Influence of double rods and interbody cages on quasistatic range of motion of the spine after lumbopelvic instrumentation.

PURPOSE: This in vitro biomechanical study compares residual lumbar range of motion (ROM) and rod strain after lumbopelvic instrumentation using 2 rods, 4 rods and interbody cages.

METHODS: Seven human cadaveric specimens were instrumented from L1 to sacrum, and pelvic screws were implanted. The pelvis was constrained and moments up to 7.5 Nm were applied to T12. Segmental L1-S1 ROM was analyzed by tracking radiopaque balls implanted in each vertebra using biplanar radiographs. Deformation within principal rods was measured by strain gauges. Four configurations were compared: 2 rods (2R), 4 rods (4R), 4 rods + ALIF at L4-L5 and L5-S1 (4R + ALIF), 2 rods + ALIF (2R + ALIF).

RESULTS: Intact average global L1-S1 ROM was 42.9° (27.9°-66.0°) in flexion-extension (FE), 35.2° (26.8°-51.8°) in lateral bending (LB), 18.6° (6.7°-47.8°) in axial rotation (AR). In FE, average ROM was 1.9° with both 4-rod configurations versus 2.5° with 2R and 2.8° with 2R + ALIF (p < 0.05). In LB, ROM ranged between 1.2° and 1.5° without significant differences. In AR, ROM was 2.5° with both 4-rod configurations versus 2.9° with 2R (p = 0.07) and 3.1° with 2R ALIF (p = 0.01). In FE, strain decreased by 64% and 65% in principal rods at L3-L4 with 4-rod. When comparing 2-rod configurations, strain decreased by 1% in flexion and increased by 22% in extension at L3-L4 when adding an ALIF at L4-L5 and L5-S1.

CONCLUSIONS: Double rods and interbody cages decrease residual ROM in FE and AT. Double rods seem efficient in limiting strain in principal rods. The use of single rods with cages at the lumbosacral junction increases strain at the first adjacent level without cage.