The Basics



There have been and currently are many theories as to how and why the orthokeratology effect happens.  The platform of our OOK retainer the BE is based on the following concepts which have been researched and published by numerous sources.  The conclusions of these published studies make up the basis for the BE which has been scientifically validated with a high degree of accuracy and reproducibility.

Squeeze Film Pressure – When a contact lens is placed on the surface of the cornea, tear layer is “sandwiched” between the lens and cornea.  This tear layer has been proven to exert a force, either negative or positive, when distributed unequally across a surface area.  The tear layer will apply negative force (pull), where it is thickest and positive force (push), where it is thinnest in an effort to achieve equalibrium across the entire surface of the fluid layer.  It is this “squeeze film pressure” that we use to shift the epithelium. 

The single most important concept to understanding the mechanics behind the BE OOK retainer is that by controlling the squeeze film forces we can bring about the desired epithelial displacement.

Epithelial Displacement – The corneal epithelium is approximately 50um thick and is composed of 5-6 layers of cells, which studies indicate will compress and displace.  Additional research shows that the epithelium has a Young’s Modulus of elasticity an order of magnitude less than the stroma.  AOK works on the concept that if we “sandwich” the fluid epithelium and the tear layer between the rigid AOK lens and the impressible Bowman’s membrane and stroma, the result will be a change in the profile of the epithelium.  In other words, the rigid lens does NOT change shape, the rigid stroma does NOT change shape, tear layer is incompressible and applies additional forces (squeeze film force), when distributed unevenly between two surfaces.  The result: the epithelium is the only compressible component and must change in shape. This TEMPORARY epithelial shifting creates our refractive change.

In a paper by Dr. Mountford et al, it was learned that because epithelial cells are primarily fluid, they will behave as a Newtownian Fluid and obey Pascal’s principle.  Pascal’s principle states that an applied force will distribute pressure evenly throughout an enclosed fluid system.  However, if an applied force is distributed unevenly on the epithelial surface for example, then squeeze film forces will shift tissue until a state of equilibrium is reached.  When pressure distributions over aspheric surfaces are allowed to find equilibrium, they always do so over the smallest surface area – a sphere!

Because the volume of tissue does not change, there is displacement from the spherically flattened central area flowing para-centrally (away from the apex).

When this pressure differential is introduced across the epithelium, the displacement of tissue from the apex towards the periphery results in a thinning of the central epithelium by up to a maximum of 20um.

Treatment Zones – With a maximum of 20um tissue displacement at the apex, further reduction of the apical radius and therefore further reduction in refractive power is only achievable over a smaller treatment diameter.  Many practitioners knowledgeable in PRK and LASIK procedures are familiar with Munnerlyn’s formula which is used to calculate the AOK treatment zone:  Ablation depth (um) = Rx (X) Diameter2***(mm)/3.

 

Spherical vs. Eccentric Corneals – OK involves the “sphericalization” of the cornea.  When apical epithelial cells shift due to tear pressure forces, they do so over the smallest possible surface area – a sphere.  However, if the pre-therapy cornea is very spherical in curvature from the apex towards the limbus, limited cell displacement can be achieved. 

This is NOT a reference to the patient’s keratometric readings.  Do not confuse spherical corneal curvature taken from K-readings with sphericity or eccentricity of the cornea as measured by a topographer along the horizontal axis, from the apex towards the limbus, to a specific cord diameter.

Orthokeratologists use Eccentricity (E-value) as an effective mathematical description of the corneal shape from apex to periphery.  It is important to confirm that a patient exhibits a minimum Eccentricity (E-value) before considering AOK therapy for a patient.  The average patient has a 0.40 – 0.45 positive e-value while a truly spherical cornea from the apex to the limbus would have a 0.00 e-value.  Therefore, a practitioner will know before performing AOK therapy on a patient whether or not they are a candidate to begin with.  Do not perform AOK on patients that exhibit low E-value’s with high Rx’s.

In numerous studies on the subject, Dr. Mountford found there is a linear relationship between eccentricity and the degree of shift in refractive error for a given corneal curvature:

The relationship between corneal eccentricity (E), apical curvature (Ro) and refractive change.

E=0.40 E=0.50 E=0.6
Initial RO Rx Reduction Rx Reduction Rx Reduction
41.00 -1.39 -2.08 -2.86
42.00 -1.50 -2.23 -3.07
43.00 -1.62 -2.41 -3.30
44.00 -1.75 -2.60 -3.54
45.00 -1.88 -2.79 -3.79
46.00 -2.02 -2.99 -4.06
Figure 1

Orthokeratology involves the sphericalization of the cornea.  It is important to remember that a cornea that is spherical in curvature from the apex to periphery (Zero or Low e-value) can achieve little or no refractive change.  Conversely, a corneal that is aspheric (High e-value) can shift a large amount of epithelial cells para-centrally (towards the limbus) resulting in a larger refractive change.