It has been proposed (Cvek 1992) that immature teeth are weakened by filling of the root canals with calcium hydroxide dressing and gutta-percha. The aim of the present study was to test the hypothesis that dentin in contact with calcium hydroxide would show a reduction in fracture strength after a certain period of time. Immature mandibular incisors from sheep were extracted and divided into two experimental groups. Group 1: the pulps were extirpated via the apical foramen. The root canals were then filled with calcium hydroxide (Calasept) and sealed with IRM(R) cement, and the teeth were then stored in saline at room temperature for 0.5, 1, 2, 3, 6, 9, or 12 months. Group 2: the pulps were extirpated and the root canals were filled with saline and sealed with IRM(R) cement. The teeth were then stored in saline for 2 months. Intact teeth served as controls and were tested immediately after extraction. All teeth were tested for fracture strength in an Instron testing machine at the indicated observation periods. The results showed a markedly decrease in fracture strength with increasing storage time for group 1 (calcium hydroxide dressing). The results indicate that the fracture strength of calcium hydroxide-filled immature teeth will be halved in about a year due to the root filling. The finding may explain the frequent reported fractures of immature teeth filled with calcium hydroxide for extended periods.

–  Thirty immature sheep incisor teeth were tested for their fracture resistance after various treatment modalities using calcium hydroxide (CH) or a mineral trioxide aggregate material (MTA®) as a root filling. The incisors, having approximately 80% of their root growth completed, were removed from jaws of slaughtered sheep and divided into four experimental groups. The pulps were extirpated from all the teeth through the open apexes. (a) Saline group: the teeth were preserved in saline for 100 days at 6°C. (b) CH group: the root canals were filled with CH and sealed apically with IRM® and stored as above. (c) MTA® group: the canals were filled with MTA® and stored as above. (d) CH + MTA® group: the canals were filled with CH and sealed with IRM®. After 30 days, the CH was replaced with MTA® and stored as above. At the end of the 100‐day storage period, all teeth were embedded in plaster of Paris and tested for fracture strength at the cervical area in an Instron® testing machine. The results showed a decrease in fracture resistance (a) of the incisors with CH in the root canals after 100 days of storage, compared to (b) teeth stored in intracanal saline and (c) teeth with 30 days of CH and then filled with MTA®, and (d) those filled with MTA in the canals. In conclusion, when CH was kept in the canals of immature sheep teeth for only 30 days followed by root filling with MTA® there was no significant decrease in strength of the root within an observation period of 100 days. This finding may be of importance in the decision of treatment plans for teeth with pulp necrosis and immature root formation.

The effect of fluoride on experimental cariogenicity in man was investigated by using bovine enamel surfaces mounted in oral prosthetic appliances. Samples of sound and presoftened enamel were exposed to simulated cariogenic or noncariogenic conditions for 1 week. The samples were periodically immersed iti vitro in solutions containing 1 ppm fluoride. For the cariogenic condition, the immersion solution was 3 % sucrose, for the noncariogenic condition, water. After the experimental period, the sample enamel was assessed for hardness change and fluoride incorporation. Under the non-cariogenic condition, partial remineralization and high fluoride incorporation were found in the presoftened enamel; both hardness change and fluoride incorporation in sound enamel were minor. The cariogenic condition contributed to higher enamel softening and higher fluoride uptake by both types of samples. The findings suggest a mechanism through which intermittent cariogenesis and remineralization in the presence of fluoride may contribute to increased tooth resistance to caries.

Plasma arc light units for curing resin composites have been introduced with the claim of relatively short curing times. The purpose of the present study was to measure and compare elution of monomers from an experimental BisGMA-TEGDMA resin and a commercial resin composite when cured with a halogen unit and when cured with a plasma arc unit. Specimens of the materials were immersed in methanol, and the amounts of monomers released with time were analyzed by HPLC. By use of Fick's laws of diffusion, the amount of eluted monomers from the specimen at infinity was estimated. The elution from resin specimens and from resin composite specimens cured with the plasma arc light unit was 7 and 4 times higher, respectively, compared to the elution from specimens cured with the halogen unit. It was concluded that the plasma arc light curing unit did not provide optimal cure when used as recommended by the manufacturer.

We investigated the bond strength between restorative resin and dentin pre-treated with mixtures of HEMA and glutaraldehyde. It is suggested that the mixture acts by forming a chemical bond of HEMA molecules to a collagen-glutaraldehyde reaction complex. Subsequently applied resin will then co-polymerize with the collagen-linked methacrylate groups. Statistical analysis of the results from varying mixtures of HEMA and glutaraldehyde revealed that the bond strength was highly dependent on the HEMA concentration, with a maximum at 35%, and nearly independent of the glutaraldehyde concentration when greater than 3%. The highest mean bond strength was about 1.8 kg/mm2, and bond strength of this order of size may be attractive for clinical use.