Сraig. Dental Materials

Fig. 8-15A selection of flowable composite resins in syringe and compule delivery systems.

MANIPULATION

Flowable composites are usually packaged in syringes or in coinpules (Fig. 8-15). These can be used for direct application to the cavity or the tooth surface; however, it is easy to exert a little pressure on the syringe and express an excessive amount of material. These materials can also be deposited from the syringe to a paper pad surface and then applied to the tooth using a balltipped applicator or a microbrush. In dispensing the material, care must be exercised to avoid introducing air bubbles, which eventually become surface voids. Depth of cure after light activation should be slightly better than that of highly filled composites.

Because the physical properties of flowable composites are lower than the restorative composites, it is generally considered that these materials should be reserved for nonfunctional tooth surface restorations. However, they provide an advantage when used as the sealant portion of a preventive resin restoration. A bonding agent should be applied to the etched enamel and light cured to provide the basic adhesion for retention. The flowable composite can then be used to cover the restored area and the exposed pits and fissures on the occlusal surface of molars and premolars. The high flow characteristics also work well in restoring minimal cavity preparations involving fissures that are cleaned or prepared with air abrasion techniques (Fig. 8-16).

GLASS IONOMERS

The final materials that need to be considered for caries prevention are glass ionomers and hybrid ionomers (resin-modified glass ionomers). Because of their documented slow release of fluoride, glass ionomers are used in cervical and Class 5 restorations in adults where esthetics is not critical. They are specifically recommended for patients with high caries risk (Table 8-2).

COMPOSITION AND REACTION

Glass ionomers are supplied as powders of various shades and a liquid. The powder is an ion-leachable aluminosilicate glass, and the liquid is a water solution of polymers and copolymers of acrylic acid. The material sets as a result of the metallic salt bridges between the Al* and Caw ions leached from the glass and the acid groups on the polymers. The reaction goes to completion slowly, with the formation of a crosslinked gel matrix in the initial set and an aluminum ion exchange strengthening the crosslinking in the final set. A chelation effect takes place with the calcium on the exposed tooth surface, creating an adhesive bond. The surfaces of new restorations should be protected from saliva during the initial set with a heavy varnish or lightcured bonding agent.

PROPERTIES

The handling characteristics and physical properties of glass ionomers can be varied to suit various clinical applications by altering the glass composition and or polyacid formulation. The properties of glass ionomers are compared qual-

Fig. 8-16 Air abrasion application for a carious fissures in a mandibular molar; B, flowable resin composite.

preventive resin restoration. A, Preoperative view of cavity preparation with air abrasion; C, restoration with

214 Chapter 8 PREVENTIVE MATERIALS

itatively with other restorative materials in Table 8-3. Properties especially noteworthy are a modulus that is similar to dentin, a bond strength to dentin of 2 to 3 MPa, an expansion coefficient comparable to tooth structure, low solubility, and fairly high opacity. The flux used in fusion of the glass contains fluoride that is released slowly to provide an anticariogenic effect on adjacent tooth structure.

Although the bond strength of glass ionomers to dentin is lower than that of composites, clinical studies have shown that the retention of glass ionomers in areas of cervical erosion are considerably better than for composites. When the dentin is conditioned (etched) using a dilute solution (15 % to 25%) of polyacrylic acid, the glass ionomer may be applied without a cavity preparation. Four-year clinical data showed a retention rate for glass ionomer cervical restorations of 75%. The surfaces of the restorations

seen in the studies were noticeably rough, and some shade mismatches were present. Pulp reaction to glass ionomers is mild; if the thickness of dentin is less than 1 mm, use a calcium hydroxide liner. Although the surface remains slightly rough, cervical restorations did not contribute to inflammation of gingival tissues. Less Streptococcus mutans exists in plaque adjacent to glass ionomer restorations.

MANIPULATION

Glass ionomers are packaged in bottles and in vacuum capsules for mechanical mixing in an amalgamator. In bulk dispensing, the powder and liquid are dispensed in proper amounts on the paper pad, and half the powder is incorporated to produce a homogeneous milky consistency. The remainder of the powder is added, and a total mixing time of 30 to 40 seconds is used with a typical initial setting time of 4 minutes. After placing the restorative and developing the correct contour, protect the surface from saliva by applying varnish or bonding agent. Trimming and finishing are done, if possible, after 2 4 hours.

