An axial measurements device is used to take axial measurements of the eye, from the anterior surface of the cornea to either the surface of the retina (ultrasound) or the retinal photoreceptors (optical). The axial measurements are typically expressed in mm (Ophthalmic Axial Length (0022,1010). Currently these measurements are taken using ultrasound or laser light. The measurements are used in calculation of intraocular lens power for cataract surgery. Axial measurements devices and software on other systems perform intraocular lens power calculations using the axial measurements in addition to measurements from other sources (currently by manual data entry, although importation from other software systems is expected in the future).
When the natural lens of the eye turns opaque it is called a cataract. The cataract is surgically removed, and a synthetic intraocular lens is placed where the natural lens was before. The power of the lens that is placed determines what the patient’s refractive error will be, meaning what power his glasses will need to be to maximize vision after surgery.
Axial measurements devices provide graphical displays that help clinicians to determine whether or not the probe used in taking the measurements is aligned properly. Annotations on the display provide information such as location of gates that assists the clinician in assessing measurement quality. High, fairly even waveform spikes suggest that the measurement producing a given graph is likely to be reliable. The quality of the graphical display is one of the factors that a clinician considers when choosing which axial length measurement to use in calculating the correct intraocular lens power for a given patient.
Axial measurements devices and software on other systems perform intraocular lens power calculations for cataract surgery patients. The power selection of intraocular lens to place in a patient’s eye determines the refractive correction (e.g. glasses, contact lenses, etc.) the patient will require after cataract surgery.
The data input for these calculations consists of ophthalmic axial length measurements (one dimensional ultrasound scans that are called “A-scans” in the eye care domain) and keratometry (corneal curvature) measurements in addition to constants and sometimes others kinds of measurements. The data may come from measurements performed by the device, on which the intraocular lens calculation software resides, or from manual data entry, or from an external source. There are a number of different formulas and constants available for doing these calculations. The selection of formula to use is based on clinician preference and on patient factors such as the axial length of the eye. The most commonly used constants, encoded by Concept Name Code Sequence (0040,A043) using CID 4237, are a function of the model of intraocular lens to be used.
The most commonly used formulas, encoded by IOL Formula Code Sequence (0022,1029) using CID 4236, for intraocular lens calculation are inaccurate in a patient who has had refractive surgery, and numerous other formulas are available for these patients. Since most of them have not been validated to date, they were not included in this document.
Intraocular lens calculation software typically provides tabular displays of intraocular lens power in association with each lens’s predicted refractive error (e.g. glasses, contact lenses, etc).
Figure CCC.2-1 Sagital Diagram of Eye Anatomy (when the lens turns opaque it is called a cataract)
Courtesy; National Eye Institute, National Institutes of Health; ftp://ftp.nei.nih.gov/eyean/eye_72.tif
Figure CCC.2-2 Eye with a cataract
Courtesy; National Eye Institute, National Institutes of Health; ftp://ftp.nei.nih.gov/eyedis/EDA13_72.tif
Figure CCC.2-3 Eye with Synthetic Intraocular Lens Placed After Removal of Cataract
This file is licensed under the Creative Commons Attribution ShareAlike 2.5 License, Author is Rakesh Ahuja, MD
Figure CCC.3-1 demonstrates an A-scan waveform – produced by an ultrasound device used for ophthalmic axial length measurement. This is referenced in the Ophthalmic Axial Measurements IOD in Attribute Referenced Ophthalmic Axial Length Measurement QC Image Sequence (0022,1033).
Figure CCC.3-1 Scan Waveform Example
Time (translated into distance using an assumed velocity) is on the x-axis, and signal strength is on the y-axis. This waveform allows clinicians to judge the quality of an axial length measurement for use in calculating the power of intraocular lens to place in a patient’s eye in cataract surgery. Figure CCC.3-1 above demonstrates a high quality scan, with tall, even spikes representing the ocular structures of interest. This tells the clinician that the probe was properly aligned with the eye. The first, double spike on the left represents anterior cornea followed by posterior cornea. The second two, more widely spaced spikes represent anterior and posterior lens. The first tall spike on the right side of the display is the retinal spike, and the next tall spike to the right is the sclera. Smaller spikes to the far right are produced by orbital tissues. Arrows at the bottom of the waveform indicate the location of gates, which may be manually adjusted to limit the range of accepted values. Note that in the lower right corner of the display two measurements are recorded. In the column labeled AXL is an axial length measurement, which on this device is the sum of the measurements for ACD (anterior chamber depth), lens, and VCD (vitreous chamber depth). The measured time value for each of the segments and a presumed velocity of sound for that segment are used to calculate the axial length for that segment. An average value for each column is displayed below along with the standard deviation of measurements in that column. The average axial length is the axial length value selected by this machine, although often a clinician will make an alternative selection.
Figure CCC.4-1 demonstrates the waveform-output of a partial coherence interferometry (PCI) device used for optical ophthalmic axial length measurement. This is referenced in the Ophthalmic Axial Measurements IOD in Attribute Referenced Ophthalmic Axial Length Measurement QC Image Sequence (0022,1033).
Figure CCC.4-1 Waveform Output of a Partial Coherence Interferometry (PCI) Device Example
Physical distance is on the x axis, and signal strength is on the y axis. What is actually measured is phase shift, determined by looking at interference patterns of coherent light. Physical distance is calculated by dividing “optical path length” by the “refractive group index” – using an assumed average refractive group index for the entire eye. The “optical path length” is derived from the phase shift which is actually observed. Similar to ultrasound, this waveform allows clinicians to judge the quality of an axial length measurement.
Figure CCC.4-1 above demonstrates a high quality scan, with tall, straight spikes representing the ocular axial length. The corneal spike is suppressed (outside the frame on the left hand side) and represents the reference 0 mm. The single spike on this display represents the signal from the retinal pigment epithelium (RPE) and provides the axial length measurement value (position of the circle marker). Sometimes smaller spikes can be observed on the left or right side of the RPE peak. Those spikes represent reflections from the internal limiting membrane (ILM, 150-350 µm before RPE) or from the choroid (150-250 µm behind RPE) respectively.
Because all classical IOL power calculation formulas expect axial lengths measured to the internal limiting membrane (as provided by ultrasound devices), axial length measurements obtained with an optical device to the retinal pigment epithelium are converted to this convention by subtracting the retinal thickness.
Figure CCC.4-1 above displays five axial length measurements obtained for each eye (one column for each eye) and the selected axial length value is shown below the line.
Figure CCC.5-1 demonstrates a typical display of IOL (intraocular lens) calculation results.
Figure CCC.2-6 IOL Calculation Results Example
On the right the selected target refractive correction (e.g. glasses, contact lenses, etc.) is -0.25 diopters. At the top of the table three possible intraocular lens models are displayed, along with the constants (CID 4237) specific to those lens models. Each row in that part of the table displays constants required for a particular formula. In this example the Holladay formula has been selected by the operator, and results are displayed in the body of the table below. Calculated intraocular lens powers are displayed with the predicted postoperative refractive error (e.g. glasses, contact lenses, etc.) for each lens. K1 and K2 on the right refer to the keratometry values (corneal curvature), in diopters, used for these calculations.