Bone Structure, Remodeling, and Biomechanics

Flashcards covering lecture notes on bone biology, histology, endochondral ossification, mechanical properties, Wolff's law, hormonal regulation, and fracture healing.

What percentage of the organic content in bone is constituted by collagen?

Collagen constitutes 90% of the organic content in bone.

What is hydroxyapatite, and what is its role in mature bone?

an inorganic matrix that mixes with collagen to stiffen mature bones.

What percentage of the body's total calcium reserve is stored within bones?

Bones store 99% of the body's calcium.

Where is trabecular (spongy) bone primarily located in long bones?

Found primarily at the proximal (top) and distal (bottom) ends of long bones.

What is the periosteum?

The periosteum is a fibrous membrane covering the outer surface of cortical bone where muscles and tendons attach.

What substances fill the internal spaces of trabecular (spongy) bone?

Trabecular bone is filled with marrow and fat.

What region of a long bone fuses to the metaphysis when growth is complete?

The epiphysis (specifically at the knee end of the femur in the provided example).

What initial tissue model serves as the foundation for endochondral ossification?

Cartilage, which is flexible, resilient, avascular, and composed mostly of collagen.

What are the key characteristics of a primary ossification center?

It is the first center to appear (normally before birth) and forms the central part of a bone, such as the diaphysis of long bones or the body of irregular bones.

Where do secondary ossification centers typically form in long bones, and when do they appear?

They form the epiphyses at both ends of a long bone and usually appear after birth.

Why do growth plates fuse after puberty?

the rate of bone deposited exceeds the rate of cartilage growth, causing growth to cease.

What structural features (indicated on the right) remain after epiphyseal cartilages fuse and growth stops?

Epiphyseal lines.

What are Harris lines in bone?

Lines of increased bone density in long bones that represent the position of the growth plate at the time of a metabolic or physical disturbance.

How is enamel hypoplasia characterized on tooth crowns?

It is characterized by transverse lines, pits, and grooves on the crown surface.

What is the primary function of osteoblasts?

Bone-forming cells responsible for synthesizing and depositing bone material.

What is the primary function of osteocytes, and where do they reside?

Osteocytes reside in lacunae and are responsible for maintaining bone tissue.

What is the primary function of osteoclasts?

Responsible for the resorption (breakdown/destruction) of bone tissue.

What are osteons, and how are they oriented to best resist mechanical stress?

The functional units of compact bone, and they resist stress best when aligned parallel to force.

What passes through the central Haversian canal of an osteon?

Blood vessels, lymph, and nerve fibers.

What is the function of Volkmann's canals?

Volkmann's canals are small canals that pierce bone tissue at right angles to connect Haversian canals into a network.

Through what microscopic channels are nutrients transported to osteocytes in lacunae?

Canaliculi.

What are the three main purposes of bone remodeling?

To regulate calcium homeostasis, repair micro-damage, and shape the skeleton during development.

How does the balance of bone remodeling differ during childhood?

During childhood, bone deposition exceeds bone resorption.

At what age range do young adults typically reach peak bone density?

Between 25 and 30 years of age.

How does the balance of bone remodeling shift in old age?

Bone resorption exceeds bone deposition.

What pathological change is seen in the mandible of the Neandertal male fossil known as 'The Old Man from La Chapelle'?

Extensive bone resorption in the mandible due to tooth loss.

What two control loops regulate bone remodeling?

A hormonal mechanism that maintains calcium homeostasis in the blood and mechanical and gravitational forces acting on the skeleton.

Which gland releases calcitonin, and what triggers its secretion?

The thyroid gland releases calcitonin in response to rising blood calcium levels.

How does calcitonin regulate blood calcium levels?

stimulates calcium salt deposition in bone, lowering blood calcium levels.

Which glands release Parathyroid Hormone (PTH), and what triggers its secretion?

The parathyroid glands release PTH in response to falling blood calcium levels.

How does Parathyroid Hormone (PTH) increase blood calcium levels?

PTH signals osteoclasts to degrade bone matrix and release $Ca^{2+}$ into the blood.

What standard concentration range is maintained for calcium homeostasis in human blood?

$$9\text{--}11\text{ mg/100 ml}$$

What is Wolff's Law?

Bone grows or remodels in response to the forces or demands placed upon it.

Where are long bones thickest along their shaft according to Wolff's Law, and why?

Midway along the shaft, because that is where bending stress is greatest.

What structural asymmetry occurs in professional tennis players who began playing in youth?

The cortical bone in their swinging humerus becomes approximately 40% thicker and stronger compared to their non-playing arm.

What is the difference between static loading and dynamic loading of bone?

Static loading applies a constant force to a material, whereas dynamic loading applies a force that fluctuates over time.

How is mechanical stress ($s$) calculated?

$$\text{Stress } (s) = \frac{\text{Force}}{\text{Area}} = \frac{F}{A}$$

How is mechanical strain ($e$) defined?

Strain is deformation measured as change in length over initial length: $\text{Strain } (e) = \frac{\Delta L}{L}$

In biomechanics, what do positive strain ($+\text{ strain}$) and negative strain ($-\text{ strain}$) represent?

Positive strain represents tension ($+\text{ tension}$) and negative strain represents compression ($-\text{ compression}$).

What is tension loading?

Tension is a type of mechanical loading in which material is pulled apart or elongated.

What deformation pattern occurs during bending load on a bone?

compression on one side of the bone and tension on the opposite side.

What causes shear loading in bone?

Opposing forces are applied to two points that are not aligned.

What does the slope of the elastic region on a stress/strain plot represent?

Stiffness (resistance to deformation).

How do tension and compression cycles mechanically stimulate bone deposition?

They generate a small electrical potential that stimulates bone deposition and increases density at points of stress.

What demographics over age 65 are affected by osteoporosis?

Osteoporosis affects 1/3 of women and 10% of men over age 65.

How does estrogen influence bone cell activity?

turns on osteoblasts to produce bone and turns off osteoclasts to prevent resorption.

What are Bone Remodeling Units (BRU)?

Tightly coupled groups of adjacent osteoblasts and osteoclasts that sequentially carry out bone resorption and deposition.

What occurs during Stage 1 (Hematoma formation) of bone fracture repair?

Torn blood vessels hemorrhage, forming a mass of clotted blood (hematoma); bone cells deprived of nutrients die, and the site becomes swollen, painful, and inflamed.

What occurs during Stage 2 (Fibrocartilaginous callus formation) of bone fracture repair?

Granulation tissue forms, capillaries invade, phagocytes clean debris, fibroblasts secrete collagen fibers to connect broken ends, and osteoblasts begin forming spongy bone.

When does bony callus formation (Stage 3) begin and complete after a fracture?

It begins 3-4 weeks after injury and continues until a firm union is formed 2-3 months later.