Pes Cavus

Chapter 26


Pes Cavus




Pes cavus describes a foot with a high arch that maintains its shape and fails to flatten out with weight bearing. By majority, the components of pes cavus are an increased calcaneal pitch and varus of the hindfoot, plantar flexion of the medial forefoot, and adduction of the entire forefoot. The predominant deformity in pes cavus may be in the hindfoot, the forefoot, or a combination of both. A precise radiographic definition of pes cavus is difficult because the deformity is made up of various components in the forefoot and hindfoot.



Etiology


Although the specific etiology of any cavus foot varies with the disease process, all forms result from muscle imbalance. Historical attempts to attribute all forms of cavus foot to a single neurologic lesion have proven overly simplistic. Bentzon2 and Hallgrimsson16 in 1939 proposed that extrinsic muscle imbalance underlay all deformities. In 1959, Duchenne9 subsequently proposed that the deformity results from an imbalance between the extrinsics and the intrinsics. Neither approach is fully satisfactory; a cavus foot is best thought of as an end result that a variety of subtle neurologic lesions can produce.


Brewerton et al5 specifically looked at the cause of pes cavus in a series of 77 patients and found subtle neurologic defects in 66% of them, leaving a large group of “idiopathic” cases. Of these, 11 of the 26 patients had a family history of pes cavus, and 7 of the 26 had nonspecific abnormalities upon electromyographic and nerve conduction velocity examination. Most cases of idiopathic pes cavus likely represent a very subtle neurologic lesion that is below clinical detection. Roughly half of the detectable lesions are variants of Charcot-Marie-Tooth disease, but a host of other less common conditions can also be discovered (Table 26-1).19



Neurologic referral is mandatory in situations that might point toward a correctable lesion of the spinal cord, such as a syrinx or spinal cord tumor. These include rapid progression, hyperreflexia, clonus, or significant asymmetry between sides in motor pattern or deformity. A new diagnosis of central neurologic disease is not uncommon in foot and ankle practice.



Charcot-Marie-Tooth Disease


Charcot-Marie-Tooth disease (CMT) should at least be considered in every patient who presents with pes cavus. CMT is not, in fact, a single disease but rather a heterogeneous group of disorders caused by inheritable defects in any of several constituent proteins of the myelin sheath of peripheral nerve. The disorder was described in general terms by the great French neurologist Jean Martin Charcot and his pupil Marie in 1886 and independently by Tooth in England later that year.6,44 Originally, Charcot attributed the disorder to a spinal defect, and it was Tooth’s subsequent work that correctly classified it as a peripheral nerve disorder. The disease is the most common inheritable defect of peripheral nerves, but approximately half of the time it represents a new sporadic chromosomal recombination error.


The nomenclature associated with CMT is confusing because of the historical lack of understanding of its cause. The archaic term peroneal muscular atrophy (PMA) was supplanted by Dyck and Lambert, who developed an extensive classification of inheritable motor neuropathies based upon their electrodiagnostic patterns in the 1960s and 1970s.12 Their scheme refers to a series of seven hereditary motor sensory neuropathies (HMSN-I through HMSN-VII).


Since 1990 there has been an explosion of understanding of the specific genetic defects underlying the CMT disorders,4 leading to a new and still evolving reclassification of the disease.


CMT-1 is the most common form and accounts for more than 50% of all cases. It is autosomal dominant and demonstrates slow nerve conduction velocities (NCVs) in the range of 10 to 30 m/sec as a result of demyelination. CMT-1 can be further subdivided.


CMT-1A accounts for 80% of CMT-1 cases and is the single most common form of the disease in general. Curiously, it is usually caused by a segmental trisomy along chromosome 17. The area contains the gene for peripheral myelin protein-22 (PMP-22), whose function remains unknown. Although some cases have been linked to alternative point mutations in PMP-22, the segmental trisomy responsible for most cases indicates CMT can be produced from a gene dosage effect. This chromosomal aberration is inheritable in an autosomal dominant fashion, but many cases seem to represent sporadic chromosomal recombination events.


CMT-1B accounts for 5% to 10% of CMT-1 patients and is associated with a point mutation in the myelin P0 gene. The phenotype is associated with a particularly aggressive form of the disease.


