The bunion tendon starts from the extensor digitorum longus muscle and ends at the base of the proximal phalanx of the thumb, with a few ending at the base of the distal phalanx of the thumb. The bunion tendon and the extensor digitorum longus tendon travel in a single fibrous sheath, and they both work to extend the thumb, while a contraction of the extensor digitorum longus tendon allows the thumb to extend dorsally in addition to abducting the thumb. If a problem occurs with the extensor digitorum longus tendon, you may experience a mild bunion with limited thumb extension, as well as tenderness and swelling, or even bruising, where the extensor digitorum longus tendon is located. However, if only one bunion tendon is ruptured, the other tendons can compensate, and the diagnosis can only be made by examining the extensor function of each finger individually.