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Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering
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Journal of Prosthodontic Research journal homepage: www.elsevier.com/locate/jpor
Original article
Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering Josef Schweiger* , Jan-Frederik Güth, Kurt-Jürgen Erdelt, Daniel Edelhoff, Oliver Schubert Department of Prosthetic Dentistry, University Hospital, LMU Munich, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 March 2019 Received in revised form 4 July 2019 Accepted 9 July 2019 Available online xxx
Purpose: The purpose of this study was to evaluate internal porosities, retentive force values and survival of cobalt–chromium (Co–Cr) alloy clasps fabricated by direct metal laser-sintering (DMLS) and compare them to conventionally cast clasps. Methods: Embrasure clasps were digitally designed fitting teeth 35 and 36 on identical metal models (N = 32). Sixteen clasps were fabricated using DMLS (group DMLS) and another sixteen clasps were additively manufactured from wax and then cast from a Co–Cr alloy (group CAST). Internal porosities were examined using micro-focus X-ray (micro-CT) and analyzed applying Kolmogorov–Smirnov test, Mann–Whitney test, and T test (significance level: p < 0.050). A universal testing machine was used to determine the retentive force values at baseline and after 1095, 5475, 10,950 and 65,000 cycles of simulated aging. Data were analyzed employing Kolmogorov–Smirnov test, one-way ANOVA, and Scheffé’s post-hoc test (significance level: p < 0.050). Survival was estimated for 65,000 cycles of artificial aging using Kaplan–Meier analysis. Results: Micro-CT analysis revealed a higher prevalence (p < 0.001), but a more homogeneous size and a significantly smaller mean (p = 0.009) and total volume (p < 0.001) of internal porosities for group DMLS. The groups showed mean initial retentive force values of 13.57 N (CAST) and 15.74 N (DMLS), which significantly declined over aging for group CAST (p = 0.003), but not for group DMLS (p = 0.107). Survival was considerably higher for group DMLS (93.8%) than for group CAST (43.8%) after 65,000 cycles of aging. Conclusions: Clasps made by laser-sintering could be an alternative to conventional cast clasps for the fabrication of removable partial denture frameworks. © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Keywords: Additive manufacturing CAD/CAM Clasp Removable partial denture Retentive force
1. Introduction Dentist Dr. F.E. Roach noted in 1930 that “[ . . . ] the clasp is the oldest and, [ . . . ] still is, and probably will continue to be, the most practical and popular means of anchoring partial dentures of the removable type” [1]. This statement is as topical in 2019 as it was almost a hundred years ago. Despite considerable progress in dental prophylaxis and modern operative, restorative, prosthetic, and implant dentistry, the removable partial denture (RPD) continues to be a reliable treatment option for a large percentage of patients. Due to increasing average life expectancy and decreasing tooth loss, requirements will tend to change from removable full dentures to RPDs, and demand is expected to increase in the future [2–4].
* Corresponding author at: Department of Prosthetic Dentistry, University Hospital, LMU Munich, Goethestraße 70, D-80336 Munich, Germany. E-mail address: [email protected] (J. Schweiger).
Being cost-effective, highly versatile [4], and commonly satisfactory to the patient regarding the overall treatment outcome [5], the RPD must be considered better than its general reputation. On the downside, there are inherent drawbacks in terms of aesthetics or biomechanical problems such as non-physiological loading [6], sensitivity and wear of abutment teeth caused by the denture [5], but also an increased susceptibility to root caries and periodontal health issues [7]. Common technical complications such as deformation and fatigue fracture of RPD clasps result in loss of retention and function and cause the need for extensive reworking and additional expenses [8,9]. To avoid adverse events, diligent treatment planning and procedure, proper aftercare, optimum hygiene, but equally important, selection of a sophisticated technique and adequate materials are fundamental preconditions for long-term success. Co–Cr alloys are a material of choice for the fabrication of RPD frameworks [10]. They have been widely used in dental applications combining excellent biocompatibility, favorable physicochemical properties [11], and low expenses. Nowadays RPD framework, that is, clasps, connectors, and support elements, is still commonly fabricated using analogue casting methods, in particular the lost-wax technique.
https://doi.org/10.1016/j.jpor.2019.07.006 1883-1958/ © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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Fig. 1. Experimental setup of the study.
The ongoing computerization of dentistry and dental technology not only creates innovative workflows [12], but also alters longstanding treatment concepts that have proven reliable for ages. Dental CAD/CAM (computer-aided design/computer-aided manufacturing) offers multiple advantages concerning improved efficiency in the dental laboratory, the application of innovative materials, and enhanced quality management, resulting in more predictability, improved longevity and reproducibility of the denture [13]. Recently, additive or generative production technologies are gaining attention, not only in the fabrication of dental models [14] or surgical templates [15], but also in the production of metal denture frameworks [16]. Additive manufacturing (AM) is highly productive but resource-efficient at the same time [12], ensuring high density and thus homogeneity of the workpiece, but also effecting considerable thermal distortion [16,17]. Selective laser-sintering (SLS) is an umbrella term describing an AM process that entails the sequentially building of material layers by fusing powder particles using laser technology. DMLS technique again is used to process metallic materials in this way [16]. Co–Cr alloys processed by means of additive manufacturing present favorable biocompatibility [18,19] as well as superior mechanical properties compared to cast alloys [19–22]. The development of accuracy is assessed positive, but still below that of subtractive manufacturing [23]. Apart from that, it must be noted that the metallurgical characteristics of any additively manufactured alloys differ significantly from that of their conventionally produced counterparts in many respects [24]. DMLS took its first steps into the fabrication of RPDs, when Williams et al. [25] processed a Co–Cr removable partial denture framework by means of this technology for clinical use in 2006. Both patient and practitioner judged the outcome satisfactory [25]. Bibb et al. [26,27] used SLM (selective laser-melting) to produce RPD framework in the same year and found accuracy and fit to be clinically adequate [25]. Since then, several investigations have dedicated to laser-sintering in RPD framework production addressing various aspects of the matter [17,19,28–37]. Campbell et al. [4] stated in a recent publication that there is need for RPD treatment strategies to evolve by, inter alia, implementing digital technologies and new materials. CAD/CAM fabricated clasps are expected to provide the preferable
