Orthodontic anchorage is the resistance to unwanted tooth movement that may occur during treatment. When a force is applied to reposition a tooth, an equal and opposite force is simultaneously exerted on the remaining teeth. If this reactionary force is left uncontrolled, it would cause the anchor teeth to shift unintentionally, potentially compromising the overall treatment outcome. Anchorage is the strategic method orthodontists use to counteract this mechanical principle, ensuring that only the target teeth move as planned.
Understanding the Mechanical Necessity
The physics governing tooth movement dictates that every force applied to shift a tooth generates a corresponding counter-force. If a force is applied to pull a crowded front tooth backward, the same amount of force is pushing the back teeth forward. Without a mechanism to resist this reactionary push, both the front and back teeth would move toward each other, a phenomenon known as reciprocal movement.
This reciprocal movement poses a significant challenge because the goal of orthodontics is often to achieve precise, differential tooth movement. For example, in a case requiring space closure, the orthodontist may intend for the front teeth to move backward significantly more than the back teeth move forward. If the reactionary force is not managed, the anchor teeth could move forward too much, resulting in a loss of the necessary space and an altered bite.
The mechanical necessity of anchorage is centered on maintaining the integrity of the anchor unit—the teeth or structures designated to stay put. Predictable tooth movement is impossible if the foundation from which the force is generated is unstable. A well-designed anchorage plan ensures that the majority of the applied force is translated into the desired movement of the target teeth, while the anchor teeth remain relatively stationary.
Classifying the Required Resistance
Orthodontic treatment planning begins with assessing the degree of resistance needed, which is categorized into three main classifications based on the desired movement ratio. This strategic planning determines which teeth are permitted to move and how much movement is acceptable for the anchor unit.
Minimum Anchorage
Minimum anchorage is used when the movement of the anchor teeth is acceptable or even beneficial to the overall treatment result. In these cases, the anchor teeth are expected to move slightly, allowing for a relatively equal, reciprocal movement between the two units under force.
Moderate Anchorage
Moderate anchorage is a mid-level classification, where some movement of the anchor unit is tolerated, but the primary goal is still to maximize the movement of the target teeth. This requires employing devices that increase the stability of the anchor unit beyond simple tooth-to-tooth splinting.
Maximum Anchorage
Maximum anchorage is the highest level of resistance required, demanding that the anchor teeth show little to no measurable forward movement. This classification is typically required when all created space must be utilized for the movement of only the target teeth, such as retracting severely protruding front teeth.
Sources of Support: Dental and Skeletal
The resistance required to stabilize teeth can be derived from two primary anatomical sources: the teeth themselves, known as dental anchorage, or the surrounding jawbone, called skeletal anchorage. Dental anchorage utilizes a group of teeth linked together to form a single, larger, and thus more resistant, anchor unit. This technique relies on the combined surface area of the roots of the anchor teeth being significantly greater than the surface area of the roots of the teeth being moved.
While effective for minimum and moderate resistance, dental anchorage has inherent limitations because the anchor teeth are still embedded in the movable bone of the dental arch. Even when splinted together, they remain susceptible to the reciprocal force, leading to a phenomenon known as “anchorage loss.” This small, unwanted movement can accumulate over the course of treatment and compromise the final bite position.
The most stable form of resistance is skeletal anchorage, which provides absolute anchorage. This is achieved by deriving support directly from the dense, immovable bone of the jaw, bypassing the periodontal ligament surrounding the teeth. Absolute anchorage completely isolates the anchor unit from the reciprocal forces, ensuring that the anchor structure does not move at all.
Tools Used to Maintain Position
A variety of physical appliances are used to translate the anchorage strategy into a mechanical reality, depending on the required source and degree of resistance.
Dental Anchorage Appliances
For intraoral dental anchorage, devices like the transpalatal arch (TPA) and the Nance arch are frequently employed to splint the upper back teeth together. The TPA is a simple wire connecting the molars across the roof of the mouth, which prevents unwanted rotation or sideways movement. The Nance arch incorporates an acrylic button that rests against the roof of the mouth, adding mucosal resistance to the dental anchorage unit. Similarly, a lingual arch connects the molars on the inside surface of the lower teeth, acting as a stable wire to prevent the forward drift of the anchor molars.
Skeletal and Extraoral Appliances
For cases demanding absolute anchorage, Temporary Anchorage Devices (TADs) represent the modern standard. These are small titanium screws or mini-implants temporarily placed directly into the jawbone. The TAD acts as a completely fixed, immovable point from which to push or pull teeth, eliminating anchorage loss. Extraoral devices, such as headgear, are traditional tools used to secure anchorage outside the mouth. Headgear uses the neck or the back of the head as the resistant source, transferring the reciprocal force to these external structures.