Cataract surgery demands extremely sophisticated and precise instrumentation. Keratomes must deliver accurate incisions without damaging adjacent corneal tissue. A poorly constructed corneal incision can induce postoperative astigmatism. Because diamond blades cause little distortion or injury to adjacent tissue,1 they have been considered the gold standard. Nonetheless, the cost of these knives (often more than $1,000 each) can be impractical for some surgeons or practices. Also, diamond blades require sterilization after their use, and they must periodically be sharpened. A retractable design shields the blade's cutting edge when it is not in use, and this sheath may retain biological material despite cleaning and sterilization between cases. Prions that cause Creutzfeldt-Jakob disease are highly resistant to denaturation and proteolysis, and conventional sterilization of nondisposable instruments may not destroy these pathogenic proteins.

Disposable instrumentation is of benefit as regards cost and the transmission of infection, but the performance of diamond blades is clearly superior. Scanning electron micrographs of corneal incisions made with steel knives have demonstrated distorted tissue, microscopic stress lines, and significantly greater damage to the incision's margins with a loss of surface epithelium compared with diamond blades.1 Such changes can activate keratocytes to a greater degree and stimulate a more aggressive stromal healing response with its sequelae of scarring, haze, contraction of the incision, and astigmatism.2 A new disposable silicon blade, however, may represent an impressive alternative to the diamond knife.

EXPERIMENTAL EVIDENCE
The Surface

In a preliminary study,3 we compared the performance of the silicon BD Atomic Edge Blade (US availability predicted for summer 2005 by company; BD Ophthalmic Systems, Franklin Lakes, NJ) with that of a diamond keratome and a disposable steel Beaver blade (BD Ophthalmic Systems). On scanning electron microscopy, the cutting edge of both the silicon and diamond blades appeared smooth and regular with similar angulations. In striking contrast, the surface of the steel blade at this magnification was pitted and uneven (Figure 1).

The Incision

To evaluate incisional morphology, we maintained human cadaveric corneas (Bristol Eye Bank, Bristol, UK) in air-interface organ culture for several days to ensure complete re-epithelialization. We pressurized corneas to 40mmHg in an artificial anterior chamber maintainer (Gebauer Medizintechnik GmbH, Neuhausen, Germany) prior to making a central incision to a defined depth and length with each blade fixed to a microdrive. We then performed scanning electron microscopy (Figure 2). The incisions made with the diamond and silicon blades were cleanly linear with a smoothly divided but intact epithelial layer. The stromal wound margins also appeared clearly defined. In contrast, the incision made with the steel blade exhibited clear irregularity, sheer induced fraying of its margin, and a superficial removal of epithelium. The walls of all three incisions' crevasses appeared comparably smooth.

Electron microscopy of a transverse section through each incision demonstrated clearly how a blade sheers through the arrays of collagen fibrils within the stroma, thereby dividing the tissue. There was little evidence of compression or significant displacement of stromal collagen fibrils with either the diamond or silicon blade (Figure 3).

Sharpness

The sharpness of a blade depends on the interaction of a complex array of variables such as friction, angulation of the blade, force on the blade, its speed of travel, tissue rotation, and the inhomogeneous elastic properties of the incised tissue. As yet, there is no universal unit for measuring sharpness. A surgeon's appreciation of this quality arises from a tactile sense of resistance to the blade's movement through tissue. To compare the sharpness of the three different blades, we defined sharpness as the force required to penetrate a given medium (N/mm2). For this purpose, we fixed each blade to a solid support with the test medium approximated to the knife's tip. We then advanced the test medium with gradually increasing force (measured by a force gauge) to the point of complete penetration. We plotted the results as a graph of force versus blade type (Figure 4). The diamond blade's superior performance was immediately apparent and required just 2.1N/mm2 (SD ±0.78) to penetrate the test medium, whereas the steel blade needed 76N/mm2 of force (SD ±3.4). Results with the BD Atomic Edge Blade fell in the middle at 40N/mm2 (SD ±2). By this metric of sharpness, the silicon blade was considerably sharper than the steel blade.

CONCLUSION
The BD Atomic Edge Blade may satisfy the cataract surgeon's desire for the perfection of a diamond knife combined with disposability and a lower cost. Disposable instrumentation no longer means a compromise in performance, and it has the added advantage of avoiding the transmission of disease.
Romesh Angunawela, MD, MRCOphth, is currently British Eye Research Foundation, Corneal Research Fellow at The Rayne Institute, St. Thomas' Hospital, London. He states that he holds no financial interest in the products or companies mentioned herein. Dr. Angunawela may be reached at +44 20 79228110; romeshi@hotmail.com.
Christoph Winkler Von Mohrenfels, MD, was Visiting Research Fellow to the Rayne Institute, St. Thomas' Hospital, London, and currently works as an ophthalmic surgeon at the University Hospital, Munich, Germany. He states that he holds no financial interest in the products or companies mentioned herein. Dr. Von Mohrenfels may be reached at christoph.wvm@web.de.
John Marshall, PhD, is Head of the Academic Department of Ophthalmology, The Rayne Institute, St. Thomas' Hospital, London. He states that he holds no financial interest in the products or companies mentioned herein. Dr. Marshall may be reached at +44 20 79228110; june.spacey@kcl.ac.uk.
1. Marshall J, Trokel S, Rothery S, Krueger RR. A comparative study of corneal incisions induced by diamond and steel knives and two ultraviolet radiations from an excimer laser. Br J Ophthalmol. 1986;70:482-501.
2. Jester JV, Petroll WM, Cavanagh HD. Corneal stromal wound healing in refractive surgery: the role of myofibroblasts. Prog Retin Eye Res. 1999;18:311-356.
3. Angunawela RI, Winkler C, Rajan MS, Marshall J. Comparison of a prototype plasma-edged ceramic blade to diamond and steel blades. Poster presented at: The AAO/SOE Joint Meeting; October 23-24, 2004; New Orleans, LA.