Bench testing of a knee joint arthrometer.

The KT1000 and KT2000 knee joint arthrometers (MEDmetric Corp, San Diego, California) have been shown to overestimate the measurement of knee joint sagittal laxity. The purpose of this study was to investigate the accuracy of the KT arthrometers as measures of anterior and posterior linear displacement. The anterior and posterior linear displacements of 3 KT arthrometers (2 KT1000 arthrometers and 1 KT2000 arthrometer) were compared with the simultaneous displacement measured by a precision linear Vernier Dial Test Indicator (Davenport Ltd, London, United Kingdom). The displacement calculated using the analog output of the KT2000 was also compared with the values on the KT2000 displacement dial. Compared with the Vernier Dial Test Indicator, the KT arthrometers overestimated anterior linear displacement by between 22% and 24%. True anterior displacement for all 3 arthrometers, as recorded by the Vernier Dial Test Indicator, was found by multiplying the KT value by 0.79. When compared with the Vernier Dial Test Indicator, the KT arthrometers underestimated posterior linear displacement by between 18% and 19%. True posterior displacement, as recorded by the Vernier Dial Test Indicator, was found by multiplying the KT1000 value by 1.17 and the KT2000 value by 1.16. The internal apparatus of the KT2000 and KT1000 knee joint arthrometers overestimates anterior displacement and underestimates posterior displacement with a predictable relative systematic error. Future validation studies should use these correction equations to assess the accuracy of the KT arthrometers. Sagittal plane knee laxity measured with the KT devices requires systematic correction for optimal accuracy.