Response to Letter to the Editor for “Femoral Bone Mineral Density Shows Stronger Correlation With Pedicle Screw Insertional Torque than Lumbar Bone Mineral Density or Hounsfield Units: A Retrospective in Vivo Study”

We thank the authors for their insightful comments regarding our study evaluating the relationship between bone mineral density (BMD), Hounsfield units (HU), and pedicle screw insertional torque. While trajectory-specific CT-based assessments represent an important and evolving approach to evaluating local bone characteristics, existing literature has generally demonstrated only moderate correlations between DEXA or conventional HU measurements and insertional torque. In our study, we extended prior work by evaluating both systemic and vertebra-specific metrics, including femoral and lumbar DEXA, as well as two HU measurements: L1 HU and HU at the instrumented vertebra. Among these, femoral BMD demonstrated the strongest correlation with insertional torque, while vertebral HU showed improved association compared to L1 HU but remained inferior to femoral BMD. Although trajectory-specific HU measurements have shown promising results, prior studies often involve different screw trajectories, such as cortical bone trajectory techniques, which may limit direct comparability to conventional pedicle screw fixation. Furthermore, these methods require standardized measurement protocols that have not yet been widely established. Importantly, screw fixation strength is influenced by both bone density and bone quality. As DEXA and HU primarily assess bone density, their moderate correlations may reflect the contribution of additional biological and structural factors. Ongoing studies incorporating laboratory and clinical variables may further clarify these relationships. We agree that trajectory-specific approaches hold promise and look forward to future investigations that integrate both systemic and local assessments to optimize surgical planning.