Stress patterns on implants in prostheses supported by four or six implants: a three-dimensional finite element analysis.

PURPOSE: Using the three-dimensional finite element method (FEM), this study compared the biomechanical behavior of the "All-on-Four" system with that of a six-implant-supported maxillary prosthesis with tilted distal implants. The von Mises stresses induced on the implants under different loading simulations were localized and quantified. MATERIALS AND METHODS: Three-dimensional models representing maxillae restored with an "All-on-Four" and with a six-implant-supported prosthesis were developed in three-dimensional design software and then transferred into FEM software. The models were subjected to four different loading simulations (full mouth biting, canine disclusion, load on a cantilever, load in the absence of a cantilever). The maximum von Mises stresses were localized and quantified for comparison. RESULTS: In both models, in all loading simulations, the peak stress points were always located on the neck of the distal tilted implant. The von Mises stress values were higher in the "All-on-Four" model (7% to 29%, higher, depending on the simulation). In the presence of a cantilever, the maximum von Mises stress values increased by about 100% in both models. CONCLUSIONS: The stress locations and distribution patterns were similar in the two models. The addition of implants resulted in a reduction of the maximum von Mises stress values. The cantilever greatly increased the stress.

Estudo pelo método dos elementos finitos 3D das tensões induzidas em implantes instalados no sistema All-on-4 / Evaluation by the 3D finite elements method of the stress distribution on implants in the All-on-4 system

A reabilitação da maxila edêntula com uso de implantes normalmente é limitada pela baixa quantidade óssea e pela presença dos seios maxilares. Na tentativa da resolução dessas condições desfavoráveis, abordagens têm sido propostas, como enxertos ósseos, fixações zigomáticas, implantes curtos e implantes inclinados. Uma alternativa conservadora é o All-on-4, que consiste em uma prótese fixa suportada por quatro implantes, sendo os dois posteriores instalados com inclinação de até 45º junto à parede anterior do seio maxilar, e os dois anteriores instalados verticalmente. Entretanto, estudos biomecânicos de apenas quatro implantes suportando uma reconstrução total fixa de maxila são raros. Por meio do método dos elementos finitos 3D, analisou-se a distribuição de tensões geradas por simulação de carga funcional nos implantes. Um modelo 3D representando uma reabilitação com All-on-4 foi construído utilizando-se o software SolidWorks. Quatro carregamentos foram aplicados: (1) carga vertical (200 N) bilateral no cantiléver (primeiros molares), segundos (150N) e primeiros pré-molares (150 N) e caninos (100 N); (2) carga (50 N) horizontal unilateral no canino, (3) carga vertical (200 N) unilateral no cantiléver; (4) carga vertical (200 N) unilateral no segundo pré-molar, simulando ausência do cantiléver. Os resultados mostraram os pontos de maior tensão sempre se localizando na plataforma do implante distal inclinado do lado de aplicação da carga, com o valor máximo de 139,4 MPa. A presença do cantiléver aumentou a tensão no implante distal em 92%. Concluiu-se que os valores máximos de tensões encontrados estão abaixo do limiar de fratura do titânio e que cantiléver deve sempre ser minimizado ou eliminado.

The rehabilitation of the edentulous maxilla using dental implants is usually limited by poor bone density and by the presence of the maxillary sinuses. Attempts to resolve these unfavorable conditions include bone grafts, zygomatic fixtures, short implants and tilted implants. A conservative alternative is the All-on-4, which consists of a fixed full prosthesis supported by four implants, with the two posterior implants tilted until 45o. However, biomechanical studies of only 4 implants supporting a full maxillary fixed reconstruction are rare. Using the 3D finite element method, the stress distribution in implants generated by the simulation of functional masticatory loads was analyzed. A 3D model of an "All-on-4" rehabilitation was built assisted by the software SolidWorks. Four different loads were applied: (1) vertical simultaneous load (200 N) on cantilever (1st molar), 2nd (150 N) and 1st molars (150 N) premolars and canines (100 N), (2) horizontal load (50 N) on unilateral canine, (3) vertical load (200 N) on unilateral cantilever; (4) vertical load (200 N) unilaterally in the 2nd premolar, simulating the absence of the cantilever. The results showed the points of greatest tension located on the platform of the distal implant, and the maximum value observed was 139.4 MPa. Cantilever maximized the tension in 92% in the distal implant. It was concluded that the maximum tension value found is lower than the tensile strength of the titanium and that cantilever must be minimized or eliminated.