cancellous bone

**Structure:** Cancellous bone consists of an interconnected network of thin plates and rods called trabeculae, with marrow spaces between them. Trabeculae are made of lamellar bone arranged in an irregular pattern. Unlike compact (cortical) bone, they generally lack Haversian systems (osteons) and receive nutrients by diffusion from the marrow space. Trabeculae align along lines of mechanical stress (Wolff's law), which allows load distribution and shock absorption.

**Location:** Epiphyses and metaphyses of long bones, vertebral bodies, iliac crest, sternum, ribs, skull diploë, and the calcaneus. In adults, red (hematopoietic) marrow persists mainly in cancellous bone of the axial skeleton and proximal femur/humerus. Bone marrow biopsy and aspirate are taken from the posterior iliac crest.

**Comparison with cortical bone:** - Cortical bone is ~80% of skeletal mass, dense, with a low surface-to-volume ratio, and is mainly mechanical/protective. - Cancellous bone is ~20% of skeletal mass, porous, and has a high surface area and a higher remodeling rate (about 5-10 times that of cortical). It is metabolically active and is the main site of mineral exchange.

**Cells:** Osteoblasts (bone formation; derived from mesenchymal stem cells), osteocytes (mechanosensing; embedded in lacunae), and osteoclasts (bone resorption; derived from monocyte/macrophage lineage, regulated by RANKL/OPG) all act on trabecular surfaces.

**Clinical significance:** - **Osteoporosis:** Trabecular bone is lost first because of its high turnover. Postmenopausal (type I) osteoporosis, driven by estrogen deficiency, preferentially affects trabecular bone. This leads to vertebral compression fractures and distal radius (Colles) fractures. Senile (type II) osteoporosis affects both cortical and trabecular bone, typically causing hip fractures. Hip fractures involve the femoral neck (mixed). - **Vertebral compression fractures:** Cause kyphosis (dowager's hump) and loss of height. - **Paget disease:** Involves a mosaic pattern of woodcut/lamellar bone. It starts with an osteolytic phase and is followed by a mixed phase and a sclerotic phase. It causes increased ALP with normal Ca and phosphate. - **Osteomyelitis:** Hematogenous spread in children typically seeds the metaphysis (sluggish, looping capillaries), which is cancellous bone. - **Osteopetrosis:** Osteoclast dysfunction (carbonic anhydrase II deficiency, impaired acidification) obliterates marrow space, causing pancytopenia, extramedullary hematopoiesis, and cranial nerve compression. - **Hyperparathyroidism:** Preferentially affects cortical bone (subperiosteal resorption, "salt-and-pepper" skull), whereas trabecular bone is relatively preserved. - **Glucocorticoid-induced osteoporosis:** Predominantly affects trabecular bone, as does hypogonadism. - **Bone marrow transplant and harvest sites:** Iliac crest cancellous bone is used as autograft because it is osteogenic, osteoinductive, and osteoconductive. - **Fracture healing:** Cancellous bone heals faster than cortical bone because of its greater vascularity and surface area. Healing can occur with little callus formation. - **Avascular necrosis:** Subchondral cancellous bone of the femoral head is a common site.

**High-yield pearls:** Trabecular bone is lost first in osteoporosis and hypogonadism. Sites rich in trabecular bone (vertebrae, distal radius, femoral neck) are most prone to fragility fractures. DEXA T-score ≤ -2.5 defines osteoporosis. Lumbar spine is DEXA's key trabecular-rich site.