Mechanical behavior and failure analysis of prosthetic retaining screws after long-term use in vivo. Part 2: Metallurgical and microhardness analysis.
OBJECTIVE
This study involved testing and analyzing multiple retrieved prosthetic retaining screws after long-term use in vivo to: (1) detect manufacturing defects that could affect in-service behavior; (2) characterize the microstructure and alloy composition; and (3) further characterize the wear mechanism of the screw threads.


METHODS
Two new (control) screws from Nobel Biocare (NB) and 18 used (in service 18-120 months) retaining screws [12 from NB and 6 from Sterngold (SG)] were: (1) metallographically examined by light microscopy and scanning electron microscopy (SEM) to determine the microstructure; (2) analyzed by energy dispersive X-ray (EDX) microanalysis to determine the qualitative and semiquantitative average alloy and individual phase compositions; and (3) tested for Vickers microhardness.


RESULTS
Examination of polished longitudinal sections of the screws using light microscopy revealed a significant defect in only one Group 4 screw. No significant defects in any other screws were observed. The defect was considered a "seam" originating as a "hot tear" during original casting solidification of the alloy. Additionally, the examination of longitudinal sections of the screws revealed a uniform homogeneous microstructure in some groups, while in other groups the sections exhibited rows of second phase particles. The screws for some groups demonstrated severe deformation of the lower threads and the bottom part of the screw leading to the formation of crevices and grooves. Some NB screws were comprised of Au-based alloy with Pt, Cu, and Ag as alloy elements, while others (Groups 4 and 19) were Pd-based with Ga, Cu, and Au alloy elements. The microstructure was homogeneous with fine or equiaxed grains for all groups except Group 4, which appeared inhomogeneous with anomalous grains. SG screws demonstrated a typical dendritic structure and were Au-based alloy with Cu and Ag alloy elements. There were differences in the microhardness of gold alloy screws from NB and SG as well as palladium alloy screws from NB.


CONCLUSIONS
Significant differences within NB retaining screws and between NB and SG screws were found for microstructure, major alloy constituents, and microhardness.
Division of Prosthodontics, Marquette University School of Dentistry, Milwaukee, WI 53233, USA. youssef14@hotmail.com
Youssef
Y
Al Jabbari
Raymond
R
Fournelle
Gerald
G
Ziebert
Jeffrey
J
Toth
Anthony
A
Iacopino
D003722:Dental Alloys; D017267:Dental Prosthesis Design; D017268:Dental Prosthesis Retention / Q000295:instrumentation*; D019094:Dental Prosthesis, Implant-Supported; D019232:Dental Restoration Failure; D019554:Dental Restoration Wear; D003799:Dental Stress Analysis; D004577:Electron Probe Microanalysis; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006244:Hardness; D006801:Humans; D008669:Metallurgy; D008855:Microscopy, Electron, Scanning; D013314:Stress, Mechanical
D003722:Dental Alloys