Toric lenses and soft CL problems
Toric Lens Lenses used to correct astigmatic/sphericocylin drical patients. Toric lenses have a ballast or “weight” on the bottom of the lens to help keep the axis where it should be. The lens may rotate after blinking but will return to the desired axis because of the ballast. Each toric lens has markings on the lens itself to help the examiner determine if the lens is fitting properly. Most toric lenses have markings or “scribe marks” inferiorly at 6:00 and 2 more scribe marks at 5 and 7. The people who will be most successful in torics will have a cylindrical axis close to 90 and 180 degrees.
Types of ballast in toric lenses Types of scribe marks on toric lenses.
People who will not do well in toric lenses: Patients with tight lids. Narrow palpebral fissure. Poor tear quality or quantity. Refractive astigmastism grater than +2.75D. Toric lenses are available from to in spherical power and to-2.50 in cylindrical power. Base curves =8.3 to 8.9 Diameters=13.8 to 14.5
Base curve and diameter is fit the same as spherical CL. Refract patient to get power needed. Transpose power to minus cylinder. Put trial lens in that power on eye. Allow the lens minutes to settle. Take VA, if VA is NOT what expected, over refract spherical/cylinder power only. Put axis to trial lens axis. Evaluate BC, D and scribe marks with slit lamp exam. If the slit lamp exam reveals the scribe marks are in good position, and BC and D are fine you are done! If BC and D are not correct, note in chart and make modifications. If hash marks are not in correct position, use the Lars principle.
Transposition Refraction= X90 Transpose to –cyl= X180 Refreaction= X90 Transpose to – cyl= X180 Remember……. If the spherical power is+/- 4.00D, You will need to convert the power to compensate for vertex distance to get the effective power.
Over-refraction If VA is not what was anticipated, you need to modify the spherical/cylindrical power of the trial lens to improve VA. AXIS MUST BE SAME AS TRIAL LENS. With the trial lens on the eye, refract the patient with spherical/cylindrical power only, to see if the VA will improve with more or less spherical/cylindrical power. If you get improvement, modify trial lens power. Trial lens power= X90 Over refraction= X90 Add over refraction to existing power after transposing X180 X90 new trial lens Trial lens power= X180 Over refraction= X180 Add over refraction to existing power after transposing. X90 X180 new trial lens Do not try to refine the axis. The axis stays the same as the trial lens!!!
LARS Principle Lars rule(principle) or clockwise/counterclockwi se rule can be used to compensate for the misalignment of the lens axis so it will match the patient’s refractive axis. Sometimes, even though there is a ballast, the CL doesn’t stay where it is supposed to and then the axis will be “off” causing blurry VA. Lars is an acronym for Left, add, right, subtract. When looking through the slit lamp, if the hash marks are rotating to the fitter’s left, the number of degrees off rotation should be added to the trial lens axis. If the hash marks are rotated to the fitter’s right, the number of degrees off should be subtracted from the trial lens axis.
LARS Picture the CL as a clock, there are 60 minutes on the clock (360 degrees). Every “minute” equals 6 degrees. Every “minute” the hash marks are “off” equals 6 degrees of misalignment. A hash mark that orients at 7 o’clock instead of 6 o’clock is 5 “minutes” off. 6 degrees X 5 minutes= 30 degrees. The axis is 30 degrees off of where it should be, causing a blurring effect and decreased VA.
Trial lens= X90 Figure 3.7= 3 minutes off=18 degrees. The hash marks are to the fitter’s left, so we ADD 18 degrees to the trial lens axis. New trial lens axis= X108. Figure 3.8, also 18 degrees off but we SUBTRACT from the axis because it is to OUR right. New trial lens axis= X 72
Bounce back test CL will center and hash marks will be on 5,6,&7 before blink. After blink, the hash marks will rotate. 2-3 seconds after blink, the hash marks should return to 5,6 & 7 and be centered on eye. If there is NO movement, the lens is too tight. You want the lens to rotate and move, just make sure it rotates and moves back where it belongs!
Perform ocular exam. Determine power, BC, and D. Put trial lens on eye for VA and fit evaluation. Test VA, do over refraction if needed. Transpose and recalculate new trial lens power. Do slit lamp exam for trial lens fit ( to flat or steep) and look where hash marks are aligned. If misaligned, apply LARS method and note new axis. Take all changes noted and apply to a new trial lens. Reapply new trial lens to eye and start all over! Hopefully all of the modifications are correct and the fit and VA is perfect.
Problems and complications can happen because of hypersensitivity, not enough oxygen delivery, bacterial, viral or fungal colonization, inflammatory response or improper fitting. We need to fit the lenses properly and educate the patient so these problems can be avoided or recognized and fixed.
Micro mechanical trauma, corneal abrasion Staining on corneal surface due to dry eye or insuffient cleaning
Corneal edema due to hypoxia CL comlications
Mechanical, torn soft CLFungal cells on corneal
conjunctivitis Subepithelial infiltrates due to chronic hypoxia, white blood cells collect in epithelial pockets.
Ulcerative bacterial keratitis pseudomonas
acanthomoeba
Giant papillary conjunctivits More GPC
Your instruction and fitting are very important for the health of the patient’s eye. Make sure the lens is fit properly. Always evaluate your fit under the slit lamp to ensure a good fitting lens. Make sure the patient know the correct way to handle and clean the lenses. You are the person responsible for teaching the patient care and handling. Never let a patient leave without understanding the rules!
See you next week for insertion and removal. Have a great week.