Horses and ponies with sidebone do not always show signs of lameness and some other cause may be responsible. Grooving the hoof at the quarters allows the foot to expand and relieve pain. A full roller shoe should also be fitted. Your vet will advise you whether 'bute' is necessary. A hook situated on the inside and bottom end of the thigh bone, on its hind leg, cups the patella and the medial patella ligament, so preventing the leg from bending.
Concussion is a contributory cause.
They can be caused by poor conformation or striking the leg with the other foot. Protective boots can help prevent splints. Both of these conditions are more common in younger horses and if suspected veterinary advice sought. Synovia is the name of the oily fluid that lubricates joints and tendons. It's secreted by a membrane that covers the joint or forms a protective sheath over a tendon.