An intra ocular lens may be placed in the space between the cornea and the iris, be attached to the iris or be placed behind the iris and pupil. Nowadays lens implants are usually placed behind the iris and pupil. The fluid filled space between the cornea and the iris is called the anterior chamber. Lens implants designed to be positioned here are therefore known as anterior chamber intra ocular lens implants (A/C IOL’s).
These implants are supported in position by S-shaped haptics (supporting limbs) that rest in the periphery of the anterior chamber where the root of the iris and the dome of the cornea meet. This is called the anterior chamber angle and this type of lens implant is therefore said to be ‘angle supported’. When using an anterior chamber lens implant it is usually necessary to make a small hole in the periphery of the iris. This is called a peripheral iridotomy (or PI).
This is because the optic of the lens, which sits immediately in front of the pupil, may obstruct the flow of aqueous fluid through the pupil. This could cause a rise in the fluid pressure within the eye (glaucoma). The iridotomy prevents this from happening. An intra ocular lens implant may be clipped or hooked to the iris. This type of lens was abandoned in the early 1980’s due to an unacceptably high rate of complications.
A newer design has more recently been devised but in the UK it is at present rarely used in the context of cataract surgery. With current techniques of cataract surgery the intra ocular lens implant is usually placed behind the iris. The “optic” (the true focusing part of the lens implant) sits just behind the pupil. This is very close to the position of the natural lens of the eye (which has been removed). The small space behind the iris is called the posterior chamber. Lenses designed for implantation into this space are therefore called posterior chamber intra ocular lens implants. Eye surgeons refer to them as P/C IOL’s.
One of the advantages of phacoemulsification (and extra capsular cataract surgery) is that part of the lens capsule is preserved. This capsule is a thin membrane like structure that surrounds the natural lens of the eye. With these particular surgical techniques the opaque natural lens (the cataract) is removed from within the capsule. The capsule remains as an empty pocket. It can then be used to support and fixate a posterior chamber lens implant. Without this support the lens implant would fall into the vitreous gel that fills the main cavity of the eye and would end up at the back of the eye.
The posterior chamber lens implant can either be placed onto the remaining capsule or actually slipped into the capsular pocket. Placing the implant onto the capsule is rather like resting the lens onto a mini trampoline. The supporting legs (haptics) of the lens implant rest in the cul-de-sac between the back of the iris and the ciliary body.
This recess is called the ‘ciliary sulcus’. Posterior chamber lens implants resting on the surface of the capsule are therefore said to be sulcus fixated, i.e. held in position by haptics resting in the ciliary sulcus. Alternatively the lens implant may be placed inside the capsular pocket. Eye surgeons call this the ‘capsular bag’. A lens held in place in this position is said to be ‘in the bag’ or capsular bag fixated. This is the probably the best place for a lens implant as it is the exact position of the natural lens of the eye.