Functional and molecular outcomes of the human masticatory muscles.

The masticatory muscles achieve a broad range of different activities such as chewing, sucking, swallowing, and speech. In order to accomplish these duties, masticatory muscles have a unique and heterogeneous structure and fiber composition, enabling them to produce their strength and contraction speed largely dependent on their motor units and myosin proteins that can change in response to genetic and environmental factors. Human masticatory muscles express unique myosin isoforms, including a combination of thick fibers, expressing myosin light chains (MyLC) and myosin class I and II heavy chains (MyHC) -IIA, -IIX, α-cardiac, embryonic and neonatal and thin fibers, respectively. In this review, we discuss the current knowledge regarding the importance of fiber-type diversity in masticatory muscles versus supra- and infrahyoid muscles, and versus limb and trunk muscles. We also highlight new information regarding the adaptive response and specific genetic variations of muscle fibers on the functional significance of the masticatory muscles, which influences craniofacial characteristics, malocclusions, or asymmetry. These findings may offer future possibilities for the prevention of craniofacial growth disturbances.

Morphofunctional compensation of masseter muscles in unilateral posterior crossbite patients.

Unilateral posterior crossbite is a widespread, asymmetric malocclusion characterized by an inverse relationship of the upper and lower buccal dental cusps, in the molar and premolar regions, on one side only of the dental arch. Patients with unilateral posterior crossbite exhibit an altered chewing cycles and the crossbite side masseter results to be less active with respect to the contralateral one. Few studies about morphological features of masticatory muscle in malocclusion disorders exist and most of these have been performed on animal models. The aim of the present study was to evaluate morphological and protein expression characteristics of masseter muscles in patients affected by unilateral posterior crossbite, by histological and immunofluorescence techniques. We have used antibody against PAX-7, marker of satellite cells, and against α-, ß-, γ-, δ-, ε- and ζ-sarcoglycans which are transmembrane glycoproteins involved in sarcolemma stabilization. By statistical analysis we have evaluated differences in amount of myonucley between contralateral and ipsilateral side. Results have shown: i) altered fibers morphology and atrophy of ipsilateral muscle if compared to the contralateral one; ii) higher number of myonuclei and PAX-7 positive cells in contralateral side than ipsilateral one; iii) higher pattern of fluorescence for all tested sarcoglycans in contralateral side than ipsilateral one. Results show that in unilateral posterior crossbite hypertrophic response of contralateral masseter and atrophic events in ipsilateral masseter take place; by that, in unilateral posterior crossbite malocclusion masticatory muscles modify their morphology depending on the function. That could be relevant in understanding and healing of malocclusion disorders; in fact, the altered balance about structure and function between ipsilateral and contralateral muscles could, long-term, lead and/ or worsen skeletal asymmetries.

Correlation between in vivo stresses and oxidation of UHMWPE in total hip arthroplasty.

The possibility of in vivo, stress-induced oxidation in orthopaedic UHMWPE has been investigated. EtO sterilised, uncrosslinked UHMWPE liners, explanted or shelf-aged, have been collected. Linear wear and wear rate were assessed and FTIR spectroscopy was employed to detect oxidation and to build up oxidation products spatial maps across the liners section. Oxidation profiles have been compared to stress distribution profiles, resulting from a FE analysis conducted on the same liners geometries and couplings. It was found that oxidised and stressed areas followed the same asymmetrical, localized distribution profile. It was therefore possible to establish a correlation between stressed areas and observed oxidation.

Interplay between surface properties of standard, vitamin E blended and oxidised ultra high molecular weight polyethylene used in total joint replacement and adhesion of Staphylococcus aureus and Escherichia coli.

We have assessed the different adhesive properties of some of the most common bacteria associated with periprosthetic joint infection on various types of ultra high molecular Weight Polyethylene (UHMWPE). Quantitative in vitro analysis of the adhesion of biofilm producing strains of Staphylococcus aureus and Escherichia coli to physically and chemically characterised standard UHMWPE (PE), vitamin E blended UHMWPE (VE-PE) and oxidised UHMWPE (OX-PE) was performed using a sonication protocol. A significant decreased bacterial adhesion was registered for both strains on VE-PE, in comparison with that observed on PE, within 48 hours of observation (S. aureus p = 0.024 and E. coli p = 0.008). Since Vitamin E reduces bacterial adhesive ability, VE-stabilised UHMWPE could be valuable in joint replacement by presenting excellent mechanical properties, while reducing bacterial adhesiveness.

