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E. B. Geukers, S. Schutte, P. P. Jonker, B. A. J. Lenseigne, F. C. T. van der Helm, H. J. Simonsz; Sensitivity Analysis for Strabismus Measurements Using Corneal Reflections. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1980.
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The reoperation rate of strabismus surgery is about 20%. The main determinant for the amount of muscle relocation is the preoperative angle of strabismus. Currently, the Prism Cover Test (PCT) is the golden standard to measure strabismus angles. Drawbacks of PCT include the limited precision, the required time and required cooperation level. Accordingly, there is potential for a more accurate and quicker strabismus measurement device. A potential principle for this device is the IR eye tracking technique of measuring the corneal reflection relative to the corneal center.
The sensitivity of the method of measuring angles of strabismus using the corneal reflection relative to the corneal center was investigated in relation to a patient’s biometric parameters and the geometry of the measurement setup. A 2D model was made of the setup and the anatomy of the eye according to Gullstrand’s eye model (Le Grand and El Hage, 1980). The sensitivity of the measurement method to variations (mean ± 2SD) in the corneal radius (Rc), the anterior chamber depth (ACD), the interpupillary distance (IPD) and the distance (d) and rotation () of the head with respect to the experimental setup were investigated.
Strabismus angles were determined theoretically using relative distances between the calculated corneal reflection and corneal center in the 2D model. The corneal reflection method was sensitive to Rc (± 5% for 7.8 ± 0.6mm), ACD (± 16% for 3.2 ± 0.8mm) and to head rotation (± 4% for 0 ± 10°). Measurement accuracy was affected little by the head distance d (± 0.2% for 1000 ± 100 mm) and IPD (± 0.1% for 60 ± 20 mm).
In a strabismus measurement device, the patient’s Rc, ACD and head rotation with respect to the camera should be accounted for, as these parameters have a significant influence on the measurement accuracy. The IPD and the head distance from the patient to the experimental setup can be neglected.
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