru Motoyoshi

The effect of cortical bone thickness on the stability of orthodontic mini-implants and on the stress distribution in surrounding bone

Cortical bone thickness (CBT) was evaluated at mini-implant placement sites in 65 orthodontic patients and was found to be directly proportional to the success rate of the mini-implant. The success rate of the mini-implant was significantly greater at sites with CBT> or =1.0mm. To examine the biomechanical effects of CBT, finite element models were made for CBT from 0.5 to 1.5mm, at 0.25-mm intervals. Cortical bone models without cancellous bone were constructed to examine the biomechanical influence on cortical bone after cancellous bone resorption. CBT influenced the stresses in the cancellous bone, but could not directly influence the stresses in the cortical bone. For CBT<1mm, the cancellous bone models exhibited von Mises stresses exceeding 6MPa, and the cortical bone models without cancellous bone showed von Mises stresses exceeding 28MPa. Greater CBT values were associated with higher mini-implant success rates. This morphometric study and mathematical simulation verify that a clinical CBT threshold of 1mm improves the success rate of mini-implants.

Cortical bone thickness in the buccal posterior region for orthodontic mini-implants

The aim of this study was to investigate cortical bone thickness in the buccal posterior region mesial and distal to the first molar, where mini-implants are often placed, and determine any differences according to location, age and sex. The subjects were 43 patients who had mini-implants placed in the posterior buccal alveolar bone as anchors for orthodontic treatment. Computed tomography was used for diagnostic imaging of the area surrounding the site of implant placement. Cortical bone thickness was measured from 1 to 15 mm below the alveolar crest at 1-mm intervals. The average cortical bone thicknesses ranged from 1.09 to 2.12 mm in the maxilla and 1.59 to 3.03 mm in the mandible. The greater the height, the thicker the cortical bone tended to be, and the mandibular cortical bone was significantly thicker than that of the maxilla. The cortical bone was thinner in females than in males in the region of attached gingiva in the maxilla mesial to the first molar. The mandible suffices as a preparation site for mini-implants, while the maxilla might be insufficient at shallow locations. Regardless of age, the initial stability of mini-implants in shallow locations in the maxilla of women should be considered.

Bone stress for a mini-implant close to the roots of adjacent teeth - 3D finite element analysis

This study aimed to evaluate stress in the bone when an orthodontic mini-implant is close to the roots of adjacent teeth using finite element models (FEMs), and to investigate the causes of the high implant failure rate in the mandible. Four FEMs were used: the implant touches nothing; the implant touches the surface of the periodontal membrane; part of the screw thread is embedded in the periodontal membrane; and the implant touches the root. The effect of cortical bone thickness was evaluated using values of 1, 2 and 3 mm. Maximum stress value and stress distribution on the bone elements was determined. Maximum stress on the bone increased when the mini-implant was close to the root. When the implant touched the root, stress increased to 140 MPa or more, and bone resorption could be predicted. Stress was higher for a cortical bone thickness of 2 mm than for other thicknesses. Cortical bone 2 mm thick had a higher risk for bone resorption. A mandible with an average cortical bone thickness of 2 mm may have a higher risk for implant loosening than a maxilla with the same degree of root proximity, which may be related to the lower success rate in the mandible.

Factors affecting the long-term stability of orthodontic mini-implants

INTRODUCTION The placement and removal torques of mini-implants were evaluated as an index of implant stability. We examined factors affecting the initial and long-term stability of mini-implants. METHODS We measured the placement and removal torques of 134 mini-implants placed in buccal posterior alveolar bone and assessed the relationships among placement and removal torques, placement period, age, sex, and cortical bone thickness. The mini-implants were machine-surfaced, 1.6 mm in diameter and 8 mm long. A torque screwdriver was used to measure the peak torque values. RESULTS AND CONCLUSIONS The placement and removal torques averaged approximately 8 and 4 N cm, respectively. A torque of 4 N cm suggests sufficient anchorage capability for mini-implants. No significant correlation between placement and removal torques was found. Placement torque was significantly related to age and cortical bone thickness in the maxilla, whereas removal torque was not significantly related to placement period, age, sex, or cortical bone thickness.

A three-dimensional measuring system for the human face using three-directional photography

This study was intended to develop a three-dimensional measuring system of the human face for clinical use, to ensure a high precision and a simple input operation by means of a personal computer and to measure the degree of its accuracy. With this system, it is possible to measure automatically two-dimensional coordinates of hundreds of grid points on photographs of the human face with an image scanner as a reading device and to calculate their three-dimensional coordinates with a computer. An orthognathic surgical case illustrates this technique in which the patients face is displayed before and after the surgery on a cathode-ray tube (CRT), with the three-dimensional coordinates obtained with this system. A cubic plaster cast with a certain degree of irregularity has been constructed to measure the precision of this system. Comparison was then made between the three-dimensional coordinates obtained with this system and the coordinates obtained with the contact three-dimensional measuring system. The mean of errors and the standard deviation were 0.04 +/- 0.24 mm for the X coordinate, 0.03 +/- 0.16 mm for the Y coordinate, and 0.08 +/- 0.23 mm for the Z coordinate. Thus the accuracy of this system is high enough for the measurement of the human face.

