Orthokeratology (often abbreviated to ortho-k) is the use of specifically designed rigid gas permeable contact lenses to alter the shape of the cornea in order to reduce or eliminate low to moderate degrees of myopia and astigmatism. Ortho-k lenses are worn at night and then removed in the morning. This makes them especially suitable for people working in dusty atmospheres or taking part in activities such as water or contact sports. There is also increasing interest in the potential use of the technique in reducing myopic progression in young patients.

Overview

The lenses have a moulding effect on the cornea by a process of epithelial redistribution. This is not permanent, hence the need for nightly wear. If the lenses are left out, typically for two or three consecutive nights depending on the original refractive error, the original prescription returns.

It can take up to seven nights for the lenses to fully correct the refraction and up to a month before the correction is stable for a useful period of time. On average, 60-70 per cent of the correction is achieved after the first night of wear; however, some people are more successful with ortho-k lenses than others.

As the effects of ortho-k lenses are fully reversible, they are a good compromise for people considering corrective laser eye surgery but who have reservations about the permanent change in corneal shape and thickness. The patient is correction-free during the day and only wears the lenses when sleeping, reducing the possible discomfort of dry eyes or variable vision during the day.

Most successful prescriptions tend to be between -1.00DS and -4.50DS and up to -1.50DC, but it is also possible to correct up to -6.00DS and, with specialist toric periphery ortho-k lenses, up to -2.50DC.

History of ortho-k

The first ortho-k designed lenses were made from PMMA material in the 1960s by George Jessen and marketed as Orthofocus. The early designs were unpredictable and led to the belief that ortho-k was more luck than science. Many groups and individuals claim to have been the first to develop the modern ortho-k solutions. Dr Richard Wlodyga and Nick Stoyan, in particular, are generally credited with developing the first reverse zone lens design in the 1990s.

It was not until computerised corneal topography became available in the 1990s that it was possible to apply the theory to create designs with repeatable results. Topography is able to accurately map the corneal surface curvature using a non-invasive, painless imaging procedure (Figure 1).

[CaptionComponent="3000"]

Modern ortho-k has developed using multicurve reverse geometry lenses that can be made by computerised sub-micron lathing machines using more stable and breathable materials.

We also have a better understanding of how the corneal shape changes using video topography and numerous worldwide studies have confirmed the effectiveness of modern ortho-k.

Why fit ortho-k lenses?

Ortho-k patients tend to be loyal customers because you are the eye care professional who enabled them to have correction-free, comfortable, clear vision during the day. Furthermore, as a planned replacement programme, practitioner and patient develop a good rapport early on in the treatment.

Myopia control

Ortho-k has also been shown in a number of longitudinal studies to reduce the speed of myopic progression in children. These studies include:

  • LORIC (Hong Kong) – In this study (Longitudinal Orthokeratology Research in Children), it was found that axial length changes were reduced by 50 per cent with ortho-k when compared with children corrected with glasses.1

  • CRAYON (Ohio State University)

    – This study (Corneal Reshaping and Yearly Observation of Nearsightedness) showed that correction with ‘normal’ RGP contact lenses or soft contact lenses result in the same change in axial length, whereas eyes fitted with orthokeratology lenses had an average of almost 60 per cent reduction in change in axial length. 2

  • ROMIO (Hong Kong).3

  • MCOS (Spain).4

  • SMART (USA).5

  • ROK Study (Australia).6

As myopia is the most common worldwide visual problem, ortho-k is proving a promising option for those wishing to influence myopic progression. By age eight in Europe and the US, approximately two per cent of children are myopic. This rises to about 30 per cent by age 12. The corresponding figures in Asia are much higher, with 12 per cent of eight-year olds being myopic and up to 90 per cent of 12-year-olds.7

Many other studies have looked into the effect of daylight, close work and computer use on children’s vision, but one thing is clear – myopia is an increasing visual problem.8

Who can fit ortho-k lenses?

Anyone who has experience in fitting rigid gas permeable (RGP) contact lenses can fit ortho-k lenses. The key requirements are a video topographer (I would suggest this is essential) and available chair time. The topography pictures are required for the initial fitting and for monitoring the progress of the corneal flattening and lens fitting. The calculation for the lens geometry may be carried out by specialist computer software and requires no advance computing knowledge.

