Mastering the Cavus Foot and Associated Neuropathies: A Comprehensive Surgical Guide

INTRODUCTION TO THE CAVUS FOOT AND NEUROPATHIC DEFORMITIES

The pes cavus (or cavovarus) foot presents one of the most formidable challenges in orthopedic foot and ankle surgery. Characterized by an abnormally high medial longitudinal arch, forefoot equinus (plantarflexion), and hindfoot varus, the deformity is rarely idiopathic. It is frequently the manifestation of an underlying neuromuscular disorder, most notably Charcot-Marie-Tooth (CMT) disease, spinal dysraphism, or cerebral palsy.

Because the altered biomechanics of the cavus foot place immense stress on the lateral column and plantar structures, patients frequently develop secondary pathologies, including stress fractures, peroneal tendinopathy, and peripheral nerve entrapments such as Tarsal Tunnel Syndrome and Interdigital (Morton’s) Neuroma. This masterclass provides an exhaustive, evidence-based framework for the evaluation and surgical reconstruction of the cavus foot, alongside the definitive management of its associated neuropathies.

SUMMARY OF CAVUS FOOT EVALUATION AND TREATMENT

The successful management of the cavus foot hinges on a systematic, algorithmic approach. The primary goal of any intervention is to produce a stable, plantigrade, and painless foot while preserving maximum joint mobility.

The Six Determinants of Prognosis and Treatment

Before embarking on surgical intervention, the orthopedic surgeon must answer six critical questions to dictate the treatment algorithm:

1. Is the deformity multiplanar or single planar?
   Cavus deformities are rarely isolated to the sagittal plane. They typically involve forefoot pronation, midfoot cavus, and hindfoot varus.
2. Is the deformity primarily forefoot, midfoot, hindfoot, or a combination of these?
   Identifying the apex of the deformity dictates the level of osteotomy or arthrodesis.
3. Is one or more of the components fixed?
   The Coleman Block Test is mandatory. If the hindfoot varus corrects when the plantarflexed first ray is allowed to drop off a block, the hindfoot deformity is flexible and driven by the forefoot. If it does not correct, the hindfoot deformity is fixed and requires direct surgical correction (e.g., calcaneal osteotomy).
4. Is the underlying cause a progressive, severe course, with or without treatment?
   Neuromuscular conditions like CMT are progressive. Surgical planning must account for future muscle degradation.
5. Are tendon transfers required to maintain the correction gained by arthrodesis or osteotomy?
   Bony correction alone will fail if the deforming muscle forces (e.g., strong peroneus longus overpowering a weak tibialis anterior) are not balanced.
6. Is there a sensory deficit?
   The presence of neuropathy drastically alters the surgical threshold. In general, the least invasive procedure that alleviates symptoms is the best treatment for a cavus foot deformity, especially in feet with sensory deficits, to minimize the risk of Charcot arthropathy or chronic ulceration.

Clinical Pearl: Triple arthrodesis in patients with neuromuscular disease may ultimately result in degenerative arthritis of adjacent joints. Furthermore, triple arthrodesis in adolescents with Charcot-Marie-Tooth disease is significantly less likely to produce long-term satisfactory results than when performed in adults 40 years or older. However, in the absence of a sensory deficit, triple arthrodesis remains a reasonable salvage choice for severe, rigid cavovarus deformity and may be the only procedure capable of producing a plantigrade foot.

SURGICAL MANAGEMENT OF THE CAVUS FOOT

Surgical reconstruction of the cavus foot is an à la carte procedure, tailored to the specific flexibility and apex of the patient's deformity.

1. Soft Tissue Releases (Steindler Stripping)

Indications: Flexible cavus deformity, often as an adjunct to bony procedures.
Technique:
* Positioning: Supine with a bump under the ipsilateral hip.
* Approach: A medial incision is made over the calcaneal tuberosity.
* Dissection: The plantar fascia, abductor hallucis, flexor digitorum brevis, and abductor digiti minimi are sharply released from their origins on the calcaneus.
* Pitfall: Avoid injury to the lateral plantar nerve, which courses deep to the abductor hallucis and can be tethered during aggressive stripping.

2. First Metatarsal Dorsiflexion Osteotomy

Indications: Fixed plantarflexion of the first ray driving a flexible hindfoot varus (positive Coleman block test).
Technique:
* Approach: Dorsomedial incision over the first metatarsal base.
* Osteotomy: A dorsal closing-wedge osteotomy is performed 1 to 1.5 cm distal to the tarsometatarsal joint.
* Fixation: The osteotomy is closed, elevating the first metatarsal head, and secured with a dorsal plate or crossed compression screws.

