Femtosecond Laser assisted Cataract surgery (Part II)- A brief review of the literature
In the first part of this series I briefly introduced my impressions of Femtosecond Laser Assisted Cataract surgery(FLACS) and outlined my reasons for being an early adopter.
I this second installment I look at the each step of the procedure and discuss how these may be an improvement over the current technique of phacoemulsification. I also take a look at the published literature to see if there exists any evidence to support the impression that FLACS may be better than phacoemulsification.
A Medline search produces 69 published studies in peer reviewed literature. Of these there are number of review articles. These studies have looked at FLACS from a number of perspectives to see if there are any objective gains to justify pursuing this approach. I will summarize the findings.
In the beginning
Back in 1967 when phacoemulsification was introduced by Charles Kelman, many questions were asked about the validity of transitioning to a new technology that was at the time perceived to be more expensive and more time consuming than extracapsular cataract surgery (ECCE).
ECCE was considered a well established procedure yielding excellent results at minimal cost. In an article written in 1981 entitled “Recent advances in cataract operations”{17} the conclusion was: “Most ophthalmic surgeons choose to carry out the standard cataract extraction because although phacoemulsification offers a more rapid rehabilitation due to smaller incisions, it requires special training and experience and expensive, complicated equipment”. Indeed, in those days performing a conventional ECCE was quicker and most certainly less costly. With time however, phacoemulsification came to completely replace ECCE even though the benefits were not that dramatically better. Now, the same questions are being asked all over again and I suspect ultimately, FLACS will come to dominate the cataract surgery scene just like phacoemulsification did with ECCE.
A new era
The current literature looks at each of the steps of FLACS and discussess the benefits if any over conventional surgery. Briefly these are:
•The laser created side ports and main wound and whether these confer any advantage over hand created ones particularly whether these wounds leak any less, lead to less surgical induced astigmatism or lead to lower rates of endophthalmitis.
•Whether the Limbal relaxing incisions performed by the laser are any better than hand created ones or better than a toric lens in the control of astigmatism.
•The capsulorhexis and whether a perfectly round rhexis lead to greater strength of the capsule, better effective lens position, lower decentration and tilt of the intraocular lens or lower rates of posterior capsular opacification.
•Whether laser assisted lens fragmenation translates into lower total phaco energy used or shorter surgical times with consequent lower rates of endothelial cell loss.
•Whether there are any specific surgical situations that benefit from FLACS or whether there are specific contraindications to its use.
•The refractive outcome and whether the deviation from the intended final refraction is any less and the pread of results tighter with FLACS.
Laser created wounds
One of the advantages of FLACS is the precision with which the main wound and side ports can be created. The move to temporal clear corneal incisions by most cataract surgeons has resulted in better refractive outcomes with lower surgically induced astigmatism but at the cost of a greater rates of endophthalmitis {1,2}. There are multiple explanations for this observed increase which include, greater propensity for wound leaks, absence of overlying conjunctiva to seal the wound, temporal location of the wound leading to wound pout during blinking and the poor corneal wound sealing properties. A recent study using anterior segment OCT showed that the internal, endothelial surface of a clear corneal incision tends to gape and show a Descemet’s detachment {3,4,5}which may be a factor which can contribute to wound leak. The same study showed that wounds created by a laser do not demonstrate this gape and so may seal better.
Additionally, these laser created wounds are precise, square shaped which have been shown to be more resistant to leaking{6}.
The laser system can also create precise limbal relaxing incisions (LRI) which can be accurately placed using the inbuilt OCT to control astigmatism. Limbal relaxing incisions need to be created at 90% depth if they are to function correctly. The traditional method for creating these is using a 600 micron knife. This means that in thicker corneas an adequate depth will not be reached and therefore the effect is less. With OCT guided placement this will no longer be a problem. Further, it is speculated that LRI’s may be more precise than toric lenses in patients with high asigmatism. This is because toric lenses lose their effect at a rate of 30% for each 10 degrees of decentration. Thus a 30 degree decentred lens has completely lost all its astigmatism reducing properties. Accurately aligning a toric lens to even 10 degrees is very challenging even using guidance systems such as SMI.
Capsulorhexis
The casulorhexis is one of the most difficult steps for the begining surgeon to master. It is extremely important that it be round, centered and of the right diameter to allow for a small overlap on the surface of the intraocular lens.
A rhexis that is too small can result in capular fibrosis that results in posterior displacement of the lens with a consequent hyperopic shift. One that is too large can lead to anterior displacement of the lens, tilt, decentration and rotation of the lens with myopic shift, astigmatism and coma. Errors in the effective lens position (ELP) is the single biggest factor that leads to a post operative refractive error{7,8}. A capulorhexis created by the femtosecond laser is perfect 100% of the time whereas a hand created one is challenging to produce perfectly every time. Further more, there is evidence that laser created rhexis are stronger and more tear resistant {9}than hand torn ones leading to less radial tears and risk of dropped nuclei which occur in 1% cases.
A laser created rhexis is also safer in patients with zonular instability such as occurs in PXF in which radial forces created by hand tearing a capsule can lead to further zonular dehiscence with increased risk of lens loss and in intumescent nuclei in which the commencement of the rhexis can lead to a radial tear.
Rates of posterior capsular opacification are known to be reduced by having a complete and uniform overlap by the anterior capsule onto the anterior lens surface. Achiving this by manual means is less reliable than by laser.
Lens fragmentation
During phacoemulsification, the lens is fragmented by a variety of techniques.
Most surgeons utilise the “divide and conquer” technique which involves creating grooves in the lens then removing quadrants and others utilise the chop technique which breaks the lens up using an instrument. In FLACS the laser is used to fragment the lens which in turn means that less phaco ultrasound energy is utilized. It has been shown that it results in a 43% reduction of ultrasound energy and a reduction of phaco time by 51% {10}. It is also known that ultrasound energy results in endothelial cell loss and that FLACS lead to less cell loss {11}.
