Bar-supported and unsupported dental implants: A QCT-FEA study in human maxilla

Bar-supported and unsupported dental implants: A QCT-FEA study in human maxilla
Luboš Řehounek, Aleš Jíra
Faculty of Civil Engineering, Department of Mechanics, Czech Technical University in Prague
Introduction
A very popular means of treating edentulism (loss of all teeth) is to prepare the patient's jaw for placement of 4 load-bearing dental implants that carry an overdenture. These implants can either be unsupported-placed individually with no structural connections, or supported-usually with a metal wire or a solid bar that acts as a stirrup that connects all 4 implants together for better distribution of masticatory forces. The former option is the standard in dental practice as it requires less customization and is less time intensive. However, the bar-supported variant finds its use if the patient's bone quality is poor, implants are located in the maxilla or the implants are divergent and additional support is needed to maintain their proper position. Moreover, bar-retained implant-supported overdentures (IODs) have a survival rate of ca. 97 % in a period greater than 7 years post-operation.
Material and Method
This work analyzes differences in stress distributions of both variants (unsupported and bar-supported). The numerical model consists of the patient-specific 3D bone model of human maxilla and of an imported .STL implant part. The implant assembly was modelled in 2 variants-bar-supported and without bars. The program used to create the models was ANSYS SpaceClaim. Position of the implants, their shape and location was modelled according to data provided by an anonymous patient. The analyzed implant variants can be seen on Fig. 1.
Fig. 1. Two analyzed variants of the implant assembly. Left-the bar-supported variant, right-only intraosseous implants.
Results
The main analyzed quantity was stress risk factor in bone. As we can see from overviews on Fig. 2, the inclusion of the bar changes the distribution of stress (but mainly its values). The bar-supported variant shows smaller displacements of the implant that is loaded thanks to the additional stiffness provided by the connections to other implants.
Fig. 2. Risk factor (RF) of both variants of the assembly at the final computation step (F=800 N). The first image shows the bar-supported variant, where stresses are better distributed and there are no cracked, crushed or plastic solid elements. The unsupported variants shows much larger pools of concentrations of stress and also some plastic elements and cracked elements. The scale for RF was chosen in the range of 0-30 % to better illustrate the stress distribution in the bone as the scale of 0-100 % would leave the reader oblivious to the small areas. Deformations magnified 50 times.
Discussion
The QCT-FEA analyses of the interaction of bone and two variants of the implant assembly (bar-supported and unsupported) showed a predictable outcome favoring the bar-supported variant. This is illustrated on Fig. 2. However, this does not mean that bar-supported variants are always better in clinical practice as the decision-making process involves many other factors (patient specific conditions like bone quality, necessity of installation of bars, time demands or financial situation of the patient). Also, this hypothetical unsupported assembly will still probably have some additional stiffness provided by the overdenture itself, so there is room for additional analyses comparing what contribution will this stiffness provide. The simulations of presented study took less than 1 minute to complete each-a time short enough to perform on-site. However, the time needed to prepare the mesh can be a concern for medical practitioners as it usually exceeds computation times by an order of magnitude.
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