Bone formation (

1
Is there an optimal force level for sutural
expansion?
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
AM University Health Science Center, Dallas,
Texas
ABSTRACT
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.
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.
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
Figure 8. Scatter plots and regression slopes for
bone formation as a function of sutural
separation between days 18-28 and days 28-38.
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.
BACKGROUND
Figure 3. Experimental timeline for records,
including animal weights, caliper measures, and
radiographs, as well as bone labels.
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.
Bone formation (µm)
MATERIALS and METHODS
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.
R.841 Bone formation18-2826.013(359.58Sutural
separation18-28) R.731 Bone formation28-3859.2
32(375.62Sutural separation28-38)
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
CONCLUSIONS

• Ideal 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
• The amount of sutural separation increases with
  increasing force levels, but the increases are
  not proportional.
• Bone formation increases with increasing force,
  but it plateaus at the highest force levels.
• Bone formation is directly related to the amount
  of sutural separation produced.