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D. R. Mukhopadhyay, R. V. North, K. E. Hamilton; Comparison of Orbscan IIz, Pentacam and Ultrasound Pachymetry in Centre and Mid-periphery. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2852.
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© ARVO (1962-2015); The Authors (2016-present)
Recent work has shown that CCT and mid-peripheral corneal thickness (MCT) affect the accuracy of applanation tonometry. This was a clinical study to compare methods of CCT and MCT measurement using Orbscan IIz (Bausch + Lomb, USA), Pentacam (Oculus, Germany) and ultrasound pachymetry (Sonomed, USA), to determine the most effective.
70 eyes of 35 normal subjects (20 female/15 male; age 30.8±10.5 years) underwent 3 corneal thickness scans using Orbscan IIz and Pentacam, and 5 measurements using ultrasound pachymetry in each of 5 corneal locations. Ultrasound MCT readings were obtained by varying fixation to change corneal location of the ultrasound probe. Mean CCT and MCT at 2.5mm from the centre superiorly, nasally, inferiorly and temporally were analysed. Repeated measures ANOVA with Bonferroni correction was used to make pairwise comparisons between measurements from each instrument at each position. Coefficient of variation (CoV) was calculated for each instrument.
Right eye results are summarised in the table below:
Orbscan and ultrasound CCT were not significantly different (p=0.996), nor were Pentacam and ultrasound CCT (p=0.11). However, Orbscan and Pentacam CCT were significantly different (p=0.001). All other comparisons between instruments were significant in the mid-periphery (p<0.05); Orbscan MCT was highest and ultrasound MCT lowest in all positions. CoV for each method ranged from 5.1 to 7.5% in all positions.
For CCT, Orbscan and ultrasound show good agreement while Pentacam is significantly lower than Orbscan. Significant differences are found between instruments in measures of four mid-peripheral locations. Therefore, the three methods cannot be interchanged reliably, particularly when considering corneal thickness beyond the centre. Ultrasound MCT values are lower than the other methods, possibly due to difficulty of correct probe placement 2.5mm out from the central cornea. CoV shows each method to have similarly low variability in all positions.
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