Biomechanical implications of lateral pterygoid contribution to biting and jaw opening in humans

The contributions of the lateral pterygoid muscle to a variety of different tasks were analysed by a linear programming mode based on the equations for static equilibrium in three dimensions and containing 12 muscles. The model was used to study lateral pterygoid activity at maximum bite force (MBF) for changes in (i) the direction and point of application of the bite force, (ii) the orientation of the masseter and medial pterygoid muscles and (iii) the slope of the articular eminence (glenoid slope). The effects on MBF of removing one or both lateral pterygoids were also examined. Lateral pterygoid provided a very important indirect contribution to some clenching forces. Under some conditions removing one lateral pterygoid muscle (simulating guarding an injured muscle) halved the MBF. Its activity at MBF was reduced as masseter was tilted more forward, the glenoid slope was made more horizontal and the bite force was made more vertical. The muscle helped to oppose (balance) the horizontal reaction forces at the bite point and joints, which potentially pushed the condyle backward. A balancing muscle is now defined as one (like lateral pterygoid) whose activity increases the output force by far more than its direct contribution to that force. In a larger model containing 16 muscles, every muscle was most active when its line of action was parallel to the output force. Finally, in a model which divided lateral pterygoid into superior and inferior heads, activity suddenly switched from the superior head to the inferior head when the angle of opening changed from 120° (forward from the vertical) to 140°.     

Maximum jaw opening capacity in adolescents in relation to general joint mobility

Mandibular jaw opening was related with general joint mobility in a non-patient adolescent group. The angular rotation of the mandible at maximum jaw opening was slightly larger in females than in males and significantly larger in hypermobile individuals. No significant relationship between linear measuring of maximal mandibular opening capacity and peripheral joint mobility was found either at active (AROM) or at passive range of mandibular opening (PROM). PROM was strongly correlated to the mandibular length. Clinical signs in the great jaw closer muscles could not be associated to decreased AROM. The mean value of the difference between PROM-AROM (DPA) was 1.2 mm. Frequent clenching and/or grinding was correlated to increased DPA only in hypermobile adolescents (r = 0.49***). Those with DPA exceeding 5mm had all reciprocal clicking.     

Analysis of jaw opening reflexes

A bstract — Electromyograms were compared with kinematograms recorded for 35 male dental students, average age 25 years, with full occlusions and without temporomandibular joint dysfunction. The records were obtained during the open-close-clench cycle rhythmically performed as the patient occluded on the molar teeth. The mechanical tracings disclosed seven subjects with centric slide and the duration of the cycle was significantly longer than normal. The kinematographic latency of the jaw opening reflex was found to be about equal to the sum of the electromyographic latency and inhibition.     

A new method for recording temporomandibular joint sounds and electrical jaw muscle activity in relation to jaw opening degree

A system for continuously quantifying of jaw movements in terms of jaw angle was tested in combination with a previously tried method for simultaneous recording of temporomandibular joint (TMJ) sounds and electromyographic (EMG) activity. The system was used to measure the degree of rotation of the lower jaw in the sagittal plane in relation to the upper jaw. It was tested on a group of healthy subjects and patients with TMJ clicking and found useful to record EMG activity and TMJ clicking in relation to opening degree and velocity of jaw movement in open-close cycles.     

Volume changes in human masticatory muscles between jaw closing and opening

Most jaw muscles are complex, multipennate with multiple components. The morphologic heterogeneity of masticatory muscles reflects their functions. We hypothesized that the volume of masticatory muscles changes between jaw closing and opening, and that there is a difference in the volume change among the muscles. Magnetic resonance images of the entire head were obtained in ten normal young adult subjects before and after maximum jaw opening. The volume changes of the masseter, medial, and lateral pterygoid muscles were measured. Only slight changes were seen in the masseter and medial pterygoid muscles. The lateral pterygoid muscle, however, significantly decreased its volume during jaw opening. The results provide normative values of muscle volume in living subjects, and suggest that the volume changes differ among jaw muscles.     

