Influence of veneering porcelain thickness and cooling rate on residual stresses in zirconia molar crowns.

OBJECTIVE: The aim of this study was to investigate the influence of increasing veneering porcelain thickness in clinically representative zirconia molar crowns on the residual stresses under fast and slow cooling protocols. METHODS: Six veneered zirconia copings (Procera, Nobel Biocare AB, Gothenburg, Sweden) based on a mandibular molar form, were divided into 3 groups with flattened cusp heights that were 1mm, 2mm, or 3mm. Half the samples were fast cooled during final glazing; the other half were slow cooled. Vickers indentation technique was used to determine surface residual stresses. Normality distribution within each sample was done using Kolmogorov-Smirnov & Shapiro-Wilk tests, and one-way ANOVA tests used to test for significance between various cusp heights within each group. Independent t-tests used to evaluate significance between each cusp height group with regards to cooling. RESULTS: Compressive stresses were recorded with fast cooling, while tensile stresses with slow cooling. The highest residual compressive stresses were recorded on the fast cooled 1mm cusps which was significantly higher than the 2 and 3mm fast cooled crowns (P<0.05). There was a significant linear trend for residual stress to decrease as veneering porcelain thickness increased in the fast cooled group (P<0.05). No significant differences were found between the various cusp heights during slow cooling (P≥0.05). SIGNIFICANCE: Cooling rate and geometric influences in a crown anatomy have substantially different effects on residual stress profiles with increasing veneering porcelain thickness compared to the basic flat plate model.

A comparison between rib fracture patterns in peri- and post-mortem compressive injury in a piglet model.

OBJECTIVES: Forensic biomechanics is increasingly being used to explain how observed injuries occur. We studied infant rib fractures from a biomechanical and morphological perspective using a porcine model. METHODS: We used 24, 6th ribs of one day old domestic pigs Sus scrofa, divided into three groups, desiccated (representing post-mortem trauma), fresh ribs with intact periosteum (representing peri-mortem trauma) and those stored at -20°C. Two experiments were designed to study their biomechanical behaviour fracture morphology: ribs were axially compressed and subjected to four-point bending in an Instron 3339 fitted with custom jigs. Morphoscopic analysis of resultant fractures consisted of standard optical methods, micro-CT (µCT) and Scanning Electron Microscopy (SEM). RESULTS: During axial compression fresh ribs did not fracture because of energy absorption capabilities of their soft and fluidic components. In flexure tests, dry ribs showed typical elastic-brittle behaviour with long linear load-extension curves, followed by short non-linear elastic (hyperelastic) behaviour and brittle fracture. Fresh ribs showed initial linear-elastic behaviour, followed by strain softening and visco-plastic responses. During the course of loading, dry bone showed minimal observable damage prior to the onset of unstable fracture. Frozen then thawed bone showed similar patterns to fresh bone. Morphologically, fresh ribs showed extensive periosteal damage to the tensile surface with areas of collagen fibre pull-out along the tensile surface. While all dry ribs fractured precipitously, with associated fibre pull-out, the latter feature was absent in thawed ribs. CONCLUSIONS: Our study highlights the fact that under controlled loading, fresh piglet ribs (representing perimortem trauma) did not fracture through bone, but was associated with periosteal tearing. These results suggest firstly, that complete lateral rib fracture in infants may in fact not result from pure compression as has been previously assumed; and secondly, that freezing of bone during storage may affect its fracture behaviour.

Compressive rib fracture: peri-mortem and post-mortem trauma patterns in a pig model.

Despite numerous studies on high impact fractures of ribs, little is known about compressive rib injuries. We studied rib fractures from a biomechanical and morphological perspective using 15, 5th ribs of domestic pigs Sus scrofa, divided into two groups, desiccated (representing post-mortem trauma) and fresh ribs with intact periosteum (representing peri-mortem trauma). Ribs were axially compressed and subjected to four-point bending in an Instron 3339 fitted with custom jigs. Morphoscopic analysis of resultant fractures consisted of standard optical methods, micro-CT (µCT) and scanning electron microscopy (SEM). During axial compression, fresh ribs had slightly higher strength because of energy absorption capabilities of their soft and fluidic components. In flexure tests, dry ribs showed typical elastic-brittle behaviour with long linear load-extension curves, followed by relatively short non-linear elastic (hyperelastic) behaviour and brittle fracture. Fresh ribs showed initial linear-elastic behaviour, followed by strain softening, visco-plastic responses. During the course of loading, dry bone showed minimal observable damage prior to the onset of unstable fracture. In contrast, fresh bone showed buckling-like damage features on the compressive surface and cracking parallel to the axis of the bone. Morphologically, all dry ribs fractured precipitously, whereas all but one of the fresh ribs showed incomplete fracture. The mode of fracture, however, was remarkably similar for both groups, with butterfly fractures predominating (7/15, 46.6% dry and wet). Our study highlights the fact that under controlled loading, despite seemingly similar butterfly fracture morphology, fresh ribs (representing perimortem trauma) show a non-catastrophic response. While extensive strain softening observed for the fresh bone does show some additional micro-cracking damage, it appears that the periosteum may play a key role in imparting the observed pseudo-ductility to the ribs. The presence of fibrous pull-out and grooving of the outer tensile surface associated with periosteal stretching suggests that the periosteum under tension is able to sustain very high strain and bridge the mouth of the extending butterfly crack, thereby contributing to the observed strain-softening behaviour.