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Relationship between Bone Apposition in the Compression Zone and the Bending Theory of Orthodontic Tooth Movement for Aligners


The philosophy of clear aligners, which is minimally invasive, avoids the use of attachments that require surgical intervention or the introduction of metal elements to perform orthodontic movements. This implies a deep study of the most important movements of aligners, which are translation movements, how much can be achieved and how to perform them without damaging the alveolar and cortical bone. To do this, we have to describe two hypotheses or theories to better understand translation movements such as expansion or distalization and how they are implemented in the use of aligners:

1-Bone apposition hypothesis:

The hypothesis of bone apposition in the compression zone, developed by Birte Melsen, proposes that orthodontic tooth movement occurs primarily through the apposition of new bone on the compression side of the alveolus.

2-Bending theory of alveolar bone:

The bending theory of alveolar bone, proposed by authors such as Grim, Baumrind and Korn, suggests that tooth movement occurs through the deformation of the cortical alveolar bone, without significant bone apposition.

3-Relationship between the two theories:

Although both theories attempt to explain orthodontic tooth movement, they are not mutually exclusive. In fact, current evidence suggests that both mechanisms (bone apposition and bending) contribute to tooth movement to varying degrees:

-Bone apposition:

  • It is the main mechanism in rapid movements and in areas of high compression.


  • It is the main mechanism in slow movements and in areas of low compression.

-Factors influencing each mechanism:

  • Magnitude of force: Higher forces favor bone apposition.

  • Duration of force: Long-duration forces favor bending.

  • Bone type: Cortical bone is more susceptible to bending than trabecular bone.

-Clinical implications:

Understanding the interaction between bone apposition and bending is important for the orthodontist, as it allows:

  • Treatment planning: Select the most appropriate movement technique for each case.

  • Predict patient response: Estimate the speed and amount of movement that can be achieved and alveolar and cortical bone remodeling time.

  • Minimize side effects: Reduce the risk of pain, inflammation, bone loss and root resorption.

4-Application of orthodontic tooth movement theories to aligner expansion

In aligner expansion, both theories, that of bone apposition in the compression zone and that of bending or bending of the alveolar bone, play an important role:

-Bone apposition:

  • The pressure exerted by the aligners on the alveolar bone stimulates bone apposition in the compression zone, which allows the expansion of the dental arch.

  • The magnitude and duration of the pressure are key to determining the amount of bone apposition that occurs


  • The flexible material of the aligners allows for a slight bending of the alveolar bone, especially in the case of teeth with short roots.

  • Bending can complement bone apposition and accelerate tooth movement

-Factors to consider:

  • Type of expansion: Maxillary expansion usually requires more bone apposition than molar distalization.

  • Aligner stiffness: Stiffer aligners generate more pressure and therefore more bone apposition.

  • Sequence of movements: Expansion can be performed in stages to control the amount of bending and minimize the risk of root resorption.

5-Amount of movement

When a tooth moves 0.25 mm, there is a combination of bone apposition and bending of the alveolar bone.

-Bone apposition:

  • Time: Bone apposition occurs in a continuous process. Osteoblasts deposit new bone in the compression zone, which can take between 12.5 and 15 days to complete for a movement of 0.25 mm.

  • Factors: The amount of new bone deposited depends on the magnitude and duration of the force applied.


  • Time: Bending is a faster process than bone apposition. It can occur in a matter of hours or days.

  • Factors: The amount of bending that occurs depends on the flexibility of the bone and the magnitude of the force applied.

-Relationship between bone apposition and bending:

  • Fast movements: In fast movements (more than 0.25 mm per month), bone apposition is the main mechanism.

  • Slow movements: In slow movements (less than 0.25 mm per month), bending is the main mechanism.

In the case of a 0.25 mm movement, it is likely that both mechanisms contribute to a similar extent.

-Additional considerations:

  • Age of the patient: Bone apposition is faster in children and adolescents than in adults.

  • Bone type: Trabecular bone is resorbed and deposited more quickly than cortical bone.

  • General health of the patient: Bone apposition can be affected by systemic diseases.


Bone apposition in the compression zone and bending of the alveolar bone are two important mechanisms that contribute to orthodontic tooth movement. Understanding both mechanisms allows the orthodontist to plan and execute more efficient and safe treatments.

Aligner expansion is based on the combination of bone apposition and bending of the alveolar bone. The orthodontist must select the most appropriate stiffness aligner for each case, taking into account the factors mentioned above.


  • Melsen, B. (1991). Biological reaction of alveolar bone to orthodontic tooth movement. The American Journal of Orthodontics and Dentofacial Orthopedics, 99(1), 5-12.

  • Fred M. Grimm, (1972) Bone bending, a feature of orthodontic tooth movement, American Journal of Orthodontics, Volume 62, Issue 4,

  • Roodman, D.(1996) “Advances in bone biology: the osteoclast”, Endocrine reviews, volume17-4, pages 308-32

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