15. Supplemental fixation of lateral mass screws: a strategy to improve fixation strength

BACKGROUND CONTEXT Elderly patients are increasingly meeting the indications for spinal fusion and are more likely to have suboptimal bone quality. Patients with suboptimal bone quality who have undergone posterior cervical fusion have increased risk of fixation failure of up to 5.4% and may require stronger forms of fixation. Adding supplemental fixation to lateral mass screws (LMS) can increase fixation as measured by pullout strength. PURPOSE This work addresses the question: can supplemental screw fixation of lateral mass screws improve pullout strength? STUDY DESIGN/SETTING Biomechanical cadaveric study. PATIENT SAMPLE Fifteen C3-C6 vertebral bodies and 15 additional motion segments (C3-C4 or C5-C6) (age: 49-67). OUTCOME MEASURES Lateral mass screw construct peak pullout strength. METHODS All specimens were CT scanned with a phantom to obtain bone mineral density (BMD). The individual vertebral bodies and the cranial body of the motion segments were implanted with bilateral lateral mass screws. Supplemental fixation was added on one side of the vertebral body using two 2.4mm diameter unicortical screws (length 8-12mm). One side of the cranial vertebral body of the motion segments was supplemented with a single 2.4mm diameter transfacet tricortical screw (length 12-16mm). Screw constructs were subjected to posterior pullout coincident with the axis of the lateral mass screw at a rate of 5mm/min until failure. RESULTS A paired comparison was made of the peak pullout force of the LMS alone and LMS with supplemental fixation. Analysis was also made comparing BMD and pullout force.In the vertebral bodies with two supplemental unicortical screws, pullout force was significantly higher in the supplemented screw side. The average pullout force was 48.2% higher with supplemental screws (865±213N vs 637±206N; p=0.0038). There was moderate correlation between pullout force and bone mineral density for both the supplemented and LMS alone (correlation coefficients of 0.34 and 0.45 respectively). In the motion segments, peak pullout force was significantly higher in the side supplemented with a tricortical transfacet screw. The pullout force was 25.9% higher with supplemental transfacet screw fixation (1055±521N vs 912±508N; p=0.047). The pullout strength of the LMS with transfacet fixation was highly correlated with BMD (correlation coefficient of 0.71) while the LMS pullout strength was moderately correlated with BMD (correlation coefficient of 0.64). CONCLUSIONS Our data confirms the hypotheses that the pullout strength of C3-C6 LMS fixation using LMS supplemented with two unicortical screws or LMS supplemented with one-tricortical transfacet screw is superior to LMS fixation alone. FDA DEVICE/DRUG STATUS Cervical Fusion System (Approved for this indication)