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Quantification of porosity and degree of mineralization of bone facilitates a better understanding of the possible effects of adaptive bone remodelling and the possible consequences for its mechanical properties. The present study set out first to give a three-dimensional description of the cortical canalicular network in the human mandibular condyle, in order to obtain more information about the principal directions of stresses and strains during loading. Our second aim was to determine whether the amount of remodelling was larger in the trabecular bone than in cortical bone of the condyle and to establish whether the variation in the amount of remodelling was related to the surface area of the cortical canals and trabeculae. We hypothesized that there were differences in porosity and orientation of cortical canals between various cortical regions. In addition, as greater cortical and trabecular porosities are likely to coincide with a greater surface area of cortical canals and trabeculae available for osteoblastic and osteoclastic activity, we hypothesized that this surface area would be inversely proportional to the degree of mineralization of cortical and trabecular bone, respectively. Micro-computed tomography was used to quantify porosity and mineralization in cortical and trabecular bone of ten human mandibular condyles.The cortical canals in the subchondral cortex of the condyle were orientated in the mediolateral direction, and in the anterior and posterior cortex in the superoinferior direction. Cortical porosity (average 3.5%) did not differ significantly between the cortical regions. It correlated significantly with the diameter and number of cortical canals, but not with cortical degree of mineralization. In trabecular bone (average porosity 79.3%) there was a significant negative correlation between surface area of the trabeculae and degree of mineralization; such a correlation was not found between the surface area of the cortical canals and the degree of mineralization of cortical bone. No relationship between trabecular and cortical porosity, nor between trabecular degree of mineralization and cortical degree of mineralization was found, suggesting that adaptive remodelling is independent and different between trabecular and cortical bone. We conclude (1) that the principal directions of stresses and strains are presumably directed mediolaterally in the subchondral cortex and superoinferiorly in the anterior and posterior cortex, (2) that the amount of remodelling is larger in the trabecular than in the cortical bone of the mandibular condyle; in trabecular bone variation in the amount of remodelling is related to the available surface area of the trabeculae.

The complex architecture of the human jaw muscles suggests regional differences in function within these muscles. This study examines the way the temporalis and masseter muscle regions are activated when free mandibular movements with various speeds and against various external leads are carried out guided by visual feedback. Electromyographic (EMG) activity was registered in six temporalis and three masseter muscle regions with bipolar fine-wire electrodes. Recordings were made during open/close excursions, protrusion/retrusion movements, and laterodeviations. During open/close excursions and protrusion/retrusion movements, an anterior and posterior temporalis part could be distinguished, whereas during laterodeviations a more complex partitioning of this muscle was observed. During the protrusion/retrusion movements and the laterodeviations, the temporalis muscle demonstrated higher EMG peak activities than the masseter muscle, and within the masseter muscle the deep masseter showed higher EMG peaks than the superficial one. In contrast to this, during the open/close excursions the masseter showed higher peak activities than the temporalis muscle, while the superficial masseter showed higher EMG peak activities than the deep masseter. Within the deep masseter, differences were also found. During open/close excursions, the anterior deep region demonstrated higher EMG peak activities than the posterior region, whereas during protrusion/retrusion and laterodeviations the posterior deep region showed higher peaks. In general, speed had a greater effect on the EMG peak activity than external load. Only during laterodeviations did speed and load equally influence peak activity in both the deep and superficial masseter. During protrusion/retrusion movements, load showed no significant effect on EMG peak activity in the masseter muscle. A general finding was that, according to task, different regions were activated preferentially. This points to a partitioning of the excitatory command of the motoneuron pool.

While the movability of the human temporomandibular joint is great, the strains and stresses in the cartilaginous structures might largely depend on the position of the mandible with respect to the skull. This hypothesis was investigated by means of static three-dimensional finite element simulations involving different habitual condylar positions. Furthermore, the influence of several model parameters was examined by sensitivity analyses. The results indicated that the disc moved together with the condyle in the anterior direction without the presence of ligaments and the lateral pterygoid muscle. By adapting its shape to the changing geometry of the articular surfaces, the disc prevented small contact areas and thus local peak loading. In a jaw-closed configuration, the influence of 30 degrees variations of the loading direction was negligible. The load distribution capability of the disc appeared to be proportional to its elasticity and was enhanced by the fibrocartilage layers on the articular surfaces.

The degree of mineralization of bone (DMB) in the mandibular condyle reflects the age and remodeling rate of the bone tissue. Quantification of DMB facilitates a better understanding of possible effects of adaptive remodeling on mineralization of the condyle and its possible consequences for its mechanical quality. We hypothesized differences in the degree and distribution of mineralization between trabecular and cortical bone and between various cortical regions. Microcomputed tomography was used to measure mineralization in 10 human mandibular condyles. Mean DMB was higher in cortical (1,045 mg hydroxyapatite/cm(3)) than in trabecular bone (857 mg/cm(3)) and differed significantly between cortical regions (anterior 987 mg/cm(3), posterior 1,028 mg/cm(3), subchondral 1,120 mg/cm(3)). The variation of DMB distribution was significantly larger in the anterior cortex than in the posterior and subchondral cortex, indicating a larger amount of heterogeneity of mineralization anteriorly. Within the cortical bone, DMB increased with the distance from the cortical canals to the periphery. Similarly, the DMB of trabecular bone increased with the distance from the surface of the trabeculae to their cores. It was concluded that the rate of remodeling differs between condylar trabecular and cortical bone and between cortical regions and that DMB is not randomly distributed across the bone. The difference in DMB between condylar cortical and trabecular bone suggests a large difference in Young's modulus.

The higher microtensile bond strength values found for specimens with a smaller cross-sectional area are often explained by the lower occurrence of internal defects and surface flaws. We hypothesized that this aberrant behavior is mainly caused by the lateral way of attachment of the specimens to the testing device, which makes the strength dependent on the thickness. This study showed that composite bars of 1x1x10, 1x2x10, and 1x3x10mm attached at their 1-mm-wide side (situation A) fractured at loads of the same magnitude, as a result of which the microtensile strength ( micro TS), calculated as F/A (force at fracture/cross-sectional area), significantly increased for specimens with decreasing thickness. Attachment at the 1-, 2-, or 3-mm-wide side (situation B) resulted in equal micro TS values (P > 0.05). Finite element analysis showed different stress patterns for situation A, but comparable patterns for situation B. Both situations showed the same maximum stress at fracture.

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A muscle model is described that uses electromyogram (EMG), muscle length and speed of contraction to predict muscle force. Physiological parameters are the Hill constants and the shape of the twitch response to a single stimulus. The model was incorporated in a jaw model of the rabbit and tested by predicting the bite force produced by the jaw muscles during mastication. The time course of the calculated force appeared to match the bite force, measured in vivo by a strain gauge, applied to the bone below the teeth. The variation in peak strain amplitude from cycle to cycle correlated with the variation predicted by the model. The peak amplitude of the integrated EMGs of individual jaw muscles showed an average correlation with peak strain of 0.41. Use of the sum of the available peak amplitudes, weighted according to their effect upon the bite force increased the correlation to 0.46; the model predicted bite forces showed a correlation of 0.57 with the strain. The increase in correlation was statistically significant. The muscle forces were calculated using a minimum number of easily obtainable constants.