IOL Calculation After LASIK. Chapter (3)

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Bertram Curtis
8 years ago
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1 the cornea (P) may be considered as the sum of the power of the anterior (Pa) and posterior (Pp) corneal surfaces, as shown in the formula: P = Pa + Pp = (n2 n1)/r1 + (n3 n2)/r2, where n1 is the refractive index of the air (= 1); n2 is the refractive index of the cornea (= 1.376); and n3 is the refractive index of the aqueous humor (= 1.336). Both pre-operatively and post-operatively, we can always calculate Pa by multiplying the corneal power (ie, measured by keratometry or simulated keratometry [Sim-K]) by (136,141) Hence: Pa = Sim-K X Once we know the pre-operative Pa, the negative-valued Pp can be easily calculated as the difference between the total and the anterior surface corneal powers, as shown in the formula: Pp = P Pa = Sim-K (Sim-K X 1.114). According to this method, the true corneal power (K) after LASIK or PRK can be calculated by adding the post-operative Pa to the preoperative Pp, which is assumed not to be significantly altered by surgery. The formula is: K = postop Pa + preop Pp = postop Sim-K X (preop Sim-K preop Sim-K x 1.114). The reliability of this method still awaits clinical confirmation. Theoretically, results are promising, on condition that the double-k formula was also used. (133) ٥٨

Both pre-operatively and post-operatively, we can always calculate Pa by multiplying the corneal power (ie, measured by keratometry or simulated keratometry [Sim-K]) by 1.114.

2 Jarade s formula. A quite similar alternative option is the formula published by Jarade and Tabbara. (142) It is also based on the consideration of both anterior and posterior corneal surfaces. It requires the pre-lasik K-value (Kpreop) and pre and post-lasik corneal radii measured by autokeratometry (respectively Ra-preop and Ra-post). The postexcimer laser K-value (K) is calculated as follows: K = Kpreop [(Nc 1) X (Ra-post Ra-preop) / (Ra-post Rapreop)], where Nc is the corneal refractive index of Again, this method needs further validation in the clinical setting. METHODS REQUIRING ONLY PREOP REFRACTION Although the cataract surgeon may find it difficult to obtain pre- LASIK or PRK K-readings in many patients especially if the clinical chart is unavailable, the preoperative refraction is easier to obtain, as this value may be retrieved even from an old pair of spectacles or a prescription. When the pre-operative refraction is the only available parameter, we have several options for calculating post-operative corneal power. Some of these (ie, Feiz-Mannis nomogram, Latkany s formulas, Masket s refractive history method), (132,143,144) are based on linear regression formulas, the results of which should be added to or subtracted from the IOL ٥٩

$The postexcimer laser K-value (K) is calculated as follows: K = Kpreop [(Nc 1) X (Ra-post Ra-preop) / (Ra-post Rapreop)], where Nc is the corneal refractive index of 1.376.$

3 power calculated using post-lasik/prk K-readings (ie, as if the eye had not undergone photorefractive surgery). The remaining formulas aim to modify the post-lasik/prk K- readings, either by correcting the keratometric index of refraction on the basis of the refractive change, (145) or by adjusting the K-readings of video-keratography and keratometry. (146) Of course, all the following methods assume that the surgically induced refractive change corresponds to the preoperative refraction. Feiz-Mannis nomogram. After confirming that K-readings from manual keratometry lead to unwanted hyperopia in eyes that underwent myopic LASIK, Feiz and collegues explored the relationship between the surgically induced refractive change at the spectacle plane and the IOL power calculation error (performed using postoperative keratometry). (138) They detected a statistically significant linear relationship for both myopic and hyperopic corrections. For previously myopic eyes, the diopters of underestimation are derived from the formula: diopters of underestimation = (0.595 X refractive change), where refractive change is the absolute value (ie, 4.5 for a patient with a pre-operative defect of D). Hence, the corrected IOL power (P) is obtained by subtracting the resulting value from the target IOL power (PTARG) (eg, for emmetropia), calculated using the post- LASIK keratometry. The formula is: ٦٠

$K-readings of video-keratography and keratometry. (146) Of course, all the following methods assume that the surgically induced refractive change corresponds to the preoperative refraction.$

4 P = PTARG (0.595 X refractive change). For previously hyperopic eyes, the formula is: P = PTARG (0.862 X refractive correction). Subsequent studies evaluating the formula have reported excellent results in about half of patients; refractive surprises have still occurred. (133,137,144) Latkany s regression formula. Latkany, (134) investigated the relationship between the error in IOL power estimation, as calculated on the basis of conventional keratometry and the amount of refractive correction performed by excimer laser. They found that such a relationship is linear and is expressed by the following formulas: using average K: -(0.46 X Pre-Rx ), and using flattest K: - (0.47 X Pre-Rx ), where Pre-Rx = pre-refractive surgery myopic spherical equivalent. The result of the regression formula is then added to the target IOL power, calculated using the corneal power measured by conventional keratometry. Although there is limited clinical experience, theoretical comparisons showed the formulas to be quite promising. (133) Masket s refractive history method. The first step is the same as in the above-mentioned formulas by Feiz and Latkany, in that the IOL power is calculated as if the eye did not undergo previous refractive surgery. Thereafter, the target IOL power obtained is adjusted according to the formula: ٦١

$Latkany, (134) investigated the relationship between the error in IOL power estimation, as calculated on the basis of conventional keratometry and the amount of refractive correction performed by$

5 IOL power adjustment = X refractive correction This value is added to the target IOL power calculation in patients who previously underwent myopic laser correction, or subtracted in patients who underwent hyperopic laser correction. The final formula is: P = PTARG X refractive correction Dr. Masket recommends using the SRK/T formula for myopic eyes and the Hoffer Q-formula for hyperopic eyes, on which his results are based. The first results appear promising in both hyperopic and myopic patients. (144) Jarade, Savini, and Camellin s formulas to calculate postrefractive surgery keratometric index of refraction. We have already stated that the conventional keratometric refractive index (n=1.3375) adopted to convert the anterior corneal curvature into diopters, is inaccurate after LASIK and PRK. Several investigators have analyzed whether changing it on the basis of the amount of myopic correction may improve IOL power calculation predictability after excimer laser surgery. In all cases, once the new keratometric index of refraction is known, the corneal power (K) is obtained using the standard formula: K=(n-1)/r, where r is the anterior corneal radius of curvature. The proposed formulas to calculate post-lasik/prk keratometric index of refraction are the following: Effective refractive index = X AMC; (145) ٦٢

$The final formula is: P = PTARG 0.326 X refractive correction +0.101. Dr.$

Intraocular Lens Power Calculation after Myopic Refractive Surgery

$Intraocular Lens Power Calculation after Myopic Refractive Surgery$ Intraocular Lens Power Calculation after Myopic Refractive Surgery Theoretical Comparison of Different Methods Giacomo Savini, MD, Piero Barboni, MD, Maurizio Zanini, MD Objective: To evaluate the reliability