Astigmatism when applied to the cornea means that the cornea is not spherical like the top of a ball, but oblong like the back of a spoon. Lenticular astigmatism means that the natural crystalline lens of the eye is not perfectly symmetrical front and back, but the crystalline lens is misshaped similar to the back of the spoon example.
Astigmatism, no matter the source, causes light passing through the cornea to focus at more than one location on the retina, causing blurred vision and sometimes ghosting. Astigmatism is a refractive error that can be corrected with glasses, contacts, or refractive surgery like Lasik, All-Laser Lasik, LASEK, PRK, and CK, but there are important limitations on the correction with refractive surgery.
P-IOLs do not have astigmatic correction available, so a P-IOL cannot correct astigmatism in the cornea nor in the lens.
It is currently impossible to correct lenticular astigmatism at the crystalline lens without removing the lens and replacing it with a new plastic or silicone intraocular lens (IOL). It is impossible to just fix the crystalline lens; the crystalline lens must be replaced. This replacement is the same process as cataract surgery, but when there is no cataract it is called Refractive Lens Exchange (RLE). Due to other considerations, RLE is not always appropriate to correct purely refractive errors.
IOLs do not have astigmatic correction available, so an IOL cannot correct astigmatism in the cornea, but can resolve astigmatism in the lens by replacement.
Corneal based refractive surgery is able to correct corneal induced astigmatism at the source. It is also possible to correct for lenticular astigmatism by "overcorrecting" in the cornea to accommodate the lenticular astigmatism. In most refractive surgery cases, the desire is to correct all astigmatism no matter the source. This provides the patient with the most clear vision possible.
Think of the eye as having two lenses, one is the cornea, the other is the crystalline lens. If the lens has an error of, say -1.00 diopter astigmatism, then the cornea could be changed +1.00 diopter to provide zero net astigmatism and a clear view. This is like saying that if you can't raise the bridge, lower the water.
If the patient has lenticular astigmatism and the lenticular astigmatism is corrected at the cornea, then when the crystalline lens is replaced with an IOL for RLE or cataract surgery, the corneal correction that includes the correction for the lenticular astigmatism becomes an "overcorrection" at the cornea.
There are many ways to accommodate this situation and much depends upon the individual patient's circumstances. Obviously, someone with significant lenticular astigmatism could simply not have refractive surgery. If the patient is nearing the age of cataract development or is fully presbyopic, RLE is a possible option. The patient could have only the astigmatism in the cornea removed, but this may provide poor vision because the astigmatism in the lens may cause blurry vision or ghosting. The patient could have both corneal and lenticular astigmatism corrected at the cornea, and then have the cornea corrected again after RLE or cataract surgery.
Lenticular astigmatism is easily diagnosed before surgery and is just one of the many considerations that must be evaluated for every patient. A patient with lenticular astigmatism should discuss the situation with his or her doctor and come to a decision that best fits the individual patient's needs and desires.
