Introduction

Root canal-treated teeth with insufficient coronal tooth structure require a custom or pre-fabricated post system to retain the core material before fabrication of an indirect restoration. Various post systems have been used previously to gain intracanal support to support an extrinsic core material. Customised fabrication of posts involves making an impression of the root canal configuration, followed by the fabrication of a post which closely adheres to the root canal morphology. Custom cast posts show better resistance to torsional stress [1, 2] in the teeth with flared and multiple roots [3, 4]. Custom posts can be fabricated by employing a direct or indirect technique from metallic or non-metallic materials [5]. Metal custom posts have previously reported acceptable success rates and mechanical qualities [6]. Superior aesthetics and fracture resistance have been attributed to ceramic posts [7]. However, posts made of zirconia have exhibited reduced attachment strength and an increased risk for root fractures [8, 9]. Custom posts can be fabricated conventionally or by a computer-aided design-computer-aided manufacturing (CAD-CAM) protocol [10,11,12,13]. Pre-fabricated posts can be fabricated from different metallic and non-metallic materials. Titanium and stainless steel posts exhibit diminished radiographic visibility and corrosion, respectively [14]. Ceramic posts demonstrate higher strength and aesthetics but diminished resin bonding [15,16,17,18,19,20,21,22]. Fibre posts report a much lower incidence of root fractures [23, 24], acceptable aesthetics and retrievability [25]. A recent systematic review provides robust evidence on pre-fabricated fibre posts’ ability to reduce the risk of secondary caries, debonding, and tooth fracture [26]. CAD-CAM-fabricated glass fibre posts have been reported to exhibit enhanced fracture resistance, bond strength, retention and aesthetics compared to pre-fabricated glass fibre posts [27,28,29,30,31]. Comparative studies between CAD-CAM fibre posts and other types of posts are required to generate evidence regarding the acceptability of CAD-CAM posts. Network meta-analysis (NMA) enables investigators to combine direct and indirect evidence to establish comparative efficacy across a network of studies. The purpose of this NMA was to compare the bond strength, fracture resistance, catastrophic failure, and cement film thickness of CAD-CAM custom fibre posts and other types of posts from in vitro studies.

Materials and methods

The protocol for the systematic review was registered with the International Prospective Register of Systematic Reviews (PROSPERO: (CRD42024501614)) and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) extension statement of NMA [32]. The study population included teeth that are endodontically treated, followed by post and core. Interventions were CAD-CAM fibre posts. Comparators were pre-fabricated fibre posts or other types of posts. The outcomes evaluated were fracture resistance, bond strength, catastrophic failures, and cement film thickness. Screening of titles and abstracts, full-text reading, and data extraction were performed independently and in duplicates by two reviewers (MMG, XYL). The risk of bias within each study was independently assessed by two reviewers (MMG, XYL) using RoBDEMAT (RoB 2.0) [33]. Disagreements and discrepancies were resolved by discussion with a third reviewer (R.K.M). Standardised mean differences (SMD) in millimetres (mm) and 95% confidence intervals were used as summary statistics for continuous outcomes. For direct comparisons, a standard pairwise meta-analysis was performed by using a random-effects (DerSimonian and Laird) model [32, 34]. If a direct comparison was based on two or more studies, heterogeneity among trials was assessed by considering the I2 statistics [32, 35]. To synthesise the available evidence by combining direct and indirect evidence from different studies, a random-effects NMA was applied [36,37,38]. The probability of each post type being the best was estimated by constructing nanograms and their surface area under the cumulative ranking (SUCRA) [38, 39]. A comparison-adjusted funnel plot was used to examine the publication bias. A statistical software programme (Stata version 15.0; (StataCorp)) was used for statistical analysis and graph generation [40].