characteristics provided by cast claps, but, moreover, to overcome some of the detriments of the conventional technology. Therefore, the digital approach must be subject to comprehensive in vitro research before clinical application. The intention of this study is therefore to evaluate internal porosities as well as the retentive force and survival of Co–Cr alloy embrasure clasps fabricated by DMLS and compare them to cast clasps. The null hypotheses were that there will be no difference concerning internal porosities, retentive force, and survival of laser-sintered clasps and the conventionally cast ones. Furthermore, it was hypothesized that artificial aging has no impact on the retentive force values of both experimental groups over the period of artificial aging. 2. Materials and methods 2.1. Fabrication of the specimens The experimental setup of the study is presented in Fig. 1. Models manufactured from a Co–Cr alloy (N = 32) were digitized and embrasure clasps were designed (CAD-Software Dental Designer RPD, 3Shape, Copenhagen, Denmark) for each model fitting teeth 35 and 36 (Fig. 2). The geometry of an embrasure clasp had been applied before [10], representing a close to in vivo setup. Undercut depths were determined using a surveyor (Ney Surveyor, Dentsply, York, PA, USA) [38] and modified matching non-precious metal alloys. The clasps were fabricated applying conventional casting (group CAST; N = 16) and DMLS technology (group DMLS; N = 16). The specimens from group CAST were initially fabricated from wax by means of AM (ProJet PD 3000, 3D-Systems, Valencia, CA, USA) and subsequently cast from Co–Cr (Remanium GM 800, Dentaurum, Pforzheim, Germany) applying the lost-wax technique. The clasps of group DMLS were produced from a Co–Cr alloy powder (EOS Cobalt Chrome RPD, EOS, Krailling, Germany) using the EOSINT M270 system (EOS) [19,34]. The particle size was 40 mm. An “offset”, meaning the gap between clasps and teeth, of +40 mm was chosen for group DMLS. Heat treatment included a 30 min stress relief annealing at 1000 C and a solution annealing at 1150 C for 20 min. Further post-processing included manual removing of support structures, and finalization by one trained certified dental technician.
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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Fig. 2. Computer-aided design of the embrasure clasp. The supporting connectors attached to the tips of the clasps were removed during post-processing.
2.2. Evaluation of internal porosity All clasps were inspected using micro-CT (WENZEL exaCT S75 HRE, Wenzel Volumetrics, Balingen, Germany) in order to determine prevalence, average volume, and total volume of internal porosities per clasp. Examination was conducted employing exaCT analysis-software (Wenzel Volumetrics). Statistical analysis was performed applying SPSS software (Statistics 23.0, SPSS Inc., Stanford, CA, USA). Level of significance was set at p < 0.05. Kolmogorov–Smirnov test, and in effect, T test or Mann– Whitney test were utilized. Homogeneity of variance was tested applying Levene’s test. 2.3. Retentive force measurements and artificial aging The retentive force values were determined at a separation speed of 1000 mm/min using a universal testing machine (Type 1445, Zwick/Roell, Ulm, Germany). After baseline testing, all specimens were subjected to artificial aging in a chewing simulator (Chewing Simulator CS-4, Mechatronic, Feldkirchen-Westerham, Germany), undergoing 1095 aging cycles (representing 1 year of clinical service with 3 times denture removal per day) and simultaneous thermocycling (5 C/55 C). During cycling, joining and separation speed were set at 20 mm/s and 60 mm/s respectively, and force was 50 N. Retentive force measurements were conducted and subsequently repeated after 5475 cycles (5 years), 10,950 cycles (10 years), and 65,000 cycles (60 years). As specimens dropped out due to failure, the measurements at 65,000 cycles were conducted with 15 (group DMLS) and 6 specimens (group CAST). For statistical evaluation (SPSS software, Statistics 23.0), level of significance was set at p < 0.05. Kolmogorov– Smirnov test, one-way ANOVA, and Scheffé post-hoc test were applied. 2.4. Estimation of survival Survival was estimated over 65,000 cycles of artificial aging (simulating 60 years of clinical service) using Kaplan–Meier analysis. After every 2500 cycles, aging was interrupted and adverse events, i.e. clasp failure/fracture, were registered. The Kaplan–Meier estimator is a non-parametric statistic tool used to approach the survival function from lifetime data [39]. 3. Results 3.1. Internal porosity Significance analysis revealed a higher prevalence (1059 porosities in group DLMS; 787 porosities in group CAST;
Fig. 3. Total number and volume distributions of internal porosities in group CAST (a) and group DLMS (b).
p < 0.001), but more uniform size of internal porosities for group DMLS (Fig. 3). Kolmogorov–Smirnov test and non-parametric Mann–Whitney test were applied. Average (p = 0.009) and total porosity volume per clasp (p < 0.001) was significantly larger in group CAST (Fig. 4). Kolmogorov–Smirnov test, and in effect, T test (average porosity volume) and Mann–Whitney test (total porosity volume) were utilized. Levene’s test revealed a violation of homogeneity of variance for average porosity volume. 3.2. Retentive force values Retentive force values for groups CAST and DLMS are given in and Fig. 5. Group CAST showed mean initial retentive force values of 13.57 N and group DMLS of 15.74 N. The values significantly changed over simulated aging for group CAST (p = 0.003), but not for group DMLS (p = 0.107), resulting in values of 6.41 N (group CAST) and 12.28 N (group DMLS) after 65,000 cycles of artificial aging. Kolmogorov–Smirnov test and one-way ANOVA were applied. Scheffé post-hoc test confirmed a noticeable difference of retentive force values for group CAST after 65,000 cycles compared to all other stages of aging.
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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Fig. 6. Kaplan–Meier curve estimating survival over 65,000 cycles of simulated aging for both testing groups.
Fig. 4. Average (a) and total volume per clasp of internal porosities (b).
Fig. 5. Retentive force values of groups CAST and DMLS at baseline and over time.
