Orthodontic braces function by applying controlled mechanical forces to reposition teeth. The strength of these appliances involves two areas: the physical resilience of the materials and the mechanical force they exert on the biological system. Physical durability ensures the appliance survives the harsh oral environment. Mechanical strength is precisely calibrated to stimulate the biological changes necessary for permanent tooth movement.
The Materials: Durability and Design Strength
Orthodontic appliances are constructed from specialized alloys chosen for their unique combination of strength, flexibility, and resistance to corrosion. Brackets, the small components bonded directly to the teeth, are frequently made from high-grade stainless steel or ceramic compounds. Stainless steel alloys, such as Type 304, are favored for their high tensile strength and ability to resist degradation from saliva and food particles.
The archwires, which connect the brackets and deliver the force, are made from materials like stainless steel, cobalt-chromium, or titanium alloys. Stainless steel wires exhibit a high tensile strength, often ranging from 1,000 to 1,800 megapascals (MPa), providing the necessary rigidity. Nickel-titanium (NiTi) archwires are prized for their superelasticity and shape memory, allowing them to deliver a light, continuous force even after being severely bent. This material selection ensures the system is durable enough to withstand chewing forces, yet flexible enough to function as a precise biological tool.
The Science of Applied Orthodontic Force
The working strength of braces is measured not by sheer power, but by the magnitude of force required to safely initiate a biological response. This force is calibrated to be light and continuous, typically measured in centi-Newtons (cN) or gram-force (gf). This gentle pressure is a deliberate choice, as heavy, sudden forces can cause tissue damage rather than controlled movement.
For most routine tooth movements, the optimal force range is between 50 and 100 centi-Newtons. The precise force is specific to the tooth type and the intended direction of movement. For instance, for the extrusion of a single front tooth, the force may be as low as 20 cN due to the less surface area available.
The strength of the system relies on the superelastic properties of wires like NiTi. These wires maintain a constant force across a significant distance of tooth travel, making the system highly efficient.
How Bone Remodeling Makes Braces Effective
The gentle mechanical force applied by the wires and brackets initiates a powerful biological process known as alveolar bone remodeling. This process converts sustained, light pressure into permanent structural change. When a tooth is pushed, the periodontal ligament (PDL) that holds the tooth in the jawbone is compressed on one side and stretched on the opposite side.
On the compression side, the pressure stimulates specialized cells called osteoclasts to become active. These cells work to resorb, or break down, the adjacent bone tissue, creating the necessary space for the tooth to move. Simultaneously, on the tension side where the PDL is stretched, different cells called osteoblasts begin to deposit new bone.
If the force applied is too high, it can compress the PDL so severely that it cuts off the blood supply, leading to a sterile tissue necrosis called hyalinization. This temporary tissue death halts tooth movement, creating a lag phase until the dead tissue is removed. The success of the treatment depends on the appliance’s ability to stimulate this delicate and continuous cycle of cellular activity within the jawbone.