The liquid in the unit-dose capsule is forced into the powder by a press and is mixed by a mechanical mixer. The mixture is injected directly into the cavity preparation with a special syringe. Working time is short and critical, so it is

imperative to place the material with a minimum of manipulation. If the gel stage of the reaction is disrupted during the early phase of the reaction, the physical properties will be very low and adhesion can be lost.

Adhere rigidly to clinical techniques for glass ionomers, maintaining isolation, using adequate etching procedures, protecting the restoration from saliva after placement, and delaying final finishing for 1 day or longer if possible.

Hybrid ionomers or resin-modified glass ionomers are used for restorations in low stressbearing areas and are recommended for patients

with high caries risk (see Table 8-3). These restorations are more esthetic than glass ionomers because of their resin content. Examples of cervically eroded teeth and hybrid ionomer restorations are shown in Fig. 8-17.

COMPOSITION AND REACTION

The powder of hybrid ionomers is similar to that of glass ionomers. The liquid contains monomers, polyacids and water. Hybrid ionomers set by a combined acid-base ionomer reaction and light-cured resin polymerization of 2- hydroxyethyl methacrylate. Placing a dentin bonding agent before inserting a hybrid ionomer is contraindicated, because it decreases fluoride release.

PROPERTIES

Hybrid ionomers bond to tooth structure without the use of a dentin bonding agent. Typically, the tooth is conditioned (etched) with polyacrylic acid or a primer before placing the hybrid ionomer. The transverse strength of a hybrid ionomer is almost double that of a standard glass ionomer. Hybrid ionomers release more fluoride than compomers and composites but almost the same as glass ionomers. Fig. 8-18 illustrates the release of fluoride ions from a standard glass ionomer and resin-modified glass ionomer over a 30day period. There is an early period of high release, which tapers after about 10 days to 1 ppm. Glass ionomers and hybrid ionomers recharge when exposed to fluoride treatments or fluoride dentifrices. Fig. 8-19 illustrates this recharge capability with a similar time-dependent release curve. In evaluating the effectiveness of this release, fluoride has been measured in plaque samples immediately adjacent to glass ionomer-based restorations (Fig. 8-20). For

these two materials from the same manufacturer, plaque adjacent to the resin-modified glass ionomer had a significantly higher fluoride content than plaque adjacent to compomer restorations at 2 days and 21 days after insertion of the restorations.

MANIPULATION

An example of a hybrid ionomer packaged in capsules is shown in Fig. 8-21. Hybrid ionomers are also packaged as powder-liquids; their manipulation is like that of standard glass ionomers. Mechanical mixing of the unit-dose capsules provides a uniform mix that has much fewer of the larger air voids that can be introduced during hand spatulation. Optimum powder/liquid ratio is critical to the long-term maintenance of physical properties and the clinical success of restorations. Unlike glass ionomer restorations, hybrid ionomers set immediately when light-cured and can be finished soon after. Glass ionomer-based

ATHLETIC MOUTH PROW

The use of athletic mouth protectors in contact sports has increased rapidly; they are routinely used in football, soccer, ice hockey, basketball, wrestling, field hockey, softball, and other sports. This increased use is justified by studies that showed that 38% of participants in sports sustained orofacial injuries and only 15% of those injured were wearing a mouth protector at the time of injury. A survey found that 62% of injuries occurred in unorganized sports.

Injuries to teeth from trauma caused by athletic activity have involved pulpitis, pulpal necrosis, resorption, replacement resorption, inter-

Fig. 8-20 Total fluoride concentration in plaque (yg fluoride per mg plaque) adjacent to resin-modified glass ionomer and compomer restorations over

21 days; restored test teeth vs. nonrestored control teeth.

Fig. 8-22 Examples of stock and mouth-formed mouth protectors. Note the protector on the top is a mouth and lip protector and those on the left and right center have had the straps cut off to allow testing of the percentage of impact absorbed.

nal hemorrhage, pulp canal obliteration, and inflammatory resorption.

As a result of the possibilities of orofacial injury, high school athletes are required to wear internal mouth protectors, and the National Collegiate Athletic Association has adopted a mouth-protector rule. As a result of these actions, more professional athletes are wearing mouth protectors. Because of the increased use of mouth protectors, it is estimated that 50,000 orofacial injuries are prevented each year.