3.3. Survival The calculated survival probability over 65,000 cycles of artificial aging was 43.8% for group CAST and 93.8% for group DMLS (Fig. 6). For group CAST nine adverse events were recorded (1 after 25,000, 2 after 27,500, 1 after 30,000, 1 after 45,000, 3 after 62,500, and 1 after 65,000 cycles). In group DMLS one failure occurred after 37,500 cycles. All adverse events were fractures of upper clasp arms, but no specific fracture patterns could be distinguished (Fig. 7a and b). 4. Discussion This study compared Co–Cr alloy embrasure clasps fabricated by DMLS and cast clasps by means of internal porosities, retentive force values over time, and the survival rate. The first part of the hypothesis must be rejected, as there was in fact a lower prevalence but more disparate size and a significantly larger mean and total volume of internal porosities for group CAST. The artificial aging revealed significant influence on retentive force
values of the clasps of group CAST, while group DMLS presented stable values over the time of simulated aging. Therefore, the hypothesis can be partly supported and partly refuted with respect to retentive force values. Concerning survival, the hypothesis can be rejected as group CAST displayed a markedly lower survival rate than group DMLS. Because additive manufacturing allows for the efficient and accurate standardized production of complex geometries, substantially saving time and resources, and continuous progress is made, ever new dental applications arise. Studies that investigated RPD framework manufactured using different AM strategies reported that the technology might be a promising option for this area of application [19,26,27,30–32,34–37]. The retentive force of RPD clasps grows with increasing cross section of the retentive clasp arm, undercut depth, and young’s modulus, and decreases with the length of the clasp arm. Especially a high degree of toughness and bending stiffness are required for adequate long-time performance [40]. Another intrinsic factor that becomes crucial in computer assisted fabrication is the software-based definition of production parameters [41]. Particularly determining the offset, i.e., the gap between underlying surface and workpiece, is a specificity of CAD/CAM production and of vital importance. Addressing this topic, pilot testing was conducted before embarking on the actual project in order to define production parameters that would generate initial retentive force values of the DLMS clasps similar to those of the CAST specimens. Eventually, an offset of +40 mm was found to be fit, meaning that the inner surfaces of the digitally designed clasps penetrated the virtual surface of the teeth by that amount. Choosing the design of an embrasure clasp [10] was considered suitable in order to create a clinically relevant situation, and high stress and fatigue exposure for the materials. Undercut depth was set at 0.2 for premolar clasps and 0.25 mm for molar clasps and thus in a range corresponding with preceding investigations [10,32,34,36]. Microstructure and porosity characteristics of laser-sintered clasps depend on multiple factors such as strength and speed of the laser, layer thickness, and the powder material used [30], and can be controlled by adjusting these parameters [42]. It has been demonstrated that cast clasps and SLM fabricated clasps that show similar mechanical properties differ significantly concerning their microstructure [30]. Alageel et al. [19] used micro-CT and found minimum porosity in cast and laser-sintered clasps at all. In line with the current investigation, they located considerably more pores in the DMLS clasps, but with a smaller average volume and a more homogeneous distribution [19]. Porosities of cast
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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Fig. 7. Fractured clasp from group CAST (a). SEM picture of the fractured surface with 100 magnification (b).
frameworks mainly result from shrinkage during solidification and pressure caused by dissolved gases [43], displaying a heterogeneous distribution of defects. Pore formation in laser-sintered clasps are depending on fabrication parameters, for instance, a long laser exposure time [11] or incompletely melted metal powders [36], showing a more uniform pattern. Separation speed values in retentive force testing varied widely between 10 mm/min [44,45], 50 mm/min [32,36,40], and 300 mm/ min [33] in similar investigations. Since it cannot be ruled out that the separation speed has a long-term impact on retentive behavior, and researchers found in another context that a clinically relevant separation speed is up to 6000 mm/min for RPDs [46], a higher separation speed of 1000 mm/min was chosen to mimic conditions close to clinical reality. Although about 10,000 separation cycles representing 10 years of clinical service have proven sufficient to receive sound results in retentive force testing [31,32], the authors of the present study have agreed to go beyond. The reason for that was to obtain meaningful results, considering that earlier research saw Co–Cr clasps fail not before 20,000 cycles, although exposed to higher stress [47]. Moreover, it can be observed that in vivo clinical usage of RPDs is often characterized by non-uniform modes of insertion and removal and thus by even heavier mechanical strain. Investigations dealing with the retentive force of RPD clasps generate heterogeneous results in general. This is mostly due to the disparate experimental setups concerning the type and geometry of the clasps or the material of the models. It is therefore less important which absolute retentive force values are provided at baseline, but how they alter over time. As it has been stated that about 5 N might be an adequate retentive force for one RPD clasp [48,49], retentive force values should not fall below. When Arda and Arikan [44] tested the retentive forces of cast Akers clasps on
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molars (M) and premolars (PM), values of Co–Cr alloy specimens dropped from between 5.6 0.04 N (M) and 6.15 0.15 N (PM) to 2.6 0.33 N (M) and 2.8 0.29 N (PM) after only 3 years of simulated aging [44]. Declining retentive force values are commonly observed in cast clasps [40,45], which coincides with the results found in this study. However, no decrease was found up until 10 years of simulated service. This again is in accordance with other researchers who found slightly increasing retentive force values in cast Co–Cr embrasure clasps over 5 years of cyclic loading [10]. Concerning retentive force values in CAM produced clasps there is only few evidences. Nakata et al. [32] tested cast Akers clasps and clasps fabricated by repeated laser-sintering and high-speed milling, a methodology that is thought combine the advantages of subtractive and additive manufacturing to enable high accuracy and surface qualities forgoing extensive manual post-processing. The researchers found the initial retentive force values to be 12.9 3.5 N for the cast Co–Cr clasps and 12.3 2.6 N for the lasersintered/milled