In-vivo degradation of poly(carbonate-urethane) based spine implants.

Fourteen explanted Dynesys® spinal devices were analyzed for biostability and compared with a reference, never implanted, control. Both poly(carbonate-urethane) (PCU) spacers and polyethylene-terephthalate (PET) cords were analyzed. The effect of implantation was evaluated through the observation of physical alterations of the device surfaces, evaluation of the chemical degradation and fluids absorption on the devices and examination of the morphological and mechanical features. PCU spacers exhibited a variety of surface damage mechanisms, the most significant being abrasion and localized, microscopic surface cracks. Evidence of oxidation and chain scission were detected on PCU spacers ATR-FTIR. ATR-FTIR, DSC and hardness measurements also showed a slight heterogeneity in the composition of PCU. The extraction carried out on the PCU spacers revealed the presence of extractable polycarbonate segments. One spacer and all PET cords visually exhibited the presence of adherent biological material (proteins), confirmed by the ATR-FTIR results. GC/MS analyses of the extracts from PET cords revealed the presence of biological fluids residues, mainly cholesterol derivatives and fatty acids, probably trapped into the fiber network. No further chemical alterations were observed on the PET cords. Although the observed physical and chemical damage can be considered superficial, greater attention must be paid to the chemical degradation mechanisms of PCU and to the effect of byproducts on the body.

Deep bite: a case report with chewing pattern and electromyographic activity before and after therapy with function generating bite.

AIM: The purpose of this case report is the concurrent evaluation of the masticatory pattern and the electromyographic activity, recorded during mastication, before and after therapy of deep bite malocclusion. CASE REPORT: An 11-year-old boy, affected by deep bite (overbite = 5 mm) was treated by the use of a functional appliance (Function Generating Bite for Deep bite correction = FGB-D). Mandibular movements during mastication of a soft and a hard bolus were recorded both before and 10 months after correction of the malocclusion. Electromyographic activity (EMG) of the masseters and anterior temporalis muscles were recorded at the same time. Chewing cycles and EMG activity were recorded with the K7 I kinesiograph (Myotronics Inc., Seattle, WA-USA). Before therapy a higher EMG activity was recorded for both masseters and anterior temporalis muscles in comparison with the results after therapy. RESULTS: The results showed a great decrease of the EMG activity of masseter and anterior temporalis muscles. Moreover, the height and width of the chewing cycles in the frontal plane increased after therapy. CONCLUSION: The functional improvement showed after therapy with FGB-D showed that the functional appliance is able to correct the dental malocclusion and the masticatory function. The orthodontic treatment should consider not only the repositioning of teeth within the dental arches but also the effects on function, especially when the malocclusion involves the muscular and skeletal structures.

Surface electromyography in the assessment of jaw elevator muscles.

Surface electromyography (EMG) allows the non-invasive investigation of the bioelectrical phenomena of muscular contraction. The clinical application of surface EMG recordings has been long debated. This paper reviews the main limitations and the current applications of the surface EMG in the investigation of jaw elevator muscles. Methodological factors associated with the recording of the surface EMG may reduce the reliability and sensitivity of this technique and may have been the cause of controversial results reported in different studies. Despite these problems, several clinical applications of surface EMG in jaw muscles are promising. Moreover, technological advances in signal detection and processing have improved the quality of the information extracted from the surface EMG and furthered our understanding of the anatomy and physiology of the stomatognathic apparatus.

Computerized axiography in TMD patients before and after therapy with 'function generating bites'.

The study evaluates the temporomandibular joint (TMJ) movements of patients with signs and symptoms of temporomandibular disorders (TMD) before and after therapy with the functional appliances of the 'function generating bite' (FGB) type. Thirty subjects suffering from TMD were selected and divided into two groups: group A (young patients: four males, nine females, mean age +/- standard deviation: 13.3+/-1.5 years); group B (adults: three males, 14 females, mean age +/- standard deviation: 23.2+/-4.4 years). A control group comprised 13 healthy subjects with perfect normal occlusion, TMD-free, was matched for age and sex with patient groups and was examined at T0 and after 12 months (T1). Computerized axiography was performed before and after therapy (average 13 months) with FGBs to evaluate any difference in condyle border movements. Results showed a statistically significant improvement after treatment, for groups A and B, in length, clicks, tracings with normal morphology, superimposition, deviations, regularity and return to starting position and speed (statistical analysis: chi-squared test) except for the symmetry of tracings which was significantly improved only for the young patient group. No statistically significant differences at time T0/T1 were found in the control group. In conclusion, the study shows that the TMJ tracings of TMD patients before and after therapy with 'FGB' significantly improve especially in young patients. FGB may be a useful appliance to improve TMJ function in young and adult TMD patients requiring orthodontic treatment.

