Before digging deeper into this topic, clarity is needed on the difference between corneal toricity and corneal astigmatism. When using the term toricity we are referring to the physical shape of the cornea and specifically how corneal curvature differs between the flat and steep meridians that are perpendicular to each other. In general, this will be the difference in curvature between the flattest meridian and the curvature along the meridian 90 degrees from the flat meridian. Astigmatism is a measure of how this difference in curvature between these two principal meridians (toricity) alters refractive power to give a dioptric cylinder refraction.
In a nutshell, toricity is a measure of shape (measured in mm) and astigmatism is a measure of refraction (measured in dioptres). This is where some of you might shout out that you use dioptres as a measure of shape with higher dioptric power indicating a steeper shape. While this is technically correct from a visualization perspective, this concept is meaningless from a geometric perspective as dioptres are not a unit of length. It is not possible to measure out a radius of say 42.00D. To allow geometric modeling the dioptre measure needs to be converted to a length in mm length using the keratometry formula: mm = 337.5/dioptre. In this example 337.5/42.00D=8.04mm. To convert back the other way just exchange the dioptre for the mm, so dioptre=337.5/mm.
Regardless of your preferred measurement unit, the reason for thinking about toricity being a shape measure and astigmatism a refractive measure is that toricity influences how the lens will fit while astigmatism is the refractive component that needs to be corrected. In this post I am going to concentrate on corneal shape, so toricity. In general, when it comes to Orthok fitting, toricity can be further defined as falling into one of two categories: Central toricity or Limbus-to-limbus toricity.
Central toricity
In central toricity the difference in curvature between the two principal meridians is limited to the central zone and outside of this zone the cornea has a similar radius of curvature between the two principal meridians. The best way to visualise this is either using the axial curvature or refractive topography map where you will see the bow tie astigmatic pattern is restricted to the central part of the map surrounded by an annulus of close to uniform green (see above image). What we are most interested in, however, is the difference in sagittal (sag) height between the flat and steep meridians in this outer zone of the cornea. If you are at all unsure about sag height and how it is used in Orthok lens fitting I suggest you read my earlier post on the topic.
Sag height is a factor of curvature, which means that in the presence of corneal toricity the steep meridian will have a deeper sag height than the flatter meridian. In central toricity, this differential is limited to the central zone of typically around 6mm diameter, and outside of this zone, the cornea returns to being more spherical and hence similar sag height between the two principal meridians. The reason this is important is that your success in Orthok lens fits is largely dependent on the uniformity in sag height in this outer zone, which is where the peripheral landing zone of the lens rests on the cornea.
Limbus to limbus toricity
In the alternative case of limbus-to-limbus toricity, as the name suggests the astigmatic bow tie pattern seen in axial curvature and refractive topography maps will extend out to the periphery. This is a good time to observe one of the fundamental differences between the axial map and tangential map views because if you switch to a tangential view you will see the bow tie appearance shrink back towards the central zone of the cornea. As a result, it can be very difficult to categorize central from limbus-to-limbus toricity using tangential maps which is why you should always use axial curvature or refractive maps for this purpose.
When considered as sag height, the difference in sag height between the two principal meridians continues out to the limbus. This means that when you are fitting a rotationally symmetrical lens, which will be the case if fitting a standard (non-toric) lens design, the landing zone of the lens will rest on a hill between the flat and steep meridians. If the sag height of the lens is calculated to align with the flat meridian, then it will be too shallow for the steep meridian and the lens will rock. When this occurs, the Ph.D. research of Dr. Vinod Masseedupally from the University of New South Wales shows that the lens will tend to decenter and likely lead to poor vision outcomes.1
Assessing type of corneal toricity
Determining corneal toricity as either being central or limbus-to-limbus from a visual appraisal of topography difference maps is fine categorically, but it doesn't help evaluate the likelihood of lens fit success or decide what type of lens to use. Instead, the most useful measure is the difference in sagittal height between the flat and steep principal meridians of the cornea at a chord length that coincides with the peripheral bearing point of the Orthok lens being fit.
In a previous post, I covered how to retrieve corneal sag height and weighted corneal sag height measurements from your corneal topography image capture, with the weighted being the better measure to use if available. To assess the effect of corneal toricity on sag height at the lens bearing point, simply record the sag height measurement along the flat meridian at the lens bearing chord and then do the same along the steep meridian and subtract the former from the latter to establish the difference. However, you will often find that there is insufficient captured data along the steep meridian due to this in most cases being close to 90 degrees where the upper and lower eyelids get in the way.
If this is the case your topographer will return an ‘out of range’ error and you will need to gradually reduce the length of the measurement chord until sufficient data is found to calculate a measurement. To create a valid flat/steep sag height difference you will now need to recalculate the sag height for the flat meridian across the same chord you end up using for the steep meridian and then subtract to calculate the difference. The measurement chord now falling short of lens bearing point is less than ideal but will be the best you can achieve with the data you have to hand. To give you more accuracy in determining toric corneal shape is the main reason for advising you to adopt strategies that maximise the area of corneal topography image capture at measurement.
Lenses to fit
For corneas with central toricity, only spherical (standard) lens designs will work. A toric lens won't work because the peripheral part of the cornea, where the landing zone of the toric Orthok lens rests, will be close to spherical and therefore misalign to its toric back surface. A spherical lens will fit in much the same way as if it were fitted to a spherical (non-toric) cornea, but will likely only be effective in correcting up to around -1.50D of corneal astigmatism.
Limbus to limbus corneal astigmatism can usually be successfully fit with standard spherical lenses up to around -1.50D but will become unstable and likely to decenter if fitted beyond this amount.1 In this case toric lenses offer a superior solution, however, the better measure to use for determining whether to fit a toric design is the difference in sag height between the flat and steep corneal meridians at a chord that coincides with the peripheral landing zone of the lens. A difference of around 50µm and beyond currently being voiced by experienced Orthok lens fitters as when a toric rather than spherical design should be used.
Limits to accept when starting out
If you are starting out I wouldn’t worry too much about your ability to detect central from limbus-to-limbus corneal toricity by topography map appearance. Instead just measure the sag height of the cornea in the two principal meridians at the bearing chord of the lens that you are going to use as described above, and if the difference between the two is greater than 50µm then consider it be a cornea that has limbus-to-limbus toricity and therefore one to be avoided until you have some experience on board.
References
- Maseedupally VK et al. Treatment zone decentration during orthokeratology on eyes with corneal toricity. Optom Vis Sci 2016;93:1101-11
About Paul
Dr Paul Gifford is a co-founder of Eyefit, an information resource to assist contact lens practitioners in all modes of practice. Learn more about him here.
