Limitations of peripheral quantitative computed tomography metaphyseal bone density measurements

Context: Peripheral quantitative computed tomography ( pQCT) measurements are frequently obtained to assess cancellous bone density in the appendicular skeleton. Large variations in bone morphology associated with skeletal development may limit the interpretation of pediatric pQCT studies based on a single slice. Objective: The objective of the study was to characterize the variability in trabecular bone density values along the length of the metaphysis. Design: The design was an analysis of pQCT bone density data. Setting: The study was conducted at a hospital radiology department. Patients: The study included 35 children with cerebral palsy aged 6-12 yr. Main Outcome Measure: Variations in cancellous bone density along the length of the proximal tibial metaphysis were measured. Results: The patterns of decay in metaphyseal trabecular bone density were different in all subjects, and the density changed from the physis to the shaft at a rate of 16.8 +/- 8.2% per 1 mm ( range 8.6-37.9% per 1 mm). The slopes of the density curve drastically changed in some children over a short period of 6 months. Even with a high correlation ( r(2) +/- 0.88) between the density of a slice located a fixed distance from the growth plate and the overall mean metaphysis density, the respective changes in density over 6 months were only moderately correlated ( r2 = 0.58). Conclusions: These results underscore the difficulty in interpreting metaphyseal pQCT bone density measurements from a single slice and highlight the need for developing pQCT acquisition techniques that provide more representative bone density determinations in the appendicular skeleton of children.