Thanks for your query.
The reasons of the protrusion of the thumb MP joint are:
Deformed joint due to previous trauma, disease of the joint or some diseases that affect the small joints only.
As for the trauma, you must be remembering if there was any trauma in the past.
If there is no history of trauma you can do the following:
Get a consultation of an Orthopedic Surgeon for clinical evaluation, investigations of the blood, urine and X-ray in two different views.
Further treatment if at all necessary will depend upon the informed discussion among you, the Doctors and your parents.
I hope this answers your query well.
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.