Since its introduction to the UK market in 1997,1 the Rose K lens system, now part of the Menicon portfolio, has become the world’s most popularly prescribed range of contact lens designs for keratoconus and irregular corneas.
The latest addition to the range is the Rose K2 XL semi-scleral lens. Many cases of irregular cornea cannot be satisfactorily fitted with conventional or large diameter corneal lenses. Utilising a semi-scleral design provides stability and, in cases of poor tolerance to corneal diameter lenses, improved comfort.
The Rose K2 XL has been developed to address these cases and complement the current range. The fitting concept is consistent with the rest of the lenses in the Rose K family. The lens is available in the UK from David Thomas Contact Lenses.
Applications
Primary indications are those cases of irregular cornea that cannot be successfully fitted with an intra-limbal design. Conditions include pellucid marginal degeneration (PMD), post-corneal graft, corneal rings, post-LASIK ectasia, dry eye and some keratoconus. Rose K2 XL is also helpful in cases where corneal lenses are poorly tolerated or situations where improved stability is required for sport or a poor working environment.
Technical features
The design and parameters of the Rose K2 XL are summarised in Table 1.
The lens is available with BOZR 8.40mm to 5.60mm. The standard total diameter (TD) is 14.60mm with an available range of 13.60mm to 15.60mm. The lens has an aspheric back optic zone which decreases as the base curve steepens. The front surface incorporates aberration control. Edge lift is defined in 0.5 steps from -3.0 decreased to +3.0 increased.
The peripheral design and peripheral edge lift can be specified separately for each individual quadrant. More marked sectorial changes in lens geometry can be achieved using asymmetric corneal technology (ACT). With ACT, changes in the lens geometry are introduced from the boundary of the optic zone to the edge of the lens. A full range of toric options is available.
The recommended material is Menicon Z and the lens is also available in Boston XO, Optimum Extra and Optimum Extreme.
Fitting procedures
Most semi-scleral designs are fitted using sagittal heights to achieve full corneal clearance. The Rose K2 XL is fitted using the same philosophy as intra-corneal diameter lenses.
The initial trial lens selection is influenced by the corneal condition and keratometry. Table 2 shows the fitting relationship for different irregular corneal conditions. If diagnosis is uncertain, the estimated first trial lens is 0.2mm steeper than mean K.
In keratoconus, the fitting increment related to mean K becomes increasingly flatter as the mean K steepens with worsening disease. The first trial lens selection is made by reference to Figure 1.
[CaptionComponent="1088"]Lens insertion is best achieved by supporting the lens on a large sucker or balancing the lens in a tripod between thumb, index and middle finger. The lens should be filled with saline and, for diagnostic fitting, fluorescein is added. The patient should tilt the head forward so that the face is parallel to the floor. The lens is then applied directly over the cornea so that the solution remains in the lens. If a large bubble appears, the lens should be removed, refilled and reinserted. It is recommended that the lens is removed by applying a sucker to the temporal portion of the lens and peeling away from the cornea towards the nose.
As with all lenses in the Rose K range, a five-point step-by-step approach to fitting is recommended:
- Base curve selection. Initial BOZR is selected according to the corneal condition. The optimum central fit is represented by very light feather touch at the highest point on the cornea. Very light feather touch, as seen with fluorescein, indicates that there is still approximately 25 microns of tear layer under the lens at this point. The BOZR is steepened until no touch is first observed. A lens 0.1mm flatter than this will usually give the desired minimal touch fluorescein pattern. The lens fit is confirmed after 20 minutes’ settling.
Figure 2 illustrates the optimal central fit, along with steep and flat central fits.
[CaptionComponent="1089"][CaptionComponent="1090"][CaptionComponent="1091"]- Peripheral fit. Edge lift is assessed after the optimum central fit has been attained. A higher edge lift is required compared to semi sclerals fitted using the sag principle. Optimal edge clearance ensures tear exchange beneath the lens which is characteristic of the fitting philosophy. The edge clearance is judged by a combination of fluorescein pattern observation, movement of the lens and the ease by which fluorescein passes under the lens with push-up test at 6 o’clock.
