Bone Thickness and Volume Fraction Measurements: CT, CBCT versus Histology

Method: Six mandibular condyles from 5-month-old pigs, covered by soft tissue, underwent CBCT and multislice CT scans at clinically-relevant 0.3-mm and 0.56-mm voxel-sizes, respectively. Coronal-plane CBCT/CT images of the middle condylar portion were measured for CTh of the medial and lateral cortex, and for BV/TV of adjacent trabecular bone.  A series of BV/TV measurements were obtained by applying thresholding values based on the difference of grey-level values between cortical bone and adjacent structures, following a method reported by others (Naitoh et. al, Int J Oral Maxillofac Implants. 2010). Undecalcified, 50µm thick, coronal histological sections of the same condylar portion were measured for CTh and BV/TV using line and grid tools in ImageJ. Variations of average CTh among tools (CBCT/CT/histology) and locations (lateral/medial), and reliability of CBCT/CT measurements were assessed by mixed-model ANOVAs and intraclass correlation tests (ICC), respectively. BV/TV measurements from CT, CBCT were also assessed for consistency with histological measurements by ICC. 

Result: CTh from CT and CBCT were neither different (ANOVA, p>0.05) nor strongly correlated (ICC, r=0.386) with each other, but both were significantly greater (ANOVA, p<0.001) than histological measurements.  For BV/TV, not a single thresholding value resulted in BV/TV values from CBCT or CT images that were more than moderately correlated with histological measurements (ICCs, r=0.062~0.472).

Conclusion: These data indicate that mandibular condyle cortical bone thickness measurements from clinical-level CBCTs or multislice CTs may be overestimated, and neither CBCT nor multislice CT can produce trabecular BV/TV measurements that are sufficiently consistent with those from histology.