FS-CATARACT POWER

Minimising energy dissipated in the eye during cataract surgery, whether ultrasonic or laser, may lower the risk of damaging delicate intraocular tissues. But while current phaco systems allow continuous ultrasonic energy control during surgery, femtosecond (FS) laser systems do not.
However, total laser energy delivered during FS laser-assisted cataract surgery can be reduced by lowering the pre-set power of individual laser pulses, Roberto Bellucci MD, Verona, Italy, told the 2014 ESCRS Congress in London. 
Because laser energy is sensitive to light scatter, the minimum pulse power required to produce photodisruption in target tissues should vary with the transparency of tissues between the laser and target, with clearer corneas and lenses requiring less energy, he reasoned. Dr Bellucci presented results of two nomograms for adjusting FS pulse power in cataract surgery that he developed based on this principle.
For capsulotomy, Dr Bellucci adjusted pulse power based on a gross evaluation of corneal transparency. Overall, the reduction in energy in clearer corneas was slight. Adjustments varied from 7,100 to 7,400 nanojoules (nJ), with two-thirds of 79 eyes treated at 7,200nJ. He concluded that energy settings for capsulotomy are not very critical.
Potentially more significant are power adjustments for lens fragmentation, which requires much more energy delivered deeper in the eye. Dr Bellucci adjusted lens fragmentation power based on corneal and lens opacity, and lens hardness. Lens opacity was graded using the LOCS III system at the slit lamp under mydriasis, and pulse power adjusted over a range of 1,200nJ, from 7,500nJ to 8,700nJ based on total opacity. The actual study treatment range was narrower, with 67 of 79 eyes treated with 7,600 to 8,200nJ pulses, two each at 7,500, 8,500 and 8,600nJ, and none at the maximum 8,700nJ, Dr Bellucci reported. 
Fragmentation effectiveness was also rated. The 6.3 per cent of eyes that subsequently required nucleus chopping were rated “poor”; 61 per cent with easily separated nuclear fragments rated “good”; and 32.7 per cent with fragments completely separated before phaco rated “excellent”. Phaco energy was not considered because cataract hardness varied.

Fragmentation results
However, the fragmentation results were difficult to evaluate because precise numeric values for lens opacity were not identified, Dr Bellucci said. “The LOCS classification appeared of limited help in adjusting the laser energy for lens fragmentation. 
“To develop a better lens fragmentation laser energy nomogram we need more precise evaluation of lens opacity, better consideration of lens density and a more precise method to evaluate results. I think we are not there at the moment,” Dr Bellucci concluded.

Roberto Belllucci: robbell@tin.it