Biomechanical behaviour of tooth-supported fixed partial dentures by 3D FEA.

This three-dimensional finite element analysis first described stresses distribution in loaded posterior fixed partial dentures, then compared the influences of bone height, abutment roots number and pontic length on their displacements. Twelve mandibular (three to five unit) FDPs integrating periodontal ligaments were designed. Stresses were localized in connectors and cervical areas of abutments near the edentulous span. The main FDPs' displacements consisted of vertical translation increasing with low bone height. FDPs underwent simultaneously an anteroposterior displacement (about ten times smaller) towards the weakest abutment side. Splinted abutments affected this anteroposterior displacement. Span length was associated with small beam deflection.

Influence of implant length and bicortical anchorage on implant stress distribution.

BACKGROUND: Short implants present superior failure rates for everybody. PURPOSE: The aim of this theoretic study was to assess to what extent implant length and bicortical anchorage affect the way stress is transferred to implant components, the implant proper, and the surrounding bone. MATERIALS AND METHODS: Stress analysis was performed using finite element analysis. A three-dimensional linear elastic model was generated. All implants modeled were of the same diameter (3.75 mm) but varied in length, at 6, 7, 8, 9, 10, 11, and 12 mm (Brånemark System, Nobel Biocare AB, Gothenburg, Sweden). Each implant was modeled with a titanium abutment screw and abutment, a gold cylinder and prosthetic screw, and a ceramic crown. The implants were seated in a supporting bone structure consisting of cortical and cancellous bone. An occlusal load of 100 N was applied at a 30 degrees angle to the buccolingual plane. RESULTS: With the selected model and bone properties, the coronal cortical anchorage was dominating, and the bone stress concentrated to that area. CONCLUSIONS: The maximum bone stress was virtually constant, independent of implant length and bicortical anchorage. The maximum implant stress, however, increased somewhat with implant length and bicortical anchorage.

Corono-radicular reconstruction of pulpless teeth: a mechanical study using finite element analysis.

STATEMENT OF PROBLEM: Following endodontic therapy, teeth need to be protected, particularly in the cervical region, where the majority of fractures occur. The likelihood of a fracture depends on the condition of the crown and the type of reconstruction performed. PURPOSE: This simulation study was designed to compare the effect of different corono-radicular reconstruction methods on stress transmission to dental tissues. MATERIAL AND METHODS: The study software performed stress analysis of complex structures by finite element analysis. Seven 3-dimensional models were created, each representing a tooth embedded in a bony medium. The following parameters affecting corono-radicular restoration were studied: 2 levels of coronal destruction, core materials, post materials when present, and absence of post. The 2 levels of coronal tissue loss were (1) total tissue loss of the coronal dentin and (2) partial tissue loss of the coronal dentin with 2-mm surviving dentin walls. Teeth with 2 different levels of tissue loss (first study parameter) were reconstructed by 4 different techniques: nickel chromium (NiCr) cast post and core, NiCr post and composite core combination, carbon fiber post and composite core combination, and composite restoration without post. A NiCr crown covered each of the models and received a 30 degrees oblique occlusal load at a constant intensity of 100 N. The software computed the stresses (local tensile stress inducing cracks and compressive stress) for each of the models, comparing maximum intensity observed, localization, and concentration. RESULTS: Whatever the type of stress (tensile or compressive), the greatest stress was observed in the cervical region, regardless of the model. Only tensile stresses potentially responsible for fractures were compared. Cervical tensile stresses exceeded 230 Pa in the absence of a ferrule and were less than 140 Pa when a ferrule was present. In the absence of a ferrule, the NiCr composite/post combination generated greater cervical stress (254 Pa) than the cast post and core (235 Pa). Results with a ferrule showed 92 Pa for the NiCr composite/post combination and 90.5 Pa for the cast post and core. In the presence of a ferrule, the tensile stress intensities generated by the composite restoration with no root canal post (139 Pa) were 51% greater than those generated by the NiCr/composite combination and approximately 26% greater than those generated by the composite/carbon combination. CONCLUSION: Within the limitations of this study, it was confirmed that all simulated reconstructed teeth were more subject to stress in the cervical region. The absence of a cervical ferrule was found to be a determining negative factor, giving rise to considerably higher stress levels. When no ferrule was present, the NiCr post/composite combination generated greater cervical stress than cast post and cores. Nevertheless, the peripheral ferrule seemed to cancel the mechanical effect of the reconstruction material on the intensity of the stresses. With a ferrule, the choice of reconstruction material had no impact on the level of cervical stress. The root canal post, the purpose of which is to protect the cervical region, was also shown to be beneficial even with sufficient residual coronal dentin. In the presence of a root canal post, cervical stress levels were lower than when no root canal post was present. Moreover, the higher the elasticity modulus, the lower the stress levels.

Two dental implants designed for immediate loading: a finite element analysis.

PURPOSE: The aim of this study was to evaluate by finite element analysis the influence of the design of 3 different dental implants on micromovements, cervical shearing stress intensity, and stress distribution after occlusal loading. MATERIALS AND METHODS: The first investigated implant was a classical cylinder, the second was reinforced by 2 bicortical locking pins, and the third was an expanding dental implant. The parameters analyzed were the implant's geometry, the quality of the cancellous bone, and the orientation of occlusal loading. RESULTS: It was found that initial stability of the locking pin implant was greater than the initial stability of the other investigated implant designs, regardless of the quality of cancellous bone and orientation of occlusal loading; in low-rigidity cancellous bone, under a horizontal load (500 N), decreasing displacement compared to those of the other investigated implants was 16 microm. The apical expansion and locking pin implants exhibited favorable behavior regarding the distribution and intensity of cervical shearing stresses; in low-rigidity cancellous bone, under horizontal load, decreasing cervical stresses compared with those of the cylindric implant were 10 MPa for the apical expansion implant and 150 MPa for the locking pin implant. DISCUSSION: For the cylindric implant, stresses were concentrated in the neck region; for the apical expansion implant, stresses were evenly distributed from the neck to the apex of the implant. For the locking pin implant, stresses around the neck were moderate and appeared concentrated around the pins. CONCLUSIONS: Initial stability of the pin implant was greater than that of the expanding implant, but the expanding implant showed the most favorable stress distribution.