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Reply: Simulation of eye rubbing
LETTERS
Simulation of eye rubbing To estimate the clinical relevance of the study by Masket et al.,1 we have to answer the question, Does the study methodology adequately mimic what takes place when a postoperative patient rubs his or her eye? An ounce is a unit of weight or volume, not of force. A force is created solely by an accelerating mass. The authors noted the length of the gauge they used in the study but not the width, which was probably shorter than the length. However, to present the study in the most favorable way, I would assume the tip is square with a resulting area of 9 mm2. Converting ounces/millimeters2 to the more familiar lb/in2, the study tested the integrity of the incision by applying the maximum pressure 10.08 lb/in2 to the surface of the eye 0.5 mm posterior to the external incision. Rubbing the eye by the patient is usually done by applying a flattened surface of an index finger (on my finger, an area 400 mm2) against the closed eyelid. Using the same maximum weight as the study weight would create a pressure of 0.23 lb/in2. Therefore, the point pressure on the surface of the eye was more than 44 times greater than the pressure applied by a finger to the closed eyelid. If the authors had done optical coherence tomography imaging, the difference between leaking and nonleaking incisions might have been seen even though incision construction and wound architecture were not standardized. Optical coherence tomography imaging of our single-plane clear corneal incisions2 revealed that they were parabolic in profile, perhaps giving greater resistance to slippage than linear incisions, and they were longer than the cord length we were measuring. I do not believe that point pressure applied to the surface of the eye adequately simulates eye rubbing by a patient to give this study clinical relevance. I. Howard Fine, MD Eugene, Oregon, USA REFERENCES 1. Masket S, Hovanesian J, Raizma M, Wee D, Fram N. Use of a calibrated force gauge in clear corneal cataract surgery to quantify point-pressure manipulation. J Cataract Refract Surg 2013; 39:511–518 2. Fine IH, Hoffman RS, Packer M. Profile of clear corneal cataract incisions demonstrated by ocular coherence tomography. J Cataract Refract Surg 2007; 33:94–97
Reply : We would like to address the concerns expressed by Dr. Fine regarding whether our model of applying force using a calibrated gauge in proximity to a clear corneal incision (CCI) appropriately Q 2013 ASCRS and ESCRS Published by Elsevier Inc.
simulates the variable action of a patient touching or rubbing his or her eyes. Dr. Fine suggests that the ounce is a measure of only volume and not force. According to the Dictionary of Units of Measurement, ounce force is a traditional unit of force.A It is also used to describe torque. One-ounce force (ozf), as opposed to 1 ounce of mass, was used to challenge CCIs. The psi (lbf/in2) unit cited by Dr. Fine is indeed a unit of pressure. Moreover, the force gauge described in our investigation was derived from an orthodontic tool; we opted to use the original nomenclature of the manufacturer. Dr. Fine also expresses doubt that the force gauge can simulate the effect of a patient rubbing the eye. Obviously, each patient may vary where, with what, and how they rub their eye at any given moment. However, from the existing literature, we determined the induced elevation of intraocular pressure (IOP) from eye rubbing and used this as target to quantify the necessary force of the gauge, providing a reasonable and quantifiable estimation of globe deformation by which incision competence could be evaluated. Since a calibrated and quantifiable device was necessary for standardized testing, the goal of the study was to reproduce changes in IOP that might be caused by patient manipulation rather than directly simulate a digital footprint on the ocular surface, which is quite variable. McMonnies et al.1 reported similar changes in IOP using a Schiotz tonometer with a 7.35 mm2 area of indentation (total weight of 16.5 g) and application of light digital force with a mean measured finger pad print area of 168 mm2. The change in IOP caused by 1-ounce force, using the calibrated force gauge, was consistent with IOP fluctuations recorded during light and firm digital forces on the sclera. As the globe is indented and pressure rises in the anterior chamber, fluid may be displaced through the CCI as a result. Dr. Fine