Generally, IOL are implanted after the cataractous lens is removed. However, in some cases, usually after some sort of traumatic injury, the lens of the eye must be removed but the eye is not stable enough to receive an IOL. In those cases the patient is either left aphakic or a second surgery is required at a later date to implant an IOL.
If you are going in for cataract surgery it would be wise to have an IOL, as being aphakic is quite a hassle. It also makes for very poor vision at all distances, atleast in my experience (although, I've got too many eye problems to really attribute my lack of vision to any one particular cause).
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