Here is some of the most Frequently asked questions in matters relating to Keratometry, if you are doing A Scan biometry , these hints will come in Handy
Try to answer each questions & try to think about it for a minute
How much of the cornea is measured with a keratometer?
Why dose a keratometer use doubling of its images?
How should you proceed with the patient’s care?
What if a postoperative patient has a refraction of +2.00 /-2.00 x 90? Where should you cut the suture?
What measurements are necessary in determining the intraocular lens implant calculation?
How does an error in keratometry readings affect the intraocular lens calculation?
What does SRK stand for? What is it ?
Name an easy way to decrease the chance of making an errors in axial length measurement?
How much of the cornea is measured with a keratometer?
Only the central 3 mm, a peripheral corneal scar or defect may be missed by using a keratometer instead of a corneal map.
Why dose a keratometer use doubling of its images?
To avoid the problems of eye movement in determining an accurate measurement. Doubling is done with prisms.
After cataract surgery, a patient has the following refraction: +1.00 /+3.00 x 100. Does the patient have with the rule or against the rule astigmatism?
With the rule astigmatism is corrected with a plus cylinder at 90° (± 15-20°). Against the rule astigmatism is corrected with a plus cylinder at 180° (± 15 – 20°). The patient has with the rule astigmatism.
How should you proceed with the patient’s care?
Check the remaining sutures. Cutting the 11.0 suture will relax the wound and decrease the amount of astigmatism.
What if a postoperative patient has a refraction of +2.00 /-2.00 x 90? Where should you cut the suture?
Changing the refraction to plus cylinder form, you see that the patient is Plano +2.00 x 180 and has against the rule astigmatism. You cannot cut any sutures to relax the astigmatism. The only option is to do a relaxing incision of the cornea, but it is likely that the patient will tolerate glasses, especially if the refraction is close to the preoperative correction. Also, check the preoperative keratometry. The patient may have had against the rule astigmatism before surgery.
What measurements are necessary in determining the intraocular lens implant calculation?
Axial length in millimeters and keratometry readings in diopters. The desired postoperative refraction is also necessary. The SRK formula is commonly used. For emmetropia, the formula is P=A-2.5 (axial length) –0.9(Average K reading), where P equals the implant power, A is the implant constants as determined by the manufacturer, and K is the average of the keratometry readings. The A constant also can be individualized by analysis of previous cases. For each diopter of desired ametropia, add 1.25 to1.50 D. For example, if the SRK formula reveals a calculation of + 18.) D for emmetropia, implant a + 19.50D lens for –1.00D.
How does an axial error that is incorrect by 0.1 mm affect the intraocular lens calculation?
For every 0.1 mm error, the calculation is impacted by 0.25 D. Recheck the A scan if the axial length is less than 22 mm or more than 25 mm or if there is more than a 0.3mm difference in the measurement between the two eyes.
How does an error in keratometry readings affect the intraocular lens calculation?
For every error of 0.25 D, the calculation is is error by 0.25 D. Recheck the Keratometer measurements if the average corneal power is less than 40D or more than 47 D. Also check if there is a difference of more than 1 D in the average keratometer readings between eyes.
Discuss the types of formula in common use for calculation of intraocular power. Give an elense xample of each type.
Ø Theortic formulas are derived from optical priniciples and use assumes dimensions for certain parts of the eye (eg,binkhorst, colenbrander).
Ø Empiric formulas are derived by regression analysis from clinical results. A mathematical relationship between corneal curvature, axial length, and intraocular lens power can be derived (eg,SRK).
What does SRK stand for? What is it ?
The SRK (saunders, retzlaff, and kraff) formula is:
P= A-2.5 L-0.9K
Where P is the power for emmetropia, A is a lens constant, L is the axial length in millimetres, and K is the average corneal curvature in dioptres. The lens constant is an unique constant related to lens type and manufacture . it can be personalized by analysis of results with that lens
Using the SRK formula, how much of an error in intraocular lens power would result from a 0.5 D mistake during keratometry? From a 0.5 mm mistake during axial length masurement?
0.5 D error in keratometry
0.5 x 0.9 = 0.45 D, or about a half a diapter
0.5mm error in axial length
0.5 x 2.5 = 1.25 D
It should be obvious that small errors in axial length can translate into large errors in calculated lens power.
A patient presents with average keratometry readings of 43 D and an axial length of 19mm. Which IOL formula do you recommend?
Not the SRK or Binkhorst. Older theretical formulas tend to overestimate the IOL power for short eyes, while SRK underestimates it. One of the newer-generation formulas should be used, as they are much more accurate with very long or very short eyes. Examples of newer formulas include the SRKII, SRK/T, and Holladay. These formulas are comparably accurate, though SRK II doesn’t do as well in very long eyes. Only the SRK II is simple enough to calculate by hand. It is the same as the normal SRK formula, but the A constant is modified for long or short eyes.
Note that in the example above, if the SRK formula had recommended and IOL power of +32 D, the SRK II formula tells you to use +32 +3 = +35D.
SRK/T (“T” stands for “theoretical”) and Holladay are complicated theoretical formulas refined by empiric observations. Many A scan units contain computer software that calculates recommended IOL power based on one of the newer formulas.
What is the disadvantage of a plano/ convex IOL design (implanted with the convex surface closer to the cornea)?
It has more spherical aberration. A plano / convex IOL has more spherical aberration than a biconvex design. A biconvex IOL with a rear surface power that is stronger than the front surface power causes the least spherical aberration.
Name an easy way to decrease the chance of making an errors in axial length measurement?
There are many potential source of error when measuring axial length and keratometry for IOL calculations. Measuring axial length in both eyes can decrease the chance of making a serious measurement error. If the two measurements are not close to equal, the measurement should be repeated.
A patient presents for preoperative evaluation for cataract surgery. The refraction in the catractous eye is – 9.50 sphere. She has been wearing a contact lens over that eye to correct the anisometropia. Keratometry readings in the eye are 43.00 /42.50. What is a potential source of error in the IOL power calculations in this case?
The contact lens must be removed a minimum of 2 hours before keratometry. Errors caused by molding of the cornea can amount to 1 D. Some clinicians recommend removing the contact lens 2 weeks before keratometry, but others say 2 hours is enough time.
Should emmetropia be the goal in all cataract surgery?
No. Induced anisometropia may produce intolerable aniseikonia as well as inducing heterotropias. Take into consideration the status of the other eye (aphakic versus pseudophakic) and the preoperative refraction of the patient. For example, a moderate myope may wish to remain so that she can continue to read without her glasses.
Lasik formula:
Actual K= [Average pre K-1.135
(Average pre K-Average post K)
Pre K1=43.0D Post K1=31.77D
K2=42.0D K2=30.92
Average= 42.5D Average=31.34
Actual K= [42.50D-1.135(42.50-31.34)
Actual K= [42.50D-1.135X11.16]
Actual K= [42.50D-12.66]
Actual K= 29.84D