CMT-1C represents the small remainder of CMT-1 patients in whom the genetic defect is still unknown.


CMT-2 is the second most common general form of the disease and represents 20% of patients. It is autosomal dominant like CMT-1 but has dramatically different electrical findings: NCVs are near normal, and there is no evidence of demyelination. Four separate chromosomal loci have been identified, but the product proteins involved remain unknown. In general, the course of CMT-2 is more indolent than that of CMT-1.


CMT-X shows an X-linked inheritance pattern; male patients are affected and female patients are either unaffected or mildly affected carriers. It is found in 10% to 20% of all CMT cases and is associated with defects in yet another myelin constituent protein, connexin 32.


CMT-4 is an autosomal recessive form of the disease and is quite rare. It in fact encompasses a large number of described genetic defects on different chromosomal loci; no product proteins have yet been described.


The foot deformities in CMT do not result from absolute weakness of the motor units powering the foot but of their relative imbalance. Initiation of the deformity likely results from an imbalance of the failing foot intrinsics and the preserved extrinsics.3 Subsequently, a specific pattern of extrinsic motor weakness is common in CMT in which the anterior and lateral compartment musculature is selectively affected, with certain curious exceptions. The disease almost always affects the peroneus brevis but spares the peroneus longus. This was first observed clinically by Mann and Hsu33 and subsequently confirmed by Tynan et al,45 who demonstrated that the cross-sectional area of the peroneus longus was preserved on magnetic resonance imaging (MRI) of patients with the disease. An additional oddity can be observed in the anterior compartment musculature; the extensor hallucis muscle can be spared while the anterior tibialis is affected. This occurs despite the more distal location of the extensor hallucis longus (EHL) and their shared peroneal innervation.


The reasons for the unusual patterns of motor weakness in CMT remain poorly understood. The selective denervation affects certain muscles in the anterior and lateral compartments, and only very late posterior compartment involvement is seen. Denervation in CMT progresses very differently from a classic symmetric polyneuropathy, such as that encountered in diabetes. At least some speculation has been centered upon the possibility that some element of nerve compression can play a role.14


Regardless of the cause of the patterns of weakness, each of the deformities of the disease can be explained in the form of a weak agonist muscle and a more normally functioning antagonist (Table 26-2). There is no evidence of spasticity in the motor units that remain innervated.



The functional peroneus longus serves to plantar flex the first ray while the denervated anterior tibialis fails to provide any counterbalancing dorsiflexion. The first metatarsal head is depressed and a forefoot cavus results. Next to a relative talus dorsiflexion, an equinus contracture develops as the Achilles tendon (triceps surae) is unopposed by the anterior tibialis. In addition, the toe extrinsics force the toes into a clawed position that is not counterbalanced by the denervated foot intrinsics. This also serves secondarily to depress the metatarsal heads and raise the arch. The supination and adduction of the foot is worsened because the posterior tibialis is unopposed by the weakened peroneus brevis. In cases with sparing of the EHL, the claw toe deformity of the hallux is worsened even more dramatically because the patient uses the EHL to dorsiflex the foot and compensate for the weak anterior tibialis.


It is rare in clinical practice to encounter a patient at such an early stage of disease that the foot is entirely supple with no hindfoot or forefoot contractures. However, rebalancing the foot through tendon transfers can help prevent the development of further deformity if it is done early enough.38 For instance, the overpull of the peroneus longus that forces the forefoot into cavus can be eliminated by transferring the tendon to the peroneus brevis. This also serves to help oppose the tibialis posterior and prevent adduction. The emphasis in early-stage surgery on CMT should be on tendon transfers rather than lengthening motor units. Because the long-term outcome of CMT is one of progressive, inexorable weakness, strength should be preserved whenever possible.



Poliomyelitis


The last great epidemic of polio in the United States occurred in New England in 1955. Although the residual effects of the disease are now encountered with increasing rarity, it still serves as a useful model of a process that can produce either a forefoot cavus or a hindfoot cavus.8


Paralytic polio is the result of a ribonucleic acid (RNA) virus that primarily affects the thalamus, hypothalamus, motor centers of the brain stem and cerebellum, and the anterior tracts of the spinal cord. There is a wide variety of clinical presentations depending upon what portions of the central nervous system (CNS) are affected, but as a practical matter the lower extremity weakness patterns of polio come from a strikingly selective destruction of the anterior motor neurons in the spinal cord itself, preserving function both proximal and distal to the lesion.


