Factors influencing preload loss of prosthetic screws: a critical review

OBJECTIVES In implant-prosthetic rehabilitation, the unscrewing of the connecting prosthetic screw is the most frequent mechanical complication, as well as preceding the fracture of the screw itself. The goal of this literature review is to analyze the factors that influence the loss of preload of prosthetic screws. MATERIALS AND METHODS A literature search was conducted on four different databases: PubMed, Scopus, Cochrane and EBSCO. The following keywords were used to search: “Screw Preload”, “Screw Tightening”, “Abutment Screw” and “Torque Loss”. The search initially provided a total of 4270 articles of which after appropriate selection only 35 were included in the review. RESULTS The selected articles have been divided into 4 main topics: screwing techniques; type of abutment and prosthetic framework; type of prosthetic screws; use of lubricating/ sealing agents. CONCLUSIONS There are several factors that influence the loss of preload of prosthetic screws. The passivity of the structures on the implants and the control of occlusion are certainly fundamental elements for preserving the health of the implant- prosthetic components and the tightening of the screws themselves. Among the tightening techniques, several authors recommend repeating the tightening after a few minutes from the first screwing. Among the types of abutments and frameworks, the tolerances, the material and the precision of the components also play an important role. The coating of the prosthetic screws, their adaptability within the thread and the shape of the screw head are also important.The use of lubricants and sealants is a very controversial factor. In addition, saliva is the most discussed factor: for some it helps to reduce the loss of preload, for others it has no influence and for still others the reduced friction between the implant-prosthetic components given by the liquid could even be harmful given the high stresses produced. It is therefore necessary to periodically check the possible unscrewing of the prosthetic screws at a clinical level in order not to incur major complications. CLINICAL SIGNIFICANCE The fitting of the prosthesis on the implants, the occlusion and the tightening of the prosthetic screws represent the basis for favoring the longterm survival of the implant prostheses and of the implants themselves. However, several factors influence the loss of preload of the prosthetic screws. In order to prevent mechanical complications at the connection level, the clinician must know the nature of these factors in relation both to the general implant prosthesis and to the implant- prosthetic system used in the single clinical cases.