Smith T B, Kelly J R, Tesk J A
Prosthodontics Department, Naval Dental School, Bethesda, MD.
J Prosthodont. 1994 Sep;3(3):138-44. doi: 10.1111/j.1532-849x.1994.tb00144.x.
Failed crowns and failure load data were studied to gain insights into the fracture behavior of prostheses under incisal-directed, load-to-failure testing.
Incisor crowns (n = 68) were fabricated: two all-ceramic groups (feldspathic veneer on high-strength core), differing in core design, and two metal-ceramic groups, differing in metal oxidation time (30 seconds v 3 minutes). Crowns were loaded to failure on their incisal edge. Gross visual, microscopic, and elemental microprobe analyses of failed crowns were coupled with Weibull analysis of the failure load data.
Failure loads were higher for the normal oxidation time (TN) than for the extended oxidation time (TE) metal-ceramic crowns (P < .02), but both groups had indistinguishable Weibull moduli indicating the possibility of a common failure origin. Fracture behavior and Weibull results both implicated the oxide layer as being the origin of failure. The ratio of fracture loads (TE/TN) corresponded well with calculated oxide-volume ratios. Failure loads were lower for the all-ceramic than for the metal-ceramic crowns (P < .001). Fifty percent of the all-ceramic crowns failed by delamination of veneering glass alone, leaving a thin layer of residual glass on the core surface. Scanning electron microscope views showed that delamination occurred 10 to 50 microns away from the core-veneer interface. Electron microprobe elemental analysis of the core-veneer interface showed that residual core infiltration glass was not present on the core surface and that chemical alterations in the veneering glass were apparently limited to less than a 2- to 3-microns thick layer.
Failure for both restorative systems involved interfacial stresses with crack propagation occurring at or near the core-veneer interface. The weaker interface in the metal-ceramic system probably resulted from an increase in surface oxide volume, irrespective of any change in its adherence or physical properties. For the ceramic crowns, delamination crack fronts appeared to propagate through chemically unaltered veneering porcelain. Both the Weibull moduli and characteristic strengths were indistinguishable between either of the two ceramic core designs or between groups failing from delamination with or without core cracking/failure. This is consistent with delamination being the primary fracture process during failure. Clinical implications should not be drawn from results of this study because no correlation is known to have ever been established between clinical behavior and incisal load-to-failure results.
研究失败的牙冠及失败载荷数据，以深入了解在切向加载至破坏测试下修复体的断裂行为。
制作切牙牙冠（n = 68）：两个全瓷组（高强度核上的长石质贴面），核设计不同；两个金属烤瓷组，金属氧化时间不同（30秒对3分钟）。牙冠在其切缘加载至破坏。对失败牙冠进行大体视觉、微观和元素微探针分析，并对失败载荷数据进行威布尔分析。
正常氧化时间（TN）的金属烤瓷牙冠的失败载荷高于延长氧化时间（TE）的金属烤瓷牙冠（P <.02），但两组的威布尔模量无显著差异，表明可能存在共同的失效起源。断裂行为和威布尔分析结果均表明氧化层是失效的起源。断裂载荷比（TE/TN）与计算出的氧化物体积比吻合良好。全瓷牙冠的失败载荷低于金属烤瓷牙冠（P <.001）。50%的全瓷牙冠仅通过贴面玻璃分层而失败，在核表面留下一层薄薄的残余玻璃。扫描电子显微镜观察表明，分层发生在距核 - 贴面界面10至50微米处。对核 - 贴面界面进行电子微探针元素分析表明，核表面不存在残余的核浸润玻璃，贴面玻璃中的化学变化明显局限于小于2至3微米厚的一层。
两种修复系统的失效均涉及界面应力，裂纹扩展发生在核 - 贴面界面处或其附近。金属烤瓷系统中较弱的界面可能是由于表面氧化物体积增加所致，而与其附着力或物理性能的任何变化无关。对于陶瓷牙冠，分层裂纹前沿似乎在化学性质未改变的贴面瓷中扩展。两种陶瓷核设计之间或分层失败且有或无核裂纹/失效的组之间，威布尔模量和特征强度均无显著差异。这与分层是失效期间的主要断裂过程一致。本研究结果不应引申出临床意义，因为尚未发现临床行为与切向加载至破坏结果之间存在相关性。