After an initial incubation period of 6 to 20 days, the acute phase of the disease is associated with the most dramatic paralysis and lasts approximately 7 to 10 days. Clinically detectable weakness usually occurs when more than 60% of the motor neurons to a muscle group are affected. Muscle function can recover gradually thereafter; the most substantial gains occur in the first 4 months, and some return of function is usually seen up to 2 years after the illness.



Patterns of Foot Deformity in Polio



Hindfoot Cavus


The gastrocnemius–soleus complex is the critical variable in determining what variety of foot deformity will develop. The classic cavus foot deformity resulting from polio is that of a hindfoot cavus associated with a dramatically high calcaneal pitch angle. This is the result of the paralysis of the gastrocnemius–soleus complex with preservation of the remainder of the posterior compartment, the intrinsic foot musculature, and the anterior tibialis. When appropriate tension is missing from the Achilles tendon, the long toe flexors still function to depress the metatarsal heads, raising the arch. The intrinsics foreshorten the distance between the metatarsals and the calcaneus, functioning much like a bowstring to raise the calcaneal pitch. The result is a vertical posture of the calcaneus.





Other Neurologic Lesions


Although CMT and polio represent the most historically prominent etiologies of the cavus foot, a wide variety of other lesions can lead to the deformity.


Friedreich ataxia is a familial progressive ataxia in which posterior column function is steadily lost.36 It occurs in an autosomal recessive form with an earlier age of onset (11.75 years) and in a dominant form with a later age of onset (20.4 years). No cases of onset after the age of 25 have been reported. The disease is usually associated with pronounced and progressive symmetric cavus foot deformities with severe claw toe formation. In numerous instances, the foot deformities have been the presenting complaint.


A heterogeneous group of hereditary cerebellar ataxias are also associated with cavus foot, but they are less easy to categorize than Friedreich ataxia, which primarily shows spinal cord involvement.


Roussy-Lévy syndrome is a rare syndrome of cavus foot, sensory ataxia without obvious long-tract signs, peripheral motor atrophy, and kyphoscoliosis. Because it shares characteristics of both diseases, it was poorly differentiated from Friedreich ataxia and CMT for many years.37 The onset occurs very early in childhood and runs a relatively benign course.


Spinal muscular atrophy is a heterogeneous group of disorders that are usually present from birth but have some late-onset forms.41 They are characterized by an inexorably progressive loss of anterior motor neuron cells. The disorders are characterized by hypotonia and can be associated with the cavus foot deformity, although it is rarely the presenting feature.


Structural spinal cord disease often manifests with cavus foot deformity and requires a high degree of suspicion to detect. In particular, the unexplained onset of a progressive bilateral cavus deformity, or essentially any unilateral deformity, should warrant an imaging workup. Spinal cord tumors are notorious for their early absence of symptoms, and foot deformity might be the initial complaint. Syringomyelia is a cavitation in the center of the spinal cord that usually occurs in the cervical cord but can also interrupt the neural pathways to the lower extremities and result in spasticity or deformity. Diastematomyelia is a rare disorder in which a spicule of bone or fibrous band sagittally divides the spinal canal in the thoracic or high lumbar regions and separates the spinal cord into two pieces, each surrounded by dura. Because the cord and axial skeleton grow at different rates, a traction myelopathy very slowly develops as the child matures. The findings can be subtle, but making the diagnosis is critical.


Spinal dysraphism in all its forms (spina bifida, myelocele, myelomeningocele) is certainly more common and can manifest with a variety of postural foot disorders depending upon the particular patterns of involvement. Fortunately, the diagnosis is almost always well established early in life.


Cerebral palsy is, by definition, a static encephalopathy and can result in a variety of foot deformities, including pes cavus. Although the neurologic lesion is not progressive, the flexibility of the postural disorders deteriorates with time.



