High myopia refers to myopic eyes with refractive error ≥ 6.0D, and most of them belong to the category of pathological myopia. With the development of modern civilization, the number of high myopia caused by close work is increasing year by year, and the distance vision of patients is significantly reduced, which brings many inconveniences to work and life.
Due to the current level of medical technology, there is no cure for high myopia. Optical correction is the main correction method, which has the advantages of safety, simplicity and economy, among which frame glasses are the main method. However, high refractive index frames have the disadvantages of “unsightly” appearance and poor quality of vision (e.g., distorted and small vision), the former of which is often the most intolerable for most patients. Therefore, the author as a clinical ophthalmologist from the perspective of medical optometry to explain how to fit a pair of beautiful, standard high myopia frame glasses.
First, prescription
The so-called prescription is the prescription of the optometrist on the basis of the accurate prescription, combined with the patient’s eye condition, subjective requirements and other factors, the prescription to make appropriate corrections after the prescription to get the prescription. For high number of myopia, the prescription can be appropriately corrected from the following two aspects.
1, undercorrection.
It is easy to understand that reducing refractive error is the most effective and easiest way to reduce lens thickness. But not unlimited or arbitrarily reduce the patient’s prescription, the “amount” of undercorrection should be carefully prescribed by the optometrist, taking into account the specific circumstances and weighing the pros and cons. The control of undercorrection and “degree” should be based on the following principles.
(1) to meet the subjective needs of the patient: that is, the patient can be satisfied with the vision of the lens, which is the key to the success of the prescription, on this basis, a more substantial reduction in the prescription is feasible. The trial frame technique can be used to clarify whether the patient is satisfied with the corrected vision.
(2) meet the objective needs of patients: in some special cases, such as patients have driving needs or engaged in other special industries with certain requirements for vision, optometrists should grasp the bottom line of corrected vision according to the specific circumstances. This requires our optometrists to have a certain understanding of vision-related laws and regulations, industry regulations, etc.
(3) the age of the patient: mainly reflected in whether the patient has old eyesight and near vision needs.
(4) Refractive error: we should consider the “benefit” of the reduction, i.e., the ratio of the thinness of the lenses “achieved” by the reduction of a certain number of degrees to the resulting decrease in corrected vision (which can be interpreted as cost effectiveness). If the change in lens thickness is not significant after a certain reduction, but the corrected visual acuity is significantly reduced, it is necessary to consider whether to cancel the “undercorrection” program and find another way.
There are two points worth noting: first, the above four principles are not primary and secondary, and should not be considered in isolation, but according to the specific circumstances, comprehensive analysis and flexible application. Secondly, the corrected visual acuity before the “undercorrection” program is generally achieved by the trial frame, the trial frame and the patient’s final choice of eyeglass frame “eye distance” is not the same, resulting in errors in the effective refractive error, which is more pronounced in high myopia. This is more evident in high myopia. Therefore, the optometrist should compare the trial frame visual acuity and prescription visual acuity, and analyze the causes of the error in time, and summarize the experience and continuous improvement.
2, astigmatism trade-offs.
High myopia usually combined with a higher number of regular astigmatism. Most optometrists, in order to improve the success rate of prescription, often reduce or even abandon the prescription of astigmatism to a certain extent, while on the other hand, in order to achieve the target corrected visual acuity, often additional myopic spherical lens (i.e. equivalent spherical lens), eventually leading to a further increase in lens thickness.
The following is a description of cis prescription techniques for high myopia.
Most young patients have cis-regular astigmatism, which is the most common type of astigmatism we encounter in our work. We know that the vertical refractive error of cis-regular astigmatism is greater than the horizontal direction, and for myopic astigmatism, the horizontal direction is less myopic, i.e., the degree of astigmatism does not affect the thickness of the lenses in the horizontal direction if the degree of spherical lenses remains unchanged. Given that most of the current eyeglass frames are horizontal oval, the thickness of myopic lenses is mainly in the horizontal direction of the frame on both sides, while the thickness of the lenses in the vertical direction is not obvious due to the small diameter of the cut edge.
Therefore, it is important to keep the astigmatism as close as possible to or equal to the prescription, thus minimizing the myopic spherical lens, with a view to reducing the thickness of the lenses in the horizontal direction.
