October 2009

 

COVER FEATURE

 

Refractive surgery

Refractive diamond knives for today


by Matt Young EyeWorld Contributing Editor

 

Download Refractive Diamond Knives chart

 

Single edged diamond keratome fully extended; used for radial keratotomy; the broad rudder shape of the blade makes incisions straight Source: David Jory, M.D.

Roger Steinert, M.D., once referred to the femtosecond laser as a “laser scalpel,” given its ability to perform incisions. It’s an interesting notion, and one that deserves some attention. The femtosecond laser has long been compared favorably to bladed microkeratomes. But could lasers ever replace other precise cutting instruments, such as diamond blades? That’s not likely in the near future. Although the femtosecond laser has found new applications for cutting beyond its role in LASIK flap-creation, many surgeons say the diamond knife is here to stay.

A diamond is forever

In the August 2009 issue of Cornea, researchers studied top-hat shaped corneal trephination for penetrating keratoplasty. A 60-kHz femtosecond laser (Abbott Medical Optics, Santa Ana, Calif.) was used to create penetrating cuts in the top-hat design. The laser clearly is highly programmable and precise. “The [femtosecond] laser pulse energy was set to 2.50 mJ for trephination, and spot separation to 2–3 mm for the side cuts and 4 mm for the lamellar cut,” according to study co-author Thomas Kohnen, M.D., professor, ophthalmology, Johann Wolfgang Goethe University, Germany. “The laser was programmed with the following parameters to create a top-hat trephination pattern: depth of lamellar cut, inner and outer diameter (DIA) of lamellar cut, anterior DIA and posterior depth of anterior cut and posterior DIA, anterior depth and posterior depth of the posterior side cut.”

However, a diamond knife still was required to complete the surgery. “Complete penetration was performed subsequently and manually with a diamond knife because of logistic conditions,” Dr. Kohnen noted. “Complete full-thickness penetration was achieved by using a diamond knife within the [femtosecond] laser–created anterior side cut to dissect the stromal tissue bridge between the anterior side cut and the lamellar cut.”

John D. Sheppard, M.D., professor of ophthalmology, microbiology, and immunology, Eastern Virginia Medical School, Norfolk, Va., said that while exciting research is being done on using the femtosecond laser during penetrating keratoplasty, he’s still happy with a trephine and diamond knife. “I make the incision with a trephine and shelf the incision with a diamond knife,” Dr. Sheppard said. “I like to go about half depth on the trephine and then make a small shelf to give myself a seal on the interior when creating a full-thickness keratoplasty. This produces a better seal.”

Dr. Sheppard has nothing against lasers. “Lasers have increased capability in creating exactly the wound you want,” Dr. Sheppard said. He simply stands by his diamond blades. “We just ordered two more diamond knives,” said Dr. Sheppard, who declined to mention the brand names. “I live and breathe by diamond knives.” Dr. Sheppard said he enjoys diamond knife precision particularly for limbal relaxing incisions (LRIs), astigmatic keratotomy surgeries, and creating any scleral wound. “It gives a precisely reproducible scleral flap and tunnel,” Dr. Sheppard said. He also uses them for bimanual microincision cataract surgery. Like the femtosecond, they also have advantages over metal blades. “You don’t get any metal fragments in the wound,” Dr. Sheppard said. “Although those fragments are benign, they create concerns in the back of your mind over what they could do in the long run.”

Limited laser

The femtosecond, meanwhile, is limited in terms of cut depth, said Mahipal S. Sachdev, M.D., chairman and medical director, Centre for Sight, New Delhi, India. “There is a problem getting the 60-kHz femtosecond laser below 450 microns in depth,” Dr. Sachdev said. “The problem is with the optics to focus that down deeper.”

Cost also is a factor for making femtosecond laser incisions, Dr. Sachdev said. “The cost would be per patient—just to make an incision,” he said, referring to per-procedure charges. “That would increase the procedure cost without significant benefit.” There are certain incisions that still are worthwhile to perform with the femtosecond laser, such as creating incisions for Intacs (Addition Technology, Des Plaines, Ill.), Dr. Sachdev said. “We are using the femtosecond for Intacs. It makes the whole procedure very easy,” he said. Meanwhile, Dr. Sachdev uses diamond knives primarily for LRIs, suggesting that the femtosecond laser and diamond knives have their own cutting niches. “The diamond knife is sharp and the desired thickness can be calibrated,” he said.

C.M. Wavikar, D.O.M.S., Maharashtra, India, also uses diamond knives for LRIs and cataract wound construction. “Diamond knives are not getting outdated for cataract surgery because not everyone can afford toric IOLs,” Dr. Wavikar said. Dr. Wavikar uses a trapezoidal diamond knife for the main incision construction. “Using a normal keratome, there is a chance of extending the wound while withdrawing it from the eye,” Dr. Wavikar said. “With a trapezoidal diamond knife, the chance of doing that is almost zero.” Dr. Wavikar is skeptical about lasers, specifically the femtosecond, to replace cutting of more traditional knives. “Whenever you are separating the interface created by a femtosecond laser, you are actually manually dissecting which can leave behind a less-than-smooth surface,” Dr. Wavikar said. “When you are manually separating you are putting stress on the lamellae. On the contrary with diamond knives, we get a very smooth cut. Maybe the femtosecond laser needs to evolve a little bit.”

Dr. Wavikar also recommends using diamond knives over metal blades during cataract surgery, so long as diamonds are properly maintained. “With the diamond knife we have to apply the same force at all times because it does not get less sharp,” Dr. Wavikar said. “With metal, the force can differ from one blade to another.” Reproducibility, therefore, is better with a diamond knife, he said. “There is a protocol to follow with diamond knives to keep them sharp,” Dr. Wavikar said. “If you follow the protocol, they can be maintained for a very long time. Alternatively, any accident for the diamond is fatal to the diamond knife as it would become unusable.”

Editors’ note: Dr. Kohnen has no financial interests related to his study. Drs. Sheppard, Sachdev and Wavikar have no financial interests related to their comments.

Contact information

Kohnen: Kohnen@em.uni-frankfurt.de
Sachdev: +91-11-4164-4000, drmahipal@gmail.com
Sheppard: 757-622-2200, docshep@hotmail.com
Wavikar: drcmwavikar@wavikareye.com

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