Orthodontic treatment, commonly known as braces, corrects the misalignment of teeth and jaws (malocclusion). The goal is to move teeth into positions that improve chewing function and aesthetic appearance. This process relies on harnessing a specific, controlled biological response rather than simply pushing teeth with metal. The mechanism works by applying light, continuous mechanical pressure, which signals the body to remodel the surrounding bone structure. This constant force triggers cellular activities that allow the tooth to gradually slide through the jawbone into its intended location.
Components That Apply Pressure
The physical hardware of braces functions as a delivery system for mechanical forces. The primary anchor points are the brackets, small attachments bonded directly to the front surface of each tooth. Each bracket contains a slot that receives the connecting wire and dictates the angle of force applied to the tooth.
The archwire is the active component, serving as the source of the force that drives movement. Early in treatment, highly flexible Nickel-Titanium (NiTi) alloy wires are used due to their unique shape memory. The orthodontist bends this wire to fit the patient’s misaligned arch, forcing it to conform to the bracket slots. As the NiTi wire attempts to return to its original shape, it generates the light, continuous pressure required to initiate tooth movement.
As treatment progresses, the archwires are replaced with stiffer, stainless steel versions to apply stronger forces and maintain the achieved shape. Small elastic rings or thin metal wires, known as ligatures, secure the archwire firmly into the bracket slot to ensure effective force transmission. The forces applied are deliberately low, generally ranging between 50 and 100 grams, because excessive force can slow movement or damage surrounding tissues.
The Cellular Process of Tooth Movement
The mechanical forces delivered by the braces act upon the Periodontal Ligament (PDL), the soft tissue that suspends the tooth root within the bony socket. The PDL acts as a biological sensor, translating the mechanical strain from the archwire into biochemical signals that orchestrate jawbone remodeling.
When the archwire pushes a tooth, the PDL on one side of the root is squeezed against the alveolar bone, creating a zone of compression. This compression restricts blood flow and induces a localized inflammatory reaction. This state leads to the release of chemical messengers, such as prostaglandins, which signal the body to begin breaking down the bone.
These messengers activate precursor cells, causing them to differentiate into specialized bone-resorbing cells called osteoclasts. Osteoclasts dissolve the adjacent bone tissue, creating a space for the tooth to move into. If the force applied is too heavy, the intense compression can cause cell death in the PDL, temporarily halting movement until the necrotic tissue is removed in a slower process known as undermining resorption.
Simultaneously, the PDL tissue on the opposite side of the tooth root is pulled taut, creating a zone of tension. This stretching stimulates other cells within the PDL to differentiate into osteoblasts, the cells responsible for building new bone. Osteoblasts deposit new bone matrix, reinforcing the socket and filling the space left behind the moving tooth. This coordinated remodeling loop allows the tooth to relocate within the jaw structure.
Treatment Stages and Stabilization
Treatment is organized into distinct phases. The first phase, Leveling and Alignment, utilizes flexible NiTi wires to correct severe rotations, crowding, and vertical discrepancies. This stage focuses on creating a smooth, continuous arch form across all the teeth, providing a stable foundation for the next steps.
Following initial alignment, the process moves into the Working Phase, where the primary goal is to correct the relationship between the upper and lower arches, known as the bite. Stiffer, rectangular wires are employed to manage root positions, and auxiliaries like inter-arch elastics provide the traction needed to move entire segments of teeth. The final stage is Finishing and Detailing, where minor adjustments are made to ensure the teeth interlock perfectly and the roots are positioned ideally within the bone.
After active treatment concludes and braces are removed, the teeth have a natural tendency to shift back toward their original positions, a phenomenon known as relapse. This instability occurs because the newly formed bone and stretched periodontal ligament fibers require an extended period to fully reorganize and mature around the new tooth position.
The Retention Phase is a necessary continuation of treatment, requiring specialized appliances called retainers. Retainers, which can be fixed wires bonded behind the teeth or removable clear trays, mechanically hold the teeth in their final positions. This stabilization period allows bone density to solidify and the PDL fibers to adapt to their new configuration, preventing surrounding tissues from pulling the teeth back out of alignment.