Article

HYBRID LENS STRATEGIES FOR REGULAR AND IRREGULAR CORNEAS

Tips for successfully fitting today’s hybrid contact lens designs on any type of cornea.

Hybrid contact lenses have a rigid GP center that is surrounded by and fused to a soft skirt. The first patent on this type of lens was filed in 1973 by Louis J. Page; his aim was “to provide the visual acuity of the hard type lens and the comfort of the hydrophilic lens type with the possibility to incorporate multifocal visual correction.”1 Forty-five years later, the aims of this lens type are still the same, although many advances have improved their performance. This article will discuss the characteristics, indications, fitting, and care of today’s hybrid lens designs.

HYBRID LENS CHARACTERISTICS

Considering the many different types of contact lenses that are available to manage normal and irregular corneas, it is important to determine which design will produce the best result based on the anterior segment characteristics and the visual needs of each wearer. For normal and irregular corneas, hybrid contact lenses provide the same optical performance as that of corneal GP lenses. Compared to corneal GP lenses, hybrids are more comfortable, often exhibit better centration, and are more stable. These advantages are more evident in cases of irregular corneas, as the rigid portion is designed to vault over the corneal irregularity. Compared to soft lenses, hybrids are uniquely indicated when the quality of vision is affected by residual high-order aberrations or when toric soft contact lenses fail to provide stable vision. They also are indicated for compensation of presbyopia in astigmatic patients.

Compared to piggyback lens systems in which a GP lens is fit on top of a soft lens, hybrids are easier to handle, simpler to care for, and they provide increased oxygen transmission. Despite the tremendous growth in the use of scleral GP lenses, hybrids present some distinct advantages in many cases. For example, hybrid lenses allow a better tear flow, have lower apical clearance, reduce the “fogging” effect, increase oxygen transmission, and the soft skirt conforms to the scleral shape. Table 1 summarizes the advantages and disadvantages of hybrid lenses with respect to other contact lens types.

TABLE 1 PROS AND CONS OF HYBRID CONTACT LENSES COMPARED TO OTHER DESIGNS
HYBRID VERSUS GP HYBRID VERSUS SOFT HYBRID VERSUS PIGGYBACK HYBRID VERSUS SCLERAL
PROS • More comfortable
• Quicker adaptation period
• Easier to center the optics over the visual axis
• More stable quality of vision
• Vaulting results in less mechanical interaction with the cornea
• Won’t dislodge
• No risk of 3 o’clock & 9 o’clock staining
• Lower negative powers are necessary
• Unilateral wear
• Better and more stable quality of vision
• No need for lens stabilization to correct astigmatism
• Better vision for patients who have significant high-order aberrations
• More effective for correction of presbyopia in astigmatic patients
• Easier to handle
• Easier care
• Increased oxygen transmission
• Soft skirt conforms to scleral shape
• Less chance of seal-off
• Reduced “fogging” effect induced by tear debris trapped behind the lens
• Lower clearance
• Optics of the lens are closer to the entrance pupil
• Increased oxygen transmission
CONS • More difficult to apply and remove
• Longer time to settle
• Must be replaced more often
• Higher cost
• More difficult to fit
• More careful evaluation of lens-to-anterior segment required
• More difficult to apply and remove
• Less frequent replacement
• More careful evaluation of lens-to-anterior segment required
• Cannot substitute different lens materials
• More difficult to fit on very asymmetric profiles or very irregular corneas
• Must be replaced more often
• Longer time to settle

HYBRID LENS FITTING ON NORMAL CORNEAS

Hybrid contact lenses for normal corneas are indicated for correcting low-to-high myopia, hyperopia, and astigmatism (when there is a similar amount of refractive and corneal astigmatism with a difference not higher than 0.75D) as well as presbyopia. This discussion will be limited to the fitting procedures for the most recent generation of hybrid contact lenses for normal corneas available in the United States. Compared to the first generation of modern hybrid lenses, the newest generation provides a better distribution of forces between lens and cornea, which helps enhance tear exchange and lens movement. This occurs because the design has a prolate optical zone and a new junction lift area with a flatter radius compared to the reverse curvature of the soft skirt.

This design on the back surface is used in both the standard and progressive designs of the hybrid lenses for regular eyes. In addition, the progressive simultaneous vision lens is available in both center-distance and center-near designs. The center-distance lens has an adjustable center-distance optical zone that ranges in size from 1.8mm to 4.0mm and is selected based upon the photopic pupil size. The add powers range from +0.75D to +5.00D. The center-near design has a progressive zone of 3mm and is available in three add powers (+1.00D, +1.75D, and +2.50D).

