Bridge II Introduction
Surgically altered corneas can present a challenge to contact lens fitters. Following refractive surgery, the central cornea is normally flatter than the mid-peripheral and peripheral cornea. Traditional GP lenses are designed to match the average cornea which is steeper at the apical central area and flattens toward the limbus. This steep to flat shape will not properly align with an altered cornea which has been flattened centrally.
The Bridge II design was developed by Conforma to better accommodate the altered shape of the post-refractive surgery cornea. The posterior surface of the Bridge II design is a reverse aspheric and progressively steepens by approximately 3 diopters in the intermediate zone. This steepening brings the posterior lens surface into closer alignment with the intermediate cornea. Beyond the intermediate zone, the Bridge II design flattens to accommodate the more normal peripheral cornea.
To summarize, the posterior surface of the Bridge II design is flat centrally, progressively steepens in the mid-periphery and progressively flattens peripherally. The Bridge II shape has proven beneficial in fitting not only refractive surgery patients, but also corneal transplant patients with bulging grafts or elevated corneal tissue at the host-graft interface. The unique reverse aspheric Bridge II manufacturing process is only available through Conforma Contact Lenses of Norfolk, VA.
When fitting the Bridge II design, there are five primary lens fitting parameters that will need to be considered as follows:
Base Curve - Central (R1 )
Second Radius - Intermediate (R2)
Third Radius - Peripheral (R3)
Reverse Zone (RZ)
Base Curve (R1) - The base curve should be selected to generally match the average curvature of the central cornea. A corneal mapping device will provide a more accurate representation of the entire corneal surface to include central, intermediate, and peripheral topography. A keratometer will simply average the two major meridians within a limited 3.0 mm section of the central cornea. This will not accurately represent intermediate and peripheral corneal curvatures.
Your initial trial lens base curve should be selected a minimum of 1 diopter steeper than the flattest central post-op corneal readings.
This is simple a starting point and you may need to go steeper to bring the lens into alignment with the intermediate cornea. If fitting from pre-op "K" reading, select the secondary radius based on pre-op flat "K" minus 1 diopter. The base curve should be selected to approximate 2/3 of the flattening induced by the surgical procedure. Your final base curve selection will be based on lens centration, movement and fluorescein evaluation. General alignment should be your objective, but will be very difficult to achieve. Central clearance or pooling is to be expected, alignment to minimal touch in the mid-periphery and controlled peripheral edge-lift should be the desired fluorescein pattern.
Corneal transplant patients should be fit based on post-op corneal measurements only. Pre-op corneal measurements should only be considered on refractive surgery patients.
Reverse zone (RZ) - The posterior surface of the Bridge II lens is machined to progressively steepen by approximately 2 to 10 diopters in the intermediate zone, as requested by the fitter. This intermediate steepening should bring the posterior lens surface into closer alignment with corneal surface. The intersection where this surface change takes place is referred to as the Reverse Zone (RZ). From the fitting standpoint, the RZ is unlike the Optical Zone on a traditional RGP lens. In fact, it is just the opposite as it relates to parameter change and resulting affect. The diameter of the reverse zone dictates where on the posterior surface the steepening begins. A larger zone will actually have a loosening affect in the intermediate zone. A smaller zone will have a tightening affect in the intermediate area. The reverse zone should normally be approximately 2.0 mm smaller than the overall diameter. This will allow a 360-degree band (1.0 mm wide) of intermediate and peripheral curves.
Secondary Radius (R2) - The aspheric secondary radius will always be steeper than the base curve, usually be 3 diopters. A secondary curve range of 2 to 10 diopters steeper than the base curve has been successfully manufactured in the Bridge II design. The secondary curve radius is a custom parameter to be altered by the fitter as needed to improve lens to cornea relationship.
Third Radius (R3) - The aspheric third radius is normally 5 diopters (1.0 mm) flatter than the secondary radius. The third curve radius is also a custom parameter to be altered by the fitter as needed to improve lens to cornea relationship. The third curve is normally slightly flatter than the base curve, but not always. In a case where the secondary is more than 3 diopters steeper than the base, the third curve could be steeper than the base curve. A fourth radius (r^4) us optional; however, it is typically not used when fitting post-surgical patients.
Lens Diameter - The overall lens diameter of a Bridge II lens is generally large, and diameters range from 9.5 mm to 10.5 mm. The lens diameter and reverse zone must be compliant with one another. As discussed earlier, the total width of the intermediate and peripheral sections must be adequate to allow the reverse aspheric (flat-steep-flat) effect to occur. As a rule of thumb, keep the lens diameter between 9.5 mm and 10.5 mm. Then subtract 2.0 mm from the diameter to determine the reverse zone.
Example: 10.5 mm - 2.0 mm = 8.5 Reverse Zone
Lens Power - To over-refract a Bridge II trial lens is the only accurate way to determine final lens power.