LETTER TO THE EDITOR A response to M. Kern regarding Pieger S, Salman A, Bidra AS. Clinical outcomes of lithium disilicate single crowns and partial fixed dental prostheses: A systematic review. J Prosthet Dent 2014;112:22-30 Dear Editor: In regard to Dr Matthias Kern’s comments, his assessment of our reporting of his study and the differences in lithium disilicate material and discrepancies between number of patients and restorations is correct. Our conclusions, however, were based on the findings from a number of studies over a 10-year period, using standardized definitions and predetermined criteria. We appreciate the opportunity to clarify what we reported and why. 1. Given the large heterogeneity of data available on this topic, the systematic review attempted to standardize the description of anterior and posterior fixed dental prostheses (FDPs). Therefore, in our reported methodology, we had specific definitions for anterior and posterior regions. Articles that did not meet these definitions or those articles that failed to clarify anterior vs posterior regions were simply described to have not reported the data. Based on our data analysis, the authors argue that inclusion of Kern et al’s partial data on anterior or posterior region does not affect the results or conclusions of this systematic review, simply because we did not compute the survival analysis based on the region of the restoration in the mouth, due to the overall paucity and heterogeneity of these data. 2. Discrepancies in reporting the number of patients and the number of restorations is known to be a common issue with data extraction in systematic reviews.1 In our review, studies comparing different types of restorations (e.g., metal ceramic, zirconia, lithium disilicate) have included the total number of patients in their study without reporting the number of patients specific to each restoration. Thus, there were many studies with a larger number of patients than the number of lithium disilicate restorations and the authors of this systematic review have reported this issue in the discussion section as one of
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the challenges and limitations of the systematic review. Because the reader was not exposed to our entire pool of data charting, given the text limitation of a manuscript, some confusion may have been created. Nevertheless, this systematic review did not make any assumptions, correlations, or conclusions for survival/failure per patient and, therefore, this qualitative data was reported for descriptive purposes only. 3. We recognize that Empress II and e.max Press have minor differences in crystal size and physical properties, and we have described this in the introduction section of the article. However, despite these minor differences, it is important to note that they both are indeed lithium disilicate materials and several patients around the world continue to have these restorations in their mouth. The objective of this study (as represented in the title) was to simply study the clinical outcomes of lithium disilicate single crowns and FDPs. Initially, the authors intended to segregate each type of lithium disilicate material, to outline the differences in clinical behavior of these materials, and to provide this information to the reader. However, the authors did not identify enough studies for different types of lithium disilicate materials (for single crowns there were 5 studies on e.max and 3 studies on Empress II, and for FDPs, there were 2 studies on e.max and 3 studies on Empress II). Providing cumulative survival rate (CSR) from 2 to 3 studies to the reader would be futile and has a greater potential to mislead the reader. Cox regression model is contraindicated when the sample size is so small. Additionally, the authors did not perform a risk of bias analysis or any qualitative analysis on any of the included studies, simply because this would further exclude any studies and result in an inconclusive systematic review. Similarly, we did not compare results of studies that
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reported support by the manufacturers vs studies that did not receive support. Our readers and clinicians benefit when systematic reviews provide some meaningful insight about clinical outcomes, rather than reporting on inconclusive evidence drawn from thin data with multiple stratifications. 4. Similar to the discussion on different types of lithium disilicate, our original intention was to separate monolithic vs bilayered lithium disilicate restorations for single crowns and FDPs. Again, the paucity of data precluded this objective as described in the article. Each reader is clearly able to understand just by briefly looking at Tables II and III, wherein we present each study along with the type of lithium disilicate material, as well as the number of failures. This can eliminate any potential for confusion. For single crowns, data from our systematic review indicate that the type of lithium disilicate or nature of manufacturing (Empress II, e.max Press, or e.max CAD) does not affect the survival rate (CSR at 100% at 2 years and 97.8% at 5 years). For FDPs, there were only 2 studies for e.max Press that, when combined with 3 studies with other forms of lithium disilicate material, the 5-year CSR dropped to 78.1%. For single crowns, our results indicate that there were no differences in survival between various types of lithium disilicate material, which is consistent with a recently concluded randomized controlled trial by Esquivel-Upshaw et al.2 For FDPs, our results indicate a lower survival rate and our results are consistent with recent studies by Makarouna et al,3 Sola-Ruiz et al,4 and Marquardt and Strub,5 who have
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all cautioned against the use of lithium disilicate FDPs, especially in the posterior region. In conclusion, I would like to thank Dr Kern for his insightful response to our article. His questions have allowed us to clarify areas of confusion and help readers to utilize the evidence presented by us for use in clinical practice. However, I see no errors in our data analysis or results that would change our conclusions. Our conclusions are based on our findings, and our findings are simply based on the results reported by authors of various studies across the world. When the data improve in quality and quantity for FDPs, we intend to publish a subsequent systematic review on this topic with (hopefully) more promising conclusions. Until further clinical evidence is available, I continue to advise caution against using lithium disilicate FDPs. Avinash S. Bidra, BDS, MS, FACP University of Connecticut Farmington, Conn REFERENCES 1. Bidra AS. Evidence-based prosthodontics: fundamental considerations, limitations and guidelines. Dent Clin North Am 2014;58(1):1-17. 2. Esquivel-Upshaw J, Rose W, Oliveira E, Yang M, Clark AE, Anusavice K. Randomized controlled clinical trial of bilayer ceramic and metal-ceramic crown performance. J Prosthodont 2013;22:166-73. 3. Makarouna M, Ullmann K, Lazarek K, Boening KW. Six-year clinical performance of lithium disilicate fixed partial dentures. Int J Prosthodont 2011;24:204-6. 4. Sola-Ruiz MF, Lagos-Flores E, Roman-Rodriguez JL, Highsmith Jdel R, Fons-Font A, Granell-Ruiz M. Survival rates of a lithium disilicate-based core ceramic for three-unit esthetic fixed partial dentures: a 10-year prospective study. Int J Prosthodont 2013;26:175-80. 5. Marquardt P, Strub JR. Survival rates of IPS Empress 2 ceramic crowns and fixed partial dentures: results of a 5-year prospective clinical study. Quintessence Int 2006;37:253-9.