Reproducibility of surface EMG variables in isometric sub-maximal contractions of jaw elevator muscles.

The aims of this study were: (1) to develop and assess reproducibility of a new method for measuring masticatory force in the intercuspal position; (2) to test the reproducibility of surface EMG signal amplitude and spectral variables in constant force contractions of jaw elevator muscles and its dependency on inter-electrode distance. The study was performed on the masseter and temporalis anterior muscles of both sides of nine healthy volunteers. An intraoral compressive-force sensor was used to measure maximal voluntary contraction forces in the intercuspal position and to provide a visual feedback on sub-maximal forces to the subject. Three experimental sessions were performed in three days. In each session, three isometric contractions at 80% of the maximal force were sustained by the subjects for 30s. The intra-class correlation coefficient (ICC) of the maximal force measure was 71.9%. ICC of average rectified value and mean power spectral frequency of the EMG signal increased with inter-electrode distance, with values larger than 70% with 30 mm inter-electrode distance. It was concluded that surface EMG variables measured in isometric contractions of the jaw elevator muscles with the proposed force recording system show good reproducibility for clinical applications when a 30 mm inter-electrode distance is considered.

Surface EMG of jaw-elevator muscles and chewing pattern in complete denture wearers.

The aim of this study was to investigate the adaptation process of masticatory patterns to a new complete denture in edentulous subjects. For this purpose, muscle activity and kinematic parameters of the chewing pattern were simultaneously assessed in seven patients with complete maxillary and mandibular denture. The patients were analysed (i) with the old denture, (ii) with the new denture at the delivery, (iii) after 1 month and (iv) after 3 months from the delivery of the new denture. Surface electromyographic (EMG) signals were recorded from the masseter and temporalis anterior muscles of both sides and jaw movements were tracked measuring the motion of a tiny magnet attached at the lower inter-incisor point. The subjects were asked to chew a bolus on the right and left side. At the delivery of the new denture, peak EMG amplitude of the masseter of the side of the bolus was lower than with the old denture and the masseters of the two sides showed the same intensity of EMG activity, contrary to the case with the old denture. EMG amplitude and asymmetry of the two masseter activities returned as with the old denture in 3 months. The EMG activity in the temporalis anterior was larger with the old denture than in the other conditions. The chewing cycle width and lateral excursion decreased at the delivery of the new denture and recovered after 3 months.

Electromyographic evaluation of neuromuscular co-ordination during chewing in a subject with organic occlusion.

The neuromuscular co-ordination of the anterior masseter and temporal muscles during chewing has been studied. The subject analysed was 24 years old female with organic occlusion, molar and canine class 1, with 2 mm overbite and overjet, frontal disocclusion and canine protection, with no cranio-mandibular disorders. Masticatory cycles and electromyographic activity were recorded with a K6 I kinesiograph (Myotronics Inc., Seattle, WA, USA). The chewing cycles were recorded with a soft bolus and a hard bolus, on 3 consecutive days. Electromyographic analysis during masticatory cycles showed that electromyographic activity was higher in the masseter muscle homolateral to the chewing side than in the contralateral muscle, whereas the anterior temporal muscles achieved similar voltages. When chewing the hard bolus, versus the soft bolus, activity in the contralateral masseter muscle increased to a greater extent than in the homolateral masseter muscle. The results were analogous at all 3 recordings. When chewing, the subject showed good muscle co-ordination, which was constant over the 3 recordings made on 3 consecutive days. Increased activity of the contralateral masseter muscle when chewing the hard versus the soft bolus indicates the stomatognathic system's capability to adapt to load and its neuromuscular equilibrium.

Non-invasive assessment of motor unit anatomy in jaw-elevator muscles.