The ultimate goal for laser assisted lens fragmentation would be complete fragmentation that allows the lens to be aspirated with no ultrasound energy whatsoever.
Indications and contraindications
There are situations in which FLACS in its current form cannot be used. Patients with poorly dilating pupils such as those with PXF, diabetes, Posterior synechia or on miotics may not be suitable but one can envisage a microscope mounted device that can be swung into place once the pupil has been enlarged using rings or hooks.
There is no doubt that certain patients benefit more from FLACS than others.
The patient with the low endothelial count is a major beneficiary as the reduced surgical trauma and reduced phaco times will lead to less endothelial cell loss.
The patient with PXF who has pre-existing zonular weakness will benefit from the reduced stress on the zonules which arises from the creation of the capsulorhexis.
The patient with astigmatism benefits from the improved stability afforded by the accurate centration of the toric intraocular lens or the accurate placement of limbal relaxing incisions.
Refractive outcomes
One of the most important questions is whether FLACS leads to better more predictable refractive outcomes. Although the predictability of phaco surgery has improved it is still subject to occassional unpredicatble results. The Effective Lens Position (ELP) is difficult to reliably control and many factors affect it. To date there are a number of studies that look at refractive results. Some do show a trend towards more predictable refractions, particularly the paper by Nagy etal {13} which showed that the predictability of the lens power calculation was significantly better than in conventional surgery and also the work by Uly etal {12} which found similar results. By contrast, a study by Mihaltz etal found no statistically significant difference. Intuitively, one would expect that a better centred lens, with a uniform anterior capsule overlap which was less prone to tilt and decentration, should lead to more predictable and stable outcomes but more prospective studies are required to answer this question.
Coming up
In my next article I will present my own outcomes.
References
1.Bacterial endophthalmitis after small-incision cataract surgery: Effect of incision placement and intraocular lens type” by Nagaki et al. (J Cataract Refract Surg 2003;29:20–26).
2.Taban M, Behrens A, Newcomb RL, Nobe MY, Saedi G, Sweet PM, et al. Acute endophthalmitis following cataract surgery: A systematic review of the literature.Arch Ophthalmol. 2005;123:613–20.[PubMed: 15883279]
3. Xia Y, Liu X, Luo L, Zeng Y, Cai X, Zeng M, et al. Early changes in clear cornea incision after phacoemulsification: An anterior segment optical coherence tomography study. Acta Ophthalmol. 2009;87:764–8.[PubMed: 19548882]
4. Maxwell DP, Jr, Diamond JG, May DR. Surgical wound defects associated with endophthalmitis. Ophthalmic Surg. 1994;25:157–61. [PubMed: 8196919]
5. Montan PG, Koranyi G, Setterquist HE, Stridh A, Philipson BT, Wiklund K. Endophthalmitis after cataract surgery: Risk factors relating to technique and events of the operation and patient history: A retrospective case-control study. Ophthalmology. 1998;105:2171–7. [PubMed: 9855143]
6. Ernest PH, Kiessling LA, Lavery KT. Relative strength of cataract incisions in cadaver eyes. J Cataract Refract Surg. 1991;17(Suppl):668–71. [PubMed: 1955983]
7. Cekic O, Batman C. The relationship between capsulorhexis size and anterior chamber depth relation. Ophthalmic Surg Lasers 1999; 30(3): 185–190.
8. Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg 2008; 34: 368–376.
9. Frey RW, Teuma EV, O’Suilleabhain D, Elliot D, Downes Jr GR, Downes III GR et al. Evaluation of the mechanical properties of the crystalline lens capsule following photodisruption capsulotomy and continuous curvilinear capsulorrhexis. Invest Ophthalmol Vis Sci 2009; 50, E-abstract 1141.
10. Nagy Z, Takacs A, Filkorn T, Sarayba M. Initial clinical evaluation of an intraocular femtosecond laser in cataract surgery. J Refract Surg 2009; 25: 1053–1060.
11. Takacs AI, Kovacs I, Mihaltz K, Filkorn T, Knorz MC, Nagy ZZ. Central corneal volume and endothelial cell count following femtosecond laser-assisted refractive cataract surgery compared to conventional phacoemulsification. J Refract Surg 2012; 28(6): 387–391.
12. Uy HS, Hill W, Edwards KH. Refractive results after laser anterior capsulotomy. Association for Research in Vision and Ophthalmology Annual Meeting A4695 Poster #D634; Fort Lauderdale, FL 2011. www.arvo.org.
13. Filkorn T, Kovacs I, Takacs A, Horvath E, Knorz MC, Nagy ZZ. Comparison of IOL power calculation and refractive outcome after laser refractive cataract surgery with a femtosecond laser versus conventional phacoemulsification. J Refract Surg 2012; 28(8):
14. Nagy Z, Takacs A, Filkorn T, Sarayba M. Initial clinical evaluation of an intraocular femtosecond laser in cataract surgery. J Refract Surg 2009; 25: 1053–1060
15. Edwards KH, Frey RW, Naranjo-Tackman R, Villar Kuri J, Quezada N, Bunch T. Clinical outcomes following laser cataract surgery. Invest Ophthalmol Vis Sci 2010; 51, E-abstract 5394.
16. Mihaltz K, Knorz MC, Alio K, Takacs AI, Kranitz K, Kovacs I et al. Internal aberrations and optical quality after femtosecond laser anterior capsulotomy in cataract surgery. J Refract Surg 2011; 27(10): 711–716.
17. Salz JJ. Recent advances in cataract operations. West J Med;134:290-295, April 1981