Contact lenses

1. Contact lenses Week 4 Soft lens fitting techniques and care

2. Selection of lens for the patient • From the patient interview, determine what type of lens wear schedule would be best for the patient. • From the patient exam, determine if, spherical, toric or gas permeable is best for the patient. • Keep in mind, comfort and visual acuity are always first priority. Be sure to ask about occupational needs and what they hope to achieve with the CL’s. • It is your job to explain how the CL’s work and the choices they have. Take the time to explain all of the options. • Ask about distance, intermediate and near vision issues if the patient is close to or presbyopic age.

3. Base curve selection • Base curve is based on the “K” readings, however you must always evaluate the trial lens on the eye to make sure the CL has a good fit. • Base curves range from 8.0 to 9.2. Most are 8.3 to 8.8. Depending on the manufacturer, there are limitations on the BC and D. • “K” readings of 42D and lower are considered flat. “K” readings of 46D and higher are considered steep. • Base curves for soft CL’s are specified as steep (8.0 to 8.3), Median (8.4 to 8.6), and flat ( 8.7 to 9.0).

4. Power selection • Lens power selection is based on manifest refraction with a vertex distance conversion chart, if necessary. (Effective power). • A spherical lens can “mask” a small amount of astigmatism, but you will need a thicker lens or a toric lens to correct astigmatism. • To use a spherical lens on a patient with a small amount of astigmatism. You need to convert the patient’s refraction to spherical equivalent. • Also CL’s are ALWAYS in minus cylinder. You will always need to convert the plus cylinder Rx into minus cylinder. Then, do the spherical equivalent. This should only be done if the cyl is 0.75 D or less.

5. Transposition and spherical equivalent Transposition Spherical equivalent • +5.00+1.00X90 • +6.00-100X180 • -6.50+0.75X90 • -5.75-0.75X180 • -0.75+1.25X110 • +0.50-1.25X20 • +4.00+0.75X90 • +4.75-0.75X180 • +4.50 or +4.25 sph • -3.50+0.50X90 • -3.00-0.50X180 • -3.25 sph

6. Vertex distance • Always remember…. • Vertex distance is much farther when refracting a patient, you must compensate for this when fitting with a CL. • This applies to more than +/- 4.00D. • CAP rule • +8.00 sph = +8.87sph • - 6.00 sph = -5.62 sph • This is based on a 12mm VD. The VD can vary from 8-15mm.

7. Fit evaluation • Allow the CL to rest on the eye for 5 minutes before evaluating the fit. • If the patient has never worn a CL before, it will feel like an eyelash in their eye for about 5 minutes. • The lens should be centered on the eye, and re center after each blink. • The lens should have adequate movement. There should be 1mm of movement after each blink and return to center over the cornea. • If there is no movement, the lens is too tight. • If there is too much movement, the lens is too loose. • The VA should be stable and remain so before, during, and after each blink. • If the VA is not as clear as expected, do an over-refraction and check for residual astigmatism. • Dryness can cause fluctuating VA or decreased VA. CL’s may exacerbate this.

8. Fit evaluation • Check the “K”s with the CL’s on the eyes. The mires should be clear with a proper fit. • The lens diameter should extend beyond the limbus by 1 to 1.5 mm and should have an equal amount of excess material 360 degrees. • The cornea should never be exposed after a blink, this can cause problems.

9. Signs and symptoms of a loose (flat) fit • VA unstable with each blink. • Patient awareness of the lens. • Poor centering, especially low fitting. • Too much movement with each blink. • Edge “stand off”. (edges do not hug the eye) • Lens falls out. • Air bubble under the lens, especially on the periphery of the lens. • “K” mires are blurred after each blink.

10. Signs and symptoms of a tight (steep) fitting lens • Fluctuating VA, clears after each blink. • Comfortable at first, then not. • Circum corneal injection. • Circum corneal indentation. • Little or no movement upon each blink. • Keratometry mires distorted, then clear after each blink. • Air bubbles centrally, over pupil.

11. Modifications Correcting a loose fit Correcting a tight fit • Tighten/decrease the base curve but keep the diameter the same. • Increase the diameter and keep the base curve the same. • If increasing the diameter, make sure not to compromise the 1-1.5 mm corneal extend. If the lens is too large, leave the diameter and adjust the BC. • Loosen/increase the base curve, but keep the diameter the same. • Decrease the diameter and keep the base curve the same. • If decreasing the diameter, make sure not to compromise the 1-1.5 mm corneal extend. If this causes corneal exposure, leave the diameter and adjust the base curve. • Use a thinner lens, same parameters.

12. Modifications If the lens is too large If the lens is too small • Decrease the diameter, but remember…. Decreasing the diameter may inadvertently effect the BC and loosen the lens. • Always recheck your modifications with a trial lens. • Increase the diameter, but remember… increasing the diameter may inadvertently effect the BC and tighten the lens. • Recheck the modification and if necessary, change the base curve accordingly.

13. Conclusion • Use patient interview to determine lens type. • Use patient ocular exam to fit base curve, diameter and power. • Use a trial lens to evaluate the lens fit. • Take VA and over-refract if VA is not what was expected. • Make any modifications to fit and/or VA that may be necessary. • Use a trial lens to evaluate modifications. • Explain insertion and removal, care and handling and cleaning to the patient.