The success of implant treatment is not limited to Osseointegration and includes the success of the prosthesis. The restorative issues are discussed as under.
1) Excessive Cantilever
Cantilevers in implant dentistry are commonly used especially in the mandibular arch for edentulous patients receiving implant supported prosthesis. Cantilever places offset loads to the abutments and results in greater tensile and shear forces on cement or screw fixation.There are many problems which are associated with the cantilevers like:
1) Excessive Cantilever
Cantilevers in implant dentistry are commonly used especially in the mandibular arch for edentulous patients receiving implant supported prosthesis. Cantilever places offset loads to the abutments and results in greater tensile and shear forces on cement or screw fixation.There are many problems which are associated with the cantilevers like:
- Fracture of the prosthesis.
- Loss of ossointegration.
- Bone fracture.
The biomechanics of cantilevers need to be understood. It has been found that when a three unit prosthesis is supported by two implants and has a distal cantilevered pontic, the bending moments are twice those found in a prosthesis with two terminal implants. When the occlusal forces are applied on the cantilever the implant functions as a fulcrum and is subjected to axial, rotational and torsional forces. It is vital to understand the nature and effects of bending moments. Bending overload is a situation in which the occlusal forces on an implant supported prosthesis exerts a bending moment on the implant cross section at the crestal bone, leading to marginal bone loss and/or eventual implant fatigue. Overload is a well documented factor for bone resorption around implants. The factors associated with bending moments are:
- In line implant placement.
- Leverage— cantilever.
- Parafunction and heavy occlusal force.
These factors should be considered while planning. Placing implants in a triangular configuration, avoiding or reducing cantilevers, reducing the mesiodistal and buccolingual width of the final restoration and centering the occlusal contacts should be the objectives involved in the treatment.
2) Connecting implants to natural teeth and use as pier abutments
2) Connecting implants to natural teeth and use as pier abutments
The issue of connecting implants and teeth to support a fixed prosthesis is controversial and unresolved. The inherent differences in vertical and lateral movements in response to the occlusal forces and the difference in proprioception, the use of rigid connections is questionable. When an implant is a used as a pier abutment to support a prosthesis with two terminal natural teeth, under occlusal forces stands out due to the difference in mean axial movement of teeth and implants and therefore the chances of prosthesis over the natural teeth getting uncemented is much higher and breakdown of supporting tissue of the implant as it subjected to excessive forces is imminent. The use of non rigid connector is a way of reducing the stress concentration. However, it is best that the connection of natural dentition and implants to support a fixed prosthesis is avoided.
3) Absence of Passive Fit of the Prosthesis
A passive fit of the prosthesis reduces long term stresses in the superstructure, implant components and the bone adjacent to the implants. The absence of passive fit may manifested clinically as:
3) Absence of Passive Fit of the Prosthesis
A passive fit of the prosthesis reduces long term stresses in the superstructure, implant components and the bone adjacent to the implants. The absence of passive fit may manifested clinically as:
- Pain and discomfort to the patient.
- Loosening or fracture of the implant components.
In literature a passive fit should exist at the 10 micron level and is imperative to obtain optimal load distribution. The reasons for non passive framework include inaccurate impressions, improper laboratory techniques in terms of spacer application techniques, matching casting shrinkage of metal not matched with the mould expansion. The use of soldered connectors in long span fixed restorations is an excellent way to compensate the inaccuracies inherent to the laboratory procedures.
4) Improper fit at the abutment/— implant interface
It is very vital that the fit of the abutment is crosschecked radiographically prior to final delivery of the prosthesis. The microgap at the interface has both biologic and mechanical consequences. Not only does the crevice provide a niche for the microbial population but also leads to increased mechanical strain leading to fracture of the screw. At the impression stage also the radiographic verification of the impression transfer coping and then attachment of the implant analog without incorporating a rotational error is vital to prevent a rotational misfit of the abutment which also directly correlated to screw joint failure.
5) Improper Prosthetic Designing
is also a cause of failure and it reiterates the importance of planning the case with the end in mind. The choice between fixed or removable prosthesis, the use of cantilevers, the type of connectors, support and load distribution and loading protocols need to be planned for the particular patient after assessing the specific condition.
6) Improper Occlusal Scheme
This is an important cause of failure as the implant are more sensitive to occlusal trauma and leads to prosthetic failure and bone loss. The susceptibility of implant lies in the difference in stress distribution and propriception between implant and teeth. The choice of occlusal material (Acrylic,metal or ceramic), biomechanics of the prosthesis, vertical dimension, centric occlusion, lateral excursive contacts, absence of interferences are very important factors which need to be considered during planning and treatment execution.
4) Improper fit at the abutment/— implant interface
It is very vital that the fit of the abutment is crosschecked radiographically prior to final delivery of the prosthesis. The microgap at the interface has both biologic and mechanical consequences. Not only does the crevice provide a niche for the microbial population but also leads to increased mechanical strain leading to fracture of the screw. At the impression stage also the radiographic verification of the impression transfer coping and then attachment of the implant analog without incorporating a rotational error is vital to prevent a rotational misfit of the abutment which also directly correlated to screw joint failure.
5) Improper Prosthetic Designing
is also a cause of failure and it reiterates the importance of planning the case with the end in mind. The choice between fixed or removable prosthesis, the use of cantilevers, the type of connectors, support and load distribution and loading protocols need to be planned for the particular patient after assessing the specific condition.
6) Improper Occlusal Scheme
This is an important cause of failure as the implant are more sensitive to occlusal trauma and leads to prosthetic failure and bone loss. The susceptibility of implant lies in the difference in stress distribution and propriception between implant and teeth. The choice of occlusal material (Acrylic,metal or ceramic), biomechanics of the prosthesis, vertical dimension, centric occlusion, lateral excursive contacts, absence of interferences are very important factors which need to be considered during planning and treatment execution.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.