Characteristics of an extended internal fixation system for polysegmental transpedicular reduction and stabilization of the thoracic, lumbar, and lumbosacral spine

The Kluger internal fixator, with its artificial fulcrum outside the operat ive site, had to be extended for multisegmental use. Three differ ent proto types, called Central Bar (CB), Double Bar (DB II) and Double Bar IT (DB II ) were designed, which were fully compatible with the existing reduction sy stem. To evaluate the ability of these newly developed systems to provide p rimary stability in a destabilized spine, their stiffness characteristics a nd stabilizing effects were investigated in multidirectional biomechanical stability tests and compared with those of the clinically well-known Cotrel -Dubousset (CD) system. The investigations were performed on a spine tester using freshly prepared calf spines. The model tested was that of an intact straight spine followed by a defined three-column lesion simulating the mo st destabilizing type of injury. Pure moments of up to 7.5 Nm were continuo usly applied to the top of each specimen in flexion/extension, left/right a xial rotation, and left/right lateral bending. Segmental motion was measure d using a three-dimensional goniometric linkage system. Range of motion and stiffness within the neutral zone were calculated from obtained load-displ acement curves. The DB II attained 112.5% (P = 0.26) of the absolute stiffn ess of the CD system in flexion and enhanced its stability in extension by up to 144.3% (P = 0.004). In axial rotation of the completely destabilized spine, this system achieved 183.3% of the stiffness of the CD system (P < 0 .001), and in lateral bending no motion was measured in the most injured sp ecimens stabilized by the DB TI. The DB I, which was the first to be design ed and was considered to provide high biomechanical stability, did not atta in the stiffness standard set by the CD system in either flexion/extension or axial rotation of the most injured spine. The study confirms that it is worthwhile to evaluate in vitro the biomechanical properties of a newly dev eloped implant before its use in patients, in order to refine weak construc tion points and help to reduce device-related complications and to better e valuate its efficacy in stabilizing the spine.