The definition of a scleral lens is “a lens that rests exclusively on the scleral conjunctiva while vaulting the cornea and the limbus.”1,2 The only zone of the lens that bears on the sclera is the landing zone (LZ). The LZ must be accurately designed not only with regard to being spherical, toric, or quadrant-specific, but also in its extent. This area of the lens is related to the lens sagittal height (sag); together, the LZ and the sag are the two principal and critical parameters for an optimal fit.
The scleral lens sag depends on the corneal sag: the higher the corneal sag, the greater the lens sag needs to be to avoid bearing on the cornea and limbus. A higher lens sag results in greater lens pressure applied on the conjunctiva. A larger LZ width is needed to better distribute this lens pressure. As LZ width is proportional to the total diameter, a larger total lens diameter is required when making a wider LZ.
Thus, the LZ parameters are fundamental in calculating the total lens diameter, including whether the lens will be a mini-scleral or a large scleral lens.
The purpose of this article is to present a formula for calculating the scleral total diameter (TD) that will demonstrate how the LZ width influences the total lens diameter. We will also suggest a new classification of scleral lenses relating to the LZ extension.
CALCULATING TOTAL DIAMETER
Choosing the TD is crucial for an optimal fitting, and it is the first step when choosing a scleral lens. A patient’s topographic pattern and anatomic factors influence the selection of a total lens diameter. Both horizontal visible iris diameter (HVID) and limbus width strongly factor into which TD is selected. The limbus width is typically about 1.0mm.3
Additionally, the definition of a mini-scleral lens or a large scleral lens cannot be based upon the absolute value of the lens diameter, because these are defined in relation to each patient’s HVID. Although HVID and limbus width strongly influence the size of the vaulting zone chord, two other parameters determine the total lens diameter: the LZ and the last peripheral zone widths.
Consequently, total scleral lens diameter is the sum of the following components: the HVID, the limbus, the LZ, and the last peripheral zone widths (Figure 1). Hence, TD can be calculated according to this formula:4
The first two parameters, HVID and limbus width, depend on the anatomy of each patient and cannot be changed or chosen. Therefore, the only parameters that practitioners can vary and choose are the extensions on the sclera of the LZ and the last peripheral zone. Among these four parameters that constitute the scleral lens diameter, only the LZ bears on the ocular surface and is selected by the practitioner (Figure 2).
THE INFLUENCE OF LANDING ZONE WIDTH ON TOTAL DIAMETER
How much does the extension of the LZ influence the lens’ TD value? Following are some examples to calculate the overall diameter in three cases having different corneal sag but the same HVID.
First Case In the first case, let’s consider that 0.8mm is the value of the LZ width of a scleral lens. Using our formula mentioned previously:
Second Case In the second case, in which the eye has a higher corneal sag, necessitating a higher lens sag, let’s consider that 1.5mm is the value of the LZ width of a scleral lens. Using our formula:
Third Case In the third case, in which the eye has an even higher corneal sag, necessitating an even higher lens sag, let’s consider 2.0mm to be the value of the LZ width of a scleral lens. Using our formula:
Two parameters were increased from the first and second cases. The LZ width, for a more even pressure distribution over the sclera, and the last peripheral zone width to facilitate lens removal. However, the parameter that changed the most is the LZ: from 0.8mm in the first example to 1.5mm in the second example. The TD is considerably increased by 1.8mm.
Between the second and the third case, only the LZ width has been changed (by 0.5mm), increasing the TD by 1.0mm. It is evident that even if only the LZ width is changed, the total scleral lens diameter is notably increased.
SCLERAL LENS SUBDIVISION
While it is evident that LZ extension strongly influences a scleral lens’ TD, the minimum value of this area is controversial. For a “mini-scleral” lens, a minimum LZ width may arbitrarily be considered to be about 0.8mm to 1.0mm, and a maximum LZ width would be about 1.5mm. The minimum limit of about 0.8mm to 1.0mm is based on the hypothesis that a smaller width can result in a risk of discomfort and/or conjunctival vessel compression and/or bulbar conjunctival indentation.
However, the crucial value for the distinction between a mini-scleral lens and a large scleral lens is the maximum limit. This maximum value of 1.5mm is based on the hypothesis that a larger width may interfere with a more asymmetrical sclera. The outcomes of a recent report indicate that scleral asymmetry starts at the more symmetrical limbus and increases in asymmetry toward the extraocular muscles.5 However, it is not yet clear at which distance from the limbus that the sclera starts to become asymmetric.
With higher values of LZ width, we enter into the range of “large scleral” lenses. Accordingly, Table 1 shows this classification of scleral lenses.4 Based on this categorization, it is now possible to classify each of the scleral lenses in the previous examples.
|Landing Zone Width||Landing zone ≤ 1.5mm||Landing zone > 1.5mm|
In the first case, the LZ width is 0.8mm, which is the arbitrary minimum value for a scleral lens. Thus, the lens is a mini-scleral having the minimum TD value:
TDmin mini-scleral = 15.5mm
In the second case, the LZ width is 1.5mm, which is the maximum value for a lens to be classified as a mini-scleral, characterized by the maximum TD value:
TDmax mini-scleral = 17.3mm
In the third case, the LZ width is 2.00mm, classified as a large scleral lens with the following TD value:
TD large scleral = 18.3mm.
The LZ is a crucial consideration when defining a scleral lens, and its design, including its width, is significant to obtaining ideal fitting outcomes.
In calculating overall diameter, the LZ is the only parameter controlled by practitioners that bears on the sclera. The LZ width is highly associated with and directly proportional to the overall scleral lens diameter.
Finally, the new proposed definition and division of scleral lenses, mini and large, depends substantially on the magnitude of the LZ area. Scleral lens classifications cannot ignore this area that has such a strong influence on scleral lens size. CLS
- Lupelli L. Optometria A – Z. Dizionario di. Scienza, Tecnica e Clinica della Visione. First Edition. Palermo, Medical Books, 2014.
- van der Worp E. A Guide to Scleral Lens Fitting, Version 2.0 [monograph online]. Forest Grove, OR: Pacific University; 2015. Available at http://commons.pacificu.edu/mono/10/ .
- Bergmanson JPG. Clinical Ocular Anatomy and Physiology. 17th edition, Houston, Texas Eye Research and Technology Center, 2010; p 111.
- Fadel D. Modern scleral lenses: Mini versus large. Cont Lens Anterior Eye. 2017 Aug;40:200-207.
- Ritzmann M, Morrison S, Caroline P, Kinoshita B, Lampa M, Kojima R. Scleral Shape and Asymmetry as Measured by OCT in 78 Normal Eyes. Poster presented at the 2016 Global Specialty Lens Symposium, Las Vegas, 2016 Jan.