Abstracts: July-August 2016

Accuracy and reliability of Dolphin 3D voxel-based superimposition

Mohamed Bazina, DDS, MSD

Background
Superimposition of cephalograms has many uses in orthodontics, including growth evaluation and outcome assessments, but cephalograms are distorted and show incomplete two-dimensional data. Cone beam computed tomography (CBCT) provides a three-dimensional, undistorted, and more complete analysis of our patients. Superimposition of 2 CBCTs is possible by using landmarks, surfaces, or density information (voxel-based). Voxel-based superimposition is automated and uses the most image content, providing the most accurate result. Until recently such superimposition was extremely laborious, but a user-friendly voxel-based superimposition has recently been introduced.

Aim
To evaluate the accuracy and reliability of Dolphin 3D voxel-based superimposition.

Methods
This was a retrospective study using existing scans of 31 surgical orthodontic patients. The sample included 19 females and 12 males with a mean age of 21. Each patient had a pre-surgical (T1) and a post-surgical (T2) scan taken within 12 months. Surgical patients were used due to lack of expected growth to reduce outcome bias. The volumes were superimposed using voxel-based methods from Dolphin Imaging Systems and the accepted method used by Cevidanes, et al. The Cevidanes method, considered as the gold standard, uses two different open-source programs and takes about 3 hours to complete, while the Dolphin method takes under 5 minutes. T2 was superimposed on T1 cranial base. T2 registrations for both methods were compared to each other using the absolute closest point color map, with emphasis on 7 regions (Nasion, A point, B point, bilateral zygomatic, and bilateral gonion).

Results
Intraclass correlation showed excellent reliability (0.96). The mean differences between the two methods were less than 0.21 mm (voxel size = 0.38). The least difference was in the left zygomatic area with 0.09 mm ±0.07, while the largest was in the right gonion region with 0.21 mm ±0.13.

Conclusion
Dolphin 3D voxel-based superimposition, a fast and user-friendly method, is accurate and reliable.

Screw-type device diameter and orthodontic loading influence adjacent bone remodeling

Christian Francis, DDS, MS, University of Kentucky

Background
Utilization of screw-type devices for orthodontic anchorage continues to increase. These screws have been designed progressively narrower to allow for interradicular placement, but failure rates are significantly higher than those of wider endosseous implants. Bone remodeling adjacent to these anchorage screws is critical to maintaining a healthy bone-screw interface and sustaining orthodontic forces.

Purpose
The purpose of this study was to evaluate the effect of diameter and orthodontic loading of a screw-type device on supporting bone remodeling.

Research Design
Anchorage devices (n = 70) with 1.6 mm, 2.0 mm, 3.0 mm, and 3.75 mm diameters were placed into edentulous sites in skeletally mature beagle dogs following premolar extraction and healing. In a split-mouth design, devices on one side were loaded (2N) utilizing calibrated coil springs. Epifluorescent bone labels were given i.v. prior to sacrifice. Bone-implant sections (~ 70 µm) were prepared using undecalcified methods. Bone formation rate (BFR, %/yr) and other histomorphometric variables were assessed using imaging software.

Results
The BFR varied by jaw. The mean BFR ranged from 10.93%/yr. to 38.91%/yr. The BFR was significantly (p <0.05) lower in bone adjacent to the 1.6 mm diameter screws compared to bone adjacent to the 2.0 mm, 3.0 mm, and 3.75 mm diameter screws. BFR was lower adjacent to loaded 1.6 mm screws compared to non-loaded 1.6 mm screws (p <0.01) or loaded 2.0-3.75 mm diameter screws (p <0.01). No significant differences in BFR were noted, regardless of loading condition, between the 2.0 mm, 3.0 mm, and 3.75 mm diameter screws.

Conclusion
We detected a dramatic reduction in a critical biologic parameter, bone remodeling, in a controlled experimental design. While orthodontic loading of 2N did not alter bone remodeling associated with screws of 2.0 mm diameter or larger, it did decrease bone remodeling adjacent to the 1.6 mm screws.  2.0 mm diameter or larger machined screws may be more likely to maintain a healthy bone-implant interface under typical ortho-dontic forces.

The effects of corticotomies on frontonasal suture expansion and bone modeling in mature rabbits

Brittany M. Wright-Graves, DDS, MS

Background
Although expansion of complex adult sutures with continuous forces has been shown to be possible, the amounts of expansion obtained were limited. Whether the bony sutural interface can be altered to enhance sutural separation has not been experimentally investigated.

Purpose
This split-skull study was designed to evaluate whether corticotomies enhance bone modeling and reduce the resistance of sutural articulations during expansion in mature rabbits

Materials and Methods
Nine adult female rabbits, 8 to 9 months old, had miniscrew implant (MSI) supported expansion devices with 150 g open-coil nickel-titanium springs placed bilaterally across the frontonasal sutures. Corticotomies were performed, anterior and posterior to the frontonasal suture, on one randomly chosen side. The other side served as the control. Sutural separation was measured bi-weekly for 7 weeks. Using µCT scans of each specimen, bone material density and bone volume fraction were measured. Qualitative histologic analyses of the tissues were performed using H&E staining.

Results
Most (94.4%) of the MSIs remained stable throughout the experiment. There was significantly (p <.05) more sutural separation on the corticotomy side (3.73 mm) than the control side (2.83 mm). Bone volume fraction was 5.1% less, and bone density was 1.6% less on the corticictomy than control side. The amount of expansion that occurred was negatively correlated (R = .860; p = <.001) with bone density. Blinded histological evaluations showed increased numbers of osteoblasts along the bone fronts on the corticotomy side. Compared to the control side, the sutural margins on the corticotomy side exhibited greater numbers of elongated Sharpey’s fiber insertions, greater amounts of immature woven bone, and more osteocytes.

Conclusion
Mature sutures expanded with adjunctive corticotomies undergo 31% more separation than sutures expanded without corticotomies, with the amount of expansion that occurs being inversely related to bone density.

Abstracts are courtesy of Dentsply GAC.