Abstract and Introduction
Biometry has become one of the most important steps in modern cataract surgery and, according to the Royal College of Ophthalmologists Cataract Surgery Guidelines, what matters most is achieving excellent results. This paper is aimed at the NHS cataract surgeon and intends to be a critical review of the recent literature on biometry for cataract surgery, summarising the evidence for current best practice standards and available practical strategies for improving outcomes for patients. With modern optical biometry for the majority of patients, informed formula choice and intraocular lens (IOL) constant optimisation outcomes of more than 90% within ±1 D and more than 60% within ±0.5 D of target are achievable. There are a number of strategies available to surgeons wishing to exceed these outcomes, the most promising of which are the use of strict-tolerance IOLs and second eye prediction refinement.
On 29 November 1949, Harold Ridley implanted the first prosthetic intraocular lens (IOL) into a human eye. Ridley's 1952 paper describes the new technique and documents the first ever audit of refractive outcomes after cataract surgery with IOL implantation. In the first audit cycle, comprising two patients, both were found to have highly myopic outcomes (−21 and −15 dioptres spherical equivalent, DSE), resulting from excessive power of the first IOL design. This was modelled on the natural crystalline lens with a radius of curvature for the anterior and posterior surfaces of 10 and 6 mm, respectively. Unfortunately, the higher refractive index of the polymethyl methacrylate material in comparison to the crystalline lens was overlooked and the IOL was more powerful than intended. A new design with a power of +24 dioptres (D) was developed, and the next 25 patients all received this standard IOL. The second audit cycle found that with this lens the median post-operative refractive error was -2.25 DSE (range −10.5 to +4.5 DSE).
For the next two decades, IOL implantation remained controversial because of 'chaotic experimentation and defective IOL design and manufacture' and efforts were focussed on reducing the complications of IOL implantation. During this period, surgeons typically implanted the same power IOL into every eye with the intention of restoring the patient's pre-cataract refractive status, and although this approach worked well for many patients, large refractive surprises were common with poor correlation between pre- and post-operative refractive error. As IOL implantation became accepted as the standard of care for cataract surgery attention turned to improving refractive outcomes using IOL power prediction algorithms based on biometric measurements of individual patients' eyes, resulting in around 70–80% achieving a post-operative refraction within ±1 D of the intended target.[3,5] The superior clinical outcomes of phakoemulsification over previous surgical procedures, particularly in relation to post-operative astigmatism, the improved accuracy and repeatability of measurements provided by devices such as the IOL Master (Carl Zeiss Meditec AG, Oberkochen, Germany) and Lenstar LS-900 (Haag-Streit AG, Koeniz, Switzerland), and modern third- and fourth-generation IOL power prediction formulas, which use up to seven pre-operative variables, have improved the predictability of refractive outcomes and transformed routine cataract surgery from a procedure intended simply to restore sight to a refractive procedure.
Biometry has become one of the most important steps in modern cataract surgery. Gale et al. found that with appropriate formula selection, optical axial length measurement, and optimisation of IOL constants 87% of patients achieved an outcome within ±1 D of target. The Royal College of Ophthalmologists, in its most recent Cataract Surgery Guidelines, has adopted a standard of 85% within ±1 D of target and 55% within ±0.5 D of target.
This paper is aimed at the NHS cataract surgeon and intends to be a critical review of the recent literature on biometry for cataract surgery, summarising the evidence for current best practice standards and available practical strategies for improving outcomes for patients. It is assumed that both optical and ultrasound axial length measurement devices are available, appropriate measurement protocols and quality control procedures are established, and that a mechanism for ascertaining refractive outcomes and performing audit is in place.
Eye. 2014;28(2):118-125. © 2014 Nature Publishing Group