How does pain affect jaw muscle activity? The Integrated Pain Adaptation Model.

Pain and limitation of movement are two cardinal symptoms of temporomandibular disorders but it is unclear how one influences the other. The relationship between pain and movement is clinically significant but controversial with two major theories having been proposed: the Vicious Cycle Theory and the Pain Adaptation Model. The Vicious Cycle Theory proposes a vicious cycle between pain and muscle activity. This theory has little scientific basis but underpins many management strategies. The Pain Adaptation Model is more evidence-based and proposes that pain causes changes in muscle activity to limit movement and protect the sensory-motor system from further injury. The Pain Adaptation Model has many positive features but does not appear to explain the relation between pain and muscle activity in all situations. We propose that the relationship is influenced by the functional complexity of the sensory-motor system and the multidimensional nature of pain. This new Integrated Pain Adaptation Model states that pain results in a new recruitment strategy of motor units that is influenced by the multidimensional (i.e., biological and psychosocial) components of the pain experience. This new recruitment strategy aims to minimize pain and maintain homeostasis. This model emphasizes the individual reaction to pain and suggests a tailored approach towards management.

Pharmacokinetics of EMLA cream 5% application to oral mucosa.

Plasma concentrations of lidocaine and prilocaine were measured following the application of a 5% eutectic mixture of local anesthetics (EMLA) topical anesthetic cream to the oral mucosa of twelve subjects. For each subject, a total of 8 g of EMLA was occluded to 18 cm2 of buccal mucosa for 30 min. Analysis was carried out by high-pressure liquid chromatography, and results showed peak concentrations at 40 min for lidocaine and prilocaine. The maximum concentration measured in any subject was 418 ng/ml for lidocaine and 223 ng/ml for prilocaine, well below known toxic levels. No adverse local effects were observed from a 30-min application of EMLA. A follow-up pilot study assessing the clinical efficacy of EMLA for achieving sufficient analgesia for restorative procedures showed that the cream was successful in 75% of subjects tested.

Tissue conditioners protected by a poly(methyl methacrylate) coating.

Interim soft denture liners or conditioners may be valuable therapeutic materials. The life of these liners varies, but it can be extended by the use of a poly(methyl methacrylate) coating material (Monopoly). This study evaluated the ability of Monopoly to prevent water absorption and plasticizer loss from an underlying tissue conditioner (Visco-gel). Disk specimens of Visco-gel coated with Monopoly were immersed in water that was sampled at 24 hours, 1 week, 2 weeks, and 1 month. Uncoated Visco-gel samples acted as controls. Water absorption was determined gravimetrically at each time interval and decanted water was subjected to separation by reversed phase high-performance liquid chromatography for component identification. All samples suffered significant initial weight loss followed by a trend toward weight gain in the uncoated control group, probably because of water absorption. The Monopoly coating appeared to reduce this effect. Plasticizer loss from the tissue conditioner was below quantifiable limits by high-performance liquid chromatography, but methyl methacrylate monomer loss from the Monopoly coating was demonstrated. Some clinical implications are discussed.

Bioavailability of components of resin-based materials which are applied to teeth.

Chemical components of many materials used in dental practice can move into the local biophase, where they can have beneficial or adverse effects. The strongest indirect evidence that components of resin-based materials used in dentistry can move into the biophase are the many reports of allergic dermatitis in dental personnel. Direct measurement of component release has shown that triethylene glycol dimethacrylate (TEGDMA), hydroxyethyl methacrylate (HEMA), and, in the case of some orthodontic cements, bis-glycidyl methacrylate and benzoyl peroxide can move into an aqueous medium from a range of resin-based materials which are applied to teeth as part of oral care. In the case of resin composite restorations, HEMA and TEGDMA are available in microgram quantities via the salivary surface in the minutes and hours after clinical placement and via dentin and pulp in the hours and days after placement. Fortunately, moderate thickness of dentin protects pulp tissue against local toxicity. There are no data which suggest that systemic toxicity is a risk with any of these materials. There are some case reports of allergic responses to the monomers in patients, but the incidence of such responses appears at present to be much lower than that in dental personnel.

Diffusion of monomers from bonding resin-resin composite combinations through dentine in vitro.

