The development of a child’s eyesight depends on correct brain stimulation during the first years of its life.
At birth, all the cerebral structure that supports the visual function is already present.
But its performance depends on receiving, in the first years of life (and especially in the first months), high quality images, correctly and symmetrically focused by the two eyes.
If not, there will be an incorrect development of visual capabilities, leading to the onset of amblyopia (lazy eye).
Refractive errors predominate among the most frequent causes of a child’s imperfect visual development, and in principle they should be corrected or compensated with glasses. Strabismus, which most of the times is also caused by an uncorrected refractive error, comes in second.
The most frequent refractive errors are hypermetropia (the eye focuses the image behind the retina), myopia (the eye focuses the image in front of the retina) and astigmatism (the eye presents two different focuses and this double focusing produces a blurred image).
In adults and in older children (over 7 – 8 years old) with refractive errors, the consequences of non-correction are only reduced visual acuity and eyestrain many times associated with headaches. However, in younger children, the permanent wear of glasses is many times fundamental to prevent and/or treat amblyopia and strabismus.
Thus, in infants, the rejection of corrective glasses can have harmful and irreversible consequences. Therefore, it is very important to know and to optimize the factors that can increase the compliance of this age group.
The most important factor in a child’s glasses is prescription.
An accurate prescription is essential, especially in children. In adults, the prescription is usually based on subjective criteria: the patient decides which lens offers the best vision and comfort. In children, particularly in the younger, the prescription of corrective lenses is based on objective criteria. The measurement of the refractive error should be made after applying drops that inhibit the automatic focusing of the eye (cycloplegia), and prescribing criteria are more frequently based on the individual experience (and on the integration of other clinical data, such as oculomotor balance) than on guidelines issuing from clinical evidence. For this reason, the prescription should always be given by an ophthalmologist, preferably one with practice and experience on the evaluation of children.
The selection of the frame and the fitting of the lenses are also very important factors in a child’s glasses, in order to ensure comfort and efficacy every day.
The frame
It should be light, comfortable and perfectly adapted. It is important to take into account that a child’s nose is normally little developed, and so it requires a bridge specifically designed. Another critical aspect concerns the temple arms: they should be adjusted so that they prevent the frame from sliding along the nose, but without causing any discomfort on the ears.
The lenses
The most important is the optical quality of the lenses, in order to ensure perfect vision. They should also be comfortable: light and thin, produced with a high refractive index and low density material. Discomfort is one of the most frequent causes of rejection of corrective glasses during childhood.
The lenses should offer a high safety index: it is important that their material is resistant to impact, because the risk of traumatisms and accidents is higher in children.
They should guarantee good protection against harmful radiations for the visual system, filtering out 100% of the UVA and the UVB. It is important to remind that about 80% of the exposure to UV radiation occurs during de first two decades of life.
Finally, the lenses should have an effective scratch-resistant hard coating, because children are usually less careful with their glasses.
Aesthetic factors are less important in younger children and durability is also a secondary aspect, because the most important is in fact the continuous wear of corrective lenses with optical quality, which should meet the needs of a visual system in development.
Which material or materials should be chosen to produce lenses for children?
In general, organic materials have a clear advantage (shatter-resistance) over glass. Polycarbonate gives additional advantages over organic resins (standard plastics): in fact, polycarbonate has a special chemical structure that provides optimal characteristics for the production of ophthalmic lenses.
Polycarbonate
It is different from all the other plastics. Its molecules are composed of extremely long chains of atoms that slide forwards and backwards over one another. This chemical structure generates an extremely resistant material that bends or deforms without breaking (the energy of any impact deforms the lens instead of breaking it). The resistance of polycarbonate to impact is very high, when compared to the resistance of other materials: for example, it is 21 times superior to the one of CR-39.
Besides, the characteristics of polycarbonate enable the production of thin (with high refractive index) and light (low gravity) lenses that are very comfortable. Moreover, polycarbonate ensures natural UV protection, because it filters out 100% of the radiations below 380 nm.
Finally, polycarbonate lenses offer an excellent optical quality, although presenting more lateral chromatic aberration (low Abbe number) in high power corrective lenses.
The great disadvantage of polycarbonate for the production of lenses is its low
scratch-resistance: a hard coating, preferably with a double layer, is systematically applied.
Conclusion
Polycarbonate is certainly the most appropriate material for the production of ophthalmic lenses, due to its chemical structure and physical characteristics. After the addition of a scratch-resistant hard coating, it is particularly advantageous in corrective lenses for children and in eyewear for sportsmen or sportswomen.