Question

Under what type of loading and direction, the human cortical bone exhibits highest ultimate strength?

• A. Compressive, longitudinal
• B. Tensile, axial
• C. Bending, longitudinal
• D. Torsion, axial

Hint:

Bone Biomechanics Rule of Thumb: - Human bones are designed primarily for weight-bearing. Therefore, they are always strongest under compression and weakest under shear/torsion. - They are highly anisotropic: longitudinal strength along the shaft is far superior to transverse strength across the shaft.

Answer 1

The Correct Option is A

Concept: Human cortical (compact) bone is a highly anisotropic and dense structural biomaterial forming the hard outer shell of long bones. Its mechanical behavior, stiffness, and ultimate failure strength depend heavily on both the orientation of the applied force vector relative to the bone's microstructure (osteon alignment) and the nature of the mechanical load (compression, tension, or shear).

Step 1: Analyzing directional anisotropy.
Cortical bone is structurally reinforced along its longitudinal axis by columnar units called osteons (Haversian systems), which run parallel to the long axis of the bone to handle physiological weight-bearing. Because of this specialized structural layout, cortical bone is significantly stronger and stiffer when loaded along its longitudinal axis than when loaded transversely (across the osteons).

Step 2: Comparing load types.
Like many porous, mineralized ceramic-matrix composite materials (such as concrete or hydroxyapatite blocks), bone handles crushing forces much better than stretching or twisting forces. Extensive biomechanical testing reveals the hierarchical structural strength limits of cortical bone as: \[ \text{Strength}_{\text{Compression}} > \text{Strength}_{\text{Tension}} > \text{Strength}_{\text{Shear}} \]
• Longitudinal compressive strength of adult cortical bone reaches roughly $190\text{-}200\text{ MPa}$.
• Longitudinal tensile strength is significantly lower, topping out around $130\text{-}150\text{ MPa}$.
• Shear, bending, and torsional loading profiles introduce transverse stress components that shear the weak interfaces between osteons, causing failure at much lower thresholds. Consequently, human cortical bone exhibits its absolute highest ultimate strength under longitudinal compressive loading, which matches Option (A).