Initial Lens Selection and Evaluation This family of hybrid designs can be fit empirically using keratometry, subjective manifest refraction, and horizontal visible iris diameter (HVID)—without the need for a fitting set. An online calculator is available to help with initial lens selection. The base curve should be 0.10mm or 0.50D steeper than the flat meridian. The soft skirt curve is selected based on the HVID; use a medium (8.1mm) skirt for HVIDs ≥ 11.8mm and a flat (8.4mm) skirt for HVIDs < 11.8mm. The power of the single-vision lenses and the power of the distance correction in the multifocal lenses can be calculated by adding –0.50D to the sphere of the subjective manifest refraction. For example, for a manifest refraction of –1.00 –1.00 x 180, the power to order would be –1.50D; for a manifest refraction of +1.00 –1.00 x 180, the power to order would be +0.50D. In higher refractive powers, you would also need to compensate for the vertex distance.

Choose between the center-distance and center-near designs depending on patients’ visual demands. Traditionally, a center-distance lens is selected for low-to-moderate presbyopes or for larger pupil size, whereas a center-near design is for more advanced presbyopia. A combination of both can be used to optimize vision. For the center-near design, a lower add power is typically needed compared to the manifest refraction add power because of the double asphericity of the lens surface. This can help prevent over-correcting at near and can enhance distance vision. For the center-distance design, the add power should be equal to the spectacle add. The center-distance zone size should be 1mm smaller compared to the photopic pupil size to optimize vision at near and far.

Although these lenses are intended for presbyopes, the customizable center-distance optics make them possible choices for accommodative disorders and myopia management in pediatric patients.

These hybrid designs can be evaluated very much as soft lenses are in that the goal is good centration and movement. For troubleshooting, evaluate the fit by placing saline and a few drops of regular fluorescein (high-molecular-weight fluorescein is not required) in the bowl of the lens and immediately check the pattern. Quite simply, an optimal fit will exhibit good centration, with the optical zone over the pupil (Figure 1); 0.5mm to 1.00mm of movement with each blink; and the GP portion in alignment with the cornea.

Figure 1. A well-centered hybrid lens on a regular cornea.

Follow-up Evaluation At the follow-up visit, it is important to evaluate the fit of the lens after at least three hours of wear. It also helps to apply fluorescein using a strip to the outer edge of the soft skirt; there will be fluorescein under a well-fit lens after one to two minutes.

The base curve (BC) determines the lens-to-cornea fitting relationship. Adjust the BC if excessive bearing or vault is observed. As mentioned previously, aim for a slightly steeper (0.50D) than alignment fit; however, this may be increased if there is decentration or higher amounts of corneal astigmatism.

The soft skirt controls centration and movement. If the skirt curve is too steep, the force of the lens will bear on a very small area, resulting in central pooling and possible seal-off by the soft skirt. If there is minimal movement, tear stagnation, late-day/removal discomfort, or difficulty with removal, the skirt needs to be flattened. If the skirt is too flat, the GP portion will settle too heavily onto the cornea, resulting in central bearing with unwanted lens decentration. If there is excessive movement, fluting of the skirt, lens awareness, or decentration, steepen the skirt.

Be sure to also remove the lens and evaluate the cornea. There may be a light, transient indentation ring at the junction of the soft and GP portions of the lens. A deep impression ring may indicate a tight fit and should be modified.

Perform an over-refraction to determine any necessary power changes. Also determine whether there is any residual astigmatism and whether its value is higher than expected based on the difference between the refractive and the corneal astigmatism. It can be useful to perform keratometry or topography with the lens on the eye. If lens flexure occurs, a lens with increased thickness in the midperipheral zone can be re-ordered.

With the progressive lens design, optical centration of the lens is essential to obtain the best visual result for distance and near. Performing topography over the lens can help evaluate optical centration by observing the position of the progressive zone with respect to the line of sight.2 If the vision is not satisfactory with a well-centered lens, the powers can be modified using the same common rules as for the majority of soft progressive lenses.2 For example, to improve near vision, add +0.25D sphere power or increase the add in the nondominant eye. To improve distance vision, add –0.25D in the dominant eye or refit using the center-distance progressive design. The center-distance progressive design allows fitters to personalize the fit based on patients’ pupil size. When distance and near vision are both unsatisfactory and the lens is decentered (normally in a temporal-inferior direction), it is possible to steepen the skirt curvature or BC by 0.25D, being careful to avoid a tight lens. If this is not effective, a change of contact lens type is suggested.

