Katja Narva, DDS, from the Institute of Dentistry at the University of Turku studied in her doctoral thesis the reasons for denture break-ups, and the influence of different fibre reinforcements in reinforcing denture acrylics.
The total number of dentures included in the study was 51. Part of them were so-called ‘problem dentures’; i.e., they had fractured several times before. In the clinical part of the study, the dentures were reinforced with Stick fibres with the longest follow-up time being over four years. Within the four years, two maxillary complete dentures had broken and four dentures had a hairline fracture. Although the hairline fractures did not hinder the use of the dentures, they were considered as broken in the study. Of the 51 fibre reinforced dentures, 88% were completely undamaged while only approximately 30% of those dentures repaired with acrylic resin or reinforced with conventional metal wire remained undamaged.
"As we started the clinical part of the study, a fibre reinforcement was a whole new concept, and the current Stick product had not yet been launched. The technicians carrying out the repairs were not familiar with fibres, and in fact – according to our present knowledge – some things were done incorrectly. In some dentures the fibres had either been placed too far from the edge of the denture or had been inadequately wetted. Regardless of these mistakes, the fibre-reinforced dentures did extremely well in the study," says Narva. "When after four years the fibre-reinforced dentures were compared with corresponding dentures reinforced with metal wire or not reinforced at all, the durability of the fibre-reinforced dentures was found to be much better," she says.
Clinical and mechanical studies of the fibres
The clinical study showed that glass fibre reinforcement works well if it is appropriately wetted and placed in the right position. "In repairing the denture, the fibre has to be placed perpendicularly against the possible fracture line, i.e. in maxillary dentures close to the tensile stress area caused by the antagonist. Also, the fibres must be wetted very carefully by manipulating them by hand during the wetting phase in order for the acrylic resin mixture to spread evenly between the fibres and for the fibre reinforcement to ‘open’ properly inside the acrylic resin. Naturally, the fibres must be wetted with the acrylic resin mixture, not with pure monomer," Narva emphasises.
"Unidirectional Stick fibre reinforcement reinforces the denture most effectively, because all the reinforcing fibres come to use. However, if you are not sure about the direction the potential fracture runs, it is recommendable to use few layers of the fibre-reinforced net and place them in different directions, so the fibres reinforce the denture from several directions," says Narva.
The study included also four sets of mechanical tests, the first being impact strength test. The denture must be able to withstand impacts, such as those caused when it is accidentally dropped on the floor. The study showed that the glass fibre reinforcement reinforced the impact strength of acrylic denture base polymer10 times.
"Next, we decided to test whether fibres would be a suitable material for making clasps," says Narva. The glass fibre clasps were tested with a fatigue stress testing device while the clasps were immersed in water. The fibre clasps did not break in the tests, even after 150,000 loading cycles. "We noticed, however, that fibres are too flexible a material for clasps. In order to be able to keep the denture in place, the clasp has to be rigid enough not to give way. Fibre is a durable material, but it is not the best possible material for clasps due to its flexibility," comments Narva on the results.
Stick fibre reinforcement by far the best material
In the three-point bending strength and fatigue stress tests, identical test specimens were used, but they were tested with different testing methods. The test specimens were reinforced either with Ribbond polyethylene fibre, FibreKor glass fibre, Stick and everStick glass fibre reinforcement. The control group consisted of acrylic resin test specimens that had not been reinforced at all. In the bending test a testing device measures the point where the test specimen breaks.
"everStick was the strongest in the bending strength test, but in this test all three different glass fibre products yielded very similar test results. The fatigue strength test describes best the clinical situation while denture fractures are nearly always caused by material fatiguing. In this test, by contrast, the test specimens yielded very different results. This test produced one of the main discoveries of the study when we noticed that Stick was the most durable material when placed on both the tensile and the compression side. The possibility of the Stick fibre to spread inside the acrylic denture base resin reinforced the test specimens from a significantly larger area than other fibre reinforcements and thereby overlooks even slightly misplacement of the fibre reinforcement. Stick fibre reinforcement works extremely well, and even a partial reinforcement with it will be sufficient if the fibre is adequately wetted and placed in the correct position," says Narva. everStick reinforced the acrylic denture base resin just as well, but only if they were placed on the pulling tension side – i.e., the side where bending occurs. If they were placed on the compression side no equalling advantage arise. FibreKor and Ribbond functioned markedly inferior, and according to Katja Narva’s study results, they can be ruled out.
"Based on this study as a whole, I would recommend using Stick fibres in reinforcing acrylic resin dentures," says Narva.
Katja Narva, DDS, will defend her doctoral thesis on the reinforcement of dentures at the University of Turku on 26 November 2004.
Correct technique is very important in placement of fibres, and the success of the operation depends on the skills of the technician. The fibres are often placed in an incorrect position, they have been wetted inadequately, there are too few of them, etc., and, as a consequence, the denture is not reinforced in the best possible way. Adequately wetted and positioned Stick fibres are superior materials for reinforcing dentures.