also suggests that OCT is a better means of evaluating incision construction. While OCT imaging has been well used and is excellent for describing incision morphology, it has been a somewhat disappointing tool with regard to incision function. We note descriptions of internal and external wound gape, stromal misalignment, and Descemet detachment, but we are aware of few published studies that relate OCT imaging to incision competence (such as wound leakage associated with hypotony) in human patients. Calladine et al.2 reported that wound architectural features such as gaping and misalignment were affected by changes in IOP. Conversely, Torres et al.3 did not observe wound leakage via OCT in patients who had CCIs. However, IOP values in that study were in the normal range (no observed hypotony; 1 day postop IOP Z 16.4 mm Hg G 3.7 [SD]). Previous research studies have used OCT imaging with 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2013.07.020
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human donor eyes and in animal models to evaluate morphologic changes in corneal incisions with fluctuations in IOP.4,5 We are also aware of the early postoperative curvilinear morphology of the incisions as noted by Fine et al.6 on the first postoperative day. However, those findings have been reported by others and are observed to be transient only, perhaps in relationship to incision tissue edema.7 Based on the above, we maintain that the calibrated force gauge has clinical utility to evaluate the competence of wound architecture at the time of cataract surgery in a standardized quantifiable method.d Samuel Masket, MD, John Hovanesian, MD, Michael Raizman, MD, Daniel Wee, MD, Nicole Fram, MD REFERENCES 1. McMonnies CW, Boneham GC. Experimentally increased intraocular pressure using digital forces. Eye Contact Lens 2007; 33:124–129 2. Calladine D, Packard R. Clear corneal incision architecture in the immediate postoperative period evaluated using optical coherence tomography. J Cataract Refract Surg 2007; 33:1429–1435 3. Torres LF, Saez-Espinola F, Colina JM, Retchkiman M, Patel MR, Agurto R, Garcia G, Diaz JL, Huang D, Schanzlin DJ, Chayet AS. In vivo architectural analysis of 3.2 mm clear corneal incisions for phacoemulsification using optical coherence tomography. J Cataract Refract Surg 2006; 32:1820–1826 4. Rao B, Zhang J, Taban M, McDonnell P, Chen Z. Imaging and investigating the effects of incision angle of clear corneal cataract surgery with optical coherence tomography. Opt Express 2003; 11:3254–3261. Available at: http://chen.bli.uci.edu/publications/ j52_cornealincisionoptisexpres2003.pdf. Accessed June 16, 2013 5. McDonnell PJ, Taban M, Sarayba M, Rao B, Zhang J, Schiffman R, Chen Z. Dynamic morphology of clear corneal cataract incisions. Ophthalmology 2003; 110:2342–2348 6. Fine IH, Hoffman RS, Packer M. Profile of clear corneal cataract incisions demonstrated by ocular coherence tomography. J Cataract Refract Surg 2007; 33:94–97 _ Bayhan HA, C‚elik H, Bostancı Ceran B. Anterior segment 7. Can I, optical coherence tomography evaluation and comparison of main clear corneal incisions in microcoaxial and biaxial cataract surgery. J Cataract Refract Surg 2011; 37:490–500
OTHER CITED MATERIAL A. Rowlett R. How Many? A dictionary of Units of Measurement. Chapel Hill, NC, Center for Mathematics and Science Education, University of North Carolina at Chapel Hill, 2005. Available at: http://www.unc.edu/wrowlett/units/. Accessed June 16, 2013
Intraocular lens power calculation after photorefractive surgery: Modified double-K method In their article on intraocular lens (IOL) calculation after photorefractive surgery, Saiki et al.1 determined the regression equation between the mean preoperative anterior keratometry (K) values in the 3.0 mm
zone (Km) and the mean postoperative posterior corneal power in the 6.0 mm zone (K6 mm) in 72 postLASIK eyes and found a coefficient of determination (R2) of 0.73. Although R2 provides some information about the goodness of fit of a model, it does not guarantee that the model fits the data well enough (a high R2 can occur in the presence of a misspecified functional form of a relationship or in the presence of outliers that distort the true relationship).2 Furthermore, to validate the regression model, it is necessary to check whether its predictive performance remains at a similar level when applied to data that were not used in model estimation. This validation was not performed. With regard to the IOL