Mini- and Micro-incision Cataract Surgery – A Critical Review of Current Technologies

European Ophthalmic Review, 2009,3(2):52-7

Abstract:

Modern cataract surgery is striving for smaller and smaller incisions with the aim of making clear corneal incisions that are as safe and opographically stable as possible. Recent innovations in both phacoemulsification (phaco) and intraocular lens (IOL) technology have made micro-incision cataract surgery, defined as <2mm incision, safe and effective. Bi-axial sleeveless micro-phaco has recently been joined by sleeve-armed micro-co-axial micro-phaco, made possible by the development of slim-shaft strong-bevel phaco needles armed with micro sleeves that run flush with an enlarged needle head. Such tip technology allows for a highly efficient and safe high-flow, high-vacuum phaco through incisions as small as 1.4mm by providing high influx and suppressing surge while avoiding mechanical and thermal tissue damage. Two tips have so far been made available for mini- (2.2–2.4mm) and micro-incision cataract surgery (MICS) (1.4–1.6mm, depending on the incision architecture used). With the micro-tip supplemented by additional flow through an infusion spatula (‘infusion-assisted’ or ‘hybrid’ phaco), excessive flow and vacuum rates may be used, resulting in a two-fold efficiency as mirrored by the reduced phaco power required. IOL technology is lagging behind phaco technology. The challenge is to avoid trade-offs with regard to implant stability and aftercataract formation, as well as optical performance. Current MICS-IOLs are mostly hydrophilic acrylic one-piece constructions with insufficiently sharp posterior optic edges and broad haptic–optic junctions, both of which features compromise the optic-edge barrier effect. Recently, a hydrophobic three-piece IOL has been made available, which features a slim haptic junction and an exquisitely sharp optic edge and also allows for optional optic entrapment into a posterior capsulorhexis for lasting eradication of after-cataracts.
Keywords: Micro-incision cataract surgery (MICS), micro-co-axial microphacoemulsification, high-flow high-vacuum phaco, surge suppression, mechanical and thermal tissue damage, MICS lenses, optic-edge barrier effect, sharp posterior optic edge, slim haptic–optic junction, posterior optic ‘button-holing’
Disclosure: The author has no conflicts of interest to declare.
Received: July 28, 2009 Accepted September 20, 2009 Citation European Ophthalmic Review, 2009,3(2):52-7
Correspondence: Rupert Menapace, Professor of Ophthalmology, Intraocular Lens Service, Department of Ophthalmology, Waehringer Guertel 16-18, A 1090 Vienna, Austria. E: rupert.menapace@meduniwien.ac.at

Recent developments in cataract surgery have been dominated by efforts to further down-size the incision for phaco-emulsification (phaco) and intraocular lens (IOL) implantation. This article demonstrates and discusses the benefits and downsides of further down-sizing the cataract incision, the requirements regarding phaco and IOL technology, the pros and cons of the co-axial and bi-axial approaches and the currently available state-of-the-art co-axial instrumentation and implants.

Is There a Need for Further Down-sizing of the Cataract Incision?
Cataract incisions must fulfil two requirements:

• Deformation resistance (wound stability): to be safe, an incision must not open when manipulated. In practice, a patient may rub the eye with a finger tip. Temporally located incisions are particularly exposed to such deformation and must be designed accordingly. Deformation resistance depends on the size and construction of the incision; it increases as it gets smaller and longer and when it incorporates a scleral portion.

• Topographic neutrality (corneal stability): incisions must not induce corneal shape changes. Smaller incisions cause asymmetrical changes, which are properly picked up only with corneal topography. Topographical stability may be considered relevant within a pupillary zone of 5mm.

We have demonstrated that temporally located tunnel incisions with a scleral portion (temporal sclero-corneal incisions [SCIs]) are the best option to provide both adequate deformation resistance1 against digital massage and topographical neutrality within a 5mm zone.2 This is true for incision sizes up to 4mm.3

If temporally located sclero-corneal incisions up to a size of 4mm fulfil the requirements, why then should we struggle to further minimise the cataract incision? The answer is that the aforementioned is true only for sclero-corneal incisions. We have also demonstrated that 3mm-wide clear corneal incisions (CCIs) are notsafe enough and induce asymmetrical corneal flattening adjacent to the incision that encroaches on the 5mm optical zone.4 With temporal-superiorly or superiorly located incisions, this sectorial flattening effect increases significantly.5 While SCIs, due to conjunctival healing, allow for permanent sealing within one week, CCIs take months to do so, thereby exposing the patient to a risk of endophthalmitis for an extended time. However, corneal incisions have become very popular since they are easy and fast to create, exclude intra-operative ballooning of the conjunctiva and are cosmetically appealing while avoiding patient disturbances due to conjunctival bleeding or foreign body sensations. However, to be safe and astigmatically neutral CCIs must be further down-sized. How small must a safe CCI be? Deformation resistance of a CCI depends on the width and length of the incision. The smaller the incision, the shorter the incision can be while providing for the same amount of resistance against digital massage. Practically, a 2x1.5mm incision should fulfil these requirements. Smaller incisions further optimise safety, as long as they are not over-stressed during phaco and IOL insertion. Topographical impact on the central corneal zone has been shown to be negligible with a CCI size of 2.0mm or smaller.6

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Keywords: Micro-incision cataract surgery (MICS), micro-co-axial microphacoemulsification, high-flow high-vacuum phaco, surge suppression, mechanical and thermal tissue damage, MICS lenses, optic-edge barrier effect, sharp posterior optic edge, slim haptic–optic junction, posterior optic ‘button-holing’
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