Biomechanics of Clear Aligners: How do clear aligners move teeth through the bone? (Dr Niki Shah)
Osteoblasts, Osteoclasts, Clear Aligners, Proligner
Clear aligner treatment is an orthodontic treatment with a series of clear plastic dental appliances called clear aligners, otherwise known as clear bracers or trays, manufactured to tightly fit the shape and position of a patient's teeth, with each step in the series applying gentle, consistent pressure to the patient's teeth to progressively move the teeth, improving their aesthetic and functionality.
The treatment’s popularity stems from clear aligner’s capability to treat most types of malocclusions discreetly and conveniently, combined with improved comfort over the use of traditional orthodontic appliances. However, some complex cases are still better suited for traditional fixed braces, or a combination of appliances.
To learn more about the specifics of clear aligner treatment limitations, read our guide on movement classification.
Let’s look closer at the specifics.
In-Clinic Steps
Much of the beauty of clear aligner treatment is in the simplicity of the clinical process:
The doctor captures for the position and shape of the patient’s teeth with impressions or digital scans.
The doctor submits the case to Proligner with a prescription on how the teeth should be straightened.
Proligner develops a custom series of virtual models of the patient’s teeth showing how they progress from crooked to straight. Each step corresponds to a set of clear aligners made off models that are 3D printed.
The patient fits the aligners and wears them all day and all night, only taking them out to eat, drink, and brush their teeth. Unlike treatment with traditional metal bracers, patients are not required to visit their doctor to progress, rather moving to the next step at the prescribed time, usually every 2 weeks. Patients also tend to prefer aligners because of the convenience over metal bracers, as well as their relative comfort and clarity, meaning most people don’t even notice an aligner is being worn. These factors amount to a significant advantages for both the doctor and patient.
Osteoclasts and Osteoblasts
The process of how teeth move is fascinating. Clear braces apply light consistent pressure to individual teeth, differing at each step of the treatment, slowly pressing them towards to their final prescribed position. Each tooth is bound by a periodontal ligament, which is surrounded by fluid and tiny fibres connecting the teeth to the bone pocket. As soon as the clear bracers are worn, light pressure is applied to the teeth. The fluid in the periodontal ligament initially prevents the tooth from moving but after a few moments is squeezed from the area. As the ligament is only about a quarter of a millimetre thick the tooth only moves a tiny distance with each set of clear aligners.
For many years orthodontic tooth movement was explained by the compression-tension theory.
The pressure applied by the clear aligner on the tooth compresses the blood vessels in the ligaments on one side against the bone pocket and dilates the blood vessels on the other side. After about 5 hours of sustained pressure the compressed ligaments respond by producing enzymes and chemical messengers. After a day or two of continued pressure the enzymes and chemicals stimulate the production of bone cells, called osteoclasts and osteoblasts.
Osteoclasts dissolve the bone near the compressed ligament while osteoblasts grow new bone on the opposite side of the bone pocket. If the clear aligners are removed for an extended period, the process is interrupted, and the ligaments pull the tooth back towards its original position. This results in treatment having to start again. This is why patients are prescribed to wear the clear aligners for at least 22 hours per day, especially for the first few days of a new step. Note that the rate of movement varies considerably between individuals, usually taking longer for adults than for children and adolescents.
Once the osteoclasts have dissolved the bone it is time to change to the next set of aligners and repeat the process of progressively moving the teeth a little bit with each clear aligner until the series of aligners is complete and the teeth are in the final prescribed position. However, the process has not finished! Patients must continue to be disciplined and wear retainers as osteoblasts take longer to grow new bone to hold the teeth in the desired place than it takes for osteoclasts to dissolve it. The amount of time that retainers must be worn depends on the individual, with factors such as age and the complexity of movement received, requiring up to 2 years being necessary. Another consideration for continued retainer wear to maintain patients’ desired teeth position is that people’s teeth will continue to move throughout their life.
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