THREE-DIMENSIONAL VISUALISATION OF BONE REMODELLING USING PATIENT SPECIFIC FINITE ELEMENT MODELS

Abstract

Periprosthetic bone density (BD) changes can be tracked using computed-tomography (CT) assisted osteodensitometry. Patient-specific computer-generated models allow for good visualisation of density changes in bone. We describe techniques for generating smooth and realistic finite element (FE) models that contain both BD and geometry from quantitative CT data using cubic Hermite elements.

FE models were created for three patients who had a total hip replacement. CT-scans were performed at 10 days, one year, and 3 years after the operation and calibrated using a synthetic hydroxyapatite phantom. FE models of the proximal femur were automatically generated from the CT data. Each model had on average 300 tri-cubic Hermite elements. Models were least squares fitted to the entire dataset. BD data was also sampled and fitted using the same cubic interpolation functions. Density was displayed using a colour spectrum.

Realistic patient-specific FE models were obtained. Density and changes in BD were easy to identify. The error in the geometric fitting (RMS distance between data points and the model surface) was generally less then 0.5 mm. The average error for the density fitting (RMS difference between each density data point and the interpolation function value at the same point) was 61.64 mg/ml or 3.08%.

CT osteodensitometry’s potential use as a clinical tool for monitoring changes to BD can be significantly enhanced when used in conjunction with realistic patient-specific finite element (FE) models. Realistic models can be generated with an economic use of scan data, thus keeping radiation dosage down.