What a Knee Scan Actually Shows

Most people arrive at an appointment with at least one scan, and a reasonable expectation that it holds the answer. Scans are genuinely useful — but each one answers a narrow question, and none of them answers the question patients most want settled, which is what to do next.

This page explains what each type of imaging shows, what it misses, and why a report can read alarmingly while the knee it describes is doing fine.

What each scan is for

X-ray

The first test in almost every knee, and for arthritis usually the only one needed.

An X-ray shows bone. It does not show cartilage directly — cartilage is invisible on a plain film — so cartilage loss is inferred from the gap between the bones. As cartilage thins, the gap narrows. That narrowing, along with bone spurs, hardening of the bone beneath the joint surface, and the overall alignment of the leg, is what an X-ray contributes.

One detail matters more than most people realise: the film should be taken standing up. Joint space narrowing only becomes properly apparent when the joint is loaded. A knee X-rayed lying down can look substantially better than the same knee X-rayed standing, because without body weight through it the gap opens up. If your films were taken lying down, that is worth mentioning.

MRI

An MRI shows soft tissue — the cartilage itself, the menisci, the cruciate and collateral ligaments, the tendons, and changes within the bone that an X-ray cannot register.

It is the right test for a suspected meniscus tear, a ligament injury, or a picture that does not add up. It is often not necessary for straightforward osteoarthritis, where a standing X-ray read alongside the examination already answers the questions that determine treatment.

MRI’s sensitivity is also its difficulty. It finds a great deal, including a great deal that is not causing trouble.

CT

CT shows bone in three dimensions, with detail no other test matches.

In this practice its main use is planning. For a knee replacement planned on the Mako system, a CT scan taken beforehand is used to build a three-dimensional model of your knee, so implant size and position are decided as a virtual pre-operation before any incision is made. The VELYS system is imageless and maps the knee in theatre instead, so no planning CT is required. Which system is used depends on the hospital and theatre list.

CT is also used for complex fractures and for assessing bone stock in revision surgery.

Ultrasound

Ultrasound images tendons and bursae in real time, and unlike the others it can watch a structure move. It is the appropriate test for a suspected tendon problem or a bursitis, and it can guide an injection precisely into the right space.

It cannot see inside the joint properly, so it has little role in assessing arthritis.

The thing worth understanding about scan findings

Here is the fact that reframes most imaging reports:

Arthritis shows on the X-rays of more than half of people over 65. Only about one in seven of those have symptoms worth treating.

The same holds on MRI, more strongly. Meniscal tears, cartilage defects and bone marrow changes are common findings in people with no knee pain at all. Scan an asymptomatic fifty-five-year-old and you will usually find something to report.

This has two consequences worth taking seriously.

A scan finding is not automatically the cause of your pain. A report listing a degenerate meniscal tear, a cartilage defect and a Baker’s cyst is describing a knee that has aged. Whether any of those is what hurts is a separate question, answered by the examination and the history rather than by the scan.

A frightening-sounding report is not a prognosis. Words like “severe”, “full-thickness” and “extensive” describe appearance, not disability. Plenty of people with severe changes on paper walk comfortably, and some with modest changes are significantly limited. What the knee stops you doing is the measure that matters.

This is also why a scan cannot tell you whether to have a knee replacement. It establishes which compartments are worn — which determines whether a partial or total replacement is even an option. It does not establish whether an operation is the right choice for you. Do I Need a Knee Replacement? works through that separately.

When more imaging genuinely changes something

A scan is worth doing when the answer would change the plan. In practice that means:

• Deciding between a partial and a total replacement — which needs the state of all three compartments and the cruciate ligaments established, so CT or MRI is often added to the X-ray
• A suspected meniscus tear or ligament injury, particularly a knee that locks, catches or gives way
• A knee that does not fit the pattern — pain out of proportion to the films, rapid deterioration, night pain that is constant and unrelated to position
• Planning robotic surgery on the Mako system, where the CT is the plan
• Suspected infection or a fracture, where imaging is part of urgent assessment

And when it does not:

• Repeating an MRI that was done recently and has not changed
• Imaging to “check” arthritis already established on a standing X-ray, when the treatment question is about symptoms rather than anatomy
• Scanning a knee before trying the treatment that would be recommended regardless

What to bring to your appointment

Bring the images, not only the reports — on a disc, or the access details for the radiology practice’s online portal. A report is one radiologist’s summary; the images allow the films to be reviewed directly, which occasionally leads somewhere different.

Also bring any previous knee imaging you have, even if it is years old. Comparing a current film with an older one shows the rate of change, which is often more informative than either film alone.

If you are not yet sure what is wrong with your knee, Is It Knee Arthritis or Something Else? works through the alternatives, and Where Your Knee Hurts covers what the location of pain suggests.

Appointments can be made on (08) 6332 6365 or through the contact page. A GP referral is usually needed to claim the Medicare rebate for a specialist consultation.