Orthodontic treatment aims to reposition teeth by applying controlled forces over time. For this movement to be successful, a stable foundation is required from which to push or pull. This stable foundation is known as anchorage, and it represents the resistance to any unintended tooth movement. Anchorage is the method an orthodontist uses to ensure that only the target teeth move in the desired direction.
The Mechanical Principle of Anchorage
The physics governing tooth movement is rooted in Newton’s Third Law of Motion: for every action, there is an equal and opposite reaction. When an orthodontic appliance applies a force to move one tooth, an equal amount of reactive force is simultaneously exerted in the opposite direction, distributed across the anchor structures. If the anchor unit is not adequately secured, the reactive force will cause the anchor teeth to move undesirably. Orthodontic anchorage is the strategic planning and application of devices to manage this inevitable reactive force. The goal is to maximize the desired tooth movement while minimizing any displacement of the anchor teeth.
Levels of Movement Control
Orthodontists classify the required stability of the anchor unit into different levels to guide treatment planning. Minimum anchorage, or reciprocal movement, allows the teeth being moved and the anchor teeth to move equally toward each other, typically used to close gaps. Moderate anchorage is a middle ground where some anchor unit movement is acceptable, but the primary movement focuses on the target teeth. Maximum anchorage is required when the anchor unit must remain virtually stationary. This level of control is necessary when all available space, such as that created by an extraction, must be used exclusively for moving the target teeth.
Devices Used to Secure Anchorage
A range of appliances and techniques are employed to achieve the necessary levels of resistance, varying from devices within the mouth to those utilizing the patient’s skeletal structure. Intra-arch methods use the teeth themselves to create a stronger foundation by joining multiple teeth together into a single, robust anchor unit, known as compound anchorage. Appliances like a transpalatal arch, which connects molars across the roof of the mouth, or a Nance button, which uses an acrylic pad against the palate, reinforce the anchor teeth to better resist the reactive forces.
Traditional methods sometimes relied on extra-oral devices, which engage structures outside the mouth to provide stability. Headgear, for instance, uses the back of the neck or the top of the head to redirect and absorb the reactive forces. While effective, these devices require consistent patient cooperation, often needing ten or more hours of wear per day to be successful.
The most precise method for achieving maximum stability involves Temporary Anchorage Devices (TADs), also known as mini-implants or mini-screws. These small, biocompatible screws are temporarily placed directly into the jawbone. Because TADs are fixed directly to the skeleton, they act as an immovable reference point, completely bypassing the limitations of using teeth as anchors. This skeletal anchorage eliminates the reciprocal reactive forces unavoidable in teeth-based systems. TADs enable complex tooth movements that were previously difficult, offering predictable and efficient results before being simply and painlessly removed.