Failure analysis of fractured dental zirconia implants

Transcription

1 M. Gahlert D. Burtscher I. Grunert H. Kniha E. Steinhauser Failure analysis of fractured dental zirconia implants Authors affiliations: M. Gahlert, H. Kniha, Private Dental Clinic, Munich, Germany D. Burtscher, I. Grunert, Clinical Department of Prosthetic Dentistry, Innsbruck Medical University, MZA, Innsbruck, Austria E. Steinhauser, Department of Precision and Micro- Engineering/Engineering Physics, Munich University of Applied Sciences, Munich, Germany Corresponding author: Dr. Michael Gahlert Private Dental Clinic Theatinerstr. 1 D Muenchen Germany Tel.: þ Fax: þ m.gahlert@knihagahlert.de Key words: failure analysis, fracture, mechanical failure, overloading occlusion, zirconia implant Abstract Objectives: The purpose of the present study was the macroscopic and microscopic failure analysis of fractured zirconia dental implants. Methods: Thirteen fractured one-piece zirconia implants (Z-Look3) out of 170 inserted implants with an average in situ period of months (range from 20 to 56 months, median 38 months) were prepared for macroscopic and microscopic (scanning electron microscopy [SEM]) failure analysis. These 170 implants were inserted in 79 patients. The patient histories were compared with fracture incidences to identify the reasons for the failure of the implants. Results: Twelve of these fractured implants had a diameter of 3.25 mm and one implant had a diameter of 4 mm. All fractured implants were located in the anterior side of the maxilla and mandibula. The patient with the fracture of the 4 mm diameter implant was adversely affected by strong bruxism. By failure analysis (SEM), it could be demonstrated that in all cases, mechanical overloading caused the fracture of the implants. Inhomogeneities and internal defects of the ceramic material could be excluded, but notches and scratches due to sandblasting of the surface led to local stress concentrations that led to the mentioned mechanical overloading by bending loads. Conclusions: The present study identified a fracture rate of nearly 10% within a follow-up period of months after prosthetic loading. Ninety-two per cent of the fractured implants were so-called diameter reduced implants (diameter 3.25 mm). These diameter reduced implants cannot be recommended for further clinical use. Improvement of the ceramic material and modification of the implant geometry has to be carried out to reduce the failure rate of small-sized ceramic implants. Nevertheless, due to the lack of appropriate laboratory testing, only clinical studies will demonstrate clearly whether and how far the failure rate can be reduced. Date: Accepted 19 March 2011 To cite this article: Gahlert M, Burtscher D, Grunert I, Kniha H, Steinhauser E. Failure analysis of fractured dental zirconia implants. Clin. Oral Impl. Res. 23, 2012; doi: /j x The long-term prognosis of endosseous oral implants has been documented extensively for fully and partially edentulous patients (Adell et al. 1990; Lekholm et al. 1994). The natural appearance of replaced single missing teeth in the aesthetic zone of the maxilla is still one of the most complex challenges in implant dentistry and requires extensive experience of the surgeon (Zitzmann & Marinello 1999; den Hartog et al. 2008). In this context, the discussion on dental zirconia implants has emerged increasingly in the last few years (Haubenreich et al. 2005; Wenz et al. 2007; Andreiotelli et al. 2009). Recently, increasingly more companies have been offering different dental implants made of zirconiumdioxid. The main reasons for clinical use are the biocompatible characteristics of the material (Ichikawa et al. 1992; Scarano et al. 2004), aesthetic aspects like tooth colour, the good scientific results in the comparable osseointegration to titanium implants (Sennerby et al. 2005; Gahlert et al. 2007, 2009; Kohal et al. 2009; Rocchietta et al. 2009) and the ability to be machined. Some authors also report on the good stability of one-piece zirconia dental implants in vitro and in vivo (Kohal et al. 2002; Oliva et al. 2007), but there are fewer clinical evaluations on the long-term results during the loading period of the new generation of zirconia dental implants (Mellinghoff 2006). One of the most undesirable complications in implant dentistry is the fracture of the implant. In comparison with reports on fractures of aluminium oxide ceramic implants (Schlegel et al. 1994), reports on the fractures of one-piece zirconia implants are rare (Mellinghoff 2006; Andreiotelli & Kohal 2009; Silva et al. 2009). The aim of this study was to evaluate the failure mechanism of 13 fractured dental zirconia implants by clinical, macroscopic and scanning 2011 John Wiley & Sons A/S 287