3. Calcaneal Osteotomy (Dwyer or Lateralizing)

Indications: Fixed hindfoot varus.
Technique:
* Approach: Oblique lateral incision posterior to the sural nerve and peroneal tendons.
* Osteotomy: A lateral closing-wedge (Dwyer) osteotomy or a lateralizing sliding calcaneal osteotomy is performed.
* Fixation: The calcaneal tuberosity is shifted laterally and slightly superiorly, then fixed with one or two large-fragment cannulated screws inserted from the heel pad.

4. Tendon Transfers

Indications: Muscle imbalance, typically a strong peroneus longus and posterior tibial tendon overpowering weak dorsiflexors and evertors.
Techniques:
* Peroneus Longus to Brevis Transfer: Decreases the plantarflexion force on the first ray and augments hindfoot eversion.
* Jones Transfer: Transfer of the Extensor Hallucis Longus (EHL) to the neck of the first metatarsal, with IP joint arthrodesis of the hallux, to elevate the first ray and correct clawing.
* Posterior Tibial Tendon (PTT) Transfer: Routed through the interosseous membrane to the dorsum of the foot (usually the lateral cuneiform) to provide active dorsiflexion in drop-foot presentations.

5. Triple Arthrodesis (Salvage)

Indications: Severe, rigid, multiplanar deformity with degenerative joint disease; failed joint-sparing reconstructions.
Technique:
* Approach: An extensile lateral approach (Ollier) combined with a medial approach to the talonavicular joint.
* Preparation: The subtalar, calcaneocuboid, and talonavicular joints are denuded of cartilage. Wedges of bone are resected to correct the varus and cavus (typically a laterally based wedge).
* Fixation: Rigid internal fixation using large cannulated screws for the subtalar joint and plates/screws for the transverse tarsal joints.

Surgical Warning: Over-resection of the talonavicular joint can lead to severe shortening of the medial column. Always prioritize restoring the talocalcaneal angle and achieving a plantigrade, stable construct over anatomical perfection of the arch.

TARSAL TUNNEL SYNDROME (TTS)

Patients with cavovarus feet are highly susceptible to Tarsal Tunnel Syndrome due to the altered tension on the tibial nerve as it courses behind the medial malleolus. TTS can also arise from space-occupying lesions (ganglions, lipomas, varicosities) or severe pronation deformities.

Clinical Evaluation

• Symptoms: Burning, tingling, and numbness radiating into the plantar aspect of the foot and toes. Pain is often exacerbated by prolonged standing or walking.
• Signs: A positive Tinel’s sign over the tarsal tunnel. The dorsiflexion-eversion test (stretching the tibial nerve) often reproduces symptoms.
• Diagnostics: Electromyography (EMG) and Nerve Conduction Studies (NCS) confirm the diagnosis. MRI is highly recommended to rule out space-occupying lesions, which account for a significant percentage of failed conservative treatments.

Surgical Technique: Tarsal Tunnel Release

Indications: Failure of conservative management (orthotics, NSAIDs, injections), presence of a space-occupying lesion, or progressive motor weakness (intrinsic muscle atrophy).

Step-by-Step Approach:
1. Positioning: Supine with the leg externally rotated. A thigh tourniquet is applied.
2. Incision: A curvilinear incision begins 2 cm proximal to the medial malleolus, coursing distally and anteriorly along the course of the tibial nerve toward the navicular tuberosity.
3. Superficial Dissection: The flexor retinaculum (laciniate ligament) is identified and carefully divided longitudinally.
4. Nerve Decompression: The posterior tibial nerve is identified proximal to the retinaculum and traced distally. The surgeon must identify its three main branches: the medial plantar nerve, the lateral plantar nerve, and the medial calcaneal nerve.
5. Deep Release (Critical Step): The deep fascia of the abductor hallucis muscle must be completely released. The medial and lateral plantar nerves pass through separate fibro-osseous tunnels beneath this muscle. Failure to release the deep fascia of the abductor hallucis is the most common cause of failed TTS surgery.
6. Closure: The skin and subcutaneous tissues are closed. The flexor retinaculum is not repaired, to prevent recurrent compression.

Pitfall: The medial calcaneal nerve has highly variable anatomy. It may branch proximal to the flexor retinaculum or pierce it directly. Meticulous dissection is required to avoid iatrogenic neuroma formation.

