Femtosecond laser for cataract surgery

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The role of femtosecond lasers in cataract surgery is to assist or replace several aspects of the manual cataract surgery. These include the creation of the initial surgical incisions in the cornea, the creation of the capsulotomy, and the initial fragmenting (breaking up) of the lens. The femtosecond laser may also produce incisions within the peripheral cornea to aid the correction of pre-existing astigmatism. The preliminary results are promising.[1][2][3][4][5][6]

History[edit]

Femtosecond lasers have been used successfully in ophthalmic surgery since 2001.[7][8] The technology has been applied widely, most notably in LASIK (Laser In-Situ Keratomileusis) refractive surgery. In FemtoLASIK the laser replaces a mechanical device (microkeratome) to create a precise corneal flap preparing the eye for the secondary laser ablation in order to change the patient’s refractive error. There are several benefits; Femtosecond lasers have been noted to be more precise than microkeratomes, with fewer likely collateral tissue effects.[9][10] This has contributed to more precise, reproducible and safe LASIK outcomes.

In applications of the femtosecond lasers to dissection of a corneal flap for refractive surgery, the intrastromal cutting is performed at predetermined depth relative to the corneal surface applanated against a contact lens. In cataract surgery though cutting is performed inside and on the surface of the crystalline lens. Since lens position and orientation differs from eye to eye, precise 3-dimensional imaging is required to define location of the lens capsule in order to properly apply the laser cutting patterns. Image-guided laser cataract surgery was first conceptualized by D. Palanker and M. Blumenkranz in 2005.[11] A system based on this approach was developed and tested by OptiMedica Corp. in 2005 - 2010. This system includes integrated Optical Coherence Tomography and femtosecond laser for cataract surgery[12]

The Femtosecond laser procedure was first used clinically in cataract surgery by Professor Zoltan Nagy in Budapest, Hungary (Europe) in 2008. This was followed by Dr Steven Slade in the USA (2010) and Dr. Harvey Uy in Asia (2009) and Dr Michael Lawless in Australia (2011).[13]

Clear Corneal Incisions[edit]

Access to the cataract is initiated through small incisions (around 2mm in length) made into the peripheral cornea. Known as Clear Corneal Incisions (CCI) they remain the preferred method for surgeons accessing the anterior chamber of the eye during cataract surgery. Previously large incisions (approximately 5-7mm in length) were made into the sclera (white part of the eye). This is a highly vascularized part of the eye and required particular consideration for older patients on blood thinning therapy. To ensure that the incisions were adequately sealed, sutures were required following surgery. The need for sutures prolonged the recovery process. Routine CCIs are generally considered to be self-sealing (that is no sutures are required). In comparison to scleral incisions, CCIs have reduced some potential complications and increase the speed of recovery.[14][15][16] The smaller incisions will also impact on the visual outcome for the patient.

Routinely incisions are created manually by introducing a sharp, sterile blade into the cornea. Despite the perceived advantages of CCI’s over large, scleral incisions there appears to have been an increase in the incidence of endophthalmitis (postoperative infection within the eye), which has been directly linked to the use of these CCIs.[17][18] The use of a manual blade makes it difficult to control the length and architecture of the incision which may affect the stability of the wound under pressure. Following the surgery this could manifest by the corneal wound leaking increasing the potential risk of infection. Although the rate appears to be very low, endophthalmitis may be devastating with both clinical and financial implications arising from this complication. The additional cost of treating endophthalmitis including hospital stay has been estimated at €3,688 in one European study.[19] This does not include costs to society such as loss of productivity for patients and carers. It therefore remains a priority to minimize all potential risks.

Laboratory studies on cadaver eyes have shown that the femtosecond laser produces consistent and stable incisions.[20] This has been attributed both to the controlled, reproducible generation of the incisions and the configuration of the corneal wound created.[21] It would suggest that laser-cut, clear corneal incisions offer the potential of less risk of wound leak thereby further reducing the risk of infection following surgery. This will require a large amount of data to confirm and definitive results will not be present for some time.

