When orthodontic treatment concludes, teeth often shift back, a phenomenon known as relapse. A retainer is a maintenance device designed to secure the results achieved by braces or aligners. This appliance holds the teeth in their newly corrected positions, preventing them from drifting back to their original misalignment. While retainers are effective for stabilization, their capacity to reverse significant tooth movement that has already occurred is limited.
The Biological Basis of Orthodontic Relapse
Teeth move back toward their initial positions because the surrounding biological structures possess a “memory.” The periodontal ligaments (PDLs) are fibrous tissues connecting the tooth root to the jawbone, which are stretched and reorganized during active tooth movement. These fibers exert a constant, low-level tension, causing the teeth to spring back once the braces are removed.
The alveolar bone, which supports the teeth, undergoes continuous remodeling involving specialized cells called osteoclasts and osteoblasts. During orthodontic correction, the bone is actively remodeled, but stabilization around the new tooth positions takes time. This instability makes the teeth highly susceptible to relapse, compounded by persistent forces like chewing, swallowing, and tongue pressure.
Stabilization Versus Correction
The difference between stabilization and correction is fundamental to understanding a retainer’s purpose. Stabilization is the retainer’s intended role: passively holding the teeth to allow the supporting bone and soft tissues to solidify around the new alignment. Retainers are not engineered to apply the continuous, controlled force necessary for major tooth movement, unlike braces or aligners.
If the shift in alignment is very minor, such as less than a millimeter after a few days of missed wear, a removable retainer might feel tight. It can sometimes exert enough pressure to nudge the teeth back, but this mild self-correction is uncomfortable and represents the limit of a retainer’s corrective ability. Forcing an ill-fitting retainer onto teeth that have shifted noticeably can cause discomfort, damage the appliance, and potentially worsen the alignment.
Once a significant shift or rotation has occurred, the retainer becomes ineffective for correction. The relapse is too extensive for a passive appliance to overcome, and the proper course of action is retreatment. This typically involves a limited run of clear aligners or braces to safely move the teeth back into their ideal positions.
Types of Retainers and Their Function
Retainers are generally categorized into removable and fixed types, each with a distinct structure and wear schedule.
Removable Retainers
Clear plastic retainers, often called Essix retainers, are vacuum-formed to fit snugly over the entire arch of the teeth, resembling a thin, transparent mouthguard. They are popular for their near invisibility but are not easily adjustable and require replacement if noticeable shifting takes place.
The traditional Hawley retainer consists of a metal wire across the front of the teeth, embedded in an acrylic base that sits against the palate or floor of the mouth. Because the wire is bendable, an orthodontist can sometimes make small adjustments to address very minor movements. Both removable types initially require full-time wear, transitioning to nighttime use for what is often a lifetime commitment.
Fixed Retainers
Fixed retainers, sometimes called bonded or permanent retainers, consist of a thin wire custom-shaped and cemented directly to the back surfaces of the front teeth, most commonly the lower arch. Since they are non-removable, they provide continuous retention and eliminate the risk of forgetting to wear the appliance. However, they require diligent cleaning with floss threaders to prevent plaque buildup, and they only stabilize the specific teeth to which they are bonded.