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Rationale behind Twin-block incline
READERS' FORUM
Letters to the editor* Rationale behind Twin-block incline read with interest the case report entitled “Severe Class II Division 1 malocclusion in an adolescent patient, treated with a novel sagittal-guidance Twin-block appliance” (Li P, Feng J, Shen G, Zhao N. Am J Orthod Dentofacial Orthop 2016;150:153-66). In the Discussion section, the authors pointed out that Clark1,2 originally introduced a 45 inclination on the blocks in his 1982 and 1988 articles. The original Twin-block traction technique relied on extraoral traction and intermaxillary elastics from the Concorde face-bow to enhance the inclined plane effect of the 45 incline on the blocks. A year later in 1989, Trenouth3 introduced the concept of forward reflex posturing using a 70 incline on the blocks to create an artificial forward bite of accommodation. This meant that the extraoral traction and intermaxillary elastics on the original Clark appliance could be dispensed with, because they were no longer necessary for the appliance to function efficiently. Most of the time during the day is spent with the mandible in the rest position, with tooth contact only occurring during swallowing, speech, and mastication.4 A 70 block incline maintains the mandible in a forward position even when it is open in the rest position. This is because the hinge axis of opening is approximately 70 to the occlusal plane as demonstrated by Posselt5 on mandibular movement. This allows 24-hour forward posturing when the masticatory system is at rest as well as in function. Thus, the Twin-block is more a postural appliance than a functional one. Such forward posturing has been shown to lead to distraction of the mandibular condyle from the glenoid fossa.6 There is a clear mechanism to explain the resulting skeletal response that is well documented in the literature.4,6 This forms the basis of the tropic premise postulated by Mew7-9 that a jaw that is constantly postured forward will grow forward. Michael J. Trenouth Preston, United Kingdom
I
Am J Orthod Dentofacial Orthop 2017;151:232 0889-5406/$36.00 Ó 2017 by the American Association of Orthodontists. All rights reserved.
http://dx.doi.org/10.1016/j.ajodo.2016.10.014
* The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.
232
REFERENCES 1. Clark WJ. The twin block traction technique. Eur J Orthod 1982;4: 129-38. 2. Clark WJ. The twin block technique. A functional orthopedic appliance system. Am J Orthod Dentofacial Orthop 1988;93: 1-18. 3. Trenouth MJ. A functional appliance system for the correction of Class II relationships. Br J Orthod 1989;16:169-76. 4. Trenouth MJ. Twin-block appliance therapy: design and mode of action. Cranio-View 2000;9:31-8. 5. Posselt U. Studies in mobility of the human mandible. Acta Odontol Scand 1952;10(Suppl):3-160. 6. Trenouth MJ. Distraction of the mandibular condyle during Twinblock appliance treatment. Cranio-View 2005;14:25-31. 7. Mew JR. The aetiology of malocclusion: can the tropic premise assist our understanding? Br Dent J 1981;151:296-302. 8. Mew JR. Factors influencing mandibular growth. Angle Orthod 1986;56:31-48. 9. Mew JR. The postural basis of malocclusion: a philosophical overview. Am J Orthod Dentofacial Orthop 2004;126:729-38.
