A Novel Keratoconic Piggyback Fitting Utilizing a SiH Lens & a Synergeyes KC Hybrid

Abstract: A 27 year old female keratoconic, with a history of gas permeable lens intolerance, was wearing soft toric lenses with best visual acuity of 20/25 OD and 20/300 OS. Various lenses were refit with poor results. The new Synergeyes hybrid design gave good visual acuity but excessive injection and moderate discomfort. The problem was minimized by uniquely piggybacking the Synergeyes lens with a silicone hydrogel lens. This modality, with a predicted Dk/t of 27 X 10-9 (barrers), falls within the acceptable levels of oxygen needed for daily wear.  The patient was able to comfortably wear this combination for 10 hours daily.  This modality was since duplicated in 3 additional cases.

Case History: Patient SM, a 27 year old African-American female was referred for a keratoconic refitting. She reported extreme discomfort with gas permeable lenses in the past and was currently wearing soft toric lenses although she had compromised visual acuity.  She had no medical problems and was using no medications. There was no family keratoconic history.

Her current lenses were B&L Soft 66 torics of 8.5 BC and a power  of -3.00 -1.25 x 70 OU. Her best visual acuity with over refraction was 20/25 OD and 20/300 OS. All external findings and retinal findings were normal.  Biomicroscopy revealed mild corneal thinning OU with a fine central opacity and fleischer’s ring OS.  All other examination findings were unremarkable.

Figure 1. Corneal topography with a steep, oval cone OS (left side of picture).

The patient was satisfied with the comfort and visual acuity of the right eye with the habitual lens. A diagnostic fitting for the left eye of a Rose K gas permeable Boston ES with the standard periphery gave a well centered, minimal apical touch fit with good movement and edge lift. A 6.2 mm BC, 8.7 TD, contact lens power of -7.00D gave 20/40+ acuity. However, the patient’s initial comfort reaction was poor and based on her past contact lens history, she refused a gas permeable lens.

All soft torics OS gave inadequate visual acuity. In late 2006, the Synergeyes keratoconic fitting set became available. [1] This lens is a hybrid design with a gas permeable center of material Parflufocon D (Paragon HDS) with a hydrophilic hemiberfilcon (27% water) skirt. The published DK (oxygen permeability) is 100 for the gas permeable center. The  DK for the soft skirt is approximately 9.  The lens has available base curves of 5.7 mm – 7.1 mm in 0.2 mm steps with a range of powers Plano to -20.00D in .50D steps. and a 14.5 mm total diameter with three available secondary curves designated as steep, medium, or flat.

With the Synergeyes KC lens the hope was to provide the optics of the gas permeable lens with improved comfort for the patient. SM was fit with a 6.3 mm BC, 8.2 SC , -6.50D Synergeyes KC lens giving 20/30 VA with plano over refraction. The lens initially centered well and had minimal areas of central touch. Initial comfort was excellent. The lens was dispensed with the Optifree system. Maximum wearing time was to be 8 – 10 hrs and a follow up visit was scheduled for one week later.

Upon return, the patient complained of excess injection increasing during the day, with burning and discomfort upon removal. The lens demonstrated little movement even with “push up”. Traditionally, the hybrid lens designs may show increased movement as the base curve is steepened. In the case of Synergeyes, the secondary curve may also be steepened to increase movement. The lens was reordered with a 6.1mm BC, 7.9 mm SC and -7.50D Rx. Again the lens initially exhibited good movement, and slight apical clearance centrally.

After two weeks  with a maximum wearing time of 6 hours, the injection remained. Switching to a Clear Care peroxide system and further steepening of the base curve gave no relief. In a last attempt, it was decided to try a silicone hydrogel lens as a piggyback under the Synergeyes. An 8.4 mm base curve Vistakon Oasys (Dk of 103) was used. It demonstrated good movement, centration, and comfort. A -2.00D Rx was selected for easy handling. The -2.00D will also present a slightly flatter anterior surface. The 6.1 mm Synergeyes positioned on top of the silicone hydrogel lens showed apical clearance, good centration and movement and a plano over refraction giving 20/30- VA.

Discussion:

Piggyback fits have frequently been utilized to alleviate lens discomfort and/ or corneal disturbance with gas permeable lenses fit to keratoconic corneas. [2, 3] The Synergeyes hybrid lens design has become a valuable alternative for fitting astigmatic and irregular corneas. However, a disadvantage to this modality is that some patients exhibit excessive peripheral lens tightening despite proper fitting efforts. Unique to this design and the previous Softperm hybrid (Ciba) is that decreasing the base curve radius may increase lens movement by increasing peripheral lens edge “bounce”. Despite all efforts, some patients exhibit excessive injection, discomfort and lens removal
difficulties. This results in minimal or forced wearing times, conjunctival and cornea disturbances, and possible complete lens failure with this modality.

In this presented case, a piggyback silicone hydrogel of published DK 103 was selected. The 8.4 mm base curve and 14.0 mm TD often fits well on the keratoconic cornea. A low minus lens was selected for handling ease. The low minus silicone hydrogel has a tendency to present a slightly flatter corneal surface as measured by keratometry or topography. The Synergeyes lens of 6.1 mm BC, 7.9 mm SC, and -7.50D Rx atop the silicone hydrogel, had more apical clearance, due to the flatter topography and decreased peripheral lens compression as a result of the silicone hydrogel apparently elevating the Synergeyes’ mid-periphery. This may result in reduced injection and increased comfort and ease of removal. In this case, the patient was able to wear this piggyback combination for 10-12 hours a day with much improved comfort and a significant reduction of Synergeyes induced hyperemia.

