FEMTOSECOND IOL CENTRATION

Femtosecond laser capsulotomy provides a measurable improvement in spherical equivalent compared with manual capsulorrhexis, a new study shows. The result is very close to the theoretical improvement expected with a perfect capsular opening, and suggests that femtosecond technology could be a vital tool for investigating other factors affecting cataract surgery visual outcomes by removing the lens position error induced by irregular capsulorrhexes, Warren E Hill MD, Mesa, Arizona, US, told the annual meeting of the American Academy of Ophthalmology.

“We did this study as something of a sanity check. We know a perfect capsulorrhexis improves visual outcomes, but we don’t know how much. What this work did is provide numbers that show how much,†Dr Hill said. Dr Hill noted that a less than optimal capsulorrhexis may allow for changes to the effective lens position, affecting its distance from the principal plane of the cornea and its effective power. A very small capsulorrhexis may result in the lens being displaced posterior, creating a hyperopic shift, while a large rhexis can allow the lens to move forward as the forces of capsular bag contraction are brought to bear, resulting in a myopic shift. And while other factors also affect visual outcomes, including the accuracy of biometry, lens power calculation formulae and half-dioptre power steps in available IOLs, Dr Hill’s research indicates that the capsular opening is the surgical factor with the most influence overall. In absolute terms, capsule opening on average contributes 0.42 D error compared with a 0.02 error for the measurement of axial length by optical biometry, 0.25 D for autokeratometry and 0.17 D for the tolerance of IOL manufacturing for the specific IOL mentioned. The theoretical contribution of each factor to the absolute error is determined by taking the square root of the sum of the squares of the mean absolute error each factor. By this method, Dr Hill predicted that eliminating the contribution of capsule opening deviations would reduce the observed average absolute error of 0.58 D by about 0.18 D, to about 0.38 D. Partnering with Harvey Uy MD, Manila, Philippines, Dr Hill tested the hypothesis in a prospective study in which Dr Uy performed a standardised cataract procedure with the only difference being that one group received anterior capsulotomies using a LensAR system and the other received manual capsulorrhexis.

For all patients, axial length and keratometry readings were done with the IOLMaster, or ultrasound for axial length if it couldn’t be done with the IOLMaster. Lens power was calculated with the same formula. Phacoemulsification was carried out with an Alcon Infiniti using OZil, and an AcrySof SA60AT lens was implanted. A first analysis at six months found a mean spherical equivalent of 0.02 in 44 lasertreated patients compared with -0.21 in 62 manually treated patients (p=0.034), with 81 per cent of the laser group and 75 per cent of the manual group within 0.5 D of the target. A later analysis involving 249 laser and 123 manual patients showed a mean deviation of -0.21 D for the laser compared with 0.55 D for the manual group (p<0.001). This translates to an observed absolute error of 0.42 for the laser group and 0.59 for the manual group, Dr Hill said. In this larger group, 78.7 per cent of the laser patients and 52.8 per cent of the manual eyes were within 0.5 D of the target (p=0.003). “It’s absolutely uncanny that the numbers we got from the study so closely match what we predicted. Yes, the capsulorrhexis does affect the refractive outcome, and yes, the femtosecond laser does an amazing job with the capsulorrhexis,†Dr Hill said.

Overall, the results suggest that a perfectly round, centred and sized capsulotomy contributes about 0.15 to 0.2 D, Dr Hill said. He also noted that at six months Dr Uy also observed less fibrosis and contraction around the capsule opening in the eyes treated with the femtosecond laser.

Unmasking hidden factors

Beyond directly improving visual outcomes, reducing deviations induced by irregular capsule openings will make it possible to better study how other factors influence lens position and refractive outcomes. For example, the difference in ½ and ¼ dioptre steps in lens power on visual outcomes is virtually impossible to measure because there is so much variability due to IOL power calculation formulas, which remain the weakest link in the calculation process. “The great thing about the femtosecond laser is it will unmask a lot of this. Instead of mathematical noise, we will be able to isolate variables,†Dr Hill said. A study presented by Juan Batlle MD, Santo Domingo, Dominican Republic, comparing 29 eyes with manual capsulotomy with 39 eyes with femtosecond laser capsulotomies cut by an OptiMedica laser found that the lenses in the laser group were better centred one month after surgery.

“This data shows we are able to consistently perform a perfectly centred, perfectly cantered capsulorrhexis with a standard diameter. It is almost like a cookie cutter; we can do it every time in a predictable fashion.†Dr Batlle emphasised the importance of the “Liquid Optic Interface†in the achievement of a “completed†capsulotomy with 100 per cent penetration of the femtosecond laser energy avoiding “skip lesions†when using the ordinary applanation technology. The next step is to use the technology to better predict effective lens position, he added. Stephen Slade MD, Houston, US, suggested that the precision of capsulotomy placement would make it possible to determine how centring the opening on the visual axis, capsule anatomy or corneal centre might affect lens position compared with centring on the pupil centre, as is now common practice. Centring the lens on the optical axis certainly improves the performance of multifocal lenses. Tailoring the size might also help fine-tune depth of placement.

Improved fluidics

“Phaco technology has matured over the years and we now have better safety and efficiency through better power modulation and improved fluidics. This decreases the amount of energy being delivered into the eye and protects the corneal endothelium. Nuclear cataracts one and two almost require no power at all and it is a mere phacoaspiration. The introduction of torsional or transverse phacoemulsification has also enabled us to emulsify almost any type of nucleus with less risk to the cornea and the posterior capsule,†he said.

Dr Mostafa said that one of the advantages of traditional phacoemulsification is its versatility. “Even with compromised corneas, small pupils, extremely mature cataracts and posterior subcapsular cataracts, it is still possible to perform a proper capsulorrhexis, to easily remove the nucleus with the least impact on the corneal endothelium thanks to the newer modalities of power modulation and fluidics and improved techniques,†he said. There is no contraindication for conventional phacoemulsification except total corneal opacity or total subluxation of lens. Summing up, Dr Mostafa said that phacoemulsification remains a tried, cost effective and trusted technology that is continuing to deliver excellent clinical results for patients worldwide.