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Philipp M Prahs, Nadezhda Cvetkova, Horst Helbig, David Maerker; Postoperative Head Tracking: Repeatability of Measurements Using an Inertial Measurement Unit. Invest. Ophthalmol. Vis. Sci. 2016;57(12):1082. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Controversy exists about the need for patients to maintain a given posture after intraocular surgery with gas tamponade while reliable methods to monitor patient posture are not commonly available. This study aims to evaluate the precision obtained by head tracking using an Inertial Measurement Unit (IMU) compared to conventional angle measurements in the assessment of head orientation.
An IMU (InvenSense MPU6050) was coupled with a microcontroller and a bluetooth low energy transmitter. The device was mounted on a hard eye shield and fixated on the head of a subject. The subject was asked to lie on a motorized operating table while its back was elevated 0, 15, 30 , 45 and 60 degrees from the suppine position. In a first experiment the subject was moved to each position ten times while the pitch angle corresponding to the subjects back elevation was calculated from quaternion sensor output of the Inertial Measurement Unit by applying an appropiate Euler angle sequence. In a second experiment an additional IMU was attached to the contralateral temple while cycling through the above positions sensor outputs of both IMUs were recorded. In a third experiment the subject was sitting up and the hard eye shield with attached IMU was removed and reattached ten times to simulate the application of eye drops. Quaternion sensor outputs were recorded similarly. A Bland Altman analysis of agreement was performed for all three experiments.
In the first experiment pitch angle results calculated from the IMUs quaternion sensor output were 2.5 +/- 2.7, 15.5 +/- 3.8, 31.8 +/- 3.4, 45.8 +/- 2.9, 63.4 +/- 4.2 (mean +/- SD) for the 0, 15, 30, 45, 60 degrees back elevation positions respectively. The mean difference of angular measurement to IMU sensor output was 1.8 degrees (range -4.7 to 8.9). Replacing manual angular measurements by an additional independent IMU resulted in a smaller mean difference of -0.4 degrees with a smaller standard deviation of 1.3 (range -2.9 to 2.4, n=50). By removing and reattaching the IMU attached to the eye shield repeatedly mean difference increased to 3.9 degrees with a larger standard deviation of 2.5 (range -1.2 to 7.1).
Inertial Measurement Units provide sufficient precision to monitor head orientation after intraocular surgery. Further investigations to correlate surgical outcomes to postoperative patient posture data are desireable.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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