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The in vitro reliability of the CODA MPX30 as the basis for a method of assessing the in vivo motion of the subtalar joint.

BACKGROUND
The considerable variation in subtalar joint structure and function shown by studies indicates the importance of developing a noninvasive in vivo technique for assessing subtalar joint movement. This article reports the in vitro testing of the CODA MPX30, an active infrared marker motion analysis system. This work represents the first stage in the development of a noninvasive in vivo method for measuring subtalar joint motion during walking.


METHODS
The in vitro repeatability of the CODA MPX30 system's measurements of marker position, simple and intermarker set angles, was tested. Angular orientations of markers representing the position of the talus and the calcaneus were measured using a purpose-designed marker placement model.


RESULTS
Marker location measurements were shown to vary by less than 1.0 mm in all of the planes. The measurement of a 90° angle was also found to be repeatable in all of the planes, although measurements made in the yz plane were shown to be consistently inaccurate (mean, 92.47°). Estimation of segmental orientation was found to be repeatable. Estimations of marker set orientations were shown to increase in variability after a coordinate transform was performed (maximum SD, 1.14°).


CONCLUSIONS
The CODA MPX30 was shown to produce repeatable estimations of marker position. Levels of variation in segmental orientation estimates were shown to increase subsequent to coordinate transforms. The combination of the CODA MPX30 and an appropriate marker placement model offers the basis of an in vivo measurement strategy of subtalar joint movement, an important development in the understanding of the function of the joint during gait.