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the challenges and limitations of the systematic review. Because the reader was not exposed to our entire pool of data charting, given the text limitation of a manuscript, some confusion may have been created. Nevertheless, this systematic review did not make any assumptions, correlations, or conclusions for survival/failure per patient and, therefore, this qualitative data was reported for descriptive purposes only. 3. We recognize that Empress II and e.max Press have minor differences in crystal size and physical properties, and we have described this in the introduction section of the article. However, despite these minor differences, it is important to note that they both are indeed lithium disilicate materials and several patients around the world continue to have these restorations in their mouth. The objective of this study (as represented in the title) was to simply study the clinical outcomes of lithium disilicate single crowns and FDPs. Initially, the authors intended to segregate each type of lithium disilicate material, to outline the differences in clinical behavior of these materials, and to provide this information to the reader. However, the authors did not identify enough studies for different types of lithium disilicate materials (for single crowns there were 5 studies on e.max and 3 studies on Empress II, and for FDPs, there were 2 studies on e.max and 3 studies on Empress II). Providing cumulative survival rate (CSR) from 2 to 3 studies to the reader would be futile and has a greater potential to mislead the reader. Cox regression model is contraindicated when the sample size is so small. Additionally, the authors did not perform a risk of bias analysis or any qualitative analysis on any of the included studies, simply because this would further exclude any studies and result in an inconclusive systematic review. Similarly, we did not compare results of studies that
THE JOURNAL OF PROSTHETIC DENTISTRY
April 2015
reported support by the manufacturers vs studies that did not receive support. Our readers and clinicians benefit when systematic reviews provide some meaningful insight about clinical outcomes, rather than reporting on inconclusive evidence drawn from thin data with multiple stratifications. 4. Similar to the discussion on different types of lithium disilicate, our original intention was to separate monolithic vs bilayered lithium disilicate restorations for single crowns and FDPs. Again, the paucity of data precluded this objective as described in the article. Each reader is clearly able to understand just by briefly looking at Tables II and III, wherein we present each study along with the type of lithium disilicate material, as well as the number of failures. This can eliminate any potential for confusion. For single crowns, data from our systematic review indicate that the type of lithium disilicate or nature of manufacturing (Empress II, e.max Press, or e.max CAD) does not affect the survival rate (CSR at 100% at 2 years and 97.8% at 5 years). For FDPs, there were only 2 studies for e.max Press that, when combined with 3 studies with other forms of lithium disilicate material, the 5-year CSR dropped to 78.1%. For single crowns, our results indicate that there were no differences in survival between various types of lithium disilicate material, which is consistent with a recently concluded randomized controlled trial by Esquivel-Upshaw et al.2 For FDPs, our results indicate a lower survival rate and our results are consistent with recent studies by Makarouna et al,3 Sola-Ruiz et al,4 and Marquardt and Strub,5 who have
Letter to the Editor
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all cautioned against the use of lithium disilicate FDPs, especially in the posterior region. In conclusion, I would like to thank Dr Kern for his insightful response to our article. His questions have allowed us to clarify areas of confusion and help readers to utilize the evidence presented by us for use in clinical practice. However, I see no errors in our data analysis or results that would change our conclusions. Our conclusions are based on our findings, and our findings are simply based on the results reported by authors of various studies across the world. When the data improve in quality and quantity for FDPs, we intend to publish a subsequent systematic review on this topic with (hopefully) more promising conclusions. Until further clinical evidence is available, I continue to advise caution against using lithium disilicate FDPs. Avinash S. Bidra, BDS, MS, FACP University of Connecticut Farmington, Conn REFERENCES 1. Bidra AS. Evidence-based prosthodontics: fundamental considerations, limitations and guidelines. Dent Clin North Am 2014;58(1):1-17. 2. Esquivel-Upshaw J, Rose W, Oliveira E, Yang M, Clark AE, Anusavice K. Randomized controlled clinical trial of bilayer ceramic and metal-ceramic crown performance. J Prosthodont 2013;22:166-73. 3. Makarouna M, Ullmann K, Lazarek K, Boening KW. Six-year clinical performance of lithium disilicate fixed partial dentures. Int J Prosthodont 2011;24:204-6. 4. Sola-Ruiz MF, Lagos-Flores E, Roman-Rodriguez JL, Highsmith Jdel R, Fons-Font A, Granell-Ruiz M. Survival rates of a lithium disilicate-based core ceramic for three-unit esthetic fixed partial dentures: a 10-year prospective study. Int J Prosthodont 2013;26:175-80. 5. Marquardt P, Strub JR. Survival rates of IPS Empress 2 ceramic crowns and fixed partial dentures: results of a 5-year prospective clinical study. Quintessence Int 2006;37:253-9.
THE JOURNAL OF PROSTHETIC DENTISTRY