specimens. Values dropped for all groups after aging, though the CAM clasps exhibited a significantly smaller decline [32]. Torii et al. [35] found a similar pattern when investigating Akers clasps that were fabricated in a comparable way. The four groups of CAM clasps exhibited baseline retentive force values of 16.1 0.8 N, 12.3 2.6 N, and 21.5 5.3 N, depending on different manufacturing parameters, and the cast clasps demonstrated an initial retentive force of 12.9 3.5 N. The values of the CAM clasp groups dropped by 14.3 to 30.8% after 10,000 cycles of aging, whereas the cast specimens declined by 41.1% of their retentive performance. The authors also pointed at the influence of software parameters such as the offset. The evidence that cast clasps tend to lose more of their retentive force over time is in line with the findings of the present study, where the AM manufactured clasps showed initial retentive force values of 15.74 2.76 N which dropped by 21.98% to 12.28 4.79 N after 65,000 cycles of aging. The cast specimens went from 13.57 2.88 N to 6.41 1.29 N and thus lost 52.76% of their retentive force in the same period. For up to 10 years, the retentive force values were relatively constant for both groups. This phenomenon might be attributed to the highly standardized experimental setup, the relatively small undercut depth, and the smooth surface of the Co–Cr models. The superior structural homogeneity described above might be an explanation for the smaller standard deviations found in most of the retentive force values of the DMLS group. With regard to survival, Alageel et al. [19] proved that the microstructure homogeneity of DMLS produced clasps leads to a higher fatigue resistance, which might also explain the current results. Kim et al. [34] demonstrated that clasps fabricated using DMLS were more flexible and therefore less prone to cyclic fatigue. As the first fracture in the present study occurred after more than 20 years of simulated service at all, it can nevertheless be deducted that both fabrication technologies are reliable concerning longtime service. In principle, it can be observed that even if a trained expert executes the manual processing steps within a merely digital workflow under standardized conditions, they represent a critical source of error, whereas the CAM processes seem more predictable [50]. In other words, forgoing manual processing steps using CAD/ CAM techniques is likely to provide higher precision. This is also to be expected in the AM fabrication of RPD framework, representing tangible benefit. Another major advantage of the CAD/CAM fabrication of RPD framework is the ease of reproduction from stored data in the event of failure. Hence milling is likely to produce smoother surfaces and higher accuracy than AM fabrication [23,51] the combination of both technologies [32,36,51] might be a reasonable approach. However,
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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further simplifying the methodology could be a long-term objective. Researchers repeatedly emphasized that not only CAM technology but also specialized and comprehensive CAD software needs to be further developed [28,29]. An optimum construction software must provide tools that allow for the exact calculation of the retentive force for every clasp and in total, and the best possible common insertion direction. This should be done considering crown-to-root ratio, tooth mobility, and individual anatomical characteristics. The software is also expected to enable the design of reciprocal non-retentive arms averting any lateral deflection of the tooth during insertion and removal. Therefore, forthcoming evolution must focus on materials, technology, and software in order to facilitate reasonable in vivo application of the technique. 5. Conclusion Within the limitations of this investigations, the following conclusions can be deduced: 1. Laser-sintered (DMLS) clasps displayed a smaller volume and a more homogeneous distribution of internal porosities compared to the cast specimens. 2. The retentive force values of the DMLS clasps showed superior consistency over time. 3. The long-term survival of the DMLS specimens was considerably higher than that of the cast clasps. Based on these findings, additively fabricated RPD framework might offer a real alternative to conventional cast framework, propelling removable partial prosthodontics into the digital era. As there is currently no software available that provides all the tools desirable and reasonable to reliably achieve the retentive behavior easily and precisely as requested, future efforts should address this topic. Apart from that, research on the long-term performance in a clinical setting is needed. Conflict of interest The authors declare that there is no conflict of interest. Acknowledgments The authors thank EOS GmbH for the research grants received in the course of this study. References [1] Roach FE. Principles and essentials of bar clasp partial dentures. J Am Dent Assoc 1930;17:124–38. [2] Douglass CW, Watson AJ. Future needs for fixed and removable partial dentures in the United States. J Prosthet Dent 2002;87:9–14. [3] Hummel SK, Wilson MA, Marker VA, Nunn ME. Quality of removable partial dentures worn by the adult U.S. population. J Prosthet Dent 2002;88:37–43. [4] Campbell SD, Cooper L, Craddock H, Hyde TP, Nattress B, Pavitt SH, et al. Removable partial dentures: the clinical need for innovation. J Prosthet Dent 2017;118(1–4):273–80. [5] Frank RP, Milgrom P, Leroux BG, Hawkins NR. Treatment outcomes with mandibular removable partial dentures: a population-based study of patient satisfaction. J Prosthet Dent 1998;80:36–45. [6] Maxfield JB, Nicholls JI, Smith DE. The measurement of forces transmitted to abutment teeth of removable partial dentures. J Prosthet Dent 1979;41:134–42. [7] Mojon P, Rentsch A, Budtz-Jørgensen E. Relationship between prosthodontic status, caries, and periodontal disease in a geriatric population. Int J Prosthodont 1995;8:564–71. [8] Hofmann E, Behr M, Handel G. Frequency and costs of technical failures of clasp- and double crown-retained removable partial dentures. Clin Oral Investig 2002;6:104–8. [9] Saito M, Notani K, Miura Y, Kawasaki T. Complications and failures in removable partial dentures: a clinical evaluation. J Oral Rehabil 2002;29:627–33.
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Effect of two heat treatments on mechanical properties of selective-laser-melted Co-Cr metal-ceramic alloys for application in thin removable partial dentures. J Prosthet Dent 2018;119:1028.e1–6. [37] Soltanzadeh P, Suprono MS, Kattadiyil MT, Goodacre C, Gregorius W. An in vitro investigation of accuracy and fit of conventional and CAD/CAM removable partial denture frameworks. J Prosthodont 2019;28:547–55, doi: http://dx.doi.org/10.1111/jopr.12997. [38] JM Ney Company. Planned partials: the combined edition of the ney surveyor book, the ney partial denture book. University of Michigan; 1963. [39] Meier P. Nonparametric estimation from incomplete observations AU—Kaplan, E. L. J Am Dent Assoc 1958;53:457–81.
