Mizuki Inaba

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.

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.

A preliminary study of the effects of low-intensity pulsed ultrasound exposure on the stability of orthodontic miniscrews in growing rats

BACKGROUND Orthodontic miniscrews placed in growing subjects often loosen during orthodontic treatment. The ability to place miniscrews, regardless of age, would be clinically beneficial. OBJECTIVES To assess the effects of low-intensity pulsed ultrasound (LIPUS) on the stability of orthodontic miniscrews in growing rats. MATERIALS/METHODS The mobility of miniscrews after placement was recorded and the miniscrew-bone interface was examined histomorphometrically using tibiae of seven male Sprague-Dawley rats (aged 6 weeks). Field-emission scanning electron microscopic images were used to evaluate the bone-miniscrew interface, and a mobility test device was used to assess the stiffness of miniscrew placement. Fourteen custom-made miniscrews with 1.4mm diameters and 4.0mm lengths were placed in the right and left tibiae. LIPUS was used to stimulate right tibiae at the sites of miniscrew placement, and left tibiae were left untreated as controls. RESULTS Significantly lower mobility was observed in the LIPUS-treated group compared with the control group (P < 0.05). Histomorphometric evaluation indicated that bone-miniscrew adhesion was significantly better in the LIPUS-treated group than in the control group (P < 0.05). LIMITATIONS This in vivo study used tibiae rather than jaw bones because the jaw bones of 6-week-old rats were too small to allow miniscrew placement. CONCLUSIONS LIPUS was able to increase the bone-miniscrew contact and reduce the mobility of miniscrews in growing subjects. IMPLICATIONS LIPUS may accelerate the bone healing process after miniscrew placement in growing subjects and can reduce the latent period.

Measuring bone density at orthodontic miniscrew implantation sites using microcomputed tomography.

PURPOSE This study was performed to determine the accuracy of measurements of bone mineral density (BMD) and cortical bone thickness (CBT) at miniscrew implantation sites in small animals and to verify the usefulness of in vivo microcomputed tomography (micro-CT). MATERIALS AND METHODS Rat femurs were scanned before and after placing miniscrews using in vivo micro-CT. The images were superimposed using a subtraction method with bone volume measurement software. At each screw site, the total BMD was calculated as the average BMD of a cylinder 1.6 mm in diameter and 2.0 mm in depth, while the cortical BMD was the average BMD of a cylinder 1.6 mm in diameter and 1.0 mm in depth. CBT was measured three times on transaxial images of the rat femurs, and the average value was used. All miniscrews were placed using the maximum torque, verified with a digital torque tester. To verify the usefulness and accuracy of in vivo micro-CT, CBT on micro-CT images was compared with that measured on histologic sections. RESULTS Significant correlations were observed between placement torque and cortical BMD (R = 0.572), total BMD (R = 0.732), and CBT (R = 0.788). There was a significant correlation between CBT measured via field-emission scanning electron microscopy images and CBT measured with in vivo micro-CT (R = 0.974). CONCLUSIONS The BMD over a narrow range can be measured accurately in small animals with high reproducibility of the trabecular structure using a combination of high-resolution in vivo micro-CT and image superimposition using a three-dimensional subtraction method.

Enhancement of Orthodontic Anchor Screw Stability Under Immediate Loading by Ultraviolet Photofunctionalization Technology

PURPOSE Ultraviolet (UV)-mediated photofunctionalization technology is intended to enhance the osseointegration capability of titanium implants. There are concerns about orthodontic anchor screws loosening under immediate loading protocols in adolescent orthodontic treatment. The purpose of this in vivo study was to evaluate the effects of photofunctionalization on the intrabony stability of orthodontic titanium anchor screws and bone-anchor screw contact under immediate loading in growing rats. MATERIALS AND METHODS Custom-made titanium anchor screws (1.4 mm in diameter and 4.0 mm in length) with or without photofunctionalization pretreatment were placed on the proximal epiphysis of the tibial bone in 6-week-old male Sprague-Dawley rats and were loaded immediately after placement. After 2 weeks of loading, the stability of the anchor screws was evaluated using a Periotest device, and the bone-anchor screw contact ratio (BSC) was assessed by a histomorphometric analysis using field-emission scanning electron microscopy. RESULTS In the unloaded group, Periotest values (PTVs) were ~25 for UV-untreated screws and 13 for UVtreated screws (P < .01), while in the immediate-loading group, PTVs were 28 for UV-untreated screws and 16 for UV-treated screws (P < .05). Significantly less screw mobility was observed in both UV-treated groups regardless of the loading protocol. The BSC was increased ~1.8 fold for UV-treated screws, compared with UV-untreated screws, regardless of the loading protocol. CONCLUSION Photofunctionalization enhanced the intrabony stability of orthodontic anchor screws under immediate loading in growing rats by increasing bone-anchor screw contact.

Risk factors for failure of orthodontic mini-screws placed in the median palate

This study investigated the stability of mini-screws placed in the median palate. The study included 25 patients (7 males, 18 females; mean age, 23.4 ± 5.6 years; age range, 15.0-34.5 years) who had mini-screws placed during orthodontic treatment at Nihon University School of Dentistry Dental Hospital. Mini-screws (diameter, 2.0 mm; length, 9.0 mm) were placed in the median palatal region; the first screw was inserted mesiodistally at the distal contact of the maxillary first molar, and the second screw was placed 6-9 mm mesial to the first screw. Immediately after placement, the placement sites were carefully examined with cone-beam computed tomography and a Periotest device. Screw stability was not related to perforation of the nasal cavity, patient age, or patient sex. The success rate was significantly higher in patients with screw-suture distances of 1.5-2.7 mm than in those with distances of 0-1.4 mm. Moreover, mini-screws could be stabilized when palatal cortical bone thickness was ≥1.5 mm. The success rate was significantly higher in the group with insertion depths of ≥4.5 mm. These results indicate that primary stability of mini-screws requires sufficient cortical bone thickness, insertion depth, and screw-suture distance.

A preliminary study of effects of low-intensity pulsed ultrasound (LIPUS) irradiation on dentoalveolar ankylosis

The purpose of this experiment was to investigate whether low-intensity pulsed ultrasound (LIPUS) irradiation can inhibit dentoalveolar ankylosis in transplanted rat teeth. LIPUS irradiation (the pulsed ultrasound signal had a frequency of 3.0 MHz, a spatial average intensity of 30 mW/cm2, and a pulse ratio of 1:4) was performed on the face over the re-planted teeth of rats for 4 weeks. After the rats were euthanized, we measured mobility (Periotest value [PTV]) of the transplanted and control teeth using a Periotest. Finally, we performed histological evaluation to detect ankylosis. PTVs tended to be significantly lower for re-planted teeth than for control teeth. Histological evaluation revealed that the roots of all re-planted teeth were coalescent with alveolar bone. Furthermore, no ankylosis was observed in three-fifths of the re-planted teeth following LIPUS irradiation. These results indicate the potential efficacy of LIPUS to inhibit dentoalveolar ankylosis.