Pseudoainhum refers to congenital annular bands or constrictions resulting from trauma or linked to other diseases. Congenital bands usually constrict an extremity, but may encircle any portion of the body, and result from congenital collagen dysplasia and not from scarring. The bands frequently are associated with other congenital or developmental anomalies. The radiographic manifestations of ainhum are diagnostic. Initially, a radiolucent band can be observed constricting the base of the involved toe, with distal swelling.
Osteolysis develops in the distal and middle phalanges, with a characteristic tapering effect.
Ultimately, the bone narrows until it fractures and autoamputates. The radiographic appearances in pseudoainhum are similar or identical to true ainhum. Histologically, ainhum shows fissuring and epidermal hyperkeratosis and parakeratosis, which is followed by a fibrotic reaction under the deepening fissure. The fibrosis is predominately composed of collagen. As scar tissue contracts, it constricts and narrows neurovascular bundles.