James A.A. Hood

Impact-fracture energy of human premolar teeth

The fracture strength of contralateral pairs of premolar teeth was determined by an in vitro method wherein impact loads of equal energy were applied to the medial aspects of the buccal cusps. Absorbed energy values quantified the fracture resistance of unprepared teeth and teeth restored with MOD amalgams, MOD amalgam overlays, MOD gold overlays, and full gold crowns. It was established that unprepared lower premolar teeth had a greater resistance to fracture than upper premolar teeth, and lower first premolar teeth were more resistant to fracture than the other premolar teeth; the fracture resistance of premolar teeth restored with MOD amalgams was significantly reduced compared with unprepared control teeth; MOD amalgam overlays in premolar teeth produced fracture resistances not significantly different from those for the unprepared controls; and restoration of premolar teeth with MOD gold overlays and full gold crowns enhanced fracture resistance to a level much greater than that of the unprepared control teeth.

Stress relaxation properties of human dentin

OBJECTIVES This study compared the stress relaxation properties of moist and air-dried human dentin to determine if air drying modifies this mechanical property. METHODS Cylindrical dentin specimens, prepared from human canine and maxillary incisor teeth, were subjected to a controlled compressive stress applied at a constant strain rate to a predetermined maximum value, and the stress relaxation response recorded. Groups of moist and air-dried specimens were tested at both low and high stress. After a period of 1 wk, the incisor specimens were retested. RESULTS The dentin exhibited a linear stress relaxation with the logarithm of time. Air-dried dentin consistently demonstrated a decreased stress relaxation. When subjected to a second high load, the air-dried dentin displayed significantly slower stress relaxation. Air-dried incisor dentin was found to be significantly stiffer than air-dried canine dentin. SIGNIFICANCE These data suggest that the stress relaxation properties of human dentin are modified by air drying and by previous exposure to a constant compressive strain.

Thermal debonding of ceramic brackets: An in vitro study

Thermal debonding has been developed to overcome the problems of enamel damage and high forces when debonding ceramic orthodontic brackets. However, the temperature changes with thermal debonding have the potential to damage the tooth tissues. The principal aims of this study are, first, to investigate the effects of resin type, resin thickness, and debonding force on the temperature changes in human premolars during thermal debonding of ceramic brackets and, second, to record the sites of bond failure and damage to the tooth surface. Ceramic brackets were attached to each specimen by using one of four types of bonding resin in a controlled thick or thin resin layer. The ceramic debonding unit (Dentaurum, Pforzheim, Germany) was used to thermally debond the brackets with either a 40 or 80 Nmm torsional force. Higher temperature changes at the pulpal wall (> 10 degrees C in some 40 Nmm torsional force specimens) always occurred with Concise (3M Dental Products, St. Paul, Minn.) and Transbond (Unitek/3M Dental Products, Monrovia, Calif.) resins, and lower temperature changes (< 5 degrees C) with Quasar (Rocky Mountain Orthodontics, Denver, Colo.) and Ortho. B.S. (Dentaurum, Pforzheim, Germany) resins. In general, resin thickness was not significantly associated with buccal surface or pulpal wall temperature changes. However, temperature changes at the pulpal wall were significantly associated with the temperature changes at the buccal surface (r = 0.76), with the temperature of the thermal debonder blade for thin resin layer specimens (r = 0.50), and the time required to debond the bracket for both thick (r = 0.74) and thin (r = 0.63) resin layer specimens. In most specimens, the site of bond failure occurred at the bracket-resin interface. There was no evidence of enamel damage after bracket removal.

Stress and deflection of three different pontic designs

Summary Two-dimensional photoelasticity was used to analyze stresses in models of three-unit fixed partial dentures. Three designs of pontics were tested: a conventional pontic, a sanitary pontic, and a modified sanitary pontic. The maximum shear stresses for the soldered joints between the pontic and the abutment preparations under a 60 pound load in decreasing order were: the conventional pontic (2,400 p.s.i. mesially and 1,920 p.s.i. distally), the sanitary pontic (1,200 p.s.i. mesially and 960 p.s.i. distally), and the modified sanitary pontic (720 p.s.i. mesially and 720 p.s.i. distally). The modified sanitary pontic reduced the stresses at the solder joints and the maximum deflection at the center as compared to the sanitary pontic. Furthermore, the amount of gold alloy required by the modified sanitary pontic is significantly less than that required by the conventional pontic.

Thermocycling, silane priming, and resin/porcelain interfaces—an electrical leakage study

Abstract High 24-hour tensile and shear bond strengths have been reported for silane-primed resin/porcelain interfaces, but bonds often failed after prolonged water storage. This study was designed to determine the influences of both a silane primer and thermocycling on interfacial leakage between a light-cured BisGMA resin and dental porcelain. Porcelain discs measuring 10 mm in diameter and 1.5 mm in thickness were fired. Axial cylindrical cavities were prepared through the discs with a diamond bur. Four groups of eight discs each were restored in the following modes: groups 1 and 2 with light-cured bonding agent and light-cured microfilled resin, and groups 3 and 4 with a silane primer followed by the same bonding agent and resin. Specimens were water-stored for 24 hr, after which those in groups 2 and 4 were thermocycled 200 times between 4°C and 60°C. Specimens were then sealed to steel anodes and placed in 1% KCl, maintained at 21°C, around a stainless steel cathode. Leakage currents were recorded daily for 30 days and after six months, by application of a 10 V potential difference between the cathode and each anode. Positive and negative controls gave massive and nil readings, respectively. The mean leakages were consistently greatest in group 2 (being significantly greater than for the other groups). Results suggest that silane priming minimizes leakage at the resin/porcelain interface after thermal cycling.

THE EFFECT OF ENVIRONMENTAL PRESSURE CHANGES DURING DIVING ON THE RETENTIVE STRENGTH OF DIFFERENT LUTING AGENTS FOR FULL CAST CROWNS

STATEMENT OF PROBLEM The effect of pressure cycling on the bond strength of cement luting agents is largely unknown. PURPOSE This study investigated the effect of pressure cycling on the retention of full cast crowns to extracted teeth. MATERIAL AND METHODS Sixty extracted single-rooted premolar teeth had full cast crowns cemented, 20 with a zinc phosphate cement, 20 with a glass ionomer, and 20 with a resin cement. After 7 days of storage, each of the teeth in the experimental groups was pressure cycled 15 times from 0 to 3 atmospheres (304 KPa), after which the force required to dislodge the crowns was tested in an Instron testing machine. RESULTS A significant difference was found (Students t test; p > 0.01) between the force required to remove the crowns in the zinc phosphate control (142.10 +/- 36.42 N) and experimental (15.93 +/- 11.13 N) groups and the glass ionomer cemented control (186.33 +/- 24.33 N) and experimental (91.50 +/- 33.07 N) groups; no difference was found between the resin cemented control (291.15 +/- 78.48 N) and experimental (281.32 +/- 85.43 N) groups. CONCLUSION This study showed that the retention of full cast crowns to extracted teeth is reduced after pressure cycling if the crowns are cemented with either zinc phosphate cement or glass ionomer cement. Dentists should consider using a resin cement when cementing crowns and fixed partial dentures for patients, such as divers, who are likely to be exposed to pressure cycling.