IntroductionHow 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 resorptionA commonly thought negative sequela of heavy force is root resorption. The etiology of root resorption is multifactorial, and importantly, no definitive evidence links

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 forceFrom 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

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 literatureA thorough review of the literature shows hundreds of contrasting articles in various

The biology of orthodontic tooth movementpart 3: the importance of magnitude

50 Orthodontic practice Volume 5 Number 2

CONTINUING EDUCATION

Dr. Michael S. Stosich delves into the clinical consequences of force magnitude

Educational aims and objectivesThis article aims to discuss force magnitude and its possible side effects.

Expected outcomesCorrectly 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.

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

Figure 1: Finite model of an incisor and force distribution11

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.merriam-webster.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.

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).

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

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, super-elastic 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.

Figure 3: Root resorption in maxillary incisors12 Figure 2: .012, .014, .016 NiTi wires

CO

NT

INU

ING

ED

UC

AT

ION

Volume 5 Number 2 Orthodontic practice 51

SummaryWebster’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