How are ortho-k lenses made?

Ortho-k lenses are RGP lenses with a flat central zone and a reverse geometry curve. They have a similar shape to a post-Lasik cornea (Figure 2).

[CaptionComponent="3001"]

Component features are:

  • Flat central zone – this provides central positive pressure to help flatten the central cornea to achieve the required correction. This zone is made as large as possible to reduce the chance of haloes but will be smaller with higher levels of myopia.

  • Reverse curve zone – this provides a negative pressure to steepen the corneal mid-periphery. The reverse zone is a step-free connection between the flat central zone and the peripheral alignment zone. This curve is mathematically calculated to avoid a change in the sag of the lens.

  • Peripheral aspheric zone – this gives an alignment fit to the peripheral cornea. It is calculated in the same way as a normal aspheric RGP lens and is defined by the base curve of a ‘normal’ lens and the eccentricity of the cornea.

  • Bevel – this gives edge lift to allow free tear exchange under the lenses and to aid removal in the morning. It is usually around 0.5mm wide.

  • Total diameter – the total diameter of ortho-k lenses is around 0.5mm smaller than the vertical corneal diameter and is usually between 10.5mm and 11.0mm.

The tear film in the centre is very thin (less than 20 microns) and becomes thicker towards the mid-periphery and lid margins. The lens should not sit on the central cornea and must have a thin tear film underneath to enable the positive and negative pressures (that promote epithelial redistribution) to work correctly. A typical ortho-k fluorescein picture is of a central ‘touch’ zone (dark) with concentric rings of mid-peripheral clearance (bright), peripheral alignment (dark) and edge lift (bright) centred on the pupil (Figure 3).

[CaptionComponent="3002"]

If the tear film thickness is measured from the centre to the edge of the lenses there is a significant difference in the measurements (Figure 4). In the centre there should be a maximum of 10 microns, whereas in the tear reservoir there will be about 40 microns. It is usually not possible to view a fluorescein stained tear film thinner than 20 microns and Figure 4 shows why the typical concentric circles picture occurs. Everything above the red line is visible and everything under the red line is dark. The different tear film thicknesses cause pressure changes across the cornea and result in the required change in corneal morphology.

[CaptionComponent="3003"]

Lenses are fitted with peripheral alignment and have a larger diameter than most RGP lenses. The sag of an ortho-k lens will be the same as that of a normal RGP lens designed to fit the same cornea.

Materials

Ortho-k lenses are now more usually made from materials with high oxygen permeability, such as Boston XO or Boston OX2. These materials are suitable for extended wear because they meet the permeability required for wearing lenses with a closed eye (Boston XO has a Dk of 100 and Boston XO2 a Dk of 141).

These newer high-permeability materials require intensive cleaning. As the lenses are worn at night, there is no lid movement across the lens during wear and therefore they can be prone to surface deposition. A daily surface cleaner suitable for RGP lenses (such as Boston Advance, Total Care) should be used every morning. The lenses can then either be stored in a suitable RGP soaking solution or disinfected using a one-step peroxide system (such as AOSept or Easysept). Regular protein removal is also recommended and is something the practitioner needs to advise on as wear continues. If the recommended care regime is followed, it should be possible to wear the same lenses for up to 12 months.

To tell the difference between the right and left lenses most suppliers use different colours. As the lenses are not usually worn during the day, the cosmetic impact of the different colours on light eyes is not noticeable.

Patient selection

The fitting and aftercare of ortho-k lenses requires a significant amount of chair time, which the patient and practitioner must be prepared for. The patient must understand what happens to their eyes while wearing the lenses and confirm full compliance with the fitting and aftercare procedure. It is only through regular wear of the lenses and reliable attendance of aftercare appointments that they will achieve their desired freedom from correction during the day. You cannot have part-time correction with ortho-k lenses – it is all or nothing. When the lenses are not worn, the myopia will return.

