Lecture 3

What is bone modelling a result of?

Bone modelling is a result of formation through osteoblastic activity on selective bone surfaces and resorption through osteoclastic activity on other bone surfaces.

How do flat bones grow?

Flat bones grow due to bone formation on their leading outer surfaces and resorption from the inside.

How do long bones grow?

Long bones grow through a process of endochondral ossification, where cartilage is replaced by bone tissue, primarily at the growth plates located at each end of the bone. Epiphyses expand and grow similarly; through proliferation of chondrocytes in articular-epiphyseal growth cartilage.

What is endochondral ossification?

Endochondral ossification is the process through which long bones grow in length by chondrocyte proliferation in the growth plate and hypertrophy followed by osteogenesis.

How does growth in girth of bones occur ?

A net accrual of new bone (formation – resorption > 0) on the outer surfaces (intramembranous ossification) while bone from the inner surfaces is subjected to a net loss (formation – resorption < 0)

What initiates osteoclastogenesis during trabecular bone remodelling?

Osteoclastogenesis is initiated by signalling pathways that activate osteoclasts, with RANKL assisting through upregulation via PTH.

What do osteoblasts do in trabecular bone remodelling?

Osteoblasts deposit osteoid on the exposed bone matrix after the osteoclastic phase, leading to new bone formation.

What activates osteoblasts in trabecular bone remodelling?

Growth factors from the bone matrix and factors produced from osteoclasts

What is the reversal phase of trabecular bone remodelling

The reversal phase is the transition period between resorption and formation, where osteoclasts undergo apoptosis and osteoblast precursors are recruited to the site to deposit osteoid

What are the 6 phases of trabecular bone remodelling

Quiescence → resorption → reversal → formation → mineralisation → quiescence

How does cortical bone remodel ?

Osteoclasts displace bone lining cells within the haversian canal causing a tunnel to form. A resorption cavity forms which blood vessels invade. The reversal zone initiates mesenchymal cells to differentiate into osteoblasts. Formation of osteoblasts leads to deposition of concentric lamellae sheets → forms new osteon

What is the difference between the cutting cone and the closing cone

The cutting cone refers to the leading edge of the bone remodeling process where osteoclasts resorb bone, while the closing cone is the trailing edge where osteoblasts fill in the resorption cavity with new bone.

What is the fracture gap

The space between fractured bone ends

What is a bridging callus

A fibrous tissue that forms in the fracture gap, stabilizing the broken bone ends during healing.

What is a critical-sized defect

A bone defect that is too large to heal spontaneously

Describe the concept of interfragmentary strain

Interfragmentary strain refers to the mechanical stress experienced at the fracture site during healing, influenced by the stability of the fixation and the gap between bone fragments.

True or false: if there is a small fragment gap, the bone experiences a larger strain

True as smaller gaps lead to less compliance of the bridging callus tissue, increasing the strain on the bone.

What are the phases of indirect union in fracture healing?

1. Inflammation and oedema

2. Soft callus formation

3. Hard callus formation

4. Callus remodelling

What is the difference between a soft callus formation and a hard callus formation

Soft callus formation is the initial phase of fracture healing where cartilage/fibrous tissue mix is formed at the fracture site, while hard callus formation involves the conversion of this mix into cartilage and bone, providing greater stability.

What happens during direct union formation?

Direct union occurs with a small gap (less than 0.1 mm) and limited mobility, involving direct osteonal remodelling without the formation of callus.

What is the difference between stress and strain?

Stress refers to the internal force per unit area applied to the bone, while strain is the deformation or displacement of the bone resulting from applied stress.

What is axial compression?

Axial compression refers to the act of putting pressure on bone, attempting to make it shorter, often seen in weight-bearing situations.

Differentiate between concentric axial compression and eccentric axial compression

Concentric axial compression involves uniform pressure applied along the central axis of the bone, while eccentric axial compression applies pressure off-center, leading to uneven stress distribution.

What is tension

Tension refers to the force that is applied to a bone in a manner that attempts to elongate or stretch it, counteracting compressive forces.

What results from a bending moment of force on bone?

A bending moment of force creates compressive stress on the concave side of the bone and tensile stress on the convex side.

What form of compression causes bending moment of force

Eccentric axial compression

What is the torsion moment of force

Torsion moment of force is the twisting force applied to a bone that causes shear stress, resulting in a twist of the bone about its long axis

Define shear stress

Oblique or parallel (as in pushing a part of bone to the left and the part just below that to the right) internal stresses/strains

Explain how the activities of bone cells lead to the maintenance of adult bones

Bone cells, including osteoblasts, osteocytes, and osteoclasts, work together to remodel and maintain bone tissue. Osteoblasts build new bone, osteoclasts resorb old bone, and osteocytes communicate mechanical stresses to regulate this process, ensuring bone strength and adaptability.

How do bones adapt to mechanical loading

Increased loading causes net bone mass accrual by promoting formation and suppressing resorption, leading to thickening of trabeculae

Unloading stimulates resorption and suppresses formation, leading to thinning of trabeculae and/or cortex and thus a net bone loss