Biomechanical comparison of cervical interbody cage versus structural bone graft

Clinically, cervical interbody cages provide fusion rates equivalent to structural bone grafting. No published studies have biomechanically compared cages with grafts.

We sought to compare the stability offered by threaded interbody cages versus structural bone graft and to evaluate the additional stability provided by adding a one- or two-level anterior plate to both interbody techniques.

Nondestructive nonconstraining repeated-measures in vitro flexibility tests were performed on surgically instrumented specimens. SUBJECT SAMPLE: Sixteen human cadaveric specimens were separated into two groups (specimens receiving graft and specimens receiving cage) with matched bone mineral density.

Angular range of motion (ROM), neutral zone (NZ) and elastic zone (EZ) were quantified to assess stability. Student's t tests compared outcomes between and within groups.

Quasistatic nonconstraining torques (maximum 1.5 Nm) induced flexion, extension, lateral bending and axial rotation while angular motion was recorded stereophotogrammetrically. Specimens were tested normal, after discectomy, with graft or cage, with two-level plate and with one-level plate.

Graft alone and cage alone reduced ROM and EZ but not NZ to within normal. In both groups, adding a one- or two-level plate significantly reduced motion in all modes of loading. There were no significant differences in motion parameters between groups whether plated or unplated. A two-level plate provided significantly better stability than a one-level plate.

The interbody cage performed equivalently to the structural graft. Substantial increases in stability can be gained for either interbody technique by adding an anterior plate.