power calculation, 28 eyes were included (7 IOL models). The authors indicated that optimized IOL constants from the User Group for Laser Interference Biometry (ULIB)A were used for the calculations with the modified double-K method presented by them and the other methods mentioned. We would like to have confirmation that the constants a0, a1, and a2 for the Haigis formula (and therefore for the Haigis-L method) and the constant SF for the Holladay formula recommended by the ULIB web site were entered into the American Society of Cataract and Refractive Surgery IOL power calculator. If only the A-constant was entered, the remaining constants were obtained by the software included in that calculator, according to a standard relationship between them, leading to erroneous results in the calculation of the prediction errors.B Saiki et al. used the Km from the sagittal map created by the corneal tomographer (based on the anterior radius of curvature and according to data shown in Table 1 in the article calculated with the standard keratometric index) as the Kpost for “optical calculation.” The double-K method uses the preoperative keratometry to estimate the effective lens position (ELP), thus avoiding the third-generation formula's ELP-related error. However, this method does nothing to compensate postoperative corneal power.3,4 Ten years ago, Aramberri4 obtained the Kpost by the clinical history method. Postoperative keratometric data taken from any standard axial map created by any tomographer will include the typical error in corneal power after photorefractive surgery, as such measurements are based on the keratometric index (which generates erroneous results when applied to an operated cornea) and are also performed in a slightly paracentral area introducing the radius measurement error, with significant influence on a cornea with its natural prolate shape altered by the surgical procedure.3 If a noncompensated postoperative standard K power
J CATARACT REFRACT SURG - VOL 39, SEPTEMBER 2013
Simulation of eye rubbing To estimate the clinical relevance of the study by Masket et al.,1 we have to answer the question, Does the study methodology adequately mimic what takes place when a postoperative patient rubs his or her eye? An ounce is a unit of weight or volume, not of force. A force is created solely by an accelerating mass. The authors noted the length of the gauge they used in the study but not the width, which was probably shorter than the length. However, to present the study in the most favorable way, I would assume the tip is square with a resulting area of 9 mm2. Converting ounces/millimeters2 to the more familiar lb/in2, the study tested the integrity of the incision by applying the maximum pressure 10.08 lb/in2 to the surface of the eye 0.5 mm posterior to the external incision. Rubbing the eye by the patient is usually done by applying a flattened surface of an index finger (on my finger, an area 400 mm2) against the closed eyelid. Using the same maximum weight as the study weight would create a pressure of 0.23 lb/in2. Therefore, the point pressure on the surface of the eye was more than 44 times greater than the pressure applied by a finger to the closed eyelid. If the authors had done optical coherence tomography imaging, the difference between leaking and nonleaking incisions might have been seen even though incision construction and wound architecture were not standardized. Optical coherence tomography imaging of our single-plane clear corneal incisions2 revealed that they were parabolic in profile, perhaps giving greater resistance to slippage than linear incisions, and they were longer than the cord length we were measuring. I do not believe that point pressure applied to the surface of the eye adequately simulates eye rubbing by a patient to give this study clinical relevance. I. Howard Fine, MD Eugene, Oregon, USA REFERENCES 1. Masket S, Hovanesian J, Raizma M, Wee D, Fram N. Use of a calibrated force gauge in clear corneal cataract surgery to quantify point-pressure manipulation. J Cataract Refract Surg 2013; 39:511–518 2. Fine IH, Hoffman RS, Packer M. Profile of clear corneal cataract incisions demonstrated by ocular coherence tomography. J Cataract Refract Surg 2007; 33:94–97
Reply : We would like to address the concerns expressed by Dr. Fine regarding whether our model of applying force using a calibrated gauge in proximity to a clear corneal incision (CCI) appropriately Q 2013 ASCRS and ESCRS Published by Elsevier Inc.
