Intra-ocular lens implants which are well-centered can provide better quality of vision for our cataract surgery patients. In the ideal case, the visual axis and the pupillary center would be the same and the IOL, particularly a muti-focal IOL with concentric diffractive rings, would be centered here. Most patients do not have this ideal situation and it has been proposed by Jack Holladay, MD, MSEE that we center the IOL midway between the visual axis and the pupillary center. Other surgeons such as Bob Osher MD and Michael Snyder MD have advocated using mitoic agents to constrict the pupil and alignment of Purkinje images to aid in centration.
Purkinje images are due to a change in the refractive index of materials, where most of the light passes through, but a small amount is reflected back. The interfaces where the refractive index changes are the anterior and posterior corneal surfaces and the anterior and posterior lens surfaces.
In this video, we will use the Purkinje images to assist in centration and alignment of the multifocal toric IOL. This has been shown in multiple studies like this and this, to be an effective way to judge IOL centration. In our operating rooms, ophthalmic microscopes with co-axial light illumination are particularly helpful in IOL centration when the patient is under topical anesthesia and can fixate on these lights.
The four Purkinje images are:
- P1 (or P-I) which is the reflection of the light from the anterior corneal surface. This is the easiest image to see and it is easily visible throughout the entire surgery.
- P2 (or P-II) is the reflection of the light from the posterior corneal surface. This is difficult to see in most cases and is not as clinically relevant.
- P3 (or P-III) is the reflection of the light from the anterior lens / IOL surface. It is the largest of the images and it can be somewhat indistinct.
- P4 (or P-IV) is the reflection of the light from the posterior lens / IOL surface. It is the only Purkinje images which is inverted. It is easier to see with an IOL compared to a crystalline lens due to the increased difference in refractive index.
click below to see this surgical video where the Purkinje images are explained: