Chapter 9 – The Cavus Foot | Musculoskeletal Key

Chapter 9 The Cavus Foot

Paul H. Cooke and Thomas A. Ball

Historical Aspects and Modern Perspectives

Of the few articles written in the early days of orthopedics, a large proportion were about the clubfoot, including the adult clubfoot. One expert in the management of cavus, Naughton Dunn from Birmingham, England, wrote little by today’s standards, but what he did write was mostly on the subject of calcaneocavus deformity of the feet in polio – a condition treated by his eponymous triple fusion and tendon transfer¹. Dunn stated that paralytic cavus was one of the most crippling deformities of infantile paralysis. He noted that once deformity was established, treatment by splintage was difficult and usually ineffective. He observed that varus and valgus deformities tend to develop owing to the “faulty incidence of body weight or unequal muscular action.” These comments on treatment and progression remain true and relevant today, but Dunn’s outcome measure that “the results are nearly always excellent” would not pass the scrutiny of modern peer review.

In drawing lessons from the past on the subject of cavus, we must remember that in his time spina bifida and polio were rife, aging and infirmity were accepted, and that fashionable shoes and road running were the luxury of the few, not the right of all. So the etiology of the condition and the expectations of treatment have changed.

Today, overall we see fewer cases of neurological disease. In most countries patients are more likely to present with severe neurological cavus as part of Charcot–Marie–Tooth disease (CMT). Charcot–Marie–Tooth is a collection of neurological diseases and has many genetic variants. Some use the term hereditary motor and sensory neuropathy (HMSN), but most patients and clinicians prefer the eponym CMT. Other neurological diseases, such as polio and spina bifida, have all but disappeared over a generation, as a result of immunization, prenatal diagnosis, genetic counseling, and public-health measures such as the widespread prophylactic treatment of pregnant women with folic acid.

The range of patients presenting with cavus to an orthopedic clinic is wide. Nevertheless, the majority of patients with cavus are never referred for an orthopedic opinion; they either live with mild deformity and symptoms by simple modification or restriction of footwear and activity, or receive treatment from podiatrists and/or orthotists.

In tertiary practice, we mostly see patients with neurological disease as a result of CMT or cerebral palsy, who suffer the consequences of severe deformity. Nevertheless, in both specialist and general orthopedic practice, we also frequently see patients with subtle cavus and its consequences. They may complain of pain and varus instability when engaged in running and other physical pursuits, with limitation of the ability to wear more fashionable shoes, and secondary claw toes.

Definition, Classification, and Etiology

The definition of cavus is loose – being any foot with a high arch. Forefoot plantaris is the defining feature (Figure 9.1). Plantaris refers to plantar flexion of the forefoot relative to the hindfoot, and produces an increase in the height of the arch. Plantaris may occur in combination with any deformity of the hindfoot (calcaneus, equinus, varus, or valgus).

Figure 9.1 A case of a symptomatic moderate cavus foot showing forefoot plantaris.

Cavus may be subdivided into physiological or pathological. Physiological cavus represents an extension of the normal range of arch elevation and is usually mild. Pathological cavus occurs secondary to neuromuscular imbalance, and is usually more severe.

Cavus feet can also be divided anatomically into three groups, in order of severity:

1. The “straight” cavus foot – where the arch is elevated but there is no other anatomical abnormality.
2. The inverted cavovarus foot – where the arch is high and the heel is inverted.
3. The complex foot – with multiple deformities affecting the whole foot. This includes feet with equino-cavovarus or calcaneo-cavovarus deformity.

Cavus feet tend to deteriorate, with increasing symptoms and deformity. Even those that start as mild cavus can deteriorate to a more severe anatomical and symptomatic group. This deterioration is much more likely in cases where there is an underlying neurological cause.

Deformity does not improve without treatment, so once symptoms arise there is no place for withholding treatment – whether by conservative or surgical means².

