Keywords

Introduction

Optical biometer OA-2000 (Tomey Corporation, Nagoya Japan) can measure all biometric parameters needed for pre-cataract surgery: axial length, anterior chamber depth, lens thickness, corneal radius of curvature over a 2.5 mm and 3.0 mm diameter, corneal topography, central corneal thickness, pupil diameter, and Corneal Diameter (Fig. 21.1 and Table 21.1). It is done almost automatically by one measurement in short time. The touch screen enables intuitive operation, measurement, data checking, IOL calculation, and data output within one compact unit. The OA-2000 supports doctors to reduce stress on patients and accurate and smooth preoperative examination and planning for high-quality cataract surgery by these features on daily practice [1].

Fig. 21.1
A 2-part illustration. Left, a photo of the optical biometer O A-2000. Right, an illustration has a photo of a patient looking into the optical biometer and a schematic illustration of light focusing on the eyeball marks the coronal shape, pupil diameter, and corneal diameter.

Optical biometer OA-2000

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Table 21.1 Specifications of OA-2000
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Axial Length Measurement

The touch-alignment system is employed on OA-2000. It can provide a stable measurement and a high reproducibility between operators because OA-2000 aligns itself automatically to patient eye once an operator only touches the center of the pupil on the screen. Operators can measure all data easily without any special skills (Fig. 21.2) [2].

Fig. 21.2
A photo of an index finger touching a screen displays a diagram of an eye with a list of parameters on either side. A bar code is on the right.

Touch alignment, easy operation. http://vimeo.com/259629303?width=640&height=480

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OA-2000 achieves a high signal–noise ratio (SNR) and high transmitting on opacity parts by using swept-source coherence tomography method. In addition, long coherence length allows high penetration and high success rate on long axial length myopic eyes (Fig. 21.3) [3]. Additionally, OA-2000 obtains not only A-scan wave form but also B-scan image of 1 mm width on retina by 2D scanning. On a measurement result of normal eye (Fig. 21.4), SNR is high and standard deviation (SD) is very low because the measurement light isn’t reduced as the crystalline lens is clear. It means the measurement is very stable.

Fig. 21.3
A screenshot of the results of the cataract eye with long axis length. Left, a fluctuating line with values for Kerato and D I A. Right, a list of values for axial, A C D, lens, and S N R. The axial length is marked as 32.27 millimeters.

Results of cataract eye with long axis length

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Fig. 21.4
A screenshot of the measurement result of the normal eye. A fluctuating line for A scan waveform and B scan images is on the left. A table lists the axial, A C D, lens, and S N R with the values of S D highlighted are in the right. Multiple options are at the top and bottom.

Measurement result of normal eye

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On the other hand, there are some cases where it is difficult to obtain the signal around the center of the retina as shown in Fig. 21.5a. In such case, OA-2000 measures the axial length by detecting a stronger signal from peripheral area automatically. When the retinal signal cannot be detected even with that way, V scan (Fig. 21.6) is performed instead of horizontal scan to find less opacity position so that the retinal signal can be detected. High success rate is achieved by these techniques in combination.

Fig. 21.5
2 screenshots. A, a fluctuating line marks the low signal and stronger signal. The value of S N R is 449. B, a fluctuating line marks R P E, I L M, and choroid sclera. The value of S N R is 22.

Wave forms and B-scan images on retina of cataract eyes. (a) Lens central opacity. (b) Multi-peaks wave form

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Fig. 21.6
2 schematic representation of the frontal view of the eyeball. A, a double-headed arrow pointing left and right is in the inner circle. B, a V-shaped arrow is in the inner circle.

Scanning methods on OA-2000. (a) Horizontal scan. (b) Vertical scan

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In the case of dense cataract with low SNR, multi-peaks retinal waveforms may appear as Fig. 21.5b shows. OA-2000 detects retinal pigment epithelium (RPE) automatically by an original algorithm based on signal appearance ratio and peak analysis results. It is recommended to judge it comprehensively including the results of other examinations, when it is suspected if the detected retinal peak is correct or not, for example, miss-detecting epiretinal membrane as RPE. Caliper function (Fig. 21.7) allows operators to select RPE position manually if the auto-detected RPE is not correct.

Fig. 21.7
A screenshot of the caliper function has a fluctuating line marking S N R equal to 22. The measurements of axial, S D, and S N R are in the top and axial, A C D, lens, and patchy are below. Options for initial position, apply, and cancel are at the bottom.

Caliper function

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For converting optical path lengths (OPLs) to geometrical distances, an original conversion formula was established by clinical dataset of ultrasound biometer and OA-1000, how to be performed based on the way by Haigis [4, 5]. OPLs of OA-2000 are in good agreement with OA-1000, which is the previous model; OA-2000 also uses the same conversion formula.

Clinical Cases

Clinical cases of cataract eyes are introduced in Fig. 21.8.

