繰返し荷重により縦割れになった修復歯の疲労解析

Fatigue analysis of restored teeth longitudinally cracked under cyclic loading.

抄録

目的:

有限要素解析（FEA）と応力-寿命（S-N）法により，修復歯の疲労挙動，特に繰返し機械的負荷による縦歯根割れの発生機構を検討する．

OBJECTIVE: To investigate the fatigue behavior of restored teeth, in particular the mechanisms of longitudinal dentinal cracking under cyclic mechanical loading, using finite element analysis (FEA) and the stress-life (S-N) approach.

方法:

レジン複合材料で修復した10本の歯根充填型小臼歯に，縦割れを生じさせる段階的応力反復荷重を負荷した．破壊荷重と各荷重レベルで完了したサイクル数を記録した．FEAを用いて，グローバルサイクル荷重下における各コンポーネントの応力振幅を予測した．FEA では，象牙質と複合材料の界面について，無傷と剥離の両方の状態を考慮した．また，破壊メカニズムの解明のため，予測された応力集中を破壊パターンと比較した．さらに，S-N 法を用いて，修復された歯の異なるコンポーネントの寿命を予測した．累積疲労損傷は，異なる応力振幅の下で過ごした寿命の割合の合計で表された．

METHODS: Ten root-filled premolars restored with resin composites were subjected to step-stress cyclic loading to produce longitudinal cracks. Fracture loads and number of cycles completed at each load level were recorded. FEA was used to predict the stress amplitude of each component under the global cyclic load. Both intact and debonded conditions were considered for the dentin-composite interface in the FEA. The predicted stress concentrations were compared with the fracture patterns to help elucidate the failure mechanisms. The S-N approach was further used to predict the lifetimes of the different components in the restored teeth. Cumulative fatigue damage was represented by the sum of the fractions of life spent under the different stress amplitudes.

結果:

平均破壊荷重770±45N，平均破壊回数32,297±12,624回で，50％の試料に縦割れが認められた．象牙質縦割れは空洞の線角付近から始まり，根元に向かって縦方向に進展するようであった．象牙質-複合材料界面の寿命係数の和は，象牙質の寿命係数が1よりはるかに小さいのに対し，約7000回で1を超え，象牙質の破壊に先立って界面剥離が生じることが示唆された．このことは，割れた試料の界面が広がっていることを示すマイクロCT画像からも支持された．また，剥離した歯では，FEAにより窩洞の歯肉壁に象牙質応力集中が認められ，周期的荷重試験で認められた縦割れの位置と一致した．また，象牙質は30,000サイクルで寿命の和が1に近くなり，実験値とほぼ同じであった．

RESULTS: Longitudinal cracks were seen in ~50% of the samples with a mean fracture load of 770 ± 45 N and a mean number of cycles to failure of 32,297 ± 12,624. The longitudinal dentinal cracks seemed to start near the line angle of the cavity, and propagated longitudinally towards the root. The sum of fractions of life spent for the dentin-composite interface exceeded 1 after ~7000 cycles when that for dentin was much lower than 1, indicating that interfacial debonding would occur prior to dentin fracture. This was supported by micro-CT images showing widened interfacial space in the cracked samples. In the debonded tooth, FEA showed dentinal stress concentrations at the gingival wall of the cavity, which coincided with the longitudinal cracks found in the cyclic loading test. The sum of fractions of life spent for dentin was close to 1 at ~30,000 cycles, similar to the experimental value.

意義:

歯根充填歯では，繰返し荷重による象牙質の縦割れの前に，象牙質-複合材界面の剥離が起こる可能性がある．このような事象のおおよその発生時期は，FEAによる応力を用いた疲労解析によって推定することができる．したがって，この方法は，根充歯の修復設計の寿命を評価するために使用することができる．

SIGNIFICANCE: Debonding of the dentin-composite interface may occur prior to longitudinal cracking of dentin in root-filled teeth under cyclic loading. The approximate time of occurrence for these events could be estimated using fatigue analysis with stresses provided by FEA. This methodology can therefore be used to evaluate the longevity of restoration designs for root-filled teeth.