NITINOL SUTURES

An injectable metal clip may reduce time spent constructing intraocular sutures by up to 20 times, greatly simplifying procedures including pupilloplasty, iris fixation of intraocular lenses, encircling band fixation, and anchoring glaucoma drainage devices, Michael Erlanger MD told the innovators session of the ASCRS annual symposium. The circular, shape-memory titaniumnickel alloy 0.007-inch nitinol (nickel titanium alloy) suture wire is five times stronger than prolene and 38 times stronger than nylon. Proven in neurosurgery and cardiac stent applications, it is highly biocompatible and can be delivered onehanded through needles as small as 30 gauge for intraocular procedures. And unlike filament sutures, the wire clip can be bent and returned to its original shape, said Dr Erlanger, who is developing the technology at the University of Colorado, Denver, US, and has a patent pending on the injectable Nitinol suture with Jeffrey Olson MD. “This is very useful when you are working in the eye. You don’t have to use ocular gymnastics to get in a Seipser suture,†Dr Erlanger said. In animal tests, pupilloplasty was performed 16 times faster with the nitinol suture than with a modified Seipser slip knot, and the need for limbal-to-limbal passes was eliminated, making the procedure less technically challenging. Similarly, IOL iris fixation times were reduced 20-fold.

Biocompatible

When properly prepared, the titanium-nickel nitinol alloy is highly resistant to corrosion, Dr Erlanger noted. The compound has been used extensively in cardiovascular stents as well as orthopaedic and dental devices. While nitinol can be formed into any shape, for intraocular use a circular double coiled clip may be most useful. Dr Erlanger has tested it in 0.5mm and 1.0mm suture sizes. He has developed an injector based on a syringe. Nitinol’s memory characteristics allow the suture to be straightened and injected through a straight or curved needle. It resumes its circular shape as it leaves the injector, allowing it to be anchored in tissue as it comes out. The suture can be manipulated with forceps after injection. Pre-set sutures can be sprung open to grasp haptics, glaucoma shunts, rings or other devices. In a three-month biocompatibility study involving five Yucatan mini pigs, five eyes receiving nitinol sutures and five receiving modified Seipser knots showed no significant differences in corneal specular cell counts, corneal thickness measured by OCT, corneal histology staining, or retinal histology measured by cell counts of the ganglion cell, inner nuclear and outer nuclear layers, Dr Erlanger reported (Inv Ophthalmol Vis Sci 2011). However, mean surgical time for the nitinol eyes was one minute 18 seconds compared with 19 minutes 38 seconds for the modified Seipser knot eyes (p<0.005). Other successful tests include anchoring an Ahmed glaucoma drainage shunt, Retisert fixation, installation of encircling bands and IOL fixation, Dr Erlanger said. He believes this technology will find broad application in ophthalmic surgery.