Fig. 21.8
A 3-part illustration in A, B, and C has a close-up photo of the eye on the left and a screenshot on the right. The screenshot has fluctuating lines on the left and a table listing values on the right.figure 8

Clinical cases of cataract eyes, Provided by Dr. Hitoshi Tabuchi, Chief director of Department of Ophthalmology, Tsukazaki Hospital. (a) Hypermature cataract eye. The nuclear sclerosis is Emery class V. SDs for axial length, ACD, and lens show small numbers even with relatively low SNRs; measurement result is stable. (b) Cortex and posterior subcapsular cataract eye. The nuclear sclerosis is Emery class III. SDs for axial length, ACD, and lens show small numbers even with relatively low SNRs; measurement result is stable.. (c) Cortex and posterior subcapsular cataract eye. The nuclear sclerosis is Emery class II-III. The retinal signal is very low because the opacity of the cortex and the posterior subcapsular is strong. However, SDs are small enough and the measurement result is stable

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OA-2000 includes the function of wireless connection with ultrasound biometer AL-4000 (Fig. 21.9). It is useful when measurement is difficult by an optical biometer due to subject eyes conditions. The measurement results of AL-4000 can be transferred to OA-2000, and it can be used for IOL calculation on OA-2000.

Fig. 21.9
A photo of the ultrasound biometer A L-4000.

Ultrasound biometer AL-4000

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Measurement of Corneal Radius of Curvature and Topography

OA-2000 employs a Placido disc for measuring corneal radii and topography, which has been proven being an accurate measurement method and used for many years [6]. The Placido-based topography can capture image in a single shot; therefore, it is hard to be affected by eye motion, when performed at the same time as the axial length measurement. Figure 21.10 shows examples of comparison of topography map between TMS-4N and OA-2000 for same eyes. The measurement range of OA-2000 is 5.5 mm which is narrower than TMS-4N. However, it is enough for evaluating visual function, and they have good consistency in that range.

Fig. 21.10
2 sets of screenshots in A and B. A topography of the eye presents the with-the-rule astigmatism eye on the left titled T M S-4 N. A close-up of the eye with color-gradient marks in it is on the right labeled O A-2000. The measurements are in the bottom and right.

Comparison between TMS-4N and OA-2000 on topography. (a) Topography of with-the-rule astigmatism eye. (b) Topography of keratoconus eye

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Important Indices of Cornea for Cataract Surgery

OA-2000 provides not only the corneal radius of curvature results but also important indices of KAI (Kerato-Asymmetry Index) and KRI (Kerato-Regularity Index) (Fig. 21.11). These indices are ranked with A (Low)/B (Slightly high)/C (High) which show possibility to be irregular corneal astigmatism. When KAI shows high value in B or C, the eye is suspected to be a deformed cornea such as in keratoconus. When KRI shows high value, the eye is suspected to be a corneal transplant or with CL-induced problems, etc. It can call to attention checking the topographic maps so that doctors can prevent postoperative troubles. In the case that these values are high, there is higher risk of insufficient visual recovery and it should be carefully considered to choose multifocal IOLs.

Fig. 21.11
A screenshot for important indices of cornea for cataract surgery. It has a close-up photo of an eye with a table listing the values. The K A I value of 106.40 and K R I value of 4 are highlighted.

Kerato-Asymmetry Index (KAI) and Kerato-Regularity Index (KRI)

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CEI (Corneal Eccentricity Index) is shown in the topographic screen (Fig. 21.12). When CEI indicates positive number, the corneal shape is prolate which is normal eye. On the other hand, when CEI indicates negative number, the corneal shape is oblate which can be observed in typical post-LASIK eyes. By checking this value, even in the case that the surgical history is unknown, doctors can realize the possibility being post-LASIK eye and adopt post-LASIK IOL formula so that they can avoid refractive surprise after surgery.

Fig. 21.12
A screenshot of the C E I on post-L A S I K eye has a close-up photo of the eye with color-gradient markings in it. Measurements for topo are on the right.

Corneal Eccentricity Index (CEI) on post-LASIK eye

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IOL Calculation and Toric Calculation

OA-2000 includes different kinds of IOL formulae, which are standard IOL formulae, toric IOL formulae, and post-Lasik IOL formulae (Table 21.1 and Fig. 21.13). In addition, function for optimization of IOL constants is included in OA-2000 and it can support to calculate personal lens constant for each surgeon (Fig. 21.14).

Fig. 21.13
A screenshot of the I O L calculation. It lists the values for average axial, A C D, lens, K 1, K 2, W T W, and target reference at the top. It has a table below listing of values of Barrett U I I, S R K slash T, haigis optimized, and Olsen. Various options are at the bottom.

IOL calculation screen

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Fig. 21.14
A screenshot of the optimization of I O L constants. It consists of 2 bar graphs for the result of registered I O L constant and the result of optimized I O L constant. I O L, manufacture, model, measurement method, O P T-contact, physician, A L L, surgeon, and common are on the left.

Optimization of IOL constants

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Toric planning function to support with axis registration is available (Fig. 21.15). It allows doctors to mark the target axis based on the reference axis of iris pattern or conjunctival blood vessels.

Fig. 21.15
A screenshot of Toric planning screen. A close-up photo of the eye with measurements for incision axis, average axial, lens, K I, S I A, A C D, W T W, and target reference. The measurements for toric planning are on the right.

Toric Planning screen

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