• Posted August 18, 2022

Taking a Shot at Pain Relief After Knee Replacement

Researchers may have found a new way to help ease the pain of knee replacement surgery: infusing morphine directly into the shin bone.

The findings come from a recent study of 48 patients undergoing total knee replacement. The investigators found that giving a morphine injection into the shin bone during the operation controlled patients' post-surgery pain better than standard treatment alone.

It also lessened their reliance on potentially addictive opioid pain medications.

For now, the tactic is only being done at a limited number of medical centers. So people having their knees replaced in the near future are unlikely to have it as an option.

But there is no reason the approach could not be widely adopted, according to senior researcher Dr. Kwan "Kevin" Park, an orthopedic surgeon at Houston Methodist Hospital.

Total knee replacement, which is usually done to treat severe arthritis, is a common procedure, Park noted. A couple of decades ago, patients would spend days in the hospital afterward, receiving high-dose opioids to control the pain.

Things have changed since then, Park said: Pain control has improved, with the help of medications that are injected before and during surgery, for example. And patients now are often discharged on the same day as surgery.

But there's still room for improvement, he noted.

One goal is to reduce the need for opioid painkillers, such as oxycodone and hydrocodone, in the days to weeks after surgery. Besides their addictive potential, the drugs can have side effects like nausea and constipation.

Opioids, Park said, "still have a role. But we want to minimize their use, and get patients off of them as soon as possible."

For the new study, Park's team used a technique called intraosseous infusion, in which medication is directly injected into bone marrow. The researchers had already studied the infusions as a way to deliver antibiotics to the shin bone during total knee replacement; the point there was to reduce the risk of infection -- a rare but serious complication, Park said.

It worked. So the researchers wanted to see whether adding morphine to the infusion could help control patients' post-surgery pain.

In the study, 48 patients were randomly assigned to one of two groups. In one, patients received standard pain control and an antibiotic infusion into the shin bone; the other group was given the same, but morphine was added to the bone infusion.

All of the patients received general anesthesia plus regional nerve blocks, which help with post-surgery pain. They were also prescribed painkillers to use after surgery -- either acetaminophen (Tylenol) for milder pain, or opioids for more severe pain.

On average, the study found, patients given the morphine infusion reported less pain in the two days after surgery: Their pain ratings on a standard scale were 40% to 49% lower -- a difference that waned a bit but persisted through day nine. The patients also used fewer opioids in the two weeks after surgery.

An orthopedic surgeon who was not involved in the study said the results sound "potentially promising."

"It's always exciting to see new studies coming out about controlling post-op pain after total knee replacement," said Dr. Cynthia Kahlenberg, a hip and knee surgeon at the Hospital for Special Surgery in New York City.

Like Park, she said that despite strides made in pain control, "many patients experience more pain than we'd like after knee replacement surgery."

Kahlenberg said larger studies are needed to see if the current findings can be replicated. She also said that many hospitals, including her own, have begun using an additional regional nerve block during surgery -- one that was not employed in this study.

"I'm not sure how much of a difference this new technique would make if the (researchers) were incorporating both of the commonly used regional anesthesia blocks rather than just one," Kahlenberg said.

Park said further studies are planned.

For now, it's not completely clear why a single bone infusion of morphine, which has quick but temporary effects, helps control post-surgery pain for days. But Park said his team believes the blunting of the initial pain is key.

The study was recently published in The Journal of Arthroplasty.

More information

The American Academy of Orthopaedic Surgeons has more on total knee replacement.

SOURCES: Kwan "Kevin" Park, MD, orthopedic surgeon, Houston Methodist Hospital, Houston; Cynthia Kahlenberg, MD, MPH, hip and knee surgeon, Hospital for Special Surgery, New York City; The Journal of Arthroplasty, June 2022