Every incision that is made in the cornea has an effect on its shape and therefore the astigmatism. For a small sub-1 mm paracentesis incision, the effect is negligible. However, for our main phaco incision the effect is significant. And in many cases we will intentionally make our phaco incision at the steep axis in order to treat the pre-existing astigmatism and sometimes even pair the phaco incision with a second incision on the same axis. Limbal Relaxing Incisions (LRIs) are additional incisions which are made in the peripheral cornea, near the limbus, with the sole purpose of treating corneal astigmatism.
LRIs are not full-thickness, but rather they are about 80% depth (or more). The purpose is to allow the corneal curvature to relax at a specific meridian while not entering the anterior chamber. LRIs are traditionally made with either fixed-depth steel blades, adjustable-depth diamond blades, or with a femtosecond laser.
Because of corneal coupling, when we flatten one meridian of the cornea, we will steepen the other meridian (90° away). For example, if the corneal values are 45.00 x 180 / 44.00 x 90 (total of 1 D of astigmatism, steep at 180°) and then we do LRIs at the 180° meridian, we can end up with values of 44.50 x 180 / 44.50 x 90. Note that now the cornea has no astigmatism since both axes measure at 44.50 and also note that the average K value before surgery is the same as the average K value after surgery, so no adjustment to IOL calculation of spherical power needs to be made.
I recommend starting with the simple nomogram proposed by Kevin Miller MD:
- use 1 clock hour of paired incisions for 1 diopter of corneal astigmatism.
- note that 1 clock hour is 30°
- vary this with the patient age;
- do a little more in younger patients (<60)
- do a little less in older patients (>80)
- take into account the effect of your phaco incisions
- if operating temporally, more LRI arc length for WTR, less for ATR
If you’re looking for more precision, I recommend the Nichamin Age & Pachmetry Adjusted (NAPA) nomogram: LRI_Nomogram_Nichamin
Two ways to address corneal astigmatism
Corneal astigmatism can be treated at the time of the cataract surgery to improve the visual outcome and decrease reliance on glasses. The two primary ways of surgically treating astigmatism are: (1) treat it at the source by decreasing the corneal astigmatism, and (2) offset it by implanting a toric IOL which will neutralize the astigmatic effect.
Corneal relaxing incisions treat the astigmatism on the cornea itself by flattening the steep meridian so that the cornea becomes more spherical. The corneal power in that meridian is slightly weakened by creating partial depth incisions either with a blade or a laser. When these incisions are placed in the peripheral cornea we call them astigmatic keratotomies (AKs) but when they’re closer to the limbus (but still in the cornea), they are often referred to as limbal relaxing incisions (LRIs).
Limbal Relaxing Incisions versus Toric IOLs
LRIs (or AKs) should be used in eyes with corneas that have a modest amount of astigmatism but are otherwise normal. These LRIs are usually done at the time of cataract surgery and they tend to work well to treat 0.5 to 1.0 D of corneal astigmatism. They can be used for higher degrees (up to 2.0 D) but they tend not to be as accurate at this level of astigmatism.
- I favor LRIs for 0.5 to 1.0 diopter of corneal astigmatism
- I prefer toric IOLs for more than 1.0 diopter of astigmatism.
The femto AKs have the benefit of laser precision so that the depth of the AK can be made exactly at 80% or another level for the entire length of the incision. Since the femto incision does not automatically open like a diamond blade incision would, we can titrate the AK effect by opening it in stages, even during the post-op period. So, yes, the femto AKs can be better than the manual LRIs but this is only up to 1.0 D of astigmatism. After that, the toric IOLs are far better than femto AKs. Also remember that if the cornea is thin, asymmetric, or irregular then AKs should be avoided.
Do I treat the corneal astigmatism or the refraction astigmatism?
Remember that corneal astigmatism + lens astigmatism = total refractive astigmatism for the eye.
For eyes with cataracts which are about to undergo cataract surgery, we know that we will be removing the human crystalline lens. Therefore, we are concerned with just the corneal astigmatism and we treat that value with either LRIs or toric IOLs.
For eyes with no cataract (either a clear crystalline lens or a clear man-made IOL), we are concerned with the total refractive astigmatism that we determine from the refraction. If the refraction is -0.50 + 1.00 x 180, then we want to make an LRI at the steep 180 degree axis in order to decrease the astigmatism. Note that the total spherical equivalent will not change.
LRIs often have a variable effect because so many factors influence their effect:
- corneal thickness (pachymetry)
- patient age (this is hugely important)
- corneal diameter (white-to-white)
- corneal elasticity
- depth of AK incision
- arc length of AK incision
I recommend that you try using LRIs in your practice. They heal quickly since the epithelium closes within a day or two. The refractive effect starts immediately but may take a week or two to stabilize.
click below to see a video of a complete cataract surgery with paired LRIs at the end