Biomechanics of Bone 2

Question 1

Changing bone morphology through…..

Growth

Answer 1

Intramembranous & Endochondral Ossification
Bone replaces cartilage
Epiphyseal plate

Question 2

Changing bone morphology through…..

Modelling

Answer 2

Formation OR Resorption

Changes Bone shape

Question 3

Changing bone morphology through…..

Remodelling

Answer 3

Resorption AND Formation (linked together)
Bone Turnover
Metabolic or Mechanical

Question 4

Bone growth

Intramembranous

Answer 4

e.g. lateral clavicle, flat bones
• Stem cells in mesenchyme
• Differentiate into osteoblasts (Obs)
• Secrete osteoid, engulfs Obs = osteocytes
• Mineralises (spongy bone around vessels)
• Periosteum forms, with Obs present
• Lamellar bone formation

Question 5

Bone growth

Answer 5

Comprises type I collagen (mostly) and ground substance: PG macromolecules, multiadhesive glycoproteins, bone-specific, vitamin K-dependent proteins, growth factors and cytokines (IGFs, TNF-α, TGF-β, BMPs, sclerostin, IL-1, IL-6)

Question 6

BMPs

Answer 6

Induce bone growth through differentiation of mesenchymal cells into osteoblasts
Important for bone growth after surgery such as grafts, fusions, repair of defects

Question 7

bone growth - Osteoprogenitor cells

Answer 7

derived from mesenchymal stem cells: give rise to osteoblasts

Question 8

bone growth - Bone-lining cells

Answer 8

derived from osteoblasts - maintenance of osteocytes

Question 9

Bone growth - Osteocytes

Answer 9

Osteocytes can be quiescent, formative or resorptive in nature,
when resorptive they secrete matrix metalloproteinases (MMPs) that degrade the local matrix = osteocytic osteolysis - responsible for ion homeostasis (?)

Question 10

Bone growth

newborn > adult

Answer 10

Skeletal development begins in utero and continues for about 25 years
* Bone number increases
• Mineralisation increases
• Bone size increases
• Bone proportions change
• Patterns and magnitudes of forces change

• Increasing mechanical demands
• Increasing mechanical competence

Question 11

Bone growth

adult > old

Answer 11

Mechanical demands reduce

Mechanical competence declines (abruptly at menopause due to hormones)

Question 12

bone growth - Development of hand & wrist

Answer 12

3 > 4 months

Bone formation/modelling is accompanied by bone remodelling

Question 13

Bone modelling in reshaping bones – how and why

Answer 13

Resorption via osteoclastic activity on outside surface of bending
Formation via osteoblastic activity on inside surface of bending

Examples of modelling during growth (A) where the diaphyseal cross-section drifts outwards; or during adapation to loading (B) when the bone might straighten when subjected to frequent bending. (C) shows how the ‘drift’ of the central part of the shaft occurs

Question 14

Bone remodelling

Answer 14

Basic-multicellular unit erodes existing bone and then, after a period of time the eroded bone is replaced as a linked sequence of events.

Question 15

Bone remodelling

Answer 15

Basic-multicellular unit creating a secondary osteon, with subsequent mineralisation; see microradiograph (below)

Question 16

Cancellous bone surface remodelling

Answer 16

Osteoclasts ‘dig’ a trench and then osteoblasts start to refill the resorption cavity. The image (above) shows a polarised light image of a new bone packet on the underside of the trabeculum

Question 17

resorption vs formation

Answer 17

The difference between resorption and formation (bone balance) determines whether bone is gained or lost during remodelling

Question 18

Bone remodelling

Answer 18

Targeted remodelling is considered to occur to replace damaged or worn out bone that may be affected by microdamage at microscopic and sub-microscopic scales

Question 19

Physical activity and the skeleton

Wolff’s Law:

Answer 19

Wolff’s Law: Can be paraphrased as “Bone will change in mass and geometry to meet the physical demands placed upon it.”
[..it is not really a law, more a principle]

Question 20

Physical activity and the skeleton

Mechanostat theory:

Answer 20

Mechanostat theory: An extension of the ideas proposed by Wolff, in which bone adapts to function mechanically as required by detecting and responding to mechanical loads

Question 21

Physical activity and the skeleton

Answer 21

Thus, bone can change size, shape and density throughout life in order to be relevant to the demands placed upon it – whether they be increased or decreased loads (e.g. training vs immobilisation)

Question 22

balance between bone formation and resorption

Answer 22

Obviously, an understanding of how the balance between bone formation and resorption is maintained is of importance for clinical applications, but is also of general scientific value in explaining bone morphology and how animals adapt their skeletons to changing physical demands inlife