The diameter of the cornea of a newborn is on average 9.5mm. Corneal growth is completed at the end of the second year of life (approx. 11.7 mm).
DIA = 12.50; BC: 7.40 / 7.80 / 8.20
Principles of fitting:
Pediatric contact lenses, as with all lenses should show stable fitting behaviour. The Central optical zone should cover the entire area of the pupil. Since infants typically spend more time on their backs when compared to an adult, the centering of the lens will be different.
Another key factor is the handling of the contact lens – to attach and to remove the lens: a small overall diameter simplifies this procedure.
- K Readings
- IRIS Diameter (DHIV)
Choice of trial lens
BC = corneal radius + 0.30 mm
Diameter = IRIS diameter + 1, 50 mm
Power: glasses refraction (VD = 0)
|Corneal diameter (mm)||9.00||9.50||10.00||10.50|
|CL diameter (mm)||12.00||12.50||12.50||13.00|
|Flattest corneal curve (rcfl)||Base curve|
The cornea flattens as it grows, therefore the BC should be modified when the infant grows older.
Young infants may not be cooperative in the consulting room, so the following guidelines may be useful:
|Age||Base curve||CL diameter (mm)||Power (dpt.)|
The contact lens should fit centrally in front of the pupil with approx. 1mm of movement upon blink.
Choose the smallest diameter to meet these criteria.
A small bubble may be observed upon insertion which is quite normal, the bubble should disapear after a few blinks.
The push-up test helps determine how well the lens is centering. If the lens stays de-centered or gets stuck beneath the upper eyelid then increase the overall diameter by 0.50 mm or reduce the BCOR by 0,3 mm. These adjustments can be made in combination.
A steep lens will display lack of movement, in the centre is a bubble for longer than 10 minutes. You can dissolve this effect by increasing the BCOR. Generally a slighlty flat fit is preferable to a steep fit.
A tight fitting contact lens will not respond correctly to the push up test. Observations will show reduced lens movement as well as possible eye redness and irritation. The lens may also become de-centered getting itself stuck on another part of the eye.
Higher than average centre thickness is normal with high plus powered lenses, however smaller diameters associated with baby lenses help to dissolve the problem. Should the centre thickness be problematical to the fitting of the lens then flatten the BCOR by 0.50 mm
It is recommended using retinoscopy to perform a refraction. After a few hours in situ cylindrical elements of the prescription can be determined (also by using a retinascope). Astigmatism powers are available but rarely used on young infants.
Measurement of corneal diameter:
It is not possible to measure the corneal parmeters with standard instruments. In order to estimate the corneal diameter, measure the iris diameter with a ruler. Basing on this value, you can then estimate K-reading of the baby’s eye.