Results

A total of 741 articles were initially identified, of which 80 duplicates were removed, and 585 were excluded by screening the titles and abstracts. A total of 76 articles were assessed by full-text reading. Sixteen articles were selected for the quantitative analysis [27, 41,42,43,44,45,46,47,48,49,50,51,52,53,54,55]. The preferred reporting items for systematic reviews and meta-analyses (PRISMA) flow diagram (Fig. 1. Table 1 shows the characteristics of the included studies. The quality assessment of each study using the RoBDEMAT assessment tool is provided in Table 2. CAD-CAM fibre posts (CAD), pre-fabricated fibre posts (PFP), cast metal posts (CMP), and amalgam cores (AMA) were compared. For bond strength, seven in vitro studies comparing three interventions were included in the NMA (Fig. 2). CAD (SMD = 1.09 [95% CI: 0.01, 2.17]) P = 0.04 demonstrated higher bond strength when compared to PFP. Supplementary Table 1 summarises the SMD and the ranking of the interventions, while Fig. 3 shows the SUCRA ranking curves for each intervention in the network. CMP ranked the highest, followed by CAD and PFP, respectively. The results of the pairwise meta-analysis are depicted in the forest plot in Fig. 4. CAD (SMD = 2.63 [95% CI: 0.82, 4.44]) P < 0.04 demonstrated higher bond strength when compared to PFP. The network and pairwise estimates for all the interventions are summarised in Supplementary Fig. 1. Based on the comparison-adjusted forest plots, publication bias could be detected (Supplementary Fig. 2). For catastrophic failure, seven in vitro studies comparing four interventions were included in the NMA (Fig. 5). None of the interventions demonstrated a significant difference in catastrophic shortcomings compared to PFP. Supplementary Table 2 summarises the SMD and the ranking of the interventions, while Fig. 6 shows the SUCRA ranking curves for each intervention in the network. The results of the pairwise meta-analysis are depicted in the forest plot, Fig. 7. CMP (RR = 2.29 [95% CI: 1.13, 4.62]) P < 0.05 demonstrated a higher risk for catastrophic failure when compared to CAD. The network and pairwise estimates for all the interventions are summarised in Supplementary Fig. 3. CAD (RR = 0.39 [95% CI: 0.23, 0.69]) P < 0.05 demonstrated a lower risk for catastrophic failure when compared to PFP. Based on the comparison-adjusted forest plots, publication bias could be detected (Supplementary Fig. 4). For fracture resistance, seven in vitro studies comparing two interventions were included in the pairwise meta-analysis as depicted in the forest plot, Supplementary Fig. 5. None of the comparisons were statistically significant. Inconsistency was identified in the NMA for cement film thickness, and no significant findings were observed in the pairwise analysis. The network plot (Supplementary Fig. 6), SUCRA ranking curves (Supplementary Fig. 7) and the forest plot (Supplementary Fig. 8) are provided.

Fig. 1: PRISMA Flowchart describing the search strategy and identification of articles.
figure 1

PRISMA-Preferred Reporting Items for Systematic Reviews and Meta-Analysis, n-number.

Full size image
Fig. 2: Network plot depicting the network meta-analysis results for bond strength.
figure 2

CAD-CAD-CAM fibre posts PFP-Pre-fabricated fibre posts, CMP cast metal posts.

Full size image
Fig. 3: SUCRA ranking curves for bond strength.
figure 3

CAD-CAD-CAM fibre posts PFP-Pre-fabricated fibre posts, CMP cast metal posts.

Full size image
Fig. 4: Forest plot depicting the pairwise meta-analysis results for bond strength.
figure 4

CAD-CAD-CAM fibre posts PFP-Pre-fabricated fibre posts, CMP cast metal posts.

Full size image
Fig. 5: Network plot depicting the network meta-analysis results for catastrophic failures.
figure 5

CAD-CAD-CAM fibre posts PFP-Pre-fabricated fibre posts, CMP cast metal posts, AMAAmalgam cores.

Full size image
Fig. 6: SUCRA ranking curves for catastrophic failures.
figure 6

CAD-CAD-CAM fibre posts PFP-Pre-fabricated fibre posts, CMP Cast metal posts, AMA Amalgam cores.

Full size image
Fig. 7: Forest plot depicting the pairwise meta-analysis results for catastrophic failures.
figure 7

CAD-CAD-CAM fibre posts PFP-Pre-fabricated fibre posts, CMP cast metal posts, AMA amalgam cores.

Full size image
Table 1 Characteristic table describing the characteristics of the included studies.
Full size table
Table 2 Risk of bias analysis performed by ROBDemat.
Full size table

