Pedicle screw "hubbing" in the immature thoracic spine: a biomechanical and micro-computed tomography evaluation.

Background: A previous biomechanical study using adult thoracic vertebrae (both normal and osteoporotic bone density) demonstrated the deleterious effect of the pedicle screw hubbing technique. Pedicle screw "hubbing" involves seating and engaging the ventral aspect of the screw head onto the dorsal lamina cortex. This technique is postulated to provide a load-sharing effect by improving pullout resistance, as well as decreasing cephalocaudad toggling and implant loosening. We hypothesized the elastic properties of immature bone may mitigate, and perhaps enhance the purported benefits of the hubbing technique. We set out to evaluate pullout strength of fixed-head pedicle screws after hubbing versus standard insertion in the immature thoracic calf spine.

Methods: Twenty-two (n=22) single-level disarticulated fresh-frozen immature calf thoracic vertebra specimens (ranging from T2 to T13) were prepared. Twelve specimens were instrumented with pedicle screws in group I (nonhubbed) and group II (hubbed) in the opposite pedicle. Cyclic loading in a cephalocaudad direction was applied for 2000 cycles at a rate of 1 Hz. Pullout testing was performed in-line with the midline of the vertebra and peak pullout strength was measured in Newtons. Ten different specimens underwent micro-computed tomography evaluation to assess for trabecular architecture and incidence of iatrogenic microfractures.

Results: Hubbed screws resulted in significantly lower pullout strength (747±197 vs. 922±112 N, P=0.01). With the hubbing technique, the dorsal cortex demonstrated plastic deformation and conformed to the screw head in 83% of cases compared with no visible plastic deformation in the control group. Micro-computed tomography demonstrated microfractures of the dorsal cortex in 10/10 for the hubbed group compared with 1/10 for the control group.

Conclusions: This is the largest study ever performed on immature thoracic vertebra to evaluate this topic. Hubbed pedicle screws have significantly decreased pullout strength and frequently cause iatrogenic microfractures of the dorsal cortex. The unique ability of immature bone to exhibit plastic deformation did not provide a protective effect on immediate fixation strength, and the increased insertional torque during the hubbing technique should not give a false sense of added fixation. This study, along with our adult study, provides critical information to the surgeon to avoid this common misunderstanding with screw insertion technique.

Clinical Relevance: In vitro fresh-frozen immature calf spine study.