Morphological evolution of the mammalian jaw adductor complex

The evolution of the mammalian jaw during the transition from non-mammalian synapsids to crown mammals is a key event in vertebrate history and characterised by the gradual reduction of its individual bones into a single element and the concomitant transformation of the jaw joint and its incorporation into the middle ear complex. This osteological transformation is accompanied by a rearrangement and modification of the jaw adductor musculature, which is thought to have allowed the evolution of a more-efficient masticatory system in comparison to the plesiomorphic synapsid condition. While osteological characters relating to this transition are well documented in the fossil record, the exact arrangement and modifications of the individual adductor muscles during the cynodont–mammaliaform transition have been debated for nearly a century. We review the existing knowledge about the musculoskeletal evolution of the mammalian jaw adductor complex and evaluate previous hypotheses in the light of recently documented fossils that represent new specimens of existing species, which are of central importance to the mammalian origins debate. By employing computed tomography (CT) and digital reconstruction techniques to create three-dimensional models of the jaw adductor musculature in a number of representative non-mammalian cynodonts and mammaliaforms, we provide an updated perspective on mammalian jaw muscle evolution. As an emerging consensus, current evidence suggests that the mammal-like division of the jaw adductor musculature (into deep and superficial components of the m. masseter, the m. temporalis and the m. pterygoideus) was completed in Eucynodontia. The arrangement of the jaw adductor musculature in a mammalian fashion, with the m. pterygoideus group inserting on the dentary was completed in basal Mammaliaformes as suggested by the muscle reconstruction of Morganucodon oehleri. Consequently, transformation of the jaw adductor musculature from the ancestral (‘reptilian’) to the mammalian condition must have preceded the emergence of Mammalia and the full formation of the mammalian jaw joint. This suggests that the modification of the jaw adductor system played a pivotal role in the functional morphology and biomechanical stability of the jaw joint

A chewing robot based on parallel mechanism-- analysis and design : a thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering in Mechatronics at Massey University

Masticatory efficiency, dependent on number and condition of the teeth, length of time spent in chewing a bolus and the force exerted when chewing, influences an individual with the selection of food and therefore nutritionally diet. A characterisation of the masticatory efficiency could be possible with a chewing robot that simulates human chewing behaviours in a mechanically controllable way (Pap et al. 2005; Xu et al. 2005). This thesis describes such a chewing robot, developed from a biological basis in terms of jaw structure and muscles of mastication according to published articles. A six degrees of freedom parallel mechanism is proposed with the mandible as a moving platform, the skull as a fixed platform, and six actuators representing the main masticatory muscle groups, temporalis, masseter, and lateral pterygoid on the left and right side. Extensive simulations of inverse kinematics (i.e., generating muscular actuations with implementing recorded human trajectories) were conducted in SolidWorks and COSMOS/Motion to validate two mathematical models of the robot and to analyse kinematic properties. The research showed that selection of appropriate actuation systems, to achieve mandible movement space, velocity, acceleration, and chewing force, was the key challenge in successfully developing a chewing robot. Two custom designed actuation systems, for the six actuators, were developed and built. In the first approach, the muscle groups were presented as linear actuators, positioned so as to reproduce the resultant lines of action of the human muscles. However, with this design concept the spatial requirements specified from the human masticatory system made the physical building of the model impossible. The second approach used a crank mechanism based actuator. This concept did not allow a perfectly linear actuator movement that copied the muscle line of action. However, it was possible to fulfil the spatial requirements set by the human system and to allow reproduction of human chewing behaviours in relation to kinematic requirements and chewing force. The design, manufacture and testing of the entire chewing robot with crank actuators was then carried out. This included the implementation of realistic denture morphology, a mechanical jaw and the framework design for the whole system. In conclusion, this thesis research has developed successfully a mathematical and a physical robotic model. Future work on the control and sensing of the robot and design of a food retention system are required in order to fully functionalise the device

Masticatory biomechanics in the rabbit : a multi-body dynamics analysis

Acknowledgement We thank Sue Taft (University of Hull) for the µCT-scanning of the rabbit specimen used in this study. We also thank Raphaël Cornette, Jacques Bonnin, Laurent Dufresne, and l'Amicale des Chasseurs Trappistes (ACT) for providing permission and helping us capture the rabbits used for the in vivo bite force measurements at la Réserve Naturelle Nationale de St Quentin en Yvelines, France.Peer reviewedPublisher PD

mRNA expression of myosin heavy chain isoforms in the sphenomandibularis portion of the temporalis muscle

