Pes Cavus

Charcot-Marie-Tooth Disease

CMT should be considered in every patient who presents with pes cavus. CMT is not, in fact, a single disease but rather a complex spectrum of progressive disorders. It is the most common form of hereditary motor-sensory neuropathy (HMSN) with multiple genotypes attributable to defects in a variety of constituent proteins of the myelin sheath of peripheral nerves. However, approximately half of the time CMT results from a new sporadic chromosomal recombination error. Therefore, CMT cannot be excluded based on a negative family history alone. The disorder was first described in general terms in 1886 by the great French neurologist Jean Martin Charcot and his pupil Marie and independently by Tooth in England later that year. Originally, Charcot attributed the disorder to a spinal defect, while Tooth's subsequent work correctly classified it as a peripheral nerve disorder.

The nomenclature associated with CMT is confusing because of the historical lack of understanding regarding its etiology. The archaic term peroneal muscular atrophy (PMA) was supplanted in the 1960s by Dyck and Lambert, who developed an extensive classification of inheritable motor neuropathies based upon their electrodiagnostic patterns. Their scheme describes 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, leading to a new and still evolving reclassification of the disease.

CMT-1 is the most common form, accounting for more than 50% of all cases. It is autosomal dominant and demonstrates slow nerve conduction velocities (NCVs) 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 CMT. It is usually inherited in autosomal dominant fashion, as a segmental trisomy along chromosome 17 in the area encoding the gene for peripheral myelin protein-22 (PMP-22) , whose function remains unknown. This segmental trisomy chromosomal aberration is the result of a gene dosage effect .

CMT-1B accounts for 5% to 10% of CMT-1 patients and is associated with a point mutation in the myelin P ₀ 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 also autosomal dominant but has dramatically different electrical findings: NCVs are near normal, and there is no evidence of demyelination. Four separate chromosomal loci have been identified, though the product proteins involved remain unknown. In general, the course of CMT-2 is more indolent than that of CMT-1. However, CMT-2A has been shown to progress more quickly during early childhood, whereas CMT-1A has a more consistent progression during childhood and through adolescence.

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

CMT-4 is a rare autosomal recessive form of the disease that encompasses a large number of described genetic defects on different chromosomal loci.

The foot deformities in CMT do not result from absolute weakness of the motor units of the foot but rather from their relative imbalance , leading to a nonplantigrade foot. Initiation of the deformity likely results from an imbalance between the failing foot intrinsic muscles and the preserved extrinsic muscles. Subsequently, a specific pattern of 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 Hsu and subsequently confirmed by Tynan et al, 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 is observed in the anterior compartment musculature where the more distal extensor hallucis muscle is often spared while the anterior tibialis is affected despite their shared peroneal innervation. Only very late in the disease progression is posterior compartment involvement noted.

The reason for the unusual patterns of motor weakness in CMT remains poorly understood. 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 possible role of nerve compression.

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 30-2 ). There is no evidence of spasticity in the motor units that remain innervated.

Forefoot valgus results from the functional peroneus longus plantar flexing the first ray while the denervated anterior tibialis fails to provide any counterbalancing dorsiflexion. An equinus contracture develops as the Achilles tendon (triceps surae) is unopposed by the anterior tibialis. In addition, claw toes develop due to the relatively stronger toe extrinsic muscles overpowering the denervated foot intrinsic muscles. The claw toe deformity of the hallux can be worsened in extensor hallucis longus (EHL)-sparing cases as the patient recruits the EHL to dorsiflex the foot and compensate for the weak anterior tibialis. This imbalance also secondarily depresses the metatarsal heads and raises the arch, contributing to the cavus. The supination and adduction of the foot is attributable to the posterior tibialis unopposed by the weakened peroneus brevis.

It is rare in clinical orthopedic practice to encounter a patient at such an early stage of disease that the foot is entirely supple with no contractures. However, rebalancing the foot through tendon transfers can help prevent the development of further deformity and mitigate arthritic changes if performed early enough. For instance, the overpull of the peroneus longus that forces the forefoot into valgus can be eliminated by performing a tendon transfer to the peroneus brevis. This is typically combined with a transfer of the posterior tibialis tendon to the dorsum of the foot to address the cavus and adduction deformity. The surgical emphasis in early-stage CMT should be on tendon transfers rather than lengthening motor units to preserve muscle strength.

