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How Bones Work
by Tom Scheve
Introduction to How Bones Work
The human body is an incredible machine. It runs so well most of the time that
we don't have to pay much attention to any of the life-sustaining systems that are
in motion around the clock, humming along without our mindful involvement.
Right now, your body is performing vital and complicated tasks nearly too
numerous to comprehend. Fortunately, our bodies don't demand our
comprehension in order to pump the heart, oxygenate blood, regulate hormone
production, interpret sensory data and carry out every other process that keeps
our biological boats afloat.
In this article, we'll discuss one of the systems that makes life possible: the
skeletal system.
Bones prevent you from puddling on the floor in the form of a jellyfish, but what
else do they do? Bones rebuild themselves, they produce blood cells, they
protect our brains and our organs, they provide a giant system of levers that
allow us to move ourselves around, and bones also help maintain a steady
amount of calcium in our bodies.
And, even if you never make your mark on the world (or in the history books),
your bones will stick around long after you have otherwise vanished to declare to
the world: "These skeletal remains once supported skin and tissue and organs!
This person once existed!" And as the construction crew that unearthed your
bones reels back in horror, every life choice you ever made will seem -- if but for
a moment before you are shoveled into a Dumpster -- very much worthwhile.
Before we leave behind our skeletal remains to freak out future generations, we
should first learn some basics about bones: What are bones made of? What
happens when they break? And just how many of them do you have, anyway?
Bone Basics
The Bone Market
There is nothing to prevent you from legally
purchasing human bones and skulls from a vendor
in the United States. Specimens often come from
China, where unclaimed remains are sold to bone
distributors. Many skulls on the market today are
from India, which was once a primary provider of
skulls to the world market but banned the practice
of exporting human remains in 1987. (Skulls don't
come cheap: an average adult skull runs around
$500.)
There are 206 bones in the adult body. Bone is a honeycomblike grid of calcium
salts located around a network of protein fibers. These protein fibers are called
collagen.
When you patch a hole in a piece of drywall, you usually cover it with tape that
has a gummy fibrous grid, and then cover that with wall compound mortar. Bone
is made in much the same way. Collagen fibers are gummed together by a kind
of shock-absorbing glue [source: University of California-Santa Barbara]. Then,
all of this is covered and surrounded by calcium phosphate, which hardens
everything into place. Not only do bones make use calcium for strength, they also
keep some stored in reserve. When other parts of the body need a calcium
boost, the bones release the needed amount into the bloodstream.
There are two different types of bone tissue: cortical bone (the outer layer) and
cancellous bone (the inner layer). Cortical bone, also known as compact bone,
provides external protection for the inner layer against external force. It makes up
80 percent of bone mass and is dense, strong and rigid [source: Hollister].
Cortical bone is covered by a fibrous membrane called the periosteum. Think of
the periosteum as a utility vest that fits over the bone -- it has brackets and
places for muscles and tendons to attach. The periosteum contains capillaries
that are responsible for keeping the bone nourished with blood.
In the case of long bones such as the femur (the upper leg bone), the periosteum
covers the central portion of the bone but -- like a sleeveless vest -- stops short
of the cartilage tissue that resides on both ends of the bone (we'll discuss this
cartilage in a later section).
Cancellous bone, also known as trabecular or spongy bone, is the inner layer
of bone and is much less dense than cortical bone. It's formed by trabeculae,
which are needlelike structures that create a meshwork. However, instead of a
network of bone structure with periodic gaps, cancellous bone is more like a
network of connecting spaces with periodic structure. The latticework of tiny
chambers is filled either with bone marrow or connective tissue. Within these
marrow-filled spaces is where new blood cells are produced.
Though cancellous bone only makes up about 20 percent of the body's bone
mass, it plays important roles in body function. It provides structural stability and
acts as a kind of shock absorber inside the bone, but without adding too much to
the overall weight of the body.In the next section, we'll learn more about bone
marrow.
Bone Marrow
Inside the cavities of cancellous bone is soft, fatty tissue comprised of an
irregular network of blood vessels and cell types. This is called bone marrow.
