Problem Solving

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  • Lowriding1
  • Lowriding2
 

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Observation: A slightly low riding lens is the ideal position upon dispensing. The lens will then center with the eye closed. Do not make a change unless the lens is chronically low riding with eyelid closed (as demonstrated by topography) or if unacceptable ghosting persists.

Cause: The cornea becomes flatter from the apex to the periphery. This degree of corneal flattening is different for everyone, with some corneas having a greater or lesser degree of flattening. If the flattening is too great, the alignment curves will be too steep.

Solution: Loosen (flatten) the alignment curves by .50

  • Highriding1
  • Highriding2
 

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Cause: The high riding lens is usually caused either from the lens being too loose (AC too flat) or from an asymmetrical corneal shape.

Solution: If the lens is too loose, tighten (steepen) alignment
curve by 0.50 D.

Cause: Generally caused by a very spherical cornea or a cornea with against the rule cylinder.

Solution: Increase the diameter of the lens by at least 0.40mm. The recommended diameter would be 11.0mm. This assumes an HVID of at least 11.8mm.

  • Vaulting1
  • Vaulting2
 

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Observation: Vaulting occurs when excessive bearing is present in the peripheral regions (AC too steep) causing reduced central bearing. This will be obseved as central pooling or increased presence of fluorescein under the center of the lens.

Cause: The major cause of central vaulting is an alignment curve (AC) that is too steep. The more peripheral from the corneal apex, the more difficult it is to predict the rate of corneal flattening. When the alignment curve is too steep, the central portion of the lens will rise up, preventing it from applying compression to the center of the cornea. A fitting curve that is too steep can also cause central vaulting but is much less common.

Solution: Flatten the alignment curves by 0.50 D. The risk is that by loosening the alignment curves too much, centering problems can develop. If the lens is well centered, and does not appear tight in the alignment curve area, flatten the fitting curve by 0.10mm. .

Observation: An under-responder is a patient whose myopia does not reduce as anticipated. An example is a –3.00, which was reduced to –1.00 after one month of wear and then remains unchanged for 3 weeks. You will be able to refract the patient, without lenses, to 20/20 or better.

Cause: Typically, the under-responder will have vaulting in the center. Some patients will, however, respond slower than others perhaps due to different cell structure of the cornea. You do not want to rush into making a change if the exam figures are correct.

Solution: Follow the same solutions for vaulting. If no vaulting is present, recheck the original exam figures. If the fluorescein pattern looks good, wait a while longer, e.g. at least two to three weeks to allow for slow responders. If there is still no further reduction on the unaided visual acuity, increase the target power by 0.50D to 0.75D.

  • Centralislands1
  • Centralislands2
 

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Observation: Central islands are areas of distortion in the visual axis that are observed with corneal topography. If you do not use a corneal topographer in the follow-up exams, you will observe slightly distorted mires using the keratometer. This condition differs from the under-responder in that you will not be able to refract the patient, without lenses, to 20/20.

Cause: Generally caused by the fitting curve being too steep, causing the base curve (BC) to lift off too much from the central cornea. Another cause is excessive astigmatism. With corneal astigmatism present, there are unequal bearing areas where the fitting curve comes into contact with the cornea.

Solution: Flatten the fitting curve by 0.05mm to 0.10mm. This action applies pressure that is more central and will smooth out the central region. If the central disturbance is from astigmatism, then flattening the BC will help to correct this. Target the spherical equivalent of the original refraction to be plano to +1.00 assuming the patient will not have any accommodative symptoms.

Observation: This is a complication due to either mechanical irritation or physiological problems.

Cause: One major cause of central staining is a coated lens. Because of the steep Fitting Curve (FC), it is difficult to clean the central posterior surface of the lens. This will create an irritating surface, which in turn causes the staining and a tendency for lens adherence. If the BC is too flat, the reduced mechanical pressure can also cause irritation. Reduced oxygen availability can also cause central staining but this is a rare occurrence.

Solution: The first thing is to make sure the posterior surface of the lens is clean. Review the cleaning solution used. Make sure there are no dry spots. If the staining remains, steepen the BC by 0.5D.

Observation: Air bubbles are a common occurrence and typically disappear after wear. Only when staining occurs under a persistent air bubble does the lens need to be changed.

Cause: Air bubbles form when not enough solution is under the Fitting Curve. Usually the upper lids will compress the lens to the cornea and the bubbles will disappear in the morning. The fitting curve has a steep configuration, which is sometimes difficult to fill with tears. Occasionally, the resultant air bubble can encompass 270 degrees around the FC. Any staining present is due to the air bubble where the cornea is not getting the lubrication or oxygen that it needs.

Solution: If the air bubble is less than 45 degrees in length upon insertion, just monitor the next day to see if any staining occurs. If the air bubble is greater than 45 degrees, have the patient remove the lens and fill the concave surface with solution and the reinsert the lens while looking down. If a large air bubble persists, monitor the next day to see if it remains and staining persists. If so, monitor for three days to see if the bubble and staining recede. If both persist, then flatten the fitting curve 0.10mm. This will reduce the steepness of the fitting curve and reduce the air bubble. Air bubbles are usually a self-limiting condition, which require no change.

Observation: This is when the unaided visual acuity after cornea reshaping does not hold an acceptable amount of time.

Cause: Generally caused by a lens that is not centered, with the steep area almost touching the visual axis. When the cornea normally regresses, the visual axis is impacted sooner because there is less distance between the visual axis and the edge of the peripheral steep ring. If some vaulting has occurred, there will be a smaller central visual zone with a corresponding wider concentric steep ring. The cornea can only undergo a limited amount of change. Usually, the more induced change, the faster the cornea will regress. Therefore, if you have reduced –5.00 diopters of myopia, you should not expect the unaided visual acuity to hold all day. As a general rule, the lower the starting amount of myopia, the greater chance of holding for all waking hours. Emerald is not recommended for reducing myopia greater than –5.00 diopters.

Solution: If the lens is de-centered, make the appropriate modifications to the design to center the lens better. If vaulting is present, do what is required to reduce the vaulting (see “vaulting” above). Flattening the BC by 0.50 D can also prolong the holding time by making the cornea change more before a decrease in UCVA is noticed. Flattening the base curve will only be effective for a patient that is able to accommodate the additional correction early in the day.

Observation: Night ghosting is a normal observation. This usually recedes with time but may always be present to some extent.

Cause: The main cause of ghosting is when the reduced illumination at night causes the pupil to become larger than the central correction area of the cornea. This might occur even with a well-centered lens. Patients with smaller pupils will experience this very little or not at all, compared to patients with very large pupils. Another cause of ghosting is a decentered lens. This can present as ghosting reported during the day. Central islands can also give the same subjective complaints as ghosting.

Solution: Time is the answer for normal ghosting. If the lens is not centered, then follow the methods described earlier to center the lens. The optical zone of the lens can also be enlarged from 6.2 to 6.5mm. However, this might lead to a decrease in the holding time. It is recommended that you wait 1 month before increasing the size of the optical zone.

 

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