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May 31, 2017 // 17 min read

A Note on the Mechanics of Screw-Fastened Implant Connectors

Implant connector screw fatigue stress fracture screw loosening

Abstract

Implant connectors are intricate pieces of machinery that are designed to withstand functional loads during mastication. In mechanical terms these loads take the form of continuous streams of back and forth stress applications. Yet while the vast majority of implant-borne restorations bear these stresses without any detrimental effects, in some instances (in the order of a few percent) screw loosening and/or fracture will occur. Hence one may reasonably ask why such mishaps take place and how they can be prevented. The answer lies in a thorough understanding of the way a screw-fastened connector works – more specifically what a clinician should do or be careful of when screw-tightening a restoration onto an implant.

The primary function of a screw is not to bear all the stresses applied. To the contrary, by virtue of its clamping action, the screw should distribute the load onto carrier surfaces. Screw mechanics in dynamic environments center on the concepts of 'pretension' (inside the screw) and the corresponding 'preload' (of the surfaces) as well as how they decay over time once the connector is placed in function. The essential objective of any connector design is to minimize not the absolute value but the stress amplitude during cyclic loading consequent to mastication.

Therefore, for the clinician, the objective is to permit the connector – the screw – to function under optimal conditions. This translates into establishing tight contact between the carrier surfaces machined into the implant head and that of the prosthetic component. Further, these surfaces must be firmly clamped so that the force transfer between them is maximized and the stress inside the screw is relieved.

Introduction

In the early days of a discipline that would later become 'implantology', the implants (in various shapes and forms) comprised both the endosseous anchorage and the crown analog – or at least some sort of support for a restoration. Yet while separating the intra-tissular portion of the device from that which protruded into the oral cavity would have been eminently desirable to facilitate bone healing, alternatives to the "monolithic" design were not achievable as it was impossible at that time to durably link a prosthetic structure to the endosseous (or periosseous) anchorage. And indeed, how could a tooth analog - subjected to forces of several hundred Newtons - be reliably secured to a component whose maximum buccolingual width was 4 - 5 millimeters?

Micromechanics was still in its infancy and therefore the expertise was "borrowed" from the manufacturers who mastered the machining of small parts, that is, from the watchmaking industry. Viktor Kuikka who designed the early implants for Prof. Per-Ingvar Brånemark was a watchmaker by training and Reinhard Straumann as well as the engineer Franz Sutter were also involved with the watchmaking industry (Sutter et al. 1988). The connectors which V. Kuikka and F. Sutter developed are presented in Fig. 1. They are prototypical of that time's "philosophy". The Brånemark connector (Fig. 1a) was designed for a submerged implant. It is of the flat-to-flat type (Binon 2000) and features a central positioning index in the form of a six-angled nut (the hex). The Straumann connector (Fig. 1b) was designed to emerge into the oral cavity right after implant placement. It is of the cone-in-cone type and devoid of positioning index (such indices were later added, first in the form of a removable add-on part and later as a machined octagon mid-level of the cone).

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Fig. 1: a: Brånemark prototype, b: Straumann prototype. Over the years terminology has evolved in that the Brånemark prototype (a) is now referred to as a bone level implant and the Straumann prototype (b) as a tissue level implant

Fig. 2: Stacked system. Cross-sectional view (left) and schematics of a stacked system. Note that the restoration's holding screw (right) is itself screw-tightened into the head of the screw that secures the transgingival abutment collar

Fig. 3: a: Elementary abutment b: Abutment-transgingival collar complex. Note that all components (implant cylinder, abutment, screw) are co-axial

Fig. 4: Force transmission. Force transmission takes place on the horizontal surfaces (a: Brånemark prototype) or along the oblique walls of the cone (b: Straumann prototype)

Fig. 5: Force transmission and index function in the Brånemark prototype. In the flat-to-flat design, the surfaces that carry load (red) are perpendicular to the screw's long axis. Surfaces designed for indexing (blue) are parallel to the long axis

Fig. 6: Force transmission and index function in the Straumann prototype. In the cone-in-cone design, forces are transmitted via the oblique walls of the cone (red). The surfaces intended for indexing (blue) are parallel to the long axis

Fig. 7: Pretension and preload. Tightening the screw against the nut induces a slight elongation inside the shaft. At this time the screw behaves like a spring that clamps both plates against each other forcefully. Pretension is the recoiling force inside the screw shaft. Preload is the pressure that is generated at the interface of the plates. In its elementary principle, this configuration is the same as that shown in Fig. 4

Fig. 8: Effect of pretension on the tension inside the screw shaft. In the whole range up to the decompression point, due to the pretension of the screw, an increase in tension on the handles has no major effect on the tension inside the screw shaft

Fig. 9: Low vs. high pretension. A pretension of low magnitude is not protective as the loads generated during chewing might exceed the decompression point (a) and directly affect the screw (all the load is borne by the screw shaft). In contrast, high pretension maintains the chewing loads in the low range. Note that the peak loads in load case b. (A2) are higher than those in load-case a. (A1). Still the load amplitude A2 is smaller and therefore will affect joint integrity to a much lesser extent

Fig. 10: Paris-Erdogan law: da/dN = C(ΔK)ᵐ. As shown, the

Fig. 11: Principle of rotational fatigue. Due to the rotational movement under weight, the test piece (i.e. the implant-abutment complex) is subjected to a continuous flow of fully reversed sinusoidal loads

Fig. 12: Fatigue resistance. The relative fatigue resistance of natural teeth, screw-fastened metal- and screw-fastened ceramic abutments is compared. In accordance with theory, the resistance to fatigue loading augments with increasing torque

Fig. 13: Fatigue resistance. The index was removed from the connectors. In accordance with theory, fatigue resistance is not affected

Fig. 14: Surface machining. After machining, the surfaces should be brought to a high degree of finishing. Scratches and channels left by machining are not acceptable. When subjected to chewing forces, the peaks in the surface will wear off, causing the space between the abutment and the implant to increase (d). When increasing d, there will a concomitant decrease in pretension

Fig. 15: Screw tightening on a PEEK abutment. a: Original abutment. For the purpose of the demonstration, the abutment was shortened down to the rest seat of the screw head. b: Initial contact during screw-tightening. c: Tightening to 10 Ncm. The abutment is sitting well on the implant head. d: tightening to 35 Ncm. The screw head sinks into the PEEK material