OBJECT

Primary spine tumors frequently involve the C-2 vertebra. Complete resection of the lesion may require total removal of the C-2 vertebral body, pedicles, and dens process. Authors of this biomechanical study are the first to evaluate a comprehensive set of reconstruction methods after C-2 resection to determine the optimal configuration depending on the degree of excision required.

METHODS

Eight human heads (from the skull to C-6) from 4 males and 4 females with a mean age of 68 +/- 18 years at death were cleaned of tissue, while leaving ligaments and discs intact. Nondestructive flexion and extension (FE), lateral bending (LB), and axial rotation (AR) tests were conducted using a nonconstraining, pure moment loading apparatus, and relative motion across the fusion site (C1-3) was measured using a 3D motion tracking system. Specimens were tested up to 1.5 Nm at 0.25-Nm intervals for 45 seconds each. The spines were instrumented using 3.5-mm titanium rods with a midline occipitocervical plate (4.0 x 12-mm screws) and lateral mass screws (excluding C-2) at the C-1 (3.0 x 40 mm) and C3-5 levels (3.0 x 16 mm). Testing was repeated for the following configurations: Configuration 1 (CF1), instrumentation only from occiput to C-5; CF2, C-2 corpectomy leaving the dens; CF3, titanium mesh cage (16-mm diameter) from C-3 to C-1 ring and dens; CF4, removal of cage, C-1 ring, and dens; CF5, titanium mesh cage from C-3 to clivus (16-mm diameter); CF6, removal of C-2 posterior elements leaving the C3-clivus cage (spondylectomy); CF7, titanium mesh cage from C-3 to clivus (16-mm diameter) with 2 titanium mesh cages from C-3 to C-1 lateral masses (12-mm diameter); and CF8, removal of all 3 cages. A crosslink was added connecting the posterior rods for CF1, CF6, and CF8. Range-of-motion (ROM) differences between all groups were compared via repeated-measures ANOVA with paired comparisons using the Student t-test with a Tukey post hoc adjustment. A p < 0.05 indicated significance.

RESULTS

The addition of a central cage significantly increased FE rigidity compared with posterior instrumentation alone but had less of an effect in AR and LB. The addition of lateral cages did not significantly improve rigidity in any bending direction (CF6 vs CF7, p > 0.05). With posterior instrumentation alone (CF1 and CF2), C-2 corpectomy reduced bending rigidity in only the FE direction (p < 0.05). The removal of C-2 posterior elements in the presence of a C3-clivus cage did not affect the ROM in any bending mode (CF5 vs CF6, p > 0.05). A crosslink addition in CF1, CF6, and CF8 did not significantly affect primary or off-axis ROM (p > 0.05).

CONCLUSIONS

Study results indicated that posterior instrumentation alone with 3.5-mm rods is insufficient for stability restoration after a C-2 corpectomy. Either C3-1 or C3-clivus cages can correct instability introduced by C-2 removal in the presence of posterior instrumentation. The addition of lateral cages to a C3-clivus fusion construct may be unnecessary since it does not significantly improve rigidity in any direction.