Braces are a common orthodontic treatment used to correct malocclusions, which are misalignments of the teeth and jaws. The purpose of these appliances is to utilize controlled, gentle pressure over an extended period. This sustained force triggers a natural biological response within the jawbone, allowing the teeth to safely and predictably shift into new, desired positions. Understanding this process requires looking closely at both the mechanical tools that apply the force and the body’s reaction to that force.
The Essential Components of Braces
Brackets are small, precisely engineered components bonded directly to the front surface of each tooth, acting as anchor points for the system. Each bracket has a slot that receives the archwire, which is the true engine of the movement.
Metal bands are sometimes cemented around the molars at the back of the mouth. These bands provide a more robust anchor than brackets for the heavy forces sometimes needed for posterior tooth movement. Ligatures, which are small elastic O-rings or fine wires, complete the connection by securing the archwire firmly into the slot of each bracket.
Applying Precise Force: The Role of the Archwire
The archwire applies the active, continuous force necessary for tooth movement. The orthodontist initially shapes this wire to match the ideal dental arch form desired at the end of treatment. When the wire is placed into the brackets of misaligned teeth, it is bent and deflected from its ideal shape.
The wire’s inherent material properties cause it to constantly attempt to return to its original shape. This persistent effort generates the light, continuous pressure transmitted through the brackets to the teeth. Modern archwires often utilize nickel-titanium (NiTi) alloys due to their shape memory and superelastic properties.
NiTi wires can be severely bent but exert a consistent, low-level force as they recover their shape. This is an improvement over older stainless steel wires, which generated higher forces that dissipated quickly. Minimizing friction between the archwire and the bracket slot allows the force to be more efficiently delivered to the tooth for movement.
The Biological Foundation: Bone Remodeling
The mechanical force from the archwire triggers the biological process that actually moves the teeth. Teeth are suspended within the jawbone socket by the periodontal ligament (PDL), a network of fibers and cells. The forces applied by the braces act on this ligament, initiating a controlled biological response known as bone remodeling.
This remodeling involves two distinct cellular activities occurring simultaneously on opposite sides of the tooth root. On the pressure side, the PDL is compressed against the alveolar bone. This compression signals specialized cells called osteoclasts to migrate and break down bone tissue (resorption), creating space for the tooth to move.
On the opposite side, the PDL is stretched, creating a tension side. This tension stimulates osteoblasts to deposit new bone tissue, filling the void left behind as the tooth shifts position. This delicate balance between bone resorption and formation requires the force to be light and continuous. Excessive force can crush the PDL, leading to tissue damage, which halts movement and may cause side effects like root shortening.
Different Methods of Tooth Movement
Orthodontics involves precise control of the root and crown positions, and the way force is applied dictates the specific type of movement achieved.
- Tipping: The crown of the tooth moves significantly more than the root, often occurring when a single force is applied to the crown.
- Bodily Movement (Translation): The entire tooth, including the crown and the root, moves an equal distance in the same direction. This requires precise force application through the tooth’s center of resistance.
- Root Torque: A specialized movement that adjusts the angle of the tooth root without substantially changing the position of the crown. This utilizes a force couple to push the root into an ideal position.
- Rotation: Corrects a tooth turned around its long axis.
- Intrusion and Extrusion: Push the tooth deeper into or pull it slightly out of the socket, respectively.
The ability to control these distinct movements allows for the final precise alignment of the entire dental arch.