Anatomical and physiological considerations in scleral lens wear: Conjunctiva and sclera

Abstract

While scleral lenses have been fitted using diagnostic lenses or impression moulding techniques for over a century, recent advances in anterior segment imaging such as optical coherence tomography and corneo-scleral profilometry have significantly improved the current understanding of the anatomy of the anterior eye including the morphometry of the conjunctiva, sclera, and corneo-scleral junction, as well as the ocular surface shape and elevation. These technological advances in ocular imaging along with continual improvements and innovations in scleral lens design and manufacturing have led to a global increase in scleral lens prescribing. This review provides a comprehensive overview of the conjunctiva and sclera in the context of modern scleral lens practice, including anatomical variations in healthy and diseased eyes, the physiological impact of scleral lens wear, potential fitting challenges, and current approaches to lens modifications in order to minimise lens-induced complications and adverse ocular effects. Specific topics requiring further research are also discussed.

Introduction

Scleral lenses possess unique fitting characteristics that are not observed with any other contact lens modality. Unlike all other contact lenses, an optimally fitted scleral lens completely vaults the cornea and limbus, and the haptic (or landing zone) rests upon the conjunctival tissue overlying the sclera. In contrast to hybrid or soft contact lenses, the landing zone of a scleral lens does not yield to the shape of the anterior eye, so alignment of the haptic to the ocular surface is of critical importance. Additionally, although scleral lenses are fabricated from the same materials used to make corneal rigid gas permeable lenses, the entrapment of a thick layer of fluid beneath a scleral lens fundamentally alters the relationship between the lens and cornea with respect to lens fit and ocular physiology.

Currently there are no evidence-based guidelines regarding scleral lens fitting characteristics that are associated with optimal visual and physiological outcomes. This may be due, in part, to the fact that scleral lenses are primarily prescribed for eyes with severe corneal irregularity or ocular surface disease [1,2]. Given the considerable variation that is present in the clinical presentation of severe ocular disease, the development of specific guidelines for scleral lens fitting based upon these highly individual eyes has been challenging. But as increasing numbers of patients are fitted with scleral lenses [3], including those with simple ametropia and healthy eyes, general observations and trends may allow for the formulation of such guidelines.

As evidence-based guidelines are developed, observed and potential interactions between scleral lenses and the structure and function of ocular surface tissues must be considered. The purpose of this review is to outline the anatomical and physiological variations of the conjunctiva and sclera, to summarise tissue changes associated with scleral lens wear, to describe fitting approaches and lens modifications to accommodate conjunctival and scleral anomalies, and to highlight topics that require further research in order to determine how to optimise physiological outcomes with scleral lens wear.