When extensive damage occurs to the innermost layer of the periodontal ligament, competitive healing events take place. Healing from the socket wall (creating bone via bone marrow-derived cells) and healing from adjacent periodontal ligament (creating cementum and Sharpey’s fibres) occurs simultaneously.
If less than 20% of the root surface is involved, a transient ankylosis may occur, which can later be resorbed due to functional stimuli, provided the tooth in the healing period is stabilised with a splint which allows a minimum amount of mobility, or is non- splinted. In larger injuries (>4 mm2), a permanent ankylosis is created. The tooth thus becomes an integral part of the bone remodelling system, the resorbing cells being, primarily, osteoclasts. Subsequently, osteoblasts replace the resorbed areas of the root with bone.
In children, replacement resorption leads to loss of ankylosed teeth usually within 1^5 years. In adults, replacement resorption occurs more slowly, often allowing the tooth to function for many years (1^3).