Discussion

The four primary outcomes measured in this study were bond strength, catastrophic failure, fracture resistance and cement film thickness of CAD-CAM fibre posts compared to other types of posts. Bond strength is defined as the force that is required to dislodge or detach posts from root canal dentin. It can be measured by employing a push-out, pull-out, or micro tensile tests [56, 57]. However, results from recent studies have indicated that push-out tests reveal more reliable results when compared to other tests [58, 59]. Bond strength is influenced by various chemical and mechanical factors related to the dentin, post-material, post-treatment and the cement used [60,61,62,63,64,65,66,67,68,69,70,71]. CAD-CAM posts exhibited higher bond strengths compared to pre-fabricated posts. This result may be attributed to the precision fit of the CAD-CAM posts as these are customized to closely mimick the root canal configuaration in comparison to a pre-fabricated post. The close adapatation of the post morphology to the root canal space further ensures a uniform cement space [63, 72, 73]. Compatible bonding and elevated surface energy may result in better bonding for CAD-CAM and cast metal posts respectively [74, 75]. Primer coating of the post surface has been shown to enhance the surface roughness of CAD-CAM posts, whereas sandblasting can be used to the same effect in cast metal posts [63, 72]. On the contrary, pre-fabricated posts primarily depend on micromechanical retention, leading to inferior bonding [76]. Based on the overall comparison, CAD-CAM posts was ranked the highest for bond strength per the SUCRA ranking. This may be attributed to a robust manufacturing process with enhanced quality control and highly compatible bonding properties, allowing for chemical and micromechanical bonding [74].

Catastrophic failure indicates irreparable damage to the tooth restoration complex [72]. CAD-CAM posts demonstrated a lower risk of catastrophic failure than pre-fabricated posts, whereas cast metal posts demonstrated a higher risk of failure than CAD-CAM posts as per pairwise results. This may be attributed to the unequal distribution of masticatory forces in pre-fabricated posts due to the non-uniformity of the canal morphology, which leads to variable cement thickness and stress concentration [63, 67, 73]. CAD-CAM posts is custom-made to the canal configuration, resulting in a superior fit [72] and uniform cement thickness [74]. The material incompatibility concerning the similarity in the elastic modulus is worst for a cast metal post, which puts the cast metal post at high risk for catastrophic failure due to variable stress concentration [77, 78]. Further a cast metal post requires a significantly higher amount of tooth removal, resulting in weakened tooth structure [73]. Inaccuracies may also result from the casting procedure for cast metal posts [73]. CAD-CAM posts have previously demonstrated superior fatigue resistance when subjected to cyclic loading, leading to the formation of cracks and, consequently, failure [75, 77]. A higher rate of manufacturing errors is attributed to cast metal posts compared to CAD-CAM posts [63, 72].

Cement film thickness refers to the layer of adhesive cement thickness present between the dentinal wall of the root canal and the post. It can influence post adhesion by aiding in retention and fit of the post [67, 70], presence of voids, mechanical stability of stress distribution and fracture resistance [66, 79] and bond strength depending on the amount of cement film present [73, 80]. However, based on the results from our network meta-analysis, the unreliable heterogeneity proves that cement film thickness has no significant effect when comparing CAD to other post materials.

Fracture resistance indicates the ability of the post to resist fracture or withstand forces without breaking when used to restore a tooth that was previously root canal treated. Fracture resistance is measured by fracture or fatigue testing to assess the stiffness and durability of the post system in the root canal [27, 41, 43, 50, 51, 54, 55] by placing the post under different conditions or environments. Based on the meta-analysis results, fracture resistance of CAD was comparable to PFP and CMP. Material properties alone might not play a significant role in the fracture resistance of posts. Another external factor that could affect the fracture resistance of post systems in the root canals is the type of cement used. Adhesive resin cement has proven to bond better and provide support for posts to dentinal walls [80, 81]. The remaining dentin thickness supports the post. The presence of lesser remaining dentine thickness leads to a higher risk of post-fracture [82]. Accurate post-space preparation for post-fitting is an essential factor in minimising the risk of fracture of the post or tooth structure due to proper force distribution from the post towards the tooth [60, 70]. The tooth anatomy and location contribute to stress distribution from masticatory forces and functional loading [83,84,85].

The current network meta-analysis’s limitations include the inclusion of in vitro studies. However, it provides an up-to-date comprehensive comparison of the different post-types.

Further high-quality in vitro studies, including recently introduced CAD-CAM post materials, are recommended to evaluate the comparative mechanical properties of different post-types. Clinicians need to carefully evaluate the material properties of the CAD-CAM posts before making appropriate comparisons in long-term clinical studies. Subsequently, prospective cohort clinical studies may provide the most reliable evidence of different post-types survival and clinical performance.

Conclusion

CAD-CAM fibre posts demonstrate higher bond strength, lower catastrophic failure rates and similar fracture resistance compared to pre-fabricated fibre posts and cast metal posts in in vitro studies. Results from the studies need to be validated with further high-quality in vitro studies and randomised controlled clinical trials.