WEDNESDAY, Sept. 23, 2026 (HealthDay News) — A knee or hip replacement can be a lifesaver for someone in chronic pain from arthritis, but they might have a hidden effect on the brain, a new study says.

Cobalt particles from a person’s replacement joint can travel to their brain and lodge there, researchers reported recently in the journal Acta Biomaterialia.

The cobalt particles did not appear to cause any cognitive decline among patients, but were associated with brain changes linked to Alzheimer’s disease, researchers said.

Titanium particles from replacement joints were also found in people’s brains, but were not linked either with cognitive decline or Alzheimer’s pathology, researchers said.

“Total joint arthroplasty remains one of the most successful and life-changing interventions in modern medicine,” lead researcher Robin Pourzal said in a news release.

However, the new findings “suggest that wear particles from the implant, particularly in certain hip replacements involving accelerated wear, may travel beyond the joint and warrant further study,” he said. Pourzal is an associate professor and director of implant materials analysis at Rush University in Chicago.

Researchers noted that the newer materials used in today’s joint replacements make it less likely that these particles will rub off the devices and wind up in the brain.

These particles, called "wear debris," are caused by friction and movement in the joint, researchers said in background notes. Corrosion also can cause the particles to separate from the joint.

For the new study, researchers analyzed autopsy data from 700 people who donated their brains for an ongoing study of memory and aging. The people had no dementia when they entered the study, and they took annual cognitive tests until their deaths.

The research team compared 229 people who had a total hip, knee or shoulder replacement against the rest.

“For years, we’ve known that particles can deposit in tissues surrounding the joint, but this is the first study to look at the particles deposited in the brain,” Pourzal said.

Because cobalt is more likely to be found in hip replacements, the team also considered the 146 people who got a new hip separately from the 83 with knee or shoulder replacements.

Cobalt levels in brain tissue were about 9% higher among people who received a hip replacement, and nearly 4% higher in people who got a replacement knee or shoulder, the study found.

Further, brains with higher cobalt loads were more likely to show signs of Alzheimer’s disease and have elevated levels of toxic amyloid beta proteins, researchers reported.

“Implants have a very specific combination of metals such as cobalt, and when they show up together in one particle in the brain in the exact same composition that the implant is, we know it can’t come from any other source,” Pourzal said.

More research is needed to better understand the link between implant particles and brain health, researchers said. 

More information

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

SOURCES: Rush University, news release, Sept. 18, 2026; Acta Biomaterialia, Sept. 18, 2026