In addition to creating the main cataract surgical wound, femtosecond lasers are used to create limbal relaxing incisions (LRIs). When placed in specific areas of a person's cornea, LRIs can reduce the amount of astigmatism present, which can, and often does, improve post-operative uncorrected visual acuity. Typically, corneal incisions to control astigmatism have been performed by hand held blades. The major factors determining LRI effectiveness are the incision length, depth, and uniformity, the location relative to the center of the cornea, the incision geometry, the patient's age, and the amount of astigmatism. The precision of the laser created incisions should allow the surgeon greater control over the final refractive endpoint possibly leading to improved visual outcomes. Again further data is required at this point to confirm these assertions.

Capsulotomy[edit]

The femtosecond laser is able to create a near perfect, round opening in the anterior capsule by dissecting it with a spiral laser pattern crossing the anterior capsule. To avoid distortion of the incoming laser beam on gas bubbles and tissue fragments, the spiral pattern is applied first posterior to the capsule, and advances anteriorly.[22] The surgeon then is able to simply remove the tissue with surgical forceps. This has several potential benefits over the radially-created manual procedure. Both Nagy and Freidman[23][24] have shown that the strength of the capsule is as good as or greater than a manual capsulorhexis (opening created in the capsule) and the smoothness of the capsulotomy edge is similar to manually created openings. This may serve to further reduce the incidence of intraoperative tears to the capsular bag. Capsular tears are not an insignificant problem. They lead to prolonged surgery and a potential list of complications can result; including posterior extension of the tear, retained soft lens matter requiring removal, persistent uveitis, cystoid macular oedema, and secondary retinal detachment.[25][26]

In conventional cataract surgery the incidence of anterior capsular tears has been documented from 0.79% in very experienced hands to 5.3% within teaching institutions.[27][28] In the study by Marques et al.[29] 40% of anterior capsular tears extended to the posterior capsule and 20% required further surgery emphasising the potential cascade of issues that may result from this complication. Lawless in November 2012 presented data indicating a 0.2% incidence of anterior tears throughout his initial 500 cases.[30] Although further hard data may be required, reports suggest that laser cataract surgery is at least as safe and indications are that it may prove to be safer and more accurate for patients.

The capsulotomy may also impact on the visual outcome. An irregularly-shaped capsulotomy may influence the position of the implanted IOL leading to decentration and tilt which may cause a decrease in the patient’s quality of vision. The ability to create a precise, well-centred capsulotomy should therefore optimize the surgeon’s ability to achieve the patient’s anticipated visual outcome. Palanker et al. confirmed the accuracy of the laser guided surgery.[31] He showed a mean circularity of 0.942 in 29 lasered eyes compared with 0.774 in 30 manual eyes, and a twelvefold improvement in the precision of the capsulotomy diameter. Freidman[32] demonstrated that the deviation from intended diameter was 29 µm ± 26μm for laser capsulotomies and 337μm ± 258μm for a manual technique with a mean deviation from circularity of 6% and 20% respectively. Importantly Nagy and co-authors were also able to show that these results were independent of the size and shape of the eye in laser cataract cases suggesting an increased uniformity following surgery compared to manual cases.[33] Further evidence has shown that in comparison to manually created capsulotomies those created with a laser have significantly lower internal aberrations following surgery.[34][35]

Cekic[36] previously demonstrated that capsulorhexis of different sizing will lead to variation of the IOL position. It thereby follows that if a surgeon can systematically control the capsulorhexis size then the final visual outcomes will become more consistent also. This may prove to be the greatest potential advantage of laser-assisted cataract surgery although data significant to provide a clinical benefit is not available at this timepoint.

Phacofragmentation[edit]

The femtosecond laser has the capability to assist the fragmentation (breaking up) of the cataract. The laser applies a number of pulses to the lens in a pre-designed pattern which then allows the surgeon to use current technology to remove the lens matter. This additional step has been shown to reduce the average time and energy required to break up and remove the lens by approximately 50%.[37][38][39] Inherently this should make the overall procedure safer and less traumatic to the eye, which may further reduce the risk of postoperative swelling and lead also to a faster visual recovery.

