A flat foot that develops in adulthood is usually caused by a worn, weakened tendon on the inside of the ankle, called the posterior tibial tendon. It’s more common in women and in people over 40, and caught EARLY, most people manage it without surgery.
This tendon runs behind the bony bump on the inside of your ankle and under the foot, and it helps hold up the arch. When it’s overloaded and starts to fail, you feel pain along the inside of the ankle. Over months to years, the arch can sink and the heel can roll in.
Treatment depends on how far it has gone.
Early on, insoles, a brace and a strengthening program are the main treatment. Surgery is kept for feet that stay painful despite that, or that have become stiff. Recovery from surgery takes 6 MONTHS or more.
What it feels like
- Pain on the inside of the ankle and arch: along the line of the tendon, worse with walking or standing for long periods
- Swelling: behind or below the inside ankle bone
- The arch dropping: one foot looks flatter than the other, and shoes wear unevenly
- The heel rolling in: seen from behind, more of the little toes show on the affected side. Clinicians call this the “too many toes” sign
- Trouble rising onto tiptoe: standing on the affected leg alone, lifting the heel is hard or impossible
- Outer ankle pain later on: as the foot flattens, bones on the outside of the ankle can start to press together
Who gets it
- Age and sex: more common in women and after the age of 40
- Health: extra body weight, diabetes and high blood pressure all raise the risk
- Activity: high-impact sport, and years in shoes that give the foot little support
- Genes: some people are simply more prone to it
If the pain is under the heel, worst with your first steps in the morning, see plantar fasciitis. If it’s at the back of the heel, see Achilles tendinitis.
The stages in plain terms
- Stage 1, sore tendon, normal arch: the tendon is inflamed or starting to wear, but the foot still looks normal and you can rise onto tiptoe on that leg
- Stage 2, flexible flat foot: the arch has started to drop and the heel rolls in, but the foot can still be moved back into shape by hand. Single-leg tiptoe is hard or impossible
- Stage 3, stiff flat foot: the foot has set in its flattened position and can no longer be corrected by hand
- Stage 4, the ankle is involved: the ligament on the inside of the ankle gives way and the ankle itself starts to tilt
Surgeons now often call this progressive collapsing foot deformity and use a newer grading system. The idea is the same: a flexible foot has more options than a stiff one.
The earlier the arch gets support, the more options you keep.
What helps
- Arch-support insoles: off-the-shelf arch supports are a reasonable start. Custom orthotics are often used when the heel rolls in a lot. They ease pain and support the foot, but they don’t rebuild an arch that has already dropped
- A brace: a lace-up ankle brace, or a custom brace that holds the foot and ankle, takes load off the tendon. In a bad flare, a walking boot for a few weeks lets it settle
- Strengthening: a program that loads the tendon, calf and foot muscles gradually, such as resisted turning of the foot inward and heel raises built up over weeks. This is the core of early treatment
- Calf stretching: tight calves pull the foot flatter. A daily stretch against a wall helps
- Less impact: swap running and jumping for cycling or swimming while it settles
- Ice and pain relief: ice wrapped in a towel for about 20 minutes, a few times a day, and anti-inflammatory tablets if they suit you
- Weight: losing some takes load off the tendon with every step
- Patience: pain can last longer than 3 months even with early treatment. The tendon has a poor blood supply and heals slowly
Cortisone shots near this tendon are used cautiously, if at all, because of concern that they could weaken it. When they are given, it’s usually into the tendon’s sheath with ultrasound guidance. See cortisone shot side effects.
Surgery
- When it’s considered: pain that hasn’t improved after several months of good treatment, or a foot that keeps flattening
- Flexible stages: cleaning up the tendon, moving a nearby tendon to take over its job, and cutting and shifting the heel bone to realign the foot. Several procedures are often done together
- Stiff, late stages: fusion, which joins the joints below the ankle so they can’t collapse further. It gives a stable, less painful foot, but you lose side-to-side movement, so uneven ground feels harder
- Recovery: several weeks in a cast or boot, usually with no weight on the foot, then a gradual return to walking. It takes 6 months or more before healing is complete and pain is clearly better. Rehab to rebuild strength and walking is a big part of the result
The American Academy of Orthopaedic Surgeons explains the treatment options for a collapsing flat foot in more detail.
Get seen today if
- You felt a sudden pop or tearing on the inside of the ankle, and the arch dropped quickly
- The foot is red, hot and swollen, or you have a fever
- You have diabetes and the foot is suddenly warm, swollen or changing shape, even without much pain. It needs checking for a serious bone problem called Charcot foot
- You can’t put weight on the foot
Get it checked if
- One foot has become flatter than the other
- You can no longer rise onto tiptoe on one leg
- Pain on the outside of the ankle has started as the arch drops
- Pain hasn’t improved after about 3 months of insoles, a brace and exercise
- Shoes no longer fit because the foot has changed shape
A flat foot you were born with is usually harmless. One that flattens in middle age needs attention.