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[46] Ohkawa S, Okane H, Nagasawa T, Tsuru H. Changes in retention of various telescope crown assemblies over long-term use. J Prosthet Dent 1990;64:153–8. [47] Vallittu PK, Kokkonen M. Deflection fatigue of cobalt-chromium, titanium, and gold alloy cast denture clasp. J Prosthet Dent 1995;74:412–9. [48] Ahmad I, Sherriff M, Waters NE. The effect of reducing the number of clasps on removable partial denture retention. J Prosthet Dent 1992;68:928–33. [49] Sato Y, Tsuga K, Abe Y, Asahara S, Akagawa Y. Finite element analysis on preferable I-bar clasp shape. J Oral Rehabil 2001;28:413–7. [50] Schubert O, Reitmaier J, Schweiger J, Erdelt K, Güth JF. Retentive force of PEEK secondary crowns on zirconia primary crowns over time. Clin Oral Investig 2019;23:2331–8, doi:http://dx.doi.org/10.1007/s00784-018-2657-x. [51] Tan FB, Song JL, Wang C, Fan YB, Dai HW. Titanium clasp fabricated by selective laser melting, CNC milling, and conventional casting: a comparative in vitro study. J Prosthodont Res 2019;63:58–65.
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Contents lists available at ScienceDirect
Journal of Prosthodontic Research journal homepage: www.elsevier.com/locate/jpor
Original article
Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering Josef Schweiger* , Jan-Frederik Güth, Kurt-Jürgen Erdelt, Daniel Edelhoff, Oliver Schubert Department of Prosthetic Dentistry, University Hospital, LMU Munich, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 8 March 2019 Received in revised form 4 July 2019 Accepted 9 July 2019 Available online xxx
Purpose: The purpose of this study was to evaluate internal porosities, retentive force values and survival of cobalt–chromium (Co–Cr) alloy clasps fabricated by direct metal laser-sintering (DMLS) and compare them to conventionally cast clasps. Methods: Embrasure clasps were digitally designed fitting teeth 35 and 36 on identical metal models (N = 32). Sixteen clasps were fabricated using DMLS (group DMLS) and another sixteen clasps were additively manufactured from wax and then cast from a Co–Cr alloy (group CAST). Internal porosities were examined using micro-focus X-ray (micro-CT) and analyzed applying Kolmogorov–Smirnov test, Mann–Whitney test, and T test (significance level: p < 0.050). A universal testing machine was used to determine the retentive force values at baseline and after 1095, 5475, 10,950 and 65,000 cycles of simulated aging. Data were analyzed employing Kolmogorov–Smirnov test, one-way ANOVA, and Scheffé’s post-hoc test (significance level: p < 0.050). Survival was estimated for 65,000 cycles of artificial aging using Kaplan–Meier analysis. Results: Micro-CT analysis revealed a higher prevalence (p < 0.001), but a more homogeneous size and a significantly smaller mean (p = 0.009) and total volume (p < 0.001) of internal porosities for group DMLS. The groups showed mean initial retentive force values of 13.57 N (CAST) and 15.74 N (DMLS), which significantly declined over aging for group CAST (p = 0.003), but not for group DMLS (p = 0.107). Survival was considerably higher for group DMLS (93.8%) than for group CAST (43.8%) after 65,000 cycles of aging. Conclusions: Clasps made by laser-sintering could be an alternative to conventional cast clasps for the fabrication of removable partial denture frameworks. © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Keywords: Additive manufacturing CAD/CAM Clasp Removable partial denture Retentive force
1. Introduction Dentist Dr. F.E. Roach noted in 1930 that “[ . . . ] the clasp is the oldest and, [ . . . ] still is, and probably will continue to be, the most practical and popular means of anchoring partial dentures of the removable type” [1]. This statement is as topical in 2019 as it was almost a hundred years ago. Despite considerable progress in dental prophylaxis and modern operative, restorative, prosthetic, and implant dentistry, the removable partial denture (RPD) continues to be a reliable treatment option for a large percentage of patients. Due to increasing average life expectancy and decreasing tooth loss, requirements will tend to change from removable full dentures to RPDs, and demand is expected to increase in the future [2–4].
* Corresponding author at: Department of Prosthetic Dentistry, University Hospital, LMU Munich, Goethestraße 70, D-80336 Munich, Germany. E-mail address: [email protected] (J. Schweiger).
Being cost-effective, highly versatile [4], and commonly satisfactory to the patient regarding the overall treatment outcome [5], the RPD must be considered better than its general reputation. On the downside, there are inherent drawbacks in terms of aesthetics or biomechanical problems such as non-physiological loading [6], sensitivity and wear of abutment teeth caused by the denture [5], but also an increased susceptibility to root caries and periodontal health issues [7]. Common technical complications such as deformation and fatigue fracture of RPD clasps result in loss of retention and function and cause the need for extensive reworking and additional expenses [8,9]. To avoid adverse events, diligent treatment planning and procedure, proper aftercare, optimum hygiene, but equally important, selection of a sophisticated technique and adequate materials are fundamental preconditions for long-term success. Co–Cr alloys are a material of choice for the fabrication of RPD frameworks [10]. They have been widely used in dental applications combining excellent biocompatibility, favorable physicochemical properties [11], and low expenses. Nowadays RPD framework, that is, clasps, connectors, and support elements, is still commonly fabricated using analogue casting methods, in particular the lost-wax technique.
https://doi.org/10.1016/j.jpor.2019.07.006 1883-1958/ © 2019 Japan Prosthodontic Society. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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Fig. 1. Experimental setup of the study.
