Clear aligners are removable, custom-made devices used to gradually shift teeth into a straighter position. They offer an aesthetic alternative to traditional metal braces, primarily appealing due to their near-invisibility. Understanding the structure and associated components clarifies how these trays function to create complex tooth movements.
The Physical Structure of the Trays
The aligner tray is fabricated from medical-grade thermoplastic polymers, such as polyethylene terephthalate glycol (PET-G) or advanced multi-layer polyurethanes. These materials are chosen for their balance of rigidity, elasticity, and transparency, allowing them to apply consistent force while remaining discreet. High optical clarity makes the aligners appear virtually clear when worn over the teeth.
Aligners are thermoformed as a single piece to cover the entire dental arch, fitting snugly over the crowns of the teeth. They are very thin, often less than 1 millimeter in thickness, which is important for patient comfort and minimizing visible bulk. Although the material is clear, it often has a slightly matte or frosted texture. This texture helps diffuse light and prevents a highly reflective surface that might draw attention.
The polymer must be biocompatible and durable enough to withstand daily wear for one to two weeks before replacement. Over time, the material may experience a slight decrease in transparency or subtle color change due to exposure to food pigments and saliva. The short wear cycle for each tray is designed to mitigate aesthetic degradation during treatment.
Design Elements for Fit and Retention
The effectiveness of a clear aligner depends on its precision fit, achieved through advanced 3D scanning and manufacturing. The aligner’s edge, known as the trim line, plays a significant role in retention and force transmission. The trim line is where the plastic ends, and there are two main types: scalloped and straight.
A scalloped trim line is contoured to follow the gumline around each tooth. This design enhances aesthetics and may reduce gum irritation, but it can result in less retention and reduced force transmission. Conversely, a straight or high-and-flat trim line extends straight across the gums, often sitting 1 to 2 millimeters above the gingival margin. This design increases stiffness and retention, allowing for better control over complex tooth movements, though it may be slightly more visible.
Some manufacturers prefer the high-and-flat design because studies suggest it can be up to twice as retentive as a scalloped cut for aligners without attachments. This improved grip provides a uniform force distribution, necessary for controlled movements like bodily translation of the tooth root.
Associated Components and Accessories
While the tray is the primary component, treatment often involves small, temporary additions that affect the overall appearance. The most common are “attachments,” which are small bumps of tooth-colored composite resin temporarily bonded to specific teeth. These attachments act like handles, giving the aligner a point to grip and push against for challenging movements like rotations or vertical shifts.
Attachments are designed in various shapes (rectangles, circles, or ellipsoids) to facilitate specific force application. Made from a resin that matches the natural tooth shade, they are designed to be discreet, though visibility varies depending on size and location. Another component is “buttons,” which are small, round, tooth-colored or clear fixtures bonded to the teeth. Buttons serve as anchor points for rubber bands, known as elastics, which are used to correct complex bite alignment issues like overbites or underbites.
The use of attachments and buttons makes the treatment slightly more noticeable than the aligner tray alone, but they are necessary to achieve the desired clinical result. Common accessories include small, cylindrical plastic pieces called “chewies” or “aligner seaters,” used briefly to help seat the aligner fully onto the teeth, and small plastic removal tools. These tools and chewies are only used when inserting or seating the aligners and are not worn continuously.