In a busy practice, the eye care practitioner has little or no time to sit and design complex orthokeratology lenses with the next patient already waiting in the reception area. Most orthokeratology lens designing happens at the end of the day, taking away precious family time, or is deligated to another person who is not always privy to all the nuances of the fit.

EyeSpace drastically shortens the design time of even the most complex orthokeratology lens, allowing the practitioner to design the orthokeratology lens with the patient still in the chair. The visual interface simulates the lens on eye in real time and can easily be interpreted by the eye care practitioner.

This case study will show how easy it is to design a Forge orthokeratology lens for a low myope.

Introduction

The patient is a 13 year old girl who came for her annual eye examination. She is a known myope with typical myopia progression and has increased in myopia by 0.50D in both eyes over the last 12 months.

Her new prescription is:

RE: -2.25 / -0.25 x 105 VA: 6/6 (1.00)

LE: -2.25 / -0.25 x 80 VA: 6/6 (1.00)

After a discussion with the parents it was decided to fit her with orthokeratology lenses. She was then taken to the instrument room for topography. Using the Medmont E300 topographer, composite maps were taken of both eyes.

CAPTION:Medmont tangential composite map of the right eye.

CAPTION:Medmont tangential composite map of the left eye.

The composite maps gave good coverage of the entire cornea up to the limbal area. Both corneas were symmetrical with low astigmatism and average eccentricity making her an ideal candidate for orthokeratology.

More about corneal topography

Great care should be taken to ensure that corneal topography images are taken as accurately as possible to eliminate the risk of capturing contaminated data. The topography head must be properly centered and focussed and the patient must be asked to fixate on a point to ensure that the placido rings are well centered and free of any rings jams.

For more indepth information on proper topography capture techniques watch this EyeSpace instructional video.

When capturing topography maps for orthokeratology use, it is important to recognise when measured corneal irregularities are due to external influances, rather than the corneal shape itself. Using the tangential curvature map is visually the easiest way to see if any tear artifacts are visible on the map which can cause topographic errors.

The Medmont tangential map, when set to standard curvature, should ideally appear like an imaginary round island surrounded by blue sea. If another partial island is visible in the periphery or the main island is not evenly round it is most likely that the measured corneal data will contain topographic errors which can negatively affect the overall success of the orthokeratology fit.

CAPTION:In this Medmont tangential map, a tear artifact is visible in the upper right corner.

When confronted with these topographic errors the map should be retaken, taking care that no ring jam or excessive tearing is present when recapturing the map. If it is not possible to get a 'clean' image, the eroneous data can be manually removed using the build in Medmont editing function.

For more indepth information on editing Medmont topography maps watch this EyeSpace instructional video.

Designing Forge Myopia orthokeratology lenses

With the patient back in the chair the Medmont topography maps were imported into EyeSpace.

CAPTION:EyeSpace tangential composite map of the right eye.

CAPTION:EyeSpace tangential composite map of the left eye.

Measuring the HVID

Step one in designing the Forge orthokeratology lens is to determine the corneal diameter. When first selecting the topography map, EyeSpace will automatically display the build in corneal diameter measurement tool. Best practice for HVID measurement is to measure the corneal-limbal junction horizontally from the one side to the corneal-limbal junction on the oposite side (black to black) making sure that the blue calliper line passes through the center of the cornea.

If it is not possible to measure the diameter horizontally, the measurement can be done at a slight oblique angle. Measure from the bottom left to top right and repeat the measurement from the bottom right to top left, always making sure that the blue calliper line passes through the center of the cornea. If the two measurements are slightly different, use the smallest value for the final corneal HVID value.

Step 1 - Choose the lens design

With the HVID value entered, EyeSpace will promp you to choose a lens design. The lens design options might vary, depending on the EyeSpace license you are using. In this case the Forge orthokeratology lens range and specificaly the Forge Myopia lens design was chosen. The Forge Myopia lens design is ideal for cases with low amounts of myopia and astigmatism.

CAPTION:EyeSpace lens design selection window

Step 2 - Enter Spectacle prescription

Next, EyeSpace requires the spectacle prescription and vertex distance. For orthokeratology correction, care should be taken to use the lowest minus correction achieving clear vision. This is particular important when dealing with emerging presbyopes.

When dealing with high prescriptions above -6.00D it is also important to measure and enter the correct vertex distance.

