By Dr. Brad Giedd
JC 41 yo WM – Referred by local commercial doc for GP/Hybrid fitting. h/o soft toric contact lens wear – unhappy with current vision 20/40 OD, 20/30- OS (brand/parameters unknown).
MR: OD: -12.00 -2.50 X180 20/25- OS: -12.00 -3.50 X180 20/25-
Ks: OD: 45.00/47.25 @090 smooth mires OS: 45.25/47.25 @085 smooth mires
OD: SynergEyes 45.50/-3.00/8.7/14.5 SOR: -7.50 20/25+
Well-centered lens/good NaFl pattern/better visual acuity/ rejects 2° cost
OD: GP SPE Bitoric 45.00/46.75 -10.00/-11.50 9.0 Tricurve/Lenticular/HDS matl.
Well-centered Interpalpebral fit / Minimal apical clearance / 20/20- visual acuity
OS: GP SPE Bitoric 45.00/46.75 -10.50/-12.50 9.0 Tricurve/Lenticular/HDS matl.
Well-centered IP fit / Min. apical clearance / 20/20- visual acuity
The hybrid lens fit well, but as cost was a critical factor to this patient GP’s became the more cost-effective option and actually provided slightly better visual acuity. The hybrid GP did show an adequate, but not textbook NaFl pattern with some bearing in the horizontal meridian; this led me to choose a toric fitting surface to maximize lens/cornea physiology.
By Dr. Brad Giedd
PB 39 yo HF – Referred by local Lasik surgeon for GP fitting – Patient not a candidate for Lasik 2° Keratoconus – Current specs not giving adequate visual acuity 20/80 OD, 20/30- OS.
MR: OD: -2.50 -4.00 X045 20/25 OS: -1.00 -5.00 X160 20/25-
Ks: OD: 44.75/51.87 @106 distorted mires OS: 44.62/52.12 @076 distorted mires
OD: GP spherical 6.80/-8.00/8.6 20/20- Tricurve/Lenticular/HDS matl.
Well-centered lens / mild 3-9 mid-peripheral bearing / adequate edge lift
OS: GP spherical 7.00/-6.25/8.6 20/20 Tricurve/HDS matl.
Well-centered lens/ mild 3-9 mid-peripheral bearing / adequate edge lift
OD: Good vision / adequate comfort, some pm irritation / mild 3-9 staining
OS: Good vision / adequate comfort, some pm irritation / mild 3-9 staining
OD: GP SPE Bitoric 48.00/50.00 -6.50/-8.50 9.0 Toric PC’s/HDS matl.
Well-centered lens/minimal apical clearance / good edge lift 360°
OS: GP SPE Bitoric 47.00/49.00 -5.00/-7.00 9.0 Toric PC’s/HDS matl.
Well-centered lens/ minimal apical clearance / good edge lift 360°
Most keratoconus patients, especially those with smaller, central cones do well in spherical GP’s. Sometimes, however, a toric fitting surface will give better lens-cornea physiology. In this case, the spherical lens caused some symptomatic 3-9 staining that resolved with a bitoric lens and toric peripheral curve system.
By Dr. Clarke Newman
JN, a thirty-three-year-old, white male Cellist presented as a new patient with a chief complaint of decreased visual acuity and poor comfort. He wore rigid Gas Permeable (GP) lenses on a daily wear basis, and had done so for a period of eighteen years. His wearing time was roughly ten hours per day, and he presented after five hours of lens wear. His Past Medical History, Family History, and Review of Systems were wholly non-contributory. He denied taking any medications, having any system or medication allergies. His Social History included playing 1st Cello in a very large, well-known symphony orchestra. He liked to play tennis and do aerobics. JN wanted new GP lenses that “fit better.”
Current Spectacle Rx:
OD -5.00 -2.25 X 004 20/30 -2
OS -3.50 -4.25 X 161 20/30 +1
Sphero-Cylinder Over Refraction:
OD +0.00 DS 20/25 +1
OS +1.00 -1.00 X 145 20/20 -1
OD -4.75 -1.50 X 170 20/20
OS -2.00 -4.75 X 163 20/20 -1
OD 43.75 45.00 @ 008
OS 43.25 46.25 @ 075
His Horizontal Visible Iris Diameter (HVID) was noted at 11.5 mm in each eye. His Vertical Visible Aperture (VVA) was noted at 10 mm in each eye, as his upper lids covered the superior limbus by 1.5 mm in each and his lower lid was at the limbus.
