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CK is a fairly new non-laser refractive surgery technique that does not involve a lamellar cut or removal of corneal tissue. After a quiet entry onto the UK market, its success in the US has led many companies to rethink the potential patient benefits. Gareth Paterson reports

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Typically, between the ages of 40 and 45, patients approach optometrists with tales of woe regarding their inability to focus on near objects. The usual role of the optometrist is to guide their patient towards new spectacles or contact lenses. But as surgical techniques mature, other options can be discussed.

Many presbyopes have already benefited from corrective refractive surgery such as Lasik, Lasek and PRK, which can accurately correct hyperopia, myopia, and astigmatism. With this sudden freedom from distance corrective appliances, patients are now looking to correct their near vision symptoms. The fundamental differences between ametropia at distance and presbyopia mean that any surgery to alleviate near vision symptoms must take a different approach.

Heat-based refractive surgeries have been attempted in various forms during the last 100 years. Fyodorov - inventor of radial keratotomy - recently attempted to utilise heat-based corneal shrinkage to change corneal curvature. But there are problems with these 'hot wire' surgeries, as although the nichrome wire reaches temperatures as high as 600oC, when applied to the cornea it cools instantly. This results in an irregular distribution of the heat energy, meaning results are unpredictable. Conductive keratoplasty (CK) uses radio frequency (Rf) energy to generate heat at specific sites within the cornea. Studies have found the treatment to be more predictable and stable than previous surgeries.

CK is most often used to steepen the central cornea in patients with hyperopia or to treat the symptoms of presbyopia in emmetropic or mildly hyperopic presbyopes. Other applications include the treatment of unsatisfactory refractions resulting from Lasik surgery and fine-tuning the refraction after cataract and intraocular lens (IOL) surgery. Modifications in the surgical method, called the 'light touch' technique, have further improved predictability and reduced side effects.

FDA studies and the worldwide surgical record of more than 175,000 procedures have shown the CK treatment to be safe and effective.

Mechanism of Action

The CK procedure, performed using the ViewPoint CK system from Refractec, Irvine, California, is based on the delivery of a precise amount of Rf energy (350Hz) through a finely-tipped stainless steel probe - diameter 90µm - inserted into the peripheral cornea at pre-marked spots along a 360o ring of 6,7 or 8mm diameter (Figure 2).

The probe extends 450µm into the cornea and no further, as an insulated Teflon-coated governor prevents a deeper treatment (Figure 1). Heat is not applied directly to the surface of the cornea, but is generated within the tissue due to the resistance of the stromal fibres. The tissue is raised to around 65oC, which causes the collagen fibres to shrink without totally denaturing the protein. The tip acts as a heat sink carrying heat away, rather than a source that applies heat to the tissue. This is unlike laser thermal keratoplasty (LTK) and 'hotwire' techniques that heat downward from the corneal surface, heating the outermost fibres more than those deeper in the cornea.

Localised shrinkage of the collagen induces a contraction between treated areas flattening the mid-periphery, decreasing chord length and consequentially inducing steepening of the central cornea (Figure 3). When used for the treatment of astigmatism, CK is applied at one or more spots in a selected meridian to steepen that treated meridian. The process is self-limiting as the increasing denaturation of collagen reduces the efficacy of the Rf field and a reduction in temperature follows. The refractive effect of CK is determined by the number of spots treated, the number of rings treated and the diameter of that treatment ring. If all points were treated on all three rings, for maximum effect, the change of prescription would be +3.50D.

The procedure takes around one minute per ring of treatment, or about three to five minutes per eye. The cornea is marked initially with a target to identify the treatment spots. The surgeon gently applies the probe to the eye surface at the selected locations using the 'light touch technique'. The treatment could vary from eight to 32 spots, depending on the prescription. Throughout the treatment the patient may be aware of some visual distortion but should not feel any pain or discomfort.

The surgeon will then instil antibiotic and anti-inflammatory drops and a bandage contact lens, which improves post surgical comfort, and can be removed the next day. After the procedure, treatment spots initially show whitening caused by localised oedema and thermal insult, but as time passes this becomes less dense. It still appears to extend to about 80-90 per cent of the cornea depth. The spots are often linked by visible striae, which are signs of the contraction of the stromal collagen.

This can be visible soon after the treatment and in ongoing examinations, and are not a sign of complication.

Many ophthalmic professionals state this treatment gives the patient a monovision result - one eye for distance and one for near - and in that they are correct. However, the changes in curvature to the front of the eye induce a longer depth of focus due to the multifocal properties of the cornea (Figure 4) so that if the patient heals as expected there is one eye that focuses at distance and one at near, but this new depth of focus means there is a less than expected loss of distance acuity while still obtaining a satisfactory near result.

Patient Selection

Many patients enquiring into refractive surgery will be intrigued by the offer of a future without any spectacles or contact lenses.