The ongoing computerization of dentistry and dental technology not only creates innovative workflows [12], but also alters longstanding treatment concepts that have proven reliable for ages. Dental CAD/CAM (computer-aided design/computer-aided manufacturing) offers multiple advantages concerning improved efficiency in the dental laboratory, the application of innovative materials, and enhanced quality management, resulting in more predictability, improved longevity and reproducibility of the denture [13]. Recently, additive or generative production technologies are gaining attention, not only in the fabrication of dental models [14] or surgical templates [15], but also in the production of metal denture frameworks [16]. Additive manufacturing (AM) is highly productive but resource-efficient at the same time [12], ensuring high density and thus homogeneity of the workpiece, but also effecting considerable thermal distortion [16,17]. Selective laser-sintering (SLS) is an umbrella term describing an AM process that entails the sequentially building of material layers by fusing powder particles using laser technology. DMLS technique again is used to process metallic materials in this way [16]. Co–Cr alloys processed by means of additive manufacturing present favorable biocompatibility [18,19] as well as superior mechanical properties compared to cast alloys [19–22]. The development of accuracy is assessed positive, but still below that of subtractive manufacturing [23]. Apart from that, it must be noted that the metallurgical characteristics of any additively manufactured alloys differ significantly from that of their conventionally produced counterparts in many respects [24]. DMLS took its first steps into the fabrication of RPDs, when Williams et al. [25] processed a Co–Cr removable partial denture framework by means of this technology for clinical use in 2006. Both patient and practitioner judged the outcome satisfactory [25]. Bibb et al. [26,27] used SLM (selective laser-melting) to produce RPD framework in the same year and found accuracy and fit to be clinically adequate [25]. Since then, several investigations have dedicated to laser-sintering in RPD framework production addressing various aspects of the matter [17,19,28–37]. Campbell et al. [4] stated in a recent publication that there is need for RPD treatment strategies to evolve by, inter alia, implementing digital technologies and new materials. CAD/CAM fabricated clasps are expected to provide the preferable
characteristics provided by cast claps, but, moreover, to overcome some of the detriments of the conventional technology. Therefore, the digital approach must be subject to comprehensive in vitro research before clinical application. The intention of this study is therefore to evaluate internal porosities as well as the retentive force and survival of Co–Cr alloy embrasure clasps fabricated by DMLS and compare them to cast clasps. The null hypotheses were that there will be no difference concerning internal porosities, retentive force, and survival of laser-sintered clasps and the conventionally cast ones. Furthermore, it was hypothesized that artificial aging has no impact on the retentive force values of both experimental groups over the period of artificial aging. 2. Materials and methods 2.1. Fabrication of the specimens The experimental setup of the study is presented in Fig. 1. Models manufactured from a Co–Cr alloy (N = 32) were digitized and embrasure clasps were designed (CAD-Software Dental Designer RPD, 3Shape, Copenhagen, Denmark) for each model fitting teeth 35 and 36 (Fig. 2). The geometry of an embrasure clasp had been applied before [10], representing a close to in vivo setup. Undercut depths were determined using a surveyor (Ney Surveyor, Dentsply, York, PA, USA) [38] and modified matching non-precious metal alloys. The clasps were fabricated applying conventional casting (group CAST; N = 16) and DMLS technology (group DMLS; N = 16). The specimens from group CAST were initially fabricated from wax by means of AM (ProJet PD 3000, 3D-Systems, Valencia, CA, USA) and subsequently cast from Co–Cr (Remanium GM 800, Dentaurum, Pforzheim, Germany) applying the lost-wax technique. The clasps of group DMLS were produced from a Co–Cr alloy powder (EOS Cobalt Chrome RPD, EOS, Krailling, Germany) using the EOSINT M270 system (EOS) [19,34]. The particle size was 40 mm. An “offset”, meaning the gap between clasps and teeth, of +40 mm was chosen for group DMLS. Heat treatment included a 30 min stress relief annealing at 1000 C and a solution annealing at 1150 C for 20 min. Further post-processing included manual removing of support structures, and finalization by one trained certified dental technician.
Please cite this article in press as: J. Schweiger, et al., Internal porosities, retentive force, and survival of cobalt–chromium alloy clasps fabricated by selective laser-sintering, J Prosthodont Res (2019), https://doi.org/10.1016/j.jpor.2019.07.006
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Fig. 2. Computer-aided design of the embrasure clasp. The supporting connectors attached to the tips of the clasps were removed during post-processing.
2.2. Evaluation of internal porosity All clasps were inspected using micro-CT (WENZEL exaCT S75 HRE, Wenzel Volumetrics, Balingen, Germany) in order to determine prevalence, average volume, and total volume of internal porosities per clasp. Examination was conducted employing exaCT analysis-software (Wenzel Volumetrics). Statistical analysis was performed applying SPSS software (Statistics 23.0, SPSS Inc., Stanford, CA, USA). Level of significance was set at p < 0.05. Kolmogorov–Smirnov test, and in effect, T test or Mann– Whitney test were utilized. Homogeneity of variance was tested applying Levene’s test. 2.3. Retentive force measurements and artificial aging The retentive force values were determined at a separation speed of 1000 mm/min using a universal testing machine (Type 1445, Zwick/Roell, Ulm, Germany). After baseline testing, all specimens were subjected to artificial aging in a chewing simulator (Chewing Simulator CS-4, Mechatronic, Feldkirchen-Westerham, Germany), undergoing 1095 aging cycles (representing 1 year of clinical service with 3 times denture removal per day) and simultaneous thermocycling (5 C/55 C). During cycling, joining and separation speed were set at 20 mm/s and 60 mm/s respectively, and force was 50 N. Retentive force measurements were conducted and subsequently repeated after 5475 cycles (5 years), 10,950 cycles (10 years), and 65,000 cycles (60 years). As specimens dropped out due to failure, the measurements at 65,000 cycles were conducted with 15 (group DMLS) and 6 specimens (group CAST). For statistical evaluation (SPSS software, Statistics 23.0), level of significance was set at p < 0.05. Kolmogorov– Smirnov test, one-way ANOVA, and Scheffé post-hoc test were applied. 2.4. Estimation of survival Survival was estimated over 65,000 cycles of artificial aging (simulating 60 years of clinical service) using Kaplan–Meier analysis. After every 2500 cycles, aging was interrupted and adverse events, i.e. clasp failure/fracture, were registered. The Kaplan–Meier estimator is a non-parametric statistic tool used to approach the survival function from lifetime data [39]. 3. Results 3.1. Internal porosity Significance analysis revealed a higher prevalence (1059 porosities in group DLMS; 787 porosities in group CAST;
Fig. 3. Total number and volume distributions of internal porosities in group CAST (a) and group DLMS (b).
p < 0.001), but more uniform size of internal porosities for group DMLS (Fig. 3). Kolmogorov–Smirnov test and non-parametric Mann–Whitney test were applied. Average (p = 0.009) and total porosity volume per clasp (p < 0.001) was significantly larger in group CAST (Fig. 4). Kolmogorov–Smirnov test, and in effect, T test (average porosity volume) and Mann–Whitney test (total porosity volume) were utilized. Levene’s test revealed a violation of homogeneity of variance for average porosity volume. 3.2. Retentive force values Retentive force values for groups CAST and DLMS are given in and Fig. 5. Group CAST showed mean initial retentive force values of 13.57 N and group DMLS of 15.74 N. The values significantly changed over simulated aging for group CAST (p = 0.003), but not for group DMLS (p = 0.107), resulting in values of 6.41 N (group CAST) and 12.28 N (group DMLS) after 65,000 cycles of artificial aging. Kolmogorov–Smirnov test and one-way ANOVA were applied. Scheffé post-hoc test confirmed a noticeable difference of retentive force values for group CAST after 65,000 cycles compared to all other stages of aging.