There are two types of marrow: red and yellow.
Red marrow contains stem cells, unspecialized cells that can grow into different
types of specialized cells. They're responsible for replenishing and replacing cells
in the body that have been damaged or lost. (For the whole story on stem cells,
give How Stem Cells Work a read.) There are two types of stem cells found in
red marrow:
Hematopoietic stem cells (HSCs). This type of stem cell is responsible for
creating billions of new blood cells daily, at a rate of about 8 million every second
[source: Houston Museum of Natural Science]. HSCs create every type of blood
cell: red blood cells (which carry oxygen throughout the body), white blood cells
(which fight infections and kill bacteria) and platelets (which help your blood clot).
Marrow stem cells can even produce more marrow stem cells. HSCs can leave
the marrow and enter the bloodstream, where the ratio of blood cells to stem
cells is about 100,000-to-1 [source: National Institutes of Health].
Stromal stem cells. This type of stem cell generates bone cells, cartilage, fat
cells and connective tissue. Stromal stem cells are being studied for their
possible use in repairing spinal cord damage and healing disorders of the
lymphatic system.
Yellow marrow is mostly fat, and as we age, it can be found in places where red
marrow once resided -- some of the bones in our arms, legs, fingers and toes, for
instance. If the body needs more blood cells, yellow marrow can transform back
into red marrow and produce them. Some bones have a lot more red marrow
than others -- the pelvic bone, the spine's vertebrae and our ribs are all rich with
it. The body also stores iron in bone marrow.
Bone marrow can become diseased. Myeloproliferative disorders (MPDs) cause
the overproduction of immature cells from the marrow. Disorders such as aplastic
anemia and myelodysplastic syndromes (MDS) hinder the marrow's ability to
produce enough blood cells.
Several marrow diseases can be treated through stem cell transplants, which
introduce healthy stem cells to the patient's body to replace the diseased cells.
The traditional way to transplant these stem cells is to extract bone marrow from
the donor's hip bone with a syringe and introduce the material into the recipient's
body. You don't have to actually experience someone penetrating the process to
imagine how unpleasant it is. Increasingly, doctors are harvesting the marrow
stem cells from the bloodstream instead, resulting in better stem cell samples for
the recipient and less pain and discomfort for the donor.
In the next section, we'll examine some of the bones that help prevent your brain
and lungs from sliding down into your socks -- the axial bones.
Axial Bones
Bones can be broadly divided into two categories: axial and appendicular. In this
section, we'll take a look at the axial bones, so named because they form the
axis of the body. Axial bones are associated with the central nervous system and
protect delicate organs such as your heart and brain.
Axial bones include:
The cranium. Though that coconut on top of your neck feels like one big unit
with a jaw attached, the cranium is actually comprised of 22 interlocking cranial
and facial bones. These cranial plates and oddly shaped bones are held together
by joints, though these joints (quite wisely) don't allow for movement (except for
the mandible, or jawbone). Deep in your ear is the smallest bone in your body,
the stirrup. It's about the size of a grain of rice.
The spine. Your spine (also known as the vertebral column) is comprised of 33
specialized bones called vertebrae. These vertebrae provide form for the rest of
the body and protect the spinal cord. Starting from the head and moving
downward, the first seven vertebrae are cervical vertebrae, which keep your
beautiful cranium from rolling down the street every time you come to a sudden
stop. They also allow you to nod "yes" or shake your head "no." Next are the 12
thoracic vertebrae, forming the back of your rib cage. Below the thoracic
vertebrae are the lumbar vertebrae, which bear much of the body's load. Most
back muscles are connected to these workhorses. Below these is the sacrum,
which actually begins in childhood as five different vertebrae, but over time fuses
into one unit. Below this is another single unit that begins life in several pieces,
the coccyx (tailbone).
The sternum. The sternum, or breastbone, is front and center in its role as
organ-protector. It defends your heart, lungs and portions of your major arteries
from external forces. Like the coccyx or sacrum, the sternum starts off as
different sections that fuse over time into one unified piece. The sternum
provides stability to the ribs that are attached to it.