2 electron microscopic (SEM) methods. The implants were identified during a 3-year life-table analysis with a prosthetic evaluation (Burtscher et al. 2009; M. Gahlert, D. Burtscher, G. Pfundstein, H. Kühenhoff, I. Grunert & H. Kniha., unpublished data, 2010). Material and methods From October 2004 to September 2007, a total of 170 one-piece zirconia implants (Z-Look3, Z- Systems AG, Konstanz, Germany, Fig. 1) were inserted in 79 patients, whereby recruitment of the patients took place at random. The treatment covered all indications from single tooth gaps to fully edentulous jaws. Z-Look3 implants mainly consist of zirconiumdioxide (95%); yttrium and aluminium oxides contribute the rest of about 5% (DIN EN ; Z-Systems Technical Information Bulletin 2008). Implants were inserted in three different diameters of 3.25, 4 and 5 mm. Occlusal and functional relations had been analysed by clinical inspection. Thirteen of the 170 implants failed due to fracture after prosthetic loading within the monitoring period of average months (median 38 months) and a range from 20 to 56 months (Table 1). These implants were carefully removed from the bone stock by osteotomy and the implant fragments were clinically documented (Figs 2 and 3). All fractured implants were subjected to both macroscopic and microscopic failure analysis. Microscopic examination was based on light microscopy and SEM. SEM was performed using a CamScan Series 2 (Cambridge Scanning Company Ltd., Barhill, UK) at an electrode voltage of 20 kv in the secondary electron image mode. Before SEM, all implants were cleaned in an ultrasonic bath (3 min in ethanol, 5 min in distilled water, 3 min in ethanol), followed by thinfilm sputtering with gold. SEM-based fracture surface analysis was performed according to international standards (DIN EN ) and material scientific practice (Kelly et al. 1990; McCoy 2004). Results Twelve of these 13 fractured implants had been so-called diameter reduced implants measuring 3.25 mm in diameter (smallest available implant size); one implant had a diameter of 4 mm. All fractures occurred within a period between eight and 26 months after prosthetic loading (Table 1) in the frontal region of the incisors and premolars in the maxilla and mandibula. All restorations except one, which had been designed as a bridge placed on two implants in the mandible, had been implanted into the maxilla. Beneath the already mentioned implant-based bridge restoration, Fig. 1. Zirconia dental implant (Z-Look3) in two versions: 4 mm diameter (left) and diameter reduced to 3.25 mm (right). Fig. 2. Photographs showing different types of implant-based fractured restorations. Single tooth replacement (left) and a fourunit bridge (right). Table 1. Implant and patient boundary conditions referring to fractured implants Patient ID Implant position Implant diameter (mm) Implant alignment Jaw relation Time to failure after denture fixation (months) # Marginal to buccal Deep bite 12 # Marginal to buccal Normal bite 16 # Neutral Normal bite 11 # Marginal to buccal Normal bite 25 # Neutral Cross bite 26 # Marginal to buccal Normal bite 10 # Neutral Normal bite 13 # Neutral Normal bite 9 # Neutral Normal bite, bruxism 8 # Neutral Normal bite, bruxism 11 #10 32, Neutral Edge-to-edge bite 10 # Marginal to buccal Edge-to-edge bite Clin. Oral Impl. Res. 23, 2012 / John Wiley & Sons A/S

3 Fig. 3. Clinical situation after fracture of the dental zirconia implant. The shape of the fracture surface indicates that the fracture occurred due to a bending moment directed from palatal towards buccal. The implant was well osseointegrated and hadtoberemovedbymillingtoallowexplantation. Fig. 4. Lateral macroscopic view of the fracture area (patient #1). Incipient crack at the level of the first thread (marked with white arrow) another implant served as a cantilever bridge post (patient #6, Table 1); the majority of the fractured implants (n ¼ 10) served as a single tooth replacement (Figs 1 and 2). Out of the eleven patients affected by implant fracture, seven showed