LETTER TO THE EDITOR Comments 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: I would like to comment on the article by Pieger et al,1 “Clinical outcomes of lithium disilicate single crowns and partial fixed dental prostheses: a systematic review,” a recently published systematic review on lithium disilicate ceramic restorations. Table II of the article cites our own clinical study,2,3 but is not correctly described. For Kern et al,2 the table states that the number of restorations in the anterior and posterior region was not reported. That statement is incorrect. In two publications of the same patient cohort,2,3 which are both cited in the review article, we precisely described the number of restorations and also the failures regarding their locations. Thirty fixed dental prostheses (FDPs) replaced posterior teeth, while 6 FDPs replaced anterior teeth. Four of the FDPs replaced either a first premolar (3×) or a canine (1×) and so did not meet the specifications that were set by the authors for being pure anterior (canine to canine) or pure posterior (premolar and molar) restorations. Nevertheless, the remaining 32 FDPs fulfilled this (questionable) definition and could have been listed by numbers in Table II. I am concerned that readers of the review might be misled by this incorrect statement in Table II and therefore might refrain from referring to our publication because they might consider a study not reporting the location of the restorations as being of inadequate quality. Therefore, I would like to request an erratum be published regarding the listing of our study. In various study listings (4×) in Table II, the number of patients is higher than the number of restorations, which makes no sense. If there were patients with other than lithium disilicate ceramic restorations included and the exact number of patients with lithium disilicate ceramic restorations was not reported here, it should have been stated correctly that the numbers of patients with lithium disilicate was not reported. However, my biggest concern regards the pooling of data of considerably different materials and applications.

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The outdated lithium disilicate material Empress II, which has not been marketed since 2009, and the current e.max Press or e.max CAD are significantly different materials regarding crystal sizes, manufacturing technology, and physical properties, and therefore should not be combined into one simple data set. Similarly the data of veneered and monolithic lithium disilicate ceramic should not be combined into one data set as is done in Tables V and VI. In contrast, it would have been helpful to outline the differences in clinical behavior between these three groups of materials. So in my opinion this article has the potential to mislead readers by calculating pooled cumulative survival rate of very different materials without adequate statistical consideration, for example, by fitting a Cox regression model to the data, treating the material type as a covariate. If one reads the results carefully, it is obvious that with monolithic e.max Press and e.max CAD, only a few failures occurred (in cited studies 1-4 and 6, only 5 of 390 restorations failed, which corresponds to a total of only 1.3% failures). The reported outcomes of these studies would be comparable to metal-ceramic crowns and FDPs for up to 10 years. This conclusion would be quite different from the conclusion drawn currently in the review article. Matthias Kern, DMD, PhD Christian-Albrechts University at Kiel Germany REFERENCES 1. 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. 2. Kern M, Sasse M, Wolfart S. Ten-year outcome of three-unit fixed dental prostheses made from monolithic lithium disilicate ceramic. J Am Dent Assoc 2012;143:234-40. 3. Wolfart S, Eschbach S, Scherrer S, Kern M. Clinical outcome of three-unit lithium-disilicate glass-ceramic fixed dental prostheses: up to 8 years results. Dent Mater 2009;25:e63-71.

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