INTRODUCTION

Reverse geometry lenses are synonymous with modern overnight orthokeratology. However, this design concept has various applications now incorporated in corneal rigid gas permeable (RGP) and scleral lenses for irregular astigmatism, keratoconus, pellucid marginal degeneration, Terrien’s marginal degeneration, keratoglobus, post-keratoplasty and refractive surgery patients.

WHAT IS A REVERSE GEOMETRY LENS?

In a conventional lens design, the base curve is the steepest curve and the remaining curves are progressively flatter. This design is intended for the contact lens to appose the typical corneal shape where the cornea is steepest at the apex and flattens to the periphery. However, in a reverse geometry lens design, the reverse curve (the second curve after the base curve) is made steeper than the base curve while the remaining curves are progressively flatter. 

HOW DID THE REVERSE GEOMETRY LENS COME ABOUT?

Interest in the reduction of myopia with rigid lenses began in the late 1950’s when practitioners noticed that rigid contact lenses were not only controlling the progression of myopia in children, but also producing changes in the corneal curvature, refractive status, uncorrected visual acuity of adult myopic and sometimes astigmatic patients. In the early days of orthokeratology, the polymethyl methacrylate (PMMA) lens was worn during the day and the contact lenses were fitted up to 1.50D flatter-than K based on the patient’s refractive error and the practitioner’s personal preference. This conventional designed lens that was fitted flatter-than K had very limited success and was largely abandoned because of the following problems: oedema, spectacle blur, punctate staining, abrasions, corneal distortion, poor lens tolerance and increased with-the-rule astigmatism. Another reason for the failure of orthokeratology in the 1950s, was that the lens design did not take into account the physical changes of the cornea, especially the mid-peripheral corneal shape change.

In myopia orthokeratology, the shape changes from a positive shape (steeper centrally, flatter peripherally) to a negative slope (flatter centrally, steeper peripherally). By considering these corneal changes, Nick Stoyan and Richard Wlodyga developed the first reverse geometry lens in 1989, which greatly improved the effectiveness of orthokeratology using high DK material. Modern orthokeratology embraces the cornea as a visco-elastic material and as such takes into account corneal rheology which is the branch of science that deals with the flow and deformation of matter induced by stress. Reverse geometry lenses use a balance of push/pull forces and obeys a commonly known fact known as the Principle of Conservation of Energy: “Energy can neither be created nor destroyed, but can only be changed from one form to another”. This fundamental theory in orthokeratology is extended by May, Harris and Nolan to:

“Corneal power and curvature cannot be created or destroyed, only redistributed”

Orthokeratology maintains a constant arc length whenever altering refractive error and adheres to this basic principle. During orthokeratology, the BOZR pushes in (+ force) and the reverse curve pulls the cornea out (-force). In ortho-k the contact lens induces a stress that alters the shape of the cornea. The alteration in the shape is the strain response of the cornea to the stress applied by the contact lens (Figure 4). This stress/strain relationship of the cornea is linear up to a physiologic limit – allows a predictable result. Usually, the BOZR is 0.75 D flatter than the amount of myopia you intend to reduce. The reverse/relief curve is adjacent to the BOZR template and it provides a place for the displaced central cornea to move into and regulate the arc length. The radius of the reverse curve is the sagittal depth of the alignment curve as if it were a complete curve. The alignment curve is used for centration and is the principal zone to assess the fit of the lens. The peripheral curve serves tear exchange function. This design improves the centration and stability of the lens on the cornea.

THE USE OF REVERSE GEOMETRY IN RGP AND SCLERAL LENSES

Patients who are diagnosed with against-the rule (ATR) astigmatism, irregular astigmatism and corneal ectasia can be a challenge to fit with conventional contact lenses. For example, soft contact lenses may not provide adequate visual acuity and standard RGP lenses can decentre or dislodge. The intricacies of piggyback lens systems can be difficult for some patients. In addition, the location or amount of corneal irregularity often makes it impossible to achieve an adequate fit with these contact lenses. A large diameter RGP with reverse geometry offers increased mid-peripheral sagittal height to better accommodate both the large area of ATR astigmatism as well as the decentered corneal apex and provides enhanced lens comfort. If this fails, scleral lens with reverse geometry will work.

Successful RGP fitting requirements for keratoconus patients include: total pupil coverage, preventing lens adherence and decreasing lens awareness. However, to achieve these requirements can often be difficult in keratoconus patients with inferiorly-located (sagging) cones as well as oval cones. The challenges stem from the fact that the steepest portion of the cornea is generally located considerably more inferior than the corneal apex of regular corneas. Because RGPs tend to decentre themselves over the steepest portion of any cornea, they naturally drift inferiorly. This can create a suboptimal fitting relationship if the RGP optical zone bisects the pupil and results in glare and monocular diplopia. Trying to fit with apical clearance often makes things worse, since a classic combination of inferior lens displacement with a steep base curve-to-cornea fitting relationship often results in RGP lens adherence.

Clinical experience prompts the astute clinician to realise that a difference in corneal elevation greater than 325µm (between the highest peak and lowest point of elevation) will lead to limited success with corneal RGP fit stability. RGP lenses fit onto a cornea with these levels of elevation difference will rock on the eye, fall out of place intermittently and often cause discomfort and visual instability in wearers. 

A RGP lens with reverse geometry lens is fitted such that the base curve vaults the apex. This offers increased mid-peripheral sagittal height to better accommodate a decentered corneal apex and provide enhanced lens comfort. Another added feature of reverse geometry is improving overall optics by approximating the optical zone closer to the corneal plane.

Scleral lenses with reverse geometry can also solve many of the fitting problems associated with irregularly-shape corneas. Scleral lenses are large-diameter RGP lenses that vault over the cornea and rest on the conjunctiva and sclera. Large optic zones and reverse geometry designs are needed to create extra mid-peripheral steepness to avoid peripheral corneal and limbal touch. 

CLINICAL PEARLS

• The first step to choosing between a conventional RGP lens design versus a reverse geometry RGP or scleral lens is to determine the apex of the cornea. If the corneal apex is within the central 4mm of the cornea, a standard geometry lens may suffice. If the corneal apex is outside of the central 4mm, a reverse geometry lens will work better.
• Corneal topography and anterior segment OCT are beneficial prior to scleral lens fitting to determine potential areas of concern such as a protruding graft or areas of corneal elevation.
• Scleral lenses need to vault over the entire cornea, including the highest point. In patients with keratoconus, the location of corneal ectasia is usually the steepest part of cornea, but not always.
• In cases of suspected ectasia progression, add additional lens clearance to avoid corneal touch.
• In post-penetrating keratoplasty (corneal graft) cases, practitioners should determine endothelial integrity status prior to dispensing scleral lenses.  The normal endothelial cell count is 2,500cells/mm² and this decreases with age. Patients with less than 800 cells/mm² should not be fitted with scleral lenses. To assess scleral lens suitability, allow the corneal transplant patient to wear a scleral lens for four to six hours.If corneal oedema occurs, scleral lenses are contraindicated.   

CONCLUSION

The difficult issues encountered with respect to fitting conventional corneal RGP lenses on irregular corneas are inextricably linked to achieving proper lens centration and stability. A lens that decentres not only degrades visual acuity but is uncomfortable to wear. Therefore, clinical experience and knowledge of special designs such as reverse geometry are important for managing patients with irregular corneas. Reverse geometry lens designs improve patients’ quality of life, visual acuity and ocular comfort. 

REFERENCES

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