Orthodontic root resorption studied by electron microscopy.

The present study supports previous findings that root resorption takes place simultaneously with and after the elimination of hyalinized tissue. The cementoid layer and the more mature periodontal collagen fibers adjacent to cementum are possible barriers preventing root resorption. The microenvironment around hyalinized tissue is favorable for the introduction of hard-tissue resorbing cells. The findings show that the elimination of hyalinized tissue leads to the removal of the cementoid and the mature collagen thus leaving a raw cemental surface without a barrier. It is therefore hypothesized that such an area is readily attacked by odontoclasts. Once resorption lacunae are established, the cementum is resorbed from the rear as an undermining process. By continued orthodontic force application the resorption process will proceed even after all hyalinized tissue is eliminated. If the orthodontic force is discontinued or falls under a certain level, the resorption lacunae are repaired.