Biomechanics in orthodontics obeys several fundamental laws, amalgamating physics and biology. This article is the first in a series of five articles covering the fundamentals of biomechanics in orthodontics. It will enable you to better understand the physics and biological responses governing tooth movement.
Understanding the principles of biomechanics in orthodontics is essential to controlling the outcome of tooth treatment, and confidently managing your patients’ cases. While it is possible for a dentist beginning in the field of orthodontics to lean on a technician’s skill to develop a clear aligner treatment plan for basic cases, as a professional is important to understanding how to control orthodontic outcomes and to lead the development of treatment planning, particularly when attempting to correct complex malocclusions such a cross bite. For more on the complexity of treating cross bites, read this article: How to deal with crossbites in clear aligner treatment.
Each article in this series covers the following key principles for biomechanics:
The orthodontic force
Centre of resistance
Moments of force and couples
Equivalent forces
Centre of rotation
The Orthodontic Force
The main variables influencing the biological processes of alveolar modelling and remodelling include:
Mechanical: Intensity, direction, and appliance used.
Systemic: Cellular responses, hormones, vitamins, drugs, and others.
This article focuses on how the orthodontic force influences the biological processes of alveolar modelling and remodelling.
By far, more most important principle in orthodontics is the orthodontic force. The rest is simply the art of controlling that force to get the outcome desired for your patient.
Orthodontic force parameters include:
Magnitude: How intense a force is applied. Choices include, springs, elastics, wires, clear aligners, or any number of appliances used to apply a mechanical stimulus.
Direction: The most important variable.
Point of application: Where on the tooth the force is applied.
1. Magnitude
There is a theoretically defined optimal force magnitude that should be applied to teeth to result in the maximum movement while avoiding damage. The challenge is understanding how the force should be applied to different teeth, or even the same teeth between different people, to achieve the desired clinical response.
There are three zones of force values, each with distinct cellular and clinical responses.
Scientific evidence does not support the existence of optimal numerical values, rather individual differences are more important to understand than the magnitude of the force. This underlines the importance of holding knowledge on care and control of the force applied. However, it is noted that some guidelines do exist in literature defining the magnitude of force values for specific movements.
Type of Movement | Optimal Force (grams) |
Tipping | 35 - 60 |
Translation | 70 - 120 |
Root uprighting | 50 - 100 |
Rotation | 35 - 60 |
Intrusion | 35 - 60 |
Extrusion | 10 - 20 |
Proffit: Contemporary Orthodontics, 4th Edition, 2008
As a loose guide, tables like this are useful to understand what forces are considered optimal. It can be noted that for translation movements a larger force is required to be applied, tabled here as 70-120 grams, as the force load will be spread over a larger area. This is opposed to an intrusion, tabled here as only 10-20 grams, given the force load distribution is concentrated on a smaller area of ligament therefore generating greater pressure in the region, even with a light force.
It must be noted that there are numerous factors, beyond the type of movement desired, that must also be considered. Professionals who want to build their proficiency in orthodontic movements should understand the variables of the orthodontic force clearly to provide optimal outcomes for patients.
Out of the various force parameters, magnitude can be considered the most important as specialists have the most control. Given that the concepts of light/optimal and heavy forces are relative, it can be very difficult to establish the differences in force. This will now be explored.
The types of stresses and strains induced by forces are more relevant than their values. To illustrate this, imagine pushing your finger into your palm with a force of 50 grams, then imagine doing the same with a nail. The nail will certainly result in greater damage and tissue strain. This analogy applies well to orthodontics where the same force applied to a tooth with a smooth and homogeneous ligament-bone interface, which is likely to result in a predictable cellular and clinical response without occluding the blood vessels of the periodontal ligament, compared to an irregular and rough interface, which is likely to provoke tissue necrosis, will impede movement until undermining resorption removes the necrotic tissue.
The impact of force magnitude on tissue responses depends on the following factors:
a. Root size and morphology
c. Local cell population
d. Ability of cellular responses to adapt to these stimuli.
For example, healthy teeth with full support structures intact respond differently to the same force applied to teeth with alveolar bone loss. Therefore, force magnitude is only a factor to be considered, is only relative, and must be individualised according to each particular case, respecting all the variables that directly influence the biological responses observed.
Important Notes:
1. There is no scientific basis to the widely held assumption that by increasing force magnitude orthodontic movement accelerates. In fact, it usually causes problems such as increased rotations, loss of anchorage, root resorption, and the loss of control of the desired movements.
2. If a tooth is moving slower than expected, investigate the presence of any kind of mechanical obstacle such as the interference of a neighbouring or antagonistic tooth, or occlusal forces.
2. Direction:
It is important to control the direction of force to tissues. Make a mistake with this and root resorption, which is associated with intense as well as poorly directed forces, is risked.
To illustrate the point, consider a premolar being distalised towards a molar. The ideal movement would be translational, where there is constancy in the apposition and resorption of bone. Where the tooth is moved with an incorrect directional force, an inclinational movement occurs. This results in opposite areas of resorption and apposition. The problem arises when reversing the inclination to complete the distalisation by performing the root uprighting. At this stage the reabsorption and apposition areas reverse. This extends the period of treatment because of mineralisation of the osteoid tissue that normally forms in the apposition areas. This rocking movement is undesirable as it extends treatment and increases the risk of root resorption.
3. Point of Application
The point of application and the direction of force applied is closely related for achieving optimal movements, particularly translational. Its control varies depending on the appliance used. Metal braces for example, are limited in their point selection and may require the use of supplementary tools such as elastics to change the force direction, whereas clear aligners combined with attachments allow greater control, point selection, and simpler achievement of a greater range of movements without the requirement for supplementary tools given they cover the whole crown, simplify treatment.
Summary
This article has covered some biological foundations that contribute to varying individual differences in observed clinical outcomes. Pain responses, degree of mobility, speed of movement, and root resorption risks vary significantly between patients. Control of these factors is becoming easier with new technology, such as clear aligners, better 3D modelling, and the availability of orthodontic education.
Dental professionals interested in expanding their skillset in orthodontics and clear aligners are encouraged to look out for a Proligner Introduction to Clear Aligners Course near you or join the course online via live streaming: Education.
Look out for our next article in the Biomechanics in Orthodontics 101 series covering Centre of Resistance.
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