Bone Biomechanics and Forces

List the different forces which act on a typical long bone

1. Tension

2. Compression

3. Shear

4. Torsion

5. Bending

Describe the effects of tension on the microstructure of a long bone

• Non-Haversian system: regions under high tensile stress typically exhibit a lack of secondary osteons, which are commonly found in high-compression areas 

• Mineralization levels: tensile areas often show higher mineralization levels than surrounding tissues, indicating the microstructure is optimized to manage tension 

• Collagen fibre: tensile stress causes collagen fibers to align, stretch, and undergo molecular slippage, which helps manage and dissipate energy

Describe the effects of compression on the microstructure of a long bone

• Microcracks: significant damage causes microcracks to form, under extreme compression, these microcracks can coalesce, leading to "creep" (gradual, permanent deformation) or eventual fracture

• Trabecular destruction: long-term compression disrupts the trabecular network and reduces bone mineral density, increasing the risk of collapse

Describe the effects of shear on the microstructure of a long bone

• Microcracks: unique arc-shaped microcracks develop within lamella

• Cortical sensitivity: shear fatigue is a primary cause of stress fractures, as cortical bone is weaker in shear, making the microstructure highly susceptible to damage

Describe the effects of torsion on the microstructure of a long bone

• Lamellar interface failure: shear damage is caused 

• Osteon rearrangement: rearranged to strengthen the cortex 

• Mineral-collagen alignment: mineral lamellae arranged around collagen fibres to provide increased resistance 

Describe the effects of bending on the microstructure of a long bone

• Concave side undergoes compression, convex side undergoes tension

• Tensile failure: microcracks may appear in the convex side

• Compressive damage: butterfly fragments may form from the concave side 

• Accumulation of microcracks within the Haversian system 

Draw out the following forces acting on a long bone:

• Tension

• Compression

• Shear

• Torsion

• Bending

Relate biomechanical strength of a typical long bone to the diaphysis structure

• Primarily composed of cortical bone, the diaphysis is a hollow cylinder

• Highly effective at resisting torsion and bending forces, because mass is distributed away from the neutral axis

Relate biomechanical strength of a typical long bone to the epiphysis structure

• Filled with cancellous bone, capped with a thin layer of cortical bone

• This allows for the distribution of loads over a larger area at the joints, absorbing energy and transmitting stresses towards the dense cortex

Relate biomechanical strength of a typical long bone to the medullary cavity structure

• The hollow center decreases the overall mass, making the bone light, while the cortical shell provides the necessary strength

Describe how bone is flexible

• Bone is composed of 70% calcium phosphate, which provides strength, and 30% collagen, which provides flexibility

Describe how bone is malleable

• Baby and children's bones are highly malleable due to a higher percentage of collagen and a lower density of minerals compared to adult bones

Explain how bone fatigue may occur

1. Cumulative microdamage

• Repeated loading can create microcracks, especially in high-load areas such as the fetlock or tibia 

2. Muscle fatigue 

• Reduces muscle ability to reduce shock- causing higher stresses to be transferred directly to the bones 

3. Training intensity 

• Sudden increases in training speed/duration without adequate recovery time 

4. Hard surfaces 

• Training on/using hard surfaces can accelerate the fatigue process

5. Return from breaks 

• Abrupt increases in work following a period of rest (when bones may have deconditioned), can trigger sudden onset fatigue injuries 

Predict the different types of fractures that result from different biomechanical forces 

1. Tension: generally forms small, hairline fractures (stress fractures)

2. Compression: wedge, crust, burst fractures

3. Shear: oblique/diagonal fractures

4. Torsion/rotation: spiral fractures

5. Bending: oblique/transverse fractures 

Outline the stages of bone remodelling

1. Activation: osteoclast precursors are recruited to the bone surface 

2. Resorption: osteoclasts digest old or damaged mineralized bone 

3. Reversal: mononuclear cells prepare the surface for new bone formation

4. Formation: osteoblasts fill the cavity with new matrix (osteoid)

What is bone remodelling, and why is it necessary?

• A continuous, lifelong process- where mature tissue is removed (resorption), and new bone tissue is formed (ossification)

• This maintains skeletal strength, repairs microdamage and regulates calcium balance

Explain Wolff’s Law

• States that healthy human bones adapt to the loads under which they are placed, remodeling themselves over time to:

  • Become stronger when subjected to increased stress

  • Become weaker when disused 

• “Form follows function”

Which types of factor lead to bone remodelling and adaptation?

1. Mechanical factors

2. Hormonal factors

3. Lifestyle factors

Describe which mechanical factors can lead to bone remodelling and adaptation

• Increased loading (physical activity)

• Decreased loading (immobilisation/disease)

• Microdamage repair 

• Shear stress/strain 

Describe which hormonal factors can lead to bone remodelling and adaptation

• Estrogen/testosterone promote growth and maintenance 

• PTH released when calcium levels drop to stimulate osteoclasts and release calcium

• Calcitriol is crucial for maintaining calcium levels

• Growth hormone/IGFs stimulate osteoblast proliferation and bone growth 

Describe which lifestyle factors can lead to bone remodelling and adaptation

• Chronic diseases create inflammatory environments, stimulating osteoclast activity

• Smoking and alcohol inhibit osteoblast activity and promote bone loss 

• Ageing leads to decreased number of osteoblasts 

• Insufficient calcium/vitamin D intake increases PTH, resulting in bone loss

Describe the balance between bone resorption and deposition

Resorption: osteoclasts 

• These break down the bone matrix, releasing minerals (like calcium) into the bloodstream 

Deposition: osteoblasts 

• These produce new bone matrix (collagen and mineral) to replace removed tissue 

If this balance is disturbed, it can cause conditions such as osteoporosis (high resorption), or osteopetrosis (high formation)