What "flatfoot" describes
Pes planus means the medial arch is low or absent when the foot is bearing weight. It is usually accompanied by the heel tilting outward and the forefoot turning out. It comes in two functionally different forms.
Flexible flatfoot is by far the commoner. The arch is low on standing but reappears when the foot is lifted off the ground or when the person stands on tiptoe. The joints are mobile and the foot can be passively restored to a normal shape. Most flexible flat feet are painless and lifelong.
Rigid flatfoot means the arch remains flat regardless of position, and the joints do not move freely. This is less common, more likely to be symptomatic, and more likely to have a specific cause — an abnormal bony connection between tarsal bones present from childhood, longstanding arthritis, or the end stage of tendon failure. Rigid flatfoot warrants proper assessment.
Flat feet in children
Nearly all young children have flat-looking feet. The arch is filled by a fat pad, the ligaments are lax, and the visible arch typically develops through childhood into the early teens. A flexible, painless flat foot in a child who walks and runs normally is a normal finding and does not require insoles, special shoes or intervention — a point worth making because a great many devices are sold for it.
What does merit assessment in a child: a foot that is painful, a foot that is rigid rather than flexible, a marked difference between the two sides, an arch that was present and has flattened, calf tightness that limits ankle movement, or a child who avoids activity because of foot discomfort.
Adult acquired flatfoot: the one that matters
A flat foot that has been flat since childhood is one thing. An arch that collapses in adulthood is another, and it deserves attention.
The usual cause is progressive failure of the posterior tibial tendon, which runs behind the inner ankle bone and is the principal dynamic supporter of the arch. As it becomes damaged and elongated it can no longer hold the arch up, and the foot gradually flattens and rolls outward. The condition is now often described as progressive collapsing foot deformity, reflecting the fact that the whole structure changes rather than a single tendon. OrthoInfo covers it in detail.
The typical picture: pain and swelling along the inside of the ankle and arch, worse with activity; a progressively flattening arch, often on one side only; difficulty rising onto tiptoe on the affected leg, or inability to do it at all; and, later, the heel drifting outward so that more toes are visible when the foot is viewed from behind. Pain may later shift to the outer side of the ankle as the bones begin to impinge.
It is more common in middle-aged and older adults, in women, and in association with raised body weight, diabetes, hypertension and inflammatory arthritis. It is progressive, and it is much more amenable to treatment in its earlier stages — which is the single most important reason not to ignore a newly flattening, aching arch.
When a flat foot causes symptoms
Symptomatic flatfoot, of either kind, tends to produce aching along the inside of the arch and ankle, fatigue in the feet and legs after standing or walking, pain under the heel or the ball of the foot from altered load, uneven shoe wear on the inner side, and sometimes knee, hip or lower back discomfort from the altered mechanics further up.
Management
Asymptomatic flexible flatfoot needs no treatment. Not insoles, not exercises, not special footwear. Treating a foot shape that is not causing a problem is a solution in search of one.
Where there are symptoms, management is generally graded:
- Footwear with a firm heel counter, a supportive midsole and adequate torsional stiffness — a shoe that does not fold in half.
- Foot orthoses, prefabricated or custom, to support the arch and control the position of the heel. This is one of the clearer indications for a device with a functional profile; see foot orthoses.
- Calf stretching, since a tight calf both contributes to and results from a flattening foot.
- Strengthening, particularly of the posterior tibial tendon and the intrinsic muscles of the foot. Progressive loading has become a central part of managing early tendon dysfunction rather than an optional extra.
- Load management and weight, both of which change the demand on the tendon directly.
- Immobilisation in a boot or cast for a period, in acute posterior tibial tendon pain.
- Surgery, reserved for cases that fail conservative management or that have progressed to rigid deformity. Procedures range from tendon transfer and bone realignment in flexible deformity to fusion in rigid deformity, and the choice depends heavily on the stage — another argument for early assessment.
The high-arched foot, briefly
The opposite variant, pes cavus, has its own issues: a rigid foot that absorbs shock poorly, high pressure under the heel and the ball of the foot, clawing of the toes, and a tendency to ankle instability. A high arch that is new or progressing — particularly with toe clawing or weakness — should be assessed, because it is occasionally the presenting sign of an underlying neurological condition.
What to act on
See someone if an arch is flattening in adulthood, if there is pain along the inside of the ankle, if you cannot rise onto tiptoe on one leg, if one foot differs markedly from the other, if a child's flat foot is painful or rigid, or if an arch change is accompanied by numbness or weakness. Everything else in this area is usually a variation rather than a problem. See also heel pain and when a foot problem needs professional care.