CATEGORY 76 AUSTRALASIAN DENTIST The mandible has sometimes been characterized as one of the strongest bones of the human body1. However, it is not a rigid bone but a rather resilient one. This elastic behavior of the mandible is responsible for an elaborate biomechanical conduct2. The horseshoe shaped anatomy of the mandible, the activity of masticatory muscles that are inserted on the bone along with the structure of the bone itself, contribute to the deformation of the mandible1,3,4. The activity of these masticatory muscles during function or parafunction regulates the extent of deformation1. During mostly the movements of protrusion and opening of the jaw, it has been observed that the width of the mandibular arch is reduced due to the ability of the mandible to bend inwards. This deformation is defined as Median Mandibular Flexure (MMF)5-7. The contraction of masticatory muscles, especially of the lateral pterygoid muscle, enables the movements of protrusion and opening of the jaw to take place8-11. As a result there is a flexion of the mandible around the symphysis which leads to the medial movement of the condyles and to a sagittal shift of the posterior sector, which reduces the width of the arch8,10-12. There is greater deformation of the mandible during forced opening than protrusion of the jaw4,8,13,14. The anchorage of these structures on the mandible alters the shape and leads to a reduction of the arch width but it can also change the position of teeth and implants in the arch8,13,14. Regarding themuscles that are involved in the deformation of the mandible due to MMF, lateral pterygoid muscles can lead to the greatest deformation during opening of the jaw7,15. Other muscles also participate in mandibular flexure like the superior constrictor muscles, mylohyoid and platysma9. MMF has also been discovered to deform the mandible in the presence of occlusal forces and bruxism. As a result, people with more potent masticatory muscles tend to experience a stronger mandibular flexure and a greater deformation9,13,14. Prasad et al., found that people with a brachyfacial type usually experience a higher degree of deformation of the mandible, due to mandibular flexure when compared to people with a dolichofacial type16. The development of stronger masticatory muscles occurs in brachyfacial type individuals that have a shorter and more square facial morphology4,5,16,17. Similarly, Spronsen et al. concluded that people with longer faces have smaller and less potent masticatory muscles and consequently present less deformation by MMF18. It has been shown that maximum occlusal forces are associated with a greater deformation of the mandibular arch during MMF5,19. Individuals that possess a greater mandibular length, lower gonial angle and a smaller area of the symphysis tend to experience a higherMMF19. Females exhibit a greater deformation of the mandibular arch during MMF than males5. However, a different study indicated that even though females exhibit a slightly larger MMF than males, the difference is not statistically significant20,21. According to a study conducted by Ebadian et al. (2020), median mandibular flexure has been associated with bone density, as bone density decreases dimensional changes from MMF are increased21. Masticatory muscles are one of the main components that contribute to MMF, however the more strain these muscles produce the higher the density of the bone becomes21,23. Consequently the deformation of the mandibular width from the action of muscles is in a way compensated by the increase in bone density that is a result of the action of the same muscles21. Other articles have demonstrated that as age increases MMF decreases, because with an increase in aged people tend to experience greater tooth loss that leads to a higher bone density, which consequently leads to a reducedMMF11,21,24. On the contrary Chen et al. (2000), showed that an increase in age leads to a decrease in bone density and thus a higher MMF.17. Madani et al. (2011), demonstrated that MMF is not influenced by age7. Furthermore tooth loss is associated with a reduced maximum occlusal force which similarly leads to a reduced MMF25, but with a bimaxillary implant rehabilitation, higher occlusal forces are realized26. Mandibular flexion and Implant prosthesis design: An important reflection Authors: Dr. Oriol Canto-Naves: DDS, PhD. Professor of the Universitat Internacional de Catalunya – Barcelona. oriolcanto@uic.es Dr. Raul Medina-Galvez: DDS, PhD. Professor of the Universitat Internacional de Catalunya – Barcelona. Dr. Danai Maria Stasinoulia: DDS. Universitat Internacional de Catalunya – Barcelona. Dr. Josep Cabratosa-Termes: DDS, PhD. Professor of the Universitat Internacional de Catalunya – Barcelona. LINICAL Dr. Oriol Canto-Naves PROFESSIONAL DENTIST SUPPLIES - 3/8 NICOLE CLOSE BAYSWATER NORTH VIC AUSTRALIA - 03 9761 6615 - sales@profdent.com.au - ABN 69 088 275 576 WWW.PROFDENT.COM.AU
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