CONTINUING EDUCATION
The biology of orthodontic tooth movement
part 3: the importance of magnitude
Dr. Michael S. Stosich delves into the clinical consequences of force magnitude
Introduction
How important is magnitude in orthodontic
tooth movement? What is a “heavy force,”
and what is a “light force” in orthodontics?
What does it mean at the biological
level? Though much work has been done
advocating light forces for orthodontic
tooth movement, with entire systems and
philosophies built around it, what really is to
be considered heavy force, and how much
does it matter? At the most fundamental
level, the minimum amount of force
needed is enough to trigger the process
of bone remodeling around the tooth,
and the maximum is below the threshold
of hyalinization and occlusion of vascular
structures in the periodontal ligament. In
this article, I will present views grounded
in scientific research, that present various
evidence that one type of force magnitude
is better and or equal than the other, and
what clinical consequences or side effects
can result from its application.
Root resorption
A commonly thought negative sequela of
heavy force is root resorption. The etiology
of root resorption is multifactorial, and
importantly, no definitive evidence links
Educational aims and objectives
This article aims to discuss force magnitude and its possible side effects.
Expected outcomes
Correctly answering the questions on page 49, worth 2 hours of CE, will
demonstrate the reader can:
• Realize the importance of magnitude in orthodontic tooth movement.
• Define the meaning of “heavy” and “light” force in orthodontics.
• Examine risk factors regarding root resorption.
• Translate some of these concepts into clinical practice.
heavy orthodontic forces to root resorption.
Root resorption risk factors include the
severity of the presenting malocclusion,
genetics, systemic health, initial root
morphology, density of alveolar bone,
previous endodontic treatment, patient
age and sex, history of asthma or allergies,
length of treatment, the proximity of roots
to the cortical plate, extractions, duration
of force, and constant force.1,2
Owman-Moll, et al.,3 studied the
effects of applying twice the amount of
force magnitude, 50g versus 100g, related
to tooth movement and root resorption and
found the degree of root resorption did not
differ significantly with the doubled force.
Furthermore, tooth movement did not differ
significantly over a period of 7 weeks.
Light force/heavy force
Michael S. Stosich, DMD, MS, MS, has performed
orthodontic and craniofacial reconstruction
work throughout the world, but his first priority
is his patients at iDentity Orthodontics in the
Chicagoland area. With educational credentials
and training twice that required of an orthodontist, Dr.
Stosich has published and lectured throughout the United
States and abroad. His sincere interest and dedication
toward the study of stem cell tissue engineering, combined
with a rare creativity toward scientific discovery, paved the
way for Dr. Stosich to serve as lead scientist in a variety
of studies. This yielded numerous publications that lead
to important advancements in craniofacial cases. His
achievements were also awarded by the National Institutes
of Health, which endowed grants toward future study. Dr.
Stosich is also faculty at the University of Chicago Medicine.
Dr. Stosich believes in giving back to the communities he
serves and focuses on charitable giving where it can do
the most good by treating underserved and unprivileged
children through his involvement in the Smiles Change Lives
foundation, Smiles for Service, and his work on the Chicago
craniofacial team. Dr. Stosich is also involved in local
community programs linking orthodontics to philanthropy.
drstosich@identityortho.com
50 Orthodontic practice
From the light force perspective, Gonzales,
et al.,4 studied four different mesial force
applications of 10, 25, 50, and 100g on
rat maxillary first molars. The study sought
to examine the effects of varying forces on
tooth movement and root resorption. They
found increased rates of tooth movement
at the lightest force of 10g and more root
resorption with heavier forces. To translate
this into the human craniofacial structure
in terms of actual gram force was not,
unfortunately, elicited.
From a contrasting perspective,
Yee, et al.5, compared orthodontic tooth
movement under heavy (300g) and
light (50g) continuous forces in canine
retraction and found that during initial
tooth movement, force magnitudes were
unrelated, but at later periods, higher rates
Figure 1: Finite model of an incisor
and force distribution11
of tooth movement were produced with
heavy forces. The caveat to this was a loss
of retraction control, or magnitude direction,
in this study. Due to a loss of anchorage
and canine rotation, the increased rate of
tooth movement was effectively cancelled
out. Controlling direction is fundamental,
whether it be with light force or heavy force.
Indeed, the type of desired tooth movement
calls for different minimum level of force
magnitude levels. For instance, tipping,
being the mechanically easiest of tooth
movements, responds to forces as low as
35-60g.6 Bodily movement, on the other
hand, requires minimum forces of 75-120g.
This is in conflict to Tanne, et al.,7 where it
was found that 4 times the force is needed
for bodily tooth movement. McGuiness, et
al.,8 found the net force felt on the PDL to
tipping forces varies not only in magnitude,
but the type of tooth as well. As can be
seen, even deciding on the minimum force
level needed for certain types of tooth
movement is unclear.
Contrasting views in the literature
A thorough review of the literature shows
hundreds of contrasting articles in various
Volume 5 Number 2
CONTINUING EDUCATION
Figure 2: .012, .014, .016 NiTi wires
animal species and various human clinical
studies. Further variation appeared in the
type of tooth movement, the magnitude,
and the direction. Additionally, what is
classified as a heavy force (100g, 150g,
200g, etc.) in humans or a light force (10g,
20g, 30g, and so on) is not entirely clear.
