IMPLANT BIOMECHANICS

IMPLANT BIOMECHANICS

2. A strength:

Is an action or influence such as traction or pressure which, when applied to a free body, deforms it. It is expressed in newtons (N) .

3. A constraint (stress):

Stress is the internal response of a body to the application of external forces, in practice a stress is the force per unit of section applied to a body which resists an external force.

According to their directions, constraints can be classified into

• tensile stress = tension : this is the internal resistance of a body to a force attempting to pull it, to stretch it.
• compressive stress=Pressure : it is the internal resistance of a body to a force attempting to make it smaller.
• shear = splitting : it is the internal resistance of a body to a force trying to slide one part of a body over another.

2. FUNDAMENTAL BASICS IN IMPLANT BIOMECHANICS:

The purpose of implant placement is to support a prosthetic restoration; the implant unit behaves differently from the dental unit.

1. Biomechanical behavior of the tooth .

The periodontal ligament provides a flexible union between the dental surface and the bone structure; this flexible structure corresponds to physiological mobility . During functional and parafunctional loading, the periodontal ligament is well adapted to absorb these forces thanks to:

• at the disposal of the fibers which plays the role of an elastic shock absorber.
• the presence of the ground substance and blood vessels which plays the role of a hydraulic shock absorber.

The viscoelastic behavior of the ligament allows a two-stage response to the occlusal load. In the first phase, weak forces can cause a more or less significant displacement, within the limits of the clinical mobility of the tooth. In the second phase, by direct support on the bone.

• to proprioception which informs the nerve centers about the pressures exerted on the tooth in order to modulate muscular contractions by increasing or inhibiting them.

Physiological mobility of a tooth:

• -Axial mobility of a tooth : the axial mobility of a healthy tooth is on average 28µm according to Parfitt (1960).
• -The lateral mobility of a tooth : is 56 to 108µm depending on the tooth concerned.

Hypomoclion (center of rotation): In natural teeth, the presence of the viscoelastic ligament and the conical shape of the roots move the center of rotation to the apical region, allowing the stresses to be moved away from the marginal bone crest and distributed to the middle part of the root.

2. Biomechanical behavior of an implant

The biomechanical behavior of an implant must be considered from two aspects:

1.  Biomechanics specific to the implant : Linked to the nature of the material in which it is manufactured

the implant, it can be ductile and allows a certain absorption of stress or fragile fracture without benefiting from any flexibility to absorb part of the stresses.

Ductility : refers to the ability of a material to deform plastically without breaking.

Brittle fracture : is characterized by the rapid propagation of cracks.

-Titanium is ductile, alumina and zirconia are brittle. The longer the material is, the less ductile it is.

2. Biomechanical specificities of an osseointegrated implant

Compared to a natural tooth, the absence of the periodontal ligament around an osseointegrated implant is responsible for:

1. The reduction of his clinical mobility

Implants, like large fixed restorations, have virtually no clinical mobility. This lack of mobility amplifies malocclusions.

Large forces are required to cause small displacement, as it is only the elasticity of the bone that allows displacement . The forces around an implant cause a direct linear response to the load (Sekine 1986) as in the second dental phase. The elasticity of titanium combined with the application of large forces on small diameter implants can quickly become critical for crestal bone.

-Axial mobility : is on average 5µm according to Sekine.

-The lateral mobility of a tooth : is 10 to 50µm according to Sekine and less than 25 µm according to Sullivan.

2. Hypomoclion (center of rotation) displacement:

The center of rotation of an osseointegrated implant is located at the cervical zone, concentrating the stresses at the marginal bone crest.

3. Absence of mechanoreceptors

Significantly reduces the capacity for fine interdental detection, thus making it possible to install avoidance mechanisms. When functional overcontacts and/or overguiding are present on the occlusal surfaces of implant restorations, with a very low sensitivity threshold, they are not detected and therefore not avoided. This results in a large number of micro-traumas, potentially responsible for peri-implant bone loss.

3. Factors involved in implant biomechanics: 3.1- Geometric risk factors.
4. Number of implants:

If several implants support the same superstructure, the load will be shared, so the stress will be less.

Ideally it should correspond to the number of root units (RU) to be replaced; for example a canine is worth 1 RU, while a molar is worth 2 RU.

This assessment is not applied strictly; several factors are taken into account: such as the clinical and anatomical situation, as well as the design of the future prosthesis, so it is possible to have a number of implants lower than the number of URs to be replaced without a significant increase in the risk of overload.

2. Implant diameter:

Increasing the implant diameter; increases the resistance of the implant/prosthesis complex and reduces the lever arm generated by the latter.

4. Implants placed in a tripod:

The positioning of the implants in a tripod allows for a support polygon, which is much more stable than an implant alignment.

5. The height of the prosthetic restoration : A prosthetic restoration height that is too

significant creates a greater leverage arm on the implant head which will expose the components to a risk of unscrewing or fracture.

6. The position of the crown in relation to the implant :

The stresses cause a twisting moment to appear on the implants.

7. Connectivity between implant and natural tooth: will significantly increase the load on the implants.

Clinical mobility of neighboring teeth is a key factor. If their mobility is reduced,

Equilibration is almost similar to that of natural teeth. Contacts and guidance should exist but not be dominant at the start. On the other hand, if their mobility is significant, there is a high risk of occlusal overload on the implants, hence the need for equilibration

specific to embedded implants, or the obligation to use extensive support, to limit the mobility of the whole.

2. The type of bone:

Under the same load an implant has a different mobility, greater in a type IV bone than in a type I bone.

3. Implant bearing surface :

It is the surface of the implant in intimate contact with the bone which ensures:

-the quality and maintenance of osseointegration;

-the transmission of occlusal forces to the peripheral bone, Its characteristics are a function

• Dimensions and general shape of the implant (macrostructure): transverse forces applied to cylindrical implants are concentrated around the neck. Conical implants partially escape this phenomenon. Indeed, the conical shape improves rigidity, strength and distribution of lateral forces in the peripheral bone. On the other hand, if the conicity is too pronounced and if the volume of the implant is oversized compared to the dimension of the crest, the volume of peri-implant bone at the neck will be reduced, it will be poorly irrigated and a risk of resorption will be accentuated.
• Characteristics of the implant surface condition (microstructure): rough surfaces are more favorable than smooth surfaces.

4.  Occlusal risk factors, parafunctions or bruxism patients:

This is not a formal contraindication; but considered a factor of failure if certain precautions are not taken into account:

• The prosthetic restoration must be sufficiently strong through the use of appropriate components ( large implants ) in order to support a very high occlusal load.
• It is important to reduce the width of the occlusal tables and to reduce the inclination of the cusp slopes so as to bring the orientation of the load component as close as possible to the long axis of the implant.
• Support for mandibular excursion paths mainly by natural teeth , if the edentulism is embedded.
• Support for mandibular excursion paths mainly by natural teeth , if the edentulism is embedded.

5.  Technological risk factors : The different stages of the implant prosthesis transmit stresses to the implants. Thus, the materials involved in prosthetic rehabilitation, their fixation method, as well as their adaptation influence implant mechanics.

CONCLUSION

The biomechanical analysis of dental implants allows us to understand their functioning under static or cyclic loads and their fatigue behavior according to the diameter, the connection and the composition of the implants which will allow the practitioner to choose an implant capable of resisting masticatory forces.

IMPLANT BIOMECHANICS

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Composite dental fillings are discreet and long-lasting.
Interdental brushes are essential for cleaning narrow spaces.
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IMPLANT BIOMECHANICS