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.

Paralytic polio is the result of a ribonucleic acid (RNA) virus that pri-marily 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.

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. 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. 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. 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 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

The Idiopathic Cavus Foot

Despite the litany of potential known causes of the cavus deformity, the largest single group of cases encountered is symmetric and has no known cause. They can manifest because of stiffness in the hindfoot, stress fractures along the lateral column, recurrent ankle instability, or, commonly, for symptoms totally unrelated to the conformation of the arch.

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 torques 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 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 correcting 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 of 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 intrinsic musculature.

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.

Investigations

Standing anteroposterior (AP), oblique, and lateral radiographs of the foot and ankle are essential. A line drawn down the axis of the talus should pass through the axis of the first metatarsal on both AP and lateral weight-bearing images of the foot in the normal situation. The axes should ordinarily be collinear, 0 ± 5 degrees. The lateral talo–first metatarsal angle (the Meary angle) can be used to assess the severity of a forefoot plantar flexion, whereas, as opposed to other measured angles, the extent appears to correlate with the development of anteromedial ankle arthritis. The AP talo–first metatarsal angle defines the amount of forefoot adductus. The calcaneal pitch angle is elevated in cases of hindfoot cavus.

Further findings on the lateral radiographs are an increased navicular height, an increased Hibbs angle (measured by a line through the axis of the calcaneus and the first metatarsal—in normal feet the angle is 45 degrees, and in cavus feet, it is near 90 degrees), and a posterior fibula with a “flat-topped” talus. The latter appearance is an artifact because in pes cavus a standard lateral view is in fact an oblique view.

On the AP radiograph of the foot, the talocalcaneal angle is almost parallel in moderate and severe hindfoot varus. The presence of any associated metatarsus adductus should be noted by drawing of the AP talo–first metatarsal angle because this can require some degree of additional surgical attention or limit the degree of correction that can be achieved.

The extent and progression of any ankle arthritis and talar tilt is recorded on standing AP and lateral ankle radiographs. The Saltzman hindfoot alignment view can be added to assess the angles of the tibiotalar and the subtalar joint and degree of hindfoot varus.

MRI or ultrasound can occasionally be helpful to reveal any inflammation, splitting, or tears of the tendons and lateral ligaments. MRI can also detect early cartilage degeneration and osteochondral lesions when arthritis is not yet visible on plain radiographs or CT.

A digital dynamic pedobarograph can be helpful to determine a pathologic plantar pressure distribution. Associated changes are the lateralization of the line of center of pressure and increased zone of pressure under the first metatarsophalangeal joint in case of a plantar-flexed first metatarsal.

Weightbearing computed tomography (WBCT) with cone-beam acquisition is a relatively new imaging modality that has significant advantages over conventional CT. Cone-beam CT allows acquisition of data from multiple projections with one rotation of the beam around the patient, and, combined with flexible gantry movements, makes three-dimensional weight-bearing imaging of the lower extremity feasible. The technology has a low radiation dose, high image resolution, and fast image acquisition time. Other advantages include low cost and small equipment size relative to conventional CT.

WBCT is ideal for assessing complex foot and ankle deformities, including cavus deformity. Two-dimensional (2D) x-rays are dependent on patient positioning and x-ray beam projection, either of which can affect the measurements obtained on an x-ray. In contrast, WBCT is three dimensional (3D) and independent of both specimen orientation and beam projection. Obtaining images with the patient in a standing position allows more accurate assessment of alignment and articular relationships compared with non–weight-bearing CT scans.