It is important to emphasize that most of the patients have uncorrected or undercorrected astigmatism in their original lenses. Before dispensing the lenses, the patient should be informed in detail about the purpose of the astigmatism prescription and the possible discomfort, in order to obtain their subjective cooperation and improve the success rate of dispensing.
Horizontal oval frame, AB horizontal diameter, CD vertical diameter, AB>CD
Second, lens selection
The choice of lenses is also the main content to improve the appearance of frame glasses, mainly in the following aspects can be reflected.
1, lens material
Lens materials are mainly glass, resin and PC sheet (polycarbonate sheet) three.
The main disadvantage of glass lenses is that the lenses are heavy, especially in highly myopic lenses, resulting in a significant increase in the pressure of glasses on the root of the nose and back of the ears of patients, the wearer is often unable to tolerate. The latter two have been widely used in recent years due to their light texture and plasticity, occupying an absolute share of the lens market. High myopia lenses to choose the latter two is appropriate.
2, refractive index.
Refractive index refers to the ability of the lens to bend light, is the main parameter that indicates the thickness of the lens, the current market for resin lenses refractive index is generally between 1.49-1.74, the higher the refractive index the thinner the lens, more than 1.60 is called ultra-thin lenses. For high myopia, you should choose lenses with the highest possible refractive index, but the price of lenses with high refractive index is often higher due to the production process and materials. Therefore, the price factor should be integrated to choose the right high refractive index lenses, and in general, lenses with a refractive index of 1.60 or more should be chosen.
3. Special types of lenses.
High myopia should choose lenses of aspheric design.
Aspheric lenses have a specific centrifugal position, i.e. the lens gradually flattens from the center to the edge, gradually reducing the curvature, lending to the reduction of excessive refractive power at the edges and edge astigmatism. Therefore, aspheric lenses are 10-30% thinner than normal lenses at the same diopter. And aspheric lens aberration is only 1/5 of that of spherical lens, so aspheric lens has natural vision and even magnification without distortion.
Because of the difficulty of grinding and high cost of aspheric lenses made of glass, they are mostly made of resin, and most of the lenses with high refractive index are made of aspheric. This is exactly the same as the aforementioned choice of lens material and refractive index.
Third, the choice of frame
1, the geometric center distance of the frame is close to the pupil distance with lenses
As mentioned above, the thickness of myopic lenses is mainly in the horizontal direction of the frame, due to the geometric center distance of most frames is greater than the pupillary distance of the human eye, so that in many cases the temporal edge of the lens is significantly thicker than the nasal edge, and the temporal edge of the lens is precisely the most easily observed by outsiders. Therefore, choosing a frame with a geometric center distance close to the pupillary distance of the lens, so that the nasal and temporal edges of the lens are similar, can effectively reduce the thickness of the temporal edge of the lens and improve the appearance (Figure 1).
Figure 1: Temporal and nasal edges of the lenses
2. Choose a frame with a relatively small horizontal diameter
Myopic lenses are concave lenses with a thin center and thick periphery, and the further away from the center the thicker the lens is. For the frame of the horizontal oval, the smaller the cross diameter of the frame, the thinner the edge of the lens after cutting the edge.
3, the choice of the type of frame
In the choice of the type of high myopia glasses frame, full frame and half frame frames are better than frameless frames, the latter not only makes the edge of the lens thickness in front of outsiders “exposed”; and the frame is easy to deformation, resulting in the center distance of glasses and lens astigmatism axial change, affecting the correction effect.
4, shorten the distance between the mirror and the eye
The lens-eye distance refers to the distance between the lens back vertex and the front vertex of the cornea. Myopia corrective lenses are concave lenses, if the lens eye distance increases, the effective refractive error decreases (the higher the myopia decreases more), the corrected visual acuity also decreases. Therefore, the eye distance of high myopia glasses should be reduced as much as possible, and this should be taken into account in the choice of frame style and frame adjustment.
In the work of high myopia prescription, we have to integrate the above-mentioned skills and make appropriate trade-offs according to the individual situation of the prescription. In order to produce a pair of beautiful appearance, excellent correction effect of high myopia frame glasses.
Of course, given the limitations of the current manufacturing process and the inherent characteristics of the lenses, we are not yet able to fundamentally eliminate the disadvantages of thick lenses and poor imaging quality caused by high refractive error. In this regard, our optometrists should be aware of this and be good at communicating with patients to gain their trust and improve the success rate of prescription.