A small intermetatarsal bursa in the fourth web space (between the fourth and fifth metatarsal heads) is analogous to the more commonly inflamed bursae in the second and third web spaces (Morton neuroma territory). Intermetatarsal bursitis in the fourth space produces plantar forefoot pain in the lateral forefoot, distinguished from third space Morton neuroma by its more lateral location.
Fourth web space bursitis produces plantar pain at the lateral forefoot level, reproduced by squeezing the metatarsal heads laterally (Mulder sign in this space). It may be associated with a small interdigital neuroma of the fourth common plantar digital nerve, though fourth space neuromas are significantly less common than third space (80%) and second space (15%) neuromas. Ultrasound confirms an intermetatarsal bursae fluid collection in the fourth web space. Treatment with wide footwear and corticosteroid injection is often sufficient.
Intermetatarsal bursitis in the fourth web space produces lateral forefoot pain reproduced by the Mulder test in this space, mimicking a fourth space neuroma; ultrasound distinguishes a pure bursal collection (anechoic, no adjacent nerve thickening) from a true neuroma (hypoechoic fusiform nerve enlargement), guiding targeted corticosteroid injection.
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