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Fig. 6. Kaplan–Meier curve estimating survival over 65,000 cycles of simulated aging for both testing groups.
Fig. 4. Average (a) and total volume per clasp of internal porosities (b).
Fig. 5. Retentive force values of groups CAST and DMLS at baseline and over time.
3.3. Survival The calculated survival probability over 65,000 cycles of artificial aging was 43.8% for group CAST and 93.8% for group DMLS (Fig. 6). For group CAST nine adverse events were recorded (1 after 25,000, 2 after 27,500, 1 after 30,000, 1 after 45,000, 3 after 62,500, and 1 after 65,000 cycles). In group DMLS one failure occurred after 37,500 cycles. All adverse events were fractures of upper clasp arms, but no specific fracture patterns could be distinguished (Fig. 7a and b). 4. Discussion This study compared Co–Cr alloy embrasure clasps fabricated by DMLS and cast clasps by means of internal porosities, retentive force values over time, and the survival rate. The first part of the hypothesis must be rejected, as there was in fact a lower prevalence but more disparate size and a significantly larger mean and total volume of internal porosities for group CAST. The artificial aging revealed significant influence on retentive force
values of the clasps of group CAST, while group DMLS presented stable values over the time of simulated aging. Therefore, the hypothesis can be partly supported and partly refuted with respect to retentive force values. Concerning survival, the hypothesis can be rejected as group CAST displayed a markedly lower survival rate than group DMLS. Because additive manufacturing allows for the efficient and accurate standardized production of complex geometries, substantially saving time and resources, and continuous progress is made, ever new dental applications arise. Studies that investigated RPD framework manufactured using different AM strategies reported that the technology might be a promising option for this area of application [19,26,27,30–32,34–37]. The retentive force of RPD clasps grows with increasing cross section of the retentive clasp arm, undercut depth, and young’s modulus, and decreases with the length of the clasp arm. Especially a high degree of toughness and bending stiffness are required for adequate long-time performance [40]. Another intrinsic factor that becomes crucial in computer assisted fabrication is the software-based definition of production parameters [41]. Particularly determining the offset, i.e., the gap between underlying surface and workpiece, is a specificity of CAD/CAM production and of vital importance. Addressing this topic, pilot testing was conducted before embarking on the actual project in order to define production parameters that would generate initial retentive force values of the DLMS clasps similar to those of the CAST specimens. Eventually, an offset of +40 mm was found to be fit, meaning that the inner surfaces of the digitally designed clasps penetrated the virtual surface of the teeth by that amount. Choosing the design of an embrasure clasp [10] was considered suitable in order to create a clinically relevant situation, and high stress and fatigue exposure for the materials. Undercut depth was set at 0.2 for premolar clasps and 0.25 mm for molar clasps and thus in a range corresponding with preceding investigations [10,32,34,36]. Microstructure and porosity characteristics of laser-sintered clasps depend on multiple factors such as strength and speed of the laser, layer thickness, and the powder material used [30], and can be controlled by adjusting these parameters [42]. It has been demonstrated that cast clasps and SLM fabricated clasps that show similar mechanical properties differ significantly concerning their microstructure [30]. Alageel et al. [19] used micro-CT and found minimum porosity in cast and laser-sintered clasps at all. In line with the current investigation, they located considerably more pores in the DMLS clasps, but with a smaller average volume and a more homogeneous distribution [19]. Porosities of cast
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Fig. 7. Fractured clasp from group CAST (a). SEM picture of the fractured surface with 100 magnification (b).
frameworks mainly result from shrinkage during solidification and pressure caused by dissolved gases [43], displaying a heterogeneous distribution of defects. Pore formation in laser-sintered clasps are depending on fabrication parameters, for instance, a long laser exposure time [11] or incompletely melted metal powders [36], showing a more uniform pattern. Separation speed values in retentive force testing varied widely between 10 mm/min [44,45], 50 mm/min [32,36,40], and 300 mm/ min [33] in similar investigations. Since it cannot be ruled out that the separation speed has a long-term impact on retentive behavior, and researchers found in another context that a clinically relevant separation speed is up to 6000 mm/min for RPDs [46], a higher separation speed of 1000 mm/min was chosen to mimic conditions close to clinical reality. Although about 10,000 separation cycles representing 10 years of clinical service have proven sufficient to receive sound results in retentive force testing [31,32], the authors of the present study have agreed to go beyond. The reason for that was to obtain meaningful results, considering that earlier research saw Co–Cr clasps fail not before 20,000 cycles, although exposed to higher stress [47]. Moreover, it can be observed that in vivo clinical usage of RPDs is often characterized by non-uniform modes of insertion and removal and thus by even heavier mechanical strain. Investigations dealing with the retentive force of RPD clasps generate heterogeneous results in general. This is mostly due to the disparate experimental setups concerning the type and geometry of the clasps or the material of the models. It is therefore less important which absolute retentive force values are provided at baseline, but how they alter over time. As it has been stated that about 5 N might be an adequate retentive force for one RPD clasp [48,49], retentive force values should not fall below. When Arda and Arikan [44] tested the retentive forces of cast Akers clasps on
