Facet fracture

The typical fractures in the sub-axial cervical spine are usually compression fractures of the vertebral body and facet fractures. 

This can be unilateral or bilateral. Bilateral facet dislocation can often result in significant spinal cord injury. Bilateral facet dislocations with locked facets can be seen with flexion injuries and are due to significant ligamentous disruption in the posterior and middle columns (Fig. 20.13a-e). On radiographs bilateral facet dislocation is seen as anterior displacement of one vertebra with respect to the adjacent vertebra of more than 50%. Facet dislocations may be associated with facet fractures. Again these injuries are best demonstrated by CT. Spondylolisthesis is seen in association with flexion injuries and usually denotes significant soft tissue injury. Similarly retrolisthesis can be seen in extension injuries. 

Studies on the influence of polymers on the crystallization of calcium carbonates in vitro have shown that metastable liquid complexes with anionic polymers can be formed that subsequently transform to calcite. Studies of carbonate biomineralization have shown that amorphous calcium carbonate forms either transiently or as a stable phase. It has also been shown that absorbed proteins modify the fracture properties of carbonates. In particular, fracture surfaces show smooth conchoidal fracture, like glass, rather than the faceted fracture characteristic of normal crystals. This suggests that we have much to learn about the modification of the properties of crystals and amorphous solids by entrained polymer (see Fig. 3). There are parallels with the Lanxide process for... 

Finally, below 30 MPa m (Region I), striations are no longer visible, but a highly faceted fracture surface appears due to crystallographic fracture along intense slip bands. Precisely the same general appearance can be seen on stainless steel type 305 as shown in Fig. 10.23 [36]. 

Some manufacturers have experienced die above mentioned Ni3S2 scale formation phenomenon under certain gas conditions, which led to die failure of a rotating blade. One such experience involved a fracture dial was distinctly intergranular with evidence of secondary intergranular cracks or grain separation across die fracture. Intergranular facets of die fracture were sharp and distinct with little evidence of any ductile mode. The fracture appeared to have occurred in a brittle intergranular mode. 

Figure 4. Continued. STM scans of an MgO fracture surface produced in three point bend (c) a step between two (100) facets in a typical, smooth region on the compressive side of the sample. A schematic representation of the broken crystal is shown in (d). Note the contrasting distance scales in the figures.

Figure 12.4 (a) Intergranular fracture of an alumina sample showing creep cavitation due to compressive creep at 1600°C. Note closely spaced cavities along the two-grain facets. (b) Schematic of cavity formation in viscous grain boundary films as a result of applied tensile stress. 

The most common type of injury due to combined tension and extension of the cervical spine is the whiplash syndrome. However, a large majority of such injuries involve the soft tissues of the neck, and the pain is believed to reside in the joint capsules of the articular facets of the cervical vertebrae [Wallis et al., 1997]. In severe cases, teardrop fractures of the anterosuperior aspect of the vertebral body can occur. Alternately, separation of the anterior aspect of the disk from the vertebral endplate is known to occur. More severe injuries occur when the chin impacts the instrument panel or when the forehead impacts the windshield. In both cases, the head rotates rearward and applies a tensile and bending load on the neck. In the case of windshield impact by the forehead, hangman s fracture of C2 can occur. Garfin and Rothman [1983] suggested that it is caused by spinal extension combined with compression on the lamina of C2, causing the pars to fracture. 

When a force is applied to the posterosuperior quadrant of the head or when a crown impact is administered while the head is in flexion, the neck is subjected to a combined load of axial compression and forward bending. Anterior wedge frartures of vertebral bodies are commonly seen, but with increased load, burst fractures and fracture-dislocations of the facets can result. The latter two conditions are unstable and tend to disrupt or injure the spinal cord, and the extent of the injury depends on the penetration of the vertebral body or its fragments into the spinal canal. Recent experiments by Pin tar et al. [1989, 1990] indicate that burst fractures of lower cervical vertebrae can be reproduced in cadaveric specimens by a crown impact to a flexed cervical spine. A study by Nightingale et al. [1993] showed that fracture-dislocations of the cervical spine occur very early in the impact event (within the first 10 ms) and that the subsequent motion of the head or bending of the cervical spine cannot be used as a reliable indicator of the mechanism of injury. 

Frontal impacts to the head with the neck in extension will cause compression-extension injuries. These involve the fracture of one or more spinous processes and, possibly, symmetrical lesions of the pedicles, facets, and laminae. If there is a fracture-dislocation, the inferior facet of the upper vertebra is displaced posteriorly and upward and appears to be more horizontal than normal on x-ray. 

In the case of cathodicaUy protected samples, there is no superficial attack and cracks were not observed. The sample has britde surface with precipitates and porosity on the facets of the a-grains, as shown in Fig. 9.31 [115]. Only one transgranular fracture in the martensite is observed in the upper part of the micrograph. The porous areas were formed... 

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