It is a common concern that metal implants, such as hip or knee replacements, may prevent
you from having an MRI scan. In most cases, this is not true. Patients with modern joint
replacements can safely undergo MRI, depending on the materials used in the implant. It is
important to inform the radiology team about the implant before your scan.
Dr Jean de Villiers, a radiologist and director of SCP Radiology, answers some of the most
frequently asked questions, specifically around the process from referral to reporting in
radiology imaging.
What is Magnetic Resonance Imaging (MRI)?
MRI is a noninvasive imaging technique that uses powerful magnets and radio waves to
create detailed images of the body’s internal structures. Unlike Xrays or CT scans, MRI does
not involve ionising radiation and is used extensively to diagnose a wide range of conditions.
Because MRI uses strong magnetic fields, many patients ask whether it is safe to have an
MRI after a hip or knee replacement.
Can you have an MRI after a hip or knee replacement?
Yes, you can have an MRI scan on other parts of the body, as well as on the knee or hip
where the implant is. Although some older MRI scanners may not be compatible with certain
prostheses, the vast majority of MRI equipment in use today is safe and compatible with
modern hip and knee implants.
How safe is MRI if the implant is made of metal?
Most implants are made from titanium or cobaltchromium alloys. Although these materials
are metallic, they are not significantly affected by the magnetic field of an MRI scanner, nor
do they heat up during the scan. Many implants also contain hard plastic components, all of
which are designed to be compatible with MRI scanners. They are not attracted to the
powerful magnet in the same way as older or highly magnetic materials.
Dr de Villiers explains, “The vast majority of joint replacements used today are MRIsafe. The
key is that we know about them in advance, so we can adjust the scan if needed.”
What is the main challenge with MRI and an implant?
The main challenge is image quality. Metal can sometimes cause image distortion, known as
artefact, on MRI images. This may make it more difficult to assess structures close to the
implant. However, modern MRI techniques have improved significantly and can often
minimise these effects, allowing radiologists to assess surrounding tissues such as muscles
and ligaments, and to detect complications such as infection or loosening. MRI is often the
best imaging method for evaluating pain or complications after joint replacement surgery.
What happens if MRI does not produce clear diagnostic images?
In some cases, alternative imaging techniques such as CT or ultrasound may be
recommended, depending on the clinical question. However, MRI remains safe and highly
valuable for many patients with joint prostheses.
Are there implants that prevent you from having an MRI?
Certain implants and devices may be unsafe or require special precautions during MRI,
including:
Implanted pacemakers
Intracranial aneurysm clips
Cochlear implants
Certain prosthetic devices
Implanted druginfusion pumps
Neurostimulators
Bonegrowth stimulators
Any other ironbased metal implants
MRI is also contraindicated in the presence of some internal metallic objects such as bullets
or shrapnel, as well as certain surgical clips, pins, plates, screws, metal sutures or wire
mesh.
Having a hip or knee replacement does not automatically exclude you from having an MRI
scan. With modern implants and appropriate planning, MRI is both a safe and important
diagnostic tool. As technology continues to evolve, future developments are expected to
further enhance MRI compatibility with hip and knee implants, making it an even more
reliable tool for ongoing patient care.
It is crucial for patients to inform their healthcare providers about their joint replacement
before undergoing an MRI. This allows the medical team to adjust the MRI settings and take
appropriate precautions to ensure both safety and diagnostic accuracy.