- 1 Introduction
- 2 Principles of wave-front aberrometry
- 3 Intraoperative Aberrometry Technology
- 4 Results in literature
- 5 Factors Contributing Variability to IWA
- 6 Conclusions
- 7 References
Surgeons use many biometric formulas and measurements pre-operatively in hopes of minimizing refractive error after cataract exchange. Intraoperative aberrometry is an additional tool that allows surgeons to take both aphakic and pseudophakic refractive measurements in the operating room to aid in the determination of intraocular lens (IOL) power selection and placement.
Principles of wave-front aberrometry
A wave-front is the physical propagation of a ray of light. Aberrations are distortions of these visual waves caused by irregularities of the optical mechanism. In the perfect eye, wave-fronts converge onto the retina in a predictable manner. As light passes through the irregularities of an actual imperfect eye, light diverges from its ideal wave-front resulting in angle deviation. This difference can be measured at the plane of the pupil to create a topographical map .
Aberrations can be split into low and high order. Low order aberrations include factors typically addressed during an eye exam, such as prism, spherical defocus and astigmatism. These are the most clinically significant aberrations and can be corrected with glasses or contact lenses. High order aberrations are factors that may lead to disturbances in visual acuity after glasses or lenses correction and are not able to be addressed in clinic. Most often they are the result of irregularities in the cornea. They include trefoil and coma, both of which are related to irregularities of astigmatism .
Zernicke polynomial is a mathematical equation used to describe wavefront aberrations in a pyramid manner, in which more significant low order aberrations are at the base and less significant are at the top. Aberrometry can then be reported in root-mean-square (RMS) of the Zernicke polynomial. This gives a single digit value which represents the average difference of actual wave-front compared to theoretical plane wave-front .
Intraoperative Aberrometry Technology
WaveTec Vision Systems, ORange and ORA
ORange intraoperative wavefront aberrometer by WaveTec Vision Systems, Inc. was the first commercially available intraoperative wave-front aberrometer. It utilizes Talbot moiré interferometry, a system involving two gratings offset at a specific angle and distance which produces a fringe pattern as wave-fronts are diffracted through the grates. This fringe pattern is then analyzed to provide information on sphere, cylinder and axis . ORange is attached to surgical microscope and has a range from -5 to +20 D . Both aphakic and pseudophakic measurements can be taken while in the operating room to guide IOL power selection and lens placement. In 2012, WaveTec released ORange’s successor, ORA, with improved optics (super luminescent light-emitting diode as opposed to laser light), interface and algorithms . In 2014, Alcon acquired WaveTec and the ORA system. They also incorporated AnalyzOR, which compares pre-, intra-, and post-op data and allows surgeons to fine-tune their calculations to improve outcomes.
HOLOS IntraOp by Clarity is the newest available product. It utilizes the technology of a rapidly rotating micro electro-mechanical system (MEMS) mirror and quad detector to measure the magnitude of wavefront displacement. It takes up to 90 measurements per second and has a range from -5D to +16 D. Like its competitor, it attaches to the operating microscope to give intraoperative refractive measurements .
Results in literature
Routine cataract surgery
While early studies using ORange showed a post-op spherical error as low as 0.36±0.30 D, others question the reliability and precision of intraoperative wave-front aberrometry . Studies have not been conclusive about the superiority of intraoperative wavefront aberrometry (IWA) compared to traditional formulas in the uncomplicated eye, and as a result no consensus regarding the utility of IWA in routine cataract surgery has been reached.
Eyes which have undergone prior refractive surgery pose a particular challenge to surgeons determining IOL power. It was reported that less than 55% of post-LASIK eyes after cataract surgery were ±0.5D of emmetropia, compared with 70% in eyes that had not undergone previous refractive surgery . Initial studies using ORange technology in eyes that previously received myopic correcting treatment showed only 39% of eyes were within ±0.5D of emmetropia and 60% were within ±1D . However, recent studies using ORA have shown more promising results in this patient subset. Lanchulev et al showed ORA to have the highest predictive accuracy of IOL power when compared to the Haigis L formula, Shammas formula and a combination of all clinical data based off surgeon’s choice. A median absolute error of 0.35D (compared to 0.6 of surgeon’s choice, 0.53 with Haigis and 0.51 with Shammas) was reported with 67% falling within ±0.5 D and 94% falling within ±15). An additional study comparing ORA to the Masket regression formula, Haigas-L formula and Fourier-domain OCT-based formula reported a similar 0.25D median absolute error for ORA. However, this was not statistically different from the other methods in the study .
Role in Astigmatism Correction
Toric IOLs require extreme precision in axis alignment as one-third of the cylinder correction is lost for every 10 degrees of misalignment . Aberrometry can be utilized for toric lens placement in both the aphakic and pseudophakic state, as an aphakic reading will give an accurate axis while a pseudophakic reading will confirm alignment. In a study looking at IWA-guided toric IOL placement in post-refractive surgery eyes, ORA was shown to have a lower mean prediction error (0.43) compared to pre-operative calculations using the IOLMaster (0.77) and the ASCRS calculator (0.61). With ORA, 80% of eyes were ±0.75D sphere. Pre-operative measurements showed only 53% of eyes would have achieved this without ORA . An additional study regarding toric IOL placement guided by intraoperative aberrometry showed an increase in likeliness by a factor of 2.4 that mean postoperative residual refractive astigmatism would be less than 0.5D .
Limbal Relaxing Incisions (LRIs)
Pre-operative planning of LRIs using corneal topography can also be challenging. Using ORange to guide LRIs, postoperative residual refractive astigmatism was shown to be reduced by a factor of 5.7, although this trend was not statistically significant .
Factors Contributing Variability to IWA
Factors such as eyelid speculums, eyelid pressure and post-operative changes all add a degree of variability to intraoperative aberrometry. Positioning of the speculum can introduce pressure to the globe, altering the shape of the eye. Proper positioning of the speculum can help to minimize this effect. In addition, a significant difference has been noted in the cylinder of the eye immediately post-operatively vs 1 week post-operatively. Contributing factors for this effect include alterations made by the incision, stromal hydration and changes in intraocular pressure (IOP). To minimize this effect, IOP should be returned to physiologic range prior to measurements. 
While consensus has not been reached on the role of intraoperative aberrometry on the uncomplicated eye, its utility in patients with prior refractive surgery and patients in need of astigmatism correction is more established. Before using IWA, controllable factors which may contribute to variations in IWA readings should be addressed, such as speculum placement and IOP.
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