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OBJECT Cortical trajectory screw constructs, developed as an alternative to pedicle screw fixation for the lumbar spine, have similar in vitro biomechanics. The possibility of one screw path having the ability to rescue the other in a revision scenario holds promise but has not been evaluated. The objective in this study was to investigate the biomechanical properties of traditional pedicle screws and cortical trajectory screws when each was used to rescue the other in the setting of revision. METHODS Ten fresh-frozen human lumbar spines were instrumented at L3–4, 5 with cortical trajectory screws and 5 with pedicle screws. Construct stiffness was recorded in flexion/extension, lateral bending, and axial rotation. The L-3 screw pullout strength was tested to failure for each specimen and salvaged with screws of the opposite trajectory. Mechanical stiffness was again recorded. The hybrid rescue trajectory screws at L-3 were then tested to failure. RESULTS Cortical screws, when used in a rescue construct, provided stiffness in flexion/extension and axial rotation similar to that provided by the initial pedicle screw construct prior to failure. The rescue pedicle screws provided stiffness similar to that provided by the primary cortical screw construct in flexion/extension, lateral bending, and axial rotation. In pullout testing, cortical rescue screws retained 60% of the original pedicle screw pullout strength, whereas pedicle rescue screws retained 65% of the original cortical screw pullout strength. CONCLUSIONS Cortical trajectory screws, previously studied as a primary mode of fixation, may also be used as a rescue option in the setting of a failed or compromised pedicle screw construct in the lumbar spine. Likewise, a standard pedicle screw construct may rescue a compromised cortical screw track. Cortical and pedicle screws each retain adequate construct stiffness and pullout strength when used for revision at the same level.

a b s t r a c tThe formation of dry ice particles in a jet flow has been studied experimentally. The particles were produced by rapid expansion of liquid carbon dioxide through a nozzle, based on the Joule-Thomson effect. Their size distribution was measured by a laser diffraction method. The experimental results showed that the primary dry ice particles ejected from the nozzle were about 1 mm in mass median diameter. However, they grew initially in the jet flow and then became smaller due to sublimation. As a result, a bimodal size distribution was formed at increased distances from the nozzle outlet. The presence of a thermally insulated tube at the outlet of the expansion nozzle enhanced the agglomeration of the particles, whereby agglomerates of about 100 mm in mass median diameter were recorded. The agglomeration process is considered to take place by the simultaneous processes of particle deposition and reentrainment; i.e. agglomerated particles are reentrained from the layer of dry ice particles deposited on the tube walls. The agglomerate size decreased with increasing flow velocity, due to the greater detachment force applied to the deposition layer. Therefore, the flow velocity was found to be an important parameter influencing the agglomeration of dry ice particles.

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Poor powder flow leads to many problems during manufacturing and can lead to inaccurate dosing and off-specification products. Powder flowability is commonly assessed under 2 relatively high applied loads using shear cells by characterising the unconfined yield strength at a range of applied loads. For applied stresses below 1 kPa, it becomes increasingly difficult to obtain reliable values of the unconfined yield strength. The bulk cohesion and tensile strength of the powder is then obtained by extrapolating the yield locus to zero and negative loads. However, the reliability of this approximation for a given material is not known. To overcome this limitation, techniques such as the Raining Bed Method, Sevilla Powder Tester and the newly-developed Ball Indentation Method may be used.In this paper, we report our measurement results of the tensile strength of glass beads, -lactose monohydrate and various sizes of fluid catalytic cracking powders determined by the Sevilla Powder Tester and Raining Bed Method and compare them with those inferred from the Schulze Shear Cell. The results of the latter are also compared with those of the Ball Indentation Method. The outcome suggests that in the case of shear cell tests, the extrapolation of the yield locus to lower or negative loads is unsafe. The ball indentation enables the characterisation of highly cohesive powders at very low compressive loads; however extrapolation to negative loads is still not reliable. In contrast, the Sevilla Powder Tester and Raining Bed Methods are able to characterise the tensile strength directly, but high bulk cohesion poses difficulties as the internal bed failure needs to be analysed in order to reliably estimate the tensile strength. These methods provide a better understanding of powder flow behaviour at low stresses, thus enabling a greater control of manufacturing processes.