New resource could help show how eye responds to surgery.
Researchers have produced the most detailed map yet of the human cornea, the transparent dome that forms the front of the eye. Their efforts could help eye surgeons to hone their techniques and better understand postoperative complications.
"We wanted to try and make surgery safer," says Keith Meek of Cardiff University, UK, who led the project1. "People need to know what they are cutting through."
The cornea is responsible for two-thirds of the eye's focusing power. The 0.5-millimetre-thick structure is made of about 200 layers of a protein called collagen, which is arranged into fibrous bundles.
Researchers know that most bundles in each layer line up parallel to one another, and tend to run in different directions in adjacent layers. This creates a strong scaffold that holds the cornea in shape. "It's rather like plywood," says Meek. But little is known about the specific directions in which these bundles lie.
Meek and colleagues investigated this question by taking scans of unused corneas obtained from a British transplant bank. Using a technique called X-ray diffraction, which is often used to determine the structure of crystals, they worked out how the collagen bundles are arranged at more than 1,000 points across the cornea's surface.
They found that most bundles at the cornea's centre are arranged either vertically (pointing from eyebrow to cheek) or horizontally (from nose to ear). And at the edge of the cornea, most of the bundles are arranged so as to form a ring around the eye. This arrangement strengthens the region where the cornea joins the sclera - the white of the eye.
Slice and peel
Many surgical techniques involve cutting through the cornea. LASIK laser eye surgery, for example, uses a laser to carve the underlying layers of the cornea into a better shape for sharper vision. To do this, the outermost layers of the cornea are first peeled away by slicing it on three sides and pulling it back like a door. This flap is flipped back into place after surgery.
The cornea then heals itself. But, in rare cases, patients develop temporary or permanent scars in the cornea that give them hazy vision. Other problems, such as reduced night vision or a reduction in pressure inside the eye, can also occur in people who have undergone eye surgery. Not all of these complications are fully understood.
Knowing how the collagen bundles are oriented could give surgeons more control over the procedure, suggests Nigel Fullwood, who studies the cornea at the University of Lancaster, UK. "The surgical techniques are quite sophisticated," he says. "But sometimes they go strangely wrong - why?" The question hasn't yet been answered, he says.
John Marshall, a London-based eye surgeon who holds a patent on laser eye surgery, warns that corneas may look different from Meek's map when they are attached to a working eye. "It's always difficult to jump from isolated specimens to whole eyes," he says. The strain of attachment to the rest of the eyeball may change the bundles' pattern, he points out.
But having a few more hints about the cornea's detailed structure will still prove useful in understanding how it can be scarred by the surgeon's knife, says Anthony Bron, a clinical ophthalmologist at the University of Oxford, UK. "The map is not going to change surgeons' techniques," says Bron, "but it will help us to understand the problems."
Aghamohammadzadeh, H., Newton, R. H. & Meek, K. M. X-ray scattering used to map the preferred collagen orientation in the human cornea and limbus. Structure, 12, 249 - 256, doi:10.1016/j.str.2004.01.002 (2004).