Trouble Shooting Guide for Ortho-K lenses (for PDF of this page, click here)
The basic design of the third generation e Lens for Orthokeratology
1. Optic Zone (Base curve, Compression zone, BC) width 5.6 to 6.4mm
2. Fitting curve (second curve, first reverse curve, RC, FC) width 0.50 to 0.80mm
3. The first alignment zone (third curve, ACAC1, Landing curve 1) width 0.6 to 1.6mm, different lens design will have different width
4. The second alignment zone (fourth curve, AC, AC2, Landing curve 2) width 0.5-0.9mm
5. Peripheral curve - Standard radius is 11.5mm, 0.4mm width
This is the commonest de-centration problem encountered. A slight low riding lens is the ideal position upon dispensing. The lens will then center in the closed eye state. When the lens is locked down inferior or if ghosting image persists, then a change is warranted.
Possible Cause: The normal human cornea is asymmetrically aspherical and the contact lens is symmetrically spherical. The degree of corneal flattening is different for everyone with some corneas (higher e value) flattening more than others. For these people, the Alignment curve (AC) of the lens will be relatively too steep. By being too steep, the relationship of the angle of the AC in relation to the angle of the cornea will reduce the surface tension forces and therefore force the lens to become out-of-center. If the AC is very steep to the cornea, then the lens will even cause some vaulting thereby reducing the central touching pressure.
Suggestions:Flattening the AC by 0. 50D will put the lens and cornea into a more close relationship. This will increase the surface tension and improve centration. Reducing lens weight will also help with low riding lens. e. g. change lens thickness form 0. 23mm to 0. 19mmAlso, re-order a lens with wider AC thereby increasing lens overall diameter. This will loosen up the lens and give more surface touching area to help with centration.
The third generation Ortho-K lenses are tight fit design. High riding is not common.
Possible Cause: The high riding lens can be caused by a loose lens, tight upper eyelid or high corneal astigmatism.
Suggestions: If the lens is marginally high, tighten the AC by 0. 50D. Increasing the lens center thickness by 0. 03-0. 05mm will add more mass and bring the center of gravity of the lens more anterior thereby allowing gravity to bring the lens down. Order a thinner edge lens or decreasing the lens front axial edge lift by 0. 02mm will also reduce the lid action and help lens centering. Order new lens with prism ballast.
Lateral riding lenses is one of the challenging problems to solve in Ortho-K.
Possible Cause:When the de-centering forces of the lid and gravity exceed the amount of centering forces of the lens from surface tension, then the lens will become de-centered. Remember that the goal of the lids is to steer the tears to the puncta with a downward, nasal force. Against the rule astigmatism is another contributing factor. With ATR cylinder, the surface tension forces are concentrated in the 90 meridian making more lateral movement possible. The difference between the nasal and temporal topography of the cornea is also another major reason why a lens will de-center laterally.
Suggestions: What has been working well is to widen the AC or add a second AC to the standard design while reducing the width of the first AC. This increases the contact surface area and thereby increases the surface tension forces. Steepening the BC will help centration by increasing the contact area at the apex. Also, a wetter material will reduce the frictional forces of the lids thereby reducing their de-centering influences. Make sure the lens is clean because any deposit formation of the anterior surface of the lens will increase the frictional forces of the lids.
Central or Para-central Islands:
Central or para-central islands are areas of distortion in the visual axis. These can be devastating to the unaided visual acuity leading to failure of the case.
Possible Cause: One cause of central or para-central islands is the tight fitted lens causing distortion in the central cornea. This can be prevalent in previous rigid contact lens wearers especially if the lens was not well centered. Another cause is excessive astigmatism that has not been completely reduced. With corneal astigmatism present, there are unequal bearing areas where the fitting curve comes into contact with the cornea. This causes an uneven distribution of the compression forces at the apex resulting in corneal non-sphericalization.
Suggestions: If the lens is de-centered, first the lens must be made to center. Once the lens is centered then a reliable determination of the problem can be made. Flatten the fitting curve radius by 0. 50 to 1. 00D. This will decrease the lens sagital height and apply more central pressure and smooth out the central region. If the central disturbance is from under-corrected astigmatism, then flattening the BC may help to correct this.
This is a complication due to either mechanical or chemical or physiological problems.
Possible Cause: One major cause of central staining is improper cleaning procedure. Because of the steep FC, it is difficult to clean the central posterior surface of the lens. This will create a relative rough surface, which cause the staining and a tendency for lens adherence. If the BC is too flat, the mechanical pressure act on cornea can also cause irritation. This usually occurs after the second or third lens change in higher power. Reduced oxygen availability can also cause central staining that it needs to change to other material with higher dK. Allergy to the cleaning and conditioning solution should be one of the main problems.
Suggestions: The first thing is to make sure the posterior surface of the lens is clean. Make sure the patient is using good quality cotton buds to clean the posterior surface of the lens. Normal cleaning procedure utilizing the fingers usually cannot supply sufficient frictional forces to adequately clean the BC and FC area. Review the cleaning solution used. Make sure the lens is clean. If some hardly removed spot on the lens, you may ask the manufacturer’s help. If the staining remains, steepen the BC by 0. 50-1. 00D and consider changing the material to a higher dK (greater than 100) if not already being utilized. If no reason was found, change the lens solutions to other brand with different formulation.
Vaulting occurs when excessive bearing is present in the peripheral regions causing reduced central bearing. This will be seen as central pooling or increased fluorescein in the optics zone area.
Possible Cause: The major causes of central vaulting are tight lenses; either FC or AC is relatively too tight. When FC or AC is too steep, the peripheral portion of the FC or AC will press on the cornea and prevent the central portion of the lens from applying compression to the apex thereby retarding myopia reduction.
