The effect of recementation on crown retention.

PURPOSE: The object of this study was to evaluate retention of complete cast crowns cemented with different luting cements and to determine whether recemented castings are as retentive as when initially placed. MATERIALS AND METHODS: Thirty standardized Rexillium III complete crowns were made on extracted human molar teeth prepared with a modified milling machine using conventional laboratory techniques. The crowns were randomly assigned into three groups according to the luting cement used: zinc phosphate (Fleck's, Mizzy), adhesive resin (Panavia-EX, J. Morita), and glassionomer (Ketac-Cem, Aplicap, ESPE-Premier Sales). Retention was evaluated by measuring the tensile force required to dislodge the crowns from tooth preparations with an Instron testing machine after thermocycling (1,500 cycles between 5 degrees C and 55 degrees C with 1-minute dwell times). Subsequently, the tooth preparations were scraped clean and polished with flour of pumice (prophylaxis paste), and the fitting surfaces of the artificial crowns were ultrasonically cleaned and air abraded using 50-micron aluminum oxide powder prior to recementation. Results were subjected to two-way analysis of variance and Tukey's Studentized Range test. RESULTS: The analysis showed the highest mean retention for cast crowns cemented with Panavia-EX cement (314 N), but the difference was not statistically significantly different from crowns cemented with glass-ionomer cement (307 N). However, crowns cemented with zinc phosphate cement (233 N) had significantly lower values (P < 0.05). Restorations recemented with zinc phosphate had significantly lower retention (190 N) than their initial cementation (P < 0.05). CONCLUSIONS: Panavia-EX and glass-ionomer cements yielded the highest initial and recementation retentive strength, with values almost 32% higher than those obtained with zinc phosphate cement during the initial cementation and 59% higher than zinc phosphate for recementation. Zinc phosphate cement exhibited significantly lower retention for the initial and second trial of cementation.

Condylar position recorded using leaf gauges and specific closure forces.

Retruded interocclusal records were made for 40 subjects after deprogramming using leaf gauges and controlled incisal forces, which were exerted on a specially constructed occlusal force sensor. These records were used to assess the resulting displacements of the mandibular condyles from their positions in centric occlusion. The leaf gauges were found not to position the condyles inferiorly and posteriorly as has been previously reported.

In vitro study of root canal anatomy of maxillary second premolars.

The external anatomy of 435 maxillary second premolars and the internal anatomy of 300 of these premolars were studied in vitro. For the study of the internal anatomy, the teeth were decalcified and cleared. One canal was found in 67.3% of the second premolars, two canals in 32.4% and three canals in 0.3% of these premolars. A total of 90.3% had one root and 9.7% two roots. A predominance of root curvature toward the distal was found in maxillary second premolars with both one or two roots.

Root form and canal anatomy of maxillary first premolars.

The external and internal anatomy of 240 extracted, maxillary first premolars was studied. The external anatomy was studied by measuring each tooth and by observing the direction of the root curvatures from the facial and proximal aspects. The internal anatomy of the pulp cavity was studied by a method of making the teeth translucent. A total of 55.8% of the teeth had a single root, 41.7% had two roots and 2.5% had three roots. Considering all of the first premolars, 17.1% had one canal, 80.4% had two canals and 2.5% had three canals.

Morphologic study of the maxillary molars. Part II: Internal anatomy.

The internal anatomy of three hundred and seventy (370) decalcified and cleared human maxillary molars was studied. Seventy-five percent of the first molars, 58% of the second molars and 68% of the third molars studied presented three (3) root canals and 25% of the first molars, 42% of the second molars and 32% of the third molars presented four (4) root canals. The authors observed that the incidence of two root canals in the mesiobuccal root was higher in second maxillary molars than in first maxillary molars.

A laboratory assessment of recording reliability and analysis of the K6 Diagnostic System.

