Abstract

Fracture of the implant is one of the possible complications affecting dental implants; it is a rare event but of great clinical relevance. The aim of the present study was to perform a scanning electron microscopy (SEM) fractography evaluation of 7 International Team for oral Implantology (ITI) hollow implants removed because of fracture. The most common clinical risk factors, such as malocclusion, bruxism, and cantilevers on the prosthesis, were absent. Seven fractured ITI hollow implants were retrieved from 5 patients and were analyzed with the use of SEM. SEM analysis showed typical signs of a cleavage-type fracture. Fractures could be due to an association of multiple factors such as fatigue, inner defects, material electrochemical problems, and tensocorrosion.

Introduction

Because Branemark introduced the concept of osseointegration in the 1960s,1 the treatment of total or partial edentulism by implant-supported prosthesis represents an effective and often even better alternative to traditional prosthetic restorations.29 Finding out the factors affecting long-term clinical outcomes for dental implants (ie, careful selection of patients, analysis of implant site characteristics, implant microstructure and macrostructure properties, and type of prosthesis) led to a wide variety of proposed treatment modalities and implant systems. Among them, the International Team for oral Implantology (ITI) Bonefit implant system (Straumann, Waldenburg, Switzerland) has traditionally provided an efficient and long-lasting functional service. Three different shapes—hollow cylinder, hollow screw, and full-body screw—were available in 2 series: 1-part, transmucosal implants for 1-stage surgery, and 2-part implants for 2-stage, submerged technique.5,8,1015 The holes have been theorized to enhance the percentage of direct bone-to-implant contact by enabling bone growth inside the implant. Fracture of the implant is one of the complications that can provoke treatment failure. It is a rare event but clinically important, resulting in problems for both patient and clinician. Factors that might lead to the implant fracture can be due to defects in the materials (rare because of standardized quality control rules during fabrication), misfit or poor quality of superstructure components, unfavorable loading biomechanics, characteristics of the implant site, the presence of malocclusion including bruxism, and bone resorption.16 It can hardly be established whether bone resorption causes the fracture—with bone loss yielding a higher stress concentration on the implant—or follows it, or whether it may be caused by superimposition of an eventual infection or is induced by cracks in the titanium structure. The radiographic aspect of this bone loss has a typical feature, called cupping, which extends from the alveolar process apically to the fracture line.1619 Implants in premolar and molar regions are at greater risk, especially if a cantilever on the prosthesis is present, as it enhances load on implants with vertical or lateral forces.13,16,17,1926 The use of multiple vs single implants to replace a molar has been debated for a long time because the molar occlusal surface is much greater than the implant diameter, but wider implants and some prosthetic measures have allowed molar-replacing implants to achieve long-term successful results.13,14,25,26 The presence of malocclusion is a usual finding in patients with fractured implants, as they are potential causes of bending overload, defined as “a situation in which occlusal forces on an implant-supported prosthesis exert a bending moment on the implant cross-section at the crestal bone, leading to marginal bone loss and/or eventual implant fatigue failure.”21 Some authors report a greater percentage of fracture in partially edentulous patients than in totally edentulous patients, owing to the different biomechanics induced by various anatomic conditions. In totally edentulous patients, superstructures are more often made with acrylic resin, and implants are placed on a curved line that follows the alveolar ridge; however, in partial edentulism, especially posteriorly, implants are positioned along a straighter line, and the straighter the line, the higher is the bending overload.18,21,24 Cases of ITI fractured implants present some common traits: the hollow type appears to be the weakest, and the fracture line always involves one or more holes and is typically located below the level corresponding to the end of the abutment screw.13,16,19,21 

The aim of the present study is to perform a scanning electron microscopy (SEM) fractography evaluation of 7 ITI hollow implants removed for fracture.

Materials and Methods

Seven ITI 3.75 × 12 mm hollow implants (Straumann) were retrieved, as the result of a fracture, from 5 patients (3 males and 2 females) with a mean age of 45 years (range, 33–49 years). All patients were nonsmokers and nonbruxers. All implants had been used to restore single teeth in the premolar area of both jaws. Implants became fractured after a mean period of 36 months (range, 22–46 months). The coronal part of the fractured portion was cleaned in an ultrasonic bath of ethyl-alcohol and vapor at 1.2 Mpa (180°C). Fractured surfaces were investigated with SEM (LEO Electron Microscopy mod. 435 VP, UK) by SE1 mode (secondary electrons). The specimens were included in resin (Technovit 7200 VLC, Kulzer, Wehrheim, Germany), sectioned with a high-precision diamond disc into 2 halves, ground on 600-grit metallurgic paper, and polished through 2-micron diamonds. The surfaces were then electrolytically etched until all grains were visible, and the specimens were observed under light microscopy (Axiolab, Zeiss, Jena, Germany) and SEM (LEO Electron Microscopy mod. 435 VP, UK). The SEM digital images were electronically elaborated (Adobe PhotoShop 5.0, Adobe Systems Inc, San Jose, Calif) to underline the results of the investigation.

