Abstract 22

Measuring Patellar Kinematics 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 seek an answer to the following question: Can the in-vivo kinematics of the patellar be measured following total knee replacement (TKR) surgery using single-plane Radiostereometric analysis (RSA) fluoroscopy? Many complications following TKR are associated with abnormal patellar tracking and measuring its motion would enable the collection of evidence-based data to allow the quantitative comparison between different patellar treatments.

A human subject was implanted with a Profix TKR (Smith & Nephew, Warsaw, IN) and tantalum RSA markers were implanted in the anterior patellar, in the femur slightly proximal to the femoral component, in the tibia slightly distal to the tibial component and in the tibial polyethylene. A standard RSA examination, conventionally used for migration and wear measurements, was performed with the UmRSA software (RSA Biomedical AG, Umeå, Sweden) to initially determine the relative positions of the implanted RSA markers. The human subject was then asked to ascend a single-step during low-dose fluoroscopy screening under full weightbearing conditions. Fluoroscopy images were acquired at 7.5 frames per second and allowed a fully dynamic reconstruction of patellofemoral motion during the step ascent activity.

The patellar was found to translate 10 mm laterally relative to the femur between 0-30 degrees knee flexion, prior to translating 15 mm medially between 30-80 degrees of knee flexion. During 0-80 degrees of knee flexion, the tibia was found to rotate internally 16 degrees. We postulate that the patellar translated medially after engagement in the femoral groove at 30 degrees of knee flexion due to the internal rotation of the tibia exerting forces on the patellar ligament that connects the anterior tibial tubercle with the patellar apex (the distal region of patellar).

The patellar was also found to rotate 3 degrees laterally between 0-30 degrees knee flexion before rotating 5 degrees medially between 30-80 degrees knee flexion. This type of movement is in contrast to the normal patellar, of which in-vitro studies predict1 that little or no patellar rotation occurs during knee flexion. This work demonstrates that it is possible to measure in-vivo patellar kinematics using single-plane RSA fluoroscopy. The in-vivo measurements performed in this work are in accordance with the results of previous in-vitro measurements that have been reported in the literature¹. This work is part of a greater study that is analysing the patellofemoral kinematics of groups of patients following implantation with different types of TKR.

^1. Chew J.T.H, Stewart N.J., Hanssen A.D., Luo Z.-P., Rand A., An K.-N.,

Differences in patellar tracking and knee kinematics among three different designs.
Clin. Orthop. 2003;345:87-98

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