MRI After Hip and Knee Replacement

The short answer

Two different questions get tangled together here, and separating them clears up almost all of the confusion.

So having a joint replacement does not stop you having an MRI. What it changes is how useful that MRI will be, and only for the joint itself.

Is it actually safe?

The theoretical worry is a reasonable one: an MRI machine generates a powerful magnetic field, and metal in that field can vibrate and heat up. That is where the concern comes from.

In practice, with a modern joint replacement, it is not an issue:

I tell my patients their implant is MRI-safe once it is well fixed to the bone, and I mean it. If you need a scan, have the scan.

The real problem: artifact

The genuine limitation is not safety. It is image quality.

An MRI builds its picture from a very precisely controlled magnetic field. Metal distorts that field, and the distortion shows up on the image as artifact — a large area of signal loss and blurring around the implant.

Metal suppression and MARS MRI

There are tools for this. Metal suppression sequences, often called MARS — metal artifact reduction sequence — are specialised scanner settings built to reduce the distortion.

They genuinely help, and if an MRI around an implant is necessary they are worth asking for specifically. But they reduce the problem rather than remove it. A MARS MRI of a replaced joint is still a compromised picture compared with a scan of a joint that has never been operated on.

Scanning the replaced joint itself

This is the situation where expectations need setting. Someone comes in with a knee replaced elsewhere, it hurts, and they ask whether they can have an MRI to find out why.

You can. But it is genuinely difficult to evaluate a joint by MRI after it has been replaced, and the scan often does not answer the question it was ordered for.

A painful replacement is usually worked up starting with X-rays, which show implant position and fixation clearly and are not affected by the metal in the same way — along with a careful history and examination, and laboratory testing where infection is a consideration. What the right investigation is depends on what the suspected cause is. There is more on the possible causes in knee replacement complications and hip replacement complications.

MRI somewhere else in your body

This is by far the most common version of the question, and the answer is reassuring.

If you need an MRI of your spine, your brain, your shoulder, or anything else, a hip or knee replacement does not stop you. The artifact from an implant is a local effect around the implant — it does not degrade a scan of a different region. People with joint replacements have spine MRIs constantly.

If you have been putting off a scan you need because you were worried the implant ruled it out, it does not.

What to tell the imaging centre

Always tell them. Not because it is likely to be a problem, but because it lets them screen you properly and adjust the scan.

This is routine for any imaging centre and it is a short conversation, not a hurdle.

Not the same as the pre-surgery question

Worth separating, because the two questions sound alike and have opposite answers.

This page is about MRI after a joint has been replaced. Whether you need an MRI before deciding on a replacement is a different question with a different answer — for most patients a properly taken X-ray is what decides it, and an MRI adds little. That is covered in do you need an MRI for a hip or knee replacement.