Spiral Fractures

Another type of injury in children is the undisplaced spiral fracture of the distal tibial shaft in children who have just begun walking, the Toddler s fracture. There is often no or minimal history of injury and children of this age poorly localise. X-rays can be normal in up to a third of patients. The radiograph can be repeated after 10 days to 2 weeks of immobilisation. Scintigraphy will demonstrate the fracture and its location usually before it is visible on an X-ray, but scintigraphy is not the accepted mechanism of investigation. 

Fig. 8.2a-e. The pattern of a fracture is determined by the forces applied to produce it. a Spiral fracture is the result of twisting/torsional forces, b Oblique fracture due to compressive forces, c Oblique fracture (with small transverse element) due to the bone being held fast and an uneven bending force applied distal to the fracture, d Transverse fracture due to bending force about a fulcrum, e-a Butterfly fragment due to compression and bending forces... 

Fig. 8.3. Spiral fracture of the femoral shaft. Note AP view of hip and lateral view of knee. Fracture is rotated...

Intra-operative and post-operative imaging of fractures stabilised with intra-medullary devices requires views in at least two planes, not only to visualise the adequacy of the bone alignment but also to check the position of the implants. Elastic intramedullary nails may appear to be within the bone in one plane but may be seen to lie outside the bone when viewed from a different angle (Fig. 9.4a,b). The nail will have either failed to traverse the fracture site (more common in long spiral fractures) or the sharp tip of the nail will have penetrated the cortex. This is particularly likely around the calcar (femoral neck) and lesser trochanter because the procurva-tum of the femur tends to guide the medial nail in a dorsal direction. Imaging of the hip joint is difficult but crucial to avoid this mistake. 

Transverse fractures result from direct blows. Twisting injuries produce the more common spiral fracture. In infants, this injury should always rouse suspicion of child abuse. The position of the bony fragments will be determined by the pull of adjacent muscles. For proximal metaphyseal fractures, the rotator cuff abducts the proximal fragment while the distal fragment is adducted by the pectoralis muscle. 

Fractures of the head and neck of the fibula occur rarely in isolation and are more commonly associated with proximal tibial fractures, especially compression, bicondylar and subcondylar fractures. A spiral fracture of the proximal fibula is often associated with an ankle fracture, resulting from an external rotation force. These fractures are known as Maisonneuve fractures. Fibula head fractures can occur from direct impact, valgus stress (associated with a tibial condylar fracture), and varus injuries. Varus stresses can cause avulsion of the fibular styloid at the site of the biceps tendon and fibular (lateral) collateral ligament. Peroneal nerve injury is not uncommon with these injuries. Dislocation of the proximal fibula is often missed on the initial radiograph. 

Fig. 21.9. Nonossifying fibroma. Well defined eccentric lytic lesion with a spiral fracture extending up the tibial diaphysis...

Fig. 21.20. Gorham disease. Two spiral fractures sustained following only minor trauma. Progressive lysis developed over the subsequent 12 months...

Fig. 2.16 Spiral fracture of a drive shaft of a scooter, starting at surface damage indicated by arrow [20], a mating halves, b separated halves...

When the pressure is increased much above 0.23 GPa, stick-slip extrusion occurs. At an extrusion pressure of 0.23 GPa, the extrusion stops when the viscosity increases sufficiently that the extrusion force cannot overcome the resistance to extrusion. During slip in stick-slip extrusion, a spiral fractured extrudate is produced that is similar to spiral products found in shear fracture of polymeric melts during extrusion (12). The flow profile in the solid state extruded rods is a deep shear parabola (10, 13, 24) (Figure 2) which suggests that stick-slip may arise from shear fracture of the extrudate. 

Fig. 9,20 Spiral bevel ring gear assembly from the forward transmission gear box of Boeing Vertol 234-LR (Chinook) aircraft registration G-BWFC which crashed into the sea off the Shetland Isles in 1986. Note the peripheral and radial fractures in the gear, which appeared to be responsible for the crash. There was evidence of fretting and galvanic corrosion which may have been responsible for initiation of the fracture sequence...

Looking at the melt fracture of specific polymers, we see many similarities and a few differences. Polystyrene extrudates begin to spiral from smooth at t 105 N/m2, and at higher shear stresses, they are grossly distorted. Visual observations show a wine glass entrance pattern with vortices that are stable at low stress values and spiral into the capillary and subsequently break down, as t is increased. Clearly, melt fracture is an entrance instability phenomenon for this polymer. 

Spiral mills create a high velocity helix of gas that rotates around the center of the jet mill. Solids are introduced via a venturi feed injector (Fig. 8.2) and become entrained in the turbulent helical flow. The resulting high energy collisions between particles as well as with the particles and the mill internals fracture particles to micron and submicron size. 

The term melt fracture has been applied from the outset [9,13] to refer to various types of visible extrudate distortion. The origin of sharkskin (often called surface melt fracture ) has been shown in Sect. 10 to be related to a local interfacial instability in the die exit region. The alternating quasi-periodic, sometimes bamboo-like, extrudate distortion associated with the flow oscillation is a result of oscillation in extrudate swell under controlled piston speed due to unstable boundary condition, as discussed in Sect. 8. A third type, spiral like, distortion is associated with an entry flow instability. The latter two kinds have often been referred to as gross melt fracture. It is clearly misleading and inaccurate to call these three major types of extrudate distortion melt fracture since they do not arise from a true melt fracture or bulk failure. Unfortunately, for historical reasons, this terminology will stay with us and be used interchangeably with the phase extrudate distortion. ... 

Extrudate distortion cannot generally be taken as evidence for melt fracture. For example, a commonly observed spiral-like distortion originates from vortex... 

Conductor coils are commonly of a multifilament or cable structure instead of unifilar to make them more flexible and resistant to fracture. Unifilar conductor is a single wire wound spirally around a central axis (Fig. 1.12), whereas a multifilar cable consists of two or more wires wound together in parallel spirally around a central axis (Fig. 1.13) [1]. In a multifilar coil, each conductor strand is individually coated with a thin layer of insulator, and the entire helical coil is then insulated again with a thicker conventional insulator (Fig. 1.13) [1]. 

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