HYBRID LENS FITTING ON IRREGULAR CORNEAS

Several peer-reviewed articles support the use of hybrid contact lenses to manage irregular corneas.3-6 As with the previous section, the discussion here will focus on the most recent generation of hybrid contact lenses designed for irregular corneas that are available in the United States.

This latest generation incorporates reverse-geometry designs to vault over ectasias without bearing. These hybrid lenses feature two fitting parameters: vault, which replaces the BC; and soft skirt curvature. The vault corresponds to the rigid portion of the lens and defines its relative depth measured in microns (µm). The soft skirt is the only portion of this design that comes into contact with the anterior segment, and its curvature modulates the cornea-to-lens fitting relationship at the rigid-soft junction zone.

This family of hybrid lenses has two different designs: one typically indicated for prolate corneas and one typically indicated for oblate corneas. The design for prolate corneas is available with nine different vaults and four different soft skirts that are all relatively flatter compared to the soft skirt used in first-generation designs; this is because the soft skirt of the latest-generation lenses uses a silicone hydrogel material that is less capable of maintaining the elevation of the central rigid portion. A flatter soft skirt is also necessary to prevent lens seal-off. This design has a fixed lift curve (that connects the vault with the rigid landing zone) up to a vault of 250µm and a variable lift curve starting at a vault of 300µm to accommodate the increasing need of vault depth.

Hybrid Lens Fitting Pearls

To obtain optimal results with hybrid lenses, it is important to remember that:

• Lens movement is important, but tear exchange is more important.

• The rigid portion must align with the cornea when fitting the hybrid lenses for regular corneas, whereas clearance over the apex is necessary to maintain comfort, tear exchange, and corneal health with the hybrid lenses for irregular corneas.

• To confirm the relationship between the lens and the anterior surface, perform a follow-up evaluation after three hours of wear to allow for lens settling.

Initial Lens Selection and Evaluation These hybrid lenses must be fit using a diagnostic set. The first lens to use is the 250µm vault with a medium skirt curvature. Compared with the most recent generation of hybrid lenses for regular corneas, in which the lenses should be applied with just a few drops of fluorescein instilled in the bowl of the lens, the family for irregular corneas must be filled completely to the top of the bowl with preservative-free saline and fluorescein to prevent the formation of air bubbles under the lens after application. Be careful not to push the lens too forcefully onto the eye during application, as too much pressure on application can cause apical touch and bearing of the rigid landing zone.

Before evaluating the fit, it is important to wait a minimum of 30 minutes and then to confirm the results after about three hours. This time is necessary because these lenses have a settling behavior similar to that of scleral contact lenses, with 50% of settling occurring during the first hour of lens wear and stabilization after three hours; of course, this can vary significantly among individuals.7 The aim of the fitting is to provide complete apical clearance, a graded thinning of the sodium fluorescein profile at the inner landing zone of the rigid-soft junction, and landing of the soft skirt on the cornea (Figure 2).

Figure 2. Optimal fitting of a hybrid lens for irregular corneas: front (A) and profile (B) view.

The optimal apical clearance on lens application is 100µm to 150µm. When the lens is settled, an apical clearance of about 50μm is optimal (Figure 3), with light clearance at the rigid inner zone.

Figure 3. Settling of a hybrid lens for irregular corneas: central clearance at application (A) and after three hours of wear (B).

In contrast to the hybrid lenses for normal corneas, to adjust the apical clearance in the lenses for irregular corneas, modify the vault and not the soft skirt. If the vault is too high, the lens will present with a ring of bearing under the rigid landing zone (compared to an optimal vault in which the ring of bearing is under the soft skirt), which causes lens seal-off and reduces tear exchange. If the vault is insufficient, the rigid portion of the lens will bear on the corneal apex, causing mechanical irritation associated with discomfort.

The soft silicone hydrogel skirt helps to center the lens, balances the weight of the lens against the eye, and controls lens movement. The movement after a few hours of wear or at follow up may be reduced, but there may still be tear exchange, which can be evaluated in the same way as with the hybrid lenses for regular corneas. It is necessary to use a steeper soft skirt when the rigid inner landing zone lands on the cornea too aggressively. Excessive movement, edge fluting, and discomfort with blinking will occur if the skirt is too flat. A flatter soft skirt is necessary with excessive clearance of the rigid central zone or when tear exchange is reduced.

With the hybrid designs for irregular corneas, there is a positive correlation between the vault and the sagittal depth of the cornea for a chord of 8.5mm and a lower correlation between the depth of the contact lenses and of the anterior segment.8 This behavior could result because the soft skirt conforms to the scleral shape, which is different for every eye and changes the sagittal depth of each lens.

