New York (PTI): Why the lower eyelids sag with age? Well, the mystery has finally been solved -- thanks to a study which has revealed that fat expansion in the eye socket is the main culprit.

Earlier many theories sought to explain what causes the baggy lower eyelids that come with ageing. Now, a team at California University has cracked the puzzle, which they claim could help in the treatment of the condition.

"Orbital septum weakening or herniation-of-fat theory is what most plastic surgeons have been taught. However, our study showed there is actually an increase in fat with age, and it is more likely that the fat increase causes the baggy eyelids rather than a weakened ligament.

"There have been no studies to show that the orbital septum weakens," the study's lead author Dr Sean Darcy wrote in the 'Journal of Plastic and Reconstructive Surgery'.

In their study, the researchers looked at MRIs of 40 subjects (17 males and 23 females) between the ages of 12 and 80 -- they found that the lower eyelid tissue increased with age and that the largest contributor to this size increase was fat increase.

According to the researchers, fat excision should be a component of treatment for patients seeking to address this common complaint.

"A common treatment performed in the past and present is surgical excision of fat to treat a 'herniation of fat' -- meaning that the amount of fat in eye socket does not change but the cover that holds the fat in place, the orbital septum, is weakened or broken and fat slips out," Dr Darcy said.

However, added co-author Timothy Miller: "Our findings may change the way some plastic surgeons treat baggy eyes. Our study showed that a component of a patient's blepharoplasty procedure should almost routinely involve fat excision rather than these procedures."