PMC5703090_01
(a-d) Photographs of the crown and the abutment of case 1. The crown fractured mesio (m)- distally (d) in two fragments, while some minor fragments on the mesial side went missing, most likely during intraoral fracture. (b) Luting remnants can be found on the implant abutment surface. (d) The screw hole clinically filled with glass ionomer cement (*). (e and f) Higher resolution SEM images (mapped from individual SEM images) from the fracture surfaces of the fragments shown in (a) and (c). Both fragments exhibit the corresponding compression curl (circle), indicating the termination of the fracture event. (e) The palatal fragment shows clear fracture patterns on the cusp of the mesial proximal ridge (*1) and on the intaglio side of the disto-buccal margin (*2). The buccal fragment shows only minor luting remnants on the intaglio side (arrow). Most of the yellowish luting remnants are found on the zirconia abutment (b), indicating the crown-adhesive interface as the weakest link. In consequence, it seems very likely that the crown fracture was preceded by a debonding event, most likely leading to wedging of the crown and subsequent inclined shear loading in the direction assigned in (e) (dotted arrows). The fracture origin on the distal intaglio surface (*2) is thus termed a secondary event leading to abfraction of the missing marginal fragment. (e) The general direction of crack propagation (dcp) is indicated with arrows. The fracture started on the mesial side and ended on the disto-buccal side.
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The fractographic examination is presented in Figures 1 and 2.
(a) The fracture origin on the palatal fragment of Figure 1(e) (see *1). A large wear facet and damage accumulation zone can be seen on the surface (*) as well as multiple and overlaying mist and hackle regions and radial crack extension (arrows). (b) A magnification of the compression curl of Figure 1 (e) and (f), the endpoint of the fracture event on the disto-buccal side of the crown. (c) A high magnification of a fracture region close to the compression curl in (b). Small cracks are observable hypothesizing a degradation artifact in the microstructure of the resin composite crown material. (d) The secondary fracture event. Hackle indicates the dcp (arrows) radial from the fracture origin. (d) A clean intaglio surface (*) without any remnants of the luting agents, indicating an adhesive failure at the crown-adhesive interface.
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The fractographic examination is presented in Figures 1 and 2.
PMC5703090_02
(a-d) Photographs of the crown and the implant abutment of case 2. The crown fractured mesio (m)-distally (d) in two fragments. (b) The zirconia abutment was found free of luting remnants. (d) The screw hole clinically filled with glass ionomer cement (*). (e) and (f) Higher resolution SEM images (mapped from individual SEM images) from the fracture surfaces of the fragments shown in (a) and (c). Both fragments exhibit the corresponding compression curl on the mesial margins (circles), indicating the termination of the fracture event. On the opposite (distal) margins, (e) and (f) shows a sharp and tilted fracture plane, suggesting the fracture initiation site at the marginal ridge (*). A clear fracture origin cannot be located, but fine hackle lines trace back to the marginal ridge, as shown in Figure 4(d). Luting remnants can be found only on the crown intaglio surface as indicated by the arrows in (e), indicating the zirconia-adhesive interface as the weakest link. It is likely that the fracture was preceded by a debonding event, resulting in tilting of the crown and shear fracture originating from the crown margins. (f) The general direction of crack propagation (dcp) is indicated by arrows. The fracture started on the distal side and terminated on the cervical mesial margin.
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The fractographic examination is presented in Figures 3 and 4.
(a-d) High-resolution SEM images from the luting layer (Figure 3(a, b), the compression curl (Figure 3(c)) and the fracture initiation site (Figure 3(d)). While Figure 3(a) and (b) is taken from the palatal fragment, Figure 3(c) and (d) is taken from the buccal fragment. Figure 3(a) and (b) clearly show the adhesive layer on the zirconia-adhesive side as well as on the crown-adhesive side with the sandwich luting agent in between (see * in (b)). The dcp can be seen from fine hackle lines, especially in the smooth and flat adhesive layer, indicated by arrows in Figure 3(a) and (b). Figure 3(c) and (d) also shows the luting layer and the dcp, indicated by arrows. An overview of the general dcp is shown in Figure 3(f).
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The fractographic examination is presented in Figures 3 and 4.
PMC5703090_03
(a and b) Photographs of the retrieved fragments of case 3. The crown fractured mesio (m)-distally (d) in two fragments. The corresponding abutment was not available for fractographic analysis due to further patient treatment. (c) The occlusal view on the crown with the screw hole filled with glass ionomer cement (*) and the fracture plane eccentrically located on the lingual side, separating the lingual cusp (arrow). Interestingly, the fracture started on the massive and bulky lingual cusp which indicates an unbalanced masticatory loading. (d and e) Higher resolution SEM images (mapped from individual SEM images) from the fracture surfaces of the fragments shown in (a) and (b). Both fragments exhibit the corresponding compression curl on the distal side (circles), indicating the termination of the fracture event. On the opposite (mesio-lingual) side, (d) clearly shows the fracture origin (*1). A secondary fracture event is further located on the inner plane on the gingival third of the distal margin (*2). Fine hackle lines, especially in the adhesive layer (see further analysis in Figure 6), trace the general dcp as indicated in (e). The fracture started on the mesio-lingual cusp and terminated on the distal side. Luting remnants can be found on the crown intaglio surface. Some remnants broke off, as shown in (d) (arrow) and (e) (*). The luting layer tends to delaminate from either the crown or the zirconia abutment. In principle, both adhesive interfaces (but predominantly the adhesive-zirconia interface) represent the weak link leading to crown debonding.
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The fractographic examination is presented in Figures 5 and 6.
(a-e) High-resolution SEM images from the (a) fracture origin (b-e) luting layer, (f) the compression curl. While (a) is taken from the buccal fragment, (b-f) show magnifications of the lingual fragment. (a, b, d, f) Arrows indicate the dcp. (a) The fracture origin with a fracture releasing subsurface defect (*) and the radial fracture mirror and hackle region. The microstructure of the resin-based crown material did not elucidate distinct fractographic patterns. (b) The smooth adhesive layer, on the other hand, clearly indicates the dcp. Parallel, crazing-like hackle lines are found. (c) Delamination of the luting layer occurred mainly from the zirconia abutment, but also from the crown surface (arrows). (d and e) Also, the microstructure of the luting agent indicates the dcp. A magnification of the microstructure in (e) exhibits gull-wing-like microstructural features indicating the dcp. (f) The compression curl as the endpoint of the fracture event.
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The fractographic examination is presented in Figures 5 and 6.