a normal jaw relation, two an edge-to-edge bite, oneacrossbiteandoneadeepbite.onepatient with a normal bite situation suffered from bruxism. Based on the alignment and shape of the fracture surfaces already by macroscopic and light microscopic examination, it could be shown that most implants failed due to bending loads (implant of patient #1 exemplarily shown in Fig. 4). The direction of crack propagation was always from palatal, respectively, lingual towards buccal. This loading situation corresponds, e.g. to the classical bite off an apple. Incipient crack was located in 10 of the investigated 13 implants at the level of the first turn of the thread. This indicates clearly that these implants have been well osseointegrated. At one single postimplant (patient #5), the incipient crack started at the third turn of the thread (Fig. 5); at the two-post-based bridge construction (patient #10) incipient cracks were located at the fifth and eighth turn, respectively. For these specific implants, the shift of the incipient crack in the apical direction indicates that bone resorption had taken place. The location of the crack corresponds with implant integration into the bone because the highest bending moment, respectively, bending stress acts at the transition of the implant into the bone stock. In Table 2, information on the fracture type, incipient cracking and the direction of crack propagation is summarized. Macroscopic and light microscopic examinations gave no evidence that fracture of the implants occurred due to machining marks that might have been created for the connection of crowns or frameworks after implantation. By SEM, all implants were investigated for potential material defects, machining marks and the location of the onset of cracking. SEM examinations confirmed that all fractures occurred due to singular bending overload (so-called forced rupture). Material failures like pores or inclusions that are typical for ceramic materials were not found. In Figs 6 and 7, the fracture surfaces of two implants are shown. The structure of the fracture surface indicates forced rupture due to bending in a direction from the palatal towards the buccal. The structure of the fracture surface allowed the identification of the onset of the cracking site. The absence of so-called mirror zones around the crack origins (Kelly et al. 1990) indicated that no stable growth of any cracks occurred. Thus, fatigue of the implants could be excluded; all implants failed due to overload breakage (forced rupture). All onsets were placed in the lowest part of the grooves of the thread. The threaded part of the implants was roughened by sandblasting. Figs 8 and 9 display in their lower parts the outer surface of the implant with a typical sandblasted structure. As sandblasting is performed by blowing hard and edged ceramic particles with a high velocity onto the surface erratic grooves, rills and notches were formed. In Figs 8 and 9, it can be seen that the onset of the fracture of an implant (patient #11) is located at such a groove. This resulted in a twofold concentration of mechanical stresses that led to local overloading of the material and subsequent onset of cracking. Firstly, the grooves of the thread caused concentration and multiplication of mechanical stresses. Secondly, the micro-notches generated by the sandblasting process additionally led to stress concentrations due to their notching effect. The failure mechanism of a forced rupture was seen in all implants. Fatigue failure typical for metallic implant materials and therefore the purpose of normative preclinical implant test methods like ISO (2007) could be excluded. Discussion Fracture of a dental implant is always a severe complication that leads to a high level of patient 2011 John Wiley & Sons A/S 289 Clin. Oral Impl. Res. 23, 2012 /

