What are the differential diagnoses for pain affecting the sole of the foot?

Differential Diagnoses for Sole of Foot Pain

The most common cause of plantar sole pain is plantar fasciitis, but a comprehensive differential must include stress fractures, nerve entrapment, soft tissue masses, sesamoid pathology, and metatarsal disorders based on precise pain localization. 1, 2, 3

Anatomic Localization-Based Differentials

Plantar Heel Pain (Medial Calcaneal Region)

• Plantar fasciitis is the most common cause, presenting with pain on the medial aspect of the calcaneus near the sole, characteristically worse with first steps in the morning or after prolonged sitting 4, 3
• Calcaneal stress fracture causes progressively worsening pain following increased activity, with positive calcaneal squeeze test (medial-to-lateral compression) and tenderness at the lateral calcaneal wall 2
• Heel pad syndrome presents as diffuse plantar heel pain, particularly when walking barefoot 2
• Nerve entrapment (tarsal tunnel syndrome or medial calcaneal nerve) causes burning or tingling sensations in the plantar heel 2, 5

Midfoot Plantar Pain

• Tarsal coalition should be suspected in chronic midfoot pain, with radiographs showing 80-100% sensitivity for calcaneonavicular coalitions and 100% sensitivity for talocalcaneal coalitions 1
• Stress fractures of navicular or cuboid bones, though radiographs have only 12-56% sensitivity initially 1
• Plantar fascia rupture in patients with prior fasciitis or corticosteroid injections 3

Forefoot Plantar Pain

• Metatarsal stress fractures, particularly second and third metatarsals in runners and athletes 1
• Morton's neuroma causes webspace pain between metatarsal heads, though radiographs are insensitive but useful to exclude other pathology 1
• Sesamoid disorders (sesamoiditis, fracture, or bipartite sesamoid) affect the first metatarsal head region 1
• Freiberg's infraction (osteochondrosis of metatarsal head, typically second metatarsal) 1

Lateral Plantar Pain

• Painful os peroneum syndrome with peroneus longus tendon pathology presents as lateral plantar foot pain 6
• Cuboid syndrome or lateral column stress fractures 2

Systemic and Infectious Causes

Diabetic Foot Complications

• Soft tissue infection (STI) presents with non-purulent secretions, friable granulation tissue, wound undermining, and foul odor 1
• Osteomyelitis underlying diabetic foot ulcers carries high amputation risk and requires prompt diagnosis 1
• Charcot foot is an inflammatory, destructive disorder mainly involving tarsal and metatarsal joints that may coexist with or cause ulcers 1

Other Systemic Conditions

• Inflammatory arthropathies should be considered with bilateral symptoms or pain in other joints 2
• Tumors (benign or malignant) are rare but require appropriate referral 2
• Vascular compromise can cause plantar pain and necessitates proper evaluation 2

Diagnostic Algorithm

Initial Clinical Assessment

• Pain location: Precisely palpate medial calcaneus, lateral calcaneal wall, plantar fascia insertion, metatarsal heads, and webspaces 2, 7
• Pain timing: Morning pain suggests plantar fasciitis; pain with increased activity suggests stress fracture 4, 3
• Neurologic symptoms: Burning, tingling, or radiation patterns indicate nerve involvement requiring immediate subspecialist referral 2, 5
• Calcaneal compression test: Medial-to-lateral squeeze for stress fracture 2
• Passive dorsiflexion test: Elicits plantar fascia pain 3

Initial Imaging

• Weight-bearing radiographs of the foot are the first imaging study for chronic foot pain, useful for tarsal coalitions, fractures, sesamoid pathology, and Freiberg's infraction 1, 5
• Radiographs show thickened plantar fascia and fat pad abnormalities with 85% sensitivity and 95% specificity for plantar fasciitis 1

Advanced Imaging (When Initial Workup Negative)

• MRI without contrast if radiographs negative but pain persists >1 week, or for suspected stress fractures, Morton's neuroma, or soft tissue pathology 1, 5
• Technetium bone scanning for suspected stress fracture with negative radiographs, as symptoms precede radiographic findings 2
• Ultrasound can diagnose plantar fascia thickness abnormalities, Morton's neuroma, and tendon pathology 3, 6

Critical Pitfalls to Avoid

• Do not assume isolated plantar fasciitis when pain radiates proximally or has neurologic features—this demands neurologic evaluation for lumbar radiculopathy 5
• Reexamine 3-5 days post-injury if trauma history exists, as excessive initial swelling limits examination accuracy 2
• Do not rely solely on initial radiographs to exclude stress fractures given 12-56% sensitivity 1, 2
• Never inject corticosteroids near the Achilles tendon due to rupture risk 2, 8
• Consider diabetic foot complications in all diabetic patients with plantar pain, as >50% of diabetic foot wounds are infected at presentation 1
• Obtain plain radiographs first in diabetic patients before advanced imaging, as part of the initial approach recommended by IWGDF and IDSA guidelines 1