Stock, mouth-formed (boil-and-bite), and custom mouth protectors are the three types available and all provide some protection to the athlete. A few examples of stock and mouth-formed protectors are shown in Fig. 8-22. Custommade mouth protectors are usually vacuumformed from sheets of flexible, thermoplastic polymers about 14 cm square and 1.6 to 3 mm

Fig. 8-23Thermoplastic sheets for fabricating mouth protectors and fabricated mouth protectors. The one on the right is on a dental stone model; the one on the left was fabricated from a tricolored sheet.

in thickness; they may be clear or colored. Examples of these sheets and fabricated mouth protectors are shown in Fig. 8-23. In most instances the sheets are of a single material, but they may be laminates of two thermoplastic polymers. Laminated mouth protectors are fabricated so that the softer of the two layers contacts the teeth and soft tissue.

Most sheets for custom mouth protectors are vinyl acetate-ethylene copolymers (PVAc-PE). Manufacturers use several different hardnesses of the material, with copolymers containing more ethylene being harder.

The advantages of custom-made mouth protectors are (1) excellent fit, (2) comfort, (3) ease of speaking, and (4) durability; these qualities are poor for stock protectors and poor to good for mouth-formed protectors. In spite of the advantages of custom-made protectors, they are not as common as stock or mouth-formed protectors because of their higher cost.

PROPERTIES

Selected properties of thermoplastic sheets and fabricated custom-made, mouth-formed, and stock protectors are listed in Table 8-4. The Shore "A" hardness of the materials for all types of mouth protectors ranges from 70 to 80, and all have low values of water sorption and solubility.

The tear strength of the laminate is somewhat lower than single material sheets; it is not possible to test the values of the fabricated mouth protectors. With an impact of 113 N/cm, the percentage absorbed by thermoplastic sheets and custom mouth protectors placed on highstrength stone models in the area of the central incisors is essentially the same, with values of 80% to 90%.A wider range of impact absorption, 76%to 93%, is found for mouth-formed and stock protectors. These results confirm clinical studies that demonstrate no difference in the incidence of oral injuries of athletes wearing custommade, mouth-formed, or stock protectors. Thus the main advantages of the custom-made mouth protectors are those previously mentioned-fit, comfort, and ease of speaking-which should increase the probability that the protector will be tried in.

FABRICATION OF CUSTOM-MADE PROTECTORS

The fabrication of a custom-made mouth protector involves the following general steps: (1) making an alginate impression of the maxillary arch;

(2)pouring a dental stone or high-strength stone model into the impression, minus the palate;

(3)vacuum-forming a heated sheet of PVAc-PE over the model; (4) trimming the excess PVAc-PE around the model; and (5) smoothing the edges of the mouth protector.

A commercial dental vacuum machine used

to fabricate mouth protectors is shown in Fig. 8-24, A . The procedure involves clamping the PVAc-PE sheet in the frame and raising the frame to the top position, shown in the sketch in Fig. 8-24, B, and centering the model on the perforated metal platform. Turn the heater on and heat the sheet until it sags about 3 cm at the center, at which point turn the vacuum on and push the frame down to its lower position using the plastic handles. Swing the heater out of the way and turn it off, but leave the vacuum on for 30 seconds. Allow the thermoplastic sheet to cool to room temperature before removing it from the model. A thermoplastic sheet vacuum-formed over a model is shown in Fig. 8-25. Trim the excess of

Fig. 8-25 A mouth protector sheet vacuum-formed over a model is shown on the left, and a trimmed and finished protector is shown on a model at the right.

the thermoplastic sheet 3 mm short of the labial fold using a curved pair of scissors, making sure to provide clearance for the buccal and labial frenum. Smooth the edges with a wheel such as a Moore's Satin Buff Wheel or, if one is not available, with the flame from an alcohol torch, followed by adaptation with wet fingers.

Several considerations should be taken into account related to making impressions and preparing models used to fabricate mouth protectors. Remove removable appliances before making the alginate impression. If the athlete is wearing a fixed orthodontic appliance, block it out on the model with dental stone before fabricating the mouth protector. Also, if permanent teeth are still erupting, block out that space on the model.

In the absence of a vacuum, soften the thermoplastic sheet in hot water and adapt to the model using a wet sponge and wet fingers. Significantly better adaptation results from vacuumforming the protector.

If it is found necessary to equalize the occlusion when the mouth protector is worn, make the following adjustment. Gently heat the contacting surfaces of the mouth protector using an alcohol