simulates the variable action of a patient touching or rubbing his or her eyes. Dr. Fine suggests that the ounce is a measure of only volume and not force. According to the Dictionary of Units of Measurement, ounce force is a traditional unit of force.A It is also used to describe torque. One-ounce force (ozf), as opposed to 1 ounce of mass, was used to challenge CCIs. The psi (lbf/in2) unit cited by Dr. Fine is indeed a unit of pressure. Moreover, the force gauge described in our investigation was derived from an orthodontic tool; we opted to use the original nomenclature of the manufacturer. Dr. Fine also expresses doubt that the force gauge can simulate the effect of a patient rubbing the eye. Obviously, each patient may vary where, with what, and how they rub their eye at any given moment. However, from the existing literature, we determined the induced elevation of intraocular pressure (IOP) from eye rubbing and used this as target to quantify the necessary force of the gauge, providing a reasonable and quantifiable estimation of globe deformation by which incision competence could be evaluated. Since a calibrated and quantifiable device was necessary for standardized testing, the goal of the study was to reproduce changes in IOP that might be caused by patient manipulation rather than directly simulate a digital footprint on the ocular surface, which is quite variable. McMonnies et al.1 reported similar changes in IOP using a Schiotz tonometer with a 7.35 mm2 area of indentation (total weight of 16.5 g) and application of light digital force with a mean measured finger pad print area of 168 mm2. The change in IOP caused by 1-ounce force, using the calibrated force gauge, was consistent with IOP fluctuations recorded during light and firm digital forces on the sclera. As the globe is indented and pressure rises in the anterior chamber, fluid may be displaced through the CCI as a result. Dr. Fine also suggests that OCT is a better means of evaluating incision construction. While OCT imaging has been well used and is excellent for describing incision morphology, it has been a somewhat disappointing tool with regard to incision function. We note descriptions of internal and external wound gape, stromal misalignment, and Descemet detachment, but we are aware of few published studies that relate OCT imaging to incision competence (such as wound leakage associated with hypotony) in human patients. Calladine et al.2 reported that wound architectural features such as gaping and misalignment were affected by changes in IOP. Conversely, Torres et al.3 did not observe wound leakage via OCT in patients who had CCIs. However, IOP values in that study were in the normal range (no observed hypotony; 1 day postop IOP Z 16.4 mm Hg G 3.7 [SD]). Previous research studies have used OCT imaging with 0886-3350/$ - see front matter http://dx.doi.org/10.1016/j.jcrs.2013.07.020
1449
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LETTERS
human donor eyes and in animal models to evaluate morphologic changes in corneal incisions with fluctuations in IOP.4,5 We are also aware of the early postoperative curvilinear morphology of the incisions as noted by Fine et al.6 on the first postoperative day. However, those findings have been reported by others and are observed to be transient only, perhaps in relationship to incision tissue edema.7 Based on the above, we maintain that the calibrated force gauge has clinical utility to evaluate the competence of wound architecture at the time of cataract surgery in a standardized quantifiable method.d Samuel Masket, MD, John Hovanesian, MD, Michael Raizman, MD, Daniel Wee, MD, Nicole Fram, MD REFERENCES 1. McMonnies CW, Boneham GC. Experimentally increased intraocular pressure using digital forces. Eye Contact Lens 2007; 33:124–129 2. Calladine D, Packard R. Clear corneal incision architecture in the immediate postoperative period evaluated using optical coherence tomography. J Cataract Refract Surg 2007; 33:1429–1435 3. Torres LF, Saez-Espinola F, Colina JM, Retchkiman M, Patel MR, Agurto R, Garcia G, Diaz JL, Huang D, Schanzlin DJ, Chayet AS. In vivo architectural analysis of 3.2 mm clear corneal incisions for phacoemulsification using optical coherence