The estimation of fibre length in jaw-elevator muscles is important for modelling studies and clinical applications. The objective of this study was to identify, from multi-channel surface EMG recordings, the main innervation zone(s) of the superficial masseter and anterior temporalis muscles, and to estimate the fibre length of these muscles. Surface EMG signals were collected from 13 subjects with a 16-electrode linear array. The innervation zones of the masseter and anterior temporalis were identified and their variability intra- and inter-subject outlined. More than one main innervation zone location was identified in the masseter of all subjects and in the temporalis anterior of 12 subjects. Average estimated fibre lengths, for the right (left) side, were (mean+/-SD) 27.3+/-2.4 mm (27.0+/-1.7 mm) and 25.9+/-2.3 mm (26.6+/-1.6 mm), for the superficial masseter and temporalis anterior muscle, respectively. The range of innervation zone locations was up to approximately 50% of the fibre length, both within and between subjects. Fibre length estimates well matched with published data on cadavers. It was concluded that multi-channel surface EMG provides important and reliable information on the anatomy of single motor units in jaw-elevator muscles.

Surface EMG of jaw elevator muscles: effect of electrode location and inter-electrode distance.

This study addresses methodological issues on surface electromyographic (EMG) signal recording from jaw elevator muscles. The aims were (i) to investigate the sensitivity to electrode displacements of amplitude and spectral surface EMG variables, (ii) to analyse if this sensitivity is affected by the inter-electrode distance of the bipolar recording, and (iii) to investigate the effect of inter-electrode distance on the estimated amplitude and spectral EMG variables. The superficial masseter and anterior temporalis muscles of 13 subjects were investigated by means of a linear electrode array. The percentage difference in EMG variable estimates from signals detected at different locations over the muscle was larger than 100% of the estimated value. Increasing the inter-electrode distance resulted in a significant reduction of the estimation variability because of electrode displacement. A criterion for electrode placement selection is suggested, with which the sensitivity of EMG variables to small electrode displacements was of the order of 2% for spectral and 6% for amplitude variables. Finally, spectral and, in particular, amplitude EMG variables were very sensitive to inter-electrode distance, which thus should be fixed when subjects or muscles are compared in the same or different experimental conditions.

Comparison of polypropylene and polyethylene terephthalate (Dacron) meshes for abdominal wall hernia repair: a chemical and morphological study.

For the first time, by scanning electron microscopy (SEM), polypropylene (PP) excised meshes (ethylene oxide sterilized) for abdominal wall hernia repair have been shown to be greatly damaged physically, independently of the implantation time, while the polyethylene terephthalate (PET), or Dacron, ones (gamma radiation sterilized), did not undergo alterations due to the sterilization process and were not damaged, even after long implantation periods. Fourier-Transform Infrared Spectroscopy (FTIR) study of PP and PET excised meshes, as well as of their extracts with cyclohexane, has shown the presence of species, such as squalene, palmitic and stearic acid, in some cases, cholesterol, transferred from the surrounding tissues to the polymer during the implantation period. In the case of PP meshes, these small organic molecules would reduce physical and mechanical properties of the material. A hypothesis is presented to account for the better behavior (not in the clinical sense) of PET meshes.

Primary and secondary closure of the surgical wound after removal of impacted mandibular third molars: a comparative study.

Primary and secondary closure techniques after removal of impacted third molars were compared in terms of post-operative pain and swelling. Two hundred patients with impacted third molars were randomly divided into two groups of 100. Panoramic radiographs were taken to assess degree of eruption and angulation of third molars. Teeth were extracted, and in Group 1 the socket was closed by hermetically suturing the flap. In Group 2 a 5-6 mm wedge of mucosa adjacent to the second molar was removed to obtain secondary healing. Swelling and pain were evaluated for 7 days after surgery with the VAS scale. The statistical analysis (*analysis of variance for repeated measures, P < 0.05) showed that pain was greater in Group 1, although it decreased over time similarly in the two groups (P = 0.081, F(6,198) = 3.073*). Swelling was significantly worse in Group 1 (P < 0.001, F(6,198) = 44.30*). In Group 1, dehiscence of the mucosa was present in 33% of patients at day 7, and 2% showed signs of re-infection with suppurative alveolitis at 30 days. Pain and swelling were less severe with secondary healing than with primary healing.

Effects of a functional appliance on masticatory muscles of young adults suffering from muscle-related temporomandibular disorders.