The main objective of this study was to analyze by real-time quantitative polymerase chain reaction (RT-qPCR) the expression patterns of the myosin heavy chain (MHC) isoforms (MHC-I, MHC-IIa, MHC-IIx) in the sphenomandibularis portion of the temporalis muscle. We expected to find differences between the sphenomandibularis and the other portions of the temporalis that could be related to the functional characteristics of the sphenomandibularis identified by electromyography. We dissected the right temporalis muscle of ten adult human individuals (five men and five women). Samples of the anterior and posterior temporalis and of the sphenomandibularis portion were obtained from each dissected muscle. These samples were analyzed by RT-qPCR to determine the percentages of expression of the MHC-I, MHC-IIa and MHC-IIx isoforms. No significant differences were identified between the anterior and the posterior temporalis in the expression patterns of the MHC-I, MHC-IIa and MHC-IIx isoforms. However, there were significant differences between the sphenomandibularis and the anterior temporalis. Specifically, the sphenomandibularis portion had a higher percentage of expression of the MHC-I isoform (P=0.04) and a lower percentage of expression of the MHC-IIx isoform (P=0.003). The pattern of expression that we observed in the sphenomandibularis reflects a greater resistance to fatigue, a lower contraction speed, and a lower capacity of force generation in the sphenomandibularis compared to the anterior temporalis. These characteristics are consistent with electromyographic findings on the functional differences between these two portions

Choosing new ways to chew

No abstract availabl

Reorganization of jaw muscle activity during experimental jaw muscle pain

Background and Aims: Temporomandibular disorders are clinical conditions that often involve pain in the masticatory muscles, the temporomandibular jaw joint and/or associated structures. The association between muscle pain and muscle activity is often explained by uniform increases or decreases in motor unit activity throughout a muscle but recent evidence suggests more complex changes within a painful muscle. The general aim of this study was to determine if experimentally induced masseter muscle pain modifies temporalis muscle activity. Methods: 20 healthy participants received experimental pain through hypertonic saline (5% NaCl) infusion into the right masseter; pain intensity was maintained at 40-60/100 mm on a visual analogue scale (VAS). Standardized biting tasks were performed with an intraoral force transducer while single motor unit (SMU) activity was recorded from 2 intramuscular electrodes (right masseter and right temporalis). The tasks were repeated in 4 blocks: baseline 1, hypertonic saline infusion, isotonic saline infusion, baseline 2. Each block had 3 isometric biting tasks: a slow and a fast ramp jaw closing task and a 2 step-levels jaw closing task (2 force levels: step 1 and step 2). Results: 83 SMUs were discriminated from the temporalis and 58 from the masseter muscle. This study demonstrated that induced muscle pain in the right masseter can be associated with the activation of new SMUs and the silencing of other single motor units in the painful masseter muscle as well as in the right temporalis muscle, which did not receive noxious stimulation with the hypertonic saline. No differences between pain and no pain trials were found in thresholds and firing rates of SMUs from the temporalis muscle. Discussion and conclusion: The present findings are consistent with previous findings from the limb (Hodges and Tucker 2011; Tucker et al. 2009) and rather than supporting uniform increases or decreases in motor unit activity throughout a muscle, suggest that there is a reorganization of motor unit activity across the entire jaw motor system in experimental pain

Functional implications of minor mandibular asymmetry: clinical and digital research on a sample of healthy patients

OBJECTIVE.The aim of the study is to evaluate the influence that the mandibular asymmetry could have towards the other components of the Stomathognatic system and to further related structures from a functional point of view. MATERIALS and METHODS. A sample of 18 young asymmetric patients, in healthy status, was investigated with clinical evaluation and questionnaires and digital devices such as Electromiography, Stabilometry, T-scan and Formetric. Descriptive and quantitative statistical analysis were performed. RESULTS. ASIM electromyography index was significant only in a short percentage of the sample. Hypertonia of temporalis muscle in the same side of mandibular deviation and the cross-activation of the contralateral masseter were present. The data of the stabilometric platform showed that only 33% of patients had a load discrepancy between right and left side. All patients presented similar and limited postural anomalies during the Formetric examination. No significant results emerged from statistics . CONCLUSIONS. From the emerging data, mandibular asymmetry does not seem to be considered a potential risk factor for the development of functional anomalies both in the stomatognatic system and in the postural one. The only issue that should be considered is the condylar retrusion of the mandibular shortest side that is a possible destabilizing factor of the TMJ condyle-disc coordination

