Tooth tissue is damaged by decay, trauma and other factors, generally can be restored by pile core crown. There are many kinds of clinically used pile cores, such as preformed metal piles, cast metal piles, ceramic piles, fiber piles, etc. The more used piles are various metal piles, but they have many disadvantages, such as easy to lead to the occurrence of root fracture, easy corrosion, poor aesthetic properties, etc. In recent years, along with the progress of fiber-reinforced resin research, fiber piles have been widely used in clinical applications, which have good properties not found in single materials, such as corrosion resistance, high strength, fatigue resistance, high electrical insulation, and excellent biocompatibility [1]. Biomechanics is an emerging interdisciplinary discipline formed by the combination, interpenetration and integration between mechanics and biology, and is a discipline that studies mechanical problems in living organisms based on defined mechanical principles. Biomechanics and dentistry fusion, cross-formation of oral biomechanics, to study the basic scientific problems in dentistry, solve the clinical practical problems, the development of clinical technology tools. The clinical problems to be solved in the field of prosthodontics are mostly related to biomechanical contents and there are a large number of biomechanical problems. Therefore, this paper intends to review the recent progress of biomechanical research on fiber pile restorations. A comparative study between fiber piles and other types of piles Preformed metal piles and cast metal piles have been widely used in clinical practice, but there are disadvantages such as easy occurrence of root fracture, easy corrosion, and poor aesthetic properties, which can no longer meet the increasing requirements of restoration. Although fiber piles were not widely used in dental clinics until the 1990s [2], because of their unique advantages, especially their excellent biomechanical characteristics. Erik Asmussen et al [3] investigated the stresses on the dentition of three types of piles: fiberglass, preformed titanium and zirconia piles, with the magnitude of stresses becoming progressively smaller from fiberglass, preformed titanium and zirconia piles.A. Pegoretti et al [4] investigated the three-point bending strength of a fiberglass pile with a smooth cylindrical tip by means of a three-dimensional finite element method. In comparison with gold cast piles and carbon fiber piles, it was found that the gold cast piles produced the largest stress concentration at the pile-dentin interface and the glass fiber piles had the smallest stress values. The authors concluded that this was mainly due to the fact that the stiffness of the fiber pile was very close to that of the dentin, so the stress field generated by the glass fiber pile was very close to that of a normal tooth except for the cervical margin. Chayanee Chatvanitkul et al [5] also investigated the stress distribution in bent root canals restored with different pile cores using the finite element method. The results showed that the closer the elastic modulus of the pile core was to the dentin, the lower the tensile stress was and the stress was mainly concentrated on the root surface; as the elastic modulus of the pile core increased, the tensile stress gradually increased and the stress was concentrated in the pile and apical portion of the root. The authors also concluded that glass fiber pile and resin core restorations can minimize the amount of tooth abrasion and increase the reasonable retention of the pile core.Natercia R Silva et al [6] compared metal piles and fiber piles and used a three-dimensional finite element method to determine the shear stresses, and the results showed that metal piles produced a greater concentration of crown-side stresses than fiber piles and that the stresses in fiber piles were more uniformly distributed. Sung-Ho Jung et al [7] investigated the edge microleakage and fracture patterns in the cast pile core repair group, the preformed metal pile repair group, the fiber pile repair group, and the all-ceramic pile repair group by dynamic spiking experiments. The results showed that marginal microleakage was less in the fiber pile restoration and all-ceramic pile restoration groups than in the other two groups, and that the fracture pattern in the fiber pile restoration group was more favorable for retreatment of the dentition. et al [8] also experimentally confirmed that fiber pile restorations optimize the fracture strength and improve the fracture pattern. Evidence-based dentistry also summarizes the advantages of fiber piles. Joanna N. Theodosopoulou et al [9], searched 997,141,25 articles from MEDLINE, Cochrane, and EMBASE, respectively, performed a systematic evaluation. Among them, the results of randomized controlled trials suggested that carbon fiber piles were significantly biomechanically superior to precious metal piles, glass fiber piles were superior to metal threaded piles, but inferior to titanium piles and superior to quartz piles.In a systematic evaluation of articles in PubMed/Medline from 1990-2005, Dietschi D et al [10] concluded that fiber piles and composite resins can jointly resist fatigue stresses and are the most superior treatment option available. It is also less likely to produce interfacial cracks and cause severe tooth fractures after restoration failure than metal or all-ceramic piles. There are many types of fiber piles used in clinical practice, and the most commonly used are carbon fiber piles, glass fiber piles, and quartz fiber piles, which differ slightly in their biological properties. In a study by Vivian J.