Introduction: Pedicle screw fixation in the osteoporotic spine carries increased risk of loosening and failure. Increased screw insertion depth enhances pull-out strength. Yet, better guidelines for optimal screw depth in relation to screw-bone interface biomechanics and the resulting loosening risk are needed. This study evaluates the effects of screw depth on loosening risk.
Methods: Fifty osteoprotic vertebrae (L1-L5,T-Score=-3.5±0.8) were instrumented with Synthes Dual Core - Matrix monoaxial pedicle screws with 3 insertion depths: halfway through the vertebral body (mid-body), up to the anterior cortex (pericortical) and through the anterior cortex (bicortical). Specimens underwent 5000 load cycles at ±2 Nm to simulate post-operative activity. A 4 irLED marker array was attached to the pedicle screws to track motion. The screw fulcrum point (about which the screw pivots) was calculated and the location within the vertebra was identified. A t-Test evaluated differences between groups at p=0.05.
Results: Cyclic loading produced screw loosening in all groups (p<0.001). Stiffness of mid-body and pericortical screws decreased more than that of bicortical screws, with mid-body screws being significantly less than bicortical screws (p<0.05). The screw fulcrum point was 25.8±3.8, 25.3±5.2 and 13.8±4.2 mm anterior of the insertion point for mid-body, pericortical and bicortical groups, respectively (Figure 1). Average pedicle length was 21.5±2.7 mm.
Conclusions: Increased screw depth substantially enhanced screw purchase. Bicortical screws had a fulcrum point about the mid-length of the pedicle benefitting from the strong cortical shell of the pedicle associated with enhanced screw purchase. Mid-body and pericortical screws had a fulcrum point anterior to the pedicle within the traebecular region of the vertebral body, which was associated with reduced screw purchase as demonstrated by pullout tests, suggesting a weaker screw-bone interface. Data suggest that bicortical screw placement significantly enhances screw purchase and decreases post-operative screw loosening leading to improved fusion and clinical outcome.
Patient Care: A clear understanding of screw sizing may improve clinical outcomes.
Learning Objectives: By the conclusion of this session, participants should be able to: 1) Describe the importance of pedicle screw depth in regard to loosening, 2) Discuss, in small groups, the effect of pedicle screw depth on fulcrum location and loosening, 3) Understand principles underlying the selection of screw depth in lumbar posterior fixation.