To get the maximum potential from the EyeSpace Scleral lens range, it is important for the contact lens practitioner to be familiar with the anterior ocular surface anatomy and cornea-limbal shape.
The EyeSpace Scleral lens range is designed to completely vault the cornea and limbus. The bearing of the lens should be exclusive to the para limbal sclera. Therefore it is important to know the corneal and limbal diameter, which is measured clinically as the horizontal visible iris diameter (HVID). The average HVID ranges between 10.00 mm and 13.00 mm. It should also be noted that as a rule, the cornea is oval in shape, and as a result, the HVID is larger than the vertical visible iris diameter (VVID). The HVID directly affects the selection of the diameter in the EyeSpace Scleral lens range.
The rectus muscles insert in the sclera gradually further from the limbus beginning with the medial rectus at 5.50 mm (range 3.00 mm to 6.00 mm), inferior rectus 6.50 mm, lateral rectus 7.00 mm and superior rectus 7.50 mm. The line of insertion is called the spiral of Tillaux [tē-yō]. Practically, this means that the nasal side of the post limbal area presents the physical limiting factor for fitting large scleral lenses at around 22.00 - 24.00 mm in lens diameter.
Due to advances in technology such as OCT, and the need to better understand the limbal and scleral shape to improve scleral contact lens fitting, Pacific University looked at the limbal-scleral angles using OCT analysis in the scleral shape study. They imaged and analysed the three anatomical zones, the cornea, limbus, and sclera.
This area represents the cornea up to a chord length of 10.00 mm. This area can typically be analysed using standard topography methods and in normal corneas is a prolate shape. The cornea can be described using many mathematical methods such as axial curvature and power, tangential curvature and power, elevation, and eccentricity.
Initially, the limbal area and adjacent sclera were considered to have a curved shape. Recent research has indicated that this is not always the case and that this zone can be better described as a series of tangent lines in many eyes.
Meier described the limbal profile as having five different shape models:
In comparison to the population, it was evident that Profile 2 is the most commonly observed corneal-scleral shape followed by Profile 3. The different shapes also result in different sagittal heights at the level of the scleral, with Profile 1 having the greatest sagittal height, and Profile 5 the least. See figure below.
The limbal zone can be defined as the cornea-scleral angle, or limbal angle measured between the chord lengths of 10.00 mm -15.00 mm. Research has shown that this area is not rotationally symmetric (or spherical) and that a low degree of variability exists between the limbal angles as measured on the eight principal meridians of the eye.
The Scleral Shape study concluded that the average difference is approximately 1.70 degrees or 108 microns with the nasal meridian being the flattest angle followed by the temporal, superior and inferior meridians in order of flattest to steepest.
The Scleral zone can also be defined as an angle and is measured between the chord lengths of 15.00 - 20.00 mm. The variability between the scleral angles increases significantly compared with the limbal angle. The average difference of the scleral angle in the eight principal meridians is approximately 6.60 degrees or 400 microns. Taking these findings into account it is safe to assume that as the diameter of the scleral lens increases beyond the 15.00 mm chord length it will become increasingly necessary to fit a non-rotationally symmetric scleral lens.