正常人牙尖交错位紧咬牙时颞下颌关节无单元-有限元耦合法二维生物力学分析

目的用无单元-有限元耦合法模拟分析正常人牙尖交错位紧咬牙时颞下颌关节生物力学分布特征。方法采用位移加载方式,应用ANSYS8.0对建立的正常颞下颌关节有限元模型进行分析,用自行编制的耦合程序对建立的正常颞下颌关节无单元-有限元耦合模型进行分析,比较两者的应力分析结果。结果有限元模型和耦合模型应力分析结果具有较高的相似性,反映的颞下颌关节应力分布特点基本相同,但耦合模型对关节盘的应力分析更精细。结论无单元-有限元耦合法是一种有效的数值模拟方法,在生物力学研究中有推广应用前景。     

Dynamic simulation of muscle and articular properties during human wide jaw opening

Human mandibular function is determined in part by masticatory muscle tensions and morphological restraints within the craniomandibular system. As only limited information about their interactions can be obtained in vivo, mathematical modeling is a useful alternative. It allows simulation of causal relations between structure and function and the demonstration of hypothetical events in functional or dysfunctional systems. Here, the external force required to reach maximum jaw gape was determined in five relaxed participants, and this information used, with other musculoskeletal data, to construct a dynamic, muscle-driven, three-dimensional mathematical model of the craniomandibular system. The model was programmed to express relations between muscle tensions and articular morphology during wide jaw opening. It was found that a downward force of 5 N could produce wide gape in vivo. When the model's passive jaw-closing muscle tensions were adjusted to permit this, the jaw's resting posture was lower than that normally observed in alert individuals, and low-level active tone was needed in the closer muscles to maintain a typical rest position. Plausible jaw opening to wide gape was possible when activity in the opener muscles increased incrementally over time. When the model was altered structurally by decreasing its angles of condylar guidance, jaw opening required less activity in these muscles. Plausible asymmetrical jaw opening occurred with deactivation of the ipsilateral lateral pterygoid actuator. The model's lateral deviation was limited by passive tensions in the ipsilateral medial pterygoid, which forced the jaw to return towards the midline as opening continued. For all motions, the temporomandibular joint (TMJ) components were maintained in continual apposition and displayed stable pathways despite the absence of constraining ligaments. Compressive TMJ forces were presented in all the cases and increased to maximum at wide gape. Dynamic mathematical modeling appears a useful way to study such events, which as yet are unrecordable in the human craniomandibular system.     

Biomechanical responses of human temporomandibular joint disc under tension and compression

The purpose of this study was to observe the microscopic changes in the temporomandibular joint (TMJ) disc in response to tension and compression, and to study the mechanism of disc failure when subjected to mechanical stresses. The scanning electron microscope was used to observe topographic characteristics of TMJ disc samples obtained from 13 cadavers before and during biomechanical experiments. The stress relaxation experiments were conducted at different strain levels (2, 3, 4 and 6%). The uniaxial tensile experiments were carried out at a constant strain rate (0.05 mm/s). The confined compression tests were performed with 3 different indenters (2, 3 and 4mm in diameter) for 150 s and 3 h. The maximal tensile strain of the disc was 5% (nearly equal to 0.22 MPa of tensile stress) in the mediolaterally tensile direction. Typical wavelike structure of the collagen fibrils of the disc was present at 2-4% strain ranges. Tensile and shear damage to local collagen fibrils was observed in specimens of the intermediate zone and the posterior band at 6% strain level. Changes in the collagen network from a wavelike structure to distortion observed on the surface of the testing samples were reversible in the 150-s indentation, but severe, irreversible breakdown and deformation of the collagen-proteoglycan network occurred in those specimens that had been compressed for 3h. Persistent and prolonged compression inevitably resulted in irrecoverable disc failure. Mechanical stress is a crucial factor in breakdown of the TMJ disc.     

