Bone Homeostasis: Remodeling and Repair

By: Haley Drayer

What does Homeostasis mean?

• According to the medical dictionary the word homeostasis means, “the maintenance of relatively stable internal physiological conditions (as in body temperature or the pH of blood) in higher animals under fluctuating environmental conditions.”

• This term can be applied to bones and the way they remodel and repair themselves to get back to the normal structure after a fracture.

• Na+ and K+ pump

Bones seem lifeless, but did you know?

• Bone is a dynamic and active tissue• Small-scale changes in bone architecture

occur continually• Every week we recycle 5%-7% of our bone

mass• Up to ½ gram of Ca2+ may enter or leave

the adult skeleton everyday• Spongy bone (internal layer of bone) is

replaced every 3-4 years• Compact bone (external layer of bone) is

replaced every 10 years

What would happen if these facts weren’t true?

• When bone remains in place for long periods the calcium crystallizes and becomes more brittle sometimes causing fractures.

Bone Remodeling• Bone remodeling consists of bone

deposit and bone removal. • Bone deposit and bone removal are

done with the help of remodeling units.• Osteoclasts= large cells that break

down bone matrix• Osteoblasts= bone-forming cells

• In a healthy young adult the total bone mass should remain constant

• This means that the rates of bone deposit and removal are equal.

• Some parts of bone are remodeled more often than others and at different rates.

Bone Deposit• Bone deposit occurs whenever bone is injured or the bone

needs to be stronger.• Proteins, vitamin C, vitamin D, vitamin A, calcium,

phosphorus, magnesium, manganese, etc…• Pg. 188 old book.

Bone Resorption or Removal• Major contributor in this process is the

osteoclast• Osteoclasts move along a bone

surface, digging grooves called resorption bays as they break down the bone matrix.

• The part of the osteoclast that touches the bone is highly folded to form a ruffled membrane that clings tightly to the bone, sealing off the area of bone destruction.

• Osteoclasts may phagocytize the deminerialized matrix and dead osteocytes.

Control of Remodeling pg. 189-190

• Hormonal Mechanism• Parathyroid Hormone• Calcitonin

• Homeostatic Imbalance• Response to mechanical stress

Fractures• Despite their remarkable strength, bones are susceptible to

fractures, or breaks• Young ages:

• Sports injuries, automobile accidents,, and falls, etc…• Old ages:

• Most fractures are due to bones thinning and weakening

Types of Fractures• Fractures may be classified into four categories:

1. Position of the bone ends after the fracture Nondisplaced Displaced

2. Completeness of the break Complete Incomplete

3. Orientation of the break Linear Transverse

4. If the bone ends penetrate the skin Open Closed

Fractures• Fractures can be classified by the location of the fracture, the

external appearance, and/or the nature of the break.• A fracture is treated by reduction.

• Reduction is the realignment of boneClosed Reduction is when the bone ends are coaxed

into position by the physicians hands.Open Reduction is when the bone ends are secured

together surgically with pins or wires.• After a broken bone is realigned, it is immobilized

either by a cast or traction to allow the healing process to begin

Bone Repair

• A simple fracture dealing with small or medium sized bones in young adults may take six to eight weeks to heal.

• A fracture dealing with longer bones, larger bones, weight-bearing bones, and bones of elderly could be a longer healing process

First Stage of Bone Repair

1. Hematoma Formation• When a bone breaks, blood vessels in the

bone, periosteum, and surrounding tissues, are torn and hemorrhage.• As a result, a hematoma (or mass of

clotted blood) forms at the fractured site• Bone cells die because they are not

getting the nutrition they need• The tissue at the site becomes swollen,

painful, and inflamed

Second Stage of Bone Repair2. Fibrocartilaginous Callus Formation

• Within a few days, capillaries grow into the hematoma• Phagocytic cells invade the area and begin cleaning

up the debris• Fibroblasts and osteoblasts invade fracture site and

begin reconstructing bone Fibroclasts produce collagen fibers that span the

break and connect the broken bone ends Osteoblasts begin forming spongy bone

• This all helps to form the mass of repair tissue otherwise known as the fibrocartilaginous callus and this splints the broken bone

Third Stage of Bone Repair

3. Bony Callus Formation• Within a week, new bone

trabeculae begin to appear in the fibrocartilaginous callus and gradually convert it to a bony (hard) callus of spongy bone.

• Bony callus formation continues until a firm union is formed about two months later

Fourth Stage of Bone Repair

4. Bone Remodeling• Bone remodeling begins during the

formation of the bony callus and continues for several months after the bony callus is remodeled.

• The excess material is removed and compact bone is laid down to reconstruct the shaft walls.

• The final structure of the remodeled areas resembles that of the original unbroken bony region.

Bone Repair Video

• http://www.youtube.com/watch?v=qVougiCEgH8&feature=related