Introduction - Materials and methods - Results - Discussion - References.
S. Zinelis & J. MargelosBiomaterials Laboratory, School of Dentistry, University of Athens, Athens, Greece.Introduction.
Rubber dam is routinely used in endodontics and in operative dentistry for tooth isolation. Although rubber-dam clamps have been used for many years, there is still no internationally accepted standard for the production and quality evaluation of these devices (Svec et al. 1997). Currently, commercially available clamps may be divided into two categories according to the composition of the alloys used for their production. The first group is made from plain carbon martensitic steel with approximately 0.8 wt.% carbon and 600ā650 Vickers hardness, electroplated with a nickel coating. The thickness of the coating is approximately 6.5 mm and is applied in order to enhance corrosion resistance (Jedynakiewicz et al. 1985). The second group is made of martensitic stainless steel 7C27Mo (13.5 Cr, 1.0 Mo, 0.38 C, 0.55 Mn, and 0.40 Si all in wt.%) of 525 Vickers hardness (Sutton & Saunders 1996). Alternatively, the type of 420 martensitic stainless steel with minor differences in composition (12ā14 Cr, 0.15 C min., 1.0 Mn, 1.0 Si, 0.04 P and 0.03 S all in wt.%) (Davis 1993) is specified by a US government document (Medical Procurement Item Description 1983).
Although the technique of applying these devices in the mouth is straightforward, the infrequent intraoral fracture of rubber-dam clamps (Jedynakiewicz et al. 1985) is inconvenient to the dentist and patient and represents a potential hazard. This study was conducted to investigate the reasons for failure of a new rubber-dam clamp which fractured intraorally during its first use in vivo. Materials and methods.
The fractured surfaces of a new rubber-dam clamp (Hu-Friedy Co., Chicago, IL, USA) which failed during first placement into the mouth were investigated under a stereomicroscope (Elvar, Leitz, Wetzlar, Germany). Results.
The fracture occurred on the clamp bow (Fig. 1). Figure 2 demonstrates a stereomicroscopic image of the fractured surfaces with the crack originating from the groove of letter āDā engraved on the inner bow surface. Figure 3 shows the radial zone on the mating surfaces of fractured clamp whilst Figure 4 illustrates the shear lip where the fracture terminated.
Figure 1. Macroscopic view of the fractured clamp. Fracture is located on the clamp bow.Discussion.
Figure 2. Stereomicroscopic images of the fractured surfaces. Images from the inner part of the bow. Cracks originated from the bottom of an engraved groove (clearly shown on left part) and for this reason a portion of letter 'D' (the black bow between two ") was retained on the right part (original magnification 16x).
Figure 3. The surfaces of the fracture halves show the characteristic surface pattern of a radial zone which was developed during unstable crack growth. The letters 'D' and '5' represent each broken part in respect with the labeled part of Fig. 2 (original magnification 16x).
Figure 4. View of the left part under different angle and illumination. The shear-lip zone where the fracture terminated is profound (original magnification 16x).
Stereomicroscope images revealed two characteristic zones of tensile fracture; the radial zone and the outer shear-lip zone where the fracture terminated. The radial zone is produced when crack growth rate becomes rapid whilst the shear-lip zone size depends on the stress state, the properties of the material and the dimensions of the specimen (Vander Voort 1987). Tracing the cracks of radial zone backwards, the origin of fracture is determined which in this case was the groove of the letter āDā mechanically engraved on the inner surface of the dam clamp bow.
The labelling technique was implicated for crack initiation and subsequently the fatigue fracture of forged cobalt-chromium-molybdenum femoral stem (Woolson et al. 1997). The explanation of this behavior is deeply associated with residual stresses produced by thermal alterations due to the labelling technique in the heat-affected zone (HAZ). Although the extent and distribution of residual stresses remains unknown, fracture occurs many times in welded parts with externally applied stresses far below the yield strength of the material (Masubuchi 1993). Therefore, it is suggested that residual effects (residual stresses, shape alterations, etc.) of the labelling technique should also be considered with regard to the final properties of produced materials.
Although the labelling technique was associated with the fracture mechanism of this particular rubber-dam clamp, it must be acknowledged that further research is needed to point out if the effect of residual stresses introduced by the engraving procedure is actually implicated in the fracture mechanism under clinical conditions.
Davis JR (1993) Selection of wrought martensitic stainless steels. In: Olson DL, Siewert TA, Liu S, Edwards GL, eds. Welding, Brazing, and Soldering. Materials Park, OH, USA: ASM International, pp. 432-42.
Jedynakiewicz NM, Cunningham J, Williams DF (1985) Acid-etching and the fracture of rubber dam clamps. British Dental Journal 159, 121-3.
Masubuchi K (1993) Residual stresses and distortion. In: Olson DL, Siewert TA, Liu S, Edwards GL, eds. Welding, Brazing, and Soldering. Materials Park, OH, USA: ASM International, pp. 1094-102.
Medical Procurement Item Description #2-27 (January 1983) (cited in Svec et al. (1997)).
Sutton J, Saunders WP (1996) Effect of various irrigant and autoclaving regimes on the fracture resistance of rubber dam clamps. International Endodontic Journal 29, 335-43.
Svec TA, Powers JM, Ladd GD (1997) b Journal of Endodontics 23, 397-8.
Vander Voort GF (1987) Visual examination and light microscopy. In: Mills K, ed. Fractography. Materials Park, OH, USA: ASM International, pp. 98-105.
Woolson ST, Milbauer JP, Bobyn JD, Yue S, Maloney WJ (1997) Fatigue fracture of a forged cobalt- chromium-molybdenum femoral component inserted with cement. A report of ten cases. Journal of Bone and Joint Surgery (American Volume) 79, 1842-8.