Hi,
It appears you have a digital myxoid cyst. The primary cause is a degenerative joint beneath the cyst (in this case the distal interphalangeal joint of the finger). The degenerative joint allows the lubricative gel within the joint to leak and track upwards to the skin where it appears as a dome-shaped translucent swelling.
Treatment requires this “track” or connection between the affected join to the skin to be sealed. A Specialist Hand Surgeon will probably be the best doctor to treat your condition.
