Effect of Casting Procedure on Screw Loosening of Ucla Abutment in Two Implant-abutment Connection Systems

Since Bra�nemark et al published the results of a 10-year study on osseointegration in 1977, dental implants have increasingly been used to replace missing teeth. The use of osseointegrated implants had become a successful procedure for the treatment of complete, partial edentulism, and single-tooth replacements in both the anterior and posterior regions of the mouth. Although improvements in material and protocol have decreased the incidence of screw loosening, and a recent literature review reported that the incidence of screw loosening appears to be decreasing, one of the most common mechanical problems associated with dental implants is loosening of the screws. The use of screw in dental implants has some unique features. McGlumphy et al defined the screw joint as 2 parts tightened together by a screw, such as an abutment and implant being held together by a screw. A screw is tightened by applying torque. Applied torque develops a force within the screw called preload. Preload is the initial load in tension on the screw. This tensile force on the screw develops a compressive clamping force between the parts. The preload is determined by the applied torque and other factors, such as screw alloy, screw head design, manufacturer quality control, screw joint design, surface roughness, and fatigue testing. Opposing the clamping force is a joint-separating force, which attempts to separate the screw joint. Screw loosening occurs when the joint-separating forces acting on the screw joint are greater than the clamping forces holding the screw unit together. Excessive forces cause slippage between threads of the screw and threads of the bore, resulting in a loss of preload. It is not necessary to eliminate separating forces, only to minimize them. Minimizing clamping forces will act to prevent screw loosening. In single-tooth restorations, a widely used solution is the UCLA abutment. This abutment is designed to directly engage the implant and undergoes casting procedure. The cast abutment has advantages of overcoming angulation problem and esthetic problem. However, when a gold-premachined UCLA abutment undergoes casting, the abutment is exposed to the range and levels of temperatures required in the burnout and casting procedure. These manipulation may alter the abutment surfaces in contact with the implant and may lead to changes at the implant-UCLA cast abutment interface. A less than optimal fit may result in bacterial aggregation, which leads to peri-