European spine journal : official publication of the European Spine Society, the European Spinal Deformity Society, and the European Section of the Cervical Spine Research Society
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Comparative Study
Biomechanical compression tests with a new implant for thoracolumbar vertebral body replacement.
The authors present an investigation into the biomechanical functioning of a new titanium implant for vertebral body replacement (Synex). Possible indications are fractures and/or dislocations with damage of the anterior column, posttraumatic kyphosis and tumors of the thoracolumbar spine. The construction must be supplemented by a stabilizing posterior or anterior implant. ⋯ A significant (P < 0.001) correlation (R = 0.89) between Fmax and BMD was found. Synex was found to be at least comparable to MOSS concerning the compressive performance at the vertebral end-plate. A possible consequence of the significantly higher mean compression forces between 1 and 2 mm displacement might be decreased collapse of the implant into the vertebral body in vivo.
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Although several clinical applications of transpedicular screw fixation in the lumbar spine have been documented for many years, few anatomic studies concerning the lumbar pedicle and adjacent neural structures have been published. The lumbar pedicle and its relationships to adjacent neural structures were investigated through an anatomic study. Our objective is to highlight important considerations in performing transpedicular screw fixation in the lumbar spine. ⋯ The NRD was between 3.3 and 3.9 mm. Levels of significance were shown for the P < 0.05 and P < 0.01 levels. On the basis of this study, we can say that improper placement of the pedicle screw medially and inferiorly should be avoided.
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The lumbar spine is of primary importance in gait and its development is influenced by the upright posture adopted in human locomotion. However, little is known about the kinematic behavior of the lumbar spine during walking. The aim of this study was to examine (1) lumbar spine kinematics during walking, (2) the effect of walking velocity on lumbar motion patterns and (3) the coupling characteristics of rotation and bending. ⋯ Coupling of rotation and bending during walking was individually variable and dependent on walking velocity. Moreover, the smoothness of the bending-rotation path varied with walking velocity. A simplified envelope of lumbar coupling characteristics during walking is presented, and the existence of an individually variable walking speed that is characterized by a more harmonic lumbar contribution is hypothesized.
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Psoas muscle (PM) function with regard to the lumbar spine (LS) is disputed. Electromyographic studies attribute to the PM a possible role as stabilizer. Anatomical textbooks describe the PM as an LS flexor, but not a stabilizer. ⋯ Conversely, any increase of lordosis of the strip brought about by vertical downward pushing of its top, will be stabilized by tightening the pulling string in the same specific direction. As this direction corresponded with the psoas orientation, the experiments show that the PM probably functions as a stabilizer of the lordotic LS in an upright stance by adapting the state of contraction of each of its fascicles to the momentary degree of lordosis imposed by factors outside the LS, such as general posture, general muscle activity and weight bearing. The presence of multiple PM fascicles, all of about equal length, and attaching to all LS levels, facilitates this function.
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Loosening of the screws is a problem in instrumentation with pedicle screws. Coating with hydroxyapatite improves the holding characteristics for metal implants, but the possible effects on the anchorage of pedicle screws have not been described. In this study, seven patients were operated on with spinal instrumentation using four stainless steel pedicle screws. ⋯ Hydroxyapatite coating was shown to have improved the purchase of pedicle screws very effectively. By using fully coated screws, as in the present study, extraction was extremely difficult compared to extraction of conventional stainless steel screws, which were regularly loose. By reducing the area of the screws that is coated, it may be possible to achieve an enhanced purchase while extraction will be easier when compared to fully coated screws.