The NHS is failing to implement key guidelines issued by the National Institute for Clinical Excellence (NICE), a study suggests.
The NHS is supposed to use cement-based prostheses for hip replacements
NICE was set up by the government to decide what drugs and treatments should be available on the NHS.
But a study in The Lancet has found evidence to suggest that some of its recommendations are being ignored.
Andrew Dillon, chief executive of NICE, insisted the organisation was having an impact on the NHS.
The researchers looked at two pieces of guidance issued by NICE in 2000.
The first recommended that wisdom teeth should no longer be removed as a matter of routine.
The second recommended that only cement-based prostheses should be used for total hip replacements.
At the time, NICE said there was not enough evidence to support the use of non-cement based prostheses are effective.
The researchers chose these two pieces of guidance because there was data on how many of these procedures were being carried out before and after 2000.
That data showed that the number of routine wisdom teeth removals had been falling steadily since 1998. The researchers said there was no evidence the NICE guidelines had been a factor.
Their study also revealed that the number of total hip replacements using non-cement prostheses actually increased after NICE issued its guidelines.
The researchers said some doctors may not have been aware of the NICE recommendations.
They suggested the institute could be doing more to publicise its decisions.
"Our findings could reflect the type of dissemination methods used by NICE," said James Ryan, who used to based at the University of York.
"Use of more active dissemination methods could produce more effective uptake."
NICE suggested the study failed to paint an accurate picture since it looked at just two out of more than 250 clinical guidelines published over the past four years.
Andrew Dillon insisted the organisation was having an impact.
"Hundreds of thousands of patients have benefited from NICE guidance since the Institute was established in 1999," he said.
"We have been talking to people in the NHS and in our wider stakeholder community about the implementation of our guidance and we believe that by working effectively, with the NHS and with our national partners, we can help the NHS to reach even more people with effective treatments, more quickly."