What is a small, fluid-filled sack developing near a finger joint called?

A small, fluid-filled sack that develops near a finger joint is known as a myxoid cyst. These cysts typically occur at the dorsal aspect of the fingers, usually at the base of the nail, and are associated with degenerative changes in the joint. They contain a gelatinous substance, giving them a characteristic appearance and consist of a mucous-like fluid that can hold in moisture, leading to the cyst's formation.

Myxoid cysts are specifically linked to synovial tissue, which is the tissue that lines the joints and produces synovial fluid. This connection to joint areas can explain why they often appear near finger joints. The nature and composition of these cysts differentiate them from other types of growths or lesions found in similar locations, hence why myxoid cyst is the appropriate term for this condition.