INTERDIGITAL (MORTON’S) NEUROMA

Morton’s neuroma is a compressive neuropathy of the common digital nerve, most frequently occurring in the third intermetatarsal space. In the cavus foot, the steep metatarsal angle and clawed toes exacerbate the tethering of the nerve against the deep transverse metatarsal ligament.

Clinical Evaluation

• Symptoms: Sharp, burning pain in the forefoot, often described as "walking on a marble." Pain radiates into the adjacent toes.
• Signs: Mulder’s Click—a palpable click and reproduction of pain when the metatarsal heads are squeezed together while applying plantar pressure to the webspace.
• Diagnostics: Diagnosis is primarily clinical. Ultrasound or MRI can be utilized to confirm the diagnosis or rule out differential diagnoses such as metatarsophalangeal joint synovitis or a Freiberg’s infraction.

Surgical Technique: Interdigital Neurectomy

Indications: Persistent pain despite conservative measures (wide-toe box shoes, metatarsal pads, corticosteroid injections).

Dorsal vs. Plantar Approach:
The dorsal approach is the gold standard for primary neurectomy due to the avoidance of a painful plantar scar. The plantar approach is generally reserved for revision surgery or recurrent neuromas.

Step-by-Step Dorsal Approach:
1. Positioning: Supine. Ankle or calf tourniquet applied.
2. Incision: A 3 cm longitudinal dorsal incision is made centered over the affected intermetatarsal space (usually the 3rd webspace), starting at the web and extending proximally.
3. Dissection: Blunt dissection is used to separate the extensor tendons. A lamina spreader is inserted between the metatarsal heads to open the space.
4. Ligament Release: The deep transverse metatarsal ligament is identified and sharply divided. This immediately relieves the compression and allows the neuroma to bulge dorsally.
5. Nerve Resection: The common digital nerve is identified. It is traced distally to its bifurcation into the proper digital nerves, which are transected. The nerve is then traced proximally into the proximal metatarsal space.
6. Proximal Transection (Critical Step): The nerve must be placed under gentle traction and transected as far proximally as possible (at least 1-2 cm proximal to the weight-bearing pad of the metatarsal heads). The proximal stump must retract deep into the intrinsic musculature to prevent a symptomatic stump neuroma.
7. Closure: The deep transverse metatarsal ligament is left open. Skin is closed with non-absorbable sutures.

Clinical Pearl: Always send the excised nerve specimen for histopathological examination to confirm the diagnosis of perineural fibrosis and rule out other pathologies. Warn the patient preoperatively about permanent numbness in the affected webspace.

POSTOPERATIVE PROTOCOLS

Cavus Foot Reconstruction

• Weeks 0-2: Non-weight-bearing in a well-padded short leg splint. Strict elevation to minimize edema.
• Weeks 2-6: Sutures removed. Transition to a non-weight-bearing short leg cast.
• Weeks 6-10: Radiographs taken to assess osteotomy/arthrodesis healing. Transition to a CAM boot with progressive weight-bearing. Physical therapy initiated for range of motion and strengthening of tendon transfers.
• Months 3-6: Transition to supportive footwear with custom orthotics. Return to baseline activity is typically 6 to 12 months.

Tarsal Tunnel Release

• Weeks 0-2: Non-weight-bearing in a bulky Jones dressing and posterior splint in neutral dorsiflexion.
• Weeks 2-4: Sutures removed. Transition to a CAM boot. Weight-bearing as tolerated.
• Weeks 4+: Transition to normal footwear. Nerve regeneration and symptom resolution may take up to 6-12 months, especially in cases of chronic compression.

Morton's Neurectomy

• Weeks 0-2: Heel-weight-bearing only in a rigid postoperative shoe. Elevation is critical to prevent hematoma formation.
• Weeks 2-4: Sutures removed. Progressive weight-bearing in a wide, stiff-soled shoe.
• Weeks 4+: Return to normal footwear as tolerated. Desensitization massage of the dorsal scar is encouraged.

CONCLUSION

The management of the cavus foot and its associated neuropathies demands a profound understanding of lower extremity biomechanics and peripheral nerve anatomy. Whether performing a complex multi-level bony reconstruction for Charcot-Marie-Tooth disease, a meticulous release of the tarsal tunnel, or a precise interdigital neurectomy, the surgeon must adhere to strict, evidence-based principles. By systematically addressing both the structural deformities and the neurovascular compromises, the orthopedic surgeon can reliably restore function, alleviate pain, and prevent the devastating long-term sequelae of the neuropathic foot.