Comparison of strength and failure mode of seven implant systems: an in vitro test.

STATEMENT OF PROBLEM: The mechanical weakness of implant systems is a problem in the clinical situation. PURPOSE: This study experimentally evaluated how joint design influences the strength and failure mode of dental implant systems. MATERIAL AND METHODS: The strength tests focused on the systems as a whole. Seven implant systems with different joint design were evaluated. RESULTS: Strength and failure mode varied significantly between the implant systems and deep joints, in contrast to shallow joints, favored resistance to bending moments. The mean failure force ranged between 138 to 693 Newtons for the various implant systems. CONCLUSION: In construction of implant systems, joint depth should be taken into consideration.

Comparison between guided and freehand preparation.

Parallel pin retention is traditional in restorative dentistry and provides a conservative approach to removal of tooth structure, including restorations with supragingival margins. However, the question of whether pinhole preparations should be performed freehand or by using a paralleling instrument has been frequently discussed. This investigation compared the precision of freehand and instrument-guided preparations under simulated clinical conditions. The results indicated that guided preparations provided better paralleling precision than freehand preparations, but did not exclude success by the freehand approach. Both instrument preparation and freehand preparation were influenced by the dentist's dexterity and technical capability.

Clinical precision of paralleling instruments.

A method for measuring pin- or pinhole-deviation was developed and tested on nine consecutive bridge cases with parallel pin retention, in order to assess paralleling precision of guiding instruments used clinically. Measurements were performed on rubber base impressions including pinholes, involving the use of a stereomicroscope. The maximum pinhole error angle for the bridges investigated ranged between 5.9 and 16.1 degrees. The corresponding interval for abutments was 1.3-10.8 degrees. The experimental observations concerning the rather large values for the greatest pinhole deviation of fitting bridges could be satisfactorily explained theoretically. The results indicated that the magnitude of the largest permissible angular deviation for a restoration with parallel pin retention was significantly larger than that previously presented in the literature. Compared with results from a previously performed model study carried out during ideal preparatory conditions, the current investigation suggested that the paralleling precision of guiding instruments used clinically in fact decreases considerably, remaining, however, at an acceptable level.

Machinability of some dentin simulating materials.

Machinability in low speed drilling was investigated for pure aluminium, Frasaco teeth, ivory, plexiglass and human dentin. The investigation was performed in order to find a suitable test material for drilling experiments using paralleling instruments. A material simulating human dentin in terms of cuttability at low drilling speeds was sought. Tests were performed using a specially designed apparatus. Holes to a depth of 2 mm were drilled with a twist drill using a constant feeding force. The time required was registered. The machinability of the materials tested was determined by direct comparison of the drilling times. As regards cuttability, first aluminium and then ivory were found to resemble human dentin most closely. By comparing drilling time variances the homogeneity of the materials tested was estimated. Aluminium, Frasaco teeth and plexiglass demonstrated better homogeneity than ivory and human dentin.

Guiding sleeve tilt angle in paralleling instruments.

To investigate the guiding sleeve tilt angle in paralleling instruments, the diameter of guiding sleeves and drill shanks were measured for a number of instrument makes and Spirko type drill makes. Significant diameter differences between makes were found for drills as well as for sleeves. The guiding sleeve tilt angle changed considerably depending on drill--instrument combination. Selection of an unsuitably dimensioned drill could increase the sleeve tilt angle by about four times. A dimensional standardization of guiding shanks and sleeves is thus recommended.

A comparison between some paralleling instruments.

Five paralleling instruments, Paramax I, Paramax II, Pontostructor type A,P.P.-instrument and PR-ec-in-dent, were investigated concerning paralleling precision. A number of rows of holes were drilled in aluminium plates fixed to the instruments. The holes were sectioned longitudinally and their inclinations measured in a profile projector. The variance and range of these inclinations were calculated for each make with small values indicating good guiding characteristics. F-tests showed that Paramax I and II had better guiding characteristics than the other makes and that Pontostructor had better guiding precision than Prec-in-dent. All the instruments tested had maximum range values for angular deviation far below that regarded as tolerable. Not only the paralleling characteristics of such instruments should be assessed but also other requirements previously presented in the literature.

Roentgenological detection of casting defects in cobalt-chromium alloy frameworks.

A method for non-destructive X-ray investigation of casting defects in cobalt-chromium prosthetic frameworks has been developed and tested. The attenuation properties of a cobalt-chromium alloy were studied. A dental X-ray machine with a tube voltage working at 70 kVp and a focus-film distance of 45-50 cm on dental ultraspeed film made it possible to detect defects representing 10% or less of the thickness of cobalt-chromium details with dimensions between 0.6 and 3.0 mm. The frequency and site of internal defects in sixty-six frameworks were investigated. Of the sixty-six frameworks only two were without roentgenologically visible defects. In the other sixty-four frameworks 294 pores or cracks were recorded. The defects were mostly situated in the saddles. Porosities occurred in 53% of the saddlebar regions. The results confirmed the need for a non-destructive routine test of prosthetic frameworks.