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Letter

Simulation of eye rubbing

Fine, Howard I. MD

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Journal of Cataract & Refractive Surgery: September 2013 - Volume 39 - Issue 9 - p 1449
doi: 10.1016/j.jcrs.2013.07.020
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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.

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.
© 2013 by Lippincott Williams & Wilkins, Inc.