Article

RESEARCH REVIEW

THE (CORNEOSCLERAL) SHAPE OF WHAT’S TO COME

At one point in time, everyone thought that the world was flat…and then there was science! The same can be said about our thinking with regard to corneoscleral shape. With the ever-increasing utilization of scleral contact lenses, greater inquiry has been made into our understanding of the shape of the sclera. The impact of such understanding is improving our ability to fit virtually all types of contact lenses. Beyond contact lens fitting, our knowledge about corneoscleral shape is giving us a deeper awareness that goes beyond the cornea of what is going on in diseases such as keratoconus. This article is based on a talk that I presented at the 2019 International Summit of Specialty Contact Lenses in Rome.

Not What Was Expected

Some of the first impactful work regarding our understanding of corneoscleral shape was published more than 10 years ago. A research group headed by Eef van der Worp, BOptom, PhD; Tina Graf, BSc; and Patrick Caroline at Pacific University utilized newly introduced anterior segment optical coherence tomography (AS-OCT) technology to plot scleral shapes. The findings that they discovered significantly differed from previous theoretically based concepts (van der Worp et al, 2010). They further developed a manually measured scleral topography system and a model eye for anterior ocular surface shape based on a database of 96 eyes of normal subjects.

Their work sparked ongoing research along with the development of other systems to measure corneo-scleral shape in a more automated and clinically applicable way. Let’s see what we have come to understand over the subsequent decade.

Tangential Corneoscleral Angle

Some of the first and most surprising outcomes from the work done by van der Worp et al (2010) revealed that the corneoscleral transition zone, thought to be gradual and curved, was in fact tangential in geometry. The best way to describe it would be with angle measurements, not with radius of curvature.

Confirming AS-OCT work showed that the mean corneoscleral junction (CSJ) tended to be sharpest at the nasal side and became progressively flatter at the inferior, temporal, and superior junctions, while in many cases the CSJ angles were within 1º of 180º, indicating almost tangential extensions of the peripheral cornea to form the sclera (Hall et al, 2011; Hall et al, 2013).

Thinking in Terms of Sagittal Height

Further outcomes from the Pacific University scleral shape study showed that the nasal sclera was generally the flattest of the eight principle directions measured. However, rather than putting this in terms of flat versus steep, a better description is based on sagittal height: The nasal sclera is generally the highest, whereas the temporal inferior and the temporal superior zones were found to be the lowest. These descriptions have revolutionized contact lens fitting in general. Contact lens practitioners seem to be thinking increasingly in terms of sagittal height relationships between the contact lens and the ocular surface rather than in terms of curvature measurements.

Rotational Asymmetry of Scleral Shape

As we investigate the shape of the sclera along the various rotational meridians, we have come to understand that the scleral shape in most eyes is not spherical. In fact, most are not regularly toric; the majority are rotationally asymmetric in shape. DeNaeyer et at (2017) in the Scleral Shape Study Group utilized fluorescein-based scleral profilometry on a group of normal eyes and found that only about 6% of eyes were spherical, about 30% were toric, and about 65% of scleral shapes were described as asymmetric in one form or another.

Another interesting and impactful finding came out of work by Ritzmann and co-workers (2018) who discovered that the degree of rotational asymmetry increases as chord diameter increases. In other words, as we go further out onto the sclera, the shape asymmetry increases. Using AS-OCT technology evaluating 79 normal eyes, they found that at a 12.8mm chord length, the shape of the anterior eye was nearly rotationally symmetric, but at a chord of 15.0mm, the shape became more asymmetric. At a 15.0mm chord, nasal measurements revealed flatter scleral angles and concave corneoscleral transitions, whereas temporal scleral angles were steeper, with tangential or convex corneoscleral transitions.

Scleral Shape and Corneal Shape Relationships

The obvious question arises: Does corneal shape correlate with scleral shape? Consejo and Rozema (2018) published a paper evaluating the potential correlation between the scleral shape and corneal astigmatism. In astigmatic eyes, corneal and scleral asymmetry are highly correlated; however, they found that scleral and corneal shape were independent of each other in non-astigmatic eyes. This suggests that astigmatism may not be restricted to the cornea but could be considered a property of the entire eye globe.

Next let’s look at the impact of keratoconus on corneal shape. Two studies revealed some interesting findings. Piñero et al (2019) performed profilometry on 21 keratoconic eyes and 88 normal (control) eyes and found that corneoscleral profile asymmetry was greater in keratoconic eyes and that this was especially so in cases of moderate and advanced disease.

DeNaeyer et al (2019) also performed profilometry on 227 cases of corneal ectasia (83 central/114 peripheral) and on 115 normal eyes. They found that irregular scleral shape was more common in ectatic corneas than in regular corneas. Furthermore, they reported that for central cones (within 1.25mm from the corneal center), scleral lens design software suggested standard scleral lens toric haptic designs more frequently compared to quadrant-specific haptics. For peripheral cones (> 1.25mm from the corneal center), quadrant-specific toric haptic designs were significantly more frequently indicated (Figure 1). The outcomes from these studies suggest that keratoconus may extend beyond the cornea.

Figure 1. Use of standard peripheral toricity versus quadrant-specific toricity scleral lens designs based on the location of the cone apex of keratoconus patients (DeNaeyer et al, 2019).
Image courtesy of Greg DeNaeyer, OD

Clinical Measurement of Corneoscleral Shape and Designing Scleral Lenses

Today, more instruments are becoming available that allow for reliable and efficient measurement of corneoscleral shape within clinical practice. Additionally, these measurements can be utilized to design scleral lenses that more accurately contour the ocular surface. Initial efforts were described utilizing AS-OCT technology (Gemoules, 2008); however, due to the complexity of such measurements, this method has not gained great popularity.

Subsequently, fluorescein-based profilometry systems have been introduced that easily allow for clinical measurements (Piñero and Soto-Negro, 2017), and they have incorporated scleral lens design software. More recently, corneoscleral profile measurement software has been added to a Scheimpflug tomography system that has the ability to interphase with a number of scleral contact lens design software applications (Maller, 2019).

Finally, the most detailed and accurate measurement of corneoscleral shape is likely obtained from direct impression technology. The impression can then be scanned to create a highly detailed 3D model of the ocular surface, which then can be incorporated into scleral lens design software to produce the most unique and customized scleral lens (Nguyen et al, 2018).

Concluding Remarks

Research has opened up a new world with regard to our understanding of corneoscleral shape. This understanding has now allowed us to better manage our patients who wear scleral contact lenses. Continued research will provide a greater understanding of a variety of disease processes and will allow us to better care for our patients. CLS

The author acknowledges Eef van der Worp, BOptom, PhD, for his contribution to this article.

For references, please visit www.clspectrum.com/references and click on document #294.