Biomechanics of Bone

Biomechanics of Bone

Musculoskeletal Biomechanics

Topics discussed:

• Introduction to Musculoskeletal Biomechanics

• Biomechanics of Bone

• Biomechanics of Articular Cartilage

• Biomechanics of Tendons and Ligaments

• Biomechanics of Skeletal Muscle

• Anthropometry & kinematics

• Dynamics I: forces and moments

• Dynamics II: energy and power

• Gait and Stability

Biomechanics of Bone

1. Introduction

Bone biomechanics is the study of how bones respond to mechanical forces, distribute loads, and contribute to movement and stability. Bone is a highly specialized tissue that is both strong and lightweight, allowing it to withstand various stresses while remaining flexible enough to adapt to changing mechanical demands.

Understanding bone biomechanics is crucial in orthopedics, rehabilitation, sports science, prosthetic design, and biomaterials engineering, as it helps in designing implants, preventing fractures, and treating bone diseases like osteoporosis.

2. Structure & Composition of Bone

Bone has a hierarchical structure, making it both strong and adaptable.

a. Types of Bone Tissue

• Cortical (Compact) Bone:

  • Dense, strong outer layer.

  • Found in long bone shafts (e.g., femur, humerus).

  • Handles high compressive loads.

• Trabecular (Cancellous) Bone:

  • Porous, spongy inner structure.

  • Found in the ends of long bones and vertebrae.

  • Provides shock absorption and load distribution.

b. Bone Composition

Bone is a composite material made of:

• Organic matrix (30-35%): Primarily collagen (Type I), giving bone flexibility and tensile strength.

• Inorganic matrix (65-70%): Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂), providing hardness and stiffness.

• Water (5-10%): Important for nutrient transport and mechanical function.

3. Mechanical Properties of Bone

Bone exhibits different mechanical properties based on its structure and function.

a. Strength & Stiffness

• Ultimate strength: The maximum stress bone can handle before failure.

• Young’s Modulus: Bone stiffness, measuring resistance to deformation.

  • Cortical bone: High stiffness (~17 GPa).

  • Trabecular bone: Low stiffness (~0.1-2 GPa).

b. Types of Loads on Bone

Bone experiences different types of mechanical loads:

• Compression: Pressing force (e.g., vertebrae under body weight).

• Tension: Pulling force (e.g., tendons pulling on bone).

• Shear: Sliding forces (e.g., fractures in long bones).

• Torsion: Twisting forces (e.g., spiral fractures).

• Bending: Combination of tension and compression (e.g., femur during walking).

c. Stress-Strain Behavior

• Bone follows elastic and plastic deformation behavior.

• Elastic region: Bone returns to its original shape after stress.

• Plastic region: Bone deforms permanently before failure.

• Fracture point: Bone breaks when stress exceeds its ultimate strength.

4. Bone Adaptation & Remodeling (Wolff’s Law)

a. Wolff’s Law of Bone Remodeling

• Bone adapts to mechanical loads by changing its structure.

• Increased stress → Bone strengthens (e.g., athletes have denser bones).

• Decreased stress → Bone weakens (e.g., astronauts lose bone mass in space).

• Remodeling is regulated by osteoblasts (bone formation) and osteoclasts (bone resorption).

b. Factors Affecting Bone Strength

• Age: Bone density decreases with age, increasing fracture risk.

• Nutrition: Calcium, vitamin D, and protein are essential.

• Physical Activity: Weight-bearing exercises improve bone strength.

• Diseases: Osteoporosis reduces bone density and increases fragility.

5. Fracture Mechanics & Healing

a. Types of Bone Fractures

• Simple (closed) fracture: Bone breaks without piercing the skin.

• Compound (open) fracture: Bone breaks and penetrates the skin.

• Stress fracture: Repetitive microtrauma (common in athletes).

• Comminuted fracture: Bone breaks into multiple fragments.

b. Bone Healing Stages

1. Inflammatory phase (0-7 days): Blood clot formation.

2. Soft callus formation (1-3 weeks): Fibrocartilage bridge forms.

3. Hard callus formation (4-12 weeks): New bone replaces cartilage.

4. Remodeling phase (months-years): Bone regains original strength.

6. Applications in Biomedical Engineering & Orthopedics

Application	Role of Bone Biomechanics
Orthopedic Implants	Bone plates, screws, and prosthetics must match bone’s mechanical properties.
Fracture Fixation	Understanding bone stress helps in designing rods, plates, and pins.
Osteoporosis Treatment	Biomechanics helps in drug development and exercise recommendations.
Sports Medicine	Helps prevent and manage bone injuries in athletes.

7. Conclusion

The biomechanics of bone is fundamental to understanding bone function, injury mechanisms, and treatment strategies. Bone’s ability to adapt to mechanical forces ensures its structural integrity, but aging, disease, and excessive stress can lead to fractures. Advances in biomedical engineering, biomaterials, and regenerative medicine continue to improve bone repair, prosthetics, and orthopedic treatments.