Posttraumatic Cavovarus Foot Deformities


Any traumatic condition that leads to an imbalance of the intrinsic and extrinsic foot musculature can lead to a cavus deformity. The deep posterior compartment of the leg is most commonly involved in traumatic compartment syndromes, and a Volkmann contracture in that location will lead to a cavus foot with a prominent claw toe component. Crush injuries of the leg and severe burns or soft tissue loss can also have the same result, both from direct injuries to the musculature and indirectly through tibial nerve injury. Compartment syndromes confined to the foot most commonly occur with calcaneus fractures or with crush injuries to the forefoot; they have been associated with the late development of claw toes but not with cavus deformity.


Several forms of fracture malunion can also result in a fixed cavovarus deformity. Most commonly, a talar neck fracture with substantial medial comminution can fall into a varus malunion. This substantially limits subtalar joint eversion and leads secondarily to the calcaneus assuming a varus malalignment. Alternatively, a varus hindfoot can result from residual deformity from an intraarticular calcaneus fracture or even medial impaction of the tibial plafond.



Congenital Pes Cavus



Clubfoot Residuals


The cavus foot deformity is one of four components of the congenital clubfoot, easily remembered by the mnemonic CAVE (cavus, adductus, varus, equinus). Adult clubfoot residuals encountered after childhood casting usually result from a failure of early casting to adequately elevate the first ray before abducting the foot about the fulcrum of the talar head as described by Morcuende et al.35 The most severe deformities that result from improper casting technique are usually not those of residual cavus but of a rocker-bottom foot that results when the equinus is inappropriately corrected while the calcaneus remains locked under the talus; the foot then dorsiflexes through the midfoot rather than through the ankle.


A wave of enthusiasm for surgical clubfoot correction in the 1970s and 1980s is also now yielding residual effects in the adult foot and ankle population. The results of clubfoot correction surgery have proved to be substantially less reliable than once thought.30,42 A patient presenting with problems with a postsurgical clubfoot is just as likely to have overcorrection into planovalgus as residual undercorrection in cavovarus. The one constant in the surgically corrected clubfoot is a remarkable amount of stiffness in adults. In a dynamic gait analysis study, Huber and Dutoit18 specifically identified late subtalar stiffness as the primary feature associated with a poor result after childhood clubfoot surgery. It is rare that anything short of triple arthrodesis can be entertained to address the residual complaints.




Evolution of Deformity


Pes cavus must be viewed as a spectrum of deformities in which the underlying abnormality is that of an elevated longitudinal arch, but after that a variety of bony and soft tissue deformities can be present. The spectrum can range from a mild cavus foot, with flexible claw toes as the only significant clinical problem, to a severe fixed deformity, with altered weight bearing, callosities, lateral ankle laxity, stress reactions, and pain. The shape of the foot varies with the cause and duration of the motor imbalance that created the deformity. When muscular imbalance in pes cavus begins before maturation of the skeleton, there can be substantial change of healthy osseous morphology. After skeletal maturity, there is usually little or no change in the morphology.1 The cavus foot is best understood by systematically analyzing the bone deformities, the soft tissue deformities, and the specific muscle functions that are imbalanced.


The bony deformity may be predominantly in the hindfoot, the forefoot, or a combination of both. A hindfoot cavus describes an elevated pitch of the long axis of the calcaneus, which is usually greater than 30 degrees in a cavus foot (Fig. 26-1). Hindfoot cavus was a very common deformity in the era of widespread poliomyelitis; the focal nature of the disease in the anterior horn cells of the spinal cord often led to gastrocnemius weakness but sparing of the tibialis anterior and often the foot intrinsics. The resultant imbalance of forces then often led to dramatic calcaneal pitch angles and subsequent soft tissue contractures. Pure hindfoot cavus is now less common, while hindfoot varus is a predominant finding in pes cavus with underlying neurologic disease. Elevated calcaneal pitch is still encountered as a component of a combined deformity in the idiopathic cavus foot with no clear neurologic cause.



