Possible causes of a bulging bone at the little finger joint include tendon rupture, rheumatoid arthritis, tenosynovitis, chondroma, etc. 1. Tendon rupture: Trauma may lead to tendon rupture or dislocation of the interphalangeal joints, resulting in the appearance of a bulging bone at the little finger joint. 2. Rheumatoid arthritis: Rheumatoid arthritis may invade several joints and cause swelling and pain in the pinky joint. 3. Tenosynovitis: Long-term hand activities that cause strain may lead to tenosynovitis and this phenomenon. 4. Chondroma: Chondroma may appear on the bone of the little finger, which may also lead to this phenomenon. There may also be other reasons for the small thumb joint bone bulge, it is recommended to go to the regular hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.