(1) To quantify wear of two different denture tooth materials in vivo with two study designs, (2) to relate tooth variables to vertical loss.
Two different denture tooth materials had been used (experimental material = test; DCL = control). In study 1 (split-mouth, 6 test centers) 60 subjects received complete dentures, in study 2 (two-arm, 1 test center) 29 subjects. In study 1 the mandibular dentures were supported by implants in 33% of the subjects, in study 2 only in 3% of the subjects. Impressions of the dentures were taken and poured with improved stone at baseline and after 6, 12, 18 and 24 months. Each operator evaluated the wear subjectively. Wear analysis was carried out with a laser scanning device. Maximal vertical loss of the attrition zones was calculated for each tooth cusp and tooth. A mixed linear model was used to statistically analyse the logarithmically transformed wear data.
Due to drop-outs and unmatchable casts, only 47 subjects of study 1 and 14 of study 2 completed the 2-year recall. Overall, 75% of all teeth present could be analysed. There was no statistically difference in the overall wear between the test and control material for either study 1 or study 2. The relative increase in wear over time was similar in both study designs. However, a strong subject effect and center effect were observed. The fixed factors included in the model (time, tooth, center, etc. ) accounted for 43% of the variability, whereas the random subject effect accounted for another 30% of the variability, leaving about 28% of unexplained variability. More wear was consistently recorded in the maxillary teeth compared to the mandibular teeth and in the first molar teeth compared to the premolar teeth and the second molars. Likewise, the supporting cusps showed more wear than the non-supporting cusps. The amount of wear did not depend on whether or not the lower dentures were supported by implants. The subjective wear was correct in about 67% of the cases if it is postulated that a wear difference of 100 μm should be subjectively detectable.
The clinical wear of denture teeth is highly variable with a strong patient effect. More wear can be expected in maxillary denture teeth compared to mandibular teeth, first molars compared to premolars and supported cusps compared to non-supported cusps.
Laboratory data on the wear of denture tooth materials may not be confirmed in well-structured clinical trials probably due to the large inter-individual variability.
Wear of denture teeth is still a clinical problem because excessive wear can be observed in some, yet not in all patients after one year of service already . The polymer material used in denture teeth is generally less wear resistant than the material used in polymer crowns. Mostly PMMA or variations of it are used for denture teeth because these materials show excellent gloss stability, limited plaque accumulation, reduced chipping and ease of handling and manipulation. However, one laboratory study has demonstrated that these materials are prone to higher wear compared to polymer materials filled with glass fillers . Therefore, some manufacturers have launched denture teeth made of filled polymers (composite resins). A clinical study has recently shown that the clinical wear of denture teeth made of composite was significantly lower compared to the wear of denture teeth made of PMMA .
Biting forces of edentulous patients do not provide an explanation for the increased wear of denture teeth. Although the occlusal forces are mainly regulated by the masticatory muscles in edentulous patients – the receptors of the periodontal ligament controlling the chewing forces are missing – both the maximum biting force and the biting force during mastication is lower in edentulous subjects compared to dentate subjects .
Only a few clinical studies have been performed to investigate the wear of denture teeth. This may be explained by the technical difficulty in accurately measuring three-dimensional changes. For the quantification of material loss in dentistry, volumetric, mechanical and scanning electron-optical procedures have been used to date (reviews by Ref. ). In the 1970s a dial gauge and steel reference balls incorporated in the denture base were used to measure the wear in dentures. In another study calipers were used in conjunction with reference markings . This was followed by stereophotogrammetry and by reflex microscopy in the 1980s and finally by a computerized coordinate measuring device coupled with an electronic ruby tipped probe in the 1990s . In the 1990s a triangulation laser sensor, computer-controlled specimen stage and sophisticated matching software with subpixel interpolation and a scanning speed of 5000–10,000 surface points per second was developed for wear measurements . The accuracy of the laser scanner and matching software to calculate vertical differences between unprepared teeth ( n = 15) and the same teeth after crown preparation was calculated to be 10.9 (±1.9) μm . An in vitro study compared the analysis of vertical and volumetric loss of wear facets by a laser scanner (same as in the present study), by a mechanical sensor (profilometry) and by an optical sensor with each other using monochromatic aberration as measuring principle . In this study a favorable agreement was found between all three quantification methods for both volumetric and vertical loss. However, the mechanical sensor measured consistently higher values compared to the optical sensor for the volumetric loss and consistently lower values for the vertical loss.
Some studies also used laser scanning for wear analysis in denture teeth . The latter study discovered increased wear in partial dentures compared to complete dentures. Another study revealed that wear analysis can be restricted to posterior teeth . Small conventional trays can be used in conjunction with a silicone material to facilitate the removal of the impression from the posterior denture teeth.
A clinical study of three different denture teeth materials over 3 years did not reveal a significant difference in total wear by cuspal anatomy, tooth material, preferred chewing side or gender . However, there was significantly more wear for first molars and premolars compared to second molars and for maxillary teeth compared to mandibular teeth.
In the present study two different denture tooth materials were investigated for wear over 24 months. One material was a double-crossed linked polymer (DCL = control material) which was brought on the market in 1994 under the name Antaris/Postaris and the other an improved version of the first material. The improved version comprised 20% of UDMA/PMMA fillers (experimental material = test material) ( Table 1 ). In this material the polymer and the matrix were homogeneously cross-linked, which was achieved by subjecting both the pre-cross-linked polymer and the matrix to a secondary cross-linking process. In a laboratory study the experimental material exhibited less wear during tooth brushing than the DCL material . Thus, the main aim of the present multi-center study was to quantify the wear over 24 months of the DCL and experimental denture tooth material by using the replica method and a three-dimensional laser equipment.
|Enamel and dentin||Cervical|
|Experimental material test (weight%)||DCL control (weight%)||Both materials (weight%)|
|Cross-linked polymethyl methacrylate||0||59||0|
|Initiators and stabilizers||<0.5||<0.5||<0.5|