Ophthalmic Biometry using Calipers: Determination of Measurement Precision

Abstract

 

Purpose:

 

To access the precision of globe and cornea dimension measurements acquired using the Vernier caliper and the Castroviejo caliper, and to quantify the intra- and inter-operator variance and the scale dependence of the variance.

 

Methods:

 

Postmortem human eyes (n = 10, Age = 16 to 54 yrs, PMT = 18.5 to 66.5 hrs) were obtained from the Ramayamma International Eye Bank in India. The horizontal and vertical dimensions and the axial length of the globe were measured using a digital Vernier caliper (resolution: 0.01 mm). The horizontal and vertical dimensions of the cornea (white-to-white) were measured using two different calipers: a digital Vernier Caliper and a Castroviejo caliper. The measurements were performed by three operators. Each operator repeated the measurements until five measurements were obtained for each dimension.

 

Results:

 

The Castroviejo caliper is often used in Ophthalmology today. The Castroviejo caliper measures from zero to 20 mm in 1 mm increments (allowing estimates on the order of 0.5 mm). The Digital Vernier caliper measures from zero to 150 mm in 0.01 mm increments. From measurements of the globe using the digital caliper, no significant difference was observed between globe axial length, horizontal diameter and vertical diameter. Horizontal corneal diameter is greater than vertical diameter with all instruments and all operators. Variability of the digital calipers does not vary with measurement object. The measurement variability was similar across all operators. No significant difference was observed in the variability of the two devices (mean intra-operator standard deviation): digital: 0.127 ± 0.023 mm & Castroviejo: 0.094 ± 0.056 mm.

 

Conclusions:

 

The precision of mechanical ophthalmic biometry measurements from the present to more than 300 years in the past is limited to approximately 0.1 mm.  

 

Keywords: anatomy • comparative anatomy • computational modeling