Bioptics is widely performed but has never been precisely defined. Although often used to encompass the combination of various refractive procedures (even if both performed on the cornea), ‘bioptics’ has nevertheless a much narrower meaning.
Bioptics is a sequential method of treating large and complex refractive errors, aiming to improve stability and predictability, maintain a large optical zone, and preserve corneal prolate asphericity, thus limiting induced spherical aberrations. These three goals can only be achieved by the combination of refractive techniques with different mechanisms of action. This is why bioptics is normally performed using an intraocular implant [a phakic or pseudophakic intraocular lens (IOL)] associated with a modification of the corneal curvature (laser ablation, thermal shrinkage, incisions, intrastromal implant). The implant addresses most of the spherical error, while corneal surgery follows for finetuning, usually after 2–3 months.
The combinations of refractive techniques with similar mechanisms are not true bioptics. For example, the association of myopic excimer laser/intracorneal rings (both inducing a flattening of the corneal curvature) can be effective, but this combination should not be considered bioptics, because both techniques considerably alter corneal asphericity.
Bioptics with phakic intraocular lenses
Phakic IOLs and myopia, hyperopia, and presbyopia are discussed here.
Myopia: phakic intraocular lenses plus excimer laser
The combination of various models of phakic IOLs with corneal excimer laser is probably the most successful product of the bioptics concept. Myopic eyes generally have deep anterior chambers, allowing easy implantation of phakic IOLs. Moreover, subsequent laser ablation of a low residual myopic error (even if astigmatic) is simple and effective, and a wide, well centered optical zone (larger than the IOL optical zone) can be easily achieved.
Nearly all possible combinations have been investigated with good results: posterior chamber IOLs with LASIK and PRK; iris-fixated IOLs with LASIK and laser-assisted subepithelial keratectomy (LASEK); and angle-supported IOLs with LASIK and PRK. As expected, in all series laser enhancement significantly improved predictability, although in one study it proved unnecessary after IOL implantation in 43% of eyes. Most complications (pupil ovalization, anterior subcapsular opacity, iridocyclitis, dysphotopsia) were peculiar to specific phakic IOL types rather than to bioptics.
As for safety, it has been shown that LASIK maneuvers after phakic IOL implantation cause neither further damage to the endothelium nor IOL dislocation. PRK did not affect the endothelium either.
Intraocular lens-induced dysphotopsia was not worsened by laser enhancement; it was in fact improved. Residual errors contribute to halos, and their correction by corneal surgery with an adequately large optical zone is therefore beneficial. In a subjective evaluation, glare and halos were graded as ‘few’ in a study; in another series, halos after phakic IOL implantation were not increased by PRK or LASIK.
Hyperopia: iris-fixated phakic intraocular lenses plus laser-assisted in-situ keratomileusis
The correction of hyperopia by phakic IOLs is possible in only a limited number of eyes, because of shallower anterior chambers, whose depth is estimated to decrease by 20 µm per year: an anterior chamber phakic IOL in a hyperopic eye may thus require explantation 5–20 years after implantation. Such concerns account for the paucity of studies on bioptics with phakic IOLs in hyperopia. In both series, an iris-fixated phakic IOL was followed by LASIK. Iris-fixated IOL was chosen because it is supposed to be the farthest from anterior chamber angle and from the crystalline lens. The correction by IOL was aimed at leaving a myopic astigmatism, which is more easily corrected by LASIK. No additional endothelial damage was caused by LASIK; 95% of eyes were within 1 D of emmetropia, and 80% were within 0.5 D. Halos were intense in 2.6% of eyes and moderate in 36%. Predictability in this study was better than with iris-fixated toric phakic IOL without laser enhancement.
Presbyopia: angle-supported multifocal phakic intraocular lenses plus photorefractive keratectomy
Although only one case of PRK enhancement after angle-supported multifocal phakic IOL has been fully described, PRK and LASIK are frequently used to refine this form of presbyopic correction.
Bioptics with crystalline lens exchange
Crystalline lens exchange and myopia, hyperopia, and presbyopia, and bioptics after cataract surgery are discussed here.
Myopia: refractive lens exchange plus laser-assisted in-situ keratomileusis
Refractive lens exchange (improperly called clear lens extraction) consists of phacoemulsification and in-the-bag IOL implantation (as in cataract surgery) for refractive purposes. As it causes loss of accommodation, this technique is reserved for the presbyopic age. The concern about vitreoretinal complications reported in earlier studies has curbed use of refractive lens exchange in patients with myopia; however, modern phacoemulsification has shown increasing safety, and a meta-analysis has calculated a risk of retinal detachment of 1.85% in each eye 43.5 months after surgery – only slightly superior to that of nonoperated high myopic eyes (1.5%).