A variety of “3D biometrics” have been described to aid in assessing deformity and alignment on WBCT imaging. Perhaps most germane to cavus deformity is the foot and ankle offset (FAO), a “semi-automatic” 3D biometric that measures hindfoot alignment. The FAO uses the forefoot and not the tibia as a reference, which is a more accurate way of assessing how the foot interacts with the ground. The FAO corresponds to the offset between the hindfoot and forefoot midline and the center of the talus, expressed as a percentage in order to normalize it to foot size ( Fig. 30-6 ). Varus hindfeet are assigned a negative percentage and valgus hindfeet a positive percentage. The FAO has been shown to have high interobserver and intra-observer reliability. FAO also correlates well with physical examination. Other 3D biometrics have been utilized for cavus foot deformity with WBCT. These include the calcaneal offset (CO) and the hindfoot alignment angle (HAA), both of which have shown high interobserver and intra-observer reliability. The forefoot/hindfoot offset (FHO) has also been described as a metric of hindfoot alignment in cavus feet. Distance mapping is a WBCT image analysis technique that describes the 3D relative positions between joint surfaces. Finally, WBCT studies are beginning to look at 3D metrics that describe the effect of the upper leg on hindfoot alignment. Although 3D biometrics in cavus feet may assist with diagnosis and provide detailed information regarding the deformity, the extent to which these metrics correlate with clinical examination or inform surgical execution remains unclear.

Most WBCT studies to date have focused on pes planus deformity, but studies on WBCT and cavus foot deformity are beginning to be published. One study retrospectively reviewed WBCT scans in 17 CMT patients, 17 idiopathic cavus patients, and 17 normal controls. Eight 2D measurements and three 3D measurements (FAO, CO, and HAA) were obtained. The CMT and idiopathic cavus feet were significantly different from normal controls, and the CMT patients demonstrated significantly more deformity than idiopathic cavus patients based on the 3D measurements. In another retrospective case control study, 10 asymptomatic cavus feet were compared to 10 normally aligned feet. Distance mapping was performed at the ankle, subtalar, talonavicular, calcaneocuboid, naviculocuneiform, and tarsometatarsal joints. Significant differences were identified in the joint relationships between the normal feet and the cavus feet.

Nonoperative Treatment

Many cases of cavus deformity represent stable or slowly progressive deformities that are appropriately managed, at least initially, by nonoperative means. In the case of an adolescent with progressive deformity and a still-supple foot, however, there may be much to be lost by delay. Soft tissue surgery alone might manage the deformity early in the course of the disease, avoid ankle arthritis, and prevent the necessity of osteotomies or fusions.

A stretching program to maintain motion is an important component of conservative management, particularly in cases of neurologic origin. Eversion and dorsiflexion should be emphasized. Metatarsalgia might be an early presenting complaint from uncovering of the metatarsal heads as claw toes develop. Accommodative manufactured shoes with extra-depth toe boxes can be of substantial benefit.

Orthoses are a valuable adjunct to the conservative treatment of cavus feet. The reduced weight-bearing area in cavovarus feet is enlarged with orthotic devices. Typical custom foot orthotics for cavus may include an elevated heel to accommodate a tight gastrocnemius muscle and a recess under the first metatarsal head to accommodate the plantar-flexed first ray and allow some degree of hindfoot eversion. A forefoot wedge, beginning just lateral to the first metatarsal recess, extends to the lateral border of the device to mirror the forefoot pronation. Over the counter “cavus foot” orthotics are now available as well.

Basically, the more fixed the cavus deformity, the less likely it is that the patient will benefit from functional orthoses, and discomfort and calluses may develop in supported areas. In these patients, an accommodative insole with adequate cushion may relieve pain from pressure points. For lateral ankle instability, a high-top boot or an off-the-shelf ankle brace offer hindfoot stabilization. Lace-up braces are easier to fit inside a shoe or boot and stabilize the ankle comparably to plastic upright braces. Preexisting ankle instability usually worsens by high-arched orthoses amplifying hindfoot varus.

Severe muscle weakness is usually treated with full-length custom ankle–foot orthoses (AFOs) to prevent foot drop. The integration of orthotic modifications into the AFO improves proprioception and ankle stability more than the brace alone. Many patients can be managed by hinged AFOs with dorsiflexion assistance to allow a much more normal gait pattern. In some cases of equinus deformity, full clamshell braces or casts are needed because the strong or unopposed plantar flexors will overcome the correction obtained with a posterior brace or anterior strap.