Most neurologic cavus deformities are thought to occur from a forefoot-driven hindfoot varus resulting from a muscle imbalance as in CMT. The peroneus longus tendon is a direct antagonist to the tibialis anterior tendon. Often spared by the neuropathy, the strong peroneus longus muscle overpowers the affected tibialis anterior and initializes the deformity by plantar flexion of the medial forefoot (Fig. 26-2). The tibialis posterior muscle induces the hindfoot varus, and the Achilles tendon further enhances the varus stress secondary to the plantar-flexed first ray and varus heel alignment, which alters the transmission of axial forces through the ankle joint. With inversion of the hindfoot, the lateral foot supinates and the peroneus longus muscle further plantar flexes the first ray to restore the tripod position and a plantigrade foot.1 Soft tissue contracture converts a flexible to a fixed cavus deformity over time.



Toe deformities are thought to occur from early degeneration of the intrinsic muscles of the foot. Because the lumbricals are not acting to stabilize the metatarsalphalangeal (MTP) joint (intrinsics effect: MTP flexion, proximal interphalangeal [PIP] and distal interphalangeal [DIP] extension), the unopposed extensor digitorum longus hyperextends the unstable lesser toes at the MTP level while the flexor digitorum longus and brevis flex the phalanges.1 The plantar-flexed metatarsal heads and plantar fascia shortening amplify forefoot plantar flexion. The deformities might be as mild as flexible clawing of the MTP joints in association with mild flexion of the interphalangeal joints, or they can manifest as fixed claw toe deformities. Once severe fixed claw toe deformities are present, the forces from the extrinsic toe extensors serve to hold the metatarsal heads in a plantar-flexed position (Fig. 26-3). Of importance, the plantar fat pad is displaced distally in severe cases as the toes pull up into extension. Not only are the metatarsal heads driven plantarward by the deformity, but they are also deprived of their normal cushioning layer of fat.



The plantar fascia commonly develops a contracture with time in all forms of cavus foot. It is anatomically much thicker on the medial aspect of the foot, and as the contracture develops, it not only holds the longitudinal arch in an elevated position but also holds the forefoot adducted and keeps the calcaneus inverted (Fig. 26-4). Although some bony procedures can secondarily relax the plantar fascia by altering the shape of the arch, they are not always adequate by themselves.



Early in the development of many cases of forefoot cavus the deformities remain relatively flexible. The arch might not flatten while standing, but muscle forces are holding it in position rather than bone and joint contractures. This is typical, for instance, of a very young patient with Charcot-Marie-Tooth disease. The subtalar joint compensates for the forefoot deformity by falling into a varus alignment (Fig. 26-5). As the disease progresses, the capsule and interosseous ligament of the subtalar joint become contracted, and the once-flexible hindfoot deformity becomes fixed.




Biomechanical Consequences of Pes Cavus


The mechanics of all variants of the foot are similar. The axes of the talus and the calcaneus are more collinear. The talar head remains over the anterior process of the calcaneus, and the navicular moves to a superior instead of a medial position to the cuboid; the subtalar joint axis is more vertical; and the Chopart joint function is impaired.1 When the hindfoot is locked in inversion, there is less subtalar and transversal tarsal motion during gait than in the normal foot. The ability of the foot to absorb the impact of walking by pronation during the early part of stance phase is diminished, and the first metatarsal head and the lateral border of the foot are overloaded.1 A cavus foot is always stiffer than one of normal conformation.


The relative dorsiflexion of the talus within the ankle mortise is caused by plantar flexion of the medial forefoot, limits ankle dorsiflexion, and is accompanied by anterior ankle impingement. Together with the hindfoot varus position, relative talus dorsiflexion is thought to contribute to an increased anteromedial contact stress in the ankle joint. The association between lateral ankle instability, cavovarus deformity, and ankle arthritis has been discussed in the literature.13,46 A significant pressure increase and reduction of the area loaded in the ankle joint was also demonstrated in simulated pes cavus and is considered to lead to ankle arthritis in long-standing deformities with, but also without, lateral ligament instability.24


In the forefoot, the weight-bearing area beneath the metatarsal heads and heel pad is decreased, leading to substantially higher plantar pressures in both locations. In addition, because of the clawing and hyperextension of the MTP joints, the toes do not participate in weight bearing during toe-off, and power is diminished. The plantar fascia normally functions as a passive windlass mechanism to elevate the longitudinal arch, plantar flex the metatarsals, and invert the calcaneus. In the cavus foot, all three of these conditions are present permanently, and the plantar fascia becomes contracted.


