(a) correcting astigmatism with spherical contact lenses
(1) For spherical lenses above -3.00DS and corneal astigmatism below-1.00DC, spherical contact lenses can be used for correction; because
The radius of each meridian on the inner surface of spherical contact lens is the same, but the curvature of the two main meridians of astigmatism cornea is different.
At the same time, there must be a space full of tears to form a tear lens to make up for the differences between the meridians of the cornea. centre
Thick and elastic contact lenses can correct astigmatism well, sometimes even 2.00DC.
(2) for corneal astigmatism >: 1.00DC and astigmatism.
Give the wearer the power of the contact lens.
That is, the total DS=DS (original optometry ball luminosity)+1/2DC (original optometry astigmatism luminosity).
Example: the original optometry was -8.00ds-2.00dc× 180.
Then: the total DS = (-8.00)+(-1.00) =-9.00, which translates into a contact lens prescription of -8.00DS.
(3) for corneal astigmatism >: 1/4 spherical diopter, for example, -4.00DS-2.00DC× 180, special astigmatism soft should be applied.
Sexual contact lenses to correct.
(Note: Spherical contact lenses can only correct corneal astigmatism, and lens astigmatism requires astigmatism soft contact lenses. )
(2) Installation of astigmatism contact lenses
Design of astigmatism soft contact lens
Features: the axis position remains relatively stable-there can be a certain degree of mobility up and down, left and right, but the axis position cannot rotate obviously.
Comparison of different design methods
Design type: thickening, weighting and trimming type: thinning up and down.
(Gravity stability method) (truncation method) (dynamic stability method)
Design principle The prism design under the lens is to cut off a part under the lens and place the corresponding part of the lens up and down.
The square is thicker and heavier, and the missing part is fixed on the lower eyelid, which gradually becomes thinner by gravity to prevent it from sticking to the muscles of the upper and lower eyelids.
The rotate edge of that lens achieve the purpose of preventing rotation, and the lens is fixed by force.
Advantages: the astigmatism axis can be maintained, and the lens can be relatively comfortable to wear without foreign body feeling; camera lens
A relatively stable fixed position can move up and down, left and right with the eyeball, but
Unable to rotate; The average thickness decreases,
High oxygen permeability design, global patent
Disadvantages When the eyeball rotates, there is no obvious foreign body sensation between the lens and the eyelid edge.
Move; The lower part is thicker, and the lower eyelid
Have obvious foreign body sensation
(The astigmatism lens of satellite TV is thinned up and down)
Step 2 instruct
Regular astigmatism, stable lens rotation degree and lens rotation angle
3. Astigmatism calculation
[1] Calculation formula of corneal astigmatism:
Corneal astigmatism with horizontal diopter-vertical diopter = 180.
Empirical formula: (vertical radius of curvature-horizontal radius of curvature) ×5D= corneal astigmatism 180.
For example, if the vertical curvature radius of cornea is 7.8mm and the horizontal curvature radius is 8.0mm, then
Corneal astigmatism = (7.8-8.0) × 5dax180 =-1.00dax180.
Or: corneal astigmatism = (42.12d-43.25d) =-1.13dax180.
[2] Calculation formula of crystal astigmatism:
Lens astigmatism = total astigmatism obtained by optometry-corneal astigmatism
Example: 1: optometric astigmatism is-1.00 DAX 180, and corneal curvature is 8.00 mm/7.80 mm
Corneal astigmatism = (7.80-8.00) × 5 =-1.00 DAX180.
Crystal astigmatism = total astigmatism (-1.00 DAX180)-(-1.00 DAX180) = 0.
Example 2: Refractive astigmatism-1.75 DAX 180, corneal radian 7.95mm/7.95mm..
Corneal astigmatism =(7.95-7.95)×5=0
Crystal astigmatism = total astigmatism (-1.75dax180)-0 =-1.75dax180.
Example 3: Refractive astigmatism -2.00 DAX 180, corneal radian 7.80 mm/7.60 mm.
Corneal astigmatism = (7.60-7.80) × 5 =-1.00 DAX180.
Crystal astigmatism = (-2.00dax180)-(-1.00dax180) =-1.00dax180.
Example 4: Refractive astigmatism is 0, and corneal radian is 7.90 mm/7.70 mm.
Corneal astigmatism = (7.70-7.90) × 5 =-1.00 DAX180.
Crystal astigmatism = total astigmatism 0-(-1.00 DAX180) =+1.00 DAX180.
4. Wearing astigmatism contact lenses
[1] Select the appropriate lens radian.
Curvature of bottom arc of lens = corneal curvature +0.6- (Weikang lens)
[2] Observation after assembly
Elasticity: center positioning, peripheral coverage, mobility, finger pushing test.
Determination of rotation angle of test piece: positive and negative.
For example, in order to reverse 10, the axial position of the original optometry is 180, and the axial position of the astigmatism contact lens should be170;
For example, forward rotation 10, the original axial position of optometry is 180, and the axial position of astigmatism contact lens should be 10.
[3] Wear an optometry.
The order of astigmatism contact lenses should be: try-on lens spherical lens DS+ test stand spherical lens DS+ test stand astigmatism DC× axial position.
There are three axial positions: (1) is equal to the axial position on the audition table (the specimen does not rotate).
(2) Axis position on the audition frame+clockwise rotation degree
(3) Axis position on the audition screen-degree of inversion
Example 1. The diopter of the test piece is -2.75D/BC8.4mm, and the lens movement is tight. Observe that the rotation index is reversed 10.
The diopter on the optometry test bench is-2.00ds-2.00dc×180 =1.0.
The prescription of astigmatism contact lens should be: BC 8.7 mm-4.75 DS-2.00 DC×170.
Example 2. The diopter of the specimen is -3.00D/BC8.5mm, the tightness is appropriate and the rotation index is stable. The diopter on the optometry test bench is-6.00ds-1.50dc×160 =1.0.
The prescription of astigmatism contact lens should be: BC 8.5 mm-8.50 DS-1.50 DC×160.
[4] Expected residual astigmatism: When blinking, there will be 5 rotation due to the push of eyelids; Size and dispersion of residual astigmatism
The luminosity is proportional to the rotation angle, so when the patient's astigmatism is wrong,
When selecting adjacent astigmatism degrees according to the degrees, choose the gear with small astigmatism.
5. Evaluation of anti-rotation design of astigmatism soft contact lenses.
Can the 5 rotation that occurs in the blink of an eye be quickly recovered?
Whether the positioning and maneuverability are good.
How comfortable are the lenses?
What is the center thickness and oxygen permeability?