Precision Pulse Capsulotomy: Preclinical Safety and Performance of a New Capsulotomy Technology

The ability to reproducibly automate a perfectly sized and circular capsulotomy has spurred much of the current clinical interest in femtosecond laser assisted cataract surgery (FLACS). Compared to manual continuous curvilinear capsulorhexis (CCC), the femtosecond laser capsulotomy (FSLC) is highly reproducible, uniformly more circular, and has a more precise diameter. However, this comes at a much higher capital and procedural cost and disrupts the normal surgical workflow because the FLACS steps cannot be performed within the usual operative sequence. Furthermore, differing national regulations may ban or restrict the ability to balance bill patients for the additional costs associated with FLACS. An important concern has arisen from published reports of an increased rate of anterior capsule tears following FSLC^1,2. Scanning electron microscopy (SEM) of FSLC anterior capsule buttons demonstrates a rougher edge, when compared to manual CCC specimens. In addition, SEM analysis also reveals scattered aberrant laser shots that may be explained by microscopic eye movements occurring during the FSLC step¹. These might be postulated to predispose focal areas of the anterior capsular rim to radial tears caused by subsequent surgical forces.

Mynosys (Fremont, California) has developed a novel capsulotomy method and technology called Precision Pulse Capsulotomy (PPC) and trade named Zepto. A disposable handpiece and nano-engineered capsulotomy tip are powered by a small console to automatically and instantaneously create a perfectly circular capsulotomy of a precise pre-designed diameter. The tip consists of a circular nitinol ring surrounded by a thin silicone cover shaped like a miniature inverted frying pan. Nitinol is a shape memory alloy which means that a 5.0 mm diameter ring can be compressed and deformed for insertion through a clear corneal incision, but will then re-assume its natural circular shape once it emerges inside the anterior chamber (AC). After filling the AC with ophthalmic viscosurgical device (OVD), a retractable metal push rod elongates the ring and silicone shell into a narrower profile that can be inserted through the clear corneal incision. After retracting this push rod, the compressed tip resumes its native circular shape within the AC.

The surgeon gently positions the ring and surrounding suction cup onto the anterior capsular surface prior to applying a small amount of suction via the external console. Only slight suction is needed to appose the anterior capsule against the bottom edge of the nitinol ring, which has been precisely engineered at the micron scale to enable uniform capsule cutting. A rapid series of microsecond-long electrical pulses is used to create the capsulotomy. Phase transition of water molecules trapped between the capsule and nitinol edge cause the stretched capsular membrane to abruptly split circumferentially all at once. Unlike the sequential circular path of a manual or FSLC, the PPC technology mechanically and simultaneously cleaves all 360 degrees of the apposed capsule without cauterizing it. Collateral ocular tissue safety is achieved through two design features. First, the application of energy is extremely brief and confined only to the microscopic edge of the nitinol ring. Second, during activation the nitinol ring is completely covered by the silicone suction cup and further insulated by the surrounding OVD.

This device has been developed through extensive testing in animal and human cadaver eyes³. Miyake Apple view video imaging shows insignificant zonular traction while performing PPC in both rabbit and human cadaver eyes. Pre-clinical performance and safety testing in live rabbits, including intraoperative thermocouple measurements, slit lamp evaluation and histopathology, has been performed at the Moran Eye Center.

On SEM of human cadaver capsules, the PPC capsulotomy edge is smoother and more defect-free than that of a manual CCC.  We have performed extensive testing of capsulotomy edge strength by comparing PPC to both manual CCC and FSLC in paired fellow human cadaver eyes. The PPC edge is consistently stronger than either alternative.

The obvious potential advantage of Zepto PPC would be its ability to reproduce automate the capsulotomy step with a disposable instrument that is inserted in the conventional surgical sequence and in lieu of using capsulorhexis forceps. Surgeons would welcome a method to assure a perfectly sized and round capsulotomy without the workflow challenges and increased procedural time of FLACS. As with FSLC, popular indications would be for complicated cases or when using premium refractive IOLs. The lower cost should ideally make PPC available to all patients independent of affordability and regional regulations regarding balance billing. Surgeons might also opt for the efficiency of using Zepto routinely rather than only in select cases, particularly if it is on average faster and more consistent than manual capsulorhexis.

Because it is an integrated step during conventional phacoemulsification, PPC can be performed after insertion of iris expansion devices for small pupils. The tip is also designed to allow insertion through a smaller pupil if necessary. It does not require additional surgical steps such as capsular staining. If the PPC edge is indeed more tear resistant, this might improve surgical safety by reducing anterior and posterior capsular tears. Finally, the transparent silicone suction cup has a central window that is designed to permit patient fixation on the microscope light filament during positioning of the device. Being able to center the capsulotomy on the visual axis would be advantageous when implanting refractive lens implants such as toric and multifocal IOLs.

The PPC device name was chosen because in the metric scale, zepto is one million times smaller than femto. Both the small size of the instrument and the several millisecond speed of capsulotomy creation inspired this name. The device is CE mark approved, and has been submitted for FDA 510(k) approval.

REFERENCES:

1. Abell RG, Davies PE, Phelan D, et al. Anterior capsulotomy integrity after femtosecond laser-assisted cataract surgery. Ophthalmology 2014;121:17-24.
2. Chang JS, Chen IN, Chan WM, et al. Initial evaluation of a femtosecond laser system in cataract surgery. J Cataract Refract Surg 2014;40:29-36.
3. Chang DF, Mamalis N, Werner L. Precision Pulse Capsulotomy – Preclinical Safety and Performance of a New Capsulotomy Technology. Ophthalmology 2016; 123:255-264.
4. Thompson VM, Berdahl JP, Solano JM, Chang DF. Comparison of manual, femtosecond laser, and precision pulse capsulotomy edge tear in paired human cadaver eyes. Ophthalmology 2016;123:265-274.

Dr. Chang is clinical professor at the University of California, San Francisco. His relevant financial disclosure is that he has received compensation as a consultant for Mynosys, AMO, and LensAR.