Musculoskeletal
Bone Structure
Cortical, trabecular, marrow — living tissue under constant remodeling
Bone is a composite of mineralized matrix (hydroxyapatite, ~65%) and collagen (~25%), continuously remodeled by osteoclasts (resorb) and osteoblasts (build). Cortical (compact) bone forms the dense outer shell — 80% of skeletal mass, 20% of surface area. Trabecular (cancellous) bone is the spongy interior with high surface area — site of metabolic exchange and the first to lose density in osteoporosis. Marrow fills the spaces: red marrow makes blood cells; yellow marrow stores fat. Bone turns over fully every ~10 years.
- Composition65% mineral, 25% collagen, 10% water
- Cortical bone80% mass, dense, slow turnover
- Trabecular bone20% mass, high turnover, in vertebrae/ends
- Cell typesOsteoblast, osteoclast, osteocyte
- Peak bone massAge 25-30
- Annual turnover~10% of skeleton per year
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why bone structure matters
- Fracture prediction. DEXA T-score plus FRAX algorithm guides treatment.
- Hematology. Marrow biopsy diagnoses leukemia, myelofibrosis, multiple myeloma.
- Calcium homeostasis. Bone is the largest reservoir; PTH mobilizes it.
- Drug development. Anti-resorptives, anabolics, dual-action agents.
- Orthopedic surgery. Implant fixation depends on bone density.
- Pediatric growth. Growth plate biology determines stature.
- Cancer metastasis. Breast, prostate, lung, kidney, thyroid metastasize to bone.
Common misconceptions
- Bone is inert. It's the most metabolically active tissue per unit mass after liver.
- Calcium alone prevents osteoporosis. Need vitamin D, weight-bearing exercise, and often a drug.
- Fractures heal back to normal. Geometry restored; mineralization may take 1-2 years.
- NSAIDs are safe for fracture. Inhibit prostaglandins essential to early healing.
- Bisphosphonates can be taken indefinitely. 5 years for oral; reassess for drug holiday due to atypical femur fractures and ONJ.
- Osteoporosis is a women's disease. 1 in 4 men >50 will have an osteoporotic fracture; mortality higher than women.
Frequently asked questions
What's the difference between cortical and trabecular?
Cortical bone is dense (5-10% porosity), forms the diaphysis of long bones and outer shell of all bones. Trabecular bone is spongy (50-90% porosity) with a lattice of trabeculae — found in vertebral bodies, femoral neck, distal radius. Trabecular surface area is 10x cortical, so it remodels 5-8x faster and loses density first in osteoporosis.
What is the Haversian system?
The structural unit of cortical bone. A central Haversian canal carries blood vessels and nerves; concentric lamellae of mineralized collagen surround it; osteocytes sit in lacunae connected by canaliculi. Volkmann canals run perpendicular to connect Haversian systems. This architecture maximizes strength while permitting nutrient diffusion to embedded cells.
How does remodeling work?
Basic multicellular unit (BMU). Osteoclasts (multinucleated, derived from monocyte lineage) secrete H+ and cathepsin K to dissolve matrix — creates resorption pit. Osteoblasts (mesenchymal origin) follow, depositing osteoid (collagen + proteins) which mineralizes over weeks. Cycle takes ~3-6 months. Coupled in healthy bone; uncoupled in osteoporosis (resorption > formation).
What controls bone mass?
Mechanical loading (Wolff's law — bone adapts to stress); calcium/vitamin D intake; sex hormones (estrogen suppresses osteoclasts); PTH (intermittent low-dose builds bone; chronic high resorbs); calcitonin; sclerostin (osteocyte-secreted, inhibits osteoblasts). Astronauts lose 1-2% bone mass per month; bedrest comparable.
What is osteoporosis?
T-score ≤ −2.5 on DEXA scan (2.5 SD below young adult mean). Postmenopausal estrogen loss accelerates resorption — women lose 50% trabecular bone over lifetime. Fracture risk: vertebral, hip, wrist. Treatment: bisphosphonates (alendronate inhibits osteoclasts), denosumab (anti-RANKL antibody), teriparatide (PTH analog, anabolic), romosozumab (anti-sclerostin).
Red versus yellow marrow?
Red marrow is hematopoietic — produces ~500 billion blood cells daily. At birth, all marrow is red. By adulthood, red marrow is restricted to vertebrae, sternum, ribs, pelvis, skull, proximal femur/humerus. Yellow marrow (adipocytes) fills long-bone diaphyses. In severe anemia, yellow marrow can revert to red.
How do fractures heal?
Four phases. Hematoma (hours-days): clot forms; cytokines recruit cells. Soft callus (1-3 weeks): fibrocartilage bridges gap. Hard callus (3-12 weeks): woven bone replaces cartilage via endochondral ossification. Remodeling (months-years): woven bone converted to lamellar; medullary canal restored. Smoking, NSAIDs (especially COX-2), diabetes, and infection delay healing.