To date, no study has comprehensively determined changes in occlusal resistance in immature teeth, although some studies have estimated the strength of immature teeth, typically using ultimate fracture strength [13,14,15,16]. Load-to-failure testing is a simple way to estimate the ultimate strength of a specimen, but the test setting (e.g., cross-head speed or load angle) varies widely, and testing assemblies exclude the physiological environment (such as PDL space). Finite element analysis is another approach to stress analysis that uses 2- or 3-dimensional computer simulated models. This approach is useful for understanding stress distribution within a simulated graphic model, but interpretation of the results can be difficult because this method assumes that the tooth structure is an isotropic and homogeneous material. The results differ according to the type of pre-set model (e.g., linear or nonlinear, plastic or elastic). These methods cannot represent occlusal force distribution and resistance in clinical settings; hence, they have limited relevance in predicting the clinical performance of immature teeth.
In this study, we used human teeth with strain gauges attached to measure the actual stress that they experienced under occlusal forces on an immature tooth during and after apexification. The tested tooth was aligned in a 3-unit teeth assembly simulating adjacent teeth and the PDL space as a stress breaker. It has been reported that the average chewing force varies from 11 to 150 N, whereas the force peaks are 200 N in the anterior area, 350 N in the posterior area, and 1000 N in patients with bruxism . In another study, the maximum bite forces exhibited by adults in the molar area ranged from 250 to 400 N, and forces in the anterior area ranged from 140 to 170 N . Therefore, we preset the occlusal forces as 50 to 300 N.
All simulated immature teeth showed significantly less strain under all tested occlusal forces, and the CH-medicated canals showed the lowest values among the tested teeth. The inferior occlusal resistance of CH-filled teeth may be due to the absence of an apical plug to resist compressive forces. CH in a non-set, aqueous suspension is recommended as the material of choice for apexification [1, 2]. Despite the unpredictability of apical closure, intracanal CH medication can prevent the ingress of granulation tissue into the root canal and inhibit periapical osteoclastic activity [1, 2]. Occlusal force resistance might have different patterns if an apical calcified barrier is formed, but it was not possible to simulate an irregular porous apical barrier in the in vitro setup.
MTA-filled immature teeth with full-length roots also transmitted occlusal stress to the adjacent teeth. Owing to the slow hydration rate of dicalcium silicate, which is one of the main components of MTA, occlusal force measurements were performed 3 weeks after specimen preparation to maximize the compressive strength of the material . MTA is known to form a superficial apatite layer and induce intratubular mineralization that are supposed to enhance sealability of the material [20,21,22,23,24,25]. However, the firmly set material with its micromechanical retentive form in the root dentin was not directly connected to improved resistance to compressive occlusal forces. This might have been because the MTA surface acted as a stress endpoint, transmitting the strain to the adjacent canal wall. If the material surface ends at the cervical area where the enamel ends and the force accumulates, the tooth will not be able to resist the occlusal stress.
For this reason, GP and a composite restoration were placed as reinforcement materials at the cervical area of the immature teeth. The GP backfill added more interfaces inside the canal, and failed to reinforce the thin canal wall. However, a deep composite restoration extending from the MTA plug surface to the occlusal top provided improved occlusal force resistance, which was comparable to that of normal mature teeth. The results of CH-medicated roots could be interpreted as showing that the internal interface at the cervical area adversely affects the occlusal force resistance of immature teeth.
Considering the importance of the periodontium in supporting teeth during function, the depleted occlusal force resistance of immature teeth during a prolonged treatment period might result in stress shielding of the periapical area of the treated tooth, thereby potentially slowing the healing of periapical inflammation and remodeling process of the alveolar bone proper [26,27,28]. It was also reported that the reduction of occlusal force altered the development and maintenance of mechanoreceptors such as Ruffini endings in the PDL [29,30,31].
A limitation of the current experiment is that only compressive forces were simulated. When a tensile force is loaded, we speculate that the situation will be more complex. Considering the compressive yield strength of dental tissues, strain development under compressive occlusal forces would not be significant enough to cause immediate tooth failure . However, when a lateral force is applied, higher tensile stress will be generated than when a compressive load is applied to the same area. Given the thin root wall of immature teeth, such tensile overloading may cause cracks or even root fracture. Therefore, proper adjustments of occlusal surfaces should be performed to prevent these events.
Within the limitations of this study, it was confirmed that the pattern of occlusal force distribution in immature tooth differed according to the canal obturation material used for apexification. Application of an MTA plug with a deep composite resin core could increase the occlusal force resistance of immature teeth during and/or after apexification treatment. Further in-depth studies are required to optimize apexification procedures in terms of clinical performance during the treatment period.