There is wide variability in the literature
suggesting the benefits of light forces or
heavy forces. Each tooth in an individual
patient may require a certain “optimal”
force,6 which clinically cannot currently
be known. In reviewing the vast available
studies, it is reasonable to posit that the
magnitude of the force is as important as
the direction and duration of the force, with
the goal of reducing the amount of time the
tooth is under traumatic force (either light
or heavy).
Figure 3: Root resorption in maxillary incisors12
malocclusions, bracket slot size, and
many others, I will not seek to address it
here. However, the selection of the type
of initial arch wire can be discussed based
on a recent report. A comprehensive
2013 Cochrane Review examining nine
randomized controlled clinical trials with
571 participants over a period of 60 years,
published in the 2013 Cochrane Database
sought to answer which initial arch wires
were most effective for tooth alignment
during orthodontic treatment.9 Wires
examined were multi-stranded stainless
steel, conventional (stabilized) NiTi, superelastic NiTi, copper NiTi, and thermo-elastic
NiTi initial arch wires. Notably, no reliable
evidence was found in favor of a specific
arch wire material and its effect on pain,
speed of alignment, or root resorption.
Summary
Webster’s Dictionary defines a drug as a
substance intended for use in the diagnosis,
cure, mitigation, treatment, or prevention
of disease.10 Based on a consensus of
the various studies, the optimum level of
magnitude that is best for orthodontic tooth
movement remains unclear. Yet a safe and
fundamental principle, with the goal of
proper clinical magnitude selection, is the
minimization of errors and reduced patient
trauma from various force applications. It
is important to remember that each wire
we install on the patient is a prescription
drug delivery mechanism with a known
rate of force magnitude that potentially can
deliver a desired, or an undesired clinical
consequence, underlying the importance
of each wire and its clinical application. OP
How does this translate into
clinical practice?
In clinical practice, one example is
the cross-sectional wire diameter and
the wire’s material composition. The
orthodontist must decide which type
of wire(s) to employ for a given patient,
where numerous options are available,
each touting perceived advantages over
the next, and where each clinician has a
slightly biased preference. This begs the
question, Under what science is this wire
being chosen?
Is it best to begin everyone on a .012
or .014 NiTi (nickel-titanium) wire and
progress upwards to .016NiTi, .018NiTi,
then rectangular wires, then steel wire, and
so on? Since that question is plagued with
various clinical variables and presenting
Volume 5 Number 2
References
1. Topkara A, Karaman AI, Kau CH. Apical root resorption
caused by orthodontic forces: A brief review and a long-term
observation. Eur J Dent. 2012;6(4):445-453.
2. Sameshima GT, Sinclair PM. Predicting and preventing root
resorption: Part 1. Diagnostic factors. Am J Orthod Dentofacial
Orthop. 2001;119(5):505-510.
3. Owman-Moll P, Kurol J, Lundgren D. Effects of a doubled
orthodontic force magnitude on tooth movement and root
resorptions. An inter-individual study in adolescents. Eur J
Orthod. 1996;18(2):141-150.
4. Gonzales C, Hotokezaka H, Yoshimatsu M, Yozgatian JH,
Darendeliler MA, Yoshida N. Force magnitude and duration
effects on amount of tooth movement and root resorption in the
rat molar. Angle Orthod. 2008;78(3):502-509.
5. Yee JA, Türk T, Elekdağ-Türk S, Cheng LL, Darendeliler MA.
Rate of tooth movement under heavy and light continuous
orthodontic forces. Am J Orthod Dentofacial Orthop.
2009;136(2):150-151, e1-9.
6. Proffit WR, Fields HW. Contemporary Orthodontics. 3rd ed.
St. Louis, MO: Mosby; 2000: 296.
7. Tanne K, Sakuda M, Burstone CJ. Three-dimensional
finite element analysis for stress in the periodontal tissue
by orthodontic forces. Am J Orthod Dentofac Orthop.
1987;92(6):499-505.
8. McGuinness NJ, Wilson AN, Jones ML, Middleton J. A stress
analysis of the periodontal ligament under various orthodontic
loadings. Eur J Orthod. 1991;13(3):231-242.
9. Jian F, Lai W, Furness S, McIntyre GT, Millett DT, Hickman J,
Wang Y. Initial arch wires for tooth alignment during orthodontic
treatment with fixed appliances. Cochrane Database Syst Rev.
2013;4:CD007859.
10. “drug”. Merriam-Webster.com. 2011. http://www.merriamwebster.com/dictionary/drug. Retrieved May 8, 2011.
11. Hemanth M, Lodaya SD. Orthodontic force distribution:
a three-dimensional finite element analysis. World Journal of
Dentistry. 2010;1(3):159-162.
12. Leach HA, Ireland AJ, Whaites EJ. Radiographic diagnosis
of root resorption in relation to orthodontics. Br Dent J.
2001;190(1):16-22.
Orthodontic practice 51