Effusion

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Volar longitudinal view over PIP joint. An anechoic, minute effusion is seen displacing the hyperechoic joint capsule, that normally overlies the hyperechoic bone. At the apex of the head of the proximal phalanx, an interface reflex artifact is seen where anechoic synovial fluid overlies anechoic hyaline cartilage. Note that the proximal recess of the joint serves as a reserve space to facilitate flexion and extension, while the distal end of the joint capsule is attached to the base of the middle phalanx, deep to the flexor tendon.