Orthodontic mini-implant stability and the ratio of pilot hole implant diameter

One notable complication of mini-implants that are used to provide anchorage in orthodontic treatment is loosening. The aim of this study was to evaluate the relationship between mini-implant mobility during the healing phase and the prognosis for implant stability. Twenty male Wistar rats (aged 20 weeks) were used. Drills with diameters of 0.8, 0.9, 1.0, and 1.1 mm were used to make pilot holes in the rat tibiae. The inserted mini-implants (diameter 1.4 mm; spearhead 1.2 mm; halfway between maximum and minimum 1.3 mm; length 4.0 mm) were subjected to an experimental traction of force for 3 weeks. Bone-to-implant contact (BIC) was observed histologically. Another 20 male rats (aged 20 weeks) underwent an identical procedure, and the stability of the mini-implants was measured using the Periotest before and after traction. The data were statistically analysed using Scheffés test. The BIC ratios of the 0.9 and 1.0 mm groups were significantly greater than those of the other groups. The Periotest values measured 3 weeks after implant insertion were significantly lower (P < 0.05) than those measured at insertion, except in the 1.1 mm group. To obtain mini-implant stability, the hole diameter should be between 69 and 77 per cent of the diameter of the mini-implant. A significant decrease in the mobility of the mini-implants 3 weeks post-insertion implies a good prognosis for the subsequent mini-implant stability.

Low-Level Laser Therapy Enhances the Stability of Orthodontic Mini-Implants via Bone Formation Related to BMP-2 Expression in a Rat Model

OBJECTIVE The aim of this study was to investigate the stimulatory effects of low-level laser therapy (LLLT) on the stability of mini-implants in rat tibiae. BACKGROUND DATA In adolescent patients, loosening is a notable complication of mini-implants used to provide anchorage in orthodontic treatments. Previously, the stimulatory effects of LLLT on bone formation were reported; here, it was examined whether LLLT enhanced the stability of mini-implants via peri-implant bone formation. MATERIALS AND METHODS Seventy-eight titanium mini-implants were placed into both tibiae of 6-week-old male rats. The mini-implants in the right tibia were subjected to LLLT of gallium-aluminium-arsenide laser (830 nm) once a day during 7 days, and the mini-implants in the left tibia served as nonirradiated controls. At 7 and 35 days after implantation, the stability of the mini-implants was investigated using the diagnostic tool (Periotest). New bone volume around the mini-implants was measured on days 3, 5, and 7 by in vivo microfocus CT. The gene expression of bone morphogenetic protein (BMP)-2 in bone around the mini-implants was also analyzed using real-time reverse-transcription polymerase chain reaction assays. The data were statistically analyzed using Students t test. RESULTS Periotest values were significantly lower (0.79- to 0.65-fold) and the volume of newly formed bone was significantly higher (1.53-fold) in the LLLT group. LLLT also stimulated significant BMP-2 gene expression in peri-implant bone (1.92-fold). CONCLUSIONS LLLT enhanced the stability of mini-implants placed in rat tibiae and accelerated peri-implant bone formation by increasing the gene expression of BMP-2 in surrounding cells.

Mechanical anisotropy of orthodontic mini-implants

This study investigated stress distribution in the bone around orthodontic mini-implants using the finite element method and determined the difference in the stress distribution for different loading directions to identify risk factors for the loosening of mini-implants. Three-dimensional finite element models were constructed for conventional and cervical threadless mini-implants with cortical bone 1 or 3mm thick. The authors calculated the compressive stresses on the bone elements and evaluated stress distribution according to the loading direction. Directional dependency (i.e. mechanical anisotropy) was observed with the conventional mini-implant model. The compressive stress ranged from -31 to -55 MPa depending on the loading direction. In the cervical threadless model, mechanical anisotropy disappeared and the stress was reduced. Cortical bone thickness had no influence in either model. One of the risk factors for mini-implant failure might be related to mechanical anisotropy. This report suggests ways for clinicians to avoid overload traction force when conventional mini-implants are used. The cervical threadless mini-implant can reduce mechanical anisotropy to facilitate successful placement. Inserting a conventional screw deeply beyond the threaded part might be useful in stabilizing a mini-implant.

Safe placement techniques for self-drilling orthodontic mini-implants

Self-drilling mini-implants are being used more frequently as an orthodontic anchorage, but the placement torque of self-drilling mini-implants can easily become excessive in the thick, mandibular cortical bone. The purpose of this study is to examine a safe self-drilling placement technique that provides adequate placement torque for orthodontic mini-implants. The mini-implants were placed using self-drilling and pre-drilling methods into the ribs of pigs. Specimens were classified into two groups, thin and thick, with cortical bone thicknesses of 1.2 ± 0.02 and 2.0 ± 0.03 mm, respectively, and used to model the human maxillary and the mandibular bones. The peak mini-implant placement torque value was measured and the surrounding cortical bone was observed histologically. In the mandible model, the torque in the self-drilling and pre-drilling groups exceeded 10 N cm, except in one case which had a 1.3 mm diameter pilot hole. Histology revealed cracks in the surrounding cortical bone in the groups whose torque value was 10 N cm or more. Therefore, when using the self-drilling technique to place a 1.6mm diameter mini-implant in the mandibular alveolar bone, it is preferable to drill a 1.3mm diameter pilot hole first.

Friction heat during self-drilling of an orthodontic miniscrew

The aim of this study was to measure the heat generated when using a self-drilling miniscrew at speeds of 50, 100, 150, and 250 rpm. Specimens were classified into two groups: in the thin group the cortical bone thickness was 1.2 ± 0.02 mm on average and in the thick group it was 2.0 ± 0.03 mm on average. The thin group was used to model maxillary bone and the thick group to model mandibular bone in humans. The temperature in the 1.2-mm and 2.0-mm cortical bone specimens was measured according to revolution speed. As the revolution speed increased, the temperature significantly increased in both bone thicknesses. The temperature increased significantly more in the thicker cortical bone. The temperature increase in the 2.0-mm thick bone at 250 rpm exceeded 10°C, regarded as the threshold for bone damage in this study; other temperature increases were below this threshold. Installing self-drilling screws at high speeds with an implanter is not recommended; low speeds of less than 150 rpm should be used.