|
|
|
IOLeDa stands for intraocular lens (IOL) Data Analysis. It is a system for the analysis of the refractive results of IOL implantation. A variety of formulae are available to calculate the optical power of an intraocular lens that is necessary to achieve any desired level of post operative refractive error. A comprehensive method for the analysis of refractive results has not been previously available.
IOLeda is a registered trademark of Obeform Pty Ltd trading as IOLPower.
The spherical refractive error results of lens implantation can be analysed comprehensively provided the following is recorded for each case:
The IOLeDa method defines a relationship between three error values that summarise the sources of error in lens implantation – the Lens Selection Error (LSE), the Lens Power Error (LPE), and the Intention Error (IE). The three values can be calculated in two optical planes: the intra-ocular lens plane and the spectacle plane.
Shows whether the closest possible lens power to the power necessary for the stated Intended Refraction (IR) was used (that is, the lens power is the closest value possible if the LSE value is less than +/- 0.25D for 0.5 increments in lens power). Mathematically the result of LSE depends on the formula being used and the input variables: Axl; Kav; Lens Factor; Power implanted; and Intended refraction. A value may deliberately be greater than +/- 0.25DS but a note should be kept of the reason, to aid the interpretation of results. The reason would be expressed in terms of one of the input variables. The LSE value thus incorporates any adjustment in lens power implanted and quantifies the surgeons fudge factors.
Calculates the error associated with calculating the power of the lens as determined by the final refraction. The value is dependant on the Axl, Kav, Lens Factor, Power implanted, the achieved post operative refraction, and the formula used. In the spectacle plane it is the value often called the error of the predicted refraction.
The error of achieving the IR is the IE. Like LSE and LPE it can be a value in the IOL or the spectacle plane and is the exact sum of LSE and LPE in the two respective planes. In the spectacle plane it is independent of the formula and other variables. In the spectacle plane the values Axl, Kav, Lens Factor, the Power that is implanted, the intended refraction and the achieved post op refraction are used to calculate the exact IE in the intraocular lens plane. There is very little effect from differences in lens calculation formulae.
A common strategy for selecting the IOL power is that the surgeon has the patient's measurements (Axial length, Keratometry), and prior refraction available in the operating theatre to make the decision regarding the lens power that will be used - especially where there is a stock of lenses available from which to choose. The surgeon is usually presented with the results of a series of lens calculations involving a range of lens powers, often in half dioptre steps along with the predicted refraction for each power. There is usually a central value to produce emmetropia, that is a target refraction of zero is assumed.
It is common to select a lens power at the time of surgery based on its predicted refraction, and to review the predicted refraction relative to the other refractive data available for the patient. It is easy to extrapolate, with a reasonable level of precision, beyond the range of lens powers on the standard print out if needed. Alternatively the range of powers might be "centred" on a different value if this alternative value is defined before hand.
This process is fairly straightforward and works well most of the time. Where the task of measuring the eye is delegated to technical staff in the surgeon's office, the staff will be acting under instructions regarding the method of measurement and critical review of the data. The technical staff might generally assume emmetropia as the target refraction. Under certain circumstances they might discuss difficult cases with the surgeon while there is the opportunity to remeasure if needed, or to set the range of powers and associated predicted refractions for a desired level of ametropia.
The main circumstance where the predicted refraction for a lens may not be accepted as the target refraction is where the surgeon makes an adjustment of the lens power selected based on symmetry, or lack of it, and in the case of second eye surgery, after review of the results of the first eye. Under such circumstances the surgeon might expect a different refraction for the selected lens than the predicted refraction.
When reviewing the refractive results of cataract surgery it can be difficult to assess what is "going on" in some cases, especially if there is no record of the refractive result the surgeon expected to achieve. That is, in some cases the predicted refraction for a particular lens does not seem to reflect what the surgeon expects for the final refraction. A particular example is when the lens power for the second eye is selected. The solution is to specify the target refraction at the time of surgery for all cases (first and second eyes).
There is a difference between the target refraction and the predicted refraction, although it is a fairly subtle point and is something that surgeons may not consider consciously. The target refraction is a value which is arrived at based on assessment of the patients refractive status in both eyes, their refractive needs / desires, the pre-morbid refractive error, and the timing of surgery in both eyes. The predicted refraction is a calculated value based on the ocular measurements, a particular lens power, the lens position factor, and the formula used. In mathematical terms, the intended refraction is an independent variable (not calculated) whereas the predicted refraction is a dependent variable (calculated).
The target refraction is required as an input for the third generation lens formulae to determine the calculated lens power. In many cases emmetropia is assumed and thus the explicit expression of an intended refraction may be bypassed. When the results of lens power calculation are displayed showing a sequential range of lens powers with corresponding predicted refractions, a selected lens power has an associated predicted refraction and this might indicate the intended refraction. This assumption is imprecise and cannot capture information about surgeon "fudging".
The IOLeDa method sets out the relationships in the intraocular lens plane and spectacle plane between: the accuracy of selecting the lens power (the lens selection error), the accuracy of calculating the refractive results (lens power error), and the overall accuracy of the final result (intention error).
The IOLeDa method of analysis requires that the intended refraction be specified. Without it only the accuracy of calculating the refractive results can be reviewed, and surgeon fudging cannot not be calculated, unless the surgeon alters the lens factor (ACD, Surgeon Factor or A constant). What is called the "Lens Power Error" in the IOLeDa method is usually called the error of the predicted refraction and is a value in the spectacle plane. It describes the accuracy of the measurements in calculating the expected refractive result for a particular lens power, and does not capture information about any "surgeon fudging" unless a lens factor value is used that is different to the usual. The error of the predicted refraction has a corresponding value in the IOL plane.
By having the intended refraction recorded explicitly, a precise relationship is defined between three error values in the IOL plane and the corresponding three values in the spectacle plane. Without the detailed specification of the error values it is confusing to analyse results, especially if no account is made of the distinction between values in the IOL plane or the spectacle plane (that is the mathematical relationship between the lens power used, the predicted refraction and the error of the predicted refraction is not algebraic). The three error values (Lens selection error, Lens Power Error, Intention Error) make the analysis of refractive results very precise.
The key to understanding the value of the IOLeDa analysis is to appreciate the difference between the predicted refraction for a particular lens power, and the intended refraction. The extra information that needs to be recorded is simple - the Intended refraction. The intended refraction is a value arrived at by clinical judgement and not the calculated predicted refraction which is dependent on the eye measurements and the power of the implanted lens.