Our data indicate that the MTS is reliable and reproducible for the majority of the tested measurement parameters (Tables
). Since the markers on the casting dummy remained in a constant position, variation in the measurements could be traced to differences in dummy position and investigator scanning technique. While several methods of raster sterography have been published, data from this study are most comparable to data reported for Quantec systems. Investigating QSIS measurements in 200 patients with idiopathic scoliosis, Wojcik et al
. reported a SE for thoracic Q-angle of 4.8° while the thoracic Q-angle SE in our study was 0.93° [21
]. Most notably, thoracic Q-angle SE reported in this study is less than SE in Cobb angle reported in the literature (SE=1.7°) [22
]. Mean Q angle difference between the investigators (1.5°) and between trials (1.4°) regardless of position demonstrated that the measurement of curvature by the MTS can be reproduced by difference clinicians and is not limited by patient positioning. Additionally, no other parameters tested with the MTS demonstrated significant variability due to either trial or observer, with the exception of the Pelvic Tilt metric among trials. Variation in the Pelvic Tilt metric may be explained by variation in identification of the PSIS and is a limitation of the technique common to all rasterstereogrpahic technologies. However, identification of the PSIS is used only in this metric and has impact on spinal curve measurements.
Our data support previous research by Stokes and Moreland and by Liu et al
. indicating that trunk position has an effect on surface topography measurements [10
]. In contrast, Sakka et al
. found no variability in QSIS measurements due to trunk position [24
], although different metrics were tested in each of these studies. In the current study significant variability due to position was observed in the T1-S1 angle, T1-S1 deviation, T1-NC angle, and T1-NC deviation metrics (p<0.01, p=0.03, p<0.01, p<0.01, p<0.001
, respectively). This data supports that of Liu et al
., who also reported significant variability in these metrics due to forward rotation [10
]. However, this variability was only related to the 20°Foward position. This effect may be a result of the sensors which are placed on the subject’s back which decrease measurement noise and could result in dampening of variability caused by small changes in position.
Our data do not support previous research which indicates significant variability in the Kyphosis metric due to forward bending, indicating that the MTS is more reproducible with respect to this parameter than previous QSIS systems [10
]. Significant variability of VOI reported in the SH metric may be due to inconsistent manual movement of the dummy cast during testing and may also indicate the sensitivity of this metric to changes due to postural sway in the coronal plane.
Variability in the Back Height metric due to trunk position (p<0.001) has not been studied previously but variability in this metric with changes in position of the trunk is expected. Because back height is measured vertically and tilting of the dummy cast forward in the 10°F, 10°T, 20°F, and SH conditions results in variation in the back height measurement (i.e., the back height should decrease as the dummy cast is tilted forward). The observed variability in our study is expected but may indicate a limitation of this metric’s utility in patients with abnormal posture.
The use of a dummy cast is a limitation of this study because the cast may not accurately reflect surface marker movement which would be present in a human subject. Surface marker motion is common to all rasterstereography techniques but good correlation between rasterstereographic and radiographic metrics has been reported [15
]. The present study presents a confirmation of the technology and our lab will evaluate the system in a human subject in the near future.
Intra-Observer and Inter-observer ICCs indicate that the MTS is highly reliable for all tested metrics excluding Pelvic Tilt. Lyon et al
. assessed QSIS reliability using a random effects variance components model in 200 children with a diagnosis of adolescent idiopathic scoliosis and reported the highest reliability when three scans and three measurements were performed [25
]. Although our data is not directly comparable to this previous research because only one “subject” was included in our study, we report excellent reliability of the MTS regardless of different investigators, trials, and positions.