However, it may be necessary for those with a distance refraction or with astigmatism greater than 1.00DC to undergo laser correction prior to considering a CK treatment in the non-dominant eye. Another alternative for myopes will be to perform a monovision laser treatment after a successful trial with contact lenses in the non-dominant eye for three weeks.

CK, can be used to treat the minor distance prescription of low hyperopes from plano to +2.00D and not more than +1.00DC of astigmatism. But if this is required it reduces the possible near correction because the maximum amount of alteration to the corneal power is as stated, +3.50D.

A cycloplegic refraction prior to treatment on all hyperopic patients regardless of age is a necessary precaution to rule out those with latent ametropia.

Ocular Factors

A thorough ocular examination before performing CK is vital. Contra-indications to CK are very similar to those for other corneal surgeries like Lasik surgery. Those patients with manifest binocular vision problems, dry eyes or corneal pathologies are all contra-indicated. Corneal thickness is as important for the non-penetrative surgery as it is for Lasik or Lasek.

A peripheral pachymetry reading, measured at 6mm optical zone of less than 560µm can rule out a patient as the probe should not approach Descemet's membrane.

Expectations and Tolerance

Probably the most important factor for patient selection is that patients must be able to tolerate the imbalance in the vision between the eyes, and must hold realistic expectations of what can be achieved.

Near vision CK helps patients to read the mobile phone numbers and newspapers but it may not help patients who perform sustained periods of close and detailed work. They may have to continue wearing reading glasses after the treatment. As with most elective surgery, patient satisfaction is often predictable from how well they are counselled on realistic outcomes.

Results and complications

CK was developed initially as a treatment for mild hyperopia and approved by the FDA in 2002 after initial trials and subsequent studies showed a predictability and reliability in this role.

Results received from studies into its use as a treatment to help those with presbyopia have indicated that around 84 per cent of patients said they were either satisfied or very satisfied with the visual results and 98 per cent of individuals were able to see J5/N8 again indicating a high success rate.

The graphical representation of the improvement to patients' near vision shown in Figure 5 indicates the possible gains for our presbyopes. New roles for CK are being investigated as it may represent a less invasive method to control residual astigmatism after Lasik, PRK or cataract extraction. Early indications are encouraging.

Adverse events following any surgical procedure are inevitable, although certain procedures pose more risk in frequency and severity of potential complications, and patients must be aware of these. CK is known to have a lower incidence and severity of complications because, unlike procedures requiring corneal incisions such as excimer laser technology, CK is minimally invasive.

As with any surgical procedure, proper patient selection, a thorough ocular examination, together with careful discussion of the procedure's benefits, risks, and limitations are vital.

Immediately after treatment

Within the first few hours after the procedure the patient may feel a variety of symptoms including sensitivity to light, watering, mild redness and foreign body sensation that will resolve with the use of postoperative antibiotic, anti-inflammatory and lubricating drops.

Patients should wear sunglasses after the procedure if necessary. Distance vision will be blurred for a day or two but improves quickly over the course of the first week (Figure 6). The variation of corneal power of the cornea means that although distance acuity improves the acquired near vision is not lost.

Re-treatments for over- and under-correction

Due to an individual's prescription level and unique corneal healing process, a second treatment or enhancement may be needed within the first year.

Re-treatments are usually performed for induced astigmatism, under correction or over correction. These complications occur at a frequency of 5 to 8 per cent of cases, and are most commonly observed in patients under the age of 45.

Under correction is more common than over correction and can be treated with additional placement of CK spots. In studies performed immediately after the initial approval of CK the major reported side effect was of induced astigmatism. However, the use of the light touch technique in recent years has reduced this dramatically.

Patient Co-Management

The quick healing and low incidence of side effects after CK allow many patients to be managed at the practice of their local optometrist, rather than having to make repeated trips to the treatment clinic.

The role of the optometrist within the aftercare process promotes the idea that the optometrist is more than a spectacle provider. This helps to maintain the relationship that is so often lost when a patient has refractive surgery. Appointments the day after treatment, at two weeks, eight weeks to three months, six months and 12 months provide the patient with an adequate post-surgical care scheme.

References

1 Presentation on Conductive Keratoplasty (CK) for the Correction of Low to Moderate Hyperopia U.S. Clinical Trial 24-Month Results Physician Name, MD My CK Resource.com.

2 Optics Of Conductive Keratoplasty: Implications for Presbyopia by P S Hersh. Refractec.Com

3 Maximising Outcomes with Nearvision CK. Black et al. CK-centric supplement/insert to Cataract & Refractive Surgery Today, July 2005.

4 Conductive Keratoplasty for Presbyopia: 1-year Results- Stahl. Journal of Refractive Surgery.

◆ Gareth Paterson is an optometrist and professional services manager at Advanced VisionCare Refractive Surgery Clinic, Harley Street, London

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