Experimental Study Of The Effect Of Fiber Post And Core Redirection On The Fracture Resistance Of Tooth Root

In clinical practice, there are some patients with lip dump of anterior teethwhich result in irregular dentition of anterior teeth, thus influence their outlook.Orthodontics method is always adopted to treat these patients. However, somepatients wouldn’t stand the bad look of appliance and such long treatment cycles,and refused orthodontic treatment. So, aesthetic restorative treatment can beconsidered, usually root canal treatment. After cutting off the crown,we can makea crown with a post and core of some certain angle, according to the teethdislocation and occlusal conditions,in order to inprove the outlook of restoration.Typically, posts and cores made of casting alloys was used, but with the alloypost and core, aesthetics is not so good and root fracture easily happens. In recentyears, there is a novel compliance EverStick fiber post, which break through with the problem of unable to change direction of conventional fiber post, whilemake up the defect of bad outlook and fragile to fracture of metal post and core.But up to date, there is no detailed report on the influence of changing directionof post and core on the fracture resistance ability of root. The present study firstused the mechanical force loading test at static state to investigate the teethfracture pattern and maximum load after repair with compliance EverStick fiberpost, then lamellar CT technique, interactive medical image processing software(Mimics software), reverse engineering software (Unigraphics NX7.5) wereemployed to establish the three-dimensional finite element model of uppermandible central incisor post and core crown at different angle; based on thismodel, after repair of the post and core crown of upper mandible central incisor atdifferent angles, stress distribution of dentine and pericementum were analyzedand compared, which is expected to provide experimental basis for clinicalapplication.Methods:(1) Sixty in vitro upper mandible central incisors were randomly divided intoGroup A and Group B. Group A used EverStick fiber post and core to repair andGroup B used Co-Cr alloy post and core to repair. The two groups adopted thedirection changing repair at angles of0°,10°,20°,30°,40°. The maximumloading value and the tooth fracture pattern were recorded and analyzed aftermechanic loading at static state to repair with EverStick fiber post and core.(2)Lamellar CT technique, interactive medical image processing software (Mimicssoftware), reverse engineering software (Unigraphics NX7.5) were employed in corporately to establish the three-dimensional finite element model of uppermandible central incisor post and core crown at different angles.(3) Based on theinfinite element model of post and core crown repair of upper mandible centralincisors at different angles, the model was processed with UG cutting andconstriction to maintain unchanging of XY axis, and through change the length ofZ axis to obtain the post and core crown model of0°，10°，20°，30°，40°of uppermandible central incisors, The load locates at the cut end of the central incisorsand1/2of tongue lateral. Loading force direction includes0°vertical load and thetooth long axis is45°with the vertical direction. Load force is100N. Under thesetwo load, the effect of different angles and different materials on root periodontalmembrane and dentine stress distribution is analyzed.Results:(1) In both Group A and B, the maximum load decreases with the increasingof angles (In Group A: A0°> A10°> A20°>A30°> A40°，In Group B：B0°>B10°> B20°> B30°> B40°). At the same angels, the maximum load in group A isgreater than it in group B (A0°>B0°, A10°>B10°, A20°>B20°, A30°>B30°,A40°>B40°). So Group A has significant differences with group B (P<0.05).(2) The fracture for EverStick Post and Core mostly appears on the root neck(~73%) and root of1/3(~27%); The fracture for metal Post and Core mostlyappears on the apical tip of1/3(~73%) and vertical root (~66%).(3) Totally5three-dimensional finite element models of upper mandiblecentral incisor post and core crown at different angles were established.(4) Whatever angle loading from, peak stress appeared on tooth essence for metal Post and Core is greater than it for EverStick;Comparing with the stress ontwo materials at different angles, we found that, on either0°vertical or45°loading, the equivalent stress concentrated in the neck of the tooth, and expandedalong the direction of root outer surface to the apical turn; Whatever angle casesinclined, peak stress on0°vertical loading, appeared on tooth essence andPeriodontal membrane, are less than it on45°loading; Along with the increasingof redirection angle, peak stress, appeared on tooth essence and Periodontalmembrane, are also increasing, no matter on0°vertical or45°loading cases.Conclusion:(1) Redirection repairing upper mandible central incisor, EverStick Post andCore is much more effective than Co-Cr Metal Post and Core to improve fractureresistance; EverStick Post and Core with no more than30°tone tilted can meetthe commonly demand of bite force in clinical and also Co-Cr with no more than20°tone tilted.(2) The fracture for EverStick Post and Core mostly appears on the root neckof1/3, but for metal Post and Core, it mostly appears on the middle of root as avertical fold. So that EverStick Post and Core can effectively reduce theoccurrence of vertical root fold.Per the research of entity experiment and finite element analysis for differentAngle post-core crown repairing, we got a consistent conclusion that both oftooth stresses are increased with the increase of angles; Compared with metalpost and core, Plasticity EverStick fiber not only reduce the occurrence of verticalroot fold, but also effectively protect natural roots. Therefore, redirection EverStick fiber can meet the basic requirements in clinical.