Ординатура / Офтальмология / Английские материалы / Mastering theTechniques of Lens Based Refractive Surgery (Phakic IOLs)_Garg, Alio, Dementiev_2005

Ashok Garg (India)

INTRODUCTION

Lens based refractive surgery is certainly a growing segment of refractive surgery for last few years. It has gained world wide acceptance and popularity. Refractive surgery is usually classified in two categories.

a.Corneal based refractive surgery in which refractive surgery is done by lasers by corneal reshaping. PRK, Lasik and Lasek are standard procedures in this group.

b.Lens based refractive surgery in which refractive surgery is done either by altering the natural lens or by placing an intraocular lens inside the eye in front of patient’s natural lens.

Refractive lens surgery techniques are essentially the

same as for cataract surgery. It is a rewarding procedure both for the patient and the ophthalmologist. It allows the surgeon to efficiently and effectively correct otherwise untreatable refractive errors such as extreme myopia and hyperopia. Refractive lens surgery (RLS) uses the same successful and familiar techniques developed for cataract surgery to treat refractive errors. RLS is a process not a procedure.

HISTORY OF REFRACTIVE LENSES

a.Drs Strampelli and Barraquer in 1950s introduced a biconcave angle supported lens. However, these lenses were discarded due to serious angle and endothelium related complications.

b.Dr Dveli in 1980s restarted phakic myopia lenses with 4 soft angle supported loops but these lenses also had drawbacks.

c.Dr Georges Baikoff from France in 1980s introduced an angle supported myopia lens with Kelman type haptics.

d.Dr Jan worst and Fechner in 1986 introduced phakic myopia lens of iris claw design. This lens is a peripheral iris fixated anterior chamber lens which has gained popularity. It is now available as verisyse phakic IOL commercially.

e.Professor Fyodorov from Russia in 1986 introduced the concept of soft phakic lens in the space between the iris and the anterior surface of the crystalline lens

and is now available as an implantable contact lens (ICL) commercially.

Professor Charles D Kelman was a great visionary who early saw the refractive potential of cataract surgery. He understood that reliable and refined refractive outcomes would have a start with a small incision. This is the vision that led him to develop phaco. Refractive lens exchange would not have been possible without phaco.

There is close ties between the history of phakic IOLs and that IOLs in general since the days of Dr Ridley who first designed the IOL for the correction of aphakia, the potential of the IOL for the correction of ametropia was readily apparent. Today there is marked improvement in design and material of phakic IOLs which have enhanced the safety and efficacy of lens based refractive surgery with better clinical outcome.

Today phakic refractive lenses are available in three styles.

a.Anterior chamber angle fixated IOLs

b.Iris supported phakic IOL

c.Plate lens that fits between the Iris and the crystalline lens.

INDICATIONS FOR REFRACTIVE

LENS SURGERY

•Patients not suitable for Lasik/Lasek due to high powers or thin corneas (Extreme Myopia or Hyperopia)

•To correct ametropia in the Presbyopic age

•Endothelial count more than 2000 cells/cmm

•Stable refraction for one year

•Age above 18 years

•AC depth more than 3 mm.

CONTRAINDICATIONS FOR RLS

•Corneal dystrophy/Endothelial cell count <2000 cells/cmm

•Anterior chamber depth less than 3 mm

•Pathological myopia

•Presence of anterior/posterior synechiae

•History of uveitis

•Persistant glaucoma

•Evidence of nuclear sclerosis

•Family history of retinal detachment

•Diabetes mellitus.

However in above contraindications some are relative

on the discretion of the surgeon and the patient requirement.

ASSESSMENT FOR REFRACTIVE LENS

SURGERY

Preoperative

Complete and careful preoperative ophthalmic checkup is necessary for better postoperative visual outcome. Following examinations are mandatory. These include–

i.Objective and subjective refraction.

ii.Complete anterior and posterior segment examination-slit lamp, direct and indirect ophthalmoscopy are vital to rule-out any pathological conditions).

iii.Anterior chamber depth (ACD) analysis.

iv.Specular microscopy for endothelium status.

v.K-readings and topography by orbscan.

vi.Intraocular pressure (IOP) status. Ophthalmologists prefer topical anesthesia for lens

based refractive surgery as peribulbar and retrobulbar anesthesia carry the potential risk of globe perforation in high myopic eyes. Parabulbar and general anesthesia are other alternatives.

Postoperative Assessment

Postoperative assessment is crucial in every operated case of RLS for the successful refractive outcome.

Generally patients are assessed at first postoperative day (Day +1), first week, 15 days, one month, 3 months and one yearly check-ups. On each visit patient should be examined for –

•Visual acuity status

•IOP measurement

•Slit lamp examination for IOL position and enclavation.

•UCVA, BCVA and Residual refractive error.

•Postoperative astigmatism.

•Contrast sensitivity.

•Specular microscopy.

•Corneal status.

ADVANTAGES OF LENS BASED REFRACTIVE SURGERY OVER LASER CORRECTIVE PROCEDURES

•A higher range of refractive errors can be corrected specially extreme myopia and hyperopia which are not treatable with laser surgery

•Lens based refractive surgery is a potentially reversible procedure

•RLS is totally safe procedure as no structural changes are induced hence safe in patients with high refractive errors and their corneas

•Contrast sensitivity is marked better with RLS as compare to laser refractive procedures in eyes with higher refractive errors. There is marked improvement in BCVA with these lenses because of the magnification factor

•RLS is a highly skilled procedure so prevents misuse of the procedure

•RLS is quite effective in presbyopia with newly developed accommodating implants, multifocal implants and blended vision implants.

