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Letter

Difference between capsule tensile strength and capsulotomy edge strength

Snyder, Michael E. MD

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Journal of Cataract & Refractive Surgery: February 2018 - Volume 44 - Issue 2 - p 253
doi: 10.1016/j.jcrs.2017.12.013
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Chan et al.’s article1 ostensibly comparing femtosecond laser–assisted capsulotomy and manual capsulotomy edge strength is a well-executed study with a creative design. Unfortunately, it does not likely measure what it is purported to measure. The authors assume that strain testing with clips attached to the edge of the removed piece of capsule from a traditional capsulotomy or femtosecond laser–assisted cataract surgery case is measuring the stress resistance of that edge. Unfortunately, what they are more likely measuring is the tensile strength of the capsule tissue itself, not the strain resistance of the cut margin. The central removed piece of capsule has a convex edge, rather than the concave edge of the capsulorhexis margin that remains in the eye.

If Chan et al.’s strain testing clips were to have been applied to the concave edge of a capsulorhexis as the clips are separated, the concave edge would straighten before the devices would begin applying resistance. This would indeed be (at least theoretically) applying the maximum strain to the cut margin of the capsule, which is what the authors aspired to test. When the clips are applied to a convex edge, however, the maximum strain is applied to the capsule inside the outer arc of the capsule material (Figure 1) and would measure not the edge strength at breakage but rather the intrinsic tensile strength of the capsule itself. In fact, when breakage occurred in this study, it is not clear whether it occurred first at the edge and propagated inward (as the authors presume). It might be that the capsule was tearing outward toward the edge from a stress hole created in the middle of the material between the clips. In fact, no strain whatsoever is applied to the outer convex edge until the linear space between the 2 clips (the chord of the convex arc) has already been stretched. Because the clips have some physical thickness at their tip, the stress line between them would actually be between the midpoint of the clips, thus not really along the chord of the arc but rather a line segment parallel to and even more centripetal to the chord and the capsule edge. This would further increase the likelihood that the point of break would occur as a central stretch hole that extends to the periphery and not a tear at the edge propagating inward.

Figure 1
Figure 1:
Tension force (black arrows) applied to the clips (dark gray) create strain along a chord spanning the midpoint of the clip’s attachment to the capsule (dotted line), which is centripetal to the theoretical line of maximum strain (thin black line) that would occur if the attachment of the strain testing clips had no physical thickness. Even this line remains centripetal to the outer arc, which corresponds to the capsule edge (curved dashed line).

Considering these facts, it is not surprising that the capsules from 2 eyes of the same patient would have similar tensile strengths.

In addition, to prevent slippage, the force with which the clips were attached to the capsule must have static friction strong enough to prevent the capsule from sliding out from between their jaws as the tension is increased. This force can potentially compress the capsule focally and alter its native structural integrity. If any of the breakages occurred right at the edge of the clip, this might be related to mechanical alteration of the tissue rather than to the tissue tensile strength (or even purported capsulotomy edge strength). Such datapoints would have to be expunged from the dataset to even assess strength as the experiment was designed.

Based on the evidence presented, we can neither confirm nor refute the authors’ hypothesis that femtosecond laser–assisted cataract surgery and continuous curvilinear capsulorhexis edges have similar strength.

Reference

1. Chan T, Pattamatta U, Butlin M, Meades K, Bala C. Intereye comparison of femtosecond laser–assisted cataract surgery capsulotomy and manual capsulorhexis edge strength. J Cataract Refract Surg. 2017;43:480-485.
© 2018 by Lippincott Williams & Wilkins, Inc.