How's focusing maintained post cataract surgery?

ARU optometrist Keziah Latham answered this question for us...

Keziah Latham - The cataract affects the lens of the eye which sits just behind your pupil, and the lens itself is as you can imagine, a little bit like a grape. It's got a skin called a capsule and then it's full of the sort of the pulp of the grape if you like which are lens fibres. And the lens is a kind of an unusual thing in that it grows throughout life, you're continually producing new lens fibres at the edge of the lens and shoving the older fibres into the centre. Now the lens is transparent because of the regular arrangement of those fibres and we want it to be transparent so that it can let the light through to the back of the eye.

But as time goes on and you increase the number lens fibres in there, it pushes them altogether, changes the regularity, and that can lead to clouding of the lens which is a cataract. So yeah very common operation to remove that cataract. We peel away the little capsule at the front, and essentially remove all of the contents of the lens with a little sort of surgical hoover type thing and then you will put in an implant, a replacement plastic lens, to replace the power of the eye. Now, the lens in anybody under 40 or so is able to change shape a little bit, because this capsule on the outside of it is elastic, your ability to do that does decline with time. That's why we all need reading glasses by a certain age, and that again is because the lens is becoming much stiffer because it's not able to change shape as there's so many fibres in that lens.

So, by the time most people have got a cataract they're not able to change the shape of their lens anymore. So, when the lens is removed, there's a little plastic implant that is inserted kind of like a ship in a bottle into the remaining capsule of the lens. Now, in the majority of cases that is a fixed focus lens so it will be designed on the basis of measurements of the eye to give you a particular focal point, so either to be focused for distance and then you need to put reading glasses on afterwards, or possibly to set you up for reading and then you have to put distance glasses on.

Chris Smith - Can you do one of each so you can have a long one and a one short one? 

Keziah Latham - So you can get around that, you can go for monovision, like that, you can do that with contact lenses as well, and give one eye distance and one eye reading.

Chris Smith - And laser? Laser correction can do that as well. You can have one eye set for close work and one for longer.

Keziah Latham - You can do and that does cover both bases, it does mess about with your binocular vision a little bit too, so it might hamper your depth perception a bit. But the other thing that you can do with intraocular lenses, the implants that get put in, is you can have multifocal intraocular lenses. Not on the NHS, but you can have multifocal lenses, and they work by diffraction so that the single piece of plastic produces two simultaneous focal points, one for distance and one for reading.

Chris Smith - Does that get light going through it in different positions according to where you're looking then? So do you have to almost learn like you do with by bifocal lenses, you have to almost learn to use them?

Keziah Latham - No it's not quite like that, where you've got sort of an area of the lens is distance and an area is reading. You have little circles engraved on the lens so that the light that falls in between the circles might be focused at one distance, say to give you distance vision, but the light that passes through the annular ridges on the lens will give you a different focal point to be focused for reading. So you get this sort of simultaneous two things in focus at once. But it can give you slightly less crisp vision, it can affect your ability to detect low contrast, so because it's trying to do a number of things at once, it's not perfect at doing all of them.

Chris Smith - So how is the fine focusing in a lens that can't change its shape achieved then? Is that purely by just moving your pupil wider and smaller in order to achieve a pinhole effect, so you can get that fine focusing and compensate that way, as you get older? Is that happens?

Keziah Latham - To some extent yes, the older pupil is a little bit smaller so you do get more of a depth of focus because if you've got a smaller aperture, you get a wider depth of focus. But the short answer is that you will get your distance correction and then you will need a separate correction for reading, you can't have both at the same time.

Chris Smith - So there'll be a sweet spot at which your vision works best and then either side of that's a compromise?

Keziah Latham - Either side of that yep but that's what specs are for!