IOL POWER CALCULATION WITH SULCUS OR NONCAPSULAR FIXATION
Warren E. Hill, MD, FACS
The most common scenario where standard capsular fixation is not possible follows a large, unanticipated tear in the posterior capsule, often with vitreous prolapse. Typically, a rim of anterior capsule remains and a 3-piece foldable intraocular lens (IOL) is placed in the sulcus. Primary placement of a 3-piece IOL most often results in satisfactory fixation and centration. However, secondary sulcus placement at some later date may require scleral fixation for long-term stability.
Confusion sometimes follows the fact that IOL power calculations are typically carried out for placement in the capsular bag and an adjustment must be made for a more anterior position in the ciliary sulcus. Without adjustment, this would cause a myopic shift in the refractive result that is proportional to the base power of the IOL. The higher the spherical IOL power, the greater the refractive change will be for a given millimeters of axial shift. A printable chart outlining the necessary reduction of IOL power from the capsular bag to ciliary sulcus can be found online.1 Many have found it helpful is to tape this chart to the wall in their operating room as a reference when decisions must be made quickly.
Here is a broad rule of thumb recommendation for modern IOLs:
| Capsular Bag IOL Power | Ciliary Sulcus IOL Power Reduction |
| +34.00 D to +28.50 D | -1.50 D |
| +28.00 D to +17.50 D | -1.00 D |
| +17.00 D to +9.50 D | -0.50 D |
| +9.00 D to -5.00 D | No change |
If optic capture is undertaken, with the haptics in the sulcus and the optic placed posterior to the preserved capsulorrhexis, there is generally little change in the IOL power required. A simple rule of thumb is that 0.50 diopters (D) is subtracted from the calculated capsular bag power from +28.00 D to +17.50 D and 1.00 D subtracted from the capsular bag power from +28.50 D to +34.00 D. For IOL powers of +17.00 D and below, most commonly there is no change to the IOL power required.
For reverse optic capture, with the haptics in the capsular bag and the optic placed anterior to the preserved capsulorrhexis, the strategy for power adjustment is basically the same. A simple rule of thumb is that 0.50 D is subtracted from the calculated capsular bag power from +28.00 D to +17.50 D and 1.00 D subtracted from the capsular bag power from +28.50 D to +34.00 D. For IOL powers of +17.00 D and below, most commonly there is no change to the IOL power required.
For iris suture fixation, the IOL power calculated for the capsule bag is generally used.
For intrascleral haptic fixation, or scleral suture fixation, the power calculated for the capsular bag is generally used. Entering the sclera 1 mm posterior to the external location of the scleral spur has been described as a consistent way to approximate the plane of the zonules for scleral fixation. An elegant video by Dr. Ike Ahmed, in Toronto, Canada, showing this can be accessed online.2 If the scleral entry is more posterior, an appropriate amount of additional IOL must be added. For an aphakic eye, or an eye in which the IOL is completely dislocated and the power is unknown, the axial length and the keratometry can be used with the Holladay 1 formula to arrive at an approximate IOL power.
When an IOL has begun to dislocate, and an IOL exchange with scleral fixation is required, how the axial length is measured will depend on the amount of IOL dislocation, which is most commonly inferior. If the IOL has moved outside the visual axis, the axial length is measured by optical biometry in the aphakic mode. If the IOL is unstable, but the optic remains within the visual axis, optical biometry should be carried out in a way that takes into account the optic material. Most biometers will allow the operator to select between several popular materials, such as acrylic, silicone, polymethylmethacrylate (PMMA), etc.
If the material of the optic is not known, some clues can be taken from its design. Single-piece IOLs with thick haptics are most commonly made of acrylic. Single-piece PMMA IOLs are easily recognized by slender haptics that are continuous with the optic material. The year in which the IOL was implanted may also be helpful. For example, IOLs implanted in the 1980s and the early 1990s are typically made of PMMA.
When undertaking an IOL exchange, if the current refraction and the power of the implanted IOL is known, the calculation can be carried out as an axial length independent exercise using the refractive vergence formula.3 Here, power is being added to or subtracted from an existing optical system. This approach is generally preferable to recalculating the replacement IOL power using a standard vergence IOL formula (eg, as would be done for a phakic eye) where the initial issue that led to an unanticipated outcome may be propagated forward.
A more sophisticated approach that can be used to account for IOLs having different lens constants is possible using the Barrett Rx formula, which can be used to calculate the exchange powers for both spherical and toric IOLs.4 For toric IOLs, the Barrett Rx formula provides a 3-part solution: the spherical power, a company-specific model number for the toricity, and a recommendation for the ideal meridian of placement.
With careful planning, even in the difficult situation of compromised or absent capsular support, very good results are possible for long-term refractive stability and patient satisfaction.
References
1. East Valley Ophthalmology. Calculating bag vs sulcus IOL power. https://doctor-hill.com/iol-main/bag-sulcus.htm. Accessed June 4, 2018.
2. Ahmed I. Scleral spur positioning for suture fixation . YouTube. https://www.youtube.com/watch?v=o909Mxvs4Ng. Published September 24, 2017. Accessed June 4, 2018.
3. East Valley Ophthalmology. Refractive vergence formula. https://doctor-hill.com/iol-main/piggyback.htm. Accessed June 4, 2018.
4. Barrett Rx formula—outcome analysis. https://www.apacrs.org/barrett_rx105/. Accessed June 4, 2018.
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