What is the differential diagnosis (ddx) of acute pain around the sesamoid bones and along the flexor hallux longus?

Differential Diagnosis of Acute Pain Around Sesamoid Bones and Along Flexor Hallucis Longus

The differential diagnosis includes sesamoiditis, sesamoid stress fracture or acute fracture, bipartite sesamoid (symptomatic variant), flexor hallucis longus tenosynovitis, plantar plate disruption, os interphalangeus pathology, and less commonly avascular necrosis, infection, or inflammatory arthropathy. 1, 2, 3

Primary Sesamoid-Related Pathologies

Sesamoiditis

• Pain localizes under the first metatarsophalangeal (MTP) joint at the hallucal sesamoid bones 1
• Represents inflammation or stress-related changes without frank fracture 1, 2
• MRI shows bone marrow edema and stress-related changes 1
• Bone scintigraphy demonstrates increased uptake in the affected sesamoid 1

Sesamoid Fractures

• Acute fractures occur from direct trauma or sudden forceful dorsiflexion 4, 5
• Stress fractures develop from repetitive microtrauma and mechanical overload 2, 5
• CT is superior to radiographs for distinguishing true fractures from bipartite sesamoids 1, 2
• Radiographs have irregular, sharp fracture margins versus smooth, rounded margins in bipartite variants 6

Bipartite Sesamoid (Symptomatic)

• Common anatomical variant (present in 2-13% of individuals) that can become symptomatic 6, 3
• Critical pitfall: Not all bipartite sesamoids are pathologic—many are asymptomatic and require no treatment 6
• Negative bone scintigraphy with linear lucency suggests benign bipartition rather than fracture 6
• Positive bone scintigraphy indicates pathology requiring treatment (sesamoiditis, stress fracture, or inflammation) 6

Flexor Hallucis Longus-Related Pathologies

FHL Tenosynovitis

• Inflammation of the FHL tendon sheath causes pain along the tendon course and around sesamoids 3, 4
• MRI and ultrasound both demonstrate high sensitivity for acute tendon pathology 7
• Ultrasound specifically identifies tendon rupture or dislocation 7

Os Interphalangeus Complications

• Under-recognized ossicle located at the plantar interphalangeal joint (IPJ), visible radiographically in 2-13% but can exist as non-ossified nodule 3
• Located in the joint capsule separated from FHL tendon by a bursa 3
• Causes pain through altered mechanics with arthrosis, bursitis, tenosynovitis, or intractable plantar keratosis 3
• Consider MRI or CT when unexplained pain exists with friction blisters, intractable plantar keratosis, or hyperextension of IPJ, especially if no ossicle visible on radiographs 3

Soft Tissue and Joint Pathologies

Plantar Plate Disruption

• Occurs after MTP joint injury with disruption of the plantar capsular complex 7
• Best evaluated with weightbearing AP, lateral, and sesamoid axial views with contralateral comparison 7
• Fluoroscopy can assess sesamoid tracking distally with forced dorsiflexion 7
• MRI evaluates for plantar plate tears when radiographs are noncontributory 2

Less Common Causes

• Avascular necrosis of sesamoids presents with progressive pain and requires MRI for detection 2, 8, 5
• Infection causes severe pain with systemic symptoms 8, 4
• Inflammatory arthropathies (gout, rheumatoid arthritis) affect the sesamoid complex 3, 8
• Congenital absence of sesamoid (extremely rare) can cause MTP joint pain from altered biomechanics 8
• Chondromalacia and osteochondritis dissecans affect the sesamoid articular surfaces 8, 5

Diagnostic Imaging Algorithm

Initial Evaluation

• Start with weightbearing AP, lateral, and sesamoid axial views of both feet for comparison 7, 1
• The sesamoid axial view provides optimal visualization by projecting sesamoids free of the metatarsal head 1

Advanced Imaging Indications

• MRI is indicated if diagnosis remains uncertain after 6-12 weeks of conservative treatment or if symptoms worsen 2
• MRI detects bone marrow edema, stress fractures, avascular necrosis, plantar plate tears, and soft tissue pathology 1, 2
• CT distinguishes stress fractures from bipartite sesamoids and evaluates for nonunion 1, 2
• Bone scintigraphy differentiates symptomatic bipartite sesamoids (positive uptake) from asymptomatic variants (negative uptake) 1, 6

Critical Diagnostic Pitfalls

• Failing to differentiate sesamoiditis from Morton's neuroma leads to misdiagnosis 1
• Assuming all bipartite sesamoids are pathologic when many are asymptomatic anatomical variants 6
• Overlooking os interphalangeus pathology when no ossicle is visible on radiographs but clinical suspicion remains high 3
• Missing congenital sesamoid absence in patients with chronic MTP pain and negative imaging for other pathologies 8