2.2 Bone Functions, Structural Anatomy, and Bone Types
Key Takeaways
- Bone serves five functions: structural support, protection of organs, leverage for movement, hematopoiesis in red marrow, and storage of calcium, phosphorus, and magnesium.
- The adult skeleton is approximately 80% cortical bone and 20% trabecular bone by mass, but trabecular bone supplies the majority of metabolically active bone surface.
- Cortical bone is organized into osteons with central Haversian canals; trabecular bone is organized as a lattice of plates and rods with marrow-filled interstices.
- Because trabecular bone turns over far faster than cortical bone, the lumbar spine shows disease and treatment effects earlier than the predominantly cortical one-third radius site.
- Approximately 99% of total body calcium and about 85% of total body phosphorus are stored in the skeleton, making bone the body's principal mineral reservoir.
2.2 Bone Functions, Structural Anatomy, and Bone Types
Quick Answer: Bone provides support, organ protection, movement leverage, hematopoiesis, and mineral storage. Structurally it exists in two forms: dense cortical (compact) bone, about 80% of skeletal mass, organized into osteons around Haversian canals; and porous trabecular (cancellous or spongy) bone, about 20% of mass but the majority of bone surface area. That surface-area difference is why trabecular-rich sites such as the lumbar spine change first in both disease and therapy.
Why a DXA Technologist Needs Bone Anatomy
Every measurement decision in densitometry traces back to bone composition. A lumbar spine that responds to bisphosphonate therapy within a year while the one-third radius barely moves is not a scanner artifact — it is the predictable consequence of trabecular versus cortical composition. Deciding whether an osteophyte, an endplate sclerosis, or a marrow-fat change will distort a region of interest requires knowing what is actually inside the region.
The Five Functions of Bone
| Function | Mechanism | Densitometric relevance |
|---|---|---|
| Support | Rigid framework maintains body form and carries axial load | Vertebral bodies bear axial load; failure produces compression fracture |
| Protection | Encases brain, spinal cord, thoracic and pelvic viscera | Cranial and facial fractures are excluded from osteoporotic fracture definitions |
| Movement | Provides rigid levers for skeletal muscle attachment | Muscle loading is the stimulus that maintains bone mass; disuse causes rapid loss |
| Hematopoiesis | Red marrow produces erythrocytes, leukocytes, platelets | Marrow fat replacing red marrow with age alters DXA soft-tissue baselines |
| Mineral storage | Reservoir for calcium, phosphorus, magnesium, sodium | About 99% of body calcium and roughly 85% of phosphorus reside in bone |
The mineral-reservoir function is the one that makes osteoporosis possible. Serum calcium is defended tightly because cardiac conduction and neuromuscular function depend on it. When intestinal calcium absorption falls or renal losses rise, parathyroid hormone (PTH) mobilizes calcium from the skeleton. The skeleton is the account that gets drained to keep the serum number normal — which is why a patient can have perfectly normal serum calcium and severe osteoporosis at the same time.
Structural Anatomy of a Long Bone
Using the radius as the model, since it is one of the three DXA measurement sites:
- Diaphysis: the shaft, a thick cylinder of cortical bone surrounding the medullary cavity. The one-third (33%) radius ROI is placed here and is predominantly cortical.
- Metaphysis: the flaring transition between shaft and end, with increasing trabecular content.
- Epiphysis: the expanded end, largely trabecular bone under a thin cortical shell. The ultradistal forearm ROI samples this region.
- Periosteum: fibrous outer membrane carrying blood vessels and osteoprogenitor cells; responsible for appositional growth in width.
- Endosteum: thin membrane lining the medullary cavity and trabecular surfaces; a highly active remodeling surface.
- Articular cartilage: hyaline cartilage capping the epiphysis; radiolucent and not measured by DXA.
Flat bones, including the ilium and the vault of the skull, follow a sandwich pattern: two cortical tables enclosing a trabecular diploe. Vertebral bodies follow the same logic — a thin cortical shell around a trabecular core, with dense cortical endplates above and below.
Cortical Versus Trabecular Bone
This distinction is the single most testable piece of bone biology on the examination.
| Property | Cortical (compact) bone | Trabecular (cancellous, spongy) bone |
|---|---|---|
| Share of skeletal mass | ~80% | ~20% |
| Share of bone surface area | Minority | Majority of metabolically active surface |
| Porosity | Low (roughly 5–10%) | High (roughly 50–90%) |
| Microstructure | Osteons (Haversian systems): concentric lamellae around a central Haversian canal, linked by transverse Volkmann canals | Interconnected plates and rods forming a lattice; marrow fills the spaces |
| Turnover rate | Slow | Several times faster than cortical |
| Principal locations | Long bone shafts, outer shell of every bone, one-third radius | Vertebral bodies, femoral neck and trochanteric region, ultradistal radius, calcaneus, pelvis |
| Mechanical role | Resists bending and torsion | Distributes compressive load, absorbs energy |
| Typical fracture pattern | Hip shaft, one-third radius | Vertebral compression, distal radius (Colles) |
Why Surface Area Drives Everything
Remodeling happens on bone surfaces, not throughout bone volume. Trabecular bone, despite holding only about a fifth of skeletal mass, presents an enormously larger surface-to-volume ratio because of its lattice geometry. More surface means more remodeling sites, which means:
- Faster response to estrogen withdrawal. Postmenopausal (Type I) osteoporosis strips trabecular bone at 3–5% per year while cortical loss remains modest, which is why vertebral and distal radius fractures dominate the early postmenopausal decade.
- Faster response to therapy. Antiresorptive treatment produces measurable lumbar spine BMD gains within 12 months, while the one-third radius may show no significant change for years.
- Site-selection consequences. A patient with hyperparathyroidism preferentially loses cortical bone, which is precisely why the one-third radius is the recommended site in that disease — it is the site the disease attacks.
Composition of the Bone Matrix
By weight, bone is roughly 60–70% mineral, 20–25% organic matrix, and 5–10% water. The mineral phase is calcium hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, and it is the mineral phase that attenuates the DXA beam. The organic phase, osteoid, is roughly 90% type I collagen plus non-collagenous proteins including osteocalcin, osteonectin, and osteopontin.
This composition explains a DXA limitation that appears in exam questions about osteomalacia. DXA measures mineral, not matrix. In osteomalacia the matrix is normal in quantity but under-mineralized, so DXA reports a low BMD — yet the disease, and its treatment, are entirely different from osteoporosis. DXA quantifies mineral; it does not diagnose why the mineral is low.
Woven Versus Lamellar Bone
- Woven bone has randomly oriented collagen, is deposited rapidly, and is mechanically weak. It is normal in the fetal skeleton, in fracture callus, and in Paget disease.
- Lamellar bone has collagen arranged in ordered parallel sheets, is deposited slowly, and is strong. Essentially all mature adult bone — both cortical and trabecular — is lamellar.
Rapid deposition of woven bone in Paget disease produces markedly elevated, mechanically unreliable BMD values. A vertebra with pagetic involvement reports an artifactually high BMD and must be excluded from analysis, a rule revisited in the lumbar spine chapter.
A patient on antiresorptive therapy shows a significant lumbar spine BMD increase at 18 months but no significant change at the one-third radius. What best explains the discrepancy?
Approximately what proportion of adult skeletal mass is cortical bone?
A patient with untreated osteomalacia has a lumbar spine T-score of -2.8. What does this measurement establish?