Suggestions: Find out the most tighten part of the lens from fluorescein pattern and corneal topographer. If the AC is too tight, there may also have a low riding problem. After determining which zone induces the vaulting problem, flattening that part by at 0. 50 to 1. 00D? The risk is that by loosening the AC too much, centering problems may happen.
Air bubbles are common and will typically disappear after overnight wear. Only when staining occurs under a persistent air bubble does the lens need to be changed.
Possible Cause: Air bubbles formed when not enough solution is under the FC. Usually the upper lids will compress the lens to the cornea during nightwear and they will disappear in the morning. The FC has a very steep configuration, which is sometimes difficult to fill with tears and therefore will trap some air inside. Sometimes 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 necessary lubrication or oxygen.
Suggestions: 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, remove the lens and fill the concave surface with solution and then have the patient reinsert the lenses while looking downward. If a large air bubble persists monitor the next day to see if the bubble and staining recedes. If the bubble and staining persists then flatten the FC radius by at least 0. 50D. This will reduce the steepness of the FC and reduce the air bubble. Air bubbles look bad but are usually a self-limiting condition, which require no change.
When a lens is de-centered, the fitting relationships are changed and you will have a portion of the lens applying too much pressure on the cornea. This area will cause indentation and therefore distortion which will show up as a lens imprint and as ring jam in the placido disk image.
Possible Cause: Most lens imprints are associated with a de-centered lens. Centering the lens will eliminate most of the imprints. Too tight of fitting curve is also a cause of circular imprint.
Suggestions: If the lens is de-centered, make the appropriate lens modification. If the lens is centered and any imprinting is present, loosen the fitting curve or alignment curve by at least 0. 50D.
An under-responder is a patient whose myopia does not reduce as anticipated. An example is a -3. 00D, which is reduced to –1. 00 after one month of wear and has not changed for 3 weeks.
Possible Cause: Everyone’s cornea is different in elasticity, rigidity, diameter, thickness, intro-ocular pressure, tear quality, flexibility etc. Some patients respond more than anticipated so if only –4. 00D of correction is targeted on a –4. 75D myope, sometimes the full correction is realized. Conversely, sometimes the –4. 00D myope will only reduce –3. 00D. This is normal variation.
Suggestions: First make sure the fluorescein pattern and lens centration are good and there is no vaulting. If there is vaulting taking place, correct it to get the appropriate fitting relationship. If the fluorescein pattern looks good wait a while longer, at least two to three weeks to allow for slow responders. When the Ortho-K effect is stable and no more improvement was observed in two to three weeks, then increase the target power by flattening the BC by 0. 75 to 1. 00D. After waiting for another two to three weeks and the patient is still not responding, considering the Ortho-K endpoint is reached in this patient.
Reduced Holding Time:
This is when the unaided visual acuity does not hold an acceptable amount of time.
Possible Cause: The major cause is 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 so much change, usually the more induced change the faster the cornea will regress. Therefore if you have reduced –6. 00 of myopia, do not expect the UVA to hold all day. As a general rule, the lower the starting amount of myopia, the greater chance of holding all waking hours. The holding time is shorter in patients with larger pupil. Some patients will just not hold regardless of what their starting power.
Suggestions: If the cause is a de-centered lens, make the appropriate modifications to the design to center the lens better. If vaulting is present, again do what is required to reduce the vaulting. If the lens is well fit and the holding time is still unsatisfactory, consider having the patient wear the lens as a day lens with the option of removing the lens to do certain specific activities while the VA holds. Remember, not everyone is going to be successful with Ortho-K.
Ghosting At Night:
Night ghosting is a normal observation. This usually recedes with time but will always be present to some extent.
Possible Cause: The main cause of ghosting is when the reduced illumination at dim light causes the pupil to become larger than the central applanation area of the cornea. This will occur even with a well-centered lens. Patients with smaller pupils will not experience this to the extent of patients with very large pupils. Another cause is a de-centered lens. This can also cause ghosting during the day. Central islands can also give the same subjective complaints as ghosting.
Suggestions: Assure the patient that this will reduce with time. If the lens is not centered, then take the appropriate measures to center the lens. Usually this problem can be eliminated by proper pre-treatment consultation.
Central Lock Down:
This condition is when the lens is stuck centrally on the cornea and not moving. If this problem can not be solved, central staining and adverse physiological effects can occur. Use lubrication drops to get the lens moving. Sometimes manual manipulation of the lens is required to initiate movement of the lens. There will usually be an indentation of the lens on the cornea after the lens is removed.
Possible Cause: The primary cause is when the FC or AC are too tight. This causes the lens to stick. Another contributing factor is a coated lens especially on the posterior surface of the BC. If the lens tightens up enough to lock down centrally, a change must be made to loosen up the lens.
Suggestions: Loosen the tight FC or AC by at least 0. 50D. Make sure the posterior surface of the lens is being cleaned properly with good quality cotton buds. Go over cleaning techniques again to reinforce the importance of a clean lens. Generally, if the lens is well centered, change the FC if a change is required. If the lens is de-centered, usually the AC needs to be changed. Flattening the PC in the office will help in some cases and should be tried before a new lens is ordered.
Orthokeratology is not a simple mathematic calculation. There are many variables of which we do not have control. You must try and learn from error to improve the fitting skill by changing the lens design. We have enough experience to make you a good lens but not having that enough knowledge to give you optometric education. In case our customers need help in contact lens design and application. We might try our best to give suggestion (trouble shooting guide) for you to make your own decision.