Metaphyseal trabecular bone separation is bimodal

Introduction/Objectives Micro-computed tomography with morphometric analysis is the gold standard methodology for skeletal phenotyping of small animal models of bone disease. Metaphyseal trabecular bone is the most common site of assessment. The 2010 guidelines for the assessment of bone microstructure recommend a minimal set of parameters to be reported: bone volume fraction (BV/TV), trabecular number (Tb.N), thickness and separation (Tb.Sp). We use three osteoporosis models (rat
spinal cord injury, mouse ovariectomy and mouse ageing) to demonstrate metaphyseal Tb.Sp is bimodal both in the femur of the tibia. We propose that metaphyseal Tb.Sp should be reported as two distinct values, preliminarily named Tb.Sp1 (representing thickening/thinning of trabeculae) and Tb.Sp2 (representing loss of centrally located trabeculae).MethodsBV/TV, Tb.N and Tb.Sp analyses were
performed on metaphyseal trabecular bone VOIs. The outputs from Tb.Sp analysis are the volumeweighted average Tb.Sp, Tb.Sp histogram, and Tb.Sp dataset. Metaphyseal Tb,Sp histograms appeared bimodal (or multimodal), which is particularly clear in osteoporotic datasets. A global threshold (350µm) was applied, separating the two peaks in the distribution, enabling the acquisition of Tb.Sp1 and Tb.Sp2 VOIs. The Tb.Sp1 (or Tb.Sp2) VOI was applied to the original dataset and the Tb.Sp analysis was rerun, generating results for Tb.Sp1 (or Tb.Sp2) . Results For the age-induced osteoporosis datasets (14-, 36- and 62-weeks-old), a monotonic decrease in BV/TV and Tb.N, and increase in Tb.Sp was observed (p < 0.01) confirming age-induced osteoporosis. Tb.Sp histograms were multimodal. Segmenting the Tb.Sp dataset
based on sphere diameter enabled the distinction of effects due to intra-trabecular thinning (first peak) captured by Tb.Sp1, from those due to complete resorption of centrally-located trabeculae, which widens the metaphyseal marrow cavity, captured by Tb.Sp2. All 3 types of Tb.Sp were different (p<0.01), indicating that both trabecular thinning/thickening and marrow cavity expansion contribute to Tb.Sp. In the 62-weeks-old dataset Tb.Sp2 became dominant. Similar results were observed for ovariectomy and spinal cord injury models. Conclusions We envision that this methodological update will enable a more sensitive distinction of skeletal phenotypes.