Authors' response
T
hank you for the interest shown in our case report and giving us some comments on the rationale of the Twin-block incline. The appliance that we developed was named the sagittal-guidance Twin-block appliance because of the bite blocks between the upper and lower jaws. However, the mechanism of treatment might be quite different from the classic Twin-block appliance. The fact that we use it as a functional appliance, not just a postural one, could be the most controversial opinion between us. We use a 70 incline on the blocks to maintain the mandible in a forward position even when open in the rest position. Also, we recommend that all patients should wear this appliance 24 hours a day for 12 months. It not only stimulates the forward posturing adaptation of the masticatory system at rest as well as in function, but also stimulates the growth of mandible and the adaptative remodeling of the fossa and condyle. Through 3-dimensional cephalometric superimposition, we found significant length increments in the mandible but did not observe the distraction of the mandibular condyle from the glenoid fossa. In the maxilla, the superior-posterior force on the bonded appliance could result in the distalization of the whole maxillary posterior dentition and a certain degree of inhibition of the maxilla. After removal of the appliance, a 2-3 mm space between the maxillary canines and first premolars was usually observed. The relief of crowding was a common
Letters to the editor* Rationale behind Twin-block incline read with interest the case report entitled “Severe Class II Division 1 malocclusion in an adolescent patient, treated with a novel sagittal-guidance Twin-block appliance” (Li P, Feng J, Shen G, Zhao N. Am J Orthod Dentofacial Orthop 2016;150:153-66). In the Discussion section, the authors pointed out that Clark1,2 originally introduced a 45 inclination on the blocks in his 1982 and 1988 articles. The original Twin-block traction technique relied on extraoral traction and intermaxillary elastics from the Concorde face-bow to enhance the inclined plane effect of the 45 incline on the blocks. A year later in 1989, Trenouth3 introduced the concept of forward reflex posturing using a 70 incline on the blocks to create an artificial forward bite of accommodation. This meant that the extraoral traction and intermaxillary elastics on the original Clark appliance could be dispensed with, because they were no longer necessary for the appliance to function efficiently. Most of the time during the day is spent with the mandible in the rest position, with tooth contact only occurring during swallowing, speech, and mastication.4 A 70 block incline maintains the mandible in a forward position even when it is open in the rest position. This is because the hinge axis of opening is approximately 70 to the occlusal plane as demonstrated by Posselt5 on mandibular movement. This allows 24-hour forward posturing when the masticatory system is at rest as well as in function. Thus, the Twin-block is more a postural appliance than a functional one. Such forward posturing has been shown to lead to distraction of the mandibular condyle from the glenoid fossa.6 There is a clear mechanism to explain the resulting skeletal response that is well documented in the literature.4,6 This forms the basis of the tropic premise postulated by Mew7-9 that a jaw that is constantly postured forward will grow forward. Michael J. Trenouth Preston, United Kingdom
I
Am J Orthod Dentofacial Orthop 2017;151:232 0889-5406/$36.00 Ó 2017 by the American Association of Orthodontists. All rights reserved.
http://dx.doi.org/10.1016/j.ajodo.2016.10.014
* The viewpoints expressed are solely those of the author(s) and do not reflect those of the editor(s), publisher(s), or Association.
232
REFERENCES 1. Clark WJ. The twin block traction technique. Eur J Orthod 1982;4: 129-38. 2. Clark WJ. The twin block technique. A functional orthopedic appliance system. Am J Orthod Dentofacial Orthop 1988;93: 1-18. 3. Trenouth MJ. A functional appliance system for the correction of Class II relationships. Br J Orthod 1989;16:169-76. 4. Trenouth MJ. Twin-block appliance therapy: design and mode of action. Cranio-View 2000;9:31-8. 5. Posselt U. Studies in mobility of the human mandible. Acta Odontol Scand 1952;10(Suppl):3-160. 6. Trenouth MJ. Distraction of the mandibular condyle during Twinblock appliance treatment. Cranio-View 2005;14:25-31. 7. Mew JR. The aetiology of malocclusion: can the tropic premise assist our understanding? Br Dent J 1981;151:296-302. 8. Mew JR. Factors influencing mandibular growth. Angle Orthod 1986;56:31-48. 9. Mew JR. The postural basis of malocclusion: a philosophical overview. Am J Orthod Dentofacial Orthop 2004;126:729-38.
Authors' response
T
hank you for the interest shown in our case report and giving us some comments on the rationale of the Twin-block incline. The appliance that we developed was named the sagittal-guidance Twin-block appliance because of the bite blocks between the upper and lower jaws. However, the mechanism of treatment might be quite different from the classic Twin-block appliance. The fact that we use it as a functional appliance, not just a postural one, could be the most controversial opinion between us. We use a 70 incline on the blocks to maintain the mandible in a forward position even when open in the rest position. Also, we recommend that all patients should wear this appliance 24 hours a day for 12 months. It not only stimulates the forward posturing adaptation of the masticatory system at rest as well as in function, but also stimulates the growth of mandible and the adaptative remodeling of the fossa and condyle. Through 3-dimensional cephalometric superimposition, we found significant length increments in the mandible but did not observe the distraction of the mandibular condyle from the glenoid fossa. In the maxilla, the superior-posterior force on the bonded appliance could result in the distalization of the whole maxillary posterior dentition and a certain degree of inhibition of the maxilla. After removal of the appliance, a 2-3 mm space between the maxillary canines and first premolars was usually observed. The relief of crowding was a common