Preload loss in gold prosthesis-retaining screws as a function of time.

PURPOSE Screwed joints are widely used in implant dentistry, and their failure is a significant problem that may be related to loss of preload. Preload is the compressive force generated across a joint when a screw is tightened and is responsible for keeping a joint closed. For a given torque, preload is limited by the frictional resistance of the contacting screw threads, flange, and opposing joint surfaces. This study tested the hypothesis that following correct placement, prosthetic gold screws lose preload over time. MATERIALS AND METHODS The study used standard Nobel Biocare components. Strain gauges mounted on a standard abutment formed a transducer to measure preload. Five sets of new prosthetic gold screws, gold cylinders, and standard abutment screws were assembled in turn on the top of an implant body, using the transducer abutment. The gold screws were tightened with a Nobel Biocare Torque Controller set to 10 Ncm, when it had an output of 12.06 +/- 0.8 Ncm with 95% repeatability. Preload was monitored for 15 hours; then the screws were removed and examined under a scanning electron microscope. RESULTS Preload ranged from 157.5 to 488.9 N (mean 319.6 N), with a mean reduction over 15 hours of 24.9%, with 40.2% of this occurring within 10 seconds of tightening. DISCUSSION Torsional relaxation of the screw shaft, embedment relaxation, and localized plastic deformation of the gold alloy and opposing titanium threads were the most likely explanation for this phenomenon. CONCLUSIONS New prosthetic gold screws suffer significant loss of preload following placement.

[1]  P I Brånemark,et al.  A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. , 1981, International journal of oral surgery.

[2]  J A Holloway,et al.  Implant screw mechanics. , 1998, Dental clinics of North America.

[3]  R L Sakaguchi,et al.  Elongation and preload stress in dental implant abutment screws. , 1995, The International journal of oral & maxillofacial implants.

[4]  Joseph Edward Shigley,et al.  Standard Handbook of Machine Design , 2004 .

[5]  T. Jemt.  Failures and complications in 391 consecutively inserted fixed prostheses supported by Brånemark implants in edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. , 1992, The International journal of oral & maxillofacial implants.

[6]  Ignace Naert,et al.  A six-year prosthodontic study of 509 consecutively inserted implants for the treatment of partial edentulism , 1992 .

[7]  I Herrmann,et al.  Osseointegrated implants for single-tooth replacement: a prospective 5-year multicenter study. , 1996, The International journal of oral & maxillofacial implants.

[8]  Robert O. Parmley.  Standard handbook of fastening and joining , 1977 .

[9]  T Jemt,et al.  Loads and designs of screw joints for single crowns supported by osseointegrated implants. , 1992, The International journal of oral & maxillofacial implants.

[10]  J I Nicholls,et al.  Tolerance measurements of various implant components. , 1997, The International journal of oral & maxillofacial implants.

[11]  I Herrmann,et al.  Osseointegrated implants for treatment of partially edentulous jaws: A 5 year prospective multicentre study , 1994 .

[12]  H. Saunders,et al.  An Introduction to the Design and Behaviour of Bolted Joints , 1983 .

[13]  M. Quirynen,et al.  An up to 6-years prosthodontic study of 509 consecutively inserted Brånemark implants for the treatment of partial edentulism , 1992 .

[14]  Masaya Hagiwara,et al.  A New Tightening Technique for Threaded Fasteners , 1994 .

[15]  E A Patterson,et al.  Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants. , 1992, The International journal of oral & maxillofacial implants.

[16]  B Rangert,et al.  Mechanical aspects of a Brånemark implant connected to a natural tooth: an in vitro study. , 1991, The International journal of oral & maxillofacial implants.