Morphogenesis of Floating Bone Segments: A Legacy of Serial Tensile Cross-Strut Microdamage in Trabecular Disconnection “Crumple Zones”?

Abstract

Trabecular bone disconnection “hotspots” of real termini (ReTm) previously mapped as loci of weakness in the female aging spine and hip may be a source of free-floating cancellous segments found in the medullary space using a bespoke, thick slice histological method for identifying ReTm. A factor in their origin is apparently microdamage proliferation (differentiated by en bloc silver staining) with occasional callus moderation. Validation of similar “floating segments” (FS) in the ex-breeder rat suggested a pilot model for a potentially common phenomenon. Following marrow elution and density fractionation of the isolated floating segments from the whole proximal rat femora, scanning electron microscopy (SEM) and elemental microanalysis (EDS) was performed. The eluent contained numbers of vertically truncated, laterally branched floating segments (<1 mm) together with bony spheroids. ReTm were a defining feature, ranging from smoothly elliptical, to coarsely textured by exposed populations of substructural calcified microparticles (<1 μm). While EDS spectra of the ReTm stumps registered little trace element distinction, their matrix mineral density was significantly lower than elsewhere. It was concluded that floating bone segments are the microarchitectural consequence of destabilised ReTm hotspot domains primed by local microdamage. Their common format of serial trabecular projections from a truncated central axis combines i) acute severance of sequential tensile cross-struts, causing chronic compression overload of axial-struts, with ii) inadequate stabilising callus, facilitating ReTm stacking into predetermined, substructural “crumple zones” of force containment, spheroidal attrition and particulate dissociation. As a catabolic outcome of altered tensile and hormonal influence, FS number may add a novel variable to cancellous bone kinetics particularly in women of relevance to fracture predisposition.