CAPTION:Step 2 in designing the Forge Myopia lens with EyeSpace is to enter the spectacle prescription and vertex distance.

Step 3 - Optimise the design

With all the given information EyeSpace will calculate the correct Forge Myopia lens for the patient. EyeSpace uses sophisticated algorithms to determine the position and tilt of the lens on the eye. It is the responsibility of the practitioner to optimise the lens fit by making small tilt and lens design changes to balance the pressure zones under the alignment curve (AC) and treatment zone (TxZ).

CAPTION:Innitial Forge Myopia orthokeratology lens designed by EyeSpace for the RE. The central tear film thickness or cTFT (circled in red) should always be between 0 and 5 microns.

RE

With the RE the contact pressure of the alignment curve is most prominent at the 5 o'clock poisiton, seen as the blue touch area in the alignment curve in the EyeSpace simulation above. This indicates that the bottom section of the alignment curve is closer to the cornea compared to the top section of the alignment curve.

This phenomenon is known as lens tilt.

Orthokeratology fitting theory dictates that an ortho-k lens will settle in a position on the eye where all the forces under the lens is in equlibrium. If more pressure is present at the bottom alignment curve compared to the top, the lens will tilt on the eye towards the top area of least pressure until all the pressure points are equilised. Taking cognisance of the unbalanced AC pressure points, the lens can be manually tilted by clicking the tilt buttons (small circled crosses) found on the EyeSpace simulation to mimic the lens movement as it will occur on the eye.

The central tear film thickness or cTFT (circled red area) should always be betweeen 0 and 5 microns. In this case the calculated cTFT was -1.5 microns, indicating the lens has too little central sag causing more touch pressure in the center treatment zone relative to the alignment curve and will most likely cause the lens to decenter. Should the patient sleep with this design, the decentered lens will cause a frowny or smiley face topography pattern and/or possible central corneal staining.

By tilting the lens towards the 11 o'clock positon (yellow circled cross) the pressure zones under the alignment curve is equalised. Visually with the EyeSpace lens simulation this is seen by the equal and opposite blue touch zones in the alignment curve.

To increase the cTFT, the z-zone of the lens is increased by 10 microns, from 280 to 290, resulting in a nett cTFT of 0.2 microns.

CAPTION:Optimised Forge Myopia orthokeratology lens design for the RE

Finally verify that the alignment curve is slighly downward sloping. Ideally the AC should be fitted a little bit steeper than the mid peripheral cornea to help “lock” the lens in place and provide good centration.

The example below shows the Forge Myopia lens design with the AC designed too steep. Notice the sharp down angle of the AC (black line) and the blue bearing zone shifted to the outer region of the AC, closest to the lens edge. A Forge Myopia lens with an AC that is too steep won't have proper seal near the return curve or z-zone area, resulting in insufficient central compression manifesting as under correction and a central island topography.

CAPTION:Example of a Forge Myopia lens with an alignment curve that is too steep.

The next example shows the Forge Myopia lens design with the AC calculated too flat. Notice the upwards angle of the AC (black line) and the blue bearing zone shifted to the inner region of the AC, closest to the return curve or z-zone. A Forge Myopia lens with an alignment curve too flat won't align or 'lock' properly on the cornea and can result in excessive lens movement manifesting as ghosting, increased astigmatism and a frowny or smiley face topography.

CAPTION:Example of a Forge Myopia lens with an alignment curve that is too flat.

Once the alignment curve is properly balanced and aligned and the cTFT value of between 0 and 5 microns is achieved, in this case 0.2 microns, the right eye's Forge Myopia lens is optimised and ready to be ordered.

LE

The same steps are followed to design the LE's Forge Myopia lens.

CAPTION:Innitial Forge Myopia orthokeratology lens designed by EyeSpace for the LE

The contact presure under the alignment curve is more pronounced at the 7 o'clock postion indicating the lens will tilt more towards the 1 o'clock position on the eye. The cTFT is measured at 0.2 microns which is perfect.

Tweaking the lens fit by tilting the lens towards the 1 o'clock position, balances the presure under the alignment curve. The AC also shows the correct amount of down angle to ensure that the lens will 'lock' into position on the cornea.

Unfortunately tilting the lens caused the cTFT to drop below the 0 micron level. To correct the fit, the z-zone is increased by 5 microns, from 280 to 285, resulting in a cTFT of 0.2 microns. With the cTFT corrected the lens design is optimised and ready for order.

