Low-arched or “flat” feet are usually normal variants of foot posture and require no interference.
Some feet simply have a low arch, but have neutral rotational and coronal plane alignment.
Others are “flat” because the hindfoot and forefoot are rotated with respect to one another more than is usual. The subtalar joint is more pronated than usual, so the hindfoot is in valgus and the Achilles tendon may be tight. The forefoot is abducted and supinated at the midtarsal joint, so that the neck of the talus, which is normally in line with the first metatarsal in both transverse and sagittal planes, comes to point below and medial to the first ray. Because the hindfoot pronation and forefoot supination are balanced in most people, the foot sits plantigrade but the head of the talus in the medial border of the foot is low and the foot appears flat. This is the “overpronated” or “planovalgus” foot – but it must be emphasised again that such feet are usually asymptomatic, and there is no evidence that they are at a predictably high risk of future symptoms if not “treated”.
It should be appreciated that the “overpronated” foot is simply exhibiting an exaggerated version of the posture the foot normally adopts in early stance phase of gait. At initial heel contact, the heel is usually neutral or slightly varus, but the tibia rotates internally as the body begins to pass above the foot and this rotates the talus internally. Through the “torque converter” function of the subtalar joint, the subtalar joint becomes pronated and the heel moves into valgus. Increasing weight transfer through the foot tends to press down on the arch. In the normal foot, the static restraints, especially the plantar fascia but also the plantar, spring and deltoid ligaments and the joint capsules, restrict the pronation of the hindfoot and the tendency of the forefoot to dorsiflex. The leg moves over the foot in mid-stance, the tibia begins to rotate externally and the previous rotation is reversed. However, the foot may remain pronated for an unusual length of time, or even the whole of stance phase, if:
- The static restraints are ineffective because of
- Generalised joint laxity (especially the normal joint laxity of childhood)
- Inflammatory arthritis
- Degeneration of the ligaments and/or the tibialis posterior tendon
- There is muscle imbalance in which peroneus brevis is stronger than tibialis posterior
There is some evidence that a foot which remains in the pronated position for an unusual proportion of the stance phase functions less efficiently as a propulsive lever in late stance phase. There are also theoretically plausible mechanisms by which a variety of foot pathologies, such as hallux valgus, hallux rigidus, interdigital neuromas, tarsal tunnel syndrome and tibialis posterior tendonopathy may arise progressively out of such a foot. There is a body of thought influenced by Root which proposes active treatment of many such feet to influence future pathology as well as current symptoms. However, there is little empirical evidence that overpronation is so directly linked to such pathological conditions of the foot, or that early biomechanical intervention alters the natural history or is cost-effective. A lot of research is needed to clarify the relationship of overpronation and specific foot conditions, but most of it requires very long follow-up.
There are also congenital foot conditions which produce a flatfoot appearance without the effects of gait:
- Congenital vertical talus, which presents at birth with a severe planovalgus deformity. It is outside the scope of this work.
- Tarsal coalition, a failure of segmentation of the cartilaginous tarsal model in early foetal development. The commonest coalitions are:
Clinically the main distinction is between:
- Feet with a low arch but no rotation into planovalgus – almost always clinically irrelevant but the patient or parents may require reassurance that this is a normal appearance.
- Flexible overpronated feet without any other pathology such as arthritis, muscular imbalance or tendonopathy. The arch reappears and the heel moves into varus on tiptoe or when the great toe is dorsiflexed (Jack test), as the plantar fascia tightens. Some of these feet rub uncomfortably on shoes or ache in the arch or at other sites. Again, reassurance that this is a normal variant may be all that is required. Symptomatic feet may benefit from orthotic treatment but there is rarely an indication for surgery. Very occasionally, there is an indication for Achilles tendon lengthening, medial displacement calcaneal osteotomy or stabilisation with a medial soft tissue plication or localised arthrodesis. The use of silastic inserts in the sinus tarsi (subtalar arthroeresis) has been promoted but the evidence for their long-term benefit is slim and serious complications have been reported.
- Overpronated feet with evidence of tibialis posterior tendonopathy
- Stiff overpronated feet, which may be simple flexible feet which have stiffened with age, or may have arthritis or muscle imbalance
- Arthritic flatfeet may require investigation to identify the rheumatological diagnosis if this has not been done. Treatment is directed at control of arthritis and protection against deforming forces with orthoses, either in-shoe or external devices.
- Neuromuscular flatfeet may occur particularly in myelomeningocoele or spastic cerebral palsy. Full neurological assessment is required. Orthoses, usually external devices such as knee-ankle-foot orthoses (KAFOs) may protect against further deformity and give support to allow walking. Some of these patients will need soft tissue procedures, tendon transfers or hindfoot fusions, usually triple fusions to prevent and treat progressive deformity.
Clinical assessment of a patient with flat feet is aimed at:
- Deciding whether there are any actual symptoms needing intervention
- Identifying underlying arthritis, neurological disease or skeletal abnormalities
- Assessing flexibility and correctibility
- Identifying any unconnected conditions which could be causing any symptoms of which the patient complains
Management often consists solely of reassurance and advice to pursue normal activities. If the patient has symptoms which may be helped by orthotic treatment, this should usually be under the supervision of a podiatrist or orthotists who can produce and manage devices. Surgical involvement is not usually required. The most important team member is the one who can restrain more therapeutically aggressive colleagues from interfering with normal feet!