The safety and effectiveness of refractive lens exchange followed by LASIK enhancement have been reported in two small and in one large (64 eyes) series, showing excellent predictability (significantly improved after laser enhancement) and no retinal complications.
Hyperopia: refractive lens exchange plus excimer laser
Refractive lens exchange for hyperopia has been more readily accepted by surgeons and patients for three main reasons: low retinal risk in nonmyopic eyes, limited efficacy of corneal surgery in correcting high hyperopia, and the difficulty of placing phakic IOLs in hyperopic eyes. In addition, lens replacement deepens the anterior chamber, thus preventing angle-closure glaucoma in the long term.
All reports agree on the high safety and efficacy of refractive lens exchange followed by LASIK or PRK. The only large series, however, is characterized by a high rate (62%) of double in-the-bag IOL implantation (‘piggyback’) because higher IOL powers were unavailable: this resulted in interlenticular opacification in one-third of piggybacked eyes. Moreover, most eyes were largely overcorrected after lens surgery, requiring deeper laser ablations. The present availability of IOL powers of up to +40 D (or at least, the implantation of the second IOL in the ciliary sulcus) prevents interlenticular opacification; in addition, improved IOL formulas and intraoperative check of IOL power (e.g. by autorefractor) increase refractive lens exchange predictability, thus reducing the amount of laser enhancement. Mild to moderate halos were reported by 27% of patients.
Presbyopic lens exchange plus excimer laser
The goal of good unaided near and distance visual acuity has led to presbyopic lens exchange, in which a multifocal or a pseudo-accommodating IOL is implanted in the capsular bag. Excimer laser enhancement is not uncommon after these IOLs.
With multifocal IOLs, especially used in hyperopia, minimal residual refractive errors affect both multifocality and visual acuity and worsen halos. Therefore, attempts were made to refine presbyopic lens exchange by PRK treatment of residual errors in 12 eyes. PRK enhancement only improved distance unaided vision, with no improvements in multifocal effect or halos. These findings cast serious doubts on the theory that emmetropia (or +0.5) leads to few or no halos with multifocal IOLs.
Pseudo-accommodating IOLs require emmetropia for optimizing results, and excimer laser enhancement is needed in up to 20% of cases. Vigo et al. (L. Vigo, E. Scandola, F. Carones, presented at ESCRS, 6–10 September 2003; Munich, Germany) have treated eight eyes (seven by LASIK, one by PRK) with a -4.5 to +2.5 residual error after pseudo-accommodating IOL implantation, achieving emmetropia, good short-term accommodation, and no dysphotopsia. Unfortunately, the procedure was later abandoned because of long-term loss of accommodative ability.
Bioptics after cataract surgery
After cataract surgery, preexisting or surgically induced astigmatisms, as well as wrongly powered IOLs (a major cause of litigation), must be adequately addressed. ‘Piggybacking’ or IOL replacement can be considered, but use of the excimer laser is the most precise and least invasive approach. LASIK has been successfully employed after phacoemulsification and extracapsular extraction, the latter, however, with a 15% rate of diffuse lamellar keratitis. In the phacoemulsification studies, 82–100% of eyes treated by LASIK ended within 1 D of residual error, and no retinal, incision, or IOL complications were observed. PRK has been also successfully used after phacoemulsification in a limited case series. Possibly because of the mean patient age (75 years), a slight overcorrection of myopia was seen after LASIK and PRK.
My personal preference is for surface ablation for two reasons. First, corneal sensitivity is reduced after the age of 60 years, and the post-PRK course is normally painless. Second, dry eye is common at that age, and LASIK may induce or exacerbate symptoms. Surgically induced astigmatism is often asymmetric, however, and laser correction is therefore less precise than in congenital astigmatism.
Reverse bioptics: intraocular lenses after excimer laser
Phakic IOLs after corneal surgery and refractive lens exchange after excimer laser are discussed here.
Phakic intraocular lenses after corneal surgery
After an unsuccessful corneal refractive procedure, the temptation of a so-called enhancement is appealing to both the surgeon and the patient; however, further corneal surgery can induce recurrent haze and ectasia and is, for many reasons, poorly predictable. Thin corneas or large regressions can be conveniently approached using ‘reverse’ bioptics, i.e. phakic IOL or lens exchange after the corneal procedure. Vaz has successfully implanted iris-fixated phakic IOLs to correct residual/regressed hyperopic or myopic errors after LASIK, PRK, radial keratotomy, or intracorneal rings; two-thirds of cases ended within 1 D of emmetropia.
