Flatfoot (pes planus)

What it means

Flatfoot, or pes planus, describes a foot where the arch along the inner edge — the natural curve that normally lifts part of the sole off the ground — is lower than usual or absent altogether, so the whole foot tends to touch the ground when standing. It's not a single disease but a description of foot shape, and it ranges from a mild, flexible variation that some people are simply born with to a more significant, sometimes rigid, change that develops over time.

Why it appears on a CT or MRI report

Flatfoot is often noted as an incidental observation on imaging done for another reason, or it may be the specific focus of a scan when someone has foot or ankle pain. Reports will typically describe whether the flattened arch is flexible (the arch reappears when not bearing weight or on tiptoe) or rigid, and whether there's any associated finding that might explain a newly developed flatfoot, such as tearing or degeneration of the posterior tibial tendon — a key tendon that helps hold up the arch — along with any related joint arthritis, ligament laxity, or bone alignment changes in the midfoot and hindfoot.

What it usually means

Flexible flatfoot present since childhood is extremely common — it affects a substantial share of the population — and in most people it causes no pain and needs no treatment at all; the foot simply has a naturally lower arch and functions well. A flatfoot that develops later in life, particularly in adults over 40, is a different story and is most often linked to gradual wear and weakening of the posterior tibial tendon, sometimes called adult-acquired flatfoot. As that tendon weakens, the arch slowly collapses further, weight distribution across the foot changes, and pain can develop along the inner ankle, the arch itself, or spread to the outer foot as the deformity progresses. Other causes include arthritis, ligament injury, or nerve conditions affecting the small muscles that support the arch.

When to follow up

A flexible, lifelong flatfoot without pain doesn't usually need any specific follow-up — supportive shoes or an arch-support insole are enough for anyone who wants extra comfort. It's worth seeing a doctor or podiatrist if a flatfoot is new, is getting more pronounced, or comes with pain, swelling along the inner ankle, or a foot shape that seems to be changing, since catching posterior tibial tendon problems early allows for bracing and physical therapy that can prevent further collapse. Sudden inability to stand on tiptoe on one foot, or rapidly worsening deformity, deserves a prompt evaluation.

A plain-language way to picture it

Think of the arch of the foot like a suspension bridge, held up by a sturdy supporting cable running along its underside. Some bridges are simply built with a lower, flatter arch from the start and hold up their traffic just fine. Others start with a normal arch, but if the main supporting cable slowly frays and stretches over years of load, the bridge itself gradually sags lower and lower, and the uneven strain can start to show up as cracks and wear at the points bearing the extra weight.