The aim of this study was to investigate the effects of an original orthodontic functional appliance [function generating bite for deep bite correction (FGB-D)] on masticatory muscle activity in subjects suffering from muscle-related temporomandibular disorders (TMD). Electromyographic (EMG) analysis was performed on 33 young adults (nine men, 24 women) to evaluate the contractile symmetry of the right and left masseter and anterior temporalis muscles. The subjects were divided into three groups: a muscle-related TMD group requiring orthodontic treatment for deep bite correction (three men, eight women) and treated with FGB-D; a muscle-related TMD group not requiring orthodontic treatment (three men, eight women) and treated with a Michigan occlusal splint; and a TMD-free group (three men, eight women) as a control group. Records were made by surface EMG of maximum voluntary teeth clenching, with and without the functional appliance or occlusal splint in place, before and after 12 months of therapy. A torque index was derived from the surface EMG recordings to estimate lateral displacement of the mandible. The results show that the FGB-D corrects the torque index and thus the lateral displacement of the mandible.

Effects of different jaw relations on postural stability in human subjects.

Authors investigated the effects of different jaws relations on body posture in a sample of 95 subjects. All subjects underwent a posturometric and stabilometric analysis using a computerized footboard. Tests were performed in three mandibular positions: centric occlusion, rest position and myocentric position, respectively determined by teeth engagement, joints position, and muscles contraction. All subjects showed variations of body posture in the different mandibular positions. Statistical analysis (analysis of variance for repeated measures) confirmed that postural variations in different jaws relations were significant: in particular, the SKN multiple comparison test showed that myocentric position improved postural balance on frontal plane with respect to the other jaw positions considered.

The induction of MMP-9 release from granulocytes by vitamin E in UHMWPE.

Ultra-high molecular weight polyethylene (UHMWPE) is a biopolymer widely used in orthopaedic implants and its oxidation is considered as major responsible for inflammation and the prosthesis failure. We have studied the effect on the activation of resting human granulocytes of the addition of Vitamin E (Vit.E, alpha-tocopherol), a natural biological antioxidant and antiinflammatory agent, to UHMWPE. We have measured changes in granulocytes morphology and respiratory burst by flow cytometry using Dihydrorhodamine 123 and matrix metalloproteinase 9 (MMP-9, gelatinase B) release and activity in the growth medium using substrate zymography following contact (60 min at 37 degrees C) with cell grade polystyrene (PS), normal UHMWPE (PE) and Vit.E added UHMWPE (PE Vit.E). FTIR analyses showed that the surfaces of PE and PE-Vit.E were not significantly different. PS, PE and PE Vit.E did not alter granulocytes morphology and respiratory burst as showed by the mean fluorescence emitted (PS=12.0+/-0.1, PE=13.0+/-0.4, PE Vit.E=14.5+/-0.1). PE Vit.E was able to increase MMP-9 release compared to PS and normal PE (215+/-16% of the control, p<0.001). The PE Vit.E-induced MMP-9 release was abolished by okadaic acid (0.5 nM), suggesting a direct role of Vit.E in the phenomenon.

Ultra-high molecular weight polyethylene oxidation reduces metalloproteinase 2 secretion in human osteoblast-like cells in vitro: a mechanism of modulation of extracellular matrix.

Ultra-high molecular weight polyethylene (UHMWPE) sterilization with gamma rays induced high oxidation levels both on the surface and in the bulk that alter its structure and mechanical properties. The oxidation process of gamma-radiated UHMWPE induces a reduction of molecular weight and, consequently, a less abrasive resistance that has been related, among others, to the failure of UHMWPE in vivo. To explain the role of cells in such events, human osteoblast-like cells were seeded onto UHMWPE and oxidized UHMWPE discs. Cellular viability and morphology were evaluated along with matrix metalloproteinases (MMPs) production and activity. Oxidized UHMWPE did not induce any significant cytotoxic effects as observed by lactate dehydrogenase activity compared to the nonoxidized form; no changes in the cell morphology after 4 and 8 days proliferation were observed. In growth medium metalloproteinase 2 (gelatinase-A, MMP-2) was produced and released by osteoblast-like cells. We observed that cells grown onto oxidized UHMWPE discs decreased the release and activity of MMP-2 after 4 and 8 days culture compared to cells grown on control and non-oxidized UHMWPE discs; metalloproteinase 9 (gelatinase-B, MMP-9) release was not significantly influenced. The absence of cytotoxic and morphological effects in the presence of a down-regulation of MMP-2 release and activity suggest that oxidized polyethylene surfaces may modulate matrix remodeling and, consequently, bone formation.