Myopia is corrected during the night-time wearing of the lenses but, in the first few days or weeks, the full correction may not be maintained for the whole day. It is important that the patient does not have too high an expectation of the lenses in the early days. However, there is usually a very positive response from the patient when, after a few nights, they no longer have to wear an optical correction for most of the day. The ‘wow’ effect may not be the same as an initial response after Lasik, as the end point is reached in progressive steps. A 60-70 per cent reduction in correction might be expected after the first night of wear, meaning a -3.00DS myope may only require a -1.00DS correction after one night. During these early days, the residual correction can easily be corrected with daily disposable lenses.

Fitting

The first fitting appointment should include a full history and symptoms (including medical, ocular and contact lens wearing history), refraction, corneal radius measurements, topography and a slit lamp examination (with obvious particular emphasis on the front surface of the eye with and without fluorescein). If the patient is currently an RGP wearer, they should leave their lenses out for 10-15 days before the fitting appointment to ensure the cornea has returned to its ‘original’ shape. Most practitioners recommend less so for soft lens wearers.

The information from this initial appointment and the data collected should help to decide upon the suitability of the patient for ortho-k fitting. It is also a useful time to make an initial judgement about how realistic the patient expectations are and it is important at this stage to keep the patient informed of the exact nature of the process and likely outcome – all supported with careful record-keeping of what has been recommended.

Below we outline the ideal fitting profile:

  • A 4-5mm diameter centrally flattened zone that is centred on the pupil to give good visual acuity and contrast sensitivity in normal lighting conditions

  • A concentric, regular steep ring zone in the mid-peripheral cornea around the central zone. The more regular the ring, the better the lens centration

  • A peripheral cornea whose geometry is unchanged

After the fitting appointment the following criteria should be considered as likely to be suitable for ortho-k:

  • Spherical prescription of between -0.75DS and -4.50DS.

  • Cylindrical prescription up to -1.25DC ‘with the rule’ or -0.75DC ‘against the rule’.

  • Astigmatic spectacle refraction should be the same as the corneal astigmatism. Like fitting RGP lenses, any lenticular astigmatism may reduce the visual acuity and quality. Contraindications for ortho-k include:

  • Higher levels of myopia might not be easily corrected due to the initial corneal eccentricity and required epithelial reformation requirement.

  • Larger pupils (notably with higher refractive error).

  • Very flat corneas (flatter than radii of 8.20mm).

  • Corneal irregularities – as might be caused by, for example, keratoconus, dystrophies, pterygia.

  • Very dry eyes – keratoconjunctivitis sicca (rheumatoid arthritis is a contraindication, as might be some tear-affecting medications).

  • Poor re-epithelialisation rates (for example in diabetes or with some medications).

The most suitable patients are myopes who have:

  • High motivation to be correction-free (professional or occupation demands, sport, leisure).

  • Requirements to take part in activities that are affected by wearing glasses or contact lenses.

  • An interest in, but concerns about, refractive surgery.

  • Myopic progression, especially in younger children.

Success may also be improved if the patient has a full understanding about how ortho-k works, shows good compliance, and is willing to attend the frequent aftercare appointments in the first few weeks and later attend half-yearly aftercares. They should also be familiar with the expectation of changing their lenses at least once a year.

Ordering the lenses

The lenses are individually made to the specifications required by each patient. When all the required information has been received by the supplier they will calculate the parameters and produce the lenses. In about 80 per cent of patients, the first lenses will be the correct fit and provide the correction required. If the patient has been correctly selected and all the measurements accurately taken, it is only necessary in a minority of cases that the lenses need changing.

To order the lenses the following information is required:

  • Refraction.

  • Back vertex distance.

  • Central corneal radii.

  • Total eccentricity at 30 degrees.

  • Eccentricity in the four meridians at 30 degrees.

  • Horizontal and vertical corneal diameter.

  • Topography picture and the make and model of the keratographer used. Now you just need to wait for the lenses to arrive. Conclusion This introduction to the topic will be followed by two further articles detailing patient progress and reflect real life patients.

A more detailed course in ortho-k technique and practice will appear next year as part of the new CET cycle. Look out for details.

Read more

Introduction to Orthokeratology – lens collection and aftercare

Experiences of orthokeratology – revealing case studies

Lisa Crouch is a UK qualified optometrist practising at Visilab GlattzentrumSwitzerland and specialising in orthokeratology and complex contact lens fitting