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5. Correcting toric prescriptions with Ortho-K

6. Orthokeratology TroubleshootingJagrut Lallu

The main topics that this article will cover are:

  • The assessment of baseline corneal astigmatism and spectacle astigmatism and Javal's Rule. Typically the internal astigmatism is -0.50 to -1.00 x 90, however, please note that internal astigmatism increases with age. This is useful to know when correcting toricity in presbyopia.
  • The calculation of resulting post-Ortho-K corneal astigmatism required to give no residual astigmatism.
  • The difference between correcting against-the-rule (ATR) vs with-the-rule (WTR) astigmatism.
  • The minimum amount of corneal astigmatism that a toric lens requires to be stable.

Before we get started:

The rules for toric orthokeratology that all practitioners should be aware of are:

  • With a rotationally symmetric (RS) Ortho-K lens on a toric cornea, 50 - 60% of corneal astigmatism will be corrected.
  • With a spherical back optic zone and toric periphery, 70 - 82.5% of corneal astigmatism will be corrected.
  • Toric BOZR's can be used to control the amount of correction in each meridian.

Assessment of Corneal Astigmatism vs Spectacle Astigmatism and orthokeratology correction

Early reports of toric orthokeratology correction showed up to 60% of WTR corneal cylinder correction using rotationally symmetric back optic lenses (). Pseudovs and Mountford carefully selected a sample of patients with with-the-rule astigmatism between 0.50D and 1.75D (). These studies were performed on central astigmatism and not limbus to limbus astigmatism.

The left-hand image below depicts a WTR cornea with limbus to limbus astigmatism, whilst the right image depicts an oblique apical astigmat. The major change in astigmatism within apical cases occurs within the central 2.00 mm chord.

RS Ortho-K lenses are therefore limited when correcting against-the-rule and limbus to limbus astigmatism. With EyeSpace Forge custom designed Ortho-K lenses, it is possible to correct all forms of astigmatism, including WTR, ATR, Oblique and limbus to limbus correction.

EyeSpace automatically calculates the need for:

  1. Spherical or Toric Optic Zone Radius
  2. Spherical or Toric Z-Zone
  3. Spherical or Toric Alignment Curve

However, the practitioner needs to assess candidacy of the patient to determine overall success.

Assessing a patients candidacy for toric orthokeratology

Some key points to note when assessing candidacy for toric prescriptions and orthokeratology are:

  • Axis: spectacle astigmatism, corneal toricity and the axis of the flat meridian of the lens must be within 10 degrees
  • If the cornea has no toricity and the spectacle astigmatism is visually significant then the patient is not a candidate for orthokeratology
  • If the above are met then the BOZR can be calculated to target the power correction in each meridian

Sometimes there are cases where we need to calculate the spectacle astigmatism in order to decide whether orthokeratology will work with a patients case. It is useful to be mindful of Javals rule. Where the spectacle astigmatism is calculated as:

Spectacle cylinder = 1.25 x corneal cylinder + (-0.50x90)

It is worthy to note that the "-0.50x90" can increase with age as this is the component of astigmatic change related to lenticular cylinder.

An example of when this rule may be applicable to a case is when we do not have a subjective refraction and we need to calculate the spectacle cylinder from our retinoscopy results and corneal topography.

Consider this example

The graph called the optical analysis shows three lines in the bottom half. To zoom into this simply draw a box using your left mouse clicker.

The ideal back vertex power, OR (ocular refraction) and Tear Power are shown. The numbers next to the word Tear depict the degrees of the meridian we are looking at and the actual power at that point. This can be seen by hovering over any part of the curve. If we look at the flat meridian (9.1/215/7.65) and lens parameters you will see a -4.087 change.

If we look at the second image, this power remains the same at -4.085 (or near enough) when the steep meridian is altered to make the central optic spherical.

If we look at the meridian 90 degrees away from this:

We can clearly see that the 9.60 mm base curve induces more myopic correction at -9.417D. This tool helps us to make sure that we are not only targeting the correct amount of astigmatism but also lets us know the amount of overcorrection or Jessen Factor required so that the patient can maintain clarity all day.

The image below shows the change in power when the steep meridian has been reduced from 9.6 to 9.1mm. This is approximately equal to a change of 2.00 D. In general with each 0.1mm of base change we note a 0.50 D change in prescription. However, this rule of thumb changes around the base range. The actual change which EyeSpace calculates, in this case, is 1.922 D.

