RIGID GAS PERMEABLE LENS APPLICATIONS, BENEFITS, AND MATERIALS
•Approximately 20% of CL patients; 12% of new fits
•Higher % in Japan (>50% of new fits)
•30-50% in CL specialty practices/medical settings
•Result of better optical quality, surface wettability & astigmatic correction capabilities
•Johnson/Schnider study:
N = 20 subjects
Each subject wore RGPs for 6 weeks and soft for 6 weeks
All were unadapted to CL wear and good soft and RGP candidates
Results:
Vision better, staining less with RGPs
8 preferred RGPs; 15 would be satisfied with them
•Oxygen Transmission 2-4 x greater
•Reduced GPC
•Reduced Ocular Infection
•Reduced Acanthamoeba Adherence
Dk = Oxygen Permeability (i.e., potential for oxygen to pass through a contact lens; equals diffusion x solubility)
Dk/L – Oxygen Transmission (i.e., the actual oxygen transmission through the lens; equals Dk divided by lens thickness {x 10}
Example: if Dk = 30 and thickness = .15mm then
Dk/L = 30/.15 x 10 – 20
EOP = Equivalent Oxygen Percentage (how much of 215 oxygen will be predicted to reach anterior cornea)
Holden/Mertz Criterion:
Category EOP Dk/L
Daily Wear 9.9% 24.1
Extended Wear 12.01% 34.3
(no resid. Swelling)
Extended Wear 17.9% 87
(ideal)
Bennett/Gordon Evaluation of Dk/L of Current Soft & RGP Materials:
•Compared 3-4 commonly used soft and rigid lenses in each of several categories including EW, DW, + and – powers
•RGPs met or exceeded Mertz/Holden criterion in almost all categories; disposable soft in – power only one to exceed criterion
•Soft extended wear (i.e., disposable and traditional) only provide 1/3 to 1/2 desired oxygen in + powers
Hay/Seal Acanthamoeba Adherence Study:
•Evaluated adherence to unworn CL materials (8 hydrogel and 2 RGP; 2 species: castellanii and polyphaga; 2 forms: cyst and trophozoite)
•Significant adherence to hydrogel samples; higher adherence to ionic vs. non-ionic; high water vs. low water
•No adherence to RGP buttons unless rinsing step omitted
•Consumer Reports
•Weiss (CL Forum): N= 10 (6 months); 7 = RGP; 2NP; 1 = soft
•Fonn/Holden (Academy J): N = 31; 15 = RGP; 7 = NP; 9 = soft
•Houston Myopia Study: after 3 years; RGP group increased approx. 0.50D in myopia; spectacle group approx. 1.50D
•Only 1% of mail order are RGPs
•RGPs are custom devices which are hard to sell in pharmacies
•RGPs are rarely price advertised
•You can use service agreement with your RGP patients
•Fewer RGP contact lens prescriptions are issued to patients:
Because of custom nature, RGP Rx’s are not filled as often: When prescribing, include the following information (Grohe)
•West practice profits: RGP vs. soft
•Ames study: 38% RGPs = 48% of the income
•Ames/Gunning: Profit per replacement lens higher
2 x as many spectacle sales
8 x as many service agreements
•Copolymers with "silicon" for oxygen permeability, methylmethacrylate for stability, wetting agents (methacrylic acid, HEMA), and cross-linking agents for stability
•First successful RGP lenses
•In higher Dk materials, wettabiity became a problem due to silicon content
•Combined fluorine with the other ingredients of S/A to enhance mucin interaction with lens surface, improving wettability and allowing manufacturers to increase Dk
•Representative low Dk (< 50) F-S/A materials include:
Fluoroperm 30 (Paragon Vision Sciences)
Boston ES –29 Dk material using Aerocor technology (i.e., low silicon, no MMA or traditional crosslinkers); can be made in a thin lens design (Polymer Technology Corporation)
•Representative High Dk (> 50) F-S/A materials include:
Fluoorocon (Fluoroperm 60 material from PBH): thin lens design with lid attachment/Korb edge
Boston 7 (Polymer Technology Corporation): 73 Dk with Aerocor technology
Menicon (Menicon): 159 or 104 Dk; good wettability despite high Dk, originated in Japan
Choosing a Lens Material
•Introduction
There are presently a number of RGP lens materials available for the contact lens practitioner to choose from. The availability of all of these different materials causes the practitioner to often wonder "What material is preferable for our patients?"
