The coordination of the power of the cornea, crystalline lens, and axial length to process a sharp retinal image of a distant object is known as emmetropization.
In the United States, more than 70 percent of the population is either emmetropic or mildly hyperopic (easily corrected with a small accommodative effort).
With age, the cornea, lens, and axial length undergo coordinated changes. Essentially, the optical components (cornea and lens) must lose refractive power as the axial length increases so that a sharp image remains focused on the retina.
The cornea, which averages 48 diopters of power at birth and has an increased elasticity, loses about 4 diopters by the time the child is 2 years of age.
One may assume that the spurt in growth of the sagittal diameter of the globe during this period pulls the cornea into a flatter curvature.
The fact that the average corneal diameter is 8.5 mm at 34 weeks of gestation, 9 mm at 36 weeks, 9.5 mm at term, and about 11 mm in the adult eye supports this “pulling, flattening” hypothesis.
On the other hand, other coordinated events also occur, such as the change of lens power and the coordinated increase in eye size (most important, an increase in axial length). The crystalline lens, which averages 45 diopters during infancy, loses about 20 diopters of power by age 6 years.
To compensate for this loss of lens power, the axial length increases by 5–6 mm in that same time frame. (In general, 1 mm of change in axial length correlates with a 3-diopter change in refractive power of the eye.)
Now let us examine a possible mechanism that could account for most of the data. As the cross-sectional area of the eye expands, there is an increased pull on the lens zonules and a subsequent flattening of the lens (the anterior lens surface is affected a bit more and the posterior lens surface a bit less), thus decreasing the overall lens power.
There also may be a related decrease in the refractive index of the lens, which also contributes to the reduction in lens power. Because the incidence of myopia starts to accelerate significantly around the age of 10, one may question whether there could be a decoupling of the previously described coordinated drop in lens power and increase in axial length. An increased amount of near work (e.g. schoolwork) is associated with a higher incidence of myopia.
It is also well known that genetic predisposition influences myopia incidence, as evidenced by the fact that more Asian children than Caucasian children are myopic.
Thus one might hypothesize that the long periods of accommodation that accompany schoolwork (ciliary body contraction) may tend to stretch and weaken the linkage between the enlarging scleral shell and the ciliary body.
If this were to happen, the lens would flatten less during eye growth. Another way of looking at this phenomenon is to theorize that with the linkage weakened, the restraining effect of the lens-zonule combination on eye growth is also weakened, which results in an increase in axial length in the myopic student.
Many studies demonstrating that the myopic eye has a greater axial length than the emmetropic eye support this idea.