Cavus, in its mildest form, is common, occurring in between 7 and 15% of people³^–⁴. The causes of cavus are shown in Table 9.1. The proportion of physiological cases appearing in an orthopedic clinic depends on geography and the special interest of the surgeon, with reports of up to 70% of cases being physiological. In the senior author’s clinic, which has a special interest in neurological diseases, there are less than 20% of physiological cases, with the remaining 80% being pathological. The proportion of cases with a neurological diagnosis increases in the special interest or tertiary clinic, but also increases as the clinician becomes more diligent in seeking a neurological cause. All authors note a significant increase in the new diagnosis of neurological disease following neurological referral from the foot clinic⁵.

Table 9.1 The causes of cavus foot

Idiopathic
Congenital	residual talipes
Congenital	arthrogryposis
Traumatic	post-compartment syndrome
	nerve injury
	burn contracture
Neurological	hereditary sensorimotor neuropathy/Charcot–Marie–Tooth disease
	cerebral palsy
	spina bifida
	polio
	myelodysplasia
	diastematomyelia
	tumor
	syringomyelia
Muscular	muscular dystrophy

The diagnosis of neurological disease is important so that treatment can be tailored to the future course of the disease and the known patterns of progression. Similarly, early diagnosis of spinal disorders, such as spinal dysraphism, diastematomyelia, and cord tumors, is important if progressive mono- or paraplegia and incontinence are to be avoided. It is not uncommon for a cavus foot to be the initiating referral of these spinal conditions.

Today, of the neurological causes of cavus (Table 9.1) the HMSNs and cerebral palsy are the commonest in the UK, with the decline of polio and spina bifida.

The types of CMT are summarized in Table 9.2, but there are many types and subtypes. Over 35 different genetic origins are described.

Table 9.2 The types of Charcot–Marie–Tooth disease

Type	Subtype	Genetics	Chromosomeabnormality	Age of presentation	Features
CMT1 Demyelinating	A	Autosomal dominant	Duplication Ch17	At 10 to 15 years. May be detected earlier in known families	Weakness, ataxia, distal sensory decrease, areflexia, cavus increases. May progress to cavovarus, equino cavovarus, or more often to calcaneo cavovarus
	B	Autosomal dominant
	C	Autosomal dominant
	D	X-linked (may also have preserved myelin – CMT2).
CMT2 Myelin preserved	A	Autosomal dominant	Duplication Chr1	Often later onset up to 20 to 30 years	Each type may present like CMT1 or with stork-leg appearance, weakness, and flat foot (due to weakness)
	B		Partial deletion Ch3
	C		?
	D		Duplication Ch7
CMT3 (Dejerine Sottas Syndrome) Demyelinating		Autosomal recessive	Variable	Presents in infancy or childhood	As CMT1, severe and progressive, associated with severe spinal deformity
CMT4, CMT5, CMT6, CMT7	All rare	Multiple	Multiple	Early – infancy or childhood	Often severe and generalized (including cranial nerves) neuropathy

For a detailed description refer to www.cmt.org.uk.

Clinically, CMT1 and CMT2 are the commonest subtypes, together accounting for around 80% of cases. They usually cause progressive lower motor neuron weakness in the limbs, with resultant deformities from adolescence or young adulthood onward.

CMT2 is often less severe and of later onset than CMT1. Some types of CMT2 present with weakness and flat foot, rather than cavus. These cases will continue with this pattern of deformity as they age, and do not go on to develop high arches – the pattern does not change. Sensory symptoms, including numbness (rare) and neuropathic pain (common), are variable.

CMT3, known as Dejerine–Sottas disease, is rare and results in limb weakness and disability in early childhood. Patients with CMT4 may suffer respiratory difficulty and hearing loss as well as weakness. The other types are extremely rare and beyond the scope of this chapter.

What Causes Cavus?

It used to be thought that cavus of the foot was secondary to idiopathic tightening of the plantar fascia. This explanation is attractive but inadequate to explain the primary and secondary problems. For example, if cavus resulted from tightness of the plantar fascia, the toes would be flexed at the metatarsophalangeal joints, and the extended MTPJs and claw toes, which nearly always accompany cavus, would not be seen.

