Fitting Guidance



Fitting Set Parameters

The Standard Fitting Set is available with 8 x 14.50mm diameter lenses comprising:

6 x STDperipheryBase Curve 7.80 to 8.80 in 0.20 mm steps
1 x FLT2peripheryBase Curve 8.20 (for Nipple Cones)
1 x STP2peripheryBase Curve 8.60 (for post-graft and other reverse-geometry corneas)

It is possible to order a powered KeraSoft® Thin lens from KeraSoft® IC Plano Trial Lens. However, if a KeraSoft® IC powered lens is ordered from a KeraSoft® Thin Plano Trial Lens, the final lens may fit differently due to the thicker profile shape.

First Choice Lens Calculation

A good approximation of the Base Curve required can be calculated using the Tangential map on a topography machine. Tangential maps are more sensitive than axial maps and give better information regarding the mid periphery and periphery. For best results, use a Normalized map with 5mm zone included and numerical values displayed. For more information, go to the Using Tangential Topography page. This calculation does NOT work with the Axial (Sagittal) maps, as the mid periphery values are different.

Compensation Factors (CF)

These are numerical values within the calculation depending on the central Sim K Readings of the cornea.

Use CF of 1.30 if both Central Sim Ks are under 7.00. This calculation works for more central cones.

Use CF of 0.80 if one or both Central Sim Ks are equal to or over 7.00. This calculation works for decentred or low cones.

For Central Cones

Base curve=Average (Flat Central Sim K + Steep 5mm ring Sim K)
2
+CF

For Low & Offset Cones

Base curve=Average (Ave of Sim K + Ave K on 5mm ring)
2
+CF

These calculations give a good indication for the first choice lens and can be downloaded in a desktop App format.

First choice lens assessment

Insert lens and allow to settle for no more than 5 minutes. It is helpful to start observing the lens immediately to see whether rotation is stable or variable. For best results, assess lenses on eye with a slit lamp using the MoRoCCo VA characteristics outlined below. Note the centration, movement and rotational stabilization and ask the patient about the comfort of the lens as it settles on eye.

MoRoCCo VA Characteristics

MovementUp to 2mm is acceptable as long as the patient is comfortable
RotationLaser mark should sit at 6 o’clock.
Rotation of the lens that does not change on upward gaze is classed as stable, indicating a Tight fit.
Rotation of the lens that changes on upward gaze is classed as unstable, indicating a Flat fit
CentrationCentered lens indicates an Optimal fit.
Decentered or dropping lens indicates a Flat fit
ComfortAn Optimal lens is comfortable. A Tight lens may be comfortable at first but will then start to be uncomfortable. Non-settling discomfort indicates a Flat fit.

If the fit appears Tight or Flat, remove and choose the next appropriate base curve. If fit appears Optimal, move straight to over-refraction, using the spectacle refraction as the starting point. If this is difficult to obtain, use the topography astigmatism as a starting point.

Over-refraction

Always assess the over-refraction straight away. Leaving lenses to settle for an hour or so will change fitting characteristics. Use 0.50DS steps or more when assessing sphere and cyl. It is often easier to assess the cyl before finalizing sphere correction, as large amounts of uncorrected astigmatism cause excessive ghosting. VA with a Tight lens will be better after the blink. VA with a Flat lens will be worse after the blink. VA should be at least the same or better than spectacle refraction. If you attain a worse result, this means the fit is not optimal.

Sector management control

It is possible for up to 2 sectors of the periphery to be modified independently of the Base Curve. The most common use of this design is to ‘tuck in’ the inferior of the lens for Low Cones/PMD or to improve contact with the sclera in complex post-graft cases. Before using SMC, try a 15.00 diameter STD configuration, as this will often resolve fit issues.

Record angles counter-clockwise around the lens as A1, A2, A3 & A4.
A1 & A2 define the beginning and end of sector 1.
A3 & A4 define the beginning and end of sector 2.
Each sector can be ordered as STD, STP 1-4 or FLT 1-4, though STP4 & FLT4 are rarely used.
Blend areas are automatically set once sector angles are defined. There must be a minimum of 30 degrees between
each sector and no more than 4 step changes between periphery values.

Diameter & Periphery Guide

KeraSoft® Thin’s increased flexibility improves drapage over the peripheral cornea. This reduces the need for peripheral changes, which can often be better managed by moving to a larger diameter lens. The guide below suggests where larger diameters or different peripheries may be appropriate. Fitting Lenses with different diameters or peripheries are available on request.

Corneal typePeripheryDiameters
Central (mild to moderate)Standard14.50
Nipple Cone (advanced)Flat 2 or Flat 314.50
Low or Decentered Cone (mild to moderate)Standard14.50 & 15.00
Low or Decentered Cone (advanced)SMC (STD:STP1 or STD:STP2)14.50 & 15.00
Post-graft | post-refractive surgeryStandard14.50 & 15.00
Complex post-graft | post-refractive surgerySteep 2 or SMC design14.50 & 15.00

Making periphery changes

Always use STD lenses before trying a lens with a different periphery.
Each periphery change is equivalent to 0.20 change in Base Curve.

Nipple Cones

If the First Choice Lens Rule gives a relatively flat Base Curve (eg. 8.60) and this is acting very flat, use that value as the peripheral fit and go down at least 2 steps for the Base Curve (ie. 8.20 FLT2).

Making periphery changes

Topography does not tend to record the peripheral cornea, so the First Choice Lens Rule is likely to provide a lens that fits the central cornea but tends to flute at the edge. Keep the Base Curve as it is and tighten periphery by 1 or 2 steps or increase diameter to 15.00.

Internal Study

To investigate the efficacy of KeraSoft® Thin, an internal study was carried out comparing KeraSoft® Thin to IC on subjects already successfully adapted to the KeraSoft® IC lens. The results demonstrated that in all cases, comfort was the same or increased significantly by the thinner design. Fit was very similar to IC and 95% of subjects experienced equal or improved VA. This was all achieved by maintaining the same parameters for the fit, demonstrating that KeraSoft® Thin could be fitted from KeraSoft® IC Fitting Lenses or powered lenses. The small number who experienced worse VA represented 3 eyes out of a sample of 48. These subjects had central keratoconus and subsequent experience of the Thin design has shown that changing fit parameters in such cases will improve VA.

Internal Study