Retentive force maintenance with titanium-nickel alloy clasps

Retentive force maintenance with titanium-nickel alloy clasps

Dental Materials Color stability varies in provisional prosthodontic resins Background.—Provisional materials for fixed prosthodontics may become disc...

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Dental Materials Color stability varies in provisional prosthodontic resins Background.—Provisional materials for fixed prosthodontics may become discolored. When patients become dissatisfied, replacement, which is costly, may be required. Five provisional prosthodontic materials were tested for color stability. Methods.—The 5 autopolymerizing provisional restorative materials included polyethyl methacrylate resins (Trim II), polymethyl methacrylate resins (Duralay and Alike), and bis-acryl methacrylate resin (Luxatemp and Integrity). A total of 105 disk-shaped specimens were prepared of these materials and then exposed to distilled water, coffee, or ultraviolet (UV) light for varying lengths of time and at various temperatures. The distilled water was at a temperature of 60°C; the coffee was at 37°C. The UV light exposure lasted 24 hours; the exposure to water or coffee lasted 20 days. The color values were determined with the use of a colorimeter before and after the immersion or exposure. The change in color (∆E) was determined for the various samples and comparisons made. Results.—Significant differences were produced in the ∆E of the various materials. After water immersion, the 2 bis-acryl methacrylate resins did not differ significantly but the 3 methyl/ethyl methacrylate resins had significantly higher ∆E values and differed significantly from each another. The highest ∆E value was noted with Trim II; the lowest ∆E value of the 3 resins was with Duralay. Coffee immersion produced significantly higher ∆E values in the Luxatemp and Integrity disks than in the Trim II, Alike, and Duralay disks, which did not differ significantly from each another. UV irradiation produced significantly higher ∆E values in the Trim II, Alike, and Duralay disks than in the Luxatemp and Integrity samples. The highest ∆E was found with Trim

II disks, and Alike had the lowest of the 3 methyl/ ethyl methacrylate resins. Discussion.—The various provisional prosthodontic materials performed differently, depending on the various testing environments. Color was more stable for the bisacryl methacrylate resins in 60°C water for 20 days than for the methyl/ethyl methacrylate-based resins. Coffee stains were more likely to occur in the bis-acryl methacrylate resins than in the methyl/ethyl methacrylate samples. Color changes after UV irradiation were significantly less notable for the bis-acryl methacrylate resins than for the methyl/ ethyl methacrylate resins. For long-term use, provisional restorations should be made of bis-acryl methacrylate resin rather than methyl/ethyl methacrylate resins. However, patients must be advised of the possibility of staining by coffee when bis-acryl methacrylate resin provisional materials are used.

Clinical Significance.—Bis-acryl-methacrylate materials such as Luxatemp and Integrity proved more color stable over time than methyl or ethyl methacrylate materials—a benefit where long periods of provisionalization are anticipated—but coffee produced considerable staining.

Sham ASK, Chu FCS, Chai J, et al: Color stability of provisional prosthodontic materials. J Prosthet Dent 91:447-452, 2004 Reprints available from FCS Chu, Reception & Primary Care Clinic, Prince Philip Dental Hosp, 34 Hospital Rd, Hong Kong, China; fax: (852) 2547-0164; e-mail: [email protected]

Retentive force maintenance with titanium-nickel alloy clasps Background.—Titanium-nickel (Ti-Ni) alloys have several unique properties that contribute to their usefulness for dental applications. Memory of shape and elastic characteristics and a fairly constant stress value that rebounds

after elastic deformation are among these attributes. Retentive clasps for removable partial dentures (RPDs) must be able to flex and return to their original form as well as retain the prosthesis. Abutment teeth must not be un-

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duly stressed, and clasps must not be permanently distorted from use. An investigation of the retentive force of various types of clasps was undertaken to determine if Ti-Ni alloy clasps maintain their initial retentive force under various conditions. Methods.—A mandibular dentiform with a horizontal plane of occlusion was used in the various tests. Left second molar metal abutment crowns were fabricated with 2 different retentive undercut depths (0.25 or 0.75 mm) and clasp sizes (0.8 or 1.4 mm). Of the 98 clasps fabricated, 14 were wrought wire clasps and 28 were made of each of the following: cobalt-chromium (Co-Cr), Type IV gold alloy, and Ti-Ni alloy. A masticatory simulator was used to cycle the clasps on and off the metal crown to simulate the insertion and removal of the RPD clasp. Retentive forces were measured initially and after each sequence. Casting defects and porosity were evaluated through radiographic analysis. Internal surfaces of the clasp arms were evaluated by scanning electron microscopy to detect metal fatigue. Comparisons between the various clasps were made. Results.—Material, cycling sequence, clasp size, and undercut all influenced the retentive force value. Over the spectrum of materials and undercuts, for the 0.8-mm wide clasps the initial retentive force was between 2.3 and 7.9 N; for the 1.4-mm wide clasps, the initial retentive force was between 3.6 and 10.4 N. For the 0.25-mm retentive undercut situations, the 1.4 mm Co-Cr alloy clasps had the highest initial retentive force and the 0.8-mm Ti-Ni alloy clasps had the lowest retentive force. A gradual decline in the retentive force of the Co-Cr and gold alloys was documented with more placement and removal cycles. The decline in retentive force for the Ti-Ni alloy clasps was significant only after the first cycling sequence. Thereafter the 0.8-mm TiNi alloy clasps tended to maintain a retentive force of about 1.8 N and the 1.4-mm Ti-Ni alloy clasps maintained a force

of about 2.6 N over the cycling sequences. Compared to the loss of retentive force with the other clasps, the loss with the Ti-Ni alloy clasps was negligible. Similar trends were noted for the 0.75-mm retentive undercut areas. Final measurements showed the Ti-Ni alloy clasps had slightly greater retentive force than the clasps made of other materials, but all clasps had a similar end-point retention value. Evaluation of the porosity of the Ti-Ni alloy clasps revealed small porosities with no large voids. Other clasps had no evidence of porosity. Compared with clasps made with the other alloys, the Ti-Ni alloy clasps were relatively rough and had obvious surface porosity. All the clasps, except those made of Co-Cr alloy, had cracks. Discussion.—Ti-Ni alloy RPD clasps were highly resistant to a loss of retention in situations with 0.25-mm (conventional) and 0.75-mm (deep) retentive undercuts. A significant decline in retentive force was found for all the clasps, except those made of the Ti-Ni alloy. Thus cast Ti-Ni alloy clasps may be appropriate for use in RPD restorations.

Clinical Significance.—Significant loss of retention with use is a fact of life for most materials used for RPD clasps. Ti-Ni seems to be the exception. If the casting process can be simplified, this may be the alloy of the future.

Kim D, Park C, Yi Y, et al: Comparison of cast Ti-Ni alloy clasp retention with conventional removable partial denture clasps. J Prosthet Dent 91:374-382, 2004 Reprints available from L Cho, Dept of Prosthodontics, Kangnung Natl Univ, 123 Jibyeon-Dong, Gangneungk Gangwon-Do 210-702, Korea; fax: 82-33-640-3103; e-mail: [email protected]

Devices Actual torque values exceed preset levels Background.—Nickel-titanium (Ni-Ti) rotary instruments may separate during endodontic therapy if the level of torque is equal to or more than the torque at fracture. Both air and electric motors that lack torque control and

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electric torque control motors are used in these instruments. When the instrument is subjected to torsional stress equal to the torque level set on the motor, the motor should stop rotating and avoid fracture. Among the various