Bitoric Cylinder Power Effect: Jill Beyer, OD, FAAO
C.R. had recently received hyperopic lasik surgery (OS) and was referred for a lens fitting for his right eye by his refractive surgeon due to anisometropia. C.R. was to be returning to his home country and was unsure whether he would proceed with a refractive surgery in the right eye. He requested a lens that would provide optimal vision for the right eye.
Test Procedures, Fitting/Refitting, Design and Ordering
Manifest Refraction with Visual Acuities:
OD: +7.25 -4.00 x 160 20/25
OS: +0.75 -1.25 x 005 20/25 (Post-Hyperopic Lasik)
Corneal Topography (Sim Ks):
OD: 39.00 @ 160; 42.25 @ 070
OS: 42.25 @ 179; 45.00 @ 089 (Post-Hyperopic Lasik)
Diagnostic Contact Lens OD:
Base Curve Radii: 41.00/39.00
Overall/Optical Diameters: 9.0/7.6mm
Over-Refraction: +5.25 -0.75 x 165 20/25
Biomicroscopy (with Contact Lenses):
A mild with-the-rule astigmatic pattern was present after fluorescein application.
New Contact Lens OD:
Base Curve Radii: 41.25/39.00
Overall/Optical Diameters: 9.0/7.6mm
Peripheral Curve Radii: Toric SCR & PCR
The initial lens order was determined by fitting “On K” in the flat meridian and fitting one diopter flatter than “steep K” in the steep meridian to provide for an adequate tear pump and to prevent lens adherence.
Proper vertexing of each meridian separately is important, Use of an optical cross quickly and easily facilitates the determination of the appropriate power and amount of compensation for the lacrimal lens induced by the contact lens.
While the Mandell-Moore Bitoric Lens Guide can be utilized, the use of the optical cross is quite easy and provides a helpful perspective on the orientation of the astigmatic correction. Additionally, the optical cross method proves that if a doctor can do simple addition and subtraction, then he or she can easily design a bitoric GP lens!
Patient Consultation and Education
Lens adaptation was discussed with this patient and advantages of GP toric use were reviewed. The patient appreciated the concept of the high oxygen permeability of the lens and the possibility of optimal visual acuity with a GP lens. Additionally, the patient appreciated the hygiene and durability aspects of corneal GP lens wear, and opted to proceed with a GP bitoric lens fit.
Follow-Up Care/Final Outcome
OD Lens Dispensing Visit:
Over-Refraction and Visual Acuity:
OD: +0.50 DS 20/25
Biomicroscopy (with Lens On):
Good lens centration with a mild with-the-rule astigmatic pattern; equal 360 degrees clearance is present peripherally.
The lens showed good stability and based on markings on the flat meridian, lens rotation was minimal.
Application and removal training was provided upon the completion of the examination and lens adaptation was reviewed.
The new CPE bitoric lens provided 20/25 vision OD after the adaptation period was completed and the patient was thrilled with the visual acuity.
Discussion/Alternative Management Options
A GP lens is visually advantageous for patients with high astigmatism. Additionally, in this case with a high hyperope, there is the benefit of the high oxygen transmission through a thick GP lens; an astigmat appreciates excellent visual acuity due to the lens optics.
Although spherical power equivalent (SPE) bitoric lenses are preferred for lens rotation purposes, and base curve toric lenses are preferred for lens optics; if neither of these options are applicable then cylindrical power effect (CPE) corneal GP bitoric lenses are still an excellent choice to provide pristine GP optics when relative lens stability is maintained. In CPE toric lens cases, the amount of corneal toricity does not equal the amount of astigmatism in the spectacle prescription thus resulting in residual astigmatism.
It is important to have the laboratory place markings on the flat meridian to monitor lens rotation and rotational stability. Often rotation is not an issue, as the toric back surface fits nicely on the toricity of the cornea. However, if rotation occurs, then the amount of residual cylinder (the difference of the astigmatism in the spectacle prescription compared to the corneal toricity) will cause fluctuation in vision similar to that of a rotating soft toric lens. Once again the advantage persists over a soft lens in that the GP lens optics are excellent, and only the remaining residual cylinder will cause visual compromise if lens rotation occurs.
With an SPE bitoric, the back surface toricity matches the amount of lens power differential, so any rotation of the lens perfectly neutralizes the astigmatism by means of the lacrimal lens. With a CPE bitoric lens, partial neutralization of astigmatism occurs due to the lacrimal lens, but any remaining residual cylinder correction needs be stable on the cornea.
While a soft lens may have been an option in this particular case, the benefits of oxygen permeability and GP lens optics were obvious enough that the use of a GP toric lens was the first choice. Additionally, this lens fit was simple and yet, the patient felt as if he was obtaining true specialty care and experiencing superb vision. Toric soft contact lenses with over +7D sphere and 4D cylinder are available in custom designs, But lens delivery can be slow, vision quality may not be adequate, and oxygen permeability and tear exchange might not be optimal.
- Edrington, TB. Rigid gas-permeable lenses for astigmatism. In Hom MM: Manual of Contact Lens Prescribing and Fitting (1st Ed.) Butterworth-Heinemann, Newton, MA, 1997:155-159.
- Lindsay RG, Westerhout DI. Toric contact lens fitting. In Phillips AJ, Speedwell L. Contact Lenses (4th ed.) Butterworth-Heinemann, Jordan Hill, Oxford, 1997: 477.
There are three easy to use guides for designing bitoric corneal GP contact lenses available on the GPLI website: The Mandell-Moore Bitoric Guide, The Quinn Bitoric Guide, and The Newman Bitoric Guide.
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