Pes Cavus

Excerpt

Pes cavus is an orthopedic condition that manifests in both children and adults. Pes cavus and pes cavovarus are often used interchangeably as the most common manifestation of the cavus foot is the cavovarus presentation. Pes cavus is a deformity that is typically characterized by cavus (elevation of the longitudinal plantar arch of the foot), plantar flexion of the first ray, forefoot pronation, and valgus, hindfoot varus, and forefoot adduction. Pes cavus is frequently a manifestation of an underlying neurological process, but there has been literature that discusses a subset of patients in whom a more subtle form of the cavus foot may present without an underlying disease process. With the understanding that most cases are related to these underlying neurological processes, it becomes necessary to not only understand the intricacies of the care of the foot deformity itself but to ensure that adequate patient history, family history, clinical exam, radiographic exam, and other necessary testing is performed to ensure that the clinician has adequately diagnosed the underlying cause before initiation of treatment.

Anatomy

The cavovarus foot can be the result of forefoot driven pathology (a flexed first ray), a deformity of the hindfoot, or some combination of both pathologies. The foot normally forms a tripod with the first metatarsal head, calcaneus, and fifth metatarsal head forming the three points of contact with the ground. In the cavus foot, flexion of one element of the tripod, the first ray, leads to tilting and overload of the other two elements.

Forefoot driven pes cavus is most often caused by neurological diseases and is the result of muscular imbalances. A weak tibialis anterior, intrinsic foot muscles, and peroneus brevis are overpowered by a stronger peroneus longus and posterior tibialis. Later these muscles may also weaken but remain in a state of contracture, producing the same effect. The attachment of the peroneus longus at the metatarsals and medial cuneiform results in plantar flexion of the first ray and forefoot pronation. When a foot with fixed forefoot pronation bears weight, the hindfoot is forced to counter-correct into supination, or varus, in order to restore the tripod.

As the deformity develops, the Achilles tendon may eventually act as an invertor of the foot and will shorten over time. The unopposed contracture of the tibialis posterior and peroneus longus will lead to subtalar joint inversion. In gait, the cavovarus foot demonstrates a compensatory heel varus, a locked midfoot, and a reduction of the flexible phase and decreased shock absorption as a result of plantar fascial tightening. Typically the arch of the foot will change shape during the gait cycle, and the shock absorption of the midstance phase is negatively affected by the tightening of the plantar fascia leading to decreased shock absorption.

Hindfoot driven cavovarus deformity is commonly the result of trauma. The causes may include malunited pilon fractures and resultant varus angulation of the distal tibial articular surface, malunion of talar neck or calcaneal fractures, or longstanding ankle or subtalar joint instability. The varus deformity will result over time as an attempt to obtain a plantigrade foot by compensation through the subtalar joint. The compensation by the subtalar joint for ankle joint deformity can be explained by the anatomy of the subtalar joint. A healthy subtalar joint can tilt away from the deformity and compensate somewhat for the primary ankle varus that often results from the traumatic etiologies listed above. This will result in an overall balanced hindfoot. A study of 226 subjects with ankle osteoarthritis found the compensation of the subtalar joint in greater than 50% of varus ankle osteoarthritis cases. This prolonged compensation may lead to a progressive and fixed deformity over time.

Cock-up deformities are also common at the metatarsophalangeal joints (MTP). Overpull of the flexor digitorum longus is a contributing factor, as is the shortening and fibrosis of the plantar fascia. As the pressure under the metatarsal heads increases a result of this cock-up deformity, the MTP joints can eventually dislocate. This increased pressure is independent of the root cause of the cavovarus foot and can be of particular concern in patients with metabolic diseases such as diabetes mellitus due to the increased foot pressure leading to concerns for ulceration.

Natural History

Prognosis highly depends on deformity severity, underlying etiology, & age at presentation. The deformity is typically slow to develop and begins before puberty. In children, the deformity is initially compensated but may become more rigid over time, leading to possible alterations in bone growth, effects on the further development of bones, and alteration of the shape and morphology of the developing foot. The most common cause of pes cavus is the hereditary motor and sensory neuropathies (HMSNs), the most common subtype being Charcot-Marie-Tooth (CMT) disease. CMT is a progressive degeneration of peripheral nerve myelin with decreased motor nerve conduction. Deformities typically worsen, and surgical treatment is often part of the treatment algorithm for these patients to prevent progression to a fixed deformity.

The lateral soft tissue structures may suffer from overload leading to instability in both the typical pes cavovarus. and the subtle cavus foot, or "underpronator." Untreated long-term lateral ankle instability has been documented as a cause of advanced arthritis of the ankle joint. Anterolateral ankle instability may also result.

Patterns of Spread

The pattern of the spread of pes cavus is dependent on the underlying etiology.

HMSNs subtypes have a heritable transmission in autosomal-dominant, autosomal recessive, and x-linked recessive patterns. The subtle cavus foot that is more commonly found in adults without underlying neurological disorders is also probably inherited, with a currently unclear pattern.