Repeatability test of C7 plumb line and gravity line on asymptomatic volunteers using an optical measurement technique.

ABSTRACT

STUDY DESIGN: Prospective cohort evaluation of C7 plumb line (C7PL) and gravity line (GL) in different standing positions in asymptomatic volunteers. OBJECTIVE: To evaluate the repeatability of C7PL and GL in different standing positions using an optical method. SUMMARY OF BACKGROUND DATA Both C7PL and GL have been used to assess spinal balance. However, due to extensive radiation exposure, the measurement repeatability for both C7PL and GL has never been systematically determined. METHODS: Thirty asymptomatic adult volunteers were enrolled in this study. Two optical markers were attached to the skin overlying the spinous processes of C7 and S1. Volunteers were instructed to stand on a force plate with their arms in 3 different positions, clavicle, supported, and neutral, for posteroanterior (PA), and lateral views. Digital photos were taken for 12 times for each position. The GL position was displayed on a computer in real time. The distance from the C7 marker to S1 marker (C7-S1) and the distance from the GL to the S1 marker (GL-S1) were measured in both PA and lateral views. The repeatability on a single subject was defined as the standard deviation of the 12 repeated measurements for each standing position. RESULTS: In the PA view, the repeatability of both C7PL and GL was 3 to 4 mm in all 3 standing positions (all P > 0.05). In the lateral view, the repeatability of C7PL increased from 6 to 8 mm whereas GL remained low at 3 to 4 mm. The GL repeatability was significantly better than that of C7PL in all 3 lateral positions (all P < 0.05). The standing position did not significantly affect repeatability for both C7PL and GL. C7PL had approximately 1-cm posterior shift in the supported position and 2-cm posterior shift in the clavicle position. The effect of various radiographic positions was less than 1 cm for all 3 standing positions in the GL measurement. CONCLUSION: The GL measurement was highly reproducible in both PA and lateral views. The repeatability was similar between C7PL and GL despite the standing positions in the PA view. The repeatability of GL was better than that of C7PL in all 3 standing positions in the lateral view. The effect of standing positions was much less in GL than in C7PL.