Almost everything that goes wrong with feet goes wrong when you put weight on them. An arch drops when you stand on it. A heel drifts outwards when it takes your body weight. A bunion twists as the foot pushes off. Loose ligaments only show themselves when something is pulling on them.

Ordinary scans miss this in two different ways.

A normal CT gives excellent 3D detail — but it is taken lying down, with no weight through the foot. A deformity that is obvious when you stand can look almost normal on the couch.

Standing X-rays do at least capture your weight going through the foot, but they squash a 3D shape into a flat picture. Bones overlap each other. The measurements depend on exactly how the foot was placed and where the X-ray tube was pointing, so two X-rays of the same foot can give different numbers.

Weight-bearing CT was made to give both at once: 3D detail, taken while you stand.

You walk into a room with a scanner that has a low, open ring at floor level:

• You step inside and stand normally, usually with both feet in, holding a support bar to steady yourself
• The ring turns around your feet and lower legs — you stand still for less than a minute
• You step out

Nothing closes around your head or body. There is no tunnel, no injection, and no need to lie down. People who find MRI scans claustrophobic usually have no trouble with this one.

The scan takes minutes rather than the half-hour an MRI needs. In one study, the whole imaging session was quicker for cone-beam CT than for plain X-rays.

This is the question patients ask most often, so it is worth answering properly.

Cone-beam CT uses less radiation than a normal CT because it scans only a small part of the body, and the feet contain no organs that are sensitive to radiation. A study in the same patients found an average dose of 0.04 mSv for cone-beam CT of the limbs, against 0.13 mSv for a normal CT.

That is reassuring, but the dose is not zero, so the scan should be requested for a clear reason rather than as a routine.

A review of 78 studies of weight-bearing CT found that 56 of them reported findings that could not be obtained from standing X-rays. Some of the most useful examples:

• The foot slipping sideways under the ankle bone (peritalar subluxation). In a collapsing flat foot, the whole foot rotates out from under the talus — and this cannot be measured reliably on a flat picture.
• The true position of the heel when you stand, measured in 3D rather than guessed from a single view.
• Twisting of the first toe bone in a bunion. A bunion is a 3D deformity, and the twisting part of it cannot be seen on ordinary X-rays. It is linked to bunions coming back after surgery.
• Small amounts of looseness between the two shin bones. After a high ankle sprain, a small gap opening up between the tibia and fibula can be missed. Because weight-bearing CT can scan both ankles at the same time, the uninjured side can be used for comparison — in people with no injury the difference between sides is usually under about 9%, and a difference above roughly 19% suggests something is wrong.
• Small amounts of looseness in the middle of the foot (a Lisfranc injury) when you stand on it, to see whether the midfoot has become unstable after an injury.
• Bone rubbing on bone — whether the heel bone is actually touching the fibula in a collapsing foot, which shows up under weight and disappears lying down.
• How much room is left in a joint when you stand on it, showing where a joint is genuinely worn down under load, rather than where it only looks worn on a flat picture.
• A clear picture of the anatomy in complicated 3D deformities.

The point is not better pictures. It is different answers.

In a study of 72 patients with severe ankle arthritis following an injury, automatic 3D measurements were compared with the usual measurements taken from standing X-rays. The 3D measurements were more reliable — and 46% of patients had a second deformity below the ankle that was making up for a deformity above it.

In the same way, in bunion surgery, correcting the twist as well as the sideways angle has been linked to better function and fewer bunions coming back — and that twist is measured on a weight-bearing CT.

Weight-bearing CT is not needed for every foot or ankle problem. It earns its place in deformity, alignment problems, small amounts of looseness in a joint, and complex or repeat surgery.

Weight-bearing CT is genuinely useful, and it is only fair to be clear about what has and has not been proven.

The review of 78 studies rated the quality of those studies as moderate — an average score of 9.8 out of 24. A wider review of 129 studies published between 2013 and 2023 showed fast-growing interest, mostly in collapsing flat foot and bunions, but most of that research describes measurements and how repeatable they are, rather than proving that patients end up better off.

What we do know: weight-bearing CT measures foot and ankle alignment more accurately and more consistently than standing X-rays, and shows things those X-rays cannot. What we do not yet know: exactly which patients get a better result because of it. That research is still going on, including work from this unit.

So it is a better measuring tool, used when it is needed — not a scan everyone should have.

Mr Matthew Welck is a Consultant Orthopaedic Foot & Ankle Surgeon at the Royal National Orthopaedic Hospital, Stanmore, and Honorary Associate Clinical Professor at UCL. Using weight-bearing CT to assess 3D foot and ankle deformity is one of his research interests, and he uses it when planning complex reconstruction, repeat surgery and custom-made surgical guides. He sees patients across North and Central London.

Weight-bearing CT is a test, not a treatment — and any scan is only worth having if it changes what happens next. If you have a deformity, an alignment problem, or symptoms that plain X-rays have not explained, an assessment can work out whether a 3D scan would actually add anything.

To arrange a consultation with Mr Matthew Welck:

• Telephone: 07547 395 270
• Email/contact form: matthewwelck.com/contact
• NHS referrals: via your GP to RNOH Stanmore
• Private appointments: North and Central London

Urgent same-day appointments are offered where possible.

This article is for general information and patient education only. It is not a substitute for being assessed, diagnosed or treated by a qualified healthcare professional.