Tear Film Power and OR Effects with Scleral Lenses
BY NANCY CHAN, OD, & JOHN MARK JACKSON, OD, MS, FAAO
With corneal GP lenses, we usually treat the lenses and tear films as having “thin” lens powers. If a lens is fit 3.00D steeper than K, we expect a –3.00D tear film. But scleral lenses and their tear films are often several hundred microns thick, so we must apply “thick” lens optics instead. The most obvious effect is in the tear film power, but the contact lens and over-refraction (OR) powers can yield surprising effects, too.
Sclerals Versus Corneal GPs
Schornack et al (2014) calculated the theoretical power of scleral tear films for 315 different combinations of these values (for examples, see Table 1). For example, for the lens fit 6.00D steep, the predicted “thin” tear lens power would be +6.00D, but the actual “thick” powers are +6.61D (200μm) and +7.25D (400μm)! For all of the examples, the actual tear film power is more plus than you would predict based on the base curve-to-cornea relationship.
|Tear Thickness||CL Power||Flat K||Steep/Flat||Tear Power||Net Increase|
The tear film has the greatest plus shift with thicker tears, steeper corneas, steeper base curve relationships, and greater plus power in the contact lens itself. The influence of the contact lens power may be surprising, but the power of a thick lens (the tear film) partly depends on the amount of vergence entering the tears, which changes with different contact lens powers.
Calculating Scleral Lens Power
But the tear film power isn’t the only thick-lens factor to consider. The power of the contact lens itself changes on the eye when the OR lens is in front of it!
Table 2 shows a few of our own calculations, which take into account effects of the OR, the contact lens, and the tear film together. It turns out that a plus OR gives the contact lens a plus power shift on-eye, while a minus OR causes a minus shift in the contact lens. This can lead to an error in the ordered contact lens power of up to 0.50D for large ORs.
|CL Power||OR||CL Power Shift||Error in Lens Order|
To correct for this, consider adding a little plus to your ordered contact lens when you have a plus OR and some minus when you have a minus OR.
Finally, as Table 1 shows, you should allow the contact lens to settle fully before doing the OR, as significant changes can happen to the tear film power as its thickness decreases with settling. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references and click on document #225.
Dr. Chan is the current cornea and contact lens resident at Southern College of Optometry. Dr. Jackson is an associate professor at Southern College of Optometry, where he works in the Advanced Contact Lens Service, teaches courses in contact lenses, and performs clinical research. You can reach him at email@example.com.