The answer depends on the particular patient to be fitted.
The accompanying chart "RGP Material Selection" shows Dr. Ed Bennett’s recommendations for material selection. It can be divided into these categories:
Þ Refractive error
Þ Corneal Topography
Þ Refits
Þ Occupation/hobbies
RGP materials can further be divided by Dk:
Þ Low Dk = 25 to 50
Þ High Dk = >50
•Refractive Error
Because of the thin center thicknesses typically demonstrated in minus power contact lenses, most myopic patients would benefit from the dimensional stability provided by low Dk FSA lenses, while still meeting (or approximating) the cornea’s daily wear oxygen requirements.
However, if corneal edema is present with a low Dk lens material, often the result of either a high corneal oxygen demand (which varies between individual patients) or a tight-fitting lens, the patient should be refit into a higher Dk material.
Hyperopic patients will benefit most from a high Dk lens material because of the greater center thicknesses present in these lens powers. For the same reason, dimension stability problems with high Dk materials are less with plus (versus minus) powers.
•Corneal Topography
Patients with moderate astigmatism (i.e., > 1.50D) benefit from the flexural resistance provided by low Dk FSA lens materials.
High astigmatic (i.e., 2.50D and greater) patients often benefit from Spherical Power Effect (SPE) or Cylinder Power Effect (CPE) bitoric designs available from Pilkington/Barnes-Hind in the higher Dk Fluorocon material.
•Refits
Former PMMA and first generation RGP lens wearers should be refit into low Dk rigid lenses. Typically, these individuals have established care habits that could be damaging to newer, softer RGP lens materials. Surface scratches, and warpage could occur, especially if these patients are not properly educated in lens care and handling techniques.
Previous hydrogel lens wearers, who have experienced deposit-related problems (redness, itching, decrease in wearing time) resulting in papillary hypertrophy, would benefit from being refit into any RGP lens material, preferably the most wettable available material. This would include low Dk FSA, FFP or hydrolyzed silicone-based materials. These materials would also be recommended for all borderline dry eye patients and mild allergy suffers.
•Occupation
Individuals who perform much near work would benefit from the highest wettability materials available (similar to the previously mentioned papillary hypertrophy patients) supplemented by frequent application of rewetting/reconditioning drops.
While athletes generally benefit most from hydrogel lens use, if this option is not satisfactory, a large RGP design which would less likely displaced, would be recommended. Many of today’s FSA lenses are available in these larger designs.
Individuals who desire (or have a need) to wear their lenses on a flexible schedule or extended-wear basis (such as nurses, police, firefighters) would benefit from a high Dk FSA material.
Pilots and flight attendants who are often exposed to less than optimum oxygen levels would benefit from high Dk RGP lens materials.
CASE ONE – Mrs. T. Pendleton is a satisfied hydrogel wearer inquiring about RGPs. What do you do???
CASE TWO – 16 year PMMA wearer, currently wearing Polycon II who is dissatisfied with vision through spectacles and contact lenses. What do you do???
CASE THREE – Patient is a silicone/acrylate wearer with dryness symptoms with inferior decentered lenses. What do you do???
CASE FOUR – Jimmy Jock is a baseball player who complains of foggy vision. SLE shows mild GPC and very deposited lenses. What do you do???
CASE FIVE – Hilda Hyperope has an RX = +6.00 – 1.75 x 180 OU. She is very motivated but has no preference for CL type. What do you do???
CASE SIX – Lucille Lifesaver is a 6D myope who desires good vision and long wearing periods as she is a nurse. What do you do???