Results

SEM analysis showed intracrystalline, cleavage-type fractures of the implants. Small areas of plastic-elastic deformation associated with periodic cracks were observed on the grain surfaces, according to the Friedel-Orlow mechanism.2729 Metallographic analysis displayed several secondary microcracks surrounding the cleavage surface. The widely different orientation of crystalline planes confirmed the hypothesis of an intracrystalline fracture. Some microscopic defects were present on the fracture surface. The cleavage plane involved the holes and proceeded along an oblique transversal direction, meaning that the cleavage plane was perpendicular to the stress long axis. SEM investigation of the fracture surface showed the following:

  • Cleavage brittle fracture with multifaceted intragranular fractures, typical in polycrystalline materials, where each facet corresponded to a single grain. “River pattern” features, as a result of the Friedel-Orlow mechanism, were also observed. These markings typical of cleavage fracture were present on the fracture's facets and were due to multiple lines that converged to a single line, much in the manner of tributaries to a river (Figures 1 and 2).

  • Several intragranular microcracks were noted in areas were the metal was machined such as apical holes of the fixture and internal coil (Figures 3 and 4). In these areas, plastic cold deformation, variable degrees of mechanical stress, and the presence of biological fluid gave rise to a tensocorrosion phenomenon (Figures 5 and 6).

Figures 1–4

Figure 1. The fracture appears multifaceted and intragranular, and typical markings of cleavage such as river pattern lines are largely represented along the fracture plane (arrows) (SEM ×282). Figure 2. Mathematical reconstruction of the previous image. The direction, the depth, and the degree of convergence of the crack lines can be appreciated (arrows). Figure 3. Intragranular microcrack of 6 microns that runs at about 50° with respect to the holes of the implant (arrow) (SEM ×1110). Figure 4. Intragranular microcrack of 28 × 6 microns that runs at about 60° with respect to the fracture surface (this phenomenon is usual in fatigue fracture mechanisms; arrow) (SEM ×1030).

Figures 1–4

Figure 1. The fracture appears multifaceted and intragranular, and typical markings of cleavage such as river pattern lines are largely represented along the fracture plane (arrows) (SEM ×282). Figure 2. Mathematical reconstruction of the previous image. The direction, the depth, and the degree of convergence of the crack lines can be appreciated (arrows). Figure 3. Intragranular microcrack of 6 microns that runs at about 50° with respect to the holes of the implant (arrow) (SEM ×1110). Figure 4. Intragranular microcrack of 28 × 6 microns that runs at about 60° with respect to the fracture surface (this phenomenon is usual in fatigue fracture mechanisms; arrow) (SEM ×1030).

Figures 5–10

Figure 5. Tip of short fatigue microcrack. The base of the microcrack shows transversal lines caused by the release of energy at the crack tip produced by mechanisms of plasticity-induced crack closure (arrows). At this level, the corrosion phenomenon is enhanced by the presence of both biological fluid and local metal strain (tensocorrosion) (SEM ×1120). Figure 6. Mathematical reconstruction of the previous image. The plastic deformation is more evident and is represented by transversal lines between the walls of the crack. Figure 7. The different pseudocolored areas show different grains, and the lines (arrows) on the grain surfaces represent crystallographic planes with the highest packing density (SEM ×3050). Figure 8. The fracture runs through the grains and tends to change direction at the grain boundary (arrows) (SEM ×1110). Figure 9. The transgranular fracture runs parallel to the fracture surface (arrows). The pseudocolored area was reconstructed mathematically (SEM ×73). Figure 10. Mathematical reconstruction of the previous image. The same crack lines that are stopped by grain boundaries are visible (arrows).

Figures 5–10

Figure 5. Tip of short fatigue microcrack. The base of the microcrack shows transversal lines caused by the release of energy at the crack tip produced by mechanisms of plasticity-induced crack closure (arrows). At this level, the corrosion phenomenon is enhanced by the presence of both biological fluid and local metal strain (tensocorrosion) (SEM ×1120). Figure 6. Mathematical reconstruction of the previous image. The plastic deformation is more evident and is represented by transversal lines between the walls of the crack. Figure 7. The different pseudocolored areas show different grains, and the lines (arrows) on the grain surfaces represent crystallographic planes with the highest packing density (SEM ×3050). Figure 8. The fracture runs through the grains and tends to change direction at the grain boundary (arrows) (SEM ×1110). Figure 9. The transgranular fracture runs parallel to the fracture surface (arrows). The pseudocolored area was reconstructed mathematically (SEM ×73). Figure 10. Mathematical reconstruction of the previous image. The same crack lines that are stopped by grain boundaries are visible (arrows).

SEM investigation of etched specimens showed the following:

  • A mean grain size of 52 microns, with intragranular crack lines that ran along several crystallographic planes and that changed direction each time they crossed a grain boundary. Crack orientation tended to be at 90° with regard to the maximum principal stress (Figures 7 and 8).

Metallographic observation showed the following:

  • The presence of transgranular crack lines that ran parallel to the fracture surface (Figures 9 and 10), indicating a brittle behavior caused by external conditions (impact loading, presence of hole) or by an unfavorable material treatment.

