The biomechanical effect of pedicle screw hubbing on pullout resistance in the thoracic spine

Abstract

The biomechanical fixation strength afforded by pedicle screws has been strongly correlated with bone mineral density. It has been postulated that "hubbing" the head of the pedicle screw against the dorsal laminar cortex provides a load-sharing effect, thereby limiting cephalocaudad toggling and improving the pullout resistance of the pedicle screw. To evaluate the pullout strength (POS) of monoaxial hubbed pedicle screws versus standard fixation in the thoracic spine. Biomechanical investigation. Twenty-two human cadaveric thoracic vertebrae were acquired and dual-energy X-ray absorptiometry scanned. Osteoporotic (n = 16) and normal (n = 6) specimens were instrumented with a 5.0 × 35-mm pedicle screw on one side in a standard fashion. In the contralateral pedicle, 5.0 × 30-mm screw was inserted with hubbing of the screw into the dorsal lamina. A difference in screw length was used to achieve equivalent depth of insertion. After 2,000 cycles of cephalocaudad toggling, screws were pulled out with the tensile force oriented to the midline of the spine and peak POS measured in newtons (N). Four additional specimens were subjected to microcomputed tomography (micro-CT) analysis to evaluate internal pedicle architecture after screw insertion. Hubbed screws resulted in significantly lower POS (290.5 ± 142.4 N) compared with standard pedicle screws (511.5 ± 242.8 N; p = .00). This finding was evident in both normal and osteoporotic vertebrae based on independent subgroup post hoc analyses (p<.05). As a result of hubbing, half of the specimens fractured through the lamina or superior articular facet (SAF). No fractures occurred on the control side. There was no difference in mean POS for hubbed screws with and without fracture; however, further micro-CT analysis revealed the presence of internal fracture propagation for those specimens that did not have any external signs of failure. Hubbing pedicle screws results in significantly decreased POS compared with conventional pedicle screws. Hubbing predisposes toward iatrogenic fracture of the dorsal lamina, transverse process, or SAF during insertion.