I have personally implanted angle-supported phakic IOLs in 12 cases of regression after myopic PRK and LASIK, which had been used improperly to address high myopic errors. Regression must be stable for at least 18 months and endothelium must be regular. Calculation of IOL power is affected by difficult subjective refraction: use of the van der Heijde formula led in my first cases almost invariably to undercorrection of myopia. I now adjust the formula by overcorrecting myopia by 10%, thus achieving better predictability. The changes induced by the surgical incision in these excimer laser-treated corneas are not relevant.
Refractive lens exchange after excimer laser
In the presbyopic age, reverse bioptics is best performed using refractive lens exchange, prudently with a monofocal IOL. Again, IOL power calculation after corneal surgery is the main issue, and many formulas have been proposed. A brilliant solution is represented by intraoperative autorefractometry, which Ianchulev et al. have recently assessed in normal eyes and in six LASIK-treated eyes, performing it during phacoemulsification, between cortical cleanup and IOL implantation. Their conclusion, in both normal and postcorneal surgery eyes, is that the spherical equivalent at autorefraction should be doubled to obtain the IOL power (e.g. with a +5 D autorefraction, a +10 D IOL should be implanted in the capsular bag). No IOL A-constant or IOL model is indicated in the paper, however.
I have found more accurate the Gills' formula (originally proposed for piggyback IOLs), in which autorefraction is multiplied by 1.3–1.5 (long eye to short eye) and +1 D is added. So, a +5 D autorefraction would indicate a +8 D IOL implant (I use it with a 118.2 A-constant). My use of the intraoperative hand-held autorefractor (Retinomax 2, Nikon, Tokyo, Japan) is as follows: when the expected IOL power is less than +8 D, autorefractometry is performed after cortical cleanup, then the is IOL chosen. In all other cases, autorefractometry is done after IOL implantation and viscoelastic removal; the IOL is then immediately exchanged if outside 1 D of desired refraction.
Bioptics for keratoconus and pellucid marginal degeneration
In keratoconus and pellucid marginal degeneration, irregular astigmatism is often associated with axial or corneal myopia. Intracorneal rings have been combined with phacoemulsification and in-the-bag IOL, with iris-fixated phakic IOLs (A. Abdel Fattah, T. El-Raggal, presented at ESCRS; 18–22 September 2004; Paris, France), and with posterior chamber phakic IOLs (E. Coskunkeven, L. Oktem, E. Ozkilik, et al., presented at ESCRS; 18–22 September 2004; Paris, France).
In cases with reduced astigmatism, pseudophakic or phakic IOL implantation should probably precede intracorneal rings (which might ultimately not be needed); in other cases, rings should be implanted first to provide reliable K readings for IOL power calculation. The published cases are few and follow-up was limited, however, and this aspect of bioptics needs (and deserves) further investigation.
Conclusion
Bioptics is worth doing. It improves vision and halos and adds no particular risks to phakic or pseudophakic IOL implantation, in either myopia or hyperopia. Reverse bioptics, with phakic IOLs or refractive lens exchange (helped by intraoperative autorefraction), is a predictable compensation for poor corneal surgery and should be preferred to laser retreatments in many cases.
The only field in which doubts persist is bioptics in presbyopic lens exchange, both with multifocal and accommodative IOLs. Here, however, the problems are in the IOLs, not in laser enhancement, which has nevertheless salvaged many eyes from poor distance uncorrected visual acuity. What bioptics cannot do is to improve halos and to maintain long-term accommodation.
The weaknesses of the present paper are several. No comparative studies between techniques exist. Most series are numerically limited, and many papers were presented but unpublished. Few studies mention dysphotopsia, and none induced aberrations.
We can imagine bioptics as the standard procedure for higher refractive errors in the future, the only drawback being increased costs. Patient age, surgeon preferences (different phakic IOLs, and LASIK vs. surface ablation), and costs will dictate the solution in each case. Improved IOL designs, laser ablation strategies, and LASIK flap creation techniques (e.g. femtosecond laser) will further expand the choices.
Summary:
Bioptics improves vision and halos and adds no particular risks to phakic or pseudophakic intraocular lens implantation in either myopia or hyperopia. Reverse bioptics, with phakic intraocular lenses or refractive lens exchange, can be used to correct regressed corneal surgery.
No comments:
Post a Comment