Effect of arm design and chemical polishing on retentive force of cast titanium alloy clasps.
OBJECTIVE
Titanium dentures have recently increased in popularity. A decrease in the retentive force of the titanium clasps has frequently been observed. This study investigated the effect of retentive arm design and chemical polishing of titanium clasps.


METHODS
Four Akers clasps with 0.25-mm and 0.5-mm undercuts were designed so that the retentive arms were placed at 1/2 and 2/3 of the undercut area. Wax patterns were fabricated and invested in phosphate-bonded Al2O3/LiAl SiO6. They were cast with CP Ti Grade 2, CP Ti Grade 3, and Ti-6Al-7Nb using an argon gas-pressure casting unit (Autocast HC-III). After sandblasting, the castings were chemically treated with an HNO3/HF solution. The retentive force (N) was measured up to 10,000 insertion/removal times. The results (n = 5) were analyzed by ANOVA/Tukey's test (alpha= 0.05).


RESULTS
The retentive forces significantly decreased with increasing immersion time in the HNO3/HF solution (p < 0.05). The retentive force of the 2/3 undercut was significantly greater than that of the 1/2 undercut (p < 0.05). Excluding the initial insertion/removal period of the 2/3 undercut, there were no significant differences among all the titanium metals tested (p > 0.05). As the number of insertion/removal times increased, the retentive force of the 2/3 undercut greatly decreased. The retentive force of the Ti-6Al-7Nb clasps for the 1/2 undercut decreased the least after 10,000 insertion/removal times.


CONCLUSIONS
Chemical treatment for titanium clasps should be performed for 1-5 minutes due to the following factors: accuracy, surface roughness, surface structure, initial retention, and stability of retention. To maintain appropriate long-term retentive force, the retentive arms should be placed in the 1/2 undercut area of the abutment tooth.
Shimpo
Hidemasa
H
Department of Removable Prosthodontics, Tsurumi University, Yokohama, Japan. shimpo-hidemasa@tsurumi-u.ac.jp
D000044:Dental Abutments; D003722:Dental Alloys / Q000737:chemistry*; D016734:Dental Clasps; D045662:Dental Etching; D003775:Dental Polishing / Q000379:methods*; D017267:Dental Prosthesis Design; D003779:Denture Design; D003781:Denture Retention; D005459:Fluorides / Q000737:chemistry; D006801:Humans; D006851:Hydrochloric Acid / Q000737:chemistry; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D017942:Nitric Acid / Q000737:chemistry; D012824:Silicic Acid / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*
D003722:Dental Alloys; C070282:Ti-6Al-7Nb alloy; D012824:Silicic Acid; D017942:Nitric Acid; C035291:hexafluorosilicic acid; D014025:Titanium; D005459:Fluorides; D006851:Hydrochloric Acid; C022102:aqua regia