Beyond mean trabecular separation: decomposing metaphyseal marrow space into local trabecular spacing and marrow cavity expansion

Micro-computed tomography ({micro}CT) is widely used to assess trabecular bone microarchitecture, with trabecular separation (Tb.Sp) among the core parameters recommended for reporting. Tb.Sp is typically expressed as a single volume-weighted mean derived from maximal sphere fitting, although the underlying distribution of local separation values is rarely examined. Here, we show that Tb.Sp distributions in metaphyseal trabecular bone are frequently non-Gaussian and bimodal or multimodal. Using {micro}CT datasets from three established models of osteoporosis, spinal cord injury (SCI), ovariectomy (OVX), and ageing, we demonstrate that this behaviour is most evident in metaphyseal trabecular bone and is less apparent in epiphyseal trabecular bone or trabecular thickness distributions. We further show that multimodal metaphyseal Tb.Sp distributions correspond to two spatially distinct contributions within the marrow space: lower-diameter local separation within the residual trabecular network, and higher-diameter regions associated with larger contiguous marrow cavities. Based on this observation, we introduce a simple extension to standard morphometric analysis in which Tb.Sp is decomposed into local trabecular separation (Tb.SpL) and marrow cavity separation (Tb.SpM). Tb.Sp decomposition revealed model-specific patterns of trabecular deterioration. SCI was characterised predominantly by increased Tb.SpM, consistent with expansion of larger marrow cavities, whereas OVX showed a more subtle or distributed alteration. Ageing showed changes in both Tb.SpL and Tb.SpM, with the higher-diameter component becoming most prominent in older animals. Together, these findings demonstrate that mean Tb.Sp can mask structurally distinct forms of metaphyseal marrow-space organisation and support reporting distributional descriptors, and where appropriate Tb.SpL and Tb.SpM, alongside conventional Tb.Sp.