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Professional papers

A report on the fitting of CRT orthokeratology lens --Theory

Published people:Internet Society Published time:2020.11.10 Download the number:0

CRT cornea shaping is the largest brand in the U.S. market. It has a long history and excellent design concept. It is quite different from VST and has good wearing effect. I have heard of many international conferences on keratoplasty in the United States. Many experts in the United States use CRT products. I finally heard that they will enter the Chinese market this year. I was lucky to get a month's trial right of easilu CRT. After several unremitting attempts, I was finally able to give a fresh report.

The design of orthokeratology lens is divided into: VST design (vision shaping treatment), which is the design patent of Boston company; CRT design (corneal refractive therapy), is the design patent of Paragon company. At present, the corneal orthosis lenses used in China are all VST designs, such as Japanese alpha, American Euclidean, Hong Kong e-an-yi, Korea's lucitte, Taiwan Hengtai and Hefei dream David. The parameters of orthokeratology lens designed by VST mainly include BC (base curve) base arc, RC (reverse curve) inversion arc, AC (alignment curve) fitting arc or positioning arc, and PC (peripheral curve) peripheral arc. In the process of fitting, we mainly evaluated the selection of AC of standard test piece, and calculated the decrease of myopia through optometry on the piece to obtain BC parameters. RC is designed by the manufacturer (RC design is often protected by the manufacturer's patent and is not disclosed).

I searched the website and downloaded the "professional fitting and"

Information Guide (Figure 1). After reading and learning, combined with our experience of several cases of fitting practice, collated into the following, to share with each peer. There are many contents, which are divided into "theory", "fitting practice" and "small experiment - Comparison of CRT and VST front surface".



1) Materials:

Let's take a look at the material characteristics of CRT: (crt100 is made of hds100 pafluocon d)

  1. Basic physical characteristics:


Refractive Index(折射率)                     1.442 (Nd at 25°C)

Luminous Transmittance+ (Green) (透光率)      95%

Wetting Angle (Receding Angle)++  (湿润角)     42°

Wetting Angle (Contact Angle)+++                 70º

Specific Gravity(比重)                        1.10

Hardness (Shore D)(肖氏硬度)                 79

Water Content (含水量)                     <1%


+ Determination of the Spectral and Luminous Transmittance, ISO 8599:1994

++ Adapted from: A New Method for Wetting Angle Measurement; Madigan, et. al., International Eyecare, 01/1998. vol. 2, no.1, p.45

+++ Sessile Drip Technique per ANSI Z80.20, 8:11 Sessile drop

A drop of normal saline is placed on the lens material and the contact angle is measured. This method is the oldest and most inaccurate of all


2. Oxygen permeability:


1 HDS100的透氧性指标








Permeability (ISO Method*) Dk x 10-11

Center Thickness (mm)


Harmonic Mean Thickness** (mm)


Oxygen Transmissibility (ISO) Dk/l x10-9

HDS 100






HDS 100






HDS 100

























* (cm2/sec) (mL O2)/ (mL x mm Hg) ISO/ANSI Method, ISO 9913-1

** Sammons, W.A., “Contact Lens Thickness and All That”, The Optician, 12/05/80.

We can also find the relevant parameters of Boston boxo material (most orthokeratology lenses in China are boxo materials) on the website, which are sorted out as follows (Table 2)




Refractive Index(折射率)



Luminous Transmittance+ (Green) (透光率)


Wetting Angle (Receding Angle)++  (湿润角)



Specific Gravity(比重)



Hardness (Shore D)(肖氏硬度)



Water Content (含水量)




The specific gravity of hds100 is smaller, the wetting angle is smaller than boxo, and other parameters are similar to boxo. Due to the small wetting angle, the wearing of lens will be more comfortable than XO material.


2) Design

CRT design has three main parameters: BC (base curve) base arc, RZD (return zone depth) turning depth and lza (landing zone angle) landing angle. (Fig. 2)


  1. Adjustable parameters of CRT (Table 3)





Overall Diameter (D)


9.5 to 12.0 mm

Central Base Curve Radius


6.50 to 10.50 mm


Optical Zone Semi Chord (OZ)


2.50 to 3.50 mm


Return Zone Width (w)


0.75 to 1.5 mm


Return Zone Depth(Δ)


to 1.0 mm


Landing Zone Radius


to infinity


Landing Zone Angle(Φ)


-25° to –50°

试戴片的范围:-31°~ -35°,是主要的调整参数之一。

Landing Zone Width (LZW)


0.5 to 2.75 mm


Edge Terminus Width (P)


0.04 mm to LZW


Dioptric Powers


-2.00 to +2.00 D


 2. Several points about CRT parameters are given

The design parameters of CRT are different from VST. Many English terms do not have corresponding Chinese terms, so I have done some translation according to my own experience.

