The newest generation of femtosecond lasers used in conjunction with phacoemulsification may offer an increase in accuracy and safety in cataract removal surgery
David Allen FRCOphth
Sunderland Eye Infirmary, UK
Removal of cataract is the most common surgical procedure performed in most countries of the developed world, and with an ageing population, that position is likely to be maintained for the foreseeable future. In England alone there were 315,000 National Health Service-funded cataract operations in 2011/12.(1) The operation is usually performed under local anaesthetic as a day-case, although there remain perverse financial incentives in the state healthcare provision in some countries where general anaesthesia and in-patient care may remain common.
At the beginning of surgery, the pupil is dilated in order to allow access to the lens, which is just behind the iris. An opening (as near to circular as possible) is created in the anterior capsule of the lens, allowing access to the lens material inside. In children and young adults, the lens material is sufficiently soft that it can simply be aspirated, but in older people, if a small incision is used, the lens material has to be either emulsified or at least broken into very small pieces to allow it to be removed. To do this, a hollow titanium needle connected to a phacoemulsification (phaco) machine is used. The needle is surrounded by a silicone sleeve with a diameter a little larger than that of the needle, thus forming a dual lumen system (Figure 1). The emulsified lens material (and the fluid it is in) is aspirated up the centre of the needle, and replacement fluid enters the eye through the space between the outside of the needle and the silicone sleeve. This whole complex is connected via a handpiece to the phacoemulsification console controlled by the surgeon using a footpedal (and there is also a touch-screen on the console for making adjustments).
Advances in technology
Sophisticated engineering and micro-electronic control systems now allow the surgeon to use this equipment in a very efficient way. There are ways of controlling the delivery of the ultrasound power that both limits potential damage to delicate intra-ocular structures, as well as minimising the possibility of causing thermal damage to the cornea at the incision owing to friction generated by the needle vibrating at around 40,000 times per second. Also, if you consider that the volume of the anterior chamber of the eye is only 0.25 ml, yet the surgeon may be asking the phaco machine to remove fluid from it at a rate that is often 35 ml/minute or higher without compromising that space, then it becomes clear that the control software and mechanical actions need to be extremely precise and responsive. Modern phaco machines are just this.
The technology described above allows the surgeon to remove a large cataractous lens – approximately 10mm in diameter and up to 5mm thick at its thickest point – through a small incision, but a replacement lens implant (IOL) is needed to restore clear vision. When phaco was first developed, the incision had to be enlarged after the cataract was removed in order to be able to insert a replacement IOL. Now, however, the surgeon is able to use a foldable or compressible IOL (usually made from an acrylic material) that can be injected into the eye through a cartridge system, which then unfolds back to the normal shape and size within the now-empty lens capsule inside the eye.
It is this combination of phaco machine and foldable IOL that constitutes micro-coaxial phaco. Coaxial refers to the design outlined above, where the outflow path and the inflow path of fluid are concentric. The ‘micro’ part is more difficult to define, but is generally taken to refer to surgery performed through an incision 2.4 mm or less, with the replacement IOL being inserted through that same incision. Some surgeons perform the surgery through incisions as small as 1.8mm, although not all IOLs can be inserted through such incisions without causing some stretch. A small number of surgeons perform what is called bi-manual or bi-axial surgery. In that system, the inflow and outflow are separated and two smaller incisions are used (approximately 1mm each) to remove the cataract and then one of these incisions is enlarged to 1.8 or 2 mm for the IOL insertion.
Why the emphasis on incision size?
