Estimation of central anterior chamber depth (ACD) is vital in newer theoretical biometric formulas for intraocular lens (IOL) power calculation,1,2,3 implantation of phakic IOL,4,5 and glaucoma studies. Contact ultrasound is the most common method currently used. Ultrasound measurements can be affected by various factors such as experience of the operator, the differences in probe handling, alignment errors, etc. Higher intra- and inter-observer variability in measurements with applanation ultrasound, increased chance of corneal abrasion and infections, and difficulty in quickly sterilizing the contact probe to an acceptable degree make non-contact optical devices a popular alternative.
Recent technologies have been introduced for the measurement of axial length (IOLMaster) and keratometry. Studies have demonstrated that errors in prediction of effective lens position (ELP) may account for 20% to 40% of the total refractive prediction error.1,3 Newer theoretical formulas such as that by Haigis11 use pre-operative ACD to predict ELP. The Orbscan II topography system (Orbtek Inc.), initially designed for corneal topography, has been demonstrated to be a useful tool in anterior segment biometry.6 This system, based on scanning-slit method, is claimed to provide accurate and reproducible measurements of ACD.7,8,9,10,11
The IOLMaster (Carl Zeiss) is based on the optical method for measuring the ACD. Anterior segment biometry with such devices has been reported to have high precision (≤5 μm), high resolution (˜12 μm), and good reliability.12,13,14 Ultrasound biometry with a 10 MHZ transducer probe has a resolution of approximately 200 to 300 μm and a precision of 150 μm.15
The aim of this study was to assess the comparability of ACD measurements obtained by applanation ultrasonography, Orbscan II, and IOLMaster and examine the correlation between them.
Patients and Methods
Eighty-one eyes of 41 consecutive patients having refractive cataract surgery for age-related cataract had ACD measurements by different methods in the following order—Orbscan II topographic analysis, IOLMaster, and contact ultrasound A-scan (Ocuscan, Alcon). This order of measurement was followed to prevent the irregularities in corneal surface caused by applanation ultrasound. With the Orbscan II, the ACD inclusive of corneal thickness (epi) was used.
For applanation ultrasound, a 10 Mhz applanation probe was used and an average of 3 consecutive consistent readings of ACD was taken. The measured ACD was corrected for the actual velocity of sound in the cornea and aqueous (1641 m/sec and 1532 m/sec, respectively). The same observer performed all measurements.
Repeated measures analysis of variance (ANOVA) was used to analyze difference between ACD measurements by the 3 methods. Difference in measurements between methods was assessed using the paired t test. The correlation between Orbscan II and IOLMaster measurements was assessed using Spearman rank conversion. A value of P<.01 was considered significant.
The mean age of patients was 72 years (range 59 to 94 years). Of the 81 consecutive eyes included in the study, 5 could not have contact ultrasound measurements. The mean ACD with ultrasound, Orbscan II, and IOLMaster was 2.87 mm (± 0.55), 3.32 mm (± 0.60), and 3.33 mm (± 0.61), respectively.
A significant difference was noted between the ACD measurements recorded by the 3 devices = 69.13, P<.01). Paired t test revealed contact ultrasound measurements to be significantly lower on average than those recorded by both Orbscan II and IOLMaster (P<.01 in both cases). The ultrasound measurements were 0.40 mm lower on average than those recorded by Orbscan II with 95% confidence interval (CI) (0.31-0.49) (paired t test = 8.87, 75df, P<.01). Ultrasound measurements were 0.43 mm lower on average than those by IOLMaster (95% CI) (0.34-0.53) (paired t test = 8.99, 75df, P<.01).
Anterior chamber depth measurements by Orbscan II and IOLMaster were not significantly different (paired t test = 0.31, 80df, P = .75). The correlation coefficient between Orbscan II and IOLMaster was 0.91 (P<.01, Spearman rank conversion).
Figures 1, 2 and 3 illustrate the correlation plots describing agreement in the ACD measurements by the 3 devices. (A line of best fit is also included in the graphs.)
This study revealed a significant difference in the ACD measurements between applanation ultrasound and the 2 optical methods. Although a statistically high correlation was noted between ACD measurements by Orbscan II and IOLMaster (correlation coefficient 0.91, P<.01), Figure 1 shows up to 1.0 mm difference in ACD by these 2 methods. Contact ultrasound has disadvantages for various reasons as discussed earlier, making the noncontact optical devices popular. Auffarth and coauthors9 compared Orbscan and ultrasound measurements of ACD in eyes prior to cataract surgery and found a high correlation between the 2 methods (correlation coefficient 0.96, P<.00001). However, immersion ultrasound was used in this study.9 Vetrugno and coauthors,8 using applanation ultrasound in their study (n = 34), have reported a constant underestimation in the ACD measurements with the Orbscan when compared with ultrasound. Whether the corneal thickness was included in the Orbscan measurement was not mentioned.
A good correlation between 3 optical methods (Orbscan, Scheimpflug imaging [Nidek EAS-1000], and optical pachymetry [Haag-Streit]) in the measurement of ACD has been reported by Koranyi and coauthors.11 Our study demonstrates a consistent underestimation of ACD as measured by applanation ultrasound when compared with those by Orbscan II and IOLMaster. The most probable explanation for this is the effect of applanation when using the hand-held contact ultrasound probe. Hoffer16 reported a mean of 3.24 mm (±0.44) in 6950 phakic eyes using a pachymeter in 60% of eyes and A-scan ultrasound in 40% of eyes. Also, there is no mention of the velocity of ultrasound used. This makes it difficult to compare the ACD measurements of our study with his.
In conclusion, we have demonstrated that applanation ultrasound gives consistently lower measurements of ACD compared with Orbscan II and IOLMaster. Applanation ultrasound method of measuring the ACD is observer dependent and, therefore, retesting with immersion methods may be appropriate in such situations. Although we noted a high degree of agreement in the mean ACD between Orbscan II and IOLMaster, further studies are needed to assess interchangeability of measurements in clinical practice. Since both are optical devices, they have the advantages of noncontact biometry and are easy to use and consume less time.
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