What are the likely causes and initial management for sharp pain under the arch of a single foot?

Sharp Pain Under the Arch of One Foot

The most likely diagnosis is plantar fasciitis, and initial management should include weight-bearing radiographs to exclude other pathology, followed by conservative treatment with rest, NSAIDs, plantar fascia and Achilles stretching exercises, and arch supports or orthotics. 1, 2

Most Likely Diagnoses

Plantar fasciitis is the predominant cause of plantar arch pain, characterized by sharp pain under the arch that typically worsens with first steps in the morning or after prolonged rest. 1, 2

Other important considerations for unilateral arch pain include:

• Plantar fascia tear - acute onset with severe pain, often following sudden forceful activity 1
• Baxter's neuropathy (compression of the inferior calcaneal nerve) - presents with burning or neurologic symptoms radiating from the heel into the arch 1, 2
• Tarsal tunnel syndrome - compression of the posterior tibial nerve causing numbness, paresthesia, and burning pain along the medial arch and plantar foot 3, 4
• Posterior tibial tendon dysfunction - causes progressive arch collapse with pain along the medial arch, associated with the "too many toes sign" when viewed from behind 5, 6
• Stress fracture (navicular, cuboid, or metatarsal) - progressive pain with increased activity, positive calcaneal squeeze test 1, 2

Initial Diagnostic Approach

Obtain weight-bearing radiographs of the foot as the initial imaging study for all patients with chronic foot pain of unknown etiology. 1, 2 Radiographs have 85% sensitivity and 95% specificity for plantar fasciitis and help exclude fractures, arthritis, and bony abnormalities. 2

Key Clinical Examination Findings

• Point tenderness location: Palpate the plantar fascia insertion at the medial calcaneal tubercle, along the medial arch, and the course of the posterior tibial nerve behind the medial malleolus 1, 2
• Calcaneal compression test: Squeeze the calcaneus medially-to-laterally to assess for stress fracture 2
• Neurologic symptoms: Assess for burning, tingling, or numbness suggesting nerve entrapment rather than mechanical pain 2, 3
• Tinel's sign: Tap over the tarsal tunnel posterior to the medial malleolus to elicit radiating paresthesias 3, 4

Advanced Imaging When Radiographs Are Negative

If radiographs are negative or equivocal but pain persists beyond one week, MRI foot without IV contrast or ultrasound are the appropriate next imaging studies. 1

• MRI without contrast is preferred for evaluating plantar fasciitis, plantar fascia tears, Baxter's neuropathy (showing denervation changes in abductor digiti minimi muscle), and tarsal tunnel syndrome 1
• Ultrasound is an equivalent alternative for plantar fasciitis and allows dynamic assessment, though it has limitations for characterizing nerve pathology 1

Initial Management Algorithm

For Plantar Fasciitis (Most Common)

Conservative treatment should include: 2

• Rest and activity modification
• NSAIDs for pain and inflammation
• Stretching exercises for both plantar fascia and Achilles tendon
• Heel cushions, arch supports, or orthotics
• Weight loss if indicated

For Suspected Nerve Entrapment

If burning, tingling, or neurologic symptoms are present, immediate subspecialist referral is essential for electromyography, nerve conduction studies, and MRI. 2, 3 Conservative management for confirmed tarsal tunnel syndrome includes activity modification, pain medications, physical therapy, and corticosteroid injections into the tarsal tunnel. 3

For Posterior Tibial Tendon Dysfunction

Treatment severity depends on the degree of dysfunction and includes immobilization, orthotics, physical therapy, or subspecialty referral for progressive deformity. 6

Critical Pitfalls to Avoid

• Do not rely solely on initial radiographs to exclude stress fractures, as sensitivity ranges from only 12-56%; symptoms often precede radiographic findings by weeks. 1, 2
• Never assume isolated plantar fasciitis when neurologic symptoms are present (burning, tingling, numbness) - this pattern demands nerve entrapment evaluation. 2, 3
• Avoid corticosteroid injections near tendons due to rupture risk; injections are contraindicated near the Achilles tendon and should be used judiciously. 2
• Reexamine patients 3-5 days post-injury if initial examination is limited by swelling, as excessive edema can obscure clinical findings. 2
• Consider the "Heel Pain Triad" - the combination of plantar fasciitis, posterior tibial tendon dysfunction, and tarsal tunnel syndrome can coexist, with failure of arch support causing traction injury to the posterior tibial nerve. 5

When to Refer

Refer to a specialist if: 2

• No improvement after 6-8 weeks of appropriate conservative treatment
• Neurologic symptoms requiring advanced diagnostic testing (EMG, nerve conduction studies)
• Consideration of surgical intervention for refractory cases

A positive Tinel's sign before surgery strongly predicts successful surgical relief after tarsal tunnel decompression, with success rates varying from 44-96%. 3