How much expansion is realistic in Aligner therapy?

All patients that visit a dental establishment are different – no two patients are alike in their medical history and oral presentations. Thus, understandably, dentists are used to treating a wide variety of cases distinguished by age, pathology, and the severity of their conditions. Therefore, it is not entirely unfeasible to fathom that the timing of dentoalveolar development and maturation can differ from person to person. Out of the many dental anomalies, maxillary growth is one that particularly engages dentists’ attention.

According to several implant studies, the growth of maxilla in the transverse plane of space is the first to cease growing. This occurs when the first bridging of the mid-palatal suture begins and not at final complete fusion. In fact, some sources report that growth is normally complete by the age of 17 years and the average transverse growth between the age of four years and adulthood is 6.9 mm. Since only a small amount of growth occurs in the transverse dimension of the maxilla throughout life, it is highly unlikely that a malocclusion, if occurred, in the permanent dentition will self-correct. This is where a technique called arch expansion can come in handy.

What is malocclusion?

To understand what malocclusion is, we need to first grasp the concept of an ideal occlusion. An ideal occlusion or colloquially known as an occlusion is the harmonious relationship of the teeth and the jaws, something that is often sought after by dentists when reproducing good form and function in a patient’s mouth.

According to Angle, in the transverse plane, occlusion is considered normal or correct when:

• Molar relationship: The mesiobuccal cusp of the maxillary first molar aligns with the buccal groove of the mandibular first molar.
• Canine relationship: The maxillary canine occludes with the distal half of the mandibular canine and the mesial half of the mandibular first premolar.
• Line of occlusion: All the teeth fall in a line of occlusion which:
  • In the upper arch, is a smooth curve through the central fossae of the posterior teeth and the cingulum of the canines and incisors.
  • In the lower arch, is a smooth curve through the buccal cusps of the posterior teeth and incisal edges of the anterior teeth.

Any deviation from these relationships may be considered a form of malocclusion. The need to treat such malocclusions is drastically high in children, around 20% in teenagers aged 12-15 years and even higher (31.3%) in adults aged 35-44 years.

The treatment of dentoalveolar crossbite and crowding caused by malocclusion, especially Class III will require arch expansion to correct it.

Arch expansion: what is it?

Essentially, arch expansion is the process of creating space for your teeth enough for them to align next to each other without twisting, turning, overlapping, or crowding. During the course of arch expansion, the teeth visible in your smile move outwards in small increments, freeing up to 4.5 mm of extra arch room to align.

What is the acceptable expansion in aligner therapy?

Various techniques have been employed to bring about arch expansion. Realistic expansion will be different for different forms of expansion.

• Removable Hawley appliance: A very small amount of skeletal expansion is achieved by this technique, and only expected in prepubertal children through the separation of the (yet-to-fuse) mid-palatal suture. Expansion is produced predominantly by tipping the molar teeth buccally.

Patients are instructed to turn the expansion screw a quarter turn (0.2 mm expansion) once a week. This rate of expansion will be monitored by measuring the distance between dimples placed into the acrylic baseplate on either side of the midline. After the expansion is complete, the Hawley may be utilized as a retaining appliance for an additional three months.

• Quad-Helix appliance: It works by a combination of buccal tipping motion and skeletal expansion in a ratio of 6:1 in prepubertal children. A collective force of 400 g can be delivered by activating the appliance by approximately 8 mm (one molar width). Patients will then be reviewed every six weeks.
• Rapid Maxillary Expansion (RME): Unlike the two slow expansion techniques mentioned above, the RME technique aims to improve the ratio of skeletal to dental movement by initiating expansion at the mid-palatal suture.

Using a rigid appliance to limit the tip of the molars, the mid-palatal suture is rapidly expanded using high forces so that dental movement can be carried out before or during the pubertal growth spurt. RME is indicated in cases where exists a transverse discrepancy equal to or greater than 4 mm, typically in adults.

• Surgical techniques: Other techniques like SARPE, expansion can be carried out at a rate of 0.5 mm a day.

The precise method prompted for expansion and the amount of expansion to be applied will depend on the nature of the crossbite, the size of the discrepancy, the age of the patients, and other factors related to dentition.