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What is Type of external root resorption that occurs in concussion and subluxation: and explain in detail?
Surface resorption: 

It is non invasive process which occurs after luxation injury. 
Small superficial resorption cavities occur within cementum and the outer dentin.
It is repair process of physical damage to calcified tissue by recruitment of  cells following removal of damaged tissues by macrophages.

Inflammatory resorption: 

This resorption occurs as a result of the necrotic pulp becoming infected in the presence of severely damaged cementum. 
This infected pulp allows bacterial toxins to migrate out through the dentinal tubules into the periodontal ligament causing resorption of both root and adjacent bone.

Replacement resorption: 

Replacement resorption occurs when there is extensive damage to periodontal ligament and cementum. 
Healing occurs from the alveolar side creating a union between tooth and bone.

Textbook of Endodontics Nisha Garg 3rd Ed P: