Biomechanics applied to orthodontics

Biomechanics applied to orthodontics

Introduction :

    The term orthodontics had not yet been born when the relationship between force and tooth movement was evident.

Today, orthodontics is practiced almost everywhere in the world and the therapeutic arsenal includes several hundred techniques that are increasingly intended to be “simpler” and “automatic”. Many authors insist on the importance of biomechanics in ODF that every orthodontist should master in order to understand, with the necessary ease and rigor, the physical and mechanical essence of the devices and techniques they use. 

I. Tissue reactions:

1. the elements present

•   Bone

We describe:

• The cortical bone which constitutes the maxillary and mandibular bony edges and limits;
• The spongy or trabecular bone that occupies the space between areas of cortical bone. 

Cortical bone has a supporting role (especially muscular), so it is much denser and is 80 or 90% calcified.

Trabecular bone plays a role as a medullary reservoir involved in hematopoiesis. Its calcification is 15 to 25%.

• Hard Laminate

It is the bone that surrounds the tooth and its ligament space. It is formed of thin compact bone perforated with numerous holes (cribriform plate).

These perforations establish communication with the neighboring medullary spaces and allow the fluid-filled ligament space to act as a hydropneumatic shock absorber for the tooth.

• Ligament space:

It is filled with fluid, cells (fibroblasts, medullary cells, bone cells, blood cells), blood vessels, nerve fibers, and of course collagen fibers constituting the desmodont: the periodontal ligament.

This ligament extends from the lamina dura where it is anchored in the cortical bone lamina, to the cementum which covers the root of the tooth.

• Cement:

It is made up of cells that produce a calcified matrix. It covers the root dentin and allows the desmodont to anchor.

2. Biological concepts of tooth movement:
3. Remodeling phenomenon:

Among all the elements present, most are subject to remodeling phenomena, the disruption of which allows the therapeutic movement of the teeth.

• Bone tissue 

The changes are taking place in very localized areas. 

Osteoclastic resorption precedes apposition according to a sequence: activation-resorption-inversion-formation (ARIF):

• activation:
  • recruitment of preosteoclasts; 
  • recognition of the bone area to be resorbed; 
  • attachment of preosteoclasts to the surface; 
  • fusion of preosteoclasts into multinucleated osteoclasts; 
• absorption:
  • osteoclasts destroy bone tissue; 
• inversion:
  • involvement of mononuclear cells such as macrophages, which establish the boundary between new bone and old bone (cement line); 
  • recruitment of preosteoblasts which differentiate into osteoblasts; 
• training:
  • apposition of osteoid tissue along the cementum line; 
  • mineralization. 

This ARIF sequence is followed by a so-called quiescence phase where the new bone is lined with border cells.

Biomechanics applied to orthodontics

Regulation of bone remodeling: 

General factors

The most important calciotropic hormones as well as 

minerals, especially calcium and phosphate ions.

Local factors:

Many are synthesized by bone cells, but others come from cells of the immune or hematopoietic system, and are found in the bone microenvironment.

• We distinguish: cytokines (interleukins), prostaglandin stimulates bone resorption. 
• Alveolodental ligament:

 The ligament is a connective tissue composed of many cells and an extracellular matrix.

  Cells 

• Fibroblasts in majority. 
• Osteoblasts at the level of the alveolar bone near the ligament. 
• Cementoblasts at the level of the dental root. 
• Cells escaped from the capillaries (lymphocytes, macrophages and mast cells) and cells from neighboring medullary spaces. 

Extracellular matrix

• Fundamental substance. 
• Periodontal fibers (collagen, reticulin, oxytalase). 

There is also a network of capillaries and nerve endings.

The alveolodental ligament plays a dual role during remodeling:

• it has its own remodeling capacity; 
• it has a regulatory role in bone remodeling
• Ligament remodeling 

We already know the role of fibroblasts in the synthesis of collagen, they would also have the same properties as macrophages with the possibility of phagocytosis, and would probably be responsible for the degradation of collagen, given the rarity of macrophages,

All of these operations are carried out in the presence of vitamin C and can be carried out simultaneously by the fibroblast.

2. Physiological migration of teeth

Throughout the life of each individual, teeth move. The movement is due to the physiological migration of the germ and the tooth.