OBJECTIVES: Previous work has demonstrated diffusion of the monomer triethylene glycol dimethacrylate (TEGDMA) from resin composite through dentine in vitro. The objective of the present work was to examine monomer diffusion from bonding resins and resin composites used in combination. METHODS: Occlusal cavities were prepared in tooth crowns and restored with bonding resin-resin composite combinations. Aqueous samples from the 'pulpal chamber' of each tooth were removed at timed intervals for analysis by reversed phase-high performance liquid chromatography and mass spectrometry. RESULTS: Bonding resins contributed to monomer diffusion. A TEDGMA-containing bonding resin used in combination with a TEGDMA-containing resin composite hastened and increased TEGDMA diffusion through dentine. A 2-hydroxyethyl methacrylate (HEMA)-containing bonding resin used in combination with a TEGDMA-containing resin composite reduced TEGDMA diffusion only slightly compared with the resin composite alone and added substantial diffusion of HEMA. CONCLUSION: The bonding resins tested contributed to monomer passage to the pulp space and did not prevent movement of monomer from resin composites to the pulp.

Effect of hydrostatic pressure on the diffusion of monomers through dentin in vitro.

In previous work, the diffusion of monomers from composite and bonding resins through dentin was demonstrated in vitro. The monomers triethylene glycol dimethacrylate (TEGDMA) and 2-hydroxyethyl methacrylate (HEMA) were identified in samples from the pulp space. In the current study, we examined the effects of two levels of positive hydrostatic pressure on the passage of resin monomers through dentin in vitro from a composite-resin/bonding-resin combination to test the hypothesis that monomer diffusion is prevented by such pressure. An occlusal cavity prepared in the tooth crown was restored with the resins. Distilled water samples from the pulpal space were removed over time and analyzed for monomer content by high-performance liquid chromatography and mass spectrometry. Positive pulpal pressure reduced but did not prevent pulpward movement of diluent monomers that leach from bonding agents and from resin composites through dentin in vitro. The degree of reduction of diffusion was greater with TEGDMA than with the lower-molecular-weight monomer HEMA.

Effect of dentine on release of TEGDMA from resin composite in vitro.

Triethylene glycol dimethacrylate (TEGDMA) is a component of some resin composites which contributes to their cytotoxicity. The presence of dentine between resin composite and test cells reduces the cytotoxicity in vitro. To determine why dentine has this protective effect, the diffusion of TEGDMA from a composite resin through dentine to the pulp space was compared with release directly into aqueous solution in vitro. Both release rate and total cumulative release of TEGDMA for the two groups, at times up to 100 days, were determined using reversed-phase HPLC. Release rate directly into water was highest in the minutes immediately after immersion and declined thereafter. However, in the tooth model, using an equivalent mass and surface area of composite resin, no TEGDMA was detectable in the pulp space until 43 min after restoration placement. The rate of diffusion through dentine from that time until day 1 was less than 1% of the highest (initial) direct release rate. The rate declined thereafter. It is relevant, however, that by day 3 the total cumulative release of TEGDMA through dentine was 60% of the direct release. Dentine therefore appears to exert its protective effect principally by retarding or 'damping' the initial high release of TEGDMA to a substantial degree.

Movement of resin cement components through acid-treated dentin during crown cementation in vitro.

This study examined the hypothesis that components of crown cements may be forced through acid-treated dentin during cementation. Freshly extracted, human third molar teeth were prepared to accept full crowns. Roots were removed to allow irrigation of the pulp chamber with saline before, during, and after crown placement with resin dentin bond and resin composite cement. Saline samples were collected and analyzed by high performance liquid chromatography to identify and quantify resin components arriving in the pulp space. Two components of the bond-cement system used were identified in the pulp space samples immediately after crown cementation. These were 2-hydroxyethylmethacrylate and 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane. The amounts of these components in the pulp space decreased when the bonding agent was cured prior to crown placement. The results of this study supported the hypothesis that crown cementing components may flow through acid-treated dentin during crown cementation.

Comparison of amalgam behaviour under static and cyclic sub-modulus of rupture loading.

The behaviour of amalgam beam specimens subjected to a sub-modulus of rupture load at 37 degrees C was compared in static and cyclic modes. Testing was performed using identical equipment, with cyclic testing conducted by intermittent load application for both tests. The specimens tested exhibited varying degrees of deformation and times to fracture in both tests. The mode of load application, either cyclic or static, did not appear to have a significant effect on the degree of amalgam deformation or time to fracture. The most important factors relating to deformation are time of total load application, creep of the specific amalgam and temperature of testing.

Dental amalgams subjected to sub-fracture stresses at 37 degrees C.

Amalgam beam specimens were subjected to sub-modulus of rupture loads at 37 degrees C. The specimens tested underwent varying degrees of deformation and ultimate fracture determined by the degree of specimen bend and time to fracture. The property most closely related to the sub-fracture loading behaviour was creep, particularly the 72-h determination of this property. Time and temperature of testing are also significant factors. The present study indicates the importance of the use of 37 degrees C testing in low-load stressing of dental amalgam. The study has confirmed the nature of failure of amalgam in relation to its rheological properties at the temperature at which amalgam functions clinically.