Anatomy of Cancellous Bone

Definition and Synonyms

Gross Anatomy and Location

• Epiphyses of long bones - the bulbous ends are largely cancellous with only a thin cortical shell over them
• Metaphyses - transitional zone between epiphysis and diaphysis contains increasing trabecular bone
• Diaphysis - a thin layer of cancellous bone lines the inner surface of the cortex, surrounding the central marrow cavity
• Vertebral bodies - cancellous bone is especially prominent here
• Short bones (carpals, tarsals) - cancellous core surrounded completely by compact bone
• Flat bones - the diploë is a thick layer of cancellous bone sandwiched between two plates of compact bone (e.g., skull)
• Junqueira's Basic Histology, 17e; Medical Physiology (Boron & Boulpaep)

Microstructure - Trabeculae

• Thin, interconnected spicules, plates, and rods of bone that form a three-dimensional lattice
• The spaces between trabeculae are interconnecting cavities filled with bone marrow (red or yellow depending on age/location)
• Trabeculae extend inward from the cortex into the medullary cavity
• Their orientation is not random - they align along lines of mechanical stress (Wolff's law), giving the lattice considerable strength without greatly increasing weight
• Medical Physiology (Boron & Boulpaep); Junqueira's Basic Histology, 17e

Surface Covering - Endosteum

• All trabecular surfaces are covered by endosteum, a thin cellular layer
• The endosteum contains osteoprogenitor cells, osteoblasts, and lining cells
• This is in contrast to the outer surface of compact bone, which is covered by periosteum
• Because cancellous bone has a much greater surface-area-to-volume ratio than cortical bone, it has far more endosteal surface available for cellular activity
• Junqueira's Basic Histology, 17e

Cellular Components

Matrix Composition

• Organic component (osteoid, ~35%): Type I collagen (~90% of osteoid), osteocalcin, osteonectin, and other glycoproteins
• Inorganic component (~65%): Hydroxyapatite crystals [Ca₁₀(PO₄)₆(OH)₂], aligned along collagen fibers
• Type I collagen provides tensile strength; hydroxyapatite provides compressive strength

Histological Organization

• Trabeculae show parallel bundles of collagen in thin layers (lamellae), with regularly spaced osteocytes between lamellae in lacunae
• Unlike compact bone, cancellous trabeculae are generally too thin to contain Haversian systems (osteons)
• Osteocytes within trabeculae communicate with surface cells and with marrow vasculature directly through canaliculi and short distances through the matrix - no need for a central Haversian canal

• Junqueira's Basic Histology, 17e

Summary Table

Cortical Bone vs. Cancellous Bone

Master Comparison Table

1. Structure

• Each osteon is a cylinder of concentric ring-like lamellae surrounding a central Haversian canal that carries blood vessels and nerves
• Adjacent osteons are connected by transverse Volkmann's (perforating) canals
• Osteocytes sit in lacunae between lamellae and communicate via canaliculi
• Between osteons lie interstitial lamellae (remnants of old osteons)

• Thin, interconnected trabeculae (spicules, plates, rods) aligned along mechanical stress lines
• Inter-trabecular spaces filled with red or yellow bone marrow
• All trabecular surfaces lined by endosteum bearing osteoblasts and osteoclasts
• Medical Physiology (Boron & Boulpaep)

2. Mineral Content and Porosity

• Cortical bone: up to 90% mineralized tissue - nearly solid calcified matrix
• Cancellous bone: only ~20% mineralized tissue - the remaining 80% is marrow, blood vessels, and mesenchymal cells

3. Surface Area and Remodeling

4. Vascularity

• Cortical bone: relies on nutrient arteries entering via the nutrient foramen, traveling longitudinally and branching into Haversian and Volkmann's canals
• Cancellous bone: directly bathed by marrow sinusoids - fenestrated capillaries allow fast exchange of metabolites between circulation and marrow tissue. The metaphysis (rich in cancellous bone) has a higher vascularity than the diaphysis, which is why metaphyseal fractures heal faster than diaphyseal (cortical) fractures
• Firestein & Kelley's Textbook of Rheumatology; Sabiston Textbook of Surgery

5. Mechanical Role

• Cortical bone: provides axial strength and rigidity for weight bearing; makes up the shaft of long bones; resists bending and torsional forces
• Cancellous bone: provides load distribution - the metaphysis transfers load from the weight-bearing joint surface down to the cortical diaphysis. The trabecular architecture follows Wolff's law - trabeculae align along principal stress lines, providing strength with minimal weight

6. Clinical Relevance

• Sabiston Textbook of Surgery; Miller's Review of Orthopaedics