Against the rule cylinder and with the rule cylinder

When correcting with the rule cylinder we are aiming to flatten the steep meridian (vertical) more than the horizontal to create a more astigmatic correction. With EyeSpace it is also possible to correct against the rule astigmatism.

Consider the following case:

A patient has a spectacle refraction of -0.50/-1.75x90

The corneal cylinder shows 0.84D x 87.6

This is approximately half of the spectacle cylinder

Javals rule yields:

(.84D x 87.6) X 1.2 + (0.50 x 90) = 1.508D total refractive cylinder.

In order to approach this lens design, we need to correct approximately double the corneal cylinder to achieve the spectacle cylinder correction.

Design considerations

Given that the prescription is only very slightly myopic (-0.50D), the flat meridian (vertical), effectively needs minimal correction (a design that looks like an alignment fit rigid contact lens). This is described below:

The steep meridian (horizontal) requires myopic orthokeratology correction only to correct the refractive cylinder. As per below:

The areas in blue indicate where the lens is bearing most on the cornea. We can see from the simulation that this lens is evenly balanced on all meridians. For corrections like this, it is important to utilise the advanced tilt functions. In order to check to see if this design will work we need to review the optical analysis:

If we look at the flat meridian we see that the spectacle target is -0.50D and the tear power target is + 0.317D. The steep meridian shows a spectacle refraction -1.75DC. We need an overcorrection (Jessen Factor) so that this mould does not regress during the day. The target for this case is-5.259DC. That is an overcorrection of -3.509D.

The refractive result shows a -2.50DC at a one month review

We can also consider whether we can mould a spherical prescription on a toric cornea. In order to answer this question, we need to look at what we are actually creating optically by moulding a toric cornea with a spherical prescription.

The corneal cylinder (or Delta K) and refractive cylinder must have some relationship both in power and axis for orthokeratology to succeed. If a significant discrepancy exists between the corneal cylinder and refractive cylinder another internal component in the eye is introducing a cylinder which is referred to as internal astigmatism. We utilise Javals rule to decide whether the patient has an ability to succeed here.

For most eyes, the corneal cylinder is largely responsible for the refractive cylinder, and by sphericalising the corneal cylinder with the Forge Ortho-K lens, the refractive cylinder is fully corrected.

If an internal cylinder is present in the optical system and the corneal cylinder is removed by the ortho-k mould, the internal cylinder will remain and will, in fact, increase the post fit ortho-k refractive cylinder (this is not common but must be considered).

Consider the case below. The corneal cylinder or Delta K is 3.53D but the patient's spectacle prescription is -3.00/-0.25 x 10. This means internal astigmatism of -3.25D is present in the optical system which corrects for the remaining -3.25D of corneal astigmatism.

When designing the Forge Ortho-K lens, EyeSpace will automatically choose to design a Forge Myopia Toric Ortho-K lens. The lens will mould the corneal shape to correct for the entered spectacle prescription and will ALSO neutralize all the corneal astigmatism present.

When the orthokeratology mould neutralizes all the corneal cylinder, the -3.25D of the internal cylinder will manifest in the post fit ortho-k refractive power.

Warning

If you introduce more than one diopter of residual astigmatism when correcting myopia, you will have a qualitative reduction in visual acuity, and it is not advisable to prescribe orthokeratology lenses.

Final acuity and refraction

It is also worthy to note that when assessing the need to remake a lens - it is not always possible to achieve a plano final result. If you are achieving 6/6 or 20/20 and have a small residual cylinder, for example, plano/-0.50x80 then it is best to not remake the lens. Always look at visual acuity as well as refraction.

References

  1. An analysis of the astigmatic changes induced by accelerated orthokeratology. 2002 Sep;85(5):284–93.
  2. Mountford J. An analysis of the changes in corneal shape and refractive error induced by accelerated orthokeratology. Int Contact Lens Clin 1997;24:128–143.
  3. Toric double tear reservoir contact lens in orthokeratology for astigmatism. 2012 Jul;38(4):245–51.
  4. Beerten R, Christie C, Sprater N, et al. Improving orthokeratology results in astigmatism. Paper presented at Global Orthokeratology Symposium; July 2005; Chicago, IL.
  5. Baertschi M. Short and long-term success with correction of high astigmatism in OK. Paper presented at Global Orthokeratology Symposium; July 2005; Chicago, IL.