This laboratory study investigated the recording reliability and analysis of the K6 Diagnostic System, a computerized instrument that records mandibular movements in three planes. Two special devices were utilized for this purpose, one commercially available and the other designed and constructed by the authors. These devices could produce linear movements and simulate head and mandibular movements, respectively. The largest distortion of the simulated mandibular movements was found in the frontal plane tracings of simulated chewing patterns. For the linear movements in an x-axis, the error of the analyzed K6 system ranged from 9.4% to 30% and in a z-axis from 0% to 15%. A considerable degree of head movement was required to produce a significant error on the K6 measurements.

Suggested modifications for the leaf wafer system.

The Woelfel leaf wafer system provides an easy and accurate way to register centric relation position, which is important in dental practice. This article suggests the following modifications of the system that should enhance the accuracy and convenience of the technique: (1) custom deform wafer before deprogramming or tripodizing the mandible, (2) mark position of leaf gauge at wafer slot, (3) secure leaf gauge to wafer with cyanoacrylate glue, and (4) write pertinent information on paper leaf gauge.

Morphologic study of the maxillary molars. 1. External anatomy.

The authors studied the external anatomy, root curvature and degree of root fusion of 744 maxillary first, second and third molars. The root lengths and root curvatures were quite similar on the first and second molars with their lingual roots curving toward the buccal on 54.6% and 52.3% of them, respectively. Complete separation of all three roots was observed on 86.4% of the first molars, 75.9% of the second molars and on only 18.8% of the third molars.

Denture base acrylic resins: friend or foe?

This article reviews the advantages and disadvantages of denture base acrylic resins. Improvements in areas such as strength and speed of cure are discussed. The authors also emphasize the necessity for investigating the effects of factors other than denture base materials on the incidence of troublesome dentures.

Simple application of anterior jig or leaf gauge in routine clinical practice.

Utilization of either of the two described methods by dentists is recommended as a simple means to routinely record or provide centric relation closures. The leaf gauge (1 to 6 mm thick) can be made from plastic or x-ray film or it can be purchased. The anterior acrylic resin jig can be made in a few minutes directly in the mouth by the dentist or an assistant or by a technician using mounted casts. Both methods avoid adaptive mandibular closure patterns and dentist-guided jaw closure as well as other commonly unrecognized errors in assessing occlusal contacts.

Improving reversible hydrocolloid impressions of subgingival areas.

The causes usually given for not having success with reversible hydrocolloid are that the impression material that goes beyond the subgingival margins often tears on removal and that the syringe material sets too fast. Both of these problems can be overcome and an extremely high rate of success can be obtained by following the technique described.

New device for accurately recording centric relation.

A new system for recording jaw relations has been described. It is easy to learn to use correctly, is pleasant to the patient while assisting the patient's neuromusculature into the desired retruded closure at the specific vertical dimension preselected by the dentist, and its uses minimal amounts of recording media. Duplicate records can be quickly made for verification, various recording media may be used, improved stability during mounting is obtained, and the method is applicable in most dentulous and partially edentulous situations. Two elements are used, a dental arch-shaped, partially performed, thin, waterproof wafer and a disposable paper leaf gauge. The paper leaf gauge is narrower and more solid than the plastic leaf gauge and thus forms a better anterior leg of the tripod with the two condyles on patient-guided terminal hinge closures. The leaf gauge of preselected thickness is inserted into a slot in the wafer that has been deformed exactly like the occlusal plane by a previous centric occlusion closure. The system is quick, economical, and less complex than most methods presently used. Even so, it accurately reproduced six check-bites on one patient according to comparisons on the Veri-check instrument (Denar Corp., Anaheim, Calif.).

Distortion and other errors in oblique cephalometric radiography.

An evaluation of image distortion and serial variation in oblique cephalometric radiography. Reduction of inaccuracies from these sources is best achieved by aligning images of the inferior border of the mandible directly underlying the structures under study.

The cultural convergence of Korean immigrants in Hawaii: an empirical test of a mathematical theory.