Joint tenderness, jaw opening, chewing velocity, and bite force in patients with temporomandibular joint pain and matched healthy control subjects

AIMS: To evaluate the effect of temporomandibular arthralgia on mandibular mobility, chewing, and bite force. METHODS: Twenty female patients (ages 19 to 45 years) with unilateral temporomandibular joint (TMJ) pain during chewing (49 +/- 27 mm on a 100-mm visual analog scale) and provocation, as well as TMJ tenderness, were studied. The TMJ conditions were classified as disc derangement disorders (n = 9), osteoarthritis (n = 7), and inflammatory disorders (n = 4). The patients were compared with matched healthy volunteers without orofacial pain or tenderness. Exclusion criteria were the presence of fewer than 24 teeth or malocclusion. The methods used were (1) algometric assessment of the pressure pain threshold (PPT) over the TMJ; (2) clinical recordings of maximum jaw opening; (3) computerized kinematic assessment of maximum vertical distance, velocity, and cycle duration during chewing of soft gum; and (4) measurement of unilateral molar bite force. RESULTS: The mean (+/- SD) PPT in the patients' painful side (69 +/- 20 kPa; P = .000001) was significantly lower than in the control subjects (107 +/- 22 kPa). Jaw opening was also significantly less (P = .00003) in the patients (42 +/- 9 mm) than in the controls (52 +/- 4 mm). Chewing cycle duration and maximum closing velocity were significantly different (P < or = .03) in the patients (948 +/- 185 milliseconds and 142 +/- 46 mm/s, respectively) versus the controls (765 +/- 102 milliseconds and 173 +/- 43 mm/s, respectively), and bite force was significantly lower (P = .000003) in the patients (238 +/- 99 N) than in the controls (394 +/- 80 N). Both bite force and jaw opening in patients were significantly correlated (P < or = .02) with PPT (r = 0.53 and 0.63, respectively). CONCLUSION: These systematic findings supplement results from acute pain experiments and confirm indications from unspecified patient groups that the clinical presence of long-standing TMJ pain is associated with marked functional impairment. This impairment might be a result of reflex adaptation and long-term hypoactivity of the jaw muscles.     

偏颌畸形下颌开闭口运动特征的研究

目的 研究发育性偏颌畸形患者下领开闭口运动的特点，探讨偏颌畸形对下颌运动功能的影响。方法 采用下颌运动轨迹描记仪(Sirognathograph,SGG)采集28例偏颌畸形者及41例咬合正常者最大开闭口运动的三维方向时间—位移信号，比较两组受试者下颌运动轨迹的形态、开闭口运动的位移及速度等参数的差异。结果 偏颌畸形患者开闭口运动轨迹在冠状面内主要分布在患侧，下颌向患侧移动的幅度明显大于向健侧移动的幅度，开口运动中下颌偏离中轴的程度较闭口运动时明显。偏颌者下颌开闭口运动三维空间内的位移、速度比正常咬合者大。结论 偏颌患者下颌运动处于能量消耗较高的状态，下颌运动功能的异常可能影响口颌系统的功能。     

Quantification of translational and gliding components in human temporomandibular joint during mouth opening

OBJECTIVE: In humans, the opening movement of the mouth requires a complex combination of rotation in the lower temporomandibular joint compartment and of translation in the upper compartment. The aim of the current study was to quantitatively assess the percentage contribution of rotation and translation movements of the mandible at maximum mouth opening in normal, healthy individuals. DESIGN: Free, habitual movements of mouth opening were recorded in 12 men and 15 women aged 19-30 years using an optoelectronic three-dimensional motion analyser. All subjects had a sound, complete, permanent dentitions with Angle Class I jaw relationships, without cast restorations or cuspal coverage, TMJ or craniocervical disorders. For each subject, the mandibular movements at the interincisor point (occlusal plane) were reconstructed, and, using suitable mathematical algorithms, divided into their rotation and gliding components. The relative contribution of the two components to the total movement was calculated for each frame of motion. In particular, the situation at maximum opening was assessed. RESULTS: At maximum mouth opening, on average, men had significantly larger displacement of the mandibular interincisor point (56 mm versus 46 mm) and angle of rotation (34 degrees versus 32 degrees), than women. The percentage of mandibular movement explained by rotation at maximum mouth opening (77%) was not influenced by sex. The degree of rotation was significantly related to the displacement of the interincisor point: in women r2 = 87%, in men, r2 = 45%. CONCLUSIONS: Overall, in normal subjects with a healthy stomatognathic apparatus, mouth opening was more determined by mandibular rotation than by translation.     