1) The section of return zone is an "s" shaped curve, connecting BC and lza, and its effect is equivalent to RC of VST design, and I directly translate it into inversion area; the function of landing zone is similar to AC arc in VST design, but I translate it into landing area or positioning arc - in fact, it is not appropriate to translate it into positioning arc, because landing zone is designed as RC Zone is a tangent line rather than an arc, but for the convenience of understanding, it is also translated as positioning arc. Lza (landing zone angle) is the angle between the cross section of the landing area and the horizontal direction. The change amount of every 1 degree is equivalent to 15 μ m of height loss change.

2) In order to avoid confusion, RZD stands for return zone depth and lza stands for landing zone angle.

3) Because LZ is tangent line, it will form a natural curl with the arcuate limbus, so CRT has no edge lift design.

4) Edge terminal, the lens edge of CRT is different from that of RGP. The edge arc of RGP is "arc", which is designed to match the radian of corneal edge to ensure edge warping and tear exchange. The design and parameters of edge arc are affected by lens base arc, production process and lens thickness. The edge of CRT is specially designed and independent of other lens parameters (because it is not arc, but tangent), which makes the edge design of CRT stable and comfortable.


3. Regular try on piece

A total of 136 CRT test pieces (Fig. 3) were used

Optic Zone(光学区) = 6.0 mm

Return Zone Width(反转区宽度) = 1.0 mm

Center thickness(中央厚度) = 0.15 mm + 0.01


3) CRT fitting process

After the routine corneal shaping examination, the first piece was selected according to the flat K and myopia reduction table (the manufacturer's "draw card", flat K check). Pull the card in the card slot of "draw card" so that the arrow points to the measured corneal flat K (Fig. 4), and then select the corresponding trial piece according to the myopia reduction at the "Mrs" sign on the right side (Fig. 5). The yellow logo indicates that the trial piece can be found in the conventional film; the pink logo indicates that the trial piece is only available in the extended trial group; the white logo indicates that the trial piece is not available in the trial piece group and needs to be customized. Find the corresponding fitting piece from the fitting box for trial wearing (Fig. 6).

图片8.pngFigure 4 "draw card" 1 provided by CRT manufacturer


图片9.pngFigure 4 "card drawing" 2 provided by CRT manufacturer


Figure 6 find the corresponding trial piece in the lens box

Adjust the lens according to the evaluation performance of the trial piece: observe the performance of BC and RZD and adjust RZD (sagittal height); observe the degree of edge warping to adjust lza.

The matching process and complexity are basically similar to VST system. The final determination of lens parameters needs to be confirmed by corneal topography after overnight trial.


4) The theoretical knowledge learning experience of CRT

1. Try on more pieces

It's both advantages and disadvantages. ——Advantages: there are many pieces to try on, large space for adjustment, very close to the actual fixation, and the success rate of trial wearing method is high. Disadvantages: so many try on lenses mean more trial lenses, regular maintenance, care, and more financial costs. (if the financial budget and staff are sufficient, please ignore it directly)


2. The optical area (treatment area) is large

The optical area of CRT is much larger than that of VST, but the LZ (landing zone) of CRT is narrower than that of VST. ——In the case of the same good positioning, the large optical area will inevitably bring more pupil coverage (for patients with large pupil in dark environment, it can solve the problem of glare at night) and better visual quality. But if LZ (landing zone) is narrow, is it easier to locate than VST? It needs to be observed in practice.


3. Lza adjustment is not linked with BC

This means that the adjustment of lens parameters has less influence on the whole and is easy to grasp.


4. The topographic map after trial wearing is the standard for adjusting lenses

Whether it is CRT or VST design, fluorescence evaluation alone can not confirm whether it is suitable or not, or we need to try on the topographic map as the film adjustment standard. This is still consistent with the international mainstream of shaping and fitting ideas.


5. The lenses are very thin

The central thickness of CRT lens is very thin, "center thickness = 0.15 mm + 0.01", that is, 0.16 mm. It is much thinner than the lens of VST system (the central thickness of VST is 0.22 ~ 0.25 mm).

The advantages of thin lens are: first, the thinner the lens, the less foreign body feeling, the more comfortable to wear; second, according to the oxygen conductivity Dk / L, thinner lens means smaller L, higher oxygen conductivity Dk / L, the less prone the cornea to hypoxia complications; third, the thinner the lens eyelid clamping force on the lens, which means that the lens is less affected by eyelid tension (almost The best news for Asian children with tight eyelids.

But at the same time, thin lens is also a disadvantage: does thin lens mean that the lens is easy to break? But CRT uses hds100 material, which is stronger than XO material and is not easy to break? It needs to be observed in practice.


6. The relationship between E value and slice selection is not mentioned in the description

How to adjust lza according to e value in CRT test? For example: is the higher the E value, the smaller lza should be selected under the same conditions? It also needs to be observed and explored.