The Synergeyes has a published center gas permeable DK of 100. The lenses’ center thickness measured as 0.17 mm giving a DK/t (oxygen transmission) of 58.8. An estimated calculation of the oxygen transmission of this piggyback system can be made with fluid dynamics formulas utilized for blood and gaseous flow through IVs and stents. The Poiseuille formula for laminar flow fluid dynamics can be simplified for clinical  purposes to resemble Ohm’s law of electrical resistance. [4,5]  Ohm’s law relates voltage drop (V) across an electrical circuit to electrical resistance ® and electric current (I) as V = IR. Fluid flow calculations can be modeled as electric circuits with P (pressure) = Q (volumetric flow rate) R (resistance).

The resistance of two lenses in series is the sum of the resistance offered by each.  For the piggyback system the total resistance is the sum of the resistance of each lens. Lens oxygen transmissibility is the reciprocal of resistance so to determine the resistance of two lenses in series, add the reciprocal of their oxygen transmissions. In addition, there is a tear layer between the two lenses that offers a small additional resistance beyond that occurring with just one lens on the cornea. The resistance of the tear layer is approximately .003 or a DK/t of 333. [6, 7]

For the presented case, the resistance of the hybrid’s gas permeable center was the reciprocal of 58.8 or .01695. The DK/t of the 103 DK silicone hydrogel with a .07 mm center was 147, giving a resistance of .00680. The total resistance for the system was .01695 + .00680 + .003 = .02675 equating to a DK/t of about 27 units (10-9 (cm/sec) (mlO2/ ml x mmHg)). This would meet the Holden and Mertz criteria 8 of a DK/t of 24 to cause zero corneal edema with contact lens daily wear.

Weissman and Ye [9] calculated oxygen tensions for tear layers between piggyback lens combinations and the anterior cornea. They estimate that maintaining an open eye tear layer oxygen tension of about 100 mm Hg would likely be physiologically tolerable for most corneas. From their calculated tables, for the Synergeyes GP center of DK 100 and DK/t of close to 60, and the silicone hydrogel with a DK/t of close to 150, the predicted tear layer oxygen tension under this piggyback combination would be close to 114 mm Hg, also satisfying the oxygen tension criteria. 

The hybrid’s periphery would offer considerably more resistance with a water content of 27%. Assuming a DK of 9, and an average edge thickness of .20 would give a DK/t of about 4.5. The underlying silicone hydrogel would have minimal effect on the already low lens oxygen supply.  However, Pilskalns, Fink, and Hill [10] compared the relative oxygen uptake rates of keratoconic corneas while wearing the Synergeyes and Sofperm hybrid lenses. The Synergeyes lens allowed significantly more oxygen to reach the cornea than the Sofperm lens at the central cornea as well as at areas 2.0mm and 4.5mm temporal to the central cornea. While the 2.0 mm temporal area was under the gas permeable portion of the lens, the 4.5 mm temporal area was under the soft portion. At this location, oxygen reaching the cornea is likely derived from oxygen passing through the soft lens component and by lateral flow of oxygen passing through the rigid lens portion, resulting in better oxygen supply in this mid-peripheral area than would be predicted by the soft lens DK/t alone. At an area 1.0 mm temporal to the limbus, oxygen supplied to this location must pass through the soft portion only, and there was no statistically significant difference in oxygen uptake between the Synergeyes of DK 9.3 and the Softperm of DK 5.5.

Conclusion:

Piggyback use for keratoconics and irregular corneas has long been valuable for solving troublesome lens adaptation cases. The new Synergeyes hybrid design is also a valuable adjunct to fitting irregular corneas. This case describes an unusual combination of the Synergeyes piggybacked with a silicone hydrogel lens. Subsequently, three other silicone hydrogel-Synergeyes piggyback cases have been successfully prescribed by the authors.

References:

1. www.Synergeyes.com website
2. Soper JW. Fitting keratoconus with piggyback and Saturn II lenses. Contact Lens Forum 1986; 1: 25 – 30.
3. Baldone JA. The fitting of hard contact lenses onto soft lenses in certain diseased conditions. Contact Lens Med Bull 1973; 6: 15 – 17
4. Personal correspondence 1989 with Dr. Irving Fatt, Univ of California Berkeley College of Optometry, and 2005 with Dr. William J Benjamin UAB Birmingham College of Optometry.
5. Fatt, I. Oxygen transmissibility considerations for a hard-soft contact lens combination. Am J Optom Physiol Opt 54 (1977), pg 666-672.
6. Samett EJ. Basic fluid dynamics principles – application to percutaneous intervention. www.Emedicine.com/radio/topic867.htm
7. Scheid TR. Piggyback fitting post intacs insertion for keratoconus. www.siliconehydrogels.org , In the Practice, April 2005: 1 -5.
8. Holden, BA, Mertz GW. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthal Vis Sci. 25 (1984): 1161-1167.
9. Weissman, BA, Ye, P. Calculated tear oxygen tension under contact lenses offering resistance in series: Piggyback and scleral lenses. Contact Lens and Anterior Eye, Volume 29, Issue 5 (2006): 231-237.
10. Pilskalns, B, Fink, B, Hill, R. Oxygen Demands with Hybrid Contact Lenses. Optpm Vis Sci (2007) , 84: 334-342.