Spongy bone

Spongy bone fills the epiphyses and metaphyses of long bones, the interior of short, flat and irregular bones (where it is called diploë in the skull) and the vertebral bodies. Trabeculae (≈0.1–0.4 mm thick) are composed of irregular lamellae with osteocytes in lacunae; they contain no osteons because no cell is far from the marrow vessels that bathe each trabecula, so nutrients diffuse through canaliculi from the surface.

Trabeculae are oriented along lines of compressive and tensile stress — the classic example is the proximal femur, where principal compressive and tensile trabecular systems cross in the neck, leaving a weaker region (Ward's triangle). The spaces between trabeculae contain red marrow (haematopoietic) — in adults mainly in the axial skeleton, sternum, ribs, vertebrae, hip bones and proximal femur and humerus.

Because its surface area per volume is large, trabecular bone is remodelled ≈5–10 times faster than cortical bone, so it is lost first in oestrogen deficiency — explaining why vertebral bodies and the distal radius (trabecular-rich) are early sites of osteoporotic fracture.