Optimal Forces

Orthodontic tooth movement (OTM) consists of three phases resulting from cell signaling in the periodontal ligament (PDL) and the interaction of these factors with osteoclasts and osteoblasts within the bone. The initial application of a force displaces the tooth and results in compression of the PDL on the side of the tooth towards which it is being moved and tension on the opposing side. This is followed by the lag phase during which no movement occurs. Biologically, this period consists of the removal of hyalinization within the PDL which is caused by blood vessel occlusion. The osteoclasts which remove the hyalinized tissue are recruited by the release of receptor activator of nuclear factor kappa-B ligand (RANKL) via the Prostaglandin E2 pathway, leading to the upregulation of osteoclasts. The layer of necrotic or hyalinization tissue must be removed before OTM can occur. In the final stage of OTM, bone resorption and linear tooth movement occur.3-5

One proposed method, and the most commonly used to efficiently move teeth, is to apply optimal forces to minimize the amount of hyalinization tissue that occurs as a result of force application. Storey suggested using a light force to minimize the necrosis of cells and tissues in the PDL, leading to smaller areas of hyalinization, faster resorption, improved quality of bone, reduced root resorption and a reduced relapse potential.6 The actual amount of this force may vary based on the tooth’s root surface anatomy, bone support, location of the tooth in the mouth, and the specific movement being attempted. This concept is now applied almost universally in orthodontic treatment with the evolution of nickel titanium (NiTi) and superelastic copper nickel titanium (CuNiTi) wires for initial leveling and aligning in orthodontic treatment. Furthermore, studies have shown that among nickel titanium wires, heat activated ones deliver force levels that are lighter and more constant compared to traditional nickel titanium wires.7