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TENSILE PATTERNS IN THE HUMAN TMJ DISC
Poster P-116
Modelling
S489
TENSILE PATTERNS IN THE HUMAN TMJ DISC J.H. Koolstra
Department of Functional Anatomy, Academic Centre for Dentistry Amsterdam (ACTA), the Netherlands
Introduction The temporomandibular joint (TMJ) disc provides a large load bearing capacity over the entire motion range of the human jaw joint. It is made out of fibrocartilage, with a markedly antero-posterior orientation of the collagen fibers [Tanaka, 2003]. This indicates that this structure is able to restrain tension in this direction better than in other directions. Biomechanical analyses have indicated that the TMJ disc is loaded predominantly with shear [Koolstra, 2005]. The present aim was to study this discrepancy by analysis of the magnitude and orientation of the tensile stresses in the disc during habitual function. The TMJ disc is connected to the superior lateral pterygoid muscle. The activity of this muscle could be of influence on tensions occurring in the disc. Moreover, it has been suggested that dysfunction of this muscle may lead to symptomatic anterior disc displacements [Tanaka, 2007]. Therefore, tensile stress was analyzed with and without the presence of this connection.
Materials and Methods Principal stress has been analyzed using a biomechanical model of the human masticatory system. In this model, the deformable cartilaginous structures in both joints are included as finite element models. For the disc a non-linear viscoelastic material model was applied [Koolstra, 2007]. The model was implemented with MADYMO (TNO Automotive Safety Solutions) software. Unloaded and loaded (40N interincisal resistance) jaw open-close movements were simulated by subsequent activation of all jaw openers and jaw closers, which had been included as Hill-type contractile units.
posterior band it appeared predominantly in the inferior layer and in the intermediate zone and the anterior band only in the superior layer. When the jaw was open the tensile stress was more mediolaterally directed. Attachment of an active lateral pterygoid muscle created much more prominent antero-posteriorly directed tensile stresses in the anterior regions, both when the jaw was opened or closed. In the posterior band, the stress pattern was much less influenced. Generally, principal tensions with an antero-posteriorly orientation had a larger magnitude than otherwise, irrespective of activation of the lateral pterygoid muscle.
Discussion The present results suggest that the observed direction of collagen fibers in the TMJ disc enables to withstand the tensile forces which are created by activity of the lateral pterygoid muscle. Furthermore, as the tensile stresses in the mediolateral direction have a lesser magnitude, it is suggested that to maintain the integrity of the disc in that direction, a dense collagen network is not obligatory.
References Koolstra et al, J. Biomech, 38: 2431-2439, 2005. Koolstra et al, J Dent Res 86: 1198-1202, 2007. Tanaka et al, Crit Rev Oral Biol Med 14: 138-150, 2003. Tanaka et al, J Biomech Eng 129: in press, 2007.
Results The patterns of tensile stress in the superior layer of the articular disc differed from the inferior one. When the lateral pterygoid was not connected to the disc, a clear antero-posteriorly directed pattern was only present when the jaw was closed. In the
16th ESB Congress, Posters
Journal of Biomechanics 41(S1)
Modelling
S489
TENSILE PATTERNS IN THE HUMAN TMJ DISC J.H. Koolstra
Department of Functional Anatomy, Academic Centre for Dentistry Amsterdam (ACTA), the Netherlands
Introduction The temporomandibular joint (TMJ) disc provides a large load bearing capacity over the entire motion range of the human jaw joint. It is made out of fibrocartilage, with a markedly antero-posterior orientation of the collagen fibers [Tanaka, 2003]. This indicates that this structure is able to restrain tension in this direction better than in other directions. Biomechanical analyses have indicated that the TMJ disc is loaded predominantly with shear [Koolstra, 2005]. The present aim was to study this discrepancy by analysis of the magnitude and orientation of the tensile stresses in the disc during habitual function. The TMJ disc is connected to the superior lateral pterygoid muscle. The activity of this muscle could be of influence on tensions occurring in the disc. Moreover, it has been suggested that dysfunction of this muscle may lead to symptomatic anterior disc displacements [Tanaka, 2007]. Therefore, tensile stress was analyzed with and without the presence of this connection.
Materials and Methods Principal stress has been analyzed using a biomechanical model of the human masticatory system. In this model, the deformable cartilaginous structures in both joints are included as finite element models. For the disc a non-linear viscoelastic material model was applied [Koolstra, 2007]. The model was implemented with MADYMO (TNO Automotive Safety Solutions) software. Unloaded and loaded (40N interincisal resistance) jaw open-close movements were simulated by subsequent activation of all jaw openers and jaw closers, which had been included as Hill-type contractile units.
posterior band it appeared predominantly in the inferior layer and in the intermediate zone and the anterior band only in the superior layer. When the jaw was open the tensile stress was more mediolaterally directed. Attachment of an active lateral pterygoid muscle created much more prominent antero-posteriorly directed tensile stresses in the anterior regions, both when the jaw was opened or closed. In the posterior band, the stress pattern was much less influenced. Generally, principal tensions with an antero-posteriorly orientation had a larger magnitude than otherwise, irrespective of activation of the lateral pterygoid muscle.
Discussion The present results suggest that the observed direction of collagen fibers in the TMJ disc enables to withstand the tensile forces which are created by activity of the lateral pterygoid muscle. Furthermore, as the tensile stresses in the mediolateral direction have a lesser magnitude, it is suggested that to maintain the integrity of the disc in that direction, a dense collagen network is not obligatory.
References Koolstra et al, J. Biomech, 38: 2431-2439, 2005. Koolstra et al, J Dent Res 86: 1198-1202, 2007. Tanaka et al, Crit Rev Oral Biol Med 14: 138-150, 2003. Tanaka et al, J Biomech Eng 129: in press, 2007.
Results The patterns of tensile stress in the superior layer of the articular disc differed from the inferior one. When the lateral pterygoid was not connected to the disc, a clear antero-posteriorly directed pattern was only present when the jaw was closed. In the
16th ESB Congress, Posters
Journal of Biomechanics 41(S1)