The neuromuscular reaction to continuous dynamic jaw extension in cases with restricted mouth opening

20 patients with severe trismus resulting from either trauma or infection have been treated using the method of "continuous dynamic jaw extension". The clinical results and the electromyographic tests show that almost complete rehabilitation of mandibular movement is possible. The treatment is most effective during the healing phase of the abscess and as an early treatment for the traumatised patient. Extension exercises should not be undertaken during the exudative phase of abscess formation. The reaction ability of the masticatory musculature obtained from the electromyographic investigations is defined in the "reactive myotonus-index". Thus the jaw extension appliance used is helpful both in the diagnosis and treatment of severe trismus.     

An electromyographic study of jaw opening and closing reflexes in man

The electromyographic activity of the right masseter and digastric muscles was recorded in 10 subjects. A jaw-opening reflex was observed shortly after a mechanical stimulation in subjects performing clenching and active jaw-opening exercises. The latency of this reflex activity was about 28 ms, coinciding with the termination of the silent period of the masseter muscle. The experiments show that this opening reflex can occur without masseter spindle-unloading and may respond to low-threshold afferents.     

Electromyographic activity of the jaw-closing muscles during jaw opening in patients with masseter muscle contracture

Contracture of the jaw-closing muscles is one of the causes of limitation of jaw opening. This study examined whether there is any difference between the EMG activities of jaw-closing muscles during jaw opening in healthy people and in patients with masseter muscle contracture (MMC), who do not have a history of trauma or infection. The patient group consisted of eleven females, 18 to 62 years old with no history of trauma or infections, with limited mouth opening due to MMC. The control group included eleven healthy females, 23 to 50 years old. The EMG activity was recorded bilaterally in the central portion of masseter muscles (Mm), the anterior portion of temporal muscles (Tm), and the anterior belly of digastric muscles (Dm). Nine out of the patient group showed obvious EMG activity in Mm during jaw opening which was different from typical EMG patterns during jaw opening of up to 40 mm in the control group. Among the nine patients, eight showed antagonistic contraction in Tm as well as Mm. The mean integral value of Mm and Tm during jaw opening in the patient group was significantly higher than in the control group (P<0.01). These results suggest that EMG activity during jaw opening in MMC patients with no history of trauma or infections is different from that in healthy people.     

Biomechanical role of peri-implant trabecular structures during vertical loading

The aim of this study was to identify the load transfer paths in cortical bone and trabecular structure of cancellous bone in the jawbones for loads from endosseous implants. Maxillae were resected from beagle dogs 6 months after implant surgery and imaged using micro-computed tomography (micro-CT). A three-dimensional structure was produced based on the CT data and peri-implant trabecular structure was observed. Load transfer paths were analyzed from the results of three-dimensional finite element analysis. Furthermore, buffer actions in bone trabeculae when strain increased during stress analysis and when loads were applied were observed. Peri-implant bone trabeculae were seen extending into the upper and lower cortical bone from the fixture. The direction of bone trabecular alignment corresponded with the load transfer paths. In addition, analysis with increased strain confirmed that trabecular structures could serve as load buffers. These results suggest that bone trabeculae supporting load transfer from implants undergo remodeling.     

In vivo cross-sectional area of human jaw muscles varies with section location and jaw position

Muscle cross-sectional area (CSA) is used as a measure for maximum muscle force. This CSA is commonly determined at one location within the muscle and for one jaw position. The purpose of this study was to establish a method to standardize the analysis of the CSA of the masticatory muscles in vivo, and to compare the CSAs along their entire length for two different jaw positions (opened and closed). The CSAs in the planes perpendicular to the long axes of the masseter, medial, and lateral pterygoid muscles were measured in ten normal young adult subjects by magnetic resonance imaging. Our results showed large differences among the muscles and a non-uniform change in CSA after jaw-opening. The method enables the CSA measurement to be standardized in vivo, and allows for a correct comparison of CSAs in different skull morphologies.