The answer is twofold: a smaller incision results in less inflammation and speedier healing, and the smaller the incision the smaller the effect on the focussing of the eye. Non-ophthalmologists (and indeed ophthalmological trainees) are usually surprised to learn how much of the focussing power of the eye is in the cornea rather than in the lens. However, a moment’s thought will confirm that the convex front surface of the cornea acts like the front surface of a convex lens – that is, light bends as it passes from air into the cornea. A normal-sighted eye has a total focussing power of around 65 dioptres to bring light from a distant object into focus on the retina. Of this 65, around 41 dioptres focussing (that is, 2/3) comes from the corneal surface. Therefore a small alteration in the shape of the cornea can have a significant impact on vision. While most well-constructed corneal incisions of 3.5mm or less should seal without the need for sutures, the literature shows (on average, as some studies differ) smaller degrees of induced astigmatism (a measure of the shape change of the cornea) as incisions reduce in size towards 2.2 mm.(2) It is not clear that there is a significant benefit from even smaller incisions, although there may be more predictability in outcome.
In order to calculate the optical strength of IOL needed in an individual eye to give good distance vision, we need to know three things: the amount of focussing done by the cornea (that is, its curvature); the distance from cornea to retina where the image is to be formed; and exactly where between these two the new IOL will sit. New generation optical biometers, based on optical coherence (or more technically, laser-based partial coherence interferometry), give us very accurate measurements of the length of the eyeball, and similar techniques in the same equipment give us an accurate measurement of the curvature of the cornea – before the surgery. It should be clear, therefore, that making the minimum disturbance possible to the cornea will help with the accuracy of IOL calculations. The one factor that we cannot measure is exactly where the IOL will be at the end of the surgery (or more importantly in the post-operative period). However, having ‘solved’ the other two issues, surgeons are now concentrating on this as the least predictable factor in improving outcomes.
While there are a variety of formulas for calculating the IOL power, each taking a slightly different approach to predicting this, it has become apparent that the biggest factor causing variability is the size, shape and position of the hole torn in the anterior capsule at the start of the surgery. As mentioned above, the normal lens is bulky, and it is a much smaller, thinner lens that replaces it in the empty lens capsule. The anterior capsule and the posterior capsule tend to fuse together, from the outer edge towards the centre over the course of a few weeks after surgery. If the opening in the anterior capsule is exactly centred, is exactly circular and is 0.5–1.0mm smaller in diameter than the optic of the new IOL, then fusion process stops at the edge of the IOL optic, which is therefore fixed in the centre with little or no tilt. If these conditions are not met, and the opening is asymmetrical or decentred, then this fusion of capsule can displace the IOL off axis or can tilt it, resulting in reduced quality of vision.(3)
The latest development in the technology of cataract removal allows precisely this. After two decades or more of false starts, there are now available several machines that allow laser-assisted cataract removal. These new systems employ femtosecond laser technology and are used to create the incisions into the eye, to cut the opening in the anterior capsule and to soften the cataract so that much less ultrasound energy is required to remove it. The precise nature of laser cutting gives a much more controlled and reproducible shape, size and location of incisions, and it is expected that this will result in more reproducible effects on astigmatism, and the precision of the capsule opening has already been shown to improve the measured quality of vision as a result of more precise placement of the IOL.(4) At the moment, these lasers have a high capital cost, as well as significant procedure costs. In addition, there need to be changes to how the surgical service is delivered, and currently their use adds time to the procedure.
While this is a very exciting development, it will be some time before we have robust data that confirm the theoretical outcome and safety improvements they promise, and before the economics of their use is refined sufficiently for them to be considered for the managed healthcare environment. Of course, if improvements are demonstrated with increased use of femto technology, it seems likely that they will also prompt additional competitive efforts to design something that can improve rhexis size, shape and centration to give similar increases in accuracy of IOL positioning with less financial commitment.
Finally, in this brief overview of technological advances improving outcomes, we come to the IOL used to replace the natural lens. The standard IOLs used by many now have molecular filters in their structure to cut out potentially damaging (to the retina) parts of the light spectrum that are filtered out by pigments in the normal lens. Over the past ten years, an increasing number of IOLs being used have had aspheric surfaces. Photographers or astronomers among readers will be aware that the best quality lenses do not have spherical surfaces (as all IOLs had until recently) but have a surface that is progressively less steep as you move away from the centre of the lens. In theory it is now possible to measure any spherical aberration in the patient’s cornea before surgery and select an IOL that has a similar but opposite amount of aberration, thereby enhancing the quality of vision afterwards. In practice, however, most surgeons who use this type of implant use one that is designed to correct a certain proportion of the average aberrations of a population.
