STATIC PRECISION, REPEATABILITY, AND ACCURACY OF AN OPTOTRAK CERTUS™ OPTICAL RIGID BODY TRACKING SYSTEM: IMPLICATIONS FOR IMPLANT MICROMOTION ASSESSMENTS

Optical tracking is an important biomechanical assessment tool that can quantify the position and orientation of rigid bodies in three-dimensional space that is commonly used to evaluate large- and small-scale displacements. One system commonly used for biomechanical assessments in research labs is the Optotrak Certus™ system (Northern Digital, Waterloo, Canada). This system can monitor marker positions with a reported accuracy of 100μm, however this accuracy is influenced by application-specific factors, and previous investigations have suggested that it may be capable of reliably quantifying much smaller displacements [1]. The purpose of this investigation is to assess the reliability of the Optotrak Certus™ by quantifying its precision, repeatability, and accuracy for use in micromotion investigations. It is hypothesized that the reliability will be better than the 50μm accuracy and 30μm repeatability limits that have previously been proposed for micromotion assessments [1]. Two rigid bodies were mounted to a micrometer test stand used to position the rigid bodies at eight locations in-, then out-of-plane relative to the camera. Both rigid body and camera referencing measurement techniques were assessed. To assess positional precision and repeatability, the test stand was sequentially placed in five pre-defined locations and the static position of the rigid bodies was captured for a duration of 10 seconds, repeated ten times for each location. Positional precision was quantified as the within-trial standard deviation of each of the in-plane and out-of-plane positional coordinates [1][2]. Repeatability was quantified as the between-trial standard deviation of the average position of each of the rigid body's positional coordinates [1][2]. Finally, the test stand was displaced from 0.1–10mm relative to its initial position using the micrometer-controlled test stand, and the static position of the rigid bodies was once again captured. Accuracy was quantified as the difference between the average measured displacement and the expected displacement for each position relative to the initial position. The precision, or within-trial standard deviation of the rigid body's position was poorer when the rigid body referencing method was used, as opposed to direct camera referencing. For these capture techniques, positional precision and repeatability ranged from 0.7μm–14.9μm and 0.8μm–9.2μm, respectively. Repeatability was generally better with the rigid body referencing technique, as opposed to the camera referencing technique ranging between 1.0μm and 3.0μm, as opposed to 0.8μm and 9.2μm, respectively. In- and out-of-plane displacement accuracies ranged from 1.8μm–25.6μm and 2.6μm–29.9μm, respectively, while repeatability ranged from 0.9μm–10.0μm (Figs1,2). The present investigation assessed the Optotrak Certus™ optical tracking system for micro-scale displacements that are relevant to in-vitro assessments of orthopaedic implant subsidence and micromotion. Micro-scale displacements reported through static position captures outperform the 50μm accuracy and 30μm repeatability thresholds that have been suggested for orthopedic micromotion assessments. This was also the first investigation to directly compare how the common practice of referencing the position of one rigid body marker triad relative to another marker impacts positional and displacement reliability, relative to direct camera referencing. For any figures or tables, please contact the authors directly.

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