TRIANGULATION OF PEDICULAR INSTRUMENTATION - A BIOMECHANICAL ANALYSIS

Tensile load-to-failure pullout tests were performed on 54 cadaveric spinal segments. The vertebrae were grouped by adjacent levels and matched for bone mineral density (g/cm2), which was measured by dual-photon absorptiometry. Triangulation of Steffee screws and CD pedicle screws was accomplished by transverse plates specifically designed to increase fixation within the same vertebra before the longitudinal Steffee plate or CD rod was applied. A transverse plate with adjustable length was also tested to accomodate variable interpedicular distances. Comparative pullout load-to-failures were as follows: laminar hook, 809 SE 99.4 N; single CD pedicular screws, 863 SE 108 N; single Steffee pedicular screw, 1245 SE 75.3 N; adjustable transverse plate, 1341 SE 114; triangulated Steffee pedicle screws with a transverse plate, 1701 SE 151 N; and triangulated CD pedicle screws with a transverse plate, 2096 SE 115 N. Three triangulated constructs with pedicle screws and a transverse plate (CD, Steffee, and Kirschner) provided significantly greater fixation than conventional pedicular or laminar hook based instrumentation systems (P < 0.05). Improved treatment of spinal deformities in the elderly and osteoporotic population is dependent on improving the fixation at the metal-bone interface of spinal implants. Particularly in osteoporotic vertebrae, the strength of fixation of two triangulated pedicle screws is better than either laminar hooks or single pedicle screws. The strength of fixation of triangulated pedicle screws connected by a transverse plate is superior to a single pedicle screw because it is dependent on the mass of bone between the screws rather than simply the amount of bone within the screw thread. This theoretically allows greater correction of spinal deformities because the triangulated constructs can achieve fixation prior to the attachments of the longitudinal member of the CD, Steffee, or Kirschner instrumentation systems.