Saggy: Demi's knees need work

She is thought to have spent more than £200,000 on plastic surgery and fitness regimes to keep her looking young.

But while Demi Moore may be successfully holding back the years on most parts of her body, there are two things she can't hold back ... her sagging knees.

While the rest of the 40-year-old actress's physique looks trim and taut, they are distinctly drooping.

And floppy skin around the knees is often a giveaway for those who have undergone cosmetic operations, says an expert.

'Knees are one of the most difficult areas to do,' explained plastic surgeon Alex Karidis. 'If you suck up the fat around the knees, you are left with excess skin. It's a bit like if you blow up a balloon and then deflate it - you can see that the rubber has been extended.'

Mr Karidis, of the Hospital of St John and St Elizabeth in North London, added: 'The problem is that if you cut the skin out, you will be left with large scars around the knees.

'The same goes if you try a thigh lift. In that case, you will be left with large ugly scars at the top of your legs - it is part of the trade-off as far as plastic surgery goes.

'As a result, it is one of the most obvious telltale signs that someone has undergone plastic surgery. Of course, some people are just blessed with good knees.' Miss Moore reportedly underwent the plastic surgery late last year to regain her slimline body. Among the procedures she was said to have had were liposuction to her hips, inner and outer thighs and stomach, estimated to have cost £16,000.

She is also reported to have had a £9,500 operation to reduce her breast implants - which she had inserted at a cost of £11,500 for her film Striptease - while at the same time having her breasts lifted.

Cosmetic surgeons also believe she has had a £3,600 brow lift, spent £10,000 on a facial skin peel with chemicals, a course of £200 collagen injections and had her teeth veneered and whitened by lasers for £6,200.

A long-term diet and fitness routine, including nutritionist, personal trainer, yoga instructor and kick-boxing champion, has cost around £165,000 - bringing the total up to £220,000.

Miss Moore's resulting toned-up looks have helped turn her life around, not least in the shape of her toyboy lover Ashton Kutcher.

At 25, the comedy star is some 23 years younger than her former husband, action movie hero Bruce Willis. But while Kutcher thinks she is the bee's knees, the actress may be ruefully reflecting that her own knees are literally something of a let-down.