posterior edge form on iols

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An analysis of the posterior edge form required on Intraocular lenses in order to assist in minimizing Postoperative Capsular Opacification (PCO)

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CONFIDENTIALThe contents of this file are the property of Dennis Murphy and the Fred Hollows Intraocular Lens Laboratory in Asmara, Eritrea, and are only to be used for the purposes of discussing IOL lens design with people who are collaborating with FH Asmara on new lens design. As such, they are not to be disclosed to people outside of this group without written authorization from the author POSTERIOR EDGE FORM ON IOLs By Dennis Murphy 15 July, 2005 Table of Contents Scope...........................................................................................................................................3 Introduction...............................................................................................................................3 The Posterior edge terminology of an IOL.............................................................................4 SQUARE EDGE.............................................................................................................................4 DISCONTINUOUS EDGE..................................................................................................................4 SHARP EDGE...............................................................................................................................5 A SAW TOOTH EDGE...................................................................................................................5 Engineering Data.......................................................................................................................6 UNITS........................................................................................................................................6 STRENGTH OF MATERIALS.............................................................................................................6 FORCE.....................................................................................................................................12 Stress Concentration...............................................................................................................13 The hydraulic wedge...............................................................................................................14 The wedge effect......................................................................................................................14 THE INCLINED PLANE..................................................................................................................14 THE INFINITE WEDGE..................................................................................................................16 Peel Strength of a Joint...........................................................................................................17 Why does a round posterior edge on the IOL contribute to PCO......................................17 WEDGE ANGLE VERSUS THE RADIUS ON THE POSTERIOR LENS EDGE....................................................18 THE LENS EPITHELIAL CELLS AS A HYDRAULIC WEDGE..................................................................18 CELL PENETRATION MECHANISM.................................................................................................19 Model of the Eye......................................................................................................................20 The Rim Effect........................................................................................................................22 CAD View of an IOL in the Capsular Bag...........................................................................23 LENS WITH A TURNED-UP RIM AROUND THE POSTERIOR SURFACE.....................................................24 Turned-Up Rim around the posterior surface 8 diopter.................................................26 Turned-Up Rim around the posterior surface 30 diopter...............................................27 Summary turned up rim IOL..........................................................................................28 SHARP POSTERIOR LENS SURFACE WITH NO RIM AROUND IT...............................................................29

2 Sharp posterior surface with no rim 8 diopter..............................................................30 Sharp posterior surface with no rim 30 diopter..............................................................30 Summary sharp edge no rim IOL...................................................................................30 COMPARISON BETWEEN A TURNED UP RIM AND A SHARP EDGE WITH NO RIM .......................................31 Lenses in the marketplace......................................................................................................32 Questions..................................................................................................................................34 1..) MAXIMUM ALLOWABLE RADIUS ON THE POSTERIOR IOL EDGE...................................................34 2..) CREASES IN THE CAPSULAR BAG.............................................................................................34 3..) A RIM OR NO-RIM ON THE POSTERIOR EDGE OF THE IOL.......................................................35 4..) MAXIMUM CLEAR OPTIC DIAMETER......................................................................................35 5..) STRESS LEVELS IN THE CAPSULE.............................................................................................35 6..) CAPSULAR BAG SHRINKAGE...................................................................................................35 7..) CLINICAL OPHTHALMIC COMMENTS........................................................................................36

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ScopeThe intent of this document is to explore the underlying mechanisms involved in a sharp edge on the posterior surface of an IOL that assist in preventing/delaying postoperative PCO. It is clearly recognized that other mechanical design features related to the haptic design also interact with the sharp posterior edge in the IOL in helping to prevent PCO. However, this document will only discuss the sharp edge parameters and mechanisms. The haptic design will be addressed in a separate document.

IntroductionConventional wisdom in the ophthalmic world seems to generally believe that a sharp edge, a square edge or a discontinuous edge on the posterior surface of the IOL can, under the right conditions, act as a barrier to the migration of residual lens epithelial cells across the anterior surface of the posterior surface of the capsular bag and hence assist in delaying, or perhaps preventing the onset of postoperative PCO. There are good engineering reasons for believing this. In addition, many studies have also tended to confirm the concept. In attempting to design a suitable sharp edge on a new design of lens one rapidly discovers that there are many unknown factors and also some rather ambiguous definitions which seem to be commonly used in describing a sharp edge on the IOL. In order to discuss the design of such a lens with both engineers and ophthalmologists it becomes necessary to establish a suitable set of standard terminology and working concepts in order that everyone is singing off the same song sheet (so to speak) ++++++++++++++++++++++++++++++++++

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The Posterior edge terminology of an IOLThe literature on preventing PCO is littered with expressions such as square edge discontinuous edge sharp edge. This rather imprecise use of terminology leads to a fair amount of confusion as to exactly what is meant and more importantly exactly what is needed on the posterior edge Square edge A square edge is a corner that forms a right angle (e.g. 90 degrees). It is clear that the posterior edge shown on the lens below is not truly a square edge.

It is possible to produce a square edge on an IOL by putting a rim around the edge of the clear optic diameter. Discontinuous edge The idea of continuity comes from mathematics in particular, calculus. The underlying idea, roughly speaking, is that we can say that a function f ( x ) is continuous on an interval I, if the graph of f ( x ) is unbroken for all x in the interval I. In this description we will only be concerned with an intuitive understanding of continuity and wont get bogged down in a mathematically rigorous discussion of the subject (interesting though it may be). As an example, the graph below shows a function discontinuity.

n

that has a sharp kink in it as well as a

Clearly we are not seeking a discontinuous edge on the posterior surface of the IOL.

5 Sharp edge In the picture below the difference between a sharp edge and a rounded edge is shown.

A sharp edge is what is actually required on an IOL in order to act as barrier to the migration of the epithelial cells across the anterior surface of the posterior capsule surface. The reason for this statement will be explored below in some detail as it has a major bearing on the manufacturing specifications of the lens. It is also of interest to the clinical scene as well. Agreement on the design specification of this edge so that it assists in preventing the onset of PCO and at the same time ensures absolute patient safety is the prime reason for this document. A Saw Tooth Edge Before we proceed we need to define another aspect of sharp. The general idea of a sharp edge is something that will cut into something else like a knife. Clearly this idea of sharp is totally unacceptable for an IOL. Sharp, as in a knife, refers to a blade with a very narrow angle between the two surfaces of the sides of the knife. In a case such as this, the cutting action takes place by the intense buildup of local stress that is concentrated directly under the extremely small radius formed in the material as the knife edge is p

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