Duchenne first suggested muscle imbalance as the cause of cavus as a result of his neurophysiological experiments⁶. This theory was later revived⁷. Price looked at patients with HMSN using computed tomography and showed intrinsic atrophy⁸, particularly in the pedal lumbricals and interossei. Two years later the Liverpool group showed correlation between MRI and histological findings⁹, finding hyperplasia of the peroneus longus and atrophy of the tibialis anterior. The sum of the anatomical, MRI, and CT studies of the leg and foot in cavus documents atrophy and contracture of the intrinsic muscles in pure cavus, and of both the intrinsic and calf muscles in some idiopathic cases and all the complex forms of cavovarus.

The concept of dysfunctional intrinsic activity causing cavus is further supported by work by Garceau and Brahms who studied 47 patients with pure cavus¹⁰. They undertook selective division of the intrinsic motor branches of the plantar nerves. This reduced arch height and improved the shape of the footprint. They followed up these cases with pre- and postoperative plaster cast models to document foot shape over time. They noted improvement in shape in 46 of the patients (Figure 9.2).

Figure 9.2 The carefully documented work of Garceau and Brahms¹⁰ shows that intrinsic denervation before maturity allows collapse of a high arch.

The contradictory nature of this evidence – that cavus is associated with atrophic intrinsic muscles, but that denervation of the intrinsics leads to improvement of cavus, indicates that we do not fully understand the effects of denervation. It is likely that chronic denervation causes contracture and hence deterioration – especially when the skeleton is growing, whereas with acute denervation there is weakness, without contracture, which decreases the tendency of the arch to foreshorten.

The cause of cavus and its associated deformities in CMT is probably a complex combination of weakness and contracture of the pathologically affected muscles. This is especially marked if there is an imbalance of power between antagonistic muscles.

Thus contracture of the intrinsic muscles usually causes the plantaris and cavus. Weakness of the peroneal musculature and dorsiflexors causes inversion and equinus, respectively, as the relatively stronger, but also weakened, calf muscles and inverters continue to act. Equinus may result from the relative overactivity of the calf muscles. Calcaneus occurs when the calf muscles are weak.

In CMT the cavus is often associated with differential plantar flexion of the first ray. The etiology for plantar flexion of the first ray is controversial. One school holds that there is relative overactivity of the peroneus longus, which in combination with the weakness of the tibialis anterior, leads to plantar flexion of the first ray. The evidence for this is based upon surface electrophysiological studies undertaken during gait. Surface electrical activity may also be caused by passive stretching of muscles, so it is equally possible that this activity reflects passive stretching of a contracture of peroneus longus secondary to antagonistic activity of the stronger tibialis posterior. The tibialis posterior is relatively preserved in CMT. This seems a more probable explanation given that the peroneal muscles are always weakened in CMT.

Although difficult, understanding the cause of the deformity is necessary to treat it and avoid recurrence. Fortunately, once a pattern of muscle imbalance and contracture has presented, that pattern remains consistent and so we do not have the added complication of changing patterns of deformity and the forces causing it.

Clinical Presentation

The clinical presentations of the different patterns of deformity vary. In simple cavus the presentation relates to the shape of the foot, with problems in finding fashionable shoes, rubbing of the dorsal bumps of the tarsometatarsal joints, metatarsalgia, and clawing of the toes.

A study of 1047 veterans with diabetes found a highly significant correlation of cavus with prominent metatarsal heads, bony prominences, and hammer and claw toes¹¹. High plantar pressures are correlated with metatarsalgia. In these cases the patients overcame the metatarsalgia by reducing activity and adopting a slow and cautious gait to avoid peak forefoot pressures¹².

The tarsus in a cavus foot is in a “close-packed” state, a state that is produced physiologically at the end of gait by means of the windlass mechanism. The windlass mechanism imparts rigidity to the foot at push-off, which is advantageous. However, the cavus foot is fixed in this rigid condition throughout the gait cycle, so shock absorption and adaption of the foot to unevenness in the floor are poor during early stance. Thus metatarsalgia, foot pain, and other more proximal disorders, including hip, knee, and back pain associated with high levels of transmitted shock, are more common in the patient with the cavus foot.