Changes in the position of the germ would be caused mainly by the growth of dental structures and the concomitant remodeling of neighboring tissues, i.e. the alveolar bone, the gingiva, the alveolodental ligament.

The consequences of this migration are mainly found in two areas

• alveolar zone; 
• desmodontal area. 

At the level of the alveolar bone

According to Baron, during physiological migration, “every bone trabecula tends to maintain its thickness constant.” As a result, he explains that each time there is resorption on the side of the cribriform lamina, there is apposition on the other side and vice versa.

At the ligament level

The desmodont is a fibrous connective tissue and therefore has a certain turnover ; the physiological migration of the teeth accentuates its rate of cellular and fibrillar renewal creating a permanent adaptation to the new position of the tooth. Throughout the movement, the width of the ligament remains constant.

Biomechanics applied to orthodontics

3. Applying force to a tooth:

3.1 Immediate effects

At the moment of application of the force, a rapid displacement of the tooth occurs corresponding to the involvement of the ligament and the desmodontal hydropneumatic system, the liquids being expelled from the pressure zones towards the tension zones and the neighboring medullary spaces. This deformation has a limit, and if the force continues, the displacement continues thanks to the possibilities of deformation of the alveolar bone.

If the force increases further, there will be a deformation of the tooth, but this goes beyond the scope of the forces used in orthodontic therapy.

If the force stops, there follows a more or less rapid return to normal.

If the force continues, new events occur in response to this new equilibrium.

3.2 Longer-term effects

It is now appropriate to distinguish different areas:

• pressure zone (with reduction of ligament space); 
• tension zone (with increase in ligament space); 
• intermediate zone where there is no variation in the ligament space but where the collagen fibers are in tension or relaxed. 

Pressure side

Area where pressure is low

The desmodontal space narrows, causing compression of the connective tissue and vessels. Vascularization is disrupted but can still occur, allowing cellular elements to reach the compressed area. The body will then try to recreate the normal desmodontal space. To do this, osteoclasts are involved which will resorb the cribriform plate corresponding to the compressed area: direct bone resorption.

Area where pressure is high

If the area is subjected to greater compression, vascularization is impossible.

There is degeneration of non-vascularized tissues and formation of a hyaline zone.

The tissues seek to recreate a new balance. But the hyaline zone is inaccessible to vascularization and cells. To be able to reabsorb it, the body uses a roundabout way: indirect resorption. The osteoclasts invade the medullary spaces near the hyaline zone, then reabsorb the alveolar wall until reaching the hyaline zone, which is then accessible to the osteoclasts. The tooth can then be moved.

Tension side

The tissue reactions are very close to those observed on the pressure side, but in the opposite direction. 

If the force is light, there is widening of the ligament and vascular spaces. Many osteoblasts appear and are active from the second day, allowing bone apposition.

If the force is heavier, there is production of many osteoclastic cells and the appearance of ligament lesions. Too great a force can cause a fibrillar tear.

After a short latency period (a few hours), osteogenic cells appear. The osteoblasts then secrete an osteoid tissue which mineralizes and allows bone apposition.

Intermediate zone

These are areas where there is no change in desmodontal thickness. On the other hand, the fibers are put under tension or released. Their bone anchorage therefore reflects either a release or a tension at the bone level.

4. Mechanisms of transforming a force into dental movement

Here we will try to analyze the mechanisms that transform a physical component, force, into histological and cellular phenomena leading to another physical phenomenon: dental movement.

When applying force to a tooth, we have seen that there is a modification of the ligament space with:

• creation of pressure and tension zones; 
• fluid movements. 

This results in deformations in these areas:

• cellular elements; 
• vascular and nervous elements; 
• elements of the extracellular matrix. 

Cellular response: remodeling

• At the bone level

Osteoblasts are fundamental cells in the regulation and coordination of bone remodeling.

They receive signals that cause the formation of collagenase which leads to the removal of collagen and allows bone resorption by osteoclasts.

• At the ligament level

The fibroblast simultaneously manages the synthesis but also the degradation of collagen by reducing:

• bone cell proliferation; 
• the production of alkaline phosphatase which is one of the fundamental enzymes of bone remodeling. 

Biomechanics applied to orthodontics

5. Ideal strength from a histological point of view

• Light force – heavy force

Many authors have tried to quantify this ideal force for which the speed of tooth movement is maximum without causing tissue damage.

