Understanding the Role of WBCT for Monitoring Bone Mineral Density after Hindfoot Fusion

Research Type: Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies Introduction/Purpose: Bone density is an essential component for adaptations in mechanical loading and alignment changes. The use of Hounsfield Units (HU) from ConeBeam CT (WBCT) datasets as a proxy for measurements of Bone Mineral Density (BMD) is widely utilized in the dentistry field, but remains under-utilized in orthopaedics. Recently, it has shown promise in describing topographical changes in bone density correlated to changes in mechanical alignment. This methodology may be further utilized to monitor BMD in the peri- and postoperative period to guide indications for supplementation and rehabilitation protocols. This study aimed to investigate the distribution and evolution of HU-based BMD in the distal tibia after immobilization following hindfoot fusion. We hypothesized that relative topographical differences in BMD could be analyzed and tracked over time. Methods: We conducted a retrospective study of 47 feet (age, 55.6 y ± 14.3, weight, 83.0 kg ± 20.8) in patients who underwent subtalar arthrodesis for Progressive Collapsing Foot Deformity (PCFD) or post-traumatic osteoarthritis. Patients were non-weightbearing for six weeks postoperatively. Preoperative and postoperative WBCT (CurvebeamAI, Hatfield, PA, USA) scans at 6 weeks and 6 months were analyzed. HU values were recorded at normalized cortical bone landmarks in the sagittal and coronal planes at 2 cm, 4 cm, and 6 cm above the tibial plafond. Descriptive statistics, paired t-tests, and repeated measures ANOVA were used to compare HU values across time points and between planes. A Bonferroni correction was applied for multiple comparisons, and statistical significance was set at p < 0.05. Results: There were no significant changes observed in overall HU values between preoperative, 6-week postoperative, and 6-month postoperative WBCT scans. However, comparison of HU values measured at the sagittal and coronal planes landmarks demonstrated significant differences in bone density. Specifically, HU values measured in the sagittal plane were significantly higher compared to the coronal plane at both 2 cm and 4 cm above the tibial plafond (p < 0.001 and p = 0.01, respectively), whereas no significant difference was observed at 6 cm (p > 0.05). Conclusion: Measurement of HU values in postoperative WBCT scans captured regional differences in HU-based BMD, demonstrating reliability in assessing topographical variations. There were no significant changes in HU-based BMD observed between the preoperative and postoperative scans, suggesting that alterations in postoperative BMD were minimal, or that the sensitivity of our methodology may be limited. Though this method may not reliably assess temporal changes of BMD, it may be valuable for tracking topographical changes in BMD influenced by abnormal alignment or gait patterns. Future studies should investigate calibration techniques to improve accuracy and clinical applicability of HU for postoperative bone density evaluation.