Correlation of Back Optic Zone Radius measurement of rigid contact lenses with radiuscope and keratometer

Purpose: To investigate the accuracy of Back Optic Zone Radius (BOZR, base curve) measurements with manual keratometer for spherical polymethylmethacrylate (PMMA) lenses.

Methods: 100 spherical PMMA contact lenses were selected randomly from the trial sets. One investigator measured the BOZR with radiuscope. The second investigator measured the BOZR with keratometer. The two investigators were masked to each other's readings. As the keratometer is designed to measure a convex corneal surface, the BOZR readings are less than actual radii. A correction factor of 0.025 was used to have corrected keratometers readings.

Results: There was a high correlation between the BOZR measurement with radiuscope and the uncorrected (Pearson's correlation coefficient, r=0.99) and corrected (r=0.99) measurements with keratometer. The mean difference between BOZR measurement with radiuscope and the uncorrected and corrected measurements with keratometer were 0.003 mm (p=0.62) and -0.024 mm (p<0.001) respectively. The 95% limits of agreement for the BOZR measurements with radiuscope and the uncorrected and corrected measurements with keratometer were -0.11 to 0.11 mm and -0.14 to 0.09 mm respectively.

Conclusion: We found that the uncorrected BOZR measurements with keratometer were similar to the BOZR measurements with radiuscope. With unavailability of radiuscope in majority of contact lens practice, keratometer can be used to measure the BOZR of contact lenses.