Evaluation of CT accuracy in orthopaedic implants geometry reconstruction

X-ray CT has the potential to provide precise and accurate data from which the mechanical properties of bone can be derived. Such data would be useful in understanding bone behaviour around metallic orthopaedic implants. Metallic objects, however, generate artifacts, typically seen as starburst streaking, which can seriously degrade the accuracy of the data. The aim of this study was to evaluate new generation CT scanners accuracy in the assessment of geometric shape and features of titanium hip joint prosthetic stems. A measuring procedure has been developed and its accuracy assessed. This procedure compares the metal stem contours obtained from the transaxial CT images to the cross section of the stem solid model provided by the producer. Results show that modern CT scanners images provide precise information about geometric shape and features of titanium orthopaedic implants. CT reconstruction accuracy, in fact, has shown to be comparable to the image pixel size (0.3 mm) which is the theoretical limit of the resolution of the CT scanner. Two hip joint titanium implants with the same geometry and a different calliper have been analyzed. Smaller calliper stem yielded slightly higher errors, suggesting that CT accuracy depends on the implant dimensions and geometry. However, for both stems, the 90% of the measured errors fell within 0.6 mm (2 pixels) which can be regarded as the lower bound for titanium stem reconstruction accuracy.