Abstract 34

HOW MUCH COMPUTED TOMOGRAPHY INFORMATION IS REQUIRED IN ORDER TO AUTO-GENERATE PATIENT-SPECIFIC FINITE ELEMENT MODELS OF THE PELVIS?

¹ Shim, V B; ² Pitto, R P; ³ Streicher, R; ¹ Anderson, I A;

¹The Bioengineering Institute, University of Auckland, New Zealand
² Department of Orthopaedic Surgery, University of Auckland, South Auckland Clinical School, Middlemore Hospital, Auckland, New Zealand
³ Stryker SA, Thalwil, Switzerland

We are developing techniques for auto-generating patient-specific models of the pelvis. Pelvis models will be used for finite element mechanical simulation as well as computer-assistedsurgery for hip replacement navigation. Our goal is to minimize radiation dosage to the patient using a limited number of computed tomography (CT) slices. Patient CT slices are mainly located in the area of interest, the acetabulum. Extra slices are obtained at specific locations above and below the acetabulum. Regions not covered by patient CT slices are described using Visible Human (NLM, Bethesda, Maryland) CT slices, transformed to match the size and orientation of the patient CT sets. Bony landmark points used in the transformation are selected automatically from the boundaries of the CT images. The finite element models utilize high order (cubic Hermite) basis functions to generate accurate and smooth meshes.

We have applied the algorithm to three patient CT sets (2 males and 1 female) which covered the whole pelvis with 8mm gaps between slices, a total of 27 slices per set. Patient specific meshes constructed using as few as 8 patient slices were constructed. Accuracy of the mesh was tested by calculating root mean squared (RMS) distance between the data points collected from the 27 CT slices ( ~ 10,000 points) and the fitted surfaces. We have generated smooth FE meshes of the pelvis using only 11 CT slices, achieving a RMS error of less than 3mm. This method will provide a very efficient way for generating models without sacrificing accuracy.

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