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CLASS III IN ORTHODONTICS



Mandibular Growth Pattern


The mandible grows downward and forward through endochondral and intramembranous ossification. Bone is deposited on the posterior surface of the ramus and inferior border of the mandible, and resorbed from the anterior ramus. The condyle is the main endochondral growth center; it elongates the mandibular ramus. The alveolar ridge grows as the teeth erupt. 

Maxillary Growth Pattern


The maxilla undergoes downward and forward displacement. It is displaced forward by the growth of the cranial base. The cranial base's synchondroses close around age 7. Appositional growth of the maxilla occurs via intramembranous ossification, when bone is deposited at its circumaxillary sutures. As the maxilla is being displaced downward and forward, there is simultaneous resorption of bone on the anterior surface of the maxilla.


Growth of Maxilla and Mandible


The growth spurt of the mandible coincides with puberty. For girls, this will occur between the ages of 10-12; for boys, this will occur between the ages of 13.6-14.5. Mandibular growth plateaus around age 14 in females and 16 in males.

Maxillary growth reaches a gradual plateau in both males and females around age 15. In males, 92% of growth is completed at age 15, and in females, 84% of growth is completed at age 15.

For these reasons, the window of opportunity for growth modification of the maxilla and mandible is prior to the growth plateau.


Early Treatment of Class III Skeletal Discrepancy


The Class III skeletal discrepancy is due to excessive downward and forward mandibular growth. About 30-40% of the time, it is seen in combination with a lack of vertical, transverse, or anterior maxillary growth. The Class III skeletal discrepancy is typically acquired through genetic inheritance, but it can be influences by environmental factors, as well. At a young age, it is difficult to predict the amount of additional mandibular growth that will occur, and when the growth will stop. However, it is unlikely to be outpaced by maxillary growth with age.

In addition, a seemingly Class I patient may experience excess mandibular growth in late adolescence to early adulthood; this cannot be predicted and treated in Phase I and may require surgical intervention.


Why Address Class III in Early Treatment Instead of Waiting Until Comprehensive Treatment?


Early intervention is recommended for Class III patients. Their negative overjet and overbite can "lock in" the occlusion and inhibit maxillary growth, which can result in a constricted maxilla and crossbite. A CO-CR discrepancy is frequently seen, especially in cases with edge-to-edge incisal occlusion. This can cause unilateral crossbite, which can result in uneven growth of the mandibular rami. Young patients with a Class III discrepancy may also struggle with ineffective mastication or speech difficulties. In Phase I treatment, we are in the window of time where growth modification can be done to minimize the skeletal discrepancy by restricting the growth of the mandible and encouraging the growth of the maxilla. After puberty, not much can be done to address the skeletal positions aside from surgical intervention.


Goals of Class III Early Treatment Intervention:


  • Minimize growth of mandible

  • Maximize growth of maxilla

  • Eliminate dental interferences

  • Prepare patient for more straightforward Phase II orthodontic treatment

  • Advise the patient and guardian(s) that the mandible may continue growing, and surgical intervention may eventually be needed


Class III Early Treatment Interventions


  • Reverse-Pull Headgear

  • Reverse-Pull Headgear is comprised of a facemask and chin cup. The maxillary molars are banded, and elastics are used to connect the bands to the appliance to facilitate maxillary protraction. The protraction of the maxilla causes tensile stress in the circumaxillary sutures. Tensile forces stimulate the proliferation of osteoblasts, and thus encourages intramembranous growth via intramembranous ossification. The chin cup minimizes the downward and forward growth of the mandible by creating compressive forces on the mandible itself and on the surfaces of the condyles. This reduces the proliferative activity of the chondrocytes, thus reducing overall mandibular growth. Over time, it will change the direction of mandibular growth to a more downward and backward direction and minimize the counterclockwise rotation of the mandible.. In addition, it will assist in the correction of a dental Class III malocclusion by orthodontically extruding and protracting the maxillary molars.

  • Frankel-III

  • The Frankel-III is an intraoral appliance that positions the mandible posteriorly. It has an upper labial pad that prevents soft tissue compression, which would minimize maxillary growth. It also has vestibular shields which prevent transverse compression by the facial muscles (ex: buccinators).

  • Reverse Twin Block

  • The Reverse Twin Block is an intraoral appliance that positions the mandible posteriorly. It is composed of bite blocks with ramps that guide the mandible into a more retruded position when occluding. It can have both dental and skeletal effects.


Class III in Comprehensive Treatment


After puberty, the ideal window of opportunity for skeletal growth modification of the maxilla and mandible has passed. However, a skeletal Class III relationship can be camouflaged dentally, sometimes through extractions (typically maxillary second premolars and mandibular first premolars) or through the use of TADs and elastics for retraction of the mandibular incisors. It is important to monitor for additional growth of the mandible, even into a patient's twenties. At this point, surgery is the only option for skeletal changes.



Citations:


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  • Proffit textbook

  • Zere, Edlira et al. “Developing Class III malocclusions: challenges and solutions.” Clinical, cosmetic and investigational dentistry vol. 10 99-116. 22 Jun. 2018, doi:10.2147/CCIDE.S134303

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  • Diéguez-Pérez, Montserrat et al. “The influence of crossbite in early development of mandibular bone asymmetries in paediatric patients.” Journal of clinical and experimental dentistry vol. 9,9 e1115-e1120. 1 Sep. 2017, doi:10.4317/jced.54110

  • Lathrop-Marshall H, Keyser MMB, Jhingree S, Giduz N, Bocklage C, Couldwell S, Edwards H, Glesener T, Moss K, Frazier-Bowers S, Phillips C, Turvey T, Blakey G, White R, Mielke J, Zajac D, Jacox LA. Orthognathic speech pathology: impacts of Class III malocclusion on speech. Eur J Orthod. 2021 Sep 25:cjab067. doi: 10.1093/ejo/cjab067. Epub ahead of print. PMID: 34562076.

  • Ikegame M, Tabuchi Y, Furusawa Y, Kawai M, Hattori A, Kondo T, Yamamoto T. Tensile stress stimulates the expression of osteogenic cytokines/growth factors and matricellular proteins in the mouse cranial suture at the site of osteoblast differentiation. Biomed Res. 2016;37(2):117-26. doi: 10.2220/biomedres.37.117. PMID: 27108881.

  • Kazuo Tanne, Yu Chieh-Li Lu, Eiji Tanaka, Mamoru Sakuda, Biomechanical changes of the mandible from orthopaedic chin cup force studied in a three-dimensional finite element model, European Journal of Orthodontics, Volume 15, Issue 6, December 1993, Pages 527–533, https://doi.org/10.1093/ejo/15.6.527

  • Konstantinos Karamesinis, Efthimia K. Basdra, The biological basis of treating jaw discrepancies: An interplay of mechanical forces and skeletal configuration, Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease, Volume 1864, Issue 5, Part A, 2018, Pages 1675-1683, ISSN 0925-4439, https://doi.org/10.1016/j.bbadis.2018.02.007.

  • Mimura H, Deguchi T. Morphologic adaptation of temporomandibular joint after chincup therapy. Am J Orthod Dentofacial Orthop. 1996 Nov;110(5):541-6. doi: 10.1016/s0889-5406(96)70063-x. PMID: 8922514.

  • English JD. Early treatment of skeletal open bite malocclusions. Am J Orthod Dentofacial Orthop. 2002 Jun;121(6):563-5. doi: 10.1067/mod.2002.124166. PMID: 12080300.

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