Complications[edit]

Laser cataract surgery has been designed to provide better visual outcomes and reduce the overall risk to the patient during surgery. The procedure though is not performed without risk of complication. The use of pressure to maintain adequate suction upon the eye throughout the laser procedure has been seen to create minor subconjunctival haemorrhages (seen on the white part of the eye). These have no impact on the surgery or the final postoperative vision and resolve in the early postoperative period. The patient is also unlikely to feel discomfort or pain as a result of these haemorrhages.

Roberts et al.[40] published data to suggest that patients with dense cataracts may be at greater risk of capsular rupture due to the additional forces created within the bag during surgery. These cases were part of the initial installation of a laser unit and the combination of surgeon awareness and unit software/hardware upgrades in the time since has likely rendered this issue both predictable and preventable.

As with the introduction of any new technology there appears also to be a learning curve to the procedure. Bali et al.[41] described, in their initial installation, a number of complications that occurred during or immediately following surgery. The majority of these complications appeared to have no impact on the visual outcome. This represents the first 200 cases of an early installation. In early 2013, the same group published their findings over the subsequent 1500 surgeries. The results presented suggest that laser cataract surgery is at least as safe, and in many cases safer, than previously published literature concerning manual surgery.[42]

Laser cataract surgery in complicated cases[edit]

The initial investigations into the use of femtosecond laser technology in cataract surgery included strict inclusion criteria. With experience the criteria has expanded and surgeons are now exploring the potential benefits of the femtosecond laser in complicated cases.

Cataract surgery in white or hypermature cataracts has been associated with a greater risk of incomplete capsulorhexis, posterior capsule rupture, endothelial cell loss and incision complications such as wound burn and therefore represents a particular challenge to the ophthalmic surgeon.[43][44] The ability to minimize this risks presents an excellent opportunity. Nagy reported the successful use of laser cataract surgery in dense, white cataracts and cataracts following significant trauma.[45] Patients with pre-existing corneal disease require additional consideration to the surgical technique to maintain optimal postoperative safety outcomes. Fuch’s endothelial dystrophy is a breakdown of the inner layer of the cornea (endothelium). Relatively common in older patients it may lead to poor vision from the swollen cornea (oedema) and may eventually require transplant surgery. By reducing the manipulation required during surgery these patients may benefit from reduced corneal endothelial damage during laser cataract surgery. Nagy et al. [46] provide a clinical case for these advantages albeit in a patient with prior corneal transplant. Patients with retinal disease may require complicated surgery to optimize their potential vision. In a small case series, Bali et al. suggest that patients undergoing vitrectomy (removal of the jelly-like fluid within the eye) and concurrent cataract surgery may benefit from the laser cataract approach.[47] The precise capsulotomy may provide additional security for the inserted lens to minimize the risk of the fluid moving to the anterior chamber (front of eye). This surgical combination represent a significant time within the eye. Reducing the potential energy and manipulation within the eye appears may reduce the possible effects of the prolonged surgery. Recently Dick and Schultz [48] suggested that the laser may be able to perform both anterior and posterior capsulotomies in paediatric cases. The creation of the posterior capsulotomy is technically difficult and may lead intraoperative and long term visual complications. The precision of the laser alleviates some of this concern. The ability of surgeons to provide significant advantages to patients at risk of major intraoperative and postoperative complications will continue to increase and may ultimately represent a substantial benefit of femtosecond technology.

Current Technology[edit]

Currently there are four lasers at or near the point of commercial release. These include Alcon LenSx (Alcon Laboratories, Ft Worth, TX, USA), OptiMedica Catalys (Optimedica Corp, CA, USA), LensAR(LensAR Inc, FL, USA) and Technolas (Technolas Perfect Vision GmbH, Germany). All laser systems share a common platform which includes an anterior segment imaging system, patient interface and femtosecond laser to image, calculate and deliver the laser pulses. The specific technology to achieve these steps differs between the individual lasers with notable differences in imaging and docking systems and laser treatment algorithms. There appears to be little, if no data to assert the enhanced ability of a single unit over the others at this time.[49] The choice for surgeons in 2012 was related to availability, technical support and cost. By mid-2013 over 120,000 eyes have been operated upon in 67 countries.

References[edit]

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