-J. Wanq et al [11] on quartz fiber piles and carbon fiber piles using acid-etching bonding system and self-etching bonding system respectively, it was found that quartz fiber piles had higher bond strength than carbon fiber piles, and acid-etching bonding system had higher bond strength than self-etching bonding system, and there was a significant decrease in bond strength from crown side to root side. Ayse D. Kececi et al [12] investigated the bond strength of translucent quartz fiber piles, opaque fiberglass piles and photoelectric fiberglass piles with two different dual-cure bonding systems (Variolink II and RelyX Unicem) and showed that the bond strength of opaque fiberglass piles with Variolink II and RelyX Unicem was better than that of carbon fiber piles with self-etching bonding system. The results showed that the bonding strength of opaque fiberglass piles with the Variolink II bonding system was the highest, suggesting that the bonding strength is affected by the type of adhesive and the type of pile. In a study by Mustafa Kalkan et al [13] on three systems of glass fiber piles, opaque fiber piles (Snowpost), semi-permeable fiber piles (FiberMaster) and photoelectric fiber piles, it was found that the bond strengths of these three systems to teeth were significantly different, with non-permeable and photoelectric glass fiber piles having similar bond strengths, which were significantly higher than those of semi-permeable glass fiber piles . In the study of the bond strength of the cervical, mesial and apical segments of the fiberglass piles, it was found that the bond strength of the cervical segment was higher than that of the mesial and apical segments in the semi-permeable and photoelectric fiberglass piles, but there was no significant difference in the non-permeable fiberglass piles. And all differences were independent of time. III. Study of fiber piles with different morphology and lengths The stress effect on the dentin tissue of the root canal wall may also vary depending on the morphology and length of the fiber piles, etc. In a systematic evaluation of articles from 1990-2005, Dietschi D et al [10] suggested that root-side bonding of fiber piles is a difficult problem due to the ovoid shape of the root canal and the influence of the microstructure of the critical dentin in the deepest part of the root canal. Poskus LT et al [14] studied the differences in pile profile: conical and gently sloping, pile surface profile: parallel and serrated surfaces, bonding agent: dual-curing bonding agent (Rely-X ARC) and self-curing bonding agent and concluded that the retention force of fiberglass piles is not influenced by the design of the pile, the roughness of the surface and the type of bonding agent, the choice of serrated Erik Asmussen et al [3] also confirmed that the stresses in tapered piles are generally higher than those in parallel piles, and that increasing the length or diameter of the pile can also reduce the stresses. There are also many studies on the effect of fiber pile length on biomechanics. m.-L. HSU et al [15] in a three-dimensional finite element study of 7, 10 and 13 mm fiber piles and metal piles concluded that when the length of the pile was changed from 13 mm to 7 mm, the stress pattern of the fiber pile did not change much, while the stress pattern of the metal pile showed a huge M-shaped change. Therefore, when using metal piles, it is required that they can be as long as possible, and this involves removing more dental tissue and reducing the dental resistance. In contrast, fiber piles have the advantage of lower length requirements.Necdet Adanir et al [17] chose maxillary central incisors with an average clinical crown length of 9 mm for their experiments, and the three types of fiber piles (Snowpost) bonded after 6 mm (less than the clinical crown length); 9 mm (equal to the clinical crown length) and 12 mm (greater than the clinical crown length) were The fracture strength analysis was obtained. The fracture strength of the 6 mm fiber pile group was significantly lower than that of the other two groups, while there was no significant difference between the 9 mm and 12 mm fiber pile groups. Marco FERRARI et al [16] investigated the effect of glass fiber pile length on the compressive and tensile stresses in the mesial incisors and surrounding tissues using the finite element method. They chose three types of fiber piles inserted 5, 7, and 9 mm into the root canal and showed that all fiber pile restorations affected the biomechanics of the incisors and increased the fracture resistance of the roots, but the different lengths of fiber pile insertion had little effect on the biomechanics of the restored teeth.Schmitter et al [18] also confirmed that the pile length had little effect on the fracture strength. Fourth, the study