title: Torque loss under mechanical cycling of long-span zirconia and titanium-cemented and screw-retained implant-supported CAD/CAM frameworks.
abstract: OBJECTIVE
  This study evaluated the screw joint stability after cyclic loading of implant-supported titanium and zirconia CAD/CAM frameworks for fixed dental prostheses (FDPs) with different retention methods.
  
  
  METHODS
  Twenty four one-piece frameworks supported by six threaded implants placed in the maxilla were fabricated using a CAD/CAM technique (NeoShape). Dry-pressed porcelain crowns were luted to the frameworks to standardize the specimens. The specimens were then divided into four groups (n = 6) according to framework material (titanium or zirconia) and retention method for the prosthesis (cement- or screw-retained): G1, Ti-cemented; G2, Ti-screw-retained; G3, Zr-cemented; and G4, Zr-screw-retained. A digital torque ratchet was used to assess the initial preload removal torque. Torque was then reapplied and the specimens were submitted to a 200 N cyclic load, at a frequency of 2 Hz, underwater in controlled temperature of 37°, and for 1 × 106 cycles. An opposing lower dental arch was fabricated using bis-acrylic resin to simulate occlusal contacts in centric. After cyclic loading, postload removal torque was measured. Preload and postload torque loss was expressed as a percentage of the initial load. Data were submitted to a linear mixed-effects model for statistical significance (α = 0.05) to evaluate the effect of cyclic loading in the screw torque loss used with frameworks of different materials and retention methods.
  
  
  RESULTS
  Significant screw torque loss (%) was found for the tested groups (before/after cyclic loading, respectively): G1 (39.77/61.83), G2 (37.57/50.96), G3 (34.87/54.10), and G4 (47.56/73.50) (P < 0.05).
  
  
  CONCLUSIONS
  The screw removal torque was significantly reduced for all groups in this study after cyclic loading the specimens. Screw-retained zirconia specimens presented the highest torque loss before and after the cyclic loadings compared with the other specimens that were tested.
authors.forename: Érica Alves
authors.initials: ÉA
authors.lastname: Gomes
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Tiossi
authors.forename: Adriana Cláudia Lapria
authors.initials: AC
authors.lastname: Faria
authors.forename: Renata C S
authors.initials: RC
authors.lastname: Rodrigues
authors.forename: Ricardo Faria
authors.initials: RF
authors.lastname: Ribeiro
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry; D017438:Bisphenol A-Glycidyl Methacrylate / Q000737:chemistry; D001732:Bite Force; D002484:Cementation / Q000379:methods*; D017076:Computer-Aided Design; D003442:Crowns; D000044:Dental Abutments; D003764:Dental Materials / Q000737:chemistry*; D003776:Dental Porcelain / Q000737:chemistry; D019094:Dental Prosthesis, Implant-Supported; D003781:Denture Retention / Q000295:instrumentation*; D003830:Denture, Partial, Fixed; D006801:Humans; D008422:Materials Testing; D019279:Resin Cements / Q000737:chemistry; D013314:Stress, Mechanical; D013696:Temperature; D013997:Time Factors; D014025:Titanium / Q000737:chemistry*; D019415:Torque; D014867:Water / Q000737:chemistry; D015040:Zirconium / Q000737:chemistry*
chemical_list: D000180:Acrylic Resins; D003764:Dental Materials; D019279:Resin Cements; C098586:luxatemp; D014867:Water; D003776:Dental Porcelain; D017438:Bisphenol A-Glycidyl Methacrylate; D015040:Zirconium; D014025:Titanium; C028541:zirconium oxide