The muscle weakness patterns seen around the foot vary with the cause of the condition, but adduction of the forefoot is commonly seen when the posterior tibialis is active in the presence of a weak peroneus brevis. Metatarsus adductus exacerbates the already considerable tendency for excessive pressure in the lateral column of the foot, and stress reactions of the fifth metatarsal can result.



Physical Examination


The patient encounter begins with a careful history of the condition and a detailed family history. Generalized lateral column pain seems to be the most common presenting symptom associated with the cavus foot. Frequently, patients report ongoing lateral hindfoot instability despite one or more previous lateral hindfoot ligament repairs or reconstructions that failed later because the predisposing hindfoot statics were not corrected.


The patient’s gait is carefully observed for the nature of ground contact, the position of the heel, and the position of the toes during stance. Any fall of the heel toward further varus as weight transfers onto the limb should be noted by observing the patient walk from behind. During swing phase, the examiner should check for the possibility of a footdrop and the use of the extensor hallucis longus as an accessory dorsiflexor, leading to a cock-up deformity of the first MTP joint.


The heel pad could be seen easily from the front (“peek a boo”) with the patient standing and feet aligned straight ahead. In a normal foot, the heel pad is not visible on the medial side of the foot when viewed from the front because of the slight amount of valgus positioning of the average heel, which places the heel pad behind the normal hindfoot.


The relative position of the hindfoot to the forefoot must be noted along with the rigidity of that relationship. The normal hindfoot will be positioned in slight valgus when standing flat and deviate into varus when rising onto the toes. The patient is asked if he or she experiences a subjective instability in the tiptoe position.


The Coleman block test can be used to determine the ability of the hindfoot to fall back into an appropriate valgus posture. The heel alone or heel and lateral column of the foot are supported on a small flat wooden block while the forefoot or the medial column, respectively, remain unsupported. If the hindfoot is not fixed and the deformity is being driven by a first ray fixed in plantar flexion, the calcaneus will noticeably tilt into valgus when viewed from behind. In theory, a foot that exhibits flexibility on the Coleman block test can be corrected by working on the forefoot deformity alone.


With the patient seated, the examiner observes active and passive range of motion of the ankle, subtalar, transverse tarsal, and MTP joints. The forefoot should also be examined after manual correction of the hindfoot varus deformity to assess the amount of fixed forefoot pronation and to determine the need for medial metatarsals dorsiflexion osteotomies. Limited and painful ankle dorsiflexion may demonstrate anterior or anteromedial ankle impingement because of the relative talar dorsiflexion within the ankle mortise. The hindfoot is assessed for any chronic ligamentous incompetence. Clinical signs of the lateral border overload range from calluses to proximal diaphyseal or metaphyseal fractures of the lateral metatarsals. Peroneal tendon pathology, including tears and subluxation, are commonly present, as is tightness of the gastrocnemius–soleus complex.


Muscle function is very carefully assessed and documented for evaluation disease progression. Special attention should be paid to the ability of the peroneus longus to selectively plantar flex the first ray because this can point to the potential for a tendon transfer to effectively assist in treatment. A patient who does not carry a known neurologic diagnosis should also undergo a neurologic screening, including testing for long-tract signs, reflexes, hamstring tightness, and any asymmetry. Intrinsic wasting is usually easier to pick up in the upper extremity, and in suspected cases of CMT or other systemic peripheral neuropathies, an examination of the intrinsic musculature of the hands is in order. Subtle disease can usually be discerned in the loss of muscle mass and strength of the first dorsal interosseous along the radial border of the second metacarpal. The patient exhibits weakness in abducting the index digit away from the midline with the rest of the hand held in a neutral position to isolate the intrinsics.


Because of the very subtle findings associated with structural disease of the spinal cord, substantial unexplained asymmetry or rapid progression of the deformity warrants a neurologic referral and corresponding imaging.

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Aug 26, 2016 | Posted by in ORTHOPEDIC | Comments Off on Pes Cavus

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