The ribs. These flat bones form a protective shield around your internal organs.
There are 24 ribs, 12 on each side of your body. They come in three different
types. From the top, the first seven sets of ribs are true ribs. They connect in the
back to the spine and connect in the front to the sternum. Next are the false ribs.
These three pairs connect in the back to the spine, but in the front they attach to
the seventh true rib, which is the last rib that connects to the sternum. Last are
the floating ribs, and these two pairs of ribs are attached to the spine like all the
others, but "float" in the front without being attached to the sternum or any other
rib.
Appendicular Bones
While the axial bones form the vertical axis of the body, the appendicular bones
are the bones that connect to this axis. Unlike axial bones, protection isn't the
function of appendicular bones; they're made for action. Let's take a look:
Bones of the shoulder. The bones that make up your shoulder girdle serve to
connect your arms to your sternum and rib cage for stability and support. You
have two clavicles (collarbones) that attach on one end to the breast plate and,
on the other end, support the shoulder blades, or scapulas. The shoulder blades
provide points of contact and attachment for many muscles and the bone of each
upper arm.
Bones of the arm and hand. The entire arm appendage has three basic
components: the upper arm, the lower arm and the hand. The upper arm is one
long bone, the humerus. The top fits neatly into the scapula, and the lower end is
connected by the elbow joint to the two bones of the lower arm: the ulna (the
bone on the same side as your little finger) and the radius (the bone on the side
of your thumb). The radius plays a larger role in your overall mobility and
function, while your ulna provides more stability. Both the ulna and the radius
connect to the wrist bones in the hand. Each hand has an impressive 27 bones:
eight carpal bones that make up the wrist, five metacarpal bones that extend the
length of your palm, and 14 phalanges that form four fingers with three bones
each along with a single two-boned thumb.
The pelvic girdle. When you sit down, all the weight of your upper body rests
ultimately on your pelvic girdle. This tough pair of hip bones protects lower
organs such as the bladder and, for women, protects the development of a fetus
and facilitates birth. The dimensions of the pelvic girdle differ fairly significantly
for men and women, as the opening in the center of the girdle must be large
enough for a child to pass through.
Bones of the thigh, leg and foot. Connecting the pelvic girdle to the lower leg is
a bone in the thigh area called the femur, the longest and strongest in the body.
About 25 percent of your total height is gained from the femur bone [source:
Houston Museum of Natural Science]. The femur connects through the knee joint
(which is covered and protected by the patella, or kneecap) to the shin bone
(tibia). Slightly smaller than the tibia is the other bone in the leg, the fibula. The
fibula is responsible for muscular connections, while the tibia makes sure your
foot and your knee don't get any farther apart from each other. Each foot has 26
bones: seven tarsal bones that make up the ankle, five metatarsal bones that
make up the body of your foot (and play a significant role in supporting your
body's weight), and 14 phalanges that form -- as is the case with your fingers -four toes with three bones each with a big toe that has two bones.
The Long and Short (and Flat and Irregular) of It: Bone Shapes
The 206 bones in the human body can roughly be divided into four categories:
long, short, flat and irregular.
Not all long bones are actually long (some bones in the fingers and toes are long
bones), but most are (such as the leg and arm bones). Long bones are identified
by the shape and structure of the bone: a slightly curved shaft capped on both
sides with hyaline cartilage, longer than it is thick. They're made mostly of
compact bone, allowing them to support great amounts of weight and withstand
pressure.
The femur provides an excellent example of the strength of long
bones. Its hollow cylindrical design allows it to provide the most possible strength
out of the materials provided (the substance of the bone itself), without making it
too heavy. The hollow internal portion of many long bones is where the marrow is
located. Long bones grow from both ends, and have a cartilage plate (also
known as epiphyseal plates) between the bone shaft and each bone end.
These plates continue growing through adolescence.
Short bones. Short bones consist mainly of spongy bone with a protective
covering of compact bone. Short bