Abstract 23

Baseline Polyethylene Tibial Component Wear Measurements using Single-Plane Radiostereometric Analysis (RSA) Fluoroscopy

James Ioppolo^1,2, Bo Nivbrant¹, Mingguo Li¹, Roger Price2, David Wood¹

¹ RSA Laboratory, Department of Surgery and Pathology (Orthopaedics), University of Western Australia

² Medical Technology and Physics Department, Sir Charles Gairdner Hospital

The aim of this study is to measure the in-vivo distal penetration of the femoral component into the polyethylene tibial component following total knee replacement (TKR) surgery. Wear debris from tibial polyethylene inserts is thought to be a major factor in aseptic loosening of TKR and fractures and delamination of the tibial polyethylene also represent a major problem. The in-vivo measurement of such wear and polyethylene damage is essential for improving the articulation and longevity in TKR.

Four human subjects were implanted with a Profix (Smith & Nephew Inc., Memphis TN) TKR, with tantalum RSA markers implanted in the femur slightly proximally to the femoral component and in the tibial polyethylene. A standard RSA examination was performed with the UmRSA software (RSA Biomedical AG, Umeå, Sweden) to determine the relative positions of the implanted RSA markers. The human subjects were asked to perform a clinical procedure that consisted of flexing and extending their leg approximately 20 degrees under full weight-bearing conditions during fluoroscopy screening. Fluoroscopy images were acquired at 7.5 frames per second and allowed fully dynamic reconstruction of tibiofemoral motion during the shortened knee flexion activity. Displacement curves are produced by examining the distance between the femoral and tibial components as a function of knee flexion. Current results are limited to initial baseline displacement curves.

The in-vivo measurement of wear has to be measured as a decrease in distance between the femoral and tibial components. In a knee replacement this is not as easy as in a hip with a circular ball joint. This for several reasons: Firstly, it is almost impossible to obtain exactly the same position of the components at two different examinations. Any difference in anteroposterior position and flexion of femur on tibia will affect the measurements. Secondly, the polyethylene will not wear in a circular spot but over the sliding distance of the femoral condyles. Articulating the knee under load whilst performing dynamic measurements and plotting displacement against the flexion angle therefore serves many purposes: To align the components reproducibly, measure wear over the entire sliding distance, compensate for various flexion angles and finally, multiple readings will give a more correct mean value for the curve. This is an ongoing, long-term clinical study. We expect that the femoral penetration, measured using displacement curves at subsequent clinical examinations, will deepen with time to represent wear of the polyethylene tibial component.

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