Why has my arch collapsed on one side?
A flat foot you have had all your life is one thing. An arch that used to be there and is now lower than the other side, usually with an ache along the inside of the ankle, is a different problem, and it is worth taking seriously early. In most adults it means the tendon that holds the arch up is failing.
The tendon that holds the arch up
The tibialis posterior runs down the inside of the leg, passes behind the bony knob of the inner ankle, and fans out to attach under the arch. With every step it locks the back of the foot rigid and lifts the arch so the calf can push you forward through a stable foot. It is a small tendon for the job it does, and its blood supply is poorest just behind the ankle bone, which is exactly where it wears.
When it becomes inflamed, stretched or partly torn it lengthens, and a lengthened tendon cannot hold the arch. The arch sinks, the heel drifts outwards and the front of the foot swings out. Doctors call this tibialis posterior dysfunction, adult acquired flat foot or, more recently, progressive collapsing foot deformity. Patients call it a fallen arch.
Why it is usually one side
Flat feet from childhood are symmetrical and painless. A failing tendon declares itself on the side that has done the most work or taken an injury: an ankle twisted years ago, a period of heavy walking or standing, a job on hard floors. It is more common in women, in people over forty, and with weight, diabetes, high blood pressure or inflammatory arthritis, though in many people no single cause is found. The other foot may follow later or never. A one-sided change of shape in an adult is the feature that should prompt an assessment rather than a shrug.
Three things to check at home
The heel-rise test. Hold a bench, lift the good foot off the floor and try to rise onto the toes of the affected leg. A healthy tendon lifts the heel and swings it inwards as you go up. A failing one struggles or cannot lift the heel at all, and the heel does not swing in.
The view from behind. Stand with your feet parallel and have someone look at your heels. On the affected side the heel leans outward and more toes show on the outer side, the "too many toes" sign.
Where it hurts. Early on, along the tendon behind and below the inner ankle bone, worse with walking and standing. Later the pain moves to the outside of the ankle as the drifting heel pinches the structures there. That shift is progression, not improvement.
Why timing matters more than usual
The condition moves through stages, and the stage decides the options. While the deformity is flexible, meaning the foot can still be pushed back into shape by hand, an orthotic, a brace and a strengthening programme control most people, and if surgery is needed it can realign the heel bone, replace the failed tendon with a neighbouring one and keep the joints moving. Once the foot has stiffened and the joints have worn, the reliable operation is a fusion, which trades movement for a stable, painless foot.
The tendon itself does not heal back to its original length once it has stretched. Treatment supports the foot and strengthens what remains; it does not wait for the tendon to recover, because it will not.
What helps now
An orthotic that supports the arch and holds the heel. Off-the-shelf is a reasonable start in the early stages; custom for a more advanced or unusual shape. This is the mainstay, and many people manage on it indefinitely.
A brace around the ankle and hindfoot in more advanced cases, to take load off the tendon through the day.
A structured strengthening programme for the tendon: slow heel raises with the heel turned inwards, progressing to a single leg, alongside stretching of the calf, because a tight calf drives the arch down. Done properly for several months, this settles symptoms in some people in the early stages.
Supportive shoes with a firm heel counter, weight management where it is relevant, and a short course of anti-inflammatories for a flare.
Not a steroid injection into the tendon. Steroid weakens tendon tissue, and this is a tendon that is already failing.
When surgery is considered
Surgery is for the foot that is still painful despite all of the above, or whose shape is still changing. In a flexible foot it is a reconstruction: the heel bone is cut and shifted back under the leg, a neighbouring tendon is moved across to do the failed one's job, a tight calf is lengthened, and sometimes the outer border of the foot is lengthened to bring the forefoot back. In a stiff foot specific joints are fused in a corrected position. Either way it is major surgery with a recovery measured in months: around six weeks with little or no weight through the foot, out of the boot from about three months, and improving for a year. That is a good reason to be assessed while the smaller operation is still on the table.
Key points
An arch that drops in adulthood, on one side, with inner ankle pain, usually means the tibialis posterior tendon is failing. It is not "just flat feet".
If you cannot rise onto the toes of that foot, or the heel does not swing inwards when you do, have it assessed.
Pain moving from the inner to the outer ankle means the deformity is progressing.
An orthotic, a brace and a proper strengthening programme control most early cases; the tendon does not regain its length on its own.
While the foot is still flexible, surgery can realign it and keep the joints; once it stiffens, fusion is more likely.
More detail. Want the full picture before your appointment? Tibialis Posterior Dysfunction covers the stages, assessment, orthotics and bracing, and what surgery involves.
Ready to discuss your treatment options?
Book a consultation with Dr Janan Chandrananth if one arch is dropping or the inside of your ankle aches with walking. The options are widest while the foot is still flexible.
