Bending (or 3-point loading) is by far the most common mechanism of fracture for lower-extremity long bones. When a long bone is impacted transversely at the diaphysis, it reacts very much like a classic beam, in that it will bend (e.g., 7,17,51,52). As it bends, the bone tissue at the site of the impact will become compressed. The tissue on the opposite side will be stretched (termed "tension"). This is often demonstrated with a standard yellow-painted wooden pencil. As one bends the pencil, the yellow paint on the concave side begins to buckle or bunch up—evidence of compression. The paint on the other
side separates—evidence of tension. Another more dramatic example of tensile and com-pressive forces in the same tissue can be seen in a simple trick. In an uninflated balloon, the molecules of rubber are content to be in close proximity with each other. As the balloon is inflated many, but not all, of the molecules are put into tension. If the balloon is not fully inflated, it can be seen that the rubber closest to the inlet, and directly opposite of the inlet, are not being stretched (invariably the color is more intense—e.g., balloon will be "darker" in these spots because the balloon wall is thicker). Essentially these
molecules are in compression. A sharp wooden skewer poked into the side of the balloon would cause it to explode because the strength of the rubber was exceeded and the tensile forces were free to rip the balloon walls apart. However, if the skewer is carefully poked through the thicker areas of the balloon, the rubber molecules will compress around the stick and attempt to seal the hole; thus, the somewhat unintuitive results seen in Fig. 5. As with the balloon, failure of bone occurs in the areas of tension. The tensile forces lead to separation of the osseous tissue and propagation of a fracture line or lines.
Most sources agree that a transverse fracture can result from bending (Fig. 6). However, there is no consensus on oblique and wedge patterns (a.k.a. butterfly or delta fractures). Many authors assert that these patterns result from some form of combined loading (with compression or with twisting). In our simple 3-point loading of bare long bones, we routinely produced oblique, transverse, and wedge fracture patterns with absolutely no axial or torsional loading whatsoever. In nearly all cases of wedge fracture, the initial point of failure was on the tension side immediately opposite the point of impact (Fig. 7). This means that the point of the wedge is on the tension side of the bone, corroborating the work of Spitz (53), who investigated pedestrian leg impacts that resulted in wedge fractures. He stated that the wedge often points in the direction of the movement of the vehicle.
In a stroke of serendipity, while cleaning fractured bone wedges for photographs, an interesting discovery was made. After being soaked in bleach, the bones were dried in an oven. The charred bleach revealed an amazing array of small fracture lines emanating from the initial tensile failure point (Fig. 8). The impact occurred at the bone surface facing the top of the photo. The solution had seeped into the cracks and then essentially burned when the heat was applied. Presumably heating of the solution also caused mild expansion of the cracks. This heat-treatment of bone may be an excellent method for elucidating or enhancing microfractures (54). It was clear from this study that there are often many nondisplaced fracture lines in seemingly simple patterns. In later studies, the periosteum was carefully dissected away from transverse and oblique fracture fragments. In addition to the transverse and oblique fractures, tension lines are clearly visible (Fig. 9). The black arrows indicate the site of impact. Thus, it appears that all three of these patterns (transverse, oblique, and wedge) are actually slightly different manifestations of tensile failure. It is important to note that almost none of these tension lines were visible on plain radiographs. However, these lines may be helpful to the forensic pathologist or anthropologist examining postmortem specimens and attempting to determine a direction of impact.
What determines whether a bending load will produce an oblique, transverse, or wedge pattern? It may be a feature of the tissue (inherent weakness in one plane) or the dynamic load (shape of impactor, slight variations in the angle or energy of impact, etc.), or some combination of factors. Perhaps the addition of compressive or torsional loads will have some effect on the pattern, but this is clearly not essential and should not be assumed when reconstructing an injury scenario from the fracture pattern.
One feature of bone that may influence fracture pattern and bending forces is the presence of stress-risers. "Stress-riser" (also referred to as stress raiser or stress concentrator) is an engineering term that describes a weak area or defect that when loaded, will tend to fail prior to surrounding areas (55). This is analogous to the weakest link in
Fig. 8. Tension lines in a stripped, bleached, and heated wedge fracture.
a chain. An example of inducing a stress-riser would be the scoring of glass. When subjected to bending loads, glass will usually fail along the score or stress-riser. In some of our studies with bare bones, there was a concern that the bones could have been scored during retrieval and cleaning. To investigate this possibility, entire surfaces of several bones were scanned into a computer prior to testing. After fractures were produced, the images were examined under magnification with different software filtering applications (56). No surface feature was found to influence the initiation or propagation of fracture lines. Since subtle or accidental scoring appeared to have no effect, an additional study of several bones was performed involving intentional scoring. In Fig. 10, a femur is shown with a pattern etched into the surface (up to 2 mm deep) by a Stryker saw. The bone was subjected to impact, and although this extreme level of etching influenced the initiation site of the fracture, it did not significantly affect the overall pattern. The tensile failure lines easily crossed the etching to give rise to a standard wedge pattern that would be expected from the indicated impact direction.
Fig. 10. Scoring bone had little effect on dynamically produced fracture pattern.
These studies of surface artifacts were too small to determine whether fracture force was affected. However, complete holes through the cortex have been shown to influence the strength and presumably the site of fracture initiation (e.g., previous fracture site or fixation pins and screws). It has been reported that 3-mm holes can decrease tibial bending strength by 40% (57). As reported by Brooks et al., Bechtol and associates (during the 1950s) drilled various sizes of holes into dog bones that were then subjected to bending loads. They found that holes in the area of the bone that was placed in tension reduced bone strength by 30%. Interestingly, as long as the ratio of the hole size to bone diameter was not more than 30%, the decrease in strength was not significantly different for holes of differing sizes. Holes in the area of the bone that was in compression had essentially no effect on strength (57).
To this point, only artificial stress-risers have been discussed. It should be noted that internal weaknesses (due to pathology) can certainly affect fracture strength, pattern, and fragmentation levels (50). Also, one cannot ignore the role that bone micro-architecture may play in fracture patterns. The difference between wedge, transverse, and oblique fracture patterns in bending may simply be due to the random alignment of Haversian and Volkmann canals, such that a certain impact parallel to these lines results in fracture energy dissipation along the canals. These natural stress risers should be explored more thoroughly.
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