Marijuana Users More Prone to Infections After Knee, Shoulder Surgeries

WEDNESDAY, Oct. 19, 2022 (HealthDay News) -- Surgeons have long advised patients to stop smoking cigarettes for several weeks before their operations to lower the risk of complications. But what about weed?

New research has found reason for worry: Marijuana users had higher infection rates after minimally invasive knee and shoulder procedures. Patients also had higher rates of deep vein thrombosis (DVT) or potentially dangerous blood clots, though those risks were not statistically significant.

“We had a fairly large number of patients who were using it for various reasons, whether it was medical or non-medical,” explained study author Dr. Jason Strelzow, an assistant professor of orthopaedic surgery at the University of Chicago.

“There was really nothing out there to guide us in terms of whether we should be advising patients to avoid the use of it around the time of their operation or not,” he said.

What the researchers found suggests guidance is sorely needed.

Patients with marijuana dependence who had shoulder procedures had infection rates that increased from 0.7% to 1.7%, and DVT rates that increased from 0.2% to 0.4%. For those having knee procedures, the rates increased from just over 1% to 2.6% for infection and from 0.2% to 0.3% for DVT. While researchers also looked at rates of pulmonary embolism (a clot that travels to the lungs) for both types of surgeries, those stayed the same.

“The worry about things like smoking is really surrounding the nicotine and the effect of nicotine on blood vessels, and the effect of nicotine on the ability for wounds to heal, and we didn't think we'd see the same thing with marijuana,” Strelzow said. The increased risk they found “was a little bit of a shock to us, to be honest,” he noted.

In the study, the researchers used a national insurance claims database, identifying more than 1.1 million knee patients and nearly 748,000 shoulder patients. Nearly 22,000 of the patients had been diagnosed with marijuana dependence.

While past studies on marijuana have looked at its impacts on pain management or heart risk, its post-surgical impact wasn't clear.

But even a simple infection can lead to the need for antibiotics, repeat doctors' visits and continued monitoring, Strelzow said.

“On the other end of the spectrum, if you have a deep infection at the time of an operation, that can lead to failure of the operation, the need for repeat revision operations, and sometimes you actually can't cure those deep infections to the point where patients are on lifelong antibiotics and they have really lost the advantage of having whatever operation they were having in the first place,” Strelzow added.

Doctors are now trying to understand what it is about marijuana that might cause these problems.

Cannabinoids, one of the active ingredients in marijuana, do appear to interact with platelets, which allow blood to clot. And cells in the blood known as fibroblasts may interact with cannabinoids and interfere with the process of bone and tissue healing, Strelzow suggested.

“I think there's still a lot to be understood in that realm, but certainly the basic science would point to the fact that the cannabinoid itself, the active ingredient, likely has a role. Much like any medication that we take, it can have interactions that are unwanted consequences,” Strelzow said.

The researchers would now like to study marijuana's impact on bone, Strelzow said. Subsequent studies could also look at dose response and how long weed has an impact.

The study raises new questions, said Dr. Alan Reznik, an orthopaedic surgeon specializing in sports medicine, and chief medical officer of Connecticut Orthopaedics and member of the American Academy of Orthopaedic Surgeons.

“I think the evidence is pretty good that there is an increased risk,” Reznik said, but the association does not prove a cause-and-effect relationship.

And in terms of incidence, the number of those with increased risk of infection is still low, Reznik noted.

What can doctors do to guard against possible complications in their patients who use marijuana?

Surgeons already have a list of risk factors they look for in patients, so if marijuana is increasing the risk of infections and clots then a doctor might add precautions for a patient who uses marijuana that they would not have for someone who has no other risks and didn’t use the drug, Reznik said.

For infection risk, that might mean treating a patient with prophylactic antibiotics for minimally invasive knee and shoulder procedures, which are typically considered low risk for infection, Reznik said. In terms of increased clot risk, that might mean prescribing an anti-clotting drug.

“It is another thing we have to add to this list of things that we now start to look for, which we never used to look for before,” Reznik said.

The findings were presented Sunday at the American College of Surgeons annual meeting, in San Diego. Findings presented at medical meetings are considered preliminary until published in a peer-reviewed journal.

Strelzow suggested doctors could ask patients to give up marijuana for a period of time, as they do with tobacco smokers.

“If you are thinking about undergoing an elective procedure, certainly trying to abstain for that six weeks to three months would be at least a starting point to reduce your risk," Strelzow said. "I think that's an area we really need more data on."

More information

The U.S. National Library of Medicine has more on knee and shoulder arthroscopy.

SOURCES: Jason Strelzow, MD, assistant professor, orthopaedic surgery, University of Chicago; Alan Reznik, MD, FAAOS, orthopaedic surgeon and chief medical officer, Connecticut Orthopaedics, and associate clinical professor, surgery, Frank H. Netter MD School of Medicine, Quinnipiac University, Conn.; Oct. 16, 2022, presentation, American College of Surgeons annual meeting