Lens based refractive surgery is a safe procedure.

There is tremendous improvement in design and material of refractive lenses which have enhanced the safety and efficacy of the procedure with better visual outcome. With the advent of liquid injectable IOLs, improved surgical techniques and better diagnostic technologies, lens based refractive surgery should be available in the armamentarium of every refractive surgeon to correct entire range of refractive errors to provide safe and stable refractive vision to the patients. RLS is quite revolutionary to provide refractive correction and maintains accommodation. With further ongoing research works and modifications in refractive lens surgery techniques, I am sure it shall be procedure of choice for catarefractive surgery in near future.

Jairo E Hoyos-Chacón

Melania Cigales

Jairo E Hoyos (Spain)

INTRODUCTION

Throughout the 20th century, refractive surgery has developed along two important lines of research: corneal refractive techniques and intraocular lens implantation. Corneal refractive procedures have perhaps gained most popularity, but have gradually during the course of history established their own limits and today are not considered the option of choice to correct high ametropia, given their poor visual outcome and lack of stability when high corrections are attempted. Intraocular lenses have proved their efficacy in cataract surgery for the correction of aphakia, but their use in phakic patients to correct ametropia, has been accompanied by complications in the midand long-term, such that researchers have had to constantly modify the original lens design.

A review of the history of phakic lenses reveals two main groups according to their site of implant: anterior chamber lenses (implanted between the cornea and iris) or posterior chamber lenses (placed between the iris and crystalline lens). The first lenses implanted were anterior chamber lenses, with angular-support lenses and irisfixated lenses developed in parallel. However, the recent appearance of posterior chamber phakic lenses has sparked a highly promising new line of investigation. We probably have yet to see significant advances in the materials and designs of intraocular lenses, as we have yet to establish the ideal implant site.

ANGULAR-SUPPORT ANTERIOR

CHAMBER PHAKIC LENSES

In the middle of the last century, Strampelli1 implanted the first phakic intraocular lens in the anterior chamber.

The lens was a rigid negative power, meniscus-shaped lens made of acrylic with three fixation points, a length of 11 to 12 mm and a 6 mm optical zone. In the years to follow, many ophthalmologists worked with intraocular phakic lenses.

In 1959, J Barraquer2 reported his results in 239 eyes, although most of the lenses implanted had to be removed due to serious postoperative complications (chronic iridocyclitis, recurrent hyphema, corneal decompensation and secondary glaucoma). After evaluating Barraquer’s series of explanted lenses, Drews3 discovered that most of these complications had been provoked by imperfections in the lens manufacturing process, including the presence of polishing wax and biological defects in the angular fixation concept itself arising from a lack of knowledge of the physiology of the anterior segment (among other things it was noted that the fixation ends of the lens were too thick).

Choyce,4 who introduced several modifications to the original designs of Strampelli and Barraquer, went on to implant over 5000 lenses up until 1985. Choyce improved the manufacturing process and thinned the haptics footplates in collaboration with the company Rayner© (London, England), and marketing the highly polymerized methacrylates (Perspex CQ) that subsequently proved to be good, biocompatible materials.

Between 1986 and 1987, Baikoff5 presented the ZB design (Domilens©, Lyon, France), a monoblock polymethylmethacrylate (PMMA) lens, obtained by adapting an anterior chamber lens to correct aphakia, the Kelman Multiflex lens. This lens had four fixation points on the angle, an anterior angulation of the haptics of 25º and an optical zone of 4.5 mm. However, this model produced endothelial damage including reduced cell density, pleomorphism and cell-free zones. The cause of endothelial damage was found to be the closeness of the lens optic to the endothelium, which varied from 0.71 to 1.51 mm.

The ZB model was replaced with the second generation ZB5M (Fig. 2.1). This lens has more flexible haptics angled at 20° and a greatly reduced central thickness of the optic (250 microns). The lens was fixed

Figure 2.1: Development of the ZB lens to the ZB 5M. The optic becomes thinner, the angulation of the haptics changes from 25º to 20º and the optical zone extends from 4.5 mm to 5.0 mm (effective size 4.0 mm)

at the iridocorneal angle by four contact zones. These modifications managed to increase the distance from the optic to the endothelium by 0.6 mm in an effort to reduce the risk of endothelial damage. This lens was also able to maintain a distance to the anterior capsule of 1.2 mm. The model was available in three diameters (12.5, 13.0 and 13.5 mm) and had a biconcave optic, its overall diameter being 5.0 mm and effective diameter 4.0 mm. The power of the lens ranged from –7.00 to –20.00 diopters (D) in 1 D increments. Later on, this lens received a “fluorine plasma” surface treatment to improve its biocompatibility to create the model ZB5MF.

Successive improvements to the ZB5MF lens gave rise to the third generation NuVita MA20 (Chiron©), available only in negative powers (Fig. 2.2). With this model, the designers tried to reduce the incidence of subjective visual disturbances described by patients, by increasing the real optical zone to 4.5 mm, reducing by 20 percent the thickness of the lens margins, changing the biconcave shape of the optic to a meniscus shape and subjecting it to “Peripheral Detail Technology” to try to reduce glare. Its footplates were redesigned and enlarged to achieve better support at the angle and thus reduce the incidence of pupillary distortion. When Baush & Lomb© (Claremont, CA, USA) took over Chiron©,

Figure 2.2: Development of the ZB5MF lens to the NuVita MA20. The effective optical zone increases from 4.0 mm to 4.5 mm and the step shape of the haptics changes to a straight design