IN VIVO MEASUREMENTS OF LOADS DURING ERGOMETER CYCLING 6 MONTHS POST-OPERATIVELY

Implant and Joint

Short Talk ST-5

S323

IN VIVO MEASUREMENTS OF LOADS DURING ERGOMETER CYCLING 6 MONTHS POST-OPERATIVELY Ines Kutzner (1), Bernd Heinlein (1, 2), Friedmar Graichen (1), Antonius Rohlmann (1), Alwina Bender (1, 3), Georg Bergmann (1)

1. Julius Wolff Institut, Charité - Universitätsmedizin Berlin, Germany; 2. ZHAW Winterthur, Biomechanical Engineering, Switzerland; 3. Berlin-Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Germany

Introduction

Methods The prosthesis was implanted using conventional surgical technique in a 63 year old male patient (weight: 100 kg) suffering from gonarthrosis of the left knee. Six months post-operatively the loading in the patient’s knee joint was measured during cycling on an ergometer. The patient first cycled at a cadence of 40 rpm while the cycling power was increased stepwise from 25 Watt to 95 Watt. Measurements were taken for about 30 seconds on each power level. Subsequently the measurements were repeated at a cadence of 60 rpm and power levels of 50, 70 and 95 Watt.

Results The loading in the knee joint during cycling depends on the power output and the cadence. The magnitudes of all six load components increased with a rising power. At a cadence of 40 rpm the maximum resultant force during one pedal revolution increased from 50 %BW (Body Weight) at 25 Watt to 150 %BW at 95 Watt (Fig. 1). An increased cadence results in a decrease of the loading. At the same power level the maximum resultant force during cycling at a cadence of 60 rpm was 22-26% lower than during cycling at 40 rpm (Fig. 1). Peak resultant forces during cycling were found at approximately 70° to 80° knee flexion during the pedal downstroke.

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Resultant Force during Cycling 160 40 rpm

Force [% BW]

After total knee replacement many patients aim to resume some sport activities. Cycling is often recommended as an activity with low joint loads [Kuster, 2000]. However these recommendations are mainly based on theoretic estimations and mathematical models [Ericson, 1986]. In order to measure the joint loads in vivo, a telemetrized knee implant was developed which enables sixcomponent load measurements in a primary total knee replacement [Heinlein, 2007]. Two metallic plates with hollow stems are separated by a small gap allowing load-dependent deformation of the inner stem. The deformation is measured by six semiconductor strain gages.

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Power [Watt]

Figure 1: Resultant force during cycling at different power levels and speeds

Discussion The data was obtained from one patient. Yet it shows that cycling is a low demanding activity for the knee joint compared to other activities. During level walking a maximum resultant force of about 260 %BW was measured in the same patient 6 months post-operatively. The data also shows that the loading is highly dependent on the chosen cadence and power. Regarding mechanical aspects, cycling at a low power level and a high cadence is best suited as a low demanding activity for rehabilitation programs after total knee replacement. The measurements have been taken 6 months postoperatively. At this time the patient might take over less than 50 % of the cycling power with his operated leg. Further measurements will give evidence how the post-operative course effects the loading during cycling. Measurements in more patients will reveal how big the individual differences are.

References Heinlein et al, J Biomech, 40:4-10, 2007. Kuster et al, Orthopäde, 29:739-745, 2000. Ericson et al, Am J Sports Med., 14:285, 1986.

Acknowledgement This project was supported by Zimmer GmbH, Winterthur, Switzerland.

Journal of Biomechanics 41(S1)