CAPTION:Optimised Forge Myopia orthokeratology lens design for the LE

Optical Analysis

EyeSpace automatically generates an Optical Analysis graph for each design. The green OR and bottom graph line respectively represents the vertexed optical refraction power and calculated tear power for each meridian. The top red graph line represents the ideal back vertex power which is also known as the Jessen factor or overshoot power.

The overshoot power is an extra compression factor that is added to the Back Optic Zone Radius (BOZR) calculation to ensure that the target myopia correction is reached. EyeSpace will automatically calculate the required overshoot power based on the corneal shape and desired myopia correction.

CAPTION: The Optical Analysis graph provide a quick snap shot of the lens power calculations along all the meridians of the eye.

With both the right and left designs the average calculated overshoot factor was 0.75D.

Step 4 - Order the lenses

With the two designs completed the lenses was ordered online via the build in EyeSpace order system.

Lens parameters ordered:

RE: Forge RS 8.4/290/7.8/11.4/+0.75

LE: Forge RS 8.4/285/7.8/11.4/+0.75

It is important to note that up to this point no lens was inserted into the patients eye, the two Forge Myopia lenses were virtually designed and ordered using only EyeSpace.

Dispensing of the Forge Myopia lenses

The lenses was dispensed to the patient two weeks later after she received contact lens training. As a rule the contact lens technician will spend 30-40 min with new contact lens patients teaching them insertion, removal and cleaning techniques after which time they will practice the precedures before taking the orthokeratology lenses home.

The following day a health assessment was done to check for any adverse reactions to the first night of lens wear. With unremarkable results, the contact lens training was reinforced and the patient was asked to return in 3 days time.

Day 4 follow up

On the forth day the patient had 6/6- (1.00-) unaided visual acuity in both eyes.

RE

The EyeSpace tangential power difference map is used to assess the shape of the orthokeratology mould, which in this case shows a well formed bull's eye pattern. The difference graph in the bottom right corner shows an even U shape indicating a well balanced and even treatement zone and clean midperipheral steepening.

CAPTION:EyeSpace tangential power difference map of the right eye after 4 nights of lens wear.

The EyeSpace axial power difference map is used to assess the optical qualities of the orthokeratology mould. The difference map in the top right corner shows a well formed and round optic zone which is essential for clear visual acuity. The bottom right difference graph shows a treatment zone size is 4.75mm and maximum myopia correction of -2.14D. The slight undercorrection explains why the visual acuity measured 6/6-.

CAPTION:EyeSpace axial power difference map of the right eye after 4 nights of lens wear.

LE

The EyeSpace tangential power difference map of the left eye shows very similar results as the right eye. The top right difference map shows a well formed bull's eye pattern and the bottom graph shows a well formed U shape.

CAPTION:EyeSpace tangential power difference map of the left eye after 4 nights of lens wear.

The EyeSpace axial power difference map of the left eye in the top right corner shows a well formed and round optic zone which is slighly decentered to the bottom right. The bottom right difference graph shows a treatment zone size is 4.92mm and maximum myopia correction of -2.49D. Although the target myopia correction is achieved, the sligh decentration of the optic zone is causing a small amount of ghosting, resulting in a uncorrected VA of 6/6-.

CAPTION:EyeSpace axial power difference map of the right eye after 4 nights of lens wear.

Day 30 follow up

The patient returned for her 1 month follow up visit. Unaided visual acuity was measured at 6/6+ (1.00+) in both eyes. The corneal health still remains unremarkable and both parents and child are happy with the results.

RE

The EyeSpace axial power difference graph now shows the treatment zone size as 4.91mm and the max myopia correction at -2.28D.

CAPTION:EyeSpace axial power difference map of the right eye after 30 nights of lens wear.

LE

The EyeSpace axial power difference map in the top right corner now shows a well centered treatment zone and the bottom graph calculates the treatment zone size as 5.09 mm and the max myopia correction at -2.25D.

CAPTION:EyeSpace axial power difference map of the right eye after 30 nights of lens wear.

Conclusion

Custom designing Forge Myopia orthokeratology lenses for normal corneas with low myopia and astigmatism has never been so easy. Following these easy steps, it is possible to test, design and order low myopia orthokeratology lenses in the same time as it would have taken to fit the patient with soft contact lenses.

For more information on designing Forge Myopia lenses with EyeSpace read the EyeSpace online manual and watch this instructional video.