Ideally there should be a peripheral band of fluorescein at the edge of the lens between 0.8 and 1.0mm wide. Since the band width may vary in different quadrants, take an average from each quadrant. Correct edge lift is important to ensure tear exchange beneath the lens. Inadequate edge lift, while initially comfortable, may cause blanching of the conjunctival vessels (Figure 3) and compromise tear exchange.
[CaptionComponent="1092"]Excessive edge lift will cause discomfort and can be observed by too wide a fluorescein band and obvious movement on blinking. Patient discomfort is a good indicator of excessive edge lift. This may also allow bubbles to encroach under the lens which gives rise to visual disturbance. Alterations to the peripheral edge lift do not appear to affect the central fit of the lens.
- Diameter. The standard recommended TD is 14.60mm. Assuming an average corneal diameter of 11.8mm, the lens should extend 1.3 to 1.5mm beyond the limbus. No modification to the BOZR seems to be required if the TD is adjusted.
- Lens location. A well located lens should sit concentrically with the limbus, extending equally onto the sclera in each quadrant. A low-riding lens, where the corneal apex is decentred down, can often be improved by increasing TD and/or flattening the BOZR. Lateral decentration does not appear to be too significant.
- Lens movement. Minimal and just discernible movement, on blinking, is ideal. Excessive movement gives poor comfort and can be alleviated by reducing edge lift or flattening the BOZR. Movement can be improved by increasing edge lift and/or steepening the BOZR.
Clinical performance
When fitting contact lenses extra-limbally with inevitable reduced tear exchange, oxygen delivery to the cornea is of concern. Michaud et al2 looked at the available oxygen at the cornea taking into account the Dk of the material, the centre thickness (CT) of the lens and thickness of the tear layer.
Several semi-scleral designs have moderate thickness, about 0.25mm, and a fitting philosophy of corneal vaulting resulting in a tear layer thickness of 150-200 microns. Rose K2 XL is typically produced with a CT of 0.14mm. The fitting philosophy of light feather touch results in an estimated tear layer of 20 microns.
The oxygen performance of this lens in Menicon Z material gives an estimated Dk/t available at the cornea of 81, which is over three times that advocated by Holden and Mertz3 for daily wear.
The pre-launch evaluation was reported by Rose in 2012.4 Seventeen practitioners from around the world fitted a total of 120 eyes including various irregular corneal conditions. Neophytes accounted for 29 per cent of cases. The investigators had varying levels of experience in fitting semi-scleral lenses. No adverse events were reported and successful fits were achieved in 75 per cent of cases.
First lens fit was achieved in 57 per cent and a second lens was required in 38 per cent of cases. The principal reason for ordering further lenses was to modify the edge lift. Analysis of the results yielded the suggested trial lens selection according to corneal condition.
Conclusions
The Rose K2 XL can be fitted to challenging irregular corneal conditions and a high success rate has been reported, often achieved with the first lens fitted. The fitting philosophy is consistent with the rest of the Rose K2 family of contact lenses and can be approached with confidence without previous experience of semi-scleral lenses.
Fitting tips
- Select initial trial lens according to keratometry and corneal condition
- Verify central feather touch by steepening base curve and observing corneal clearance
- Corneal clearance with this design may give
- disappointing visual acuity
- With correct edge clearance lens can be pushed up without difficulty
- TD is usually 3.00mm greater than corneal diameter
- Insert and remove lenses with a sucker
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References
1 Cox ND. A new system for keratoconic fitting. Optician, 1997;5630;214 28-30.
2 Michaud L, van der Worp E, Brazeau D et al. Predicting estimates of oxygen transmissibility for scleral lenses. Cont Lens Anterior Eye, 2012;35:6 266-71.
3 Holden BA and Mertz GW. Critical oxygen levels to avoid edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci, 1984;25:10 1161-7.
4 Rose P. Presentation at British Contact Lens Association Clinical Conference, May 2012.
Neil Cox is senior optometrist at Moorfields and King’s College Hospitals. He is also in private contact lens practice in central London