Many orthokeratology lens designs suggest that in determining a patient’s suitability for orthokeratology the astigmatic component of their prescription should be no more than half of the spherical component. With the availability of the Forge Myopia Toric Orthokeratology lens design, this no longer holds true.
This case shows how a patient with principally astigmatic refractive error can be corrected with non-rotationally symmetrical orthokeratology lens and get clear vision throughout the day.
A 33 year dentist visited our practice curious if orthokeratology would be appropriate for him. He had never worn contact lenses before and was motivated as his glasses would frequently fog up and slip down during his work.
His spectacle prescription was:
R: -0.25/-1.50 x 170 (6/5),
L: -0.25/-1.75 x 009 (6/5).
Unaided vision was R 6/7.5, N4 (40 cm) and L 6/10, N8 (40 cm).
Topography shows regular with-the-rule astigmatism with ΔSimK R -1.80D, L -2.20D which is only slightly more corneal cylinder than the spectacle astigmatism present. Given that the cornea closely matched with his refraction we proceeded to design an orthokeratology lens for each eye using Eyespace. Trial fitting a spherical lens on eye will be of little use in this instance as the lens design will need to fit very differently in each meridian to seal the cornea correctly and mold the cornea accurately.
The right lens parameters are as follows:
- Forge Myopia Toric: Diameter 11.80 mm (this patient has a large cornea with HVID 12.40 mm).
- Flat meridian: BC 8.10 mm, Z-Zone 245um, AC 8.10 mm
- Steep Meridian: BC 8.20 mm, Z-Zone 305um, AC 7.60 mm.
The simulation in Eyespace is shown above. Because the spherical component of this patient’s prescription is almost zero we do not require much flattening of the cornea horizontally. As a result our lens has a tear profile very similar to an alignment fit lens in this meridian. In the steep meridian however we need to correct -1.75D of myopia hence the conventional orthoK tear profile pattern.
Research suggests that fitting a lens with a spherical base curve will correct ~50% of the corneal astigmatism present (Mountford, et al 2002), however our experience is that this percentage is closer to ~70%. In this instance we need more than 1.25D of astigmatism corrected so the steep meridian’s BC is 0.10 mm flatter to ensure full astigmatic correction. This is shown in the optical analysis below which shows a higher ideal BVP (Jessen Factor) along the 70 degree meridian.
In order to stabilise the lens on the eye the alignment curve radii match the underlying corneal topography in the same way as a bitoric RGP would. In this way the Forge Myopia Toric lens can be fully customised with independent toricity in the base-curve, Z-Zone (reverse curve) and the alignment curve.
The left lens was designed in the same way with parameters as follows:
- Forge Myopia Toric: Diameter 11.80 mm (this patient has a large cornea with HVID 12.20 mm).
- Flat meridian: BC 8.20 mm, Z-Zone 245um, AC 8.20 mm
- Steep Meridian: BC 8.30 mm, Z-Zone 310um, AC 7.70 mm.
Lenses were ordered and delivered. The following photos show the lenses on the eye, illustrating the accuracy of the rotation simulation of Eyespace and the NaFl pattern in the steep meridian.
After 1 week the patient was seeing R 6/5, L 6/5+, B 6/4. He was very happy with the vision and comfort was excellent. The residual prescription still showed ~ plano/-0.50DC x 180 however. A tangential difference map shows excellent seal and centration due to the toric peripheral curves.
At 6 week review vision continued to be excellent at 6/5 R+L. Residual refraction was now R +0.25/-0.25 x 180, L plano. Axial difference maps of the right eye shows a characteristic ‘figure 8’ pattern of astigmatic correction.
It is important to note that the change in SimK does not always accurately tell us about astigmatic orthoK change as this value relates to a wider chord than the treatment zone of most orthokeratology lenses. Instead it is best to use the colour scale or click in the respective area of maximum topographical change at each meridian to assess the astigmatic correction.
Here at 1.00 mm from pupil centre we are getting -1.70D change in the steep meridian and -0.50D in the flat meridian, closely matching the residual refraction.
The patient was very pleased with the results and was asked to return in 3 months for routine review.
This report shows how significant astigmatism can now be successfully corrected with orthokeratology by using toric alignment and base curves to control the fit of the lens and the refractive change respectively.
A few situations to be wary of are when the axis of the corneal astigmatism does not match the axis of the spectacle prescription, as this may make stabilising the lens to correct the appropriate refraction difficult. Also be cautious when the spectacle prescription shows significantly more against-the-rule cylinder than the cornea as this would suggest lenticular astigmatism in present, which may be difficult to correct with OrthoK at the level of the cornea.
Much higher prescriptions than the above case can also be well corrected with orthokeratology if you are prepared to get creative!
The below image shows a mixed myopia (steep meridian)/ hyperopia (flat meridian) custom OrthoK lens designed with Eyespace that corrected a patient with a spectacle prescription of R +0.75/-4.50 x 178, L +0.25/-3.50 x 12 to give 6/6+ vision in each eye with no astigmatism present in the residual refraction!
1. Mountford J, Pesudovs K. An analysis of the astigmatic changes induced by accelerated orthokeratology. Clin Exp Optom. 2002 Sep;85(5):284-93.