4 Fig. 5. Lateral view of an implant (patient #5) showing a fracture path that indicates failure due to bending. Incipient crack at the level of the third turn of thread (marked by white arrow). Table 2. Summary to failure mode, incipient crack and crack propagation Patient ID Implant position Failure loading case Crack propagation Incipient crack #1 21 Bending Palatal buccal 1st turn of thread #2 12 Bending Palatal buccal 1st turn of thread #3 21 Bending Palatal buccal 1st turn of thread #4 21 Bending Palatal buccal 1st turn of thread #5 25 Bending Palatal buccal 3rd turn of thread #6 21 Bending Palatal buccal 1st turn of thread #7 24 Bending Palatal buccal 1st turn of thread #8 24 Bending Palatal buccal 1st turn of thread #9 22 Bending Palatal buccal 1st turn of thread #9 13 Bending Palatal buccal 1st turn of thread #10 32, 42 Bending Lingual buccal 5th (32) and 8th (42) turn of thread #11 12 Bending Palatal buccal 1st turn of thread Fig. 6. Scanning electron microscopy image of the fracture surface of an implant (patient #7). Crack propagation (indicated by white arrow) due to bending clearly noticeable by the propagation pattern on the fracture surface. discomfort and clinical problems like bone loss. The reasons for implant fractures are well described in the literature, but most of the studies deal with titanium implants (Eckert et al. 2000; Velasquez-Plata et al. 2002; Virdee & Bishop 2007). In these studies, implant fractures are related to inadequate implant design and manufacturing defects (Green et al. 2002; Stuebinger et al. 2004; Manda et al. 2008), non-passive fit of the prosthetic framework (Green et al. 2002; Stuebinger et al. 2004), physiologic or biomechanical overloading (Rangert et al. 1995; Balshi 1996; Stuebinger et al. 2004), diameter reduced implants in the posterior area (Binon 2000; Eckert et al. 2000) and bone resorption (Rangert et al. 1995; Piatelli et al. 1998). Only a few studies provide information on the mechanical failures of one-piece dental zirconia implants. However, Mellinghoff (2006) reported on 189 implants that were identical to those used in this study (Z-Look3, Z-Systems AG) over an 8.2-month mean follow-up period. Out of these 189 implants, one implant fractured within the first week after prosthetic treatment. Another study on one-piece zirconia implants published by Andreiotelli & Kohal (2009) reports that in vitro preparation of these implants leads to a statistically significant negative influence on the fracture strength of the implant. Experimental tests on this aspect were performed under combined axial and bending loading. In contrast to the study of Andreiotelli and Kohal, other authors and the implant manufacturer of the Z- System allow the preparation of the implant if grinding is carried out according to a strict guidance protocol using rotating diamond instruments (Oliva et al. 2007; Silva et al. 2009). But there are no references on clinical evaluation of the mechanical performance and fracture strength of the machined material. In general, such mechanical preparations are discussed controversially. In the present study, failure analysis showed clearly that the occurrence of the fractures was never affected by intra- or postoperative preparation. Inthepresentstudy,afracturerateofnearly 10% (13 fractured implants out of a total of 170 implants) within a follow-up period of mean months (minimum 20 up to maximum 56 months) triggered a systematic analysis of the failed implants. Twelve of the 13 fractured implants had a diameter of 3.25 mm, the smallest size available within this implant system (Z- Look3, Z-Systems AG). Only one failed implant had a diameter of 4 mm. The observed fracture rate of almost 10% seems to be in contrast to the publication of Mellinghoff (2006). But Mellinghoff reported on a short follow-up period of only 8.2 months and the present study has a mean follow-up period of months. All fractures 290 Clin. Oral Impl. Res. 23, 2012 / John Wiley & Sons A/S

5 Fig. 7. Scanning electron microscopy image of the fracture surface of an implant (patient #11). Beginning area of the crack (marked by black arrow) is located in the ground of the first turn of the thread. Crack propagation (indicated by white arrow) from palatal towards buccal due to bending. Fracture surface Surface of thread (sandblasted) Fig. 8. Scanning electron microscopy detail of Fig. 7. Onset of the crack (marked with black oval) in the lowest part of the groove of the first turn of the threaded part of the implant. Fracture surface Surface of thread (sandblasted) Fig. 9. Scanning electron microscopy detail of Fig. 8. Crack started from a notch (marked with a black oval) that was generated by the sandblasting of the implant surface. observed in this study occurred between eight and 26 months after prosthetic restoration, might explain the difference from the low fracture rate observed by Mellinghoff (2006). The manufacturer of the Z-Look3 implant system reported in the year 