The ultimate goal is to maintain and improve the quality of life of dental patients by various treatment modalities. The goal can be achieved by preventing diseases, relieving pain, improving masticatory efficiency, enhancing speech and improving appearance. To achieve this many of these objectives require the replacement or alterations of the existing tooth structure. But the main challenge is development and selection of biocompatible materials that can withstand the unique conditions of the oral environment.In recent, dental implant is a treatment of choice for prosthetic rehabilitation patients. At many times placement of conventional implants may not feasible in resorbed posterior mandibular region and an atrophic maxillary arch. Excessively resorbed maxilla and mandibular arches needs to be modified in the form of graft, nerve transposition or sinus lift, etc. Such procedures are surgically invasive and duration of treatment prolong to overcome such situation system like all on four have been introduced for full mouth rehabilitation which is time saving and high rate if patient’s acceptance. This method allows tilting of distal implants in edentulous arches which enables us in placement of longer implants, improved prosthetic support with shorter cantilever, improved inter implant distance and improved anchorage in the bone.
The All-on-Four treatment concept was developed by Paulo Malo with straight and angled multi-unit abutments, to provide edentulous patients with an immediately loaded full arch restoration with only four implants.However, all on four system results in some cantilever of the prostheses leading to non axial distribution of masticatory load, hence it become imperative to study the stress distribution around such implant system in comparison to axially placed implants.Biomechanical factors play an important role in implant success or failure. The application of occlusal forces induces stresses and strains within the implant-prosthesis complex and it affects the bone remodeling process around implants. To attain optimized biomechanical conditions for implant-supported prostheses, conscientious consideration of the biomechanical factors that influence prosthesis success is necessary.As the placement of implants with significant load bearing capacity often may be restricted to the anterior portions of the arch, cantilevers distal to the most posterior implants are often required. The presence of a load bearing cantilever increases the forces distributed to the implants, possibly up to two or three times the applied load on a single implant, due to the bending moments. To reduce the load of the terminal implant, placing short implants distal to the mental foramina and having the cantilever segments rest on the implants without being connected.The prosthetic framework material plays an important role in stress transmission to the implant-support system and the peri-implant bone region. Titanium and a cobalt-chromium alloy are widely used as prosthetic framework materials due to their biocompatibility, low cost, low density and favorable mechanical properties. Zirconia, which improves the esthetic results, has emerged as a prosthetic framework material. However, studies areincongruous regarding the stress transmitted by this material. In addition, little is known about either its strength and stress transmission when subjected to occlusal forces or its biomechanical behavior in situations with a smaller number of implants or in tilted implant situations.
Classical methods of mathematical stress analysis are extremely limited in dental patients due to dental structures that have an irregular structural form and complex loading. The finite element analysis is a modern technique of numerical stress analysis that has the great advantage of being applicable to solids of irregular geometry and heterogeneous material properties. It is therefore ideally suited for the examination of structural behavior of the oral cavity. The development in main frame computers and availability of powerful microcomputers has brought this method within the reach of students and engineers. There are so many different methods available to study stress/strains in bone and dental implants. Photoelasticity is one of the methods which provide good qualitative information pertaining to the overall location of stresses but only limited quantitative information. Strain-gauge measurements also provide accurate data regarding strains only at the specific location of the gauge. But Finite element analysis (FEA) is capable of providing detailed quantitative data at any location within mathematical model. Thus, FEA has become a valuable analytical tool in the assessment of stress/strain in bone and implant systems in dentistry.
FEA can analyze arbitrary shape, loads and supporting conditions, furthermore, the mesh can mix elements of different types, shapes, and physical properties. This great adaptability is contained within a single computer program and the selection of program type, geometry, boundary conditions, element selection are controlled by user-prepared input data. The main difficulty in simulating the mechanical behavior of dental implants lies in the modeling of human maxilla and mandible and its response to applied load. Certain expectations are needed to make the modeling and these involve many factors which will potentially influence the accuracy of the FEA results: (1) detailed geometry of the implant and surrounding bone to be modeled, (2) material properties, (3) convergence test, (4) loading conditions, (5) interface between bone and implant, (6) boundary conditions, (7) validation.The use of tilted implants facilitates the achievement of the desired position of the implants from a prosthetic point of view and creates a favorable interimplant distance. Moreover, using finite element analysis, there is a biomechanical advantage in using splinted tilted distal implants rather than axial implants supporting distal cantilever units when comparing the coronal stress. There are various studies available in the literature which evaluates stress distribution on conventionally placed dental implant or all-on-four dental implant with available method of stress evaluation. But there is no In-Vitro study available in the literature to evaluating stress distribution with FEA by comparing axially placed conventional implants with implants placed with all-on-four technique with different angulations in mandible.
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