Crosbie showed that the diffuse foot pain in cavus feet was more closely related to the severity of the cavus than to any other variable, excepting body weight¹³. Thus athletic patients with cavus have low tolerance to the repeated shocks from running and jogging, while older people, with less compliant tissues, suffer with foot discomfort in everyday life. All of these problems are in turn commoner in overweight patients.

Despite this, and while the shape of the foot and the toes may be problematic, significant midfoot pain is surprisingly uncommon in pure cavus. It can usually be treated by insoles, with adequate accommodation in capacious shoes – although pressure from the dorsal boss, which commonly forms over the apex of the arch, may be problematic in fashionable shoes.

Cavus feet with a varus heel exhibit all of the problems described for cavus feet, often in a more severe form. However, the varus heel also leads to other, more disabling, symptoms, namely ankle instability and pain, as well as lateral foot pain with overload or fracture of the fifth metatarsal. These tendencies to lateral foot pain, fracture, ankle deformity, and ankle arthritis increase with the severity of the deformity.

The heel varus is significant not only because of the acute and chronic symptoms it causes, but also as the fifth metatarsal fractures and ankle instability recur after treatment, unless the heel varus is recognized and corrected.

By the time we move on to the complex deformities, general features of the neurological disease are added to the local problems of the foot (Table 9.3).

Table 9.3 The consequences of complex cavus foot deformities with CMT

Limb pain	90%
Rubbing toes	62%
Metatarsalgia	52%
Ankle instability	31%
Lateral border pain	11%
Ankle/hindfoot pain	9%
Ulceration	1%

In our unit 399 patients with CMT were surveyed: we found that the most common feature of the disease was low- to medium-level generalized lower limb pain. This did not improve with treatment and is probably a manifestation of the painful neuropathy¹⁴. The commonest problems caused by cavus were toe and footwear problems – often with superimposed metatarsalgia. Instability was also common, and was especially disabling as it is combined with the poor proprioception, as a result of the neuropathy. Lateral foot pain was seen in 11% of cases, with many having more than one focus of pain. Of the patients with lateral foot pain, over 50% of those who had no treatment went on to develop a fifth metatarsal fracture. Of those with a fracture, half went on to have two or more fractures before receiving definitive treatment. Ulceration was fortunately rare, occurring in less than 1% of all cases. Nevertheless, it was of huge significance when it did occur.

Management: Examination and Investigation

The management of the cavus foot requires the clinician to combine the skills of the physician and surgeon. An adequate history is essential. It is important to carefully establish the patient’s symptoms, including pain, numbness, poor balance, and fatigue – as well as bladder and bowel dysfunction. Similarly the past history and the family history may give important clues as to the etiology of the deformity.

Examination starts with a general examination, including posture, and a visual examination of the back and lower limbs. Careful examination of both feet, including both the medial and lateral arch height, the posture, flexibility, and deformity of the foot, is undertaken (Figure 9.3). Callosities on the sole of the foot reflect areas of high pressure under the foot, and occur particularly under the metatarsal heads and down the lateral margin of the foot (Figure 9.3d). This is combined with examination of the soles, uppers, and inside of the shoes, which provides a simple but effective form of functional gait analysis.

(a) Note the high arches and clawed great toes with callosities;

(b) varus heels in association with the raised medial arch;

(d) In a different patient, examination of the sole shows areas of callosity, which correspond to areas of peak pressure on standing and walking.

Figure 9.3 Examination of the foot and ankle in the patient with cavovarus feet.

The alignment and mobility of each joint from the ankle to the toe joints is assessed. It is then important to systematically examine and chart the power of foot and calf muscles (Figure 9.4). This can appear daunting within the time pressures of a clinic. However, it is easy to do, rapid to perform with practice, and vitally important, because failure to diagnose imbalance will lead to poor results with recurrent deformity. Special note should be made of muscle imbalance, which usually indicates a neurological origin of the cavus.