Peroneal spastic flatfoot (PSF), also known as spastic pes planus, is a relatively uncommon but clinically significant condition characterized by a rigid flatfoot accompanied by painful spasms of the peroneal muscles. Unlike flexible flatfoot, which is typically benign and asymptomatic, PSF presents with stiffness, limited subtalar motion, and involuntary muscle contractions that hold the foot in an everted or pronated position.

Etiology

The causes of peroneal spastic flatfoot are diverse, and understanding them is essential for accurate diagnosis and effective treatment. Historically, the term “peroneal spastic flatfoot” was used broadly to describe rigid valgus deformities of the foot, but early research clarified that the condition arises from several distinct mechanisms.

Tarsal Coalition

The most common underlying cause of PSF is tarsal coalition, a congenital failure of segmentation between two or more tarsal bones. This abnormal fusion—most frequently between the calcaneus and navicular bones (calcaneonavicular bar) or between the talus and calcaneus (talocalcaneal bridge)—restricts normal hindfoot motion. As the subtalar joint becomes increasingly rigid, the peroneal muscles contract reflexively in an attempt to stabilize the foot, leading to painful spasms and a rigid flatfoot deformity.

Inflammatory and Neuromuscular Causes

A smaller subset of patients develop PSF secondary to inflammatory arthropathies, particularly rheumatoid arthritis, which can inflame the subtalar joint and trigger peroneal muscle spasm. In these cases, the deformity resembles that caused by tarsal coalition but arises from joint irritation rather than congenital bone anomalies.

Neuromuscular disorders such as cerebral palsy may also contribute to abnormal muscle tone and spasticity, resulting in a similar clinical picture. Trauma, infection, and idiopathic factors have likewise been implicated.

Accessory Talar Facets and Sinus Tarsi Pathology

Recent case reports highlight the role of accessory anterolateral talar facets (AALTF) and sinus tarsi syndrome in provoking peroneal spasm. These anatomical variants or inflammatory processes can irritate local mechanoreceptors, triggering reflexive muscle contraction and rigid flatfoot deformity.

Clinical Presentation

Patients with PSF typically present with:

• Rigid flatfoot deformity that does not correct with non–weight-bearing.
• Pain, often localized to the sinus tarsi or lateral hindfoot.
• Peroneal muscle spasm, which may be visible or palpable.
• Limited subtalar joint motion, especially inversion.
• Recurrent ankle sprains or gait abnormalities.

In children and adolescents, symptoms often emerge during the early teen years as tarsal coalitions ossify and become more symptomatic. Adults may present later, especially if arthritis develops secondary to long-standing abnormal biomechanics.

Diagnostic Evaluation

A thorough diagnostic workup is essential to differentiate PSF from other causes of rigid flatfoot and to identify the underlying pathology.

Physical Examination

Key findings include:

• A stiff, flattened medial arch.
• Pain on attempted inversion of the foot.
• Visible or palpable peroneal muscle spasm.
• Limited subtalar joint mobility.

Imaging

Radiographs are the first-line imaging modality and may reveal:

• Calcaneonavicular bars.
• Talocalcaneal bridging.
• Lipping of the talonavicular joint, which strongly suggests congenital anomalies.

CT scans provide detailed visualization of bony coalitions, while MRI is useful for detecting bone marrow edema, sinus tarsi inflammation, or soft tissue abnormalities. MRI is particularly valuable when coalition is suspected but not visible on X?ray.

Diagnostic Injections

Injection of local anesthetic into the subtalar joint or sinus tarsi can temporarily relieve spasm, confirming the joint as the source of pain and guiding treatment decisions.

Management

Treatment of PSF depends on the underlying cause, severity of symptoms, and patient age. Management typically progresses from conservative to surgical interventions.

Conservative Treatment

Initial management aims to reduce pain, inflammation, and muscle spasm:

• Rest and activity modification to reduce mechanical stress.
• Nonsteroidal anti-inflammatory drugs (NSAIDs) for pain control.
• Physical therapy, including stretching and strengthening exercises.
• Orthotic devices to support the arch and improve alignment.
• Casting or bracing, particularly in cases of tarsal coalition or severe spasm.

Case reports demonstrate that below-knee casting in plantarflexion and inversion can successfully relieve peroneal spasm and restore normal foot mechanics, even in refractory cases.

Injection Therapy

Corticosteroid injections into the subtalar joint or sinus tarsi may provide temporary relief of inflammation and spasm, though repeated injections carry risks such as cartilage degeneration.

Surgical Intervention

Surgery is indicated when conservative measures fail or when structural abnormalities require correction.

Common procedures include:

• Resection of tarsal coalition, particularly in younger patients before arthritis develops.
• Tendon lengthening to relieve adaptive shortening.
• Arthrodesis (joint fusion) for severe deformity or degenerative changes.

In cases involving AALTF or sinus tarsi pathology, resection of the accessory facet or debridement of inflamed tissue may be beneficial.

Prognosis

The prognosis for PSF varies depending on the underlying cause and timing of intervention. Early diagnosis and treatment—especially before the onset of arthritis—can restore near-normal function with minimal long-term limitations. Untreated cases, particularly those involving tarsal coalition, may progress to degenerative joint disease and chronic pain.

Case studies demonstrate that even refractory cases can achieve full recovery with appropriate immobilization and rehabilitation.

Peroneal spastic flatfoot is a complex condition arising from a variety of structural, inflammatory, and neuromuscular causes. Its hallmark features—rigid flatfoot deformity and painful peroneal muscle spasm—distinguish it from more common flexible flatfoot. Accurate diagnosis requires careful clinical examination and targeted imaging to identify underlying abnormalities such as tarsal coalition or sinus tarsi pathology. Treatment typically begins conservatively, with physical therapy, orthotics, and anti-inflammatory measures, but may progress to casting or surgical correction when necessary. With timely and appropriate management, most patients can achieve significant pain relief and restoration of normal foot function.