Biomechanical analysis of anterior scoliosis instrumentation: differences between single and dual rod systems with and without interbody structural support.

Abstract

Nondestructive biomechanical testing was performed on bovine lumbar spines instrumented with multilevel scoliosis type anterior spine constructs. To determine the biomechanical effects from the number of anterior rods (1 vs 2) and the effects of interbody structural support on construct stiffness after anterior multisegmental instrumentation. Corrective surgery using anterior instrumentation for thoracolumbar and lumbar scoliosis has been performed with single rod and, more recently, with dual rod constructs. The biomechanical effect of one- or two-rod anterior instrumentation systems on construct stiffness and the addition or absence of interbody structural support have not been defined adequately in the literature. Eight bovine lumbar spines each underwent instrumentation using four different constructs: one rod without interbody support; one rod with titanium mesh interbody support at the L2-L3, L3-L4, and L4-L5 disc spaces; two rods alone; and two rods with interbody support. Nondestructive cyclic testing in flexion-extension (+/-5 Nm), lateral bending (+/-5 Nm), and torsion (+/-2 Nm) were performed. The construct stiffness (Nm/ degrees ) of the four implant configurations was compared. With the addition of a second rod, the construct was significantly stiffer than a single rod construct in flexion (P = 0.006), extension (P = 0.02), and torsion (P = 0.01), but not in lateral bending. The addition of interbody structural support to the rod systems resulted in significantly stiffer constructs than those without cages in flexion (P = 0.03), but not in the other loading conditions (extension, lateral bending, torsion). Dual rod constructs were stiffer in torsion and flexion-extension loading than single rod constructs. Neither the number of rods nor the use of structural mesh interbody support had any effect on lateral bending stiffness. However, in a single rod system, the addition of interbody support increased stiffness in flexion. The use of structural support in dual rod constructs may be helpful in "setting" the desired lordosis, but adds little to construct stiffness.