Improved post-operative vision using advanced optical measurement techniques

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Cataract (clouding of the lens of the eye) is primarily a problem of aging; more than half of the over 65s have some degree of cataract, and cataract surgery is one of the most frequently performed operations in the UK, helping over a quarter of a million patients per year. For the vast majority, after removal of the cataractous lens, an artificial lens must be implanted to achieve good vision. The refractive power of this lens, chosen to minimize blurring at the retina, is calculated from preoperative measurements of the refractive elements of the eye, including the dimensions of the cornea, aqueous humour, lens and vitreous humour, and the curvatures at the interfaces between element.For well over a century, 'schematic eyes', or mathematical models, have been used to assess the overall optical refraction of the human eye, using assumed refractive index values for each element. The models have been refined over the years, as better information on the various required parameters has become available, and are still used today. The 21st century has seen the development of new methods for ocular measurements, in particular the application of optical coherence tomography (OCT), a low-power, non-laser, non-invasive imaging technique, to in vivo measurement of optical path length. However the very precision of the technique calls into question the standard RI values used in calculating IOL powers from schematic eyes. The quality of distance vision resulting from artificial lenses, post-surgery, is generally good, but further improvement can often be obtained by the use of spectacles, particularly for long-sighted and short-sighted patients, suggesting that assumed refractive index values are sometimes wanting.The aim of the proposed research is to allow measurement of the refractive index, in vivo, for each separate element within the eye. This will enable a more accurate calculation of the correct artificial lens power to be made for an individual cataract patient, reducing, or even avoiding altogether, the post-surgery necessity of spectacles for distance vision. Given the incidence of cataract worldwide, the benefits, in terms of cost-savings and convenience to cataract patients following surgery, would be extremely widespread.OCT is a very high resolution technique, providing detailed images of structures inside the eye, down to individual cones, only a few micrometres across, within the retina. Depth within the image always appears, however, as the product of physical distance and group index; a quantity related to, but not identical to, refractive index. The two parameters cannot be separated. To obtain the group index, and hence the refractive index, an independent measurement of physical distance is required, provided, in the proposed work, by another optical technique known as confocal imaging, in which a pinhole is positioned such that only light from a certain depth within the sample can pass through the pinhole to reach the detector.Difficulties to be overcome in developing this technique include: (a) the generation of beams with a sufficiently small focused spot diameter and depth of field to define the location of a tissue boundary within an image to about 1 micrometre, (b) elimination of image artefacts caused by motion of the eye, relative to the instrument, during signal acquisition, and (c) the correction of image aberrations resulting in blurring of the focused light spot and hence reducing measurement resolution, (d) adjustment of the measuring instrument to remain centred on, and perpendicular to, the apex of the cornea during acquisition of a dataset.Methods have been considered to address all these problems, and it is anticipated that the final version of the resulting instrument will be able to measure refractive index with accuracy and precision in the third decimal place, a major advance over the current lack of techniques for in vivo measurement of this important parameter.