Orthokeratology Contact Lenses (´ORTHO-K´)
Ortho-Keratology Contact Lenses use an opposite effect compared to other contact lenses
A more recently popular medical specialty application of contact lenses is ´ORTHOKERATOLOGY´ often abbreviated as ´Ortho-K´ . This is a concept that is already known for some decades, but has seen a somewhat wider usage only since about the millennium years.
Ortho-Keratology contact lenses have the OPPOSITE concept in contrast to other contact lens applications. Typically, it is tried to adapt i.e. fit a contact lens as closely as possible to the natural shape of the ocular surface, typically the cornea, in order to omit any change on the ocular surface tissues. With Orthokeratology lenses, however, it is distinctly intended to change the natural outer shape of the eye, typically the cornea., in order to change the light refraction and thus correct a refraction disorder of the eye.
A rigid contact lens can change the shape of the cornea
Orthokeratology takes advantage of the effect that a rigid contact lens can slightly change the outer shape of the cornea - apparently without destroying or altering its integrity and health ... this must, however be taken as a preliminary statement, since the systematic long-term experience with this lens type concept is still relatively limited.
´ORTHO-K´ Lenses are intended to change the shape of the cornea - to Produce Visual correction
The therapeutical change of the outer corneal shape takes several hours and is typically performed over night. The alteration of corneal shape is not permanent but only temporary and remains sufficiently stable during the next few hours of daytime without further wear of this or another contact lens or glasses.
In order to achieve a modification in the outer shape of the already relatively stiff cornea it is clear, that a rigid contact lens must used. To allow the necessary oxygen permeability during eye closure over night, it must be a gas permeable lens. This makes us basically end up with a rigid gas permeable (RGP) contact lens.
High-Tech Ortho-K contact Lenses are FDA approved
The possibility of changing the corneal curvature was apparently observed not long after such lenses came into use with the invention of PMMA/ Acrylic Glass. The use of hard contact lenses became more popular only after the introduction of the first hard corneal PMMA lenses that had a high oxygen permeability (RGP) and could thus be worn for longer hours.
With ordinary rigid RGP lenses, the effect of Ortho-K can be relatively unpredictable but with increased experience over the years, together with computer-assisted manufacturing and fitting techniques, the Orthokeratology procedure and effect can be relatively well controlled today. The FDA approval of an Ortho-K lens for the day-time application was given some years before the millennium and the approval for an Ortho-K lens for over-night use was given shortly after.
Correction can be made within the medium range of refractive errors for several diopters
Because the corrections are made directly at the primary refractive surface of the eye, i.e. the corneal surface, a relatively limited level of curvature change is sufficient.
This can be in the range of micrometers (thousands of a millimeter) which is already sufficient to cause a visible improvement in visual acuity. 10-20 micrometers is e.g. in the range of about one cell thickness of the conjunctiva.
For the correction of myopia (´short-sightedness´) a flattening of the corneal curvature by application of an Ortho-K lens flatter than the original corneal curvature is required. This diffuses the light before entering the eye in order to move the intra-ocular focus point further back onto the retina (please see the figure to the right).
For hyperopia (´farsightedness´) the opposite, i.e. application of a steep lens is necessary to increase the optical power and thus move the focus point from its position behind the retina in forward direction.
With Orthokeratology a correction of roughly plus and minus five diopters or even more is usually possible.
The mechanism of action of Ortho-K is not completely clear
The mechanism of action of Ortho-K contact lenses is not completely clear as yet. Probably different mechanisms act together.
The effect of Ortho-K appears to be restricted to the epithelium of the cornea and does not seem to affect the underlying connective tissue stroma.
The Epithelium of the cornea consists of several, about 8-10, layers of cells, stacked one over the other, like bricks in a wall. The cells are columnar to cuboidal in the basal layer and transform into flat squamous cells at the surface. This constitutes a stratified squamous non-keratinizing epithelium. At the surface the cells get lost into the tear film by exfoliation and respectively there is a formation of new cells in the basal and intermediate layers. The stem cells are located in corneal periphery of the limbus crypts.
It takes several days or more until the full effect is achieved
For a full correction effect of the intended shape of the cornea it takes several days or sometime weeks of, typically, overnight OK-contact lens wear. But when the full effect is achieved, it only requires some hours overnight to restore the minimal re-normalization of shape that may have occurred during day-time.
For short term effects it may be assumed that movements of water (blue arrows in the schematic drawing) within the epithelial cell layer may play a role when it is compressed (red arrow) by an Ortho-K contact lens.
On the other hand, it has been shown, that a respective stimulus from a foreign material on epithelial cells in cell culture, can influence and direct a new formation and migration of cells, i.e. OK-contact lenses may also have an influence on epithelial cell growth.
It may thus be assumed, that the shape change for full effect of Orthokeratology may, at least in part, be caused by a changed pattern in the migration of the amplifying cells in the corneal epithelium. For the necessary regular conservation of the effect overnight, after day-time without the OK-Lens, it may be assumed, that short term changes of water relocation inside the epithelium may play an additional role.
Ortho-K contact lenses require a dedicated clinical specialist and regular follow-up visits
It goes without saying, that the fitting of Orthokeratology lenses must be done by an experienced and specifically educated contact lens specialist and it also requires regular follow-up visits in order to control the health of the cornea and the safety of the Ortho-K lens.
In contrast to refractive surgery, the effect of Ortho-K appears fully reversible
The effect of Ortho-K Contact Lenses appears fully reversible according to the presently available reports. This is certainly also supported by the observation of the users, that the visual correction disappears when they stop to use their Ortho-K lenses overnight for the regular prolongation of the principally temporary Ortho-K effect.
The reversibility of the effect of vision correction by Ortho-K is certainly a principal advantage compared to the irreversible change of corneal shape in refractive surgery.
In addition, the Ortho-K process seems to affect the epithelium only, whereas refractive surgery typically ablates, i.e. removes, the anterior part of the corneal stroma. This includes e.g. Bowman´s Layer that appears to be the strongest part of the cornea due to the highly interwoven meshwork of collagen fibers that occurs here. The anterior stroma also appears to be the most viable part of the corneal stroma because the number of the ´housekeeping´ cells, the keratocytes, is highest in the anterior part of the stroma.