JN’s pupillary function, confrontation visual fields, cover test, applanation tonometry, anterior segment evaluation, and dilated fundus examination were wholly unremarkable.
JN was evaluated with the Oculus Pentacam, and the following data was collected:
His current contact lenses were eight years old at presentation, and they were iso- spherical base curve in a tri-curve design. At the time of presentation, JN used the Boston Original care system and reported good compliance. The measurements taken from his current lenses are as follows:
The lenses were moving poorly and centering poorly. With each blink, the lenses were picked up under the upper lid and then they lagged down 1.5 mm, inferior-nasally. The fluorescein pattern, especially in the left eye, showed central bearing with a toroidal pattern of about two Diopters. (See Photos of the left eye below.)
There are three basic types of toric lenses. First, there is the iso-base curve, toric front surface lens. This design is indicated in patients who have anterior corneal toricity of less than two Diopters while having a significant amount of physiological, internal residual astigmatism. These lenses are stabilized in rotation by a prism ballast or peri-ballast, and, occasionally, combined with an inferior truncation.
The second type of toric design is the spherical front surface, toric base design. This lens is indicated for patients with anterior corneal toricity that is greater than two Diopters and who have an amount of physiological residual astigmatism that is equal but opposite from the induced astigmatism created by the toric base curve. Obviously, this lens application is rarely indicated.
One classically thinks that toric base optics induces a residual cylinder that equal to one-half of the difference between the two base curve radii. However, the induced cylinder is a function the index of refraction of the lens material. The “Rule of One-Half” is based on the index of refraction of PMMA, which is 1.49. If one were to use one of the more contemporary GP lens materials, one would find that the index of refraction is less than that of PMMA — sometimes significantly less. For example, all of the Boston lens materials have an index around 1.43. These materials, when one recalculates the induced cylinder, induce half of the cylinder of a PMMA lens.
This fact is important when one considers the third type of toric lens, the bi-toric lens. This lens has a toric base to align geometrically with a toroidal anterior corneal surface, and a toric front surface to surface to correct the residual astigmatism, both induced and physiologic.
There are three types of bi-toric lenses. The first is the “Spherical Power Effect” lens, or SPE lens. First described by Sarver, this lens has a compensating cylinder on the front of the lens that exactly compensates for the induced cylinder from the toric base optics. This lens is indicated when there is no physiological residual astigmatism. A patient who has a refractive astigmatism that is equal to the anterior corneal astigmatism is a candidate for this type of toric lens. This design is very handy, as the higher the amount of astigmatism a patient has, the more likely it is that the astigmatism is due to the front of the cornea.
The second type of bi-toric lens is the “Cylinder Power Effect” lens, or CPE. This lens has a compensating cylinder on the front of the lens that compensates for both the induced and physiological astigmatism. When the total astigmatism is not equal to the induced cylinder, then a CPE lens is indicated.
The third type of bi-toric lens is the “Crossed Cylinder Effect” lens, or CCE. This lens is indicated when the anterior corneal astigmatism is mislocated by more than twenty degrees from the total refractive astigmatism. These lenses are indicated about five percent of the time. When one cannot seem to resolve the optics, the cause is often this misalignment and a CCE might help. The CCE has the principle meridians on the front of the lens rotated in alignment from the back of the lens. These lenses are difficult to make, and require a very skilled lab.
It is counter-intuitive, but a bi-toric is thinner when it is made with a lens of a lower index of refraction. This fact is due to the lower induced cylinder that must be compensated. The junctional thicknesses are significantly lower in low index material bi-toric lenses. A good material for toric lenses is a material that is low in index but very stiff so that it is machinable and dynamically stable. The Boston lens materials and the Menicon Z materials are good choices as they meet these criteria. Optimally, the material should also have a low specific gravity to keep the lens mass down. However, these materials have relatively high specific gravities. For this patient, Menicon Z was selected for the afore mentioned properties as well as the high Dk/t.
The right eye needed a slight change in the iso-Spherical base curve in a quad-curve design lens. The left eye needed a lens that had a toric base curve to promote better alignment, and a toric front surface to correct the residual astigmatism. In this case, the total refractive cylinder of the left eye was 4.25 Diopters. The anterior corneal cylinder is 3.00 Diopters. Since the principle meridians of refractive cylinder and the anterior corneal toricity are roughly aligned, a CCE was not indicated. Further, since the total refractive cylinder and the anterior corneal cylinder were not equal, so, an SPE was not indicated. Therefore, a CPE bi-toric lens is indicated.
