3516 French dentists, selected at random, were asked to record every tooth they extracted during January, 1984. They were also asked to give the reason for extraction in each case. These were assigned to eight groups: caries, periodontal diseases, eruption problems, prosthetics, trauma, orthodontics, occlusal problems, and other reasons. Data were received from 910 dentists (25.3%) relating to a total of 14,621 extractions. Overall, caries was the most frequent cause for extraction (49%), followed by periodontal diseases (32.4%) and orthodontics (8.4%). In the age group under 50, dental caries was the main reason for extraction. However, in the age group over 50, periodontal diseases became the principal reason for extraction. Orthodontic extractions were most prominent in the 6-12 and 13-20 age groups, with respective frequencies of 72.6% and 24.8% of all extractions. Eruption problems contributed most frequently in the 13-20 and 21-30 age groups, their respective percentages being 8.7% and 12.3%. Extractions resulting from trauma were most frequently noted in the under-6 age group (frequency of 8.8%). As far as the type of tooth was concerned, first and second molars taken together made up 29.6% of extractions. These were followed by pre-molars, anterior teeth, and third molars at levels of 25.8%, 29.9%, and 14.7%, respectively. The teeth most frequently extracted because of caries were the molars (40.9%); because of periodontal disease, the anteriors (49.1%); and, for prosthetic reasons, again the anteriors (57.5%). Extractions from the various regions of France showed important variations. Percentage frequencies for caries and periodontal extractions were inversely linked.

A bovine tooth model system was used to study the effect of experimental salivary pellicle on enamel erosion. Test blocks with varnish-covered control surfaces in each specimen were immersed into an acidic cola beverage (pH 2.6) for 120 min, either with or without the pellicle which was grown for 7 days by using clarified human saliva. After immersion, the pellicle was removed from some specimens with 10% sodium hypochlorite (20 h at 20 °C) and ultrasonic bath (60 s) to study its effect on erosion. All specimens were then studied in the scanning electron microscope. The specimens without pellicle (positive controls) showed gross erosion with prism core dissolution in all study blocks. The specimens with pellicle showed occasionally a filmlike integument covering the apparently intact test surfaces with adjacent enamel surfaces slightly eroded. The erosion, however, was not of the same magnitude as in the positive controls. After removing the pellicle, the enamel surfaces showed pitted appearance of the prism heads; the demineralization seemed to attack more the prism sheath areas. Thus, salivary pellicle was found to protect the underlining enamel from erosion in vitro.

Surface prismatic and aprismatic human enamel and unpolished or diamond-polished specimens of bovine enamel, with test and control surfaces on each studied block, were immersed in phosphoric acid containing cola beverage (pH 2.6) and citric or malic acid containing sports drinks (pH 2.8 and 3.4, respectively) for 15–180 min. The specimens were then prepared and studied in a JEOL 35C scanning electron microscope. In human aprismatic surface enamel an irregular type of dissolution was observed, with severely affected areas adjacent to less affected ones. Prismatic human and bovine specimens showed a characteristic dissolution where initial erosion after 15 min immersion was seen to affect specifically the prism sheath areas. Longer immersion caused dissolution of enamel prism cores followed by interprismatic areas. In bovine enamel malic acid affected least the surface ultrastructure when compared with citric and phosphoric acids after 15–30 min of immersion, but thereafter no difference was observed between the acids in causing erosion. Diamond-polished enamel was found more liable to erosion than non-polished enamel. Thus, the structure of enamel greatly modified the progression of in vitro caused erosion, in particular in human tooth specimens.

This long-term experiment in Maine, U.S.A., was designed to provide information on the best silvicultural practices for managing stands of mixed northern conifers in northeastern U.S.A. We evaluated growth and yield and changes in species composition, quality, and structure during the first 40 years of the experiment. Replicated treatments include the selection system, uniform shelterwood, unregulated harvesting, and diameter-limit cutting. The new cohort established under three-stage shelterwood was subsequently left untreated or precommercially thinned. Between-treatment differences in net volume growth were not significant (α = 0.10), though gross volume growth differed significantly for managed vs. unmanaged, selection vs. shelterwood, and shelterwood vs. diameter-limit treatments. A three-stage shelterwood method with precommercial thinning 10 years following final overstory removal resulted in good control of hardwoods and hemlock and a dramatic increase in spruce and fir. The selection system on a 5-year cutting cycle resulted in increased hemlock, spruce, and fir, with a decrease in hardwood species. If the primary goal were production, even-aged management would most likely be preferred. We recommend a two-stage shelterwood method as applied in this experiment with some modification to improve species composition and stand quality. Stand quality (proportion of stand volume in cull trees) and species composition was influenced by treatment.