of fiber pile bonding system Francesca Zicari and others [19] studied PAN, CLF, VAR, UNI and EGC several bonding agents, the results of CLF bonding strength is the highest mainly because CLF in the phosphate as a functional group of monomeric substances, this molecule can be combined with the inlay layer, and it is stable in water, this structure can effectively improve the long-term binding force. Luca Giachetti et al [20] compared the bonding strength of translucent fiber piles with the dual-cure bonding system (Excite DSC and RelyX ARC), the self-bonding dual-cure system (RelyX Unicem), and the light-cure system (Excite DSC and The effects of these three bonding systems on the bond strength of translucent fiber piles. The results showed that the light-cured system was the smallest in the comparison of bond strength at the root tip, but was not statistically different from the other two systems; while the self-bonding dual-cured system had the lowest bond strength in the crown side and root middle portion, with a statistically significant difference. Ultimately, it was concluded that there was no difference in the interfacial bonding strength between the dual-curing and light-curing systems and the root canal, which is more suitable for bonding transparent fiber piles.Fulya Toksoy Topcu et al [22] investigated self-acid-etching bonding systems (ClearfilTM SE Bond and Optibond® all-in-one) for glass fiber piles and carbon fiber piles, all-acid-etching bonding system (XP BondTM) and dual-cure bonding system (MaxcemTM), the results showed that regardless of the type of bonding agent, fiberglass piles provided stronger retention than carbon fiber piles. Ebru Özsezer Demiryürek et al [21] analyzed the bond strength of fiber piles to root canals after treatment with five surface treatment agents (5% NaOCl; Sikko Tim; 17% EDTA; 37% phosphoric acid; 10% citric acid) and self-etching resin bonding agent bonding. The results showed that the surface treatment was able to improve the bonding strength of fiber piles and dentin. The highest bond strength was found in the Sikko Tim treatment group (16.52 ± 1.73), while the Sikko treatment did not remove the root-side dentin staining layer well, suggesting that removing the staining layer to pass the dentin tubules is not recommended when using self-etching resin bonding agent. This is because when using self-etching resin bonding agents, excessive acid etching of the dentin is formed, which affects the microtensile stress and ultimately has a detrimental effect on the resin bonding. At the same time, the improvement of the bonding efficacy of self-etching resin bonding agents is mainly based on the formation of the inlay layer rather than the resin protrusion formed by the resin entering the dentin tubules.R. DE SANTIS et al [23] used tensile experiments to analyze the stress distribution at the bonding interface of carbon fiber piles and resin bonding agents and found that the pressure distribution at the middle part of the bond length was minimal, while the maximum pressure was located at the highest part of the bonding surface. The average bond strength of the bond was and the maximum bond strength was 50 MPa. the authors attribute this optimized force transfer and high retention characteristics mainly to the design of the carbon fiber pile used. V. Study of the hoop effect Laurent Pierrisnard et al [24] found by finite element method that the maximum values of both tensile and compressive stresses were mainly concentrated in the neck region, and the tensile stress in the neck was less than 140 Pa in the presence of the hoop structure and increased to more than 230 Pa in the loss of the hoop structure. Schmitter et al [18] investigated the effect of the height of the hoop structure of fiber piles on the fracture strength of restorations and showed that increasing the height of the hoop structure or bonding with resin significantly increased the fracture strength, suggesting that resin bonding of fiber piles is preferable in cases where the height of the hoop structure is insufficient. A comparative study of the fracture strength of teeth restored with cobalt-chromium metal piles, carbon fiber piles and those restored without piles showed that the increase in the height of the hoop structure produced greater fracture strength, and the fracture strength of teeth restored without piles was significantly lower than that of teeth restored with cobalt-chromium metal piles or carbon fiber piles, but the fracture status of teeth restored with cobalt-chromium alloy piles was severe. VI. Outlook Fiber piles have superiority that cannot be matched by metal piles. The combined use of fiber piles and resin-based cores can make the final restorations closer to the original structure of natural teeth. Their strength and modulus of elasticity are close to that of dentin, and they can achieve a strong bond with dentin through resin-based bonding agents, forming a coordinated whole with the tooth in the end, making the stress evenly distributed along the root and improving the fracture strength of the restored tooth. It is foreseeable that with the continuous improvement of biomechanical properties of fiber piles, fiber piles will be more widely used in dental clinics.