Fatigue failures of bar-attachment brazed joints for implant-supported overdentures

Abstract

Oral implants are increasingly being used to enhance the mastication ability of edentulous patients using removable dentures. The dentures are connected by clips to beams brazed to gold cylinders, which are screwed onto abutments attached to as many as three to six implants generally placed in the anterior region of the mouth. Cantilever beams some 7–12 mm in length support the extended molar portion of the denture. Clinical trials of these structures indicate that they usually fail at the brazed joint at the gold cylinder and the cantilever abutment brazed region some 2–3 years after placement. In this paper we investigate the nature of the clinical failure process and compare it with simple flexure and preliminary fatigue results of similar cantilever brazed joints.

Introduction

Despite the advances in modern dentistry, restorative options of cases with extensive or complete loss of teeth are usually restricted to removable complete dentures. Besides improving the aesthetics and phonetics of the patient, dentures are mandatory for a person to masticate; otherwise the nutritional intake is severely restricted and can result in a series of general health complications long-term. However, even with dentures, the ability to deliver an efficient mastication is restricted and furthermore this decreases with time as the supporting maxillary and mandibular bone recede due to the reduced biomechanical stimulation [2], [29]. Implant-supported dentures improve the biomechanical integration of the dentures by providing them with a better retention and also increase the biting force by partially relieving the gingivo-mucosal support of occlusal loads. A typical implant-supported structure onto which the denture is attached is shown in Fig. 1.

Implant overdentures usually consist of several titanium oral implants placed inside the bone on which supra-gingival abutments made of the same metal are attached. On top of the abutments noble alloy cylinders (Au–Pt–Pd) are screwed and then connected by brazing noble alloy bars (Au–Pd–Cu–Zn) between them (Fig. 2). The brazing materials are also noble alloy but with a lower melting temperature than the alloy used for the cylinders and bars. Brazing is carried out using either a gas/oxygen flame or in a pre-heated furnace in which the components are inserted. A number of factors influence the quality and strength of bar-joint and bar-attachments overdentures. The recommended technical manufacturing steps as outlined by one supplier (ITI Straumann) [12], involves the brazing of a type IV gold bar-attachment or bar-joint to a non-oxidising gold cylinder using a gold brazing material with a melting point below that of the bar. This procedure is technique sensitive and the quality of the brazed joint and its strength is dependent on the technical skill of the dental technician and the surface area of the cross-section of the bar where it is brazed to the cylinder if the “fillet” of brazing material surrounding the joint is trimmed back as is recommended by the instructions. Similarly, Mericske-Stern [16] recommends that bar-attachments should be used for distal cantilever extensions due to their greater cross-sectional area than the bar-joint. From a mechanical perspective, the removal of the “fillet” around the brazed joints is anticipated to create a stress concentration point that can weaken the joint. The method of brazing depends on the choice of the technician and can vary from brazing in a porcelain furnace with or without vacuum, to using a flame (propane/oxygen; propane/air; natural gas/oxygen; natural gas/air) as the heat source.

The attachment of the denture base to the bars is still a controversial topic, with some authors supporting a rigid connection or bar attachment [14] while others recommend a resilient connection or bar joint [18].

Clinical recommendations for maxillary implant overdentures suggest the use of a minimum of 4 implants connected by bars, with or without distal cantilever extensions, usually with a cast framework [16], [30], [15]. Use of short cantilever distal extensions of the bar attachments is reported to improve the stability of the overdenture [16].

A common problem encountered in mandibular implant overdentures is their prosthodontic maintenance [20] and despite several studies showing the occurrence of “bar fractures” [9], [13], [20], the causes, nature and locations of such failures are not reported.

A review of the properties of brazed joints, especially their fatigue behaviour, reveals relatively few studies with a wide range of failure stresses. Buston et al. [3] and Press et al. [24] report that the number of cycles to failure of brazing materials ranged from 104 to 106 in a rotating bending jig at 241 MPa, whereas Wiskott et al. [28] measured the fatigue failure stress (S–N curve) in tension to range from 30 to 70 MPa for similar brazed joints. The latter authors also pointed out the significant contributions to crack initiation of flux inclusions and brazing porosity. This was verified by Press et al. [24], who also found that surface preparation prior to brazing was a contributory factor.

Starting from this premise, this preliminary study investigates the fracture behaviour of fatigue-loaded cantilever bars and relates the results with clinically failed bar attachment systems. Although it consists of a limited number of clinical cases due to the ongoing investigation, a clear pattern of failure type and location can be identified. The emphasis of this study is to compare clinical with simple laboratory failure modes for the soldered cantilever bar attachments that are attached to dental implants.