This case shows how a normal cornea can be fitted with Forge Myopia Rotational Symmetric orthokeratology lenses for myopia control purposes and to give freedom from spectacles.
Video of case report
This female patient in her mid 20’s was referred to me for orthokeratology from an optometrist living in a town a 5 hour drive away! The patient is a student and had slowly progressed in her myopia over the past 3 years.
When she saw me, she was RE: -3.25DS (6/5), LE: -3.50DS (6/5). Her records suggested she had progressed by approximately -0.75D a year over the last 3 years. She used soft daily lenses frequently as she was an active person so orthokeratology seemed to be a logical alternative to reduce reliance on glasses and to attempt to curb her adult-onset myopia progression.
Designing Forge Myopia orthokeratology lenses
Topography using Medmont E300 showed regular corneae with little corneal cylinder and slightly higher than average eccentricities. Pupils were an average ~5mm size. This data was exported into EyeSpace to design her customised lenses.
CAPTION:Medmont corneal topography maps showing the low toric corneae of the patient, ideal for orthokeratology moulding.
This patient’s corneae have only slight toricity. However the difference in elevation (seen below in EyeSpace for the right and left eye) of the principal meridians 4mm from the pupil centre does not exceed 30um, suggesting that a rotationally symmetrical lens will fit adequately.
CAPTION:EyeSpace height maps provide a quick reference to decide if a rotationally symmetric or toric orthokeratology lens should be used. In the patient's case the elevation of the two principal meridians, 4mm from the pupil center, did not exceed 30 microns, indicating the use of a Forge Myopia Rotationally Symmetric orthokeratology lens.
After measuring the HVID at 11.6mm and 11.5mm right and left respectively we used a Forge Myopia Rotationally Symmetric (RS) design for her eyes. The lens simulations in EyeSpace are shown in the images below.
RE: Forge Myopia RS with back optic zone radius 8.6mm, z-zone 315um, alignment curve radius 8.1mm, diameter 11.2mm
CAPTION: EyeSpace design page showing the final design simulation of the Forge Myopia RS lens on the right eye.
LE: Forge Myopia RS with back optic zone radius 8.6mm, z-zone 320um, alignment curve radius 8.0mm, diameter 11.2mm
CAPTION: EyeSpace design page showing the final design simulation of the Forge Myopia RS lens on the left eye.
You will see that both lenses exhibit a ‘three-point touch’ with alignment centrally and at the periphery of the lens. This is important to spread the pressure of the lens and ensure that there is no excessive force applied to the central cornea.
Both lenses show a slightly flatter fit in the vertical meridians but as we can see there is only ~15um of tear film thickness on the inside of the alignment curve. This is not enough to justify the use of a toric periphery unless either the appearance of the lens on eye, the follow-up topographies or the visual outcome suggests the lens is sealing poorly in the vertical meridian, excessively moving, centreing poorly or causing lens edge awareness.
As we can see from the photo below of the right lens on this patient’s eye the vertical meridian still appears to be sealing vertically and providing a good ortho-K fluorescein pattern (dont forget that clinically we cannot see less than 20um of fluorescein). The diameter of the lens seems appropriate also. We can afford to go slightly larger than 95% of HVID to aid centration as the Forge Myopia design has a wide 1mm band of edge lift to ensure limbal preservation.
CAPTION: Fluorescein photo of the Forge Myopia RS lens on the right eye showing good alignment curve touch in all meridians inspite of the low corneal astigmatism.
The next photo is of a different patient’s eye with a similar prescription of -4.00DS wearing a Forge Myopia Rotationally Symmetric lens. In contrast even though this patient only has 0.75D of corneal cyl and less than 30um of clearance in the vertical alignment curve (AC) we can clearly see the excessive fluorescein in the vertical alignment zone (see blue arrow). In this case the patient needs a toric periphery for improved treatment zone and alignment curve seal and comfort.
CAPTION: In this case the fluorescein photo shows excessive clearance in the vertical alignment curve indicating the use of a Forge Myopia Toric lens.
The optical analysis of both lenses is shown below. In the flat meridiant the Jessen factor (or ‘overshoot’) is shown as the ideal back-vertex power. With these lenses this is approximately R +0.75D, L +0.50D. (note that the steep meridian will always show a higher ideal BVP with a spherical back optic zone radius).
For simplicity in a rotationally symmetrical lens the ideal BVP is averaged between the principal meridians to come up with the power of the lens, in this case R +1.00D, L +0.75D. This will provide reasonably clear vision for the patient when wearing the lenses.
Dont forget that the BOZR can be altered depending on the situation eg. you may decide to give a presbyopic patient a lens with a steeper BOZR in one eye to undercorrect their myopia and give more of a monovision correction. A higher correction (eg -5.00D) in a young patient with ample accommodation may need extra Jessen factor or ‘over-shoot’ of around 1.25D or more to achieve full-correction and maintain clear vision throughout the day.
Eyespace will automatically calculate this for you.
CAPTION: EyeSpace automatically generates an Optical Analysis graph for each lens design. The ideal Back Vertex Power (BVP) graph line denotes the Jessen factor or overshoot power used for these particular Forge Myopia lenses designed for the patient.
Due to the difficulties with this patient’s proximity to our practice we taught this patient to insert and remove lenses from an Orthokeratology trial set at her initial visit. Once the lenses had been custom designed and ordered we couriered them to the patient and scheduled a follow-up visit 1 week after her first night of wear. She was given strict instructions to attend her local optometrist if she developed any pain (other than the slight irritation experienced during the teach appointment), redness, light-sensitivity or discharge during this period.
Fortunately her first week went smoothly and she presented seeing 6/5 unaided in each eye with no discomfort or handling problems. Her vision had stabilised after 3-4 days and was offering good vision throughout the day, and only mild halos around lights at night. Her residual presciption showed +0.25D in each eye. Both corneae were clear of any insult.
The topographical analysis in EyeSpace shows a well centred treatment zone in both eyes with a maximum axial dioptral power change of R -3.3D, L -3.5D. Maximum treatment zone sizes were R 5.6mm and L 5.8mm from 0D to 0D. In reality it is probably more useful to see that from the graphs there is approximately a 4mm zone of more than -3D of change, meaning this patient’s quality of vision and contrast sensitivity should be reasonable.
The treatment zone is still a little patchy but should smooth out with another 2-3 weeks wear, this should decrease her night vision symptoms. We see a good symmetrical seal in the post-wear maps showing these rotationally symmetrical lenses are fitting well and do not require modification.
Importantly there is approximately 6D of peripheral corneal steepening relative to the centre of the pupil to provide the myopia control properties desired by this patient. The patient was very happy and was asked to return 4 weeks later for routine follow-up.
CAPTION:EyeSpace axial power difference map showing the myopia correction of the right eye.
CAPTION:EyeSpace axial power difference map showing the myopia correction of the left eye.
This is a relatively simple case showing the correction of moderate myopia with a Forge Rotationally Symmetric orthokeratology lens.