Mechanics of mechanically fastened joints in polymer–matrix composite structures – A review

T elusive dream of replacing missing teeth with artificial analogs has been part of dentistry for a thousand years. The coincidental discovery by Dr P-I Branemark and his coworkers of the tenacious affinity between living bone and titanium oxides, termed osseointegration, propelled dentistry into a new age of reconstructive dentistry. Initially, the essential tenets for obtaining osseointegration dictated the atraumatic placement of a titanium screw into viable bone and a prolonged undisturbed, submerged healing period. By definition, this required a 2-stage surgical procedure. To comply, a coupling mechanism for implant placement and the eventual attachment of a transmucosal extension for restoration was explored. The initial coronal design selected was a 0.7-mm-tall external hexagon. At its inception, the design made perfect sense, because it permitted engagement of a torque transfer coupling device (fixture mount) during the surgical placement of the implant into threaded bone and the subsequent second-stage connection of the transmucosal extension that, when used in series, could effectively restore an edentulous arch. As 20 years of osseointegration in clinical practice in North America have transpired, much has changed. The efficacy and predictability of osseointegrated implants are no longer issues.1–7 During the initial years, research focused on refinements in surgical techniques and grafting procedures. Eventually, the emphasis shifted to a variety of mechanical and esthetic challenges that remained problematic and unresolved.8–10 During this period, the envelope of implant utilization dramatically expanded from the original complete edentulous application to fixed partial dentures, single-tooth replacement, maxillofacial and a myriad of other applications, limited only by the ingenuity and skill of the clinician.11–13 The external hexagonal design, ad modum Branemark, originally intended as a coupling and rotational torque transfer mechanism, consequently evolved by necessity into a prosthetic indexing and antirotational mechanism.14,15 The expanded utilization of the hexagonal resulted in a number of significant clinical complications.8–11,16–22 To mitigate these problems, the external hexagonal, its transmucosal connections, and their retaining screws have undergone a number of modifications.23 In 1992, English published an overview of the thenavailable external hexagonal implants, numbering 25 different implants, all having the standard Branemark hex configuration.14 The external hex has since been modified and is now available in heights of 0.7, 0.9, 1.0, and 1.2 mm and with flat-to-flat widths of 2.0, 2.4, 2.7, 3.0, 3.3, and 3.4 mm, depending on the implant platform. The available number of hexagonal implants has more than doubled. The abutment-retaining screw has also been modified with respect to material, shank length, number of threads, diameter, length, thread design, and torque application (unpublished data, 1998).23 Entirely new secondand third-generation interface coupling geometries have also been introduced into the implant milieu to overcome intrinsic hexagonal deficiencies.24–28 Concurrent with the evolution of the coupling geometry was the introduction of a variety of new implant body shapes, diameters, thread patterns, and surface topography.26,27,29–36 Today, the clinician is overwhelmed with more than 90 root-form implants to select from in a variety of diameters, lengths, surfaces, platforms, interfaces, and body designs. Virtually every implant company manufactures a hex top, a proprietary interface, or both; “narrow,” “standard,” and “wide” diameter implant bodies; machined, textured, and hydroxyapatite (HA) and titanium plasma-spray (TPS) surface implants; and a variety of lengths and body shapes (Table 1). In the wide-diameter arena alone, there are 25 different offerings, 15 external hexagonal, and 10 other interfaces available in a number of configurations. 1Adjunct Professor of Prosthodontics, Graduate Prosthodontics, Indiana University, Indianapolis, Indiana; Assistant Research Scientist, Department of Restorative Dentistry, University of California at San Francisco; and Private Practice, Roseville, California.

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Implants and components: entering the new millennium.

Uncertainties and dynamic problems of bolted joints and other fasteners

The significance of tightening abutment screws and gold cylinders to osseointegrated fixtures with the correct torque is demonstrated, and a simple relationship between applied torque and screw preload is derived by use of mechanical engineering principles. The principles of a number of tightening methods are outlined and assessments made of their accuracy. The difference between optimum and design torque is highlighted. The necessity and means of achieving optimum torque to ensure a reliable joint in clinical practice is discussed.

Tightening characteristics for screwed joints in osseointegrated dental implants

Statement of problem. Clinical loading may result in micromotion and metal fatigue in apparently stable implant screw joints. This micromotion may contribute to tissue inflammation and prosthesis failure. Purpose. This study investigated dental implant screw joint micromotion and dynamic fatigue as a function of varied preload torque applied to abutment screws when tested under simulated clinical loading. Material and methods. Fifteen noble alloy single-tooth implant restorations, each containing a hexed UCLA-style gold cylinder, were randomly assigned to 3 preload groups (16, 32, and 48 N·cm). Each group consisted of 5 implants (each 3.75 × 15 mm) and 5 square gold alloy abutment screws. A mechanical testing machine applied a compressive cyclic sine wave load between 20 and 130 N at 6 Hz to a contact point on each implant crown. A liquid metal strain gauge recorded the micromotion of the screw joint interface after 100, 500, 1,000, 5,000, 10,000, 50,000, and 100,000 cycles. Baseline data at 0 N·cm were collected before the application of the specified preload torque. Results. The 16 N·cm group exhibited greater micromotion ( P P =.99) for each of the preload groups through 105 cycles. Conclusion. Under the loading parameters of this study, no measurable fatigue of the implant–abutment interface occurred. However, dental implant screw joints tightened to lower preload values exhibited significantly greater micromotion at the implant–abutment interface. (J Prosthet Dent 2001;85:47-52.)

Micromotion and dynamic fatigue properties of the dental implant–abutment interface

Part 1 Introduction to the bolted joint: basic concepts stress and strength considerations threads and their strength materials stiffness and strain considerations. Part 2 Tightening the joint (establishing the clamping force): introduction to assembly torque control of bolt preload torque and turn control stretch control preload control ultrasonic measurement of bolt stretch or tension. Part 3 The joint in service: theoretical behaviour of the joint under tensile loads behaviour of the joint loaded in tension - a closer look in-service behaviour of a shear joint joint failure self loosening fatigue failure corrosion gasketed joints and leaks. Part 4 Using the information: selecting preload for an existing joint design of joints loaded in tension the design of gasketed joints the design of joints loaded in shear.

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J. H. Bickford

Papers

An Introduction to the Design and Behaviour of Bolted Joints