LOW DOSE 3D CT BASED ASSESMENT OF COMPUTER ASSISTED AND CONVENTIONALLY PERFORMED HIP RESURFACING

Abstract

Whilst computer assistance enables more accurate arthroplasty to be performed, demonstrating this is difficult. The superior results of CAOS systems have not been widely appreciated because accurate determination of the position of the implants is impossible with conventional radiographs for they give very little information outside their plane of view.

We report on the use of low dose (approximately a quarter of a conventional pelvic scan), low cost CT to robustly measure and demonstrate the efficacy of computer assisted hip resurfacing. In this study we demonstrate 3 methods of using 3D CT to measure the difference between the planned and achieved positions in both conventional and navigated hip resurfacing.

The initial part of this study was performed by imaging a standard radiological, tissue equivalent phantom pelvis. The 3D surface models extracted from the CT scan were co-registered with a further scan of the same phantom. Subsequently both the femoral and acetabular components were scanned encased in a large block of ice to simulate the equivalent Hounsfield value of human tissue. The CT images of the metal components were then co-registered with their digital images provided by the implant manufactures. The accuracy of the co-registration algorithm developed here was shown to be within 0.5mm.

This technique was subsequently used to evaluate the accuracy of component placement in our patients who were all pre-operatively CT scanned. Their surgery was digitally planned by first defining the anterior pelvic plane (APP), which is then used as the frame of reference to accurately position and size the wire frame models of the implant. This plan greatly aids the surgeon in both groups and in the computer assisted arm the Acrobot Wayfinder uses this pre-operative plan to guide the surgeon.

Following surgery all patients, in both groups were further CT scanned to evaluate the achieved accuracy. This post-operative CT scan is co-registered to the pre-operative CT based plan. The difference between the planned and achieved implant positions is accurately computed in all three planes, giving 3 angular and 3 translational numerical values for each component.

Further analysis of the CT generated results is used to measure the implant intersection volume between the pre-operatively planned and achieved positions. This gives a single numerical value of placement error for each component. These 3D CT datasets have also been used to quantify the volume of bone resected in both groups of patients comparing the simulated resection of the planned position of the implant to that measured on the post-operative CT.

This study uses 3D CT as a surrogate outcome measure to demonstrate the efficacy of CAOS systems.