Force changes associated with different intrusion strategies for deep-bite correction by clear aligners

A deep bite presents as one of the most challenging forms of malocclusion to overcome, let alone with the help of modern-day clear aligners. It is characterized by the excessive vertical overlap of mandibular incisors by the maxillary incisors in maximum intercuspation. A deep bite is a feature of malocclusion that can be dental, skeletal, or at times, a combination of both.

A dental deep bite is often characterized by a retroclination and extrusion of the upper and lower front teeth, a loss of vertical posterior dimension or a combination of both. On the other hand, a skeletal deep bite may cause a counterclockwise rotation of the mandible.

The primary method of correction of a deep bite is by the intrusion of the anterior teeth, the extrusion of the posterior teeth, or a selective combination of the two. For patients with excessive maxillary incisor display at rest (or deep curve of Spee of the mandible) with excessive lower anterior facial height, orthodontists recommend the process of anterior intrusion.

But, this comes with a challenge. It has been shown that orthodontists struggle to achieve mandibular incisor intrusion as it is one of the least accurate and unpredictable tooth movements. And, although transformations in technology and science have led to far superior innovations in orthodontics, the correction of deep bite or rather the success of these corrections has yet to be improved despite the revolutionary clear aligner sweep.

In this review, the authors aim to investigate the clinical success related to treating deep bites with aligners.

This study was published in the Angle Orthodontist.

Force changes associated with different intrusion strategies for deep-bite correction by clear aligners.

Liu Y, Hu W.

Angle Orthod. 2018 Nov;88(6):771-778. doi: 10.2319/121717-864.1. Epub 2018 Jul 23.

What they asked

The authors aimed:

“To investigate the relationships among different intrusion patterns of clear overlay aligners and the corresponding orthodontic forces and to provide guidance for clinical treatment.”

What they did

They included five groups of removable thermoplastic-formed aligners (of equal thickness), all designed for different intrusion procedures:

• Group 0 (G0) – control group with no activation.
• Group 1 (G1) – intruded both mandibular canines by 0.2 mm.
• Group 2 (G2) – intruded the four mandibular incisors by 0.2 mm.
• Group 3 (G3) – intruded all mandibular anterior teeth (canines and incisors) with activation of 0.2 mm.
• Group 4 (G4) – intruded the canines, lateral incisors, and central incisors with different amounts of activation (0.1 mm, 0.15 mm, and 0.2 mm respectively).

Conventional transverse rectangular attachments were bonded on the mandibular first premolars, second premolars, and first molars. All the aligners were manufactured using the same process and the corresponding intrusion forces were measured using a multi-axis force/torque transducer measurement system in real time.

What they found out

These were the results of the study:

• Group 1
  • Canines experienced the largest intrusion forces, 0.92 N on the left and 0.69 N on the right.
  • Central and lateral incisors experienced 0.2 N and 0.17 N extrusive forces, respectively.
  • Left and right first premolars received 0.52 N and 0.22 N, respectively and the second premolars endured a mean extrusive force of 0.44 N, indicating that the premolars were the major anchorage teeth and incisors also provided anchorage for canine intrusion.

• Group 2
  • Lateral and central incisors experienced intrusive forces of 0.17 N and 0.23 N respectively.
  • Mean extrusive forces of 0.57 N and 0.07 N were measured on the left and right canines, which acted as anchorage teeth along with the premolars.

• Group 3
  • Canines experienced greater intrusive forces (0.28 N) than incisors (0.11 N on the lateral incisors and 0.16 N on the central incisors)
  • First premolars had the largest extrusive forces on both sides.
  • Intrusive forces on canines were only one-third of that on G1 canines.
  • Intrusive forces on the incisors were the same as that of G2 on the incisors.

• Group 4
  • Central incisors experienced no statistically significantly different forces with G4 than with G3 aligners.
  • Lateral incisors and canines experienced no intrusive forces, despite the intrusive activation of 0.15 mm and 0.1 mm, respectively.

So, they found that the canines experienced the largest intrusive force when intruded with G1 aligners alone (larger intrusive force than incisors in G3). Whereas, the first premolars endured the greatest extrusive forces when all anterior teeth were intruded with G3 aligners.

What we can conclude

As can be concluded from this review, aligners with different intrusion patterns tend to exert different forces on the teeth, primarily incisors, canines, and premolars. The final intrusion forces were also found to be closely related to the designed activation, shape and position of the attachment, and the relative movement of the adjacent teeth.

With the same amount of activation, the canines experienced the largest intrusion force when intruded alone with aligners. Upon combined intrusion, canines and incisors received different forces and the first premolars experienced the largest extrusive forces.