Introduction 14 20 29 33 35 12 19 32 34 40 41 47 49 In the case of minimal-remaining tooth substance, the retention of a crown has traditionally been provided by a core and the retention of the core by a post. Two fundamentally different techniques find widespread use for the construction of a post and core: (1) a post and core cast as a single unit and (2) a prefabricated post upon which a core is directly produced in, e.g., amalgam or resin composite. The first prefabricated posts on the market were made of stainless steel or gold-plated brass. Later, posts of titanium alloys became popular, and recent developments have introduced posts made of fiber-reinforced resin composite or ceramic. Simultaneously, adhesive dentistry, offering reliable bonding to tooth substance and to restorative materials via various pretreatments, has markedly progressed to widen the possibilities also for post and core treatment. As mentioned, the post serves to retain the restoration. However, the post should also serve to protect the remaining tooth structure. These two functions of endodontic posts may be evaluated by determining the retentive ability of the posts and the fracture resistance of endodontically treated teeth provided with post-retained restorations. The latter may also be indirectly determined by analysis of the stress-distributing characteristics of the posts. Post-related factors that have been found to exert influence on retention of posts and protection of tooth structure include: shape, length, diameter, surface design, and stiffness of the post as well as the type of luting cement used. 2 5 18 28 37 38 42 46 2 5 7 8 17 42 1 24 53 5 2 48 5 12 30 5 7 37 36 43 5 10 12 43 2 23 38 39 22 23 39 47 2 6 25 31 51 9 12 30 42 47 44 45 54 4 9 11 15 41 46 Materials and methods A questionnaire was developed and piloted, and in June 2000, it was posted, in five copies, to the members of the Danish Dental Association who worked in private practice (3,444 practitioners). The practitioners were asked to complete the questionnaire in case a patient presented with a failed post-retained restoration. Questions were asked about age and gender of the patient, tooth involved, number of posts, type, design, length, and diameter of the post(s), type of core, type of luting agent, type of restoration, height of any ferrule, degree of tooth destruction, functioning time until failure, and type of failure. α Results 1 Table 1 Distribution of type of post and cores among failed restorations Type of post and core Number Cast post and core 114 Prefabricated post with direct core 130 Prefabricated post with cast core 5 One-piece post crown 7 Total 256 Of the 260 failed post-retained restorations, 54% comprised tapered posts, 22% comprised parallel-sided posts, 20% comprised posts of a combined tapered-parallel design, and 4% comprised posts with a design that was not accounted for. The majority of the cast posts (76%) and of the stainless steel prefabricated posts (73%) were tapered. The titanium posts were equally distributed between the three designs (38, 31, and 31%), and the carbon fiber-reinforced resin composite posts were either parallel sided (50%) or of a combined design (50%). 2 Table 2 Distribution of posts of failed restorations as regards post type, post material, and post length in percent of root length Type Material Length Total 15–49% 50–74% 75–90% Cast Gold 9 39 12 60 Silver–palladium 3 18 8 29 Prefabricated Titanium 14 30 17 61 Stainless steel 13 11 1 25 Carbon fiber 0 7 2 9 Total  39 105 40 184 Of the 130 teeth with a failed restoration that had been treated with a prefabricated post and a direct core, 29% had a core made in amalgam, 59% a core made in resin composite, 3% a core made in glass ionomer cement, and 9% a core made in a material that was not accounted for. Amalgam had been used more often than resin composite for stainless steel posts (70 vs 30%), whereas resin composite had been used more often than amalgam for titanium posts (84 vs 16%) and, in all cases, for carbon fiber-reinforced resin composite posts. Of the 260 failed post-retained restorations, 40% had been luted with zinc phosphate cement, 17% with glass ionomer cement, 10% with resin cement, and 33% posts with an agent that was not accounted for. The majority of the cast posts (68%) and of the stainless steel prefabricated posts (93%) had been luted with zinc phosphate cement. Most titanium posts had been luted with zinc phosphate cement (48%) or glass ionomer cement (37%). Of the carbon fiber-reinforced resin composite posts, 90% had been luted with resin cement and 10% with glass ionomer cement. Of the 260 failed restorations 69% had a ferrule, 29% did not have a ferrule, and 2% were not accounted for. As regards the degree of tooth destruction before luting of the now failed post-retained restoration, only 25% restored teeth out of 260 were accounted for. In 47 cases, the whole clinical crown had been missing at the time of restoration, and in 18 cases, between one half and three fourths of the clinical crown had been missing. 3 Table 3 Distribution of failed post-retained restorations as regards functioning time until failure Functioning time (years) Number 0–1 15 2–5 50 6–10 43 11–20 43 21–38 27 Total 178 Discussion 4 5 Table 4 Distribution of failed post-retained restorations as regards length of post and type of failure Length (mm) Failure Total Fracture of tooth Fracture of post Loosening of post 2–5 20 2 11 33 6–10 77 20 47 144 11–20 12 12 7 31 Total 109 34 65 208 Table 5 Distribution of failed post-retained restorations as regards functioning time of restoration and type of failure Functioning time (year) Failure Total Fracture of tooth Fracture of post Loosening of post 0–1 5 4 6 15 2–5 14 15 11 40 6–10 18 8 17 43 11–20 27 2 14 43 21–38 18 3 5 26 Total 82 32 53 167 Discussion Five percent of the private practitioners that had been invited to participate in the survey did so. Possible explanations for this low response rate include the open invitation design, the relative extensive questionnaire, and the fact that the questionnaire could not be completed once and for all but had to be procured whenever a patient presented with a failed post-retained restoration. Considering that prosthodontics is not a registered specialty in Denmark, there is no reason to believe that the treatments provided by the participating practitioners were of another type or quality than those provided by private practitioners in general. The fact that the questionnaires were filled out by individual practitioners in their everyday environment and not by calibrated investigators in an optimal research facility explains that the response rate to each question was not 100% and also implies some uncertainty as to the validity of the answers, especially those that included an element of estimation, e.g., of post diameter. The abovementioned factors call for some caution when interpreting the results of the survey. 9 27 46 9 11 26 46 46 13 26 27 46 The types of posts, cores, and cements used for the failed restorations, not only reflect that preferences vary among practitioners but also that the age of the restorations vary widely, as did the materials and treatments available at the time of restoration. Given the design of this survey, i.e., we have only considered restorations that have failed, we have no knowledge of the entire population of post-retained restorations. Thus, the survey does not allow distinction between an over-representation of a given material or treatment caused by outspread use and over-representation caused by poor longevity. 21 50 21 50 A number of specific types of posts or materials were found to have significantly high prevalence in the group of post-retained restorations that failed after a relatively short functioning time: carbon fiber-reinforced resin composite posts, posts cast in silver–palladium, parallel-sided and combined tapered-parallel posts, resin composite cores, and resin cement and glass ionomer cement. Some of these posts and materials are probably found in this group of restorations that failed after a short functioning time, not because they in fact are associated with short functioning time but because they have been on the market for a relatively short time compared with the alternatives and thus are over-represented in the total group of restorations with a short functioning time. 3 21 46 50 16 46 52 42 46 21 50 9 12 30 42 44 45 47 54 Conclusions Fracture of the tooth was the most common type of failure among the failed post-retained restorations followed by loosening of the post and fracture of the post. Compared with parallel-sided posts, tapered posts more often displayed fracture of the tooth and less often fracture of the post. Post-retained restorations that had functioned for a long time before failure showed an increased risk of tooth fracture compared to loosening of the post and fracture of the post. Fracture of the post was more common among male patients than among female patients.