	Light force	Heavy force
0-1 sec	The alveolar bone deforms with the appearance of bio-electrical phenomena within the bone.
1-2 sec	Ligament fluids are expelled from the compressed area. The tooth moves into the ligament space.
3-5 sec	Pressure side Tension side Compressed Vx Dilated Vx (partially obliterated) Deformed fibers and cells	Pressure side Tension side Obliterated Vx Dilated Vx Very deformed fibers and cells, cell lysis
2 days	Tooth movement with alveolar remodeling
7-14 days		Indirect resorption reaches the hyaline zoneTooth displacement

6. Tissue damage

They can be irreversible and affect:

• the tooth (rhizalysis); 
• the periodontium (loss of epithelial attachment and marginal bone). 
• Root resorptions  : 

They are due to the high pressures and particularly affect the pressure zones during this movement:

• apex during ingression; 
• pressure side root during translation
• Periodontal lesions

We can have:

• Windows caused by excessive travel speed or heavy forces;
• Gingival fissures made with closure of extraction spaces with high speeds;
• Loss of epithelial attachment and marginal bone.

II. Biomechanics:

1. Definition of biomechanics:

Bio: Bios = life

Mechanics: Mekhani = machine

Biomechanics is physical mechanics applied to living things.

– Biological part: the tooth + periodontium

– Mechanical part: these are the systems that allow forces to be applied

2. Mechanical notions of displaced body:

2.1. Strength:

Fundamental concept of physics: Cause of the deformation of a body or the modification of its state of rest or motion. 

It is characterized by four elements:

• intensity; 
• the meaning; 
• the management; 
• the point of application. 

2.2 Force couple:

It is a set of two parallel forces, of the same intensity and opposite directions. The torque tends to cause the rotation of the solid to which it is applied.

Biomechanics applied to orthodontics

2.3 Center of resistance: hypomochlion

A force whose line of action passes through the hypomochlion causes a translation of the tooth.

The concept of the center of resistance is fundamental and its location, even approximate, is sufficient to predict movements. In our practice, it can be considered fixed and located between the middle and the apical third of the root.

2.4 Center of rotation:

Point around which the tooth performs a rotational movement.

5. Moment of strength:

M = F x D

3. Concept of anchoring:

3.1 Definition: resistance of a body to displacement

Moving the body requires a driving force > resisting force.

Stable resistance: anchor point of the force.

Moving resistance: point of application and resistance of the tooth to be moved.

3.2 DENEVREZE trinomial:

RS=Rm>F: No movement

RS=Rm<F: Equal and symmetrical displacement

F>Rs>Rm: Unequal displacement

Rs>F>Rm: Desired orthodontic movement

Rs<F<Rm: Loss of anchoring.

Biomechanics applied to orthodontics

4. CHARACTERISTICS OF AN ORTHODONTIC FORCE:

• Intensity :
  • Orthodontic forces: “Biological”.
  • Optimal strength: Displacement by bone resorption without periodontal lesions.
• Direction:

Vertical, horizontal, oblique.

• Moment/force ratio.
• Pace :

– Intermittent: Periods without any device (FEB worn 14 hours/day).

      – Continuous: long period of activity, energy very gradually decreasing.

– Discontinuous: rapid decrease in force with displacement

5. Dental Movements in ODF:

7.1. Displacements due to horizontal forces:

• Version Movement (Tipping): 
• Uprighting Movement After Version (Uprighting)
• Translational movement (bodily movement).

7.2. Displacements due to vertical forces:

• Egression: displacement of the tooth in the direction of its eruption . 
• Ingression : direction opposite to the eruption.

3. Rotation : Rotation of the crown around its axis.

6. Individual physiological factors complicating the mechanical system:

• General factors:

• Food-metabolism.
• Pregnancy 
• Age.
• Bone density.
• Cell cycle.
• Biological rhythm.
• Local factors:
  • Anatomical factors
  • Histological factors.
  • Loss of alveolar bone height.
  • Functional factors.

Conclusion :

           Knowledge of the fundamental notions of biomechanics in ODF allows us not only to make the choice as to the various orthodontic techniques that allow us to achieve our therapeutic objectives but also to know how to use them in order to treat as quickly as possible while respecting tissue integrity.

Biomechanics applied to orthodontics

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Biomechanics applied to orthodontics