Adjacent level load transfer following vertebral augmentation in the cadaveric spine.

Study Design: In vitro biomechanics.

Objective: To determine if osteoporotic vertebral compression fracture (VCF) augmentation increases adjacent level load transfer.

Summary Of Background Data: Osteoporotic VCF subsequent to augmentation may result from disease progression or increased adjacent level load transfer, or both.

Methods: There were 11 T3-T7 and 10 T8-T12 divided by lumbar bone mineral density into a normal group (No. 1; n = 11) and an osteoporotic group (No. 2; n = 10). Strain and centrum stress were measured on T4 and T6 (T3-T7), and T9 and T11 (T8-T12) during tests in the intact state, following a centrum defect, during and after an augmented VCF at T5 or T10, and during a subsequent VCF. Stiffness and strength were compared: between groups 1 and 2; among intact, defect, and augmented VCF states; and between the initial and subsequent VCF.

Results: Group 1 was stiffer than 2 in compression (P = 0.01) and flexion (P = 0.07), with no difference in adjacent level load transfer (strain P = 0.72, centrum stress P = 0.36) or strength (P = 0.07). The centrum defect reduced compressive stiffness from the intact (P = 0.001), which was partially restored following VCF augmentation (P = 0.006). There were no differences in flexion stiffness (P > or = 0.14). Adjacent level load transfer in flexion exceeded that in compression (strain P = 0.001, centrum stress P = 0.19). Initial and subsequent VCF occurred at similar forces (P = 0.26) with higher adjacent level load at subsequent (strain and centrum stress P = 0.04).

Conclusions: Augmentation of multilevel spinal segments with VCF produced by combined compression, flexion, and a centrum defect normalizes adjacent level load transfer at physiologic loads. In both normal and osteoporotic spinal segments, as loads approach those of the initial VCF, protection from augmentation is lost, and subsequent adjacent level VCFs occur from extreme loading, and not the augmentation process.