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Methods for quantifying hip prosthesis induced changes in the adjacent bone are of great interest to orthopedics. In this work, we present a semi-automated technique for measuring the differences in bone density between the prosthetic and contra lateral hips within a CT volumetric data set. In order to reliably compare the bone-density measurements between the prosthetic and the contra laterals hips, a standardized zoning was developed. Using a spherical model of the outer surface of the prosthetic cup, the superior volume of the acetabular region was subdivided into four distance zones: 0-1mm, 1-6mm, 6-11mm, and 11-16mm, respectively. Furthermore, these regions were divided into four positional zones: medial, lateral, anterior, and posterior. At the same time, the positional zones were divided into four angular regions 15, 30, 45, and 60 degrees from the apex of the acetabular cup. The bone density is computed as the average density in Hounsfield unit (HU) measured from the CT scan using all the voxels within each of the 64 zones. Preliminary analysis has been completed on 3 subjects with total hip replacement. The zonal densities on the prosthetic hip and the contra lateral hip were computed and compared. Contrary to initial expectations, a paired t-test showed no statistical significance between the prosthetic and contra lateral bone-densities at any of the four distance zones. Further analysis with a larger sample subject is needed to detect differences in bone-density between the hips in the stress/weight bearing areas in the 15 to 30 degree regions. A method for reliably and consistently measuring the bone-density within standardized zones has been developed and applied on prosthetic and contra lateral hips. The average bone-density for each of the zones takes into account the entire volumetric data set within that region, which is a considerable improvement over the subjective, user driven region of interest estimate selected within one slice practiced in previous methods. Therefore, our method will offer a more reliable metric in prosthetic changes within the bone. Furthermore, this method may be applicable to analysis of density measurements of different prosthetis and surgical procedures.
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