File image: The da Vinci Surgical System (sic) is a robotic surgical system made by the American company Intuitive Surgical. Picture From Wikipedia

Thousands of patients may be having unnecessary operations because any benefit they feel could be due to a placebo effect as much as the surgery, an expert has claimed.

Surgeon Andrew Carr's warning follows research showing procedures including keyhole knee surgery, arthritis operations and gastric balloons for the obese may all work largely because people expect them to.

Studies have shown that patients who have "sham" surgery, but believe they have been operated on, can recover almost – or completely – as well.

Professor Carr said surgeons need to discuss this with patients where there is evidence it is the placebo effect, and not the operation, which has taken away their pain.

The professor of orthopaedic surgery at the University of Oxford added: ‘Having to stop work, having to change your life, coming into hospital, all of these people dressed in blue with hats on, you're anaesthetised ...

"All of that, if we believe in placebo, is surely a set-up to create a phenomenal placebo effect.

"The correct thing has got to be to do the trials, and not to continue doing operations where we don't know whether or not there's a strong placebo component or an entire placebo component, because that means that tens or hundreds of thousands of patients are having unnecessary operations."

The surgeon made his comments following a talk at Cheltenham Science Festival called Is Pain All In The Brain? He is working with the speaker, neuroscientist Professor Irene Tracey, on the first trial of the placebo effect in shoulder surgery, which will examine a common operation to shave a spur from the shoulder bone.

In her talk, Professor Tracey said people wrongly dismiss placebos as "deception and fakery", adding: "You're manipulating people's expectations that this is going to produce some relief and so they expect it."

After the event Professor Carr said convincing surgeons of the placebo effect has been difficult. He said: "Understanding that a procedure you have done all your life is simply a placebo is tough for surgeons to take."

The experts do not suggest that transplants or amputations could be replaced with sham surgery to produce a placebo effect, which they say is seen in a minority of operations. However a review published in the BMJ medical journal found half of trials reported a placebo effect in a range of procedures.

Surgery for endometriosis, which causes painful periods and infertility in women, appears to work in part because people think it will.

So does an implant for migraines, as well as removing damaged bone in people with osteoarthritis. Research suggests a false operation has the same impact on pain.

The placebo effect is also seen in vertebroplasty, injecting liquid bone cement into the spine to hold vertebrae together.

Clare Marx, from the Royal College of Surgeons, said: "We know from studies in knee surgery that there are some procedures where a sham operation gives the same outcome at a year as the genuine operation.

"The surgical profession take research studies on board and, on the whole, when there is significant evidence, respond by changing their practice so that patients are not subjected to procedures from which they are unlikely to benefit."