FIRMS SEEK IMPROVED DENTAL IMPLANTS WITH ALGOR FEA

Full jaw FEA model shows the Interpore implant connected to an adjacent natural tooth.

Like many other industries, dentistry has seen vast technological changes over the past few years. One area that has received a large amount of interest is dental implants. Today, many patients are getting dental implants instead of removable bridgework or "false teeth".

Current Technology

There are many kinds of dental implants available. Most often, an implant consists of a cylindrical or threaded implant body which is implanted in the patient's jaw. An artificial tooth, or prosthesis, made of any one of a large variety of materials, is then attached to the implant, usually with a dental adhesive. The implant and prothesis may in turn be connected to other protheses which also can be attached to one of the patient's natural teeth.

New implant designs are constantly under development. One such new idea is being evaluated by two engineering professionals from Southern California. Mr. James Swaniger, Director of Engineering for Interpore International, a leading manufacturer of dental implants and Mr. David Dearth, President of the engineering consulting firm Applied Analysis & Technology, recently completed an exhaustive analysis of a planned improvement in implant technology using Algor finite element analysis software.

James Swaniger of Interpore International (left) and David Dearth of Applied Analysis & Technology show off their work on the new dental implant design.

A New Design

The new design is aimed at providing a uniform distribution of occlusal (biting) forces and reducing stress concentrations in the implant assembly. Dental implants that do not spread biting forces evenly may cause problems. For example if the implant concentrates the stress in its own immediate area, the patient's real teeth may not receive enough use which may compromise the bone strength around the natural teeth.

New Implant Design

Most implants are constructed from titanium, a durable metal which, while providing reliable service, does not absorb stress well. One element of the new Interpore design is flexible and is constructed as a composite assembly of an external polyoxymethylene (POM) surface molded around a titanium insert. The engineers were interested in analyzing the stress distribution of the new design to see if it was better than rigid titanium implants.

To properly represent the new design in actual use, it was necessary for Mr. Dearth to model the implant, the bridge prothesis and the mandible (jawbone). Using detailed implant geometry from Interpore, a model of the implant device was built using 2-D elasticity elements.

Jawbone Model Challenge

Modeling the jawbone presented a somewhat greater challenge. To speed the process, Mr. Dearth found a suitable mandible drawing in a dental journal. He scanned the drawing into his system with a hand scanner and transferred it to Algor's Superdraw II using a DXF file. Next, he refined the basic geometry, added the FEA mesh and combined it with the model of the implant. "The idea of scanning in the basic mandible geometry saved me quite a bit of modeling time," says Mr. Dearth.

Now Mr. Dearth was ready to begin the analysis. "Comparative analysis between the flexible and rigid designs consisted of applying static loads to the final bridge model which simulates implants connected to natural teeth," he said. "Several case studies of distributed occlusal forces were processed. The specific areas of interest in these studies were changes in the distribution of stress in the implant assembly and the result of employing the flexible element."

According to Mr. Swaniger, "An exact design load requirement for occlusal, or biting, forces is not defined. In the literature, dental implants are analyzed for various ranges of applied loads. For the purpose of parametric studies, the precise value of the applied loading is not really needed." He continues, "The results of the analysis of various models are compared to each other on a qualitative basis."

Reporting the Results

The next challenge faced by Mr. Dearth was how to best report the results of the analyses. Again, he came up with a novel approach. In his words, "For this comparative analysis, results of von Mises stress analyses were plotted along the cortical and trabecular bone lines." Cortical bone is an outer layer of hard bone material surrounding an inner region of softer bone called Trabecular bone.

"To obtain these stress distribution plots," he continues, "data files defining 'node maps' along the bone lines were created using Superview's Hide Elements and Node Numbering options. After each case run was completed, the bone line elements were displayed and smoothed von Mises stress solutions were sent to output files using Superview's Aux:Stress:Output option. The stress solutions were now stored in an ASCII file. Next, I wrote a FORTRAN utility program to sort the output using the data files as guides. The sorted stress solutions were then imported into the EXCEL spreadsheet program and graphs were plotted automatically."

Stress Reductions

The analysis results show a reduction in peak stresses of 31% in the cortical bone and 13% in the trabecular bone with the flexible element. Average stress levels were reduced 25% in the cortical bone and 8% in the trabecular bone.

Other Specialties

In addition to their work in dental implants, Applied Analysis and Technology uses Algor software to serve clients in aerospace, computer peripherals, nuclear power and many other industries. In the words of Mr. Dearth, "We have found the Algor FEA software to be both accurate and the easiest to learn. In the particular application outlined here, Algor's display features and flexible output options allowed us to tailor the results into simple graphs. Algor's many features, especially model building, allow our engineers to analyze their designs themselves without having to wait for the 'stress group' to find time on the mainframe."