PIP: This paper presents a conceptual model developed by human-communication researchers that provides a description of human cultural and cognitive processes. The processes are at once useful, informative, quantitative, and compatible with the emerging thermodynamic models of nonequilibrium processes. It uses empirical results of a survey on Korean immigrants in Hawaii to illustrate a mathematical theory of cultural convergence as described by Woelfel (1980), Woelfel and Fink (1980), and Barnett and Kincaid (1983). The convergence model of communication enables cultural processes to be subsumed within the laws of thermodynamics. Together, these models predict that the members of a social system who share information with one another about a given topic will over time develop more similar conceptions of that topic. The main purpose of this paper is to test the hypothesis which states that if cultural systems behave according to the law of convergence, then, like the particle suspended by an extended spring, the divergent subculture will tend to converge over time toward the position of maximum negentropy, which is the average (equilibrium) position for the society as a whole.^ieng

Processing complete dentures.

The processing of acrylic resin dentures is an exacting art but is relatively simple to carry out to perfection day after day, even when several dentures are processed at the same time. Nine salient factors in the proper processing of acrylic resin dentures were sequentially listed. Unfortunately, a number of these technical procedures are frequently overlooked or shortcuts may be substituted. Herein lie the causes of some hitherto unexplained clinical failures attributed to the acrylic resin denture base material. The acrylic resin denture base materials have been and are currently the best plastic for making dentures. This material has many unique assets and few faults. Its fine properties are often overlooked by the dentist who has several edentulous patients experiencing problems with dentures he made for them. Sore spots, faulty occlusion, looseness, and even breakage are most likely due to inadequate mouth conditioning or poor impressions, incorrect jaw relations, or the dentist's failure to remount the dentures prior to insertion. We have sufficient knowledge and scientific evidence to realize that most, if not all, denture problems are caused by the dentist's technique or by dentist-patient relationship. These problems should not be attributed to the acrylic resin material nor to the hard-working, conscientious dental laboratory technician. I doubt that more than 2 per cent of all complete denture difficulties experienced by frustrated patients would be solved by the introduction and availability of a new perfect denture base material.

Five-year cephalometric study of mandibular ridge resorption with different posterior occlusal forms. Part I. Denture construction and initial comparison.

The average 5 year reduction of the occluding vertical dimension (nasion to menton) for 45 patients was 3.2mm. (2.8 mm. for the anatomic group, 3.2 mm. for the semianatomic group, and 3.6 mm. for the nonanatomic group, each constituting 15 patients). Mandibular physiologic resting face height became foreshortened on the average only one third as much as the morphologic centric occlusion face height. Tabulation of adjustments for sore spots indicated that: the fewest postinsertion appointments were required for the anatomic group of patients; three fourths of 604 denture base alterations were on lower dentures; for upper and lower denture base alterations, one third were made on the borders and two thirds, on the basal seat; one sixth of all alterations were made at routine recall examinations; the fewest upper denture base alterations were for the nonanatomic patients; and the fewest adjustments of the lower denture base on the tissue surface and on the borders were done for the anatomic group of patients. This investigation has been underway for 18 years, and several more re

A longitudinal study of rest position and centric occlusion.

A fifteen-year longitudinal study of rest position and centric occlusion was done on sixteen dentulous subjects with random occlusions. In reviewing the literature the author could find no other investigation with this number of years interim. Records consisted of lateral cephalometric roentgenograms taken when the patient was in rest position and in centric occlusion. Rest position was determined by the patient saying "m" and swallowing, and electromyographically when the anterior belly of the digastric and the temporalis muscles elicited the electrical activity. Both linear and angular measurements were made. The figures from 1958, 1959, and 1960 were averaged and compared with the mean of those taken in 1973. Rest position was found to have a small range of stability in all subjects, while the vertical dimension of centric occlusion increased in ten subjects, decreased in two, and remained the same in four.