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R=.841; Bone formation18-28=26.013+(359.58*Sutural separation18-28)R=.731; Bone formation28-38=59.232+(375.62*Sutural separation28-38)

Sean Shih-Yao Liu, DDS, PhD1 & Peter H. Buschang, PhD2

1. Department of Orthodontics and Oral Facial Genetics, Indiana University School of Dentistry, Indianapolis, Indiana

2. Orthodontics Department, Baylor College of Dentistry, Texas A&M University Health Science Center, Dallas, Texas

Is there an optimal force level for sutural expansion?

ABSTRACT

BACKGROUND

MATERIALSand METHODS

Purpose: To establish the causal relationships between force magnitudes, sutural separation, and sutural bone formation. Methods: Thirty-seven six-week old rabbits were randomly assigned to four force groups (0, 50, 100, or 200 g, respectively). Constant forces were delivered for 42 days by nickel-titanium open coil springs to miniscrew implants (MSIs) placed in the frontal bone on both sides of the midsagittal suture. Inter-MSI and bone marker widths were measured biweekly to quantify sutural separation and MSI movements. Sutural bone formation was quantified based on the incorporation of fluorescent bone labels administered at days 18, 28, and 38. Results: Nine of 74 MSIs failed between days 0 and 14, including four in the controls and five in the 50 g group. A decelerating-curvilinear pattern of sutural separation was evident in the 50, 100, 200 g groups. Bone markers showed that sutural widths increased 0.6 mm, 3.2 mm, 5.1 mm, and 6.2 mm in the control, 50 g, 100 g, and 200 g groups, respectively. Significantly greater amounts of bone formation were observed between days 18-28 than days 28-38. Sutural bone formation also increased with increasing forces up to 100 g; there were no differences between 100 g and 200 g groups. Sutural separation explained 71% and 53% of variation in bone formation between days 18-28 and 28-38, respectively. Conclusions: Within limits, sutural bone formation is directly related to the amount of sutural separation, which is in turn related to the amount of force applied. The results suggest that there is a level of induced sutural separation that provides greatest amount of bone formation.

CONCLUSIONSIdeal force levels exist that produce the maximum amount of sutural bone growth. Using varying amounts of constant force anchored by MSIs and indicated by bone markers and bone labels, the following relationships were established: 1.The amount of sutural separation increases with increasing force levels, but the increases are not proportional.2.Bone formation increases with increasing force, but it plateaus at the highest force levels.3.Bone formation is directly related to the amount of sutural separation produced.

Figure 7 Bone formation between days 18-28 (BF Days 18-28) and between 28-38 (BF Days 28-38) and sutural gap between two sutural bone margins for four force groups

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While orthodontists widely use sutural expansion to increase mid-facial volume, the exact nature of relationships between force magnitude and subsequent sutural separation and bone formation remain unknown. Our goal was to better understand how sutures respond to varying amounts of force application. Histomorphometric and biometric analyses were performed to test the hypotheses that (1) sutural separation increases as a function of increasing expansion force magnitude, (2) bone formation increases as a function of expansion force magnitude, and (3) bone formation is related to sutural separation. Our clinical objective was to determine whether there is an “optimal” force that maximizes sutural bone formation. If such a force exists, it provides a potential means of making expansion therapies more effective and efficient.

Figure 2. (A) Two pilot holes drilled after exposure of the frontal bones and the midsagital suture, (B) MSIs under scanning electron microscope, (C) 3-mm long MSI, and (D) a NiTi open coil spring telescoped on a guiding wire between the two MSIs with a sliding tube used to maintain the activation level.

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Figure 3. Experimental timeline for records, including animal weights, caliper measures, and radiographs, as well as bone labels.

RESULTS

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Figure 5. Bone marker width changes of the four force groups over time.

Figure 4. Histomorhometric measures. (A) Inter-bone labels width between calcein and oxytetracycline administered at day 28 and 38 and (B) sutural gap width at day 42.

Figure 1. Schematic ventrodorsal radiographs used to evaluate inter-mini-screw implant (MSI) width, the anterior bone markers width (AB), and the posterior bone markers width. MSI Bone marker

Figure 6. Representative fluorescent bone labels of the four forces groups. 1st label (green) toward bone: oxytetracycline at day 18. 2nd label (red): calcein at day 28. 3rd label (green): oxytetracycline at day 38. (A) 0 g (control) group, (B) 50 g group, (C) 100 g group, and (D) 200 g group. Arrows: discontinuities of bone labels.

Figure 8. Scatter plots and regression slopes for bone formation as a function of sutural separation between days 18-28 and days 28-38.