2008 about 0.3% fractured implants, whereby most of these had been implanted in extreme positions (Z-Newsletter 2008). The cause for the discrepancy in the fracture rates of nearly 10% in the present study, respectively, 0.3% reported by the manufacturer cannot be explained by the authors as yet. Failure analysis of the 13 fractured implants identified bending loads directed from palatal, respectively, lingual towards buccal to be the cause of damage. Alignment of the implants was mostly neutral; five implants had been placed in a position marginal towards buccal. The Z-Look3 implant system had obtained both European and US-American market accreditation (e.g. FDA approval no. K by 8 October 2007). Even though for market approval intensive laboratory mechanical testing was performed, implant fractures accumulated at the smallest implant size (3.25 mm). This leads to the question of whether there exist reliable preclinical test protocols to predict clearly the in vivo performance of endosseous dental implants by preclinical testing. The ISO standard is an international normative standard that describes how to perform dynamic fatigue testing for single postendosseous dental implants (ISO ). In this test standard, e.g. detailed information is provided on load application onto an implant without preangled connecting parts under an angle of (30 2)1 to the longitudinal axis of the implant. The test simulates a representative case with respect to 3 mm bone loss as a worst-case for the mechanical loading of the implant. In the patient population of our study, 10 out of 13 fractured implants had been fully osseous integrated before fracture, which corresponds with the incipient crack at the level of the first turn of the threaded part of the implant (Table 2). Only three implants fractured at the level of the third, fifth and eighth turn of the thread (Table 2) and therefore had been higher loaded due to bone loss. In preclinical testing according to ISO 14801, implants shall withstand 2 million loading cycles. But this standard does not contain any performance criteria concerning a peak load limit [ISO 14801:2007]. Usually, minimum release criteria of the implant manufacturers are in the region of 200 up to 240 N. Fontijn-Tekamp et al. (2000) and colleagues published physiological biting and chewing forces that range from 60 to 75 N in the anterior dentition and from 110 to 125 N in the posterior dentition. But in maximum, the forces ranged from 140 to 170 N in 2011 John Wiley & Sons A/S 291 Clin. Oral Impl. Res. 23, 2012 /

6 the anterior dentition and from 250 to 400 N in the posterior region. The ISO test standard aims at comparisons of different implant designs, sizes and materials and is not applicable for a prediction of the later in vivo performance of an implant. Reasons therefore are the wide variability of the loading conditions in vivo and also the wide range of implant use, e.g. as single postimplant or use in combination with other implants or reconstructions. Therefore, it has to be stated that no preclinical test procedure exists that can predict clearly a risk of in vivo implant fractures. The results of the present study require further material improvements as published by Kohal et al. (2010). Kohal and colleagues showed, by an in vitro study, that alumina toughened zirconia implants have an increased mechanical stability compared with tetragonal zirconium dioxide polycrystal implants. By modifications of the implant geometry, especially at the highly loaded neck and abandonment of sandblasting at this region, an improvement of the implant strength is possible due to the reduction of detrimental stress concentrations at notches. Nevertheless, only clinical studies will demonstrate clearly how far the failure rate of these specific one-piece zirconia dental implants will decline. Acknowledgements: Conflict of Interest: No benefits have been or will be received from a commercial party related directly or indirectly to the subject matter of this article. References Adell, R., Eriksson, B., Lekholm, U., Branemark, P.I. & Jemt, T. (1990) Longterm follow-up study of osseointegrated implants in the treatment of totally edentulous jaws. The International Journal of Oral & Maxillofacial Implants 5: Andreiotelli, M. & Kohal, R.J. (2009) Fracture strength of zirconia implants after artificial aging. 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