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molars (M) and premolars (PM), values of Co–Cr alloy specimens dropped from between 5.6 0.04 N (M) and 6.15 0.15 N (PM) to 2.6 0.33 N (M) and 2.8 0.29 N (PM) after only 3 years of simulated aging [44]. Declining retentive force values are commonly observed in cast clasps [40,45], which coincides with the results found in this study. However, no decrease was found up until 10 years of simulated service. This again is in accordance with other researchers who found slightly increasing retentive force values in cast Co–Cr embrasure clasps over 5 years of cyclic loading [10]. Concerning retentive force values in CAM produced clasps there is only few evidences. Nakata et al. [32] tested cast Akers clasps and clasps fabricated by repeated laser-sintering and high-speed milling, a methodology that is thought combine the advantages of subtractive and additive manufacturing to enable high accuracy and surface qualities forgoing extensive manual post-processing. The researchers found the initial retentive force values to be 12.9 3.5 N for the cast Co–Cr clasps and 12.3 2.6 N for the lasersintered/milled specimens. Values dropped for all groups after aging, though the CAM clasps exhibited a significantly smaller decline [32]. Torii et al. [35] found a similar pattern when investigating Akers clasps that were fabricated in a comparable way. The four groups of CAM clasps exhibited baseline retentive force values of 16.1 0.8 N, 12.3 2.6 N, and 21.5 5.3 N, depending on different manufacturing parameters, and the cast clasps demonstrated an initial retentive force of 12.9 3.5 N. The values of the CAM clasp groups dropped by 14.3 to 30.8% after 10,000 cycles of aging, whereas the cast specimens declined by 41.1% of their retentive performance. The authors also pointed at the influence of software parameters such as the offset. The evidence that cast clasps tend to lose more of their retentive force over time is in line with the findings of the present study, where the AM manufactured clasps showed initial retentive force values of 15.74 2.76 N which dropped by 21.98% to 12.28 4.79 N after 65,000 cycles of aging. The cast specimens went from 13.57 2.88 N to 6.41 1.29 N and thus lost 52.76% of their retentive force in the same period. For up to 10 years, the retentive force values were relatively constant for both groups. This phenomenon might be attributed to the highly standardized experimental setup, the relatively small undercut depth, and the smooth surface of the Co–Cr models. The superior structural homogeneity described above might be an explanation for the smaller standard deviations found in most of the retentive force values of the DMLS group. With regard to survival, Alageel et al. [19] proved that the microstructure homogeneity of DMLS produced clasps leads to a higher fatigue resistance, which might also explain the current results. Kim et al. [34] demonstrated that clasps fabricated using DMLS were more flexible and therefore less prone to cyclic fatigue. As the first fracture in the present study occurred after more than 20 years of simulated service at all, it can nevertheless be deducted that both fabrication technologies are reliable concerning longtime service. In principle, it can be observed that even if a trained expert executes the manual processing steps within a merely digital workflow under standardized conditions, they represent a critical source of error, whereas the CAM processes seem more predictable [50]. In other words, forgoing manual processing steps using CAD/ CAM techniques is likely to provide higher precision. This is also to be expected in the AM fabrication of RPD framework, representing tangible benefit. Another major advantage of the CAD/CAM fabrication of RPD framework is the ease of reproduction from stored data in the event of failure. Hence milling is likely to produce smoother surfaces and higher accuracy than AM fabrication [23,51] the combination of both technologies [32,36,51] might be a reasonable approach. However,
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further simplifying the methodology could be a long-term objective. Researchers repeatedly emphasized that not only CAM technology but also specialized and comprehensive CAD software needs to be further developed [28,29]. An optimum construction software must provide tools that allow for the exact calculation of the retentive force for every clasp and in total, and the best possible common insertion direction. This should be done considering crown-to-root ratio, tooth mobility, and individual anatomical characteristics. The software is also expected to enable the design of reciprocal non-retentive arms averting any lateral deflection of the tooth during insertion and removal. Therefore, forthcoming evolution must focus on materials, technology, and software in order to facilitate reasonable in vivo application of the technique. 5. Conclusion Within the limitations of this investigations, the following conclusions can be deduced: 1. Laser-sintered (DMLS) clasps displayed a smaller volume and a more homogeneous distribution of internal porosities compared to the cast specimens. 2. The retentive force values of the DMLS clasps showed superior consistency over time. 3. The long-term survival of the DMLS specimens was considerably higher than that of the cast clasps. Based on these findings, additively fabricated RPD framework might offer a real alternative to conventional cast framework, propelling removable partial prosthodontics into the digital era. As there is currently no software available that provides all the tools desirable and reasonable to reliably achieve the retentive behavior easily and precisely as requested, future efforts should address this topic. Apart from that, research on the long-term performance in a clinical setting is needed. Conflict of interest The authors declare that there is no conflict of interest. Acknowledgments The authors thank EOS GmbH for the research grants received in the course of this study. References [1] Roach FE. Principles and essentials of bar clasp partial dentures. J Am Dent Assoc 1930;17:124–38. [2] Douglass CW, Watson AJ. Future needs for fixed and removable partial dentures in the United States. J Prosthet Dent 2002;87:9–14. [3] Hummel SK, Wilson MA, Marker VA, Nunn ME. Quality of removable partial dentures worn by the adult U.S. population. J Prosthet Dent 2002;88:37–43. [4] Campbell SD, Cooper L, Craddock H, Hyde TP, Nattress B, Pavitt SH, et al. Removable partial dentures: the clinical need for innovation. J Prosthet Dent 2017;118(1–4):273–80. [5] Frank RP, Milgrom P, Leroux BG, Hawkins NR. Treatment outcomes with mandibular removable partial dentures: a population-based study of patient satisfaction. J Prosthet Dent 1998;80:36–45. [6] Maxfield JB, Nicholls JI, Smith DE. The measurement of forces transmitted to abutment teeth of removable partial dentures. J Prosthet Dent 1979;41:134–42. [7] Mojon P, Rentsch A, Budtz-Jørgensen E. Relationship between prosthodontic status, caries, and periodontal disease in a geriatric population. Int J Prosthodont 1995;8:564–71. [8] Hofmann E, Behr M, Handel G. Frequency and costs of technical failures of clasp- and double crown-retained removable partial dentures. Clin Oral Investig 2002;6:104–8. [9] Saito M, Notani K, Miura Y, Kawasaki T. Complications and failures in removable partial dentures: a clinical evaluation. J Oral Rehabil 2002;29:627–33.
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