Root resorption occurs as a result of differentiation of macrophages into osteoclasts in surrounding tissue which, if in close proximity to the root surface will resorb the root surface cementum and underlying root dentine. This can vary in severity from evidence of microscopic pits in the root surface to complete devastation of the root surface.
Deciduous root resorption is a natural process which allows exfoliation of the primary teeth to make way for the secondary teeth. Deciduous root resorption is caused by osteoclast differentiation due to pressure exerted by the erupting permanent tooth.
Root resorption of secondary teeth can occur as a result of pressure on the root surface. This can be from trauma, ectopic teeth erupting in the path of the root, inflammation, excessive occlusal loading, aggressive tumours and growths. The most common cause in Western Society is orthodontic forces (Weiland 2003).
Roots of teeth are covered with cementum. This is a structure that resembles bone. It is however more resistant to resorption than bone. There are a number of theories as to why this is the case. The most common hypothesis is that because cementum is harder and more mineralised than bone and has anti angiogenic properties, blood vessels are inhibited from forming adjacent to cementum which in turn prevents access to osteoclasts.