Introduction: Dental trauma is very common in school-age children, primarily between the ages of 8 to 10, often resulting in a crown fracture of the upper central incisor. When endodontic treatment is required, an additional difficulty arises due to the "immaturity of the teeth", i.e. the incomplete development of the root. Treating an immature tooth with a non-vital pulp is a major challenge in paediatric dentistry. Reconstructing a complicated crown fracture often requires intracanal retention. Many clinical studies have shown that more than 50% of young permanent teeth with incomplete root development and necrotic pulp are lost in the first 10 years despite endodontic treatment, which also has a very negative impact on the child's psychosocial development. Biomechanical analysis is crucial to understanding the failure of dental trauma treatment and evaluating the materials used. Finite element analysis is often used for such complex research in biomedicine. Aim: The aim of this study was to investigate the biomechanical response of a non-vital young permanent incisor after post-endodontic therapy with regard to the developmental stage of the root using the finite element method (FEM). Material and methods: A CBCT scan was used to reconstruct a basic 3D model of the upper central incisor and surrounding bone (software package Mimics version 16.0, Materialise, Leuven, Belgium), after which a finite element mesh was created (FEM software package Abaqus, USA). Based on the personalised 3D tooth model (OMZ), several other models were created according to the root development stages: 1/2 root development (S1), 3/4 (S2), more than 3/4 (S3), fully developed root with an open apex (S4) and fully developed root with a closed apex (S5). Models were also created according to the type of intracanal retention: RelyX Post (3M ESPE Deutschland GmbH, Seefeld, Germany) (RXP); GC Fibre Post (GC, Tokyo, Japan) (GFP); everStick Post (GC, Tokyo, Japan) (ESP); metal post Unimetric 1.0 (Dentsply, Ballaigues, Switzerland) (UMP); and a positive control group without retention (GPK). A static load of 100N was applied to the palatal surface of the tooth at an angle of 45 degrees and the equivalent stresses were measured according to the Henckey von Mises (HMH) strength theory. The analysis was performed using a three-way analysis of variance (ANOVA) with the factors "material", "stage of development" and "root level (apical/cervical)". For multiple comparisons, a one-way ANOVA with Tukey adjustment was used. The significance level was set at 0.05. Statistical analysis was performed using the SPSS software package, version 25.0 (IBM, Armonk, NY, USA). Results: The mean HMN stress values within each stage of root development were statistically significantly higher at the cervical level in all models examined in the S3, S4 and S5 stages, and at the S2 stage in the ESP, GFP, GPK and OMZ models. Conversely, stress values were significantly higher at the apical level in the RXP and UMP models at the S2 stage and in all models at the S1 stage. The ESP model in the S1 and S3 stages and the GFP model in the S4 and S5 stages showed the lowest stress values at the cervical level. At the apical level, the lowest stress values were displayed by the UMP model in the S1 stage, the GFP model in the S2, S3 and S5 stages and the GPK model in the S4 stage. The OMZ model showed the highest stress values at the cervical level in all stages, while the highest stress values at the apical level occurred in the OMZ model in the S3 stage and in the RXP model in all other stages. Generally, stress within the root dentine decreased in most models at both levels from the S1 to the S5 stage, with the exception of a major deviation in the GFP and OMZ models, where the S2 stage displayed a statistically significantly lower stress than the S3 stage at the apical level. Additionally, in the UMP model, the S1 stage showed significantly lower stress than the S2 stage at the apical level, and in the ESP model, the S1 stage showed significantly lower stress than all other stages at the cervical level. Conclusion: Despite the limitations of this study, it can be concluded that the greatest stress in a young permanent maxillary central incisor occurred at the cervical level, except during the initial development of one half of the root. Post-endodontic restoration with an intracanal retention did not reduce the development of stresses within the root dentin. As root development progressed, i.e. as the thickness of the dentine wall increased, stresses generally decreased in all models. The Evesrstick post proved to be the best material choice in the earlier stages of root development, while the GC fibre post was the best choice in the later stages of root development at the cervical level. A post geometry with a combined approach of a cylindrical base coronally and a conical end apically proved to be a better choice when selecting an aesthetic intracanal retention compared to a fully conical post geometry. Metal intracanal retentions did not show significantly higher stress development within the root dentin.