Retinal Physician Article Submission Guidelines-Prescribing for Astigmatism and Presbyopia


CLASSIFIEDS

Pre-owned equipment, practices for sale, open positions, helpful practice management resources and more!

Click here to view the latest classifieds from Contact Lens Spectrum.

Article Date: 8/1/2014

Print Friendly Page
GP Insights
GP Insights

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.

TABLE 1 Tear Lens Effects on Tear Film Power
Tear Thickness CL Power Flat K Steep/Flat Tear Power Net Increase
200μm –3.00D 45.00D 3.00D flat –2.76D +0.24D
400μm –3.00D 45.00D 3.00D flat –2.53D +0.47D
200μm +3.00D 45.00D 3.00D steep +3.38D +0.38D
400μm +3.00D 45.00D 3.00D steep +3.67D +0.67D
200μm +9.00D 50.00D 6.00D steep +6.61D +0.61D
400μm +9.00D 50.00D 6.00D steep +7.25D +1.25D

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.

TABLE 2 Scleral Lens Power Shifts with Over-Refraction
CL Power OR CL Power Shift Error in Lens Order
–3.00D –10.00D –3.32D +0.32D
–3.00D +10.00D –2.47D –0.53D
–3.00D –2.00D –3.08D +0.08D
–3.00D +2.00D –2.92D –0.08D

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 jjackson@sco.edu.



Contact Lens Spectrum, Volume: 29 , Issue: August 2014, page(s): 16

Table of Contents Archives



AWS-#2