Functional implications of minor mandibular asymmetry: clinical and digital research on a sample of healthy patients

OBJECTIVE.The aim of the study is to evaluate the influence that the mandibular asymmetry could have towards the other components of the Stomathognatic system and to further related structures from a functional point of view. MATERIALS and METHODS. A sample of 18 young asymmetric patients, in healthy status, was investigated with clinical evaluation and questionnaires and digital devices such as Electromiography, Stabilometry, T-scan and Formetric. Descriptive and quantitative statistical analysis were performed. RESULTS. ASIM electromyography index was significant only in a short percentage of the sample. Hypertonia of temporalis muscle in the same side of mandibular deviation and the cross-activation of the contralateral masseter were present. The data of the stabilometric platform showed that only 33% of patients had a load discrepancy between right and left side. All patients presented similar and limited postural anomalies during the Formetric examination. No significant results emerged from statistics . CONCLUSIONS. From the emerging data, mandibular asymmetry does not seem to be considered a potential risk factor for the development of functional anomalies both in the stomatognatic system and in the postural one. The only issue that should be considered is the condylar retrusion of the mandibular shortest side that is a possible destabilizing factor of the TMJ condyle-disc coordination

Maxillary reconstruction using rectus femoris muscle flap and sagittal mandibular ramus/coronoid process graft pedicled with temporalis muscle

Maxillary reconstruction using various pedicled and free-tissue transfer techniques with bone graft or without bone graft has some drawbacks. In this study, we demonstrate maxillary reconstruction using femoris rectus muscle flap and sagittal mandibular ramus/coronoid process graft pedicled with temporalis muscle through the modified lateral lip-submandibular approach. Nine patients suffering from maxillary defects secondary to maxillary cancer ablation, who underwent maxillary reconstruction using rectus femoris muscle flap and sagittal mandibular ramus/coronoid process graft pedicled with temporalis muscle, were enrolled into this study between November 2015 and August 2017. All patients who underwent the maxillary reconstruction using femoris rectus muscle flap and sagittal mandibular ramus/coronoid process graft pedicled with temporalis muscle presented satisfactory postoperative function, with adequate mouth opening, optimal esthetic outcome and no restrictions on the diet. Every rectus femoris muscle flaps mucosalized well within five weeks. No donor site functional impairment or complications were observed. The technique is a feasible and acceptable technique for the maxillary reconstructions. It is safe, quick and simple to harvest. It also presents an optimal esthetic and satisfactory functional outcome with the advantage of low morbidity of the donor site. Combined with the three-dimension reconstruction, this technique can improve the postoperative outcomes

Translocation of the temporalis muscle for treatment of facial paralysis

Our purpose was to characterize in the rhesus monkey the structure and function of vascularized temporalis muscle flaps innervated by the facial nerve after translocation into the site of the denervated zygomaticus muscle. Animals were killed at 28 to 120 days following translocation. Control data were obtained from the contralateral side. Twenty-eight days after translocation, the time to reach peak twitch tension and one-half relaxation time were 170% of control zygomaticus muscle. Contraction times decreased with time and reached control values by 100 days. Absolute isometric tetanic tension was not different between the flap (4.29 ± 1.28 newtons; X ± SEM) and control zygomaticus (3.95 ± 0.80 newtons). Succinate oxidase activity of the flap decreased from 279 ± 18 nl O 2 /mg protein/min to control zygomaticus values (98 ± 18) by 110 days. The type 1 fiber cross-sectional area of the flap was 52% of control temporalis muscle and 150% of control zygomaticus muscle ( P < 0.05). The temporalis flap demonstrated viable structure and function and appeared useful in facial movements.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50131/1/880050704_ftp.pd