What Is Cortical Bone?

Cortical bone is the dense, compact form of bone tissue that constitutes the outer shell of most bones and the shaft of long bones such as the femur and tibia. It accounts for approximately 80 percent of total skeletal mass and provides the primary mechanical support for load transmission, muscle attachment, and protection of internal structures. With a porosity of only 5 to 15 percent, cortical bone is far denser than trabecular (cancellous) bone, which ranges from 40 to 95 percent porous and fills the interior of vertebrae and the ends of long bones.

Cortical bone is a hierarchical composite material organized across several length scales, from collagen fibrils at the nanometer level up to whole-bone geometry at the centimeter scale. Its constituent minerals, primarily hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), are arranged around a collagen scaffold to produce a tissue that resists both compression and tension. This combination of mineral stiffness and organic flexibility gives cortical bone a fracture toughness substantially greater than either component alone.

Microstructure

The defining structural unit of cortical bone is the osteon, also called a Haversian system: a cylinder 200 to 250 micrometers in diameter, composed of concentric lamellar rings of mineralized collagen surrounding a central Haversian canal that carries blood vessels and nerves. Osteons are aligned preferentially along the long axis of the bone, and their density in human bone typically falls between 10 and 25 osteons per square millimeter. Secondary osteons replace earlier bone through a remodeling cycle driven by osteoclasts (resorption) and osteoblasts (deposition), leaving behind cement lines that mark the boundaries of individual remodeling events. These cement lines influence crack propagation: cracks tend to deflect along cement line interfaces rather than pass directly through osteons, contributing to the toughening mechanisms of the tissue. The PMC review of bone mechanical properties in healthy and diseased states details how osteon morphology and collagen fiber orientation together govern the anisotropic mechanical response.

Mechanical Properties

Cortical bone is mechanically anisotropic: its elastic modulus and strength are highest along the longitudinal axis and lower in the transverse and circumferential directions. Typical longitudinal elastic modulus values range from 17,900 to 18,160 MPa, tensile ultimate stress from 92 to 135 MPa, and compressive ultimate stress from 153 to 205 MPa. Under longitudinal tension the tissue exhibits a bilinear stress-strain response, with an initial linear elastic region followed by yielding at roughly 0.6 to 0.8 percent strain before fracture at less than 3 percent strain. Fracture toughness arises from both intrinsic mechanisms, such as collagen fibril sliding and microcrack nucleation ahead of a crack tip, and extrinsic mechanisms such as crack bridging and deflection at cement lines. Research on cortical bone fracture behavior and microstructural effects shows that osteon density, mineralization gradients, and cement line morphology all shift the balance between these toughening contributions.

Biomedical Engineering Applications

Engineers draw on detailed characterization of cortical bone to design implants, prosthetics, and scaffolds that must integrate with or replace bone tissue. Finite element models using cortical bone's direction-dependent material properties predict stress distributions around hip stems, bone screws, and fracture fixation plates. Additive manufacturing techniques have been used to produce biomimetic cortical bone scaffolds that replicate the Haversian microstructure at designed length scales, targeting improved osseointegration. Ultrasonic and quantitative computed tomography methods provide non-invasive measures of cortical bone thickness, porosity, and elastic constants, enabling clinical assessment of fracture risk in osteoporosis.

Applications

Cortical bone research and characterization have applications in a range of fields, including:

• Orthopedic implant design, for matching implant stiffness to surrounding bone and reducing stress shielding
• Fracture fixation and surgical planning, using patient-specific finite element models
• Bone substitute and scaffold development, targeting osteon-scale geometry for cell ingrowth
• Ultrasonic bone densitometry and medical imaging for osteoporosis diagnosis
• Forensic and archaeological anthropology, where cortical bone properties help estimate age and loading history