Discussion

In previous papers,16,17 we have described the clinical, radiographic, and histologic features most frequently found in fractured implants. In this report, we focus, on the other hand, on the SEM fractography of fractured hollow implants.

Cleavage-type fractures are defined as rapid crack propagations along low packing density crystallographic planes (ie, where the number of links to be broken is lower and the interplanar space wider). These fractures are defined as “brittle” or “ductile” if a plastic sliding precedes them. The intracrystalline fracture changes its direction whenever it crosses a grain boundary, following the most favorably oriented crystalline plane in every grain crossing.19,21 Cleavage happens when the plastic sliding is prevented or is already over.

A correlation can be found between the type of crystalline net and the number of activated sliding planes.

Hexagonally structured polycrystalline materials (HCP), like Ti-α, have just 3 sliding planes per grain, but they still can break through cleavage. On the contrary, Ti-β is a body-centered, cube-shaped structure, and cleavage may happen along all planes. All fragile metals break through intracrystalline cleavage. Cleavage begins when the local stress level exceeds the material cohesive strength, breaking the links.21 A crystalline solid resists fracture according to the ratio E/π, where E is Young's modulus and π is a combination of stress and crack size inside the material's bulk. The relation shows a metal resistance directly proportional to the coefficient of elasticity of the metal and inversely related to the number of microcracks necessary to locally concentrate the stress. Nevertheless, a metal before it fractures generally undergoes a plastic deformation. The Griffith model, modified by Irwin and Orowan, establishes that, in materials capable of plastic deformation, the cleavage starts at the grain boundaries, in keeping with the formula: σf  =  2Es + γpa)1/2 with γs representing the total energy of atomic links broken per area unit, and γp the plastic work per surface unit; usually, γs is greater than γp.30 The grain size and the number of metal phases, generally comprising different grain sizes, play an important role. Moreover, when cyclic stresses are present, as in an oral environment, fatigue fractures are produced. In this case, crack growth, due to steady width cyclic load, yields a plastic or elastic deformation zone. A minimum number of loading cycles is necessary to enable the crack propagation, according to the Paris law:

 
formula

with c and m representing 2 material experimental constants and ΔK the plastic deformation region width able to condition directly the crack growth.30 Fatigue fractures are characterized by surface striations with small ridges perpendicular to crack propagation direction.19,21 Such striations rise at the crack apex because of the presence of slips with an inclination of ±45° in relation to the fracture plane: when the load decreases, the slip direction turns 180°; this crack growth mechanism was first reported by Friedel and Orlow.2729 On the macroscopic scale, similar cases are described in the literature: all present the fracture line located at the level of the end of the abutment screw and involving the holes that hence appear as loci minoris resistentiae, where stress due to fatigue can concentrate.13,16,17,19,21 

In our cases, no mechanical problems such as abutment screw loosening or crown decementation happened before the fracture events reported in the literature.18,22,24 Given the absence of some clinical risk factors, such as the presence of malocclusion and cantilevers, fracture may result from the association of biomechanical, structural, or electrochemical problems, listed below in order of increasing likelihood:

  1. High strain (force/surface ratio), according to the formula σ  =  F/S

  2. Defects of composition and structure of the material

  3. Material threshold value achievement dependent on temperature, according to the formula31  

  4. Tensocorrosion

  5. Electrochemical corrosion due to different red/ox potentials among materials in the oral cavity

Among all of these, tensocorrosion is particularly interesting in that it represents an accelerated partial dissolution of ions of a metal placed under tensile stress in a corrosive environment. As a result, the fatigue strength of a metal is decreased; this is also called corrosion fatigue.

This process is due to the biochemical composition of environmental fluids; in the living body and inside the abutment screw hold of the implant, biological fluids of different composition are present. The greatest difference is seen in the oxygen concentration, which is 3 times greater in the arterial blood than in the venous blood, but it is almost absent inside the screw hold cavity. Metal surfaces of osseointegrated implants are exposed to stress variations under repeated loading conditions; the surface oxide film protecting the substrate from corrosion is partially broken, thereby creating microcracks. If the oxide concentrations in the surrounding environment are adequate, an effective oxide film will be formed to protect the substrate from corrosion. During stress loading, however, corrosive fatigue is accelerated as the result of lowered oxide concentration.

Conclusion

Within the limitations of the present study, it could be concluded that under SEM analysis, the implant fracture surfaces appeared characterized mainly by typical signs of a cleavage-type fracture with transgranular path and striations. These fractures could be due to an association of factors such as fatigue, inner defects of materials, electrochemical problems, and tensocorrosion.

Abbreviations

     
  • HCP

    hexagonally structured polycrystalline

  •  
  • ITI

    International Team for oral Implantology Bonefit implant system

  •  
  • SEM

    scanning electron microscopy

Acknowledgments

This work was partially supported by the Ministry of Education, University and Research (M.I.U.R.), Rome, Italy, and by the National Research Council (C.N.R.), Rome, Italy.

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Author notes

1

Dental School, University of Pisa, Italy.

2

Dental School, University of Chieti, Italy.