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Associated Topics

Screw loosening of single implant-abutment restorations

Hana Hasson DDS, MS .


Systematic Review Conclusion

Abutment screw loosening is a rare event in single-implant restorations regardless of the geometry of implant-abutment connection if proper antirotational features and torque are employed.

Critical Summary Assessment

Limited evidence suggests a low incidence of abutment screw loosening in single-implant restorations with different internal and/or external connections.

Evidence Quality Rating

Limited Evidence

Structured Abstract

Clinical Questions:

In single-implant restorations (SIR) with different connection geometries (internal and external), what is the incidence of abutment screw loosening?

Review Methods:

The authors searched multiple databases including the Cochrane Central Trials Registry for English-only materials published from 1990 to September 2006. They also reviewed references of the retrieved papers, relevant journals, and hand searched for articles. The authors conducted the review process in two phases, applying preset inclusion and exclusion criteria to an initial search yield of 1,526 titles. Twenty seven clinical studies (2,038 patients) of various designs with a follow-up duration of at least three years were selected for inclusion . These studies, which originated from both private and university practice settings, were separated according to their implant-abutment connection, either external or internal.

Main Results:

The review was divided into two types of implants systems, external and internal connection . The external- connection implant systems included those of Biomet 3i and Nobel Biocare products. The follow-up time ranged from three to five years. The estimated percentage of complication-free SIR after three years was 97.3 percent (95 percent confidence interval [CI]). Included studies were conducted from 1996 to 2004. The internal-connection implant systems included Straumann, Astra, Mac System, Bicon and Ankylos products. All studies were conducted from 1998 to 2006. The follow-up time ranged from three to 10 years. The complication free percentage of SIR for the three groups was as follows: Straumann, 98.2 percent (95 percent CI); Astra 97.0 percent (95 percent CI); and Other 97.1 percent (95 percent CI).


Abutment screw loosening is a rare event in single implant restorations regardless of the geometry of implant-abutment connection, provided that proper antirotational features and torque are employed.

Source of Funding:

None stated


Importance and Context:

Before 1994, use of the external hex-type implant-screw connection was prevalent despite its frequent tendency to loosen within the first two years after loading. This review included studies that were conducted after changes were made to improve design, screw materials, loading and geometry of implant-abutment connection.

Strengths and Weaknesses of the Systematic Review:

The authors conducted a comprehensive search, including hand searching references and journals. Because they limited their search to English-only, they may have omitted relevant studies. The authors included the worst-case scenario (total loss) instead of recording single crown implant failure.

Strengths and Weaknesses of the Evidence:

All included studies were of different designs, including clinical cases reports and retrospective studies. The reviewers rated the studies as "C" level using the method of Jokstad et al (studies included under this rating are clinical, retrospective, and case series)1. Studies included were conducted in a university, private practice and hospital based clinics which have their own limitation in-terms of the population of patients who received the implant thus increasing the potential for some biases in the reported results. Some of the restorations were part of a fixed bridge and not single implants, which may increase the likelihood of failure if, for example, an implant and a natural tooth were bridged.

Implications for Dental Practice:

There is limited evidence that abutment screws rarely loosen in single implant restorations when proper antirotational features and torque are employed. This is no longer an issue in treatment decisions since all implants we use today have internal connections. However with regard to this systematic review both internal and external connections are reasonable options, and other factors should be considered when making treatment decisions. 1. Jokstad A, Bragger U, Brunski JB, Carr A, Naert I, Wennerberg A. Quality of dental implants. Int. Dent. J 2003;53:409-443

Critical Summary Publication Date:


These summaries are not intended to, and do not, express, imply, or summarize standards of care, but rather provide a concise reference for dentists to aid in understanding and applying evidence from the referenced systematic review in making clinically sound decisions as guided by their clinical judgment and by patient needs. American Dental Association © 2018