Determination of the Hounsfield value for CT-based design of custom femoral stems

Abstract

CT and advanced computer-aided design techniques offer the means for designing customised femoral stems. Our aim was to determine the Hounsfield (HU) value of the bone at the corticocancellous interface, as part of the criteria for the design algorithm. We obtained transverse CT images from eight human cadaver femora. The proximal femoral canal was rasped until contact with dense cortical bone was achieved. The femora were cut into several sections corresponding to the slice positions of the CT images. After obtaining a computerised image of the anatomical sections using a scanner, the inner cortical contour was outlined and transferred to the corresponding CT image. The pixels beneath this contour represent the CT density of the bone remaining after surgical rasping. Contours were generated automatically at nine HU levels from 300 to 1100 and the mean distance between the transferred contour and each of the HU-generated contours was computed. The contour generated along the 600-HU pixels was closest to the inner cortical contour of the rasped femur and therefore 600 HU seem to be the CT density of the corticocancellous interface in the proximal part of cadaver femora. Generally, femoral bone with a CT density beyond 600 HU is not removable by conventional reamers. Thus, we recommend the 600 HU threshold as one of several criteria for the design of custom femoral implants from CT data. Advances in radiology and in computerised design have allowed human anatomy to be reconstructed in three-dimensional (3D) geometrical and solid models. This has prompted orthopaedic surgeons and biomechanical engineers to develop customised joint implants in order to improve the fit between the prosthesis and the surrounding cortical bone. There have been several studies on the design of such customised femoral stems, 1-6 but the imaging techniques and the design criteria of the implants have differed. Although conventional radiographs are used in the design of femoral stems, CT is currently the most accurate method for describing the anatomy of the proximal femur. 7 On the CT image the endosteal surface appears irregular and in the proximal direction increasingly consists of can-cellous bone. The CT images reflect this by depicting a transitional zone with increasing density towards the compact cortical bone. When the surgeon prepares the canal for the femoral stem, he will remove most of the cancellous bone to allow the implant to rest on bone with sufficient mechanical strength. Our aim was to assess in Hounsfield units (HU) the CT density of the inner cortical surface of the proximal femur after this bone had been removed. One HU is defined as a number on a density scale in which the X-ray absorption of water has been assigned the value of zero and the air the value of-1000. Material and Methods We used eight fresh-frozen human cadaver femora from individuals with a mean age of 69 years (43 to 84). The specimens appeared normal on gross inspection and on radiological examination. Two plastic rods were fixed along the femur to ...

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