tomography. J Cataract Refract Surg 2006; 32:1820–1826 4. Rao B, Zhang J, Taban M, McDonnell P, Chen Z. Imaging and investigating the effects of incision angle of clear corneal cataract surgery with optical coherence tomography. Opt Express 2003; 11:3254–3261. Available at: http://chen.bli.uci.edu/publications/ j52_cornealincisionoptisexpres2003.pdf. Accessed June 16, 2013 5. McDonnell PJ, Taban M, Sarayba M, Rao B, Zhang J, Schiffman R, Chen Z. Dynamic morphology of clear corneal cataract incisions. Ophthalmology 2003; 110:2342–2348 6. Fine IH, Hoffman RS, Packer M. Profile of clear corneal cataract incisions demonstrated by ocular coherence tomography. J Cataract Refract Surg 2007; 33:94–97 _ Bayhan HA, C‚elik H, Bostancı Ceran B. Anterior segment 7. Can I, optical coherence tomography evaluation and comparison of main clear corneal incisions in microcoaxial and biaxial cataract surgery. J Cataract Refract Surg 2011; 37:490–500
OTHER CITED MATERIAL A. Rowlett R. How Many? A dictionary of Units of Measurement. Chapel Hill, NC, Center for Mathematics and Science Education, University of North Carolina at Chapel Hill, 2005. Available at: http://www.unc.edu/wrowlett/units/. Accessed June 16, 2013
Intraocular lens power calculation after photorefractive surgery: Modified double-K method In their article on intraocular lens (IOL) calculation after photorefractive surgery, Saiki et al.1 determined the regression equation between the mean preoperative anterior keratometry (K) values in the 3.0 mm
zone (Km) and the mean postoperative posterior corneal power in the 6.0 mm zone (K6 mm) in 72 postLASIK eyes and found a coefficient of determination (R2) of 0.73. Although R2 provides some information about the goodness of fit of a model, it does not guarantee that the model fits the data well enough (a high R2 can occur in the presence of a misspecified functional form of a relationship or in the presence of outliers that distort the true relationship).2 Furthermore, to validate the regression model, it is necessary to check whether its predictive performance remains at a similar level when applied to data that were not used in model estimation. This validation was not performed. With regard to the IOL power calculation, 28 eyes were included (7 IOL models). The authors indicated that optimized IOL constants from the User Group for Laser Interference Biometry (ULIB)A were used for the calculations with the modified double-K method presented by them and the other methods mentioned. We would like to have confirmation that the constants a0, a1, and a2 for the Haigis formula (and therefore for the Haigis-L method) and the constant SF for the Holladay formula recommended by the ULIB web site were entered into the American Society of Cataract and Refractive Surgery IOL power calculator. If only the A-constant was entered, the remaining constants were obtained by the software included in that calculator, according to a standard relationship between them, leading to erroneous results in the calculation of the prediction errors.B Saiki et al. used the Km from the sagittal map created by the corneal tomographer (based on the anterior radius of curvature and according to data shown in Table 1 in the article calculated with the standard keratometric index) as the Kpost for “optical calculation.” The double-K method uses the preoperative keratometry to estimate the effective lens position (ELP), thus avoiding the third-generation formula's ELP-related error. However, this method does nothing to compensate postoperative corneal power.3,4 Ten years ago, Aramberri4 obtained the Kpost by the clinical history method. Postoperative keratometric data taken from any standard axial map created by any tomographer will include the typical error in corneal power after photorefractive surgery, as such measurements are based on the keratometric index (which generates erroneous results when applied to an operated cornea) and are also performed in a slightly paracentral area introducing the radius measurement error, with significant influence on a cornea with its natural prolate shape altered by the surgical procedure.3 If a noncompensated postoperative standard K power
J CATARACT REFRACT SURG - VOL 39, SEPTEMBER 2013