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Orthodontics in Review: Paper Review, a Re-look at Class III Camouflage

Alveolar bone of the skeletal III patient

Labio-lingual alveolar bone of the lower labial segment: The authors described how the alveolar bone thickness varies from skeletal 1 to skeletal 3 cases. With the use of CBCT scans indicating skeletal 3 cases have greater bone on the lingual and palatal aspects on the anterior teeth when compared to skeletal 1 cases, however alveolar bone is thinner on the buccal aspects near the apex Wang 2012.

Vertical bone height of the lower labial segment: There appears to be less vertical bone around the lower incisors when compared to the upper incisors, This discrepancy is further enhanced by in high angle patients, who have thinner and shorter alveolar bone in all of their mandibular incisors and canines compared to those with normal and low mandibular angles.

My Thoughts

From the above statements mechanics are to be planned around biological limits. There is scope to retract the lower incisors (there is more bone lingually), however there is less bone vertically and labially. Class 3 correction therefore requires bodily retraction and avoiding extrusion mechanics. Kleber Meireles in his excellent online biomechanics course (review here) described the use of a round wire in class 3 cases with retraction resulting in labial tipping of the lower incisor apex, into an area already deficient of bone. To avoid this a wire which can produce a force couple is required, a rectangular wire or use of torquing auxiliaries.

(Image from Venugopal, A., Manzano, P. and Vaid, N.R., 2022, October. TAD driven Class III camouflage: Eight point protocol to optimize efficiency, aesthetics and stability. In Seminars in Orthodontics. Elsevier)

What to extract? premolars or third molars?

Both can facilitate class 3 correction, with premolar extractions enabling greater incisor retraction of up to 4.2mm. However extraction of third molars enables ease of assessment of the degree of incisor retraction. Factors which influence the decision of premolar or third molar extractions are; the degree of crowding, curve of Spee, reverse overjet and the high or low angle.

My Thoughts

In the decision of which teeth to extract, smaller discrepancies point towards extraction of third molars and arch distalisation, greater discrepancies necessitate premolar extraction. What I liked about this topic was that lower arch distalisation was placed into context with premolar extractions. Lower arch full distalisation is not common mechanics, but when presented can seem like the panacea of orthodontics, it was great to see the authors placing these mechanics relative to conventional treatment planning and providing a decision tree for their use.

Curve of Spee

Continuous mechanics to correct a curve of Spee result in both posterior extrusion and anterior intrusion. The ratio is altered by the virtue of high or low angle. A high angle case has lower masticatory forces and more likely to undergo posterior extrusion (usually unwanted), likewise for lower angle cases a higher masticatory force is likely to favour anterior proclination. Solution to the above; high angle cases use bite blocks on the molars to prevent extrusion, and TADs to control premolar extrusion; and in low angle cases using anterior bite blocks on the lingual surfaces of the lower anteriors to prevent anterior proclination and facilitating posterior eruption

My Thoughts

A simple re-look at the curve of the Spee mechanics with continuous mechanics. The authors have described relatively simple methods to alter the posterior extrusion / anterior intrusion ratios. We have detailed in a previous blog previously on the curve of Spee, click here.

Posterior limits to full arch distalization

The authors described anatomical limits of distalisation, the ‘mandibular posterior anatomic limit’. They defined this limit as the mylohyoid ridge (the posterior  lingual boarder of the mandible, I had to google it). On average 3mm of distalisation is possible. The high / low angle factor re-appears, with low angle cases generally having a larger distance between the second molar and the mylohyoid ridge, and the inverse for high angle patients with a smaller distance. Attempting to distalise beyond the mylohyoid ridge results in distal tip of the second molar, and therefore greater chances of relapse. A clinical recommendation was to place buccal root torque during distalisation, this can reduce early contact of the second molar root with the mylohyoid ridge. A CBCT is recommended to investigate the distance between the mylohyoid ridge and second molar as large variations have been observed in the population.

My Thoughts

I find lower arch distalisation a challenging and less predictable movement in clinical practice. here the authors have explained why lower arch distalisation can be challenging, methods of assessment and mechanics to enable the process. Specifically the use of buccal root torque accounts for the widening of the mandible posteriorly, and keeps the posterior segment in the alveolar trough.

(Image from Venugopal, A., Manzano, P. and Vaid, N.R., 2022, October. TAD driven Class III camouflage: Eight point protocol to optimize efficiency, aesthetics and stability. In Seminars in Orthodontics. Elsevier)

Limits in the transverse dimension

Class 3 cases are typically compensated through buccal dental compensation, the upper posterior teeth are buccally tipped, and the lower posterior teeth lingually tipped. A class 3 patient however has thinner buccal alveolar bone in the posterior maxilla, in comparison to a class 1 case, of around 0.35mm.  The lower second molar appears to be able to compensate more than the remaining lower posterior teeth, with greater lingual tipping. The authors recommended either extractions in the maxillary arch or skeletal expansion, without which there is likely to be over burdening of the already thin buccal alveolar bone.

My Thoughts

The authors highlighted the biological restrictions in dento-alveolar expansion of the maxillary arch in class 3 cases, with space management being recommended with changes to the biological boundaries through skeletal expansion or reduction in the need for space – through dental extractions I wonder where the limits lie for mandibular transverse compensation, and to what degree lower posterior lingual tipping maybe possible.

Verdict

Overall a great review paper from which I have taken a number of clinical tips from it. The routine use of CBCT is a contentious issue, however if I have a class 3 case in which I am planning lower arch distalisation, I would require it to assess the amount of movement possible, and therefore if it is feasible. However if I plan lower premolar extractions then it is not indicated. The use of TADs has been described in a number of topics within the paper, new for me was for the vertical management to favour anterior intrusion over premolar extrusion with continuous mechanics. The paper has given me greater awareness of where the biological limits lie in class 3 cases, dare I say it, an update to the envelope of movement possible within contemporary orthodontics in class 3 management.

The lead author Adith Venugopal has been interviewed as part of the Orthodontics in Interview Podcast, which can be found here.

TAD driven Class III camouflage: Eight point protocol to optimize efficiency, aesthetics and stability Adith Venugopal, Paolo Manzano, and Nikhilesh R Vaid Seminars in Orthodontics

Contributions

Edited/contents: Farooq Ahmed,

 

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1 comment
  • we are working with a maxillofacial to what we call minimal invasive orthodontics ( Tads, miniplates, SARPE…) and Cl III cases are 1 of our target groups. It is very beneficial to share results for all of us working in this field