Pes planus

Pes planus (plural: pedes plani) is a deformity of the foot where the longitudinal arch of the foot is abnormally flattened and can be congenital or acquired. 

Terminology

Pes planus is also known as flatfoot, planovalgus foot or fallen arches ⁷.

Epidemiology

Pes planus may occur in up to 20% of the adult population, although the majority of patients are asymptomatic and require no treatment. Approximately 10% (range 7-15%) of the population with developmental flatfoot go on to develop symptoms requiring medical attention ⁷. 

Pathology

Pes planus can be ⁷:

• congenital: normal in toddlers, may persist into adulthood
• acquired secondary to: 
  • posterior tibialis dysfunction (most common) ⁴
  • trauma
  • neuroarthropathy
  • neuromuscular disease
  • inflammatory arthritis

In the pediatric population, the degree of ligamentous laxity of the foot results in relative pes planus that resolves over time ⁵. Within the first decade, there is spontaneous development of a strong arch in most people ⁷.

Pes planus results from loss of the medial longitudinal arch and can be either rigid or flexible. These deformities are usually flexible, which means that on non-weight-bearing views, the alignment of the plantar arch normalizes.

Associations

There are several conditions associated with pes planus ^1,2:

• tarsal coalition
• certain connective tissue disorders
  • Marfan syndrome
  • Ehlers-Danlos syndrome

There is some evidence to suggest that flat feet protect against stress fractures ^ref.

Radiographic features

Plain radiograph

The longitudinal arch of the foot must be assessed on a weight-bearing lateral foot radiograph. If the patient is unable to stand or weight-bear, a simulated weight-bearing radiograph should be obtained.

Weight-bearing lateral view

In normal feet, the relationship between the talus and the 1^st metatarsal results in a straight line being formed along their axes (i.e. normal Meary's angle = 0°). Pes planus, in contrast, will show ⁷:

• loss of the normal straight-line relationship with Meary's angle >4° convex downwards
• sagging at the talonavicular or naviculocuneiform joints
• angle of the longitudinal arch >170°
• calcaneal inclination angle ≤19° ⁹
• disruption of the cyma line: appears as a "lazy S-shape" of the talonavicular and calcaneocuboid joints on both AP and lateral views; it is disrupted owing to anterior shift of the talonavicular joint ⁸

Weight-bearing dorsoplantar view

It is important to assess:

• hindfoot valgus (where the talocalcaneal angle is >35°)
• talonavicular undercoverage or subluxation (where the talonavicular coverage angle is >7°)
• forefoot abduction

Congenital vs acquired

Acquired pes planus (i.e. foot collapse) can be distinguished from congenital pes planus by carefully assessing the calcaneus and midtarsal joint: ⁶

• in the acquired form, the calcaneal pitch is at least 10°; in congenital pes planus it is less
• in the acquired form, the calcaneus is downwards-concave; in the congenital form it is downwards-convex or flat
• in the acquired form, the midtarsal joint is altered by a forward-jutting talus; in the congenital form the talus is medially displaced, but the midtalar line appears normal (i.e. it is pseudonormal)

Treatment and prognosis

Treatment depends on whether:

• there are symptoms
• it is fixed or mobile
• there are associated findings, e.g. hindfoot valgus
• there is any associated pathology

Subtalar arthroereisis can be performed in pediatric symptomatic flexible flatfoot. In fixed flatfoot, non-operative management is unlikely to be beneficial since there is a fixed relationship between osseous structures.