Calculate the power of the lens using an over-refraction only once during any fitting, even if using multiple trial lenses. A significant effect of the flat curvatures used in the hybrid designs for irregular corneas is that the negative power ordered is lower with respect to the powers that would be necessary for corneal GP contact lenses.9

The irregular cornea hybrid design for oblate corneas has the same variable lift curve design as does the design for prolate corneas, with a vault higher than 300µm but with a much flatter base curve. The fitting procedures are similar to those used with the prolate design. Use a 255µm vault and a medium skirt curvature to start. The ideal fit is 50µm to 100μm of clearance after 30 minutes from the initial fit. If there is apical touch, increase the vault by 100μm until fluorescein is observed, which then provides the recommended vault. With pooling, decrease the vault in 100μm increments until bearing is observed. Once you observe bearing, add 50µm to 100μm for apical clearance. The soft skirt and power can be evaluated and calculated using the same rules as those for the prolate design.

LENS APPLICATION, REMOVAL, AND CARE

Another important part of the fitting is to teach wearers the proper procedures for application, removal, and care of their lenses to help prevent possible dropout. Due to its size and mass distribution, application of a hybrid contact lens is similar to that of a scleral lens. Different possible application methods include the two‐finger method, the tripod method, or using a lens application device. The first two methods are more indicated for the hybrid lenses used on normal corneas because only a few drops of solution are needed in the bowl of the lens.

To apply the lenses used on irregular corneas, a device such as a ring inserter or a plunger can be more effective to support the weight of the lens when completely filled with solution. It is critical to explain to wearers the importance of filling the bowl of the lens with fluid to prevent the formation of bubbles under the lens. Make sure to verify the absence of any bubbles when the lenses are fitted. If bubbles are present, the lens must be removed and re-applied with more fluid. Another common problem encountered by neophyte wearers is applying the lenses with excessive pressure against the eye; this can create a vacuum in the post-lens tear reservoir, resulting in lens seal-off and an indentation ring on the corneal surface.

To remove the lens, use a strong narrow pinch with the very tips of the thumb and forefinger at the 5 o’clock and 7 o’clock position of the soft skirt right outside of the GP portion. To increase the grip on the lens edge and for easier lens removal, it is essential that the fingers are dry and that patients keep their eyes open for around 10 seconds to reduce the pre-lens tear film. If the lens is too slippery because of a surface coating, utilize the same technique but with a tissue placed over the finger tips.

Hybrid lens care includes digital rubbing in the palm of the hand of both the front and back lens surfaces with a multipurpose solution (MPS) or soft lens daily cleaner (non-alcohol based and non-abrasive) that must be rinsed off with saline solution before disinfection. For disinfection and storage, soft lens chemical disinfection systems (MPS) or hydrogen peroxide care systems are recommended (Note: certain MPS and hydrogen peroxide systems are not recommended for use with the hybrid lenses that are available in the United States). Patients who produce protein deposits may add an enzymatic cleaner to the care regimen. For patients who experience cloudy, blurry vision or dirty lenses during wear, it is suggested to use eyedrops that can rewet and clean the lenses as well as reduce protein buildup.

CONCLUSION

Modern hybrid contact lenses, with their improved materials, designs, and with the recommended replacement schedule of every six months, can be an effective solution for managing not only irregular corneas, but also refractive errors and presbyopia. Astigmatic patients often experience superior, more stable vision than with soft torics.10,11 Presbyopic patients who have low astigmatism can experience better visual quality when compared with other soft multifocal contact lenses.12,13 CLS

REFERENCES

  1. Page LJ, inventor; Contact lenses. U.S. patent 3,973,838. June 5, 1973. Available at: http://patents.com/us-3973838.html .
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  9. Montani G. Relationship between sagittal depth of anterior segment of irregular corneas and geometrical parameters of UltraHealth contact lenses. Poster presented at the Global Speciality Lens Symposium, Las Vegas. 2016 Jan.
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  13. Piñero DP, Carracedo G, Ruiz-Fortes P, Pérez-Cambrodí RJ. Comparative analysis of the visual performance and aberrometric outcomes with a new hybrid and two silicone hydrogel multifocal contact lenses: a pilot study. Clin Exp Optom. 2015 Sep;98:451-458.
  14. Montani G. Visual performance comparation between Duette progressive and Airoptix monthly multifocal. Poster presented at the Global Specialty Lens Symposium, Las Vegas. 2015 Jan.