The curvature of the cornea is often slightly different in different directions; the usual analogy is to say that some have a curvature more like that on a rugby ball rather than that of a soccer ball. The result is astigmatism and the spectacle lens to correct it needs to have a similarly (but oppositely) curved surface: a toric lens. IOLs have been manufactured as toric lenses for some time, but it is only recently that designs have improved so that they remain in the same orientation in the eye over a prolonged period. It should be clear that, in order to counter any difference in curvature of the cornea in a particular direction, the correcting lens needs to aligned correctly.
With spectacles, this is easily achieved, as the lens is fixed within the frame. When an IOL is inserted, however, it may rotate about its central axis in the first few days/weeks after surgery until the capsule layers fuse. Results with these latest-generation toric IOLs have been good, with a significant increase in the proportion of patients with good distance vision without any glasses compared to a standard IOL.(5) In a UK cataract population, it was recently found that between 20% and 40% of cataract patients would benefit from the use of such an IOL (depending on the threshold chosen).(6) In some European managed healthcare markets, patients are allowed to pay the additional cost of such an IOL, but not in others. In some settings, the additional costs of these IOLs are absorbed within the overall cost envelope of the cataract procedure.
Finally, there are now IOLs that can correct presbyopia – the inability to change focus between distant and near objects. Presbyopia gradually develops in the eyes of people from the age of about 45 or 50, with increasingly strong reading glasses being required up to the age of around 60–65 years. When a lens is removed from an eye and replaced with a standard (or toric) IOL, then that patient, regardless of age, will require reading glasses afterwards. Some patients are able to tolerate one eye being left short-sighted, using this for reading while the other eye is left normal-sighted and used for distance. However, it is difficult to predict which patients find this acceptable. There are now IOLs available that allow the patient to see both near and far (as well as at intermediate distances) without glasses, although they are not suitable for everyone. Very recently IOLs have come on the market that correct both presbyopia and astigmatism. These presbyopia-correcting IOLs are significantly more expensive than standard or toric IOLs and it is unlikely that they will be affordable within managed healthcare systems in the foreseeable future.
Modern cataract surgery is minimally invasive, using incisions that minimise the impact on the patient’s refraction and is usually carried out as a day-case procedure. The use of phacoemulsification is efficient, and the newest generation of femtosecond lasers used in conjunction with phaco may offer an increase in accuracy and safety. Modern intra-ocular lens implants produce a high-resolution image on the retina, and some are able to correct pre-existing astigmatism and/or presbyopia. In those patients without co-existing ocular disease, there is a high degree of satisfaction.
- Hospital Episode Statistics for NHS in England. Healthcare Resource Groups 2011-12. www.hesonline.nhs.uk/Ease/servlet/ContentServer?siteID=1937&categoryID=206 (accessed 12 January 2013).
- Masket S, Wang L, Belani S. Induced astigmatism with 2.2- and 3.0-mm coaxial phacoemulsification incisions. J Refract Surg. 2009; 5:21–4.
- Hill WE. Does the capsulorhexis affect refractive outcomes? Hill WE. In: Chang D (ed) Cataract Surgery Today. Wayne, PA: Bryn Mawr Communications LLC; 2009:78.
- Szigeti A et al. Comparison of long-term visual outcome and IOL position with a single-optic accommodating IOL After 5.5- or 6.0-mm Femtosecond laser capsulotomy. J Refract Surg. 2012; 28: 609–13.
- Hollland E et al.The AcrySof Toric intraocular lens in subjects with cataracts and corneal astigmatism: a randomized, subject-masked, parallel-group, 1-year study. Ophthalmology. 2010 Nov; 117: 2104–11.
- Khan MI, Muhtaseb M. Prevalence of corneal astigmatism in patients having routine cataract surgery at a teaching hospital in the United Kingdom. J Cataract Refract Surg 2011;37:1751–5.