The next decision is the design philosophy. There are two basic philosophies. First, is the “saddle fit.” In this design, the base curve in each meridian is aligned with the corneal curvatures. The second concept is the “low toric simulation” design. This design is preferred for patients with with-the-rule or against-the-rule astigmatism. The saddle fit works better when the astigmatism is on the oblique.
The value of the low toric simulation is that a lens centers laterally and moves easier vertically on a cornea that has roughly one Diopter of with-the-rule cylinder. The Mandell-Moore guide was created to assist the prescriber in creating a low toric simulation. However, the Mandell-Moore Guide has a fundamental flaw in that it speaks to the “flat” and the “steep” meridians, when it should reference the “horizontal” and the “vertical” meridians. The Mandell-Moore guide succeeds in creating a low toric simulation of with-the-rule astigmatism when the patient has with-the-rule astigmatism. However, it fails to do so when the patient has against-the-rule anterior corneal toricity. This author has created a system that addresses the horizontal and the vertical meridians. In this manner, the horizontal meridian is always aligned and the vertical meridian is always one Diopter flatter than the corneal curvature.
To achieve alignment, the spherical curve must be flatter than the measured corneal curvature for three reasons. First the cornea is not spherical but flattens in a prolate fashion towards the periphery. Second, the chord of the posterior optic zone is less than the corneal diameter. Third, the keratometric measurements are Diopters of power that take into account the index of refraction of the cornea and the curvature of the posterior corneal surface. What is truly needed for proper alignment of the lens to the cornea is not Diopters of power but Diopters of curvature.
JN had an HVID of 12 mm. In this author’s experience, a proper lens diameter for this HVID is 9.60 mm with a posterior optic zone diameter (POZ) of 8.00 mm. When these parameters are selected, alignment in the horizontal meridian is achieved by prescribing a curvature that is roughly one-quarter of a Diopter flatter than the measured curvature. A base curve of 7.84 mm was selected and the Back Vertex Power (BVP) was adjusted to compensate for the flatter curvature. The Manifest Refraction was – 2.00 D. Therefore, the lens power is – 1.75 D. The base curve for the vertical meridian was selected at 1.25 D flatter than the measured curvature, or 7.50 mm. The power in the vertical meridian was vertex corrected and reduced by the 1.25 D to compensate for the flatter alignment. The power in the vertical meridian of the Manifest Refraction was -6.75 D. When the correct compensations were made, the BVP was – 5.00 D.
The lens parameters ordered were the following:
|OD||9.60||8.00||7.75||—||8.40||-4.25||—||TAP||0.13||BC+0.80||BC+1.0 0.30||BC+1.50 0.20||Blue||Menicon Z|
|OS||9.60||8.00||7.84||7.50||8.40||-1.75||-5.00||TAP||0.13||BC+0.80||BC+1.0 0.30||BC+1.50 0.20||Blue||Menicon Z|
A quad-curve peripheral curve design was selected to better approximate the flattening peripheral cornea. Further, toric peripheral curves were used. When using iso-spherical or iso-aspherical peripheral curves are used, they change the base curve relationship by changing the shape of the posterior optic zone from round to ellipsoidal, thus changing the sag relationships.
These lenses were lenticulated to create thinner junctional and edge thicknesses. This design element will improve comfort and decrease the overall mass of the lens by removing plastic. Decreasing the mass will improve the chances that a lid attachment can be achieved.
The lenses were dispensed and the comforted was reported as excellent. Further, the visual acuities were improved. The acuity in the right eye was 20/20 +2, in the left 20/25 +2, and with both eyes together was 20/15 -2. The lenses centered well, and there was good alignment with the simulation of 1.00 D of with-the-rule toricity on a cornea that 3.00 D. Of particular note was the even alignment of the peripheral curve set. (See photo below)
Upon follow up, this patient reported wearing time of fifteen hours a day, good comfort, and great visual acuity.
If one understands the dynamics of toric lens prescribing, then one can significantly improve the comfort, tolerance, and visual acuity of one’s patients. Having a good understanding of all of the options available makes the prescriber better able to correct defects in patient’s lens systems that cause morbidity.