People with sprains are usually told to keep mobile
People with a sprained ankle recover faster if they are given a plaster cast, British researchers say.
A study of 600 patients found that a 10-day below-knee cast is more effective than standard treatment with a tubular bandage.
The findings, published in The Lancet, are at odds with general medical opinion that the best thing for a sprained ankle is to keep it moving.
Up to 1.5m people a year in the UK go to A&E with a sprained ankle.
The injury accounts for 3-5% of all UK emergency department attendances and varies from minor stretching of the ligament to a complete tear.
It is commonly treated with ice, elevation, tubular compression bandage and advice to exercise.
But the researchers found that a tubular bandage was consistently the worst treatment.
In patients with severe ankle sprain attending eight emergency departments in the UK, a 10-day below-knee cast was associated with a speedier recovery in terms of ankle function, pain, symptoms and activity three months later.
The second best treatment was an Aircast brace which limits movement of the ankle.
The Bledsoe boot - another type of brace which is designed to restrict movement of the ankle but allow users to walk - was no more effective than a tubular bandage and neither can be recommended as a treatment for these injuries, the researchers said.
By nine months there was no difference between the groups.
A plaster cast costs £16.46, the researchers said, compared with £39.23 for an Aircast brace, £215 for the Bledsoe and £1.44 for a tubular compression bandage, the researchers said.
Professor Sallie Lamb, an expert in rehabilitation at the University of Warwick, said the finding that immobilisation is the best strategy is "contrary to popular clinical opinion".
"Tubular compression bandage, which is currently the most commonly used of all the supports investigated, was, consistently, the worst treatment.
She said early recovery is very important to patients, so although there is little difference at nine months, the speed with which people are able to get back to normal activities is an important finding.
"I think plaster should be given as standard management."
John Heyworth, president of the College of Emergency Medicine, said the study was "practise-changing, high quality research".
"What we do with these common problems, is we look for the best evidence and up until this paper, the evidence suggested that early mobility provided a better outcome and this provides some good evidence that immobilisation can provide greater benefits."
Dr Martin Shalley, A&E consultant at Birmingham Heartlands Hospital, said they tried to avoid immobilising the ankle because of the risk of deep vein thrombosis.
"That has to be balanced and we can discuss the pros and cons with the patient and work out the best treatment programme for them."