Accuracy and repeatability of a new tono-pachymeter for measuring central corneal thickness

The recently developed noncontact tonopachymeter Tonopachy NT-530 P provides intraocular pressure values corrected for central corneal thickness (CCT). The purpose of this study was to assess the accuracy and repeatability of its CCT measurements.

The CCT measurements were obtained in 64 right eyes of 64 young healthy subjects using the Orbscan corneal topography system followed by the Tonopachy and then by ultrasound pachymetry (UP). Another sample of 31 subjects was used to test the repeatability of the tonopachy measurements in 2 separate sessions 1 week apart. To compare the tonometers and determine intersession repeatability for the tonopachymeter, differences in the readings provided by pairs of the instruments or obtained in the two sessions were plotted against mean differences. The hypothesis of zero bias was examined by a paired t-test. The coefficient of repeatability was calculated as the 95% limits of agreement (LoAs) of differences between the 2 sessions.

Measurements made using the three instruments were significantly correlated (P<0.001). Mean CCTs (±SD) measured using the Tonopachy, Orbscan (acoustic factor set at 0.92) and ultrasound pachymeter were 530.42 ± 34.96, 526.73 ± 39.53, and 550.69 ± 37.26 μm, respectively. The differences between modalities (±95% LoAs) were -3.68 ± 24.36 for Orbscan versus Tonopachy, 20.66 ± 14.69 for UP versus Tonopachy, and 23.95 ± 24.21 for UP versus Orbscan. The coefficient of repeatability for the tonopachymeter was ±15.11 μm.

The Tonopachy offers similar CCT measurements to those provided by conventional pachymeters with good repeatability.