Influence of Orbital Implant Length and Diameter on Stress Distribution: A Finite Element Analysis.

PURPOSE: A mathematical simulation of stress distribution around orbital implants was used to determine which length and diameter of implants would be best to dissipate stress.

METHODS: An integrated system for computed tomography data was utilized to create a 3-dimensional model of craniofacial structures. The model simulated implants placed in the 7, 11, and 12 o'clock positions of the orbital rim. A load of 2 N was applied to the model along the long axis of the implant (model 1) and an angle of 45° with the long axis of the implant (model 2). A model simulating an implant with a diameter of 3.75 mm and lengths of 3, 4, 6, 8, and 10 mm was developed to investigate the influence of the length factor. The influence of different diameters was modeled using implants with a length of 6 mm and diameters of 3.0, 3.75, 4.2, 5.0, and 6.0 mm. Values of von Mises equivalent stress at the implant-bone interface were computed using the finite element analysis for all variations.

RESULTS: The elements exposed to the maximum stress were located around the root of the orbital implant in model 1 or between the neck and the first thread of the orbital implant in model 2. An increase in the orbital implant diameter led to a decrease in the maximum von Mises equivalent stress values. In model 1, the reductions were 45.2% (diameter of 3.0-3.75 mm), 25.3% (diameter of 3.75-4.2 mm), 17.2% (diameter of 4.2-5.0 mm), and 5.4% (diameter of 5.0-6.0 mm). In model 2, the reductions of the maximum stress values were 51.9%, 35.4%, 19.7%, and 8.1% respectively. However, the influence of orbital implant length was not as pronounced as that of diameter. In model 1, the reductions were 28.8% (length of 3-4 mm), 19.2% (length of 4-6 mm), 9.6% (length of 6-8 mm), and 4.3% (length of 8-10 mm). In model 2, the reductions of the maximum stress values were 35.5%, 21.1%, 10.9%, and 5.4% respectively.

CONCLUSIONS: An increase in the implant diameter decreased the maximum von Mises equivalent stress around the orbital implant more than an increase in the implant length. From a biomechanical perspective, the optimum choice was an orbital implant with no less than 4.2 mm diameter allowed by the anatomy.