Costly hip and knee replacements could be averted by giving patients a "breakthrough" drug costing less than £1 a day, trial results suggest.
The treatment is said to be the first to slow the progress of osteoarthritis (OA), the wear-and-tear disease that destroys joints.
Strontium ranelate, marketed as Protelos, is a powder that is mixed with water to make a lemon-flavoured drink.
It is already used to prevent fractures in post-menopausal women with the brittle bone disease osteoporosis.
The new findings, presented at the European Congress on Osteoporosis and Osteoarthritis (ECCEO) in Bordeaux, France, showed that Protelos reduced deterioration of knee joint cartilage in a group of OA patients by a third over three years.
It also led to a significant reduction in pain and improved day-to-day mobility.
An estimated 8.5 million people in the UK are affected by OA, which results in the gradual wearing away of cartilage and bone.
The painful and disabling condition is especially common in older people and associated with long-term degradation of the joints as well as mechanical injury.
Some "rapid progressors" are predisposed to the disease and more likely to develop symptoms.
Until now, pain management has been the only available treatment short of major surgery. Each year around 140,000 hip and knee replacements are performed on the NHS in England and Wales at a cost of more than £1 billion.
Many of these operations could be delayed or avoided by taking Protelos, potentially saving the taxpayer millions of pounds, the results suggest.
Trial lead investigator Professor Cyrus Cooper, from Oxford and Southampton universities, said: "This is a major breakthrough. Osteoarthritis is a painful and debilitating condition, and for over 20 years we have been searching for a treatment that would allow us to alter the course of the disease, rather than just manage the symptoms.
"The results today ... could totally change the way we treat osteoarthritis. For the first time we have a treatment that can slow the development of this debilitating disease and could reduce or even eliminate the need for expensive and painful joint replacement surgery."
The international Phase III trial involved 1,683 mostly female OA patients with an average age of 63. They were randomly treated with either 1g or 2g daily doses of Protelos, or an inactive placebo, and their condition checked at yearly intervals for up to three years.
The best results were seen with the 2g dose. For every three years of treatment, Protelos slowed progression of OA by an amount that would be expected in one year, the research showed.
Prof Cooper said the drug produced an almost 50% reduction in the frequency of rapid progression. Patients who are rapid progressors have a five-fold increased risk of needing joint replacement.
"You would expect it might have an impact on joint replacement rates," said Prof Cooper.
Because Protelos already has a proven safety record it could rapidly be relicensed for the treatment of OA. French manufacturer Servier is understood to be applying for an altered use licence from European regulators.
The drug's ability to stem cartilage loss and slow OA progression was unexpected because osteoarthritis and osteoporosis are completely different diseases.
Pre-clinical clues from lab and animal studies led researchers to suspect it might be effective in OA.
It is thought the drug may exert an influence on stem cells that generate bone and cartilage.
Judith Brodie, chief executive of the charity Arthritis Care, said: "Many people with osteoarthritis live for years in great pain before eventually needing joint replacements.
"This is the first study to show benefit from strontium ranelate so it is early days, but if there is a treatment which can hold back the disease, delay knee replacements and help people to live more active lives for longer, then it will be good news for the millions of people in the UK with this debilitating condition."
Protelos costs £27 per month, or 90p per day.
Professor Alan Silman, from the charity Arthritis Research UK, described the findings as an "exciting development".
He added: "Although it doesn't reverse osteoarthritis, it slows down its progression in terms of X-rays, and appears to have a beneficial effect on pain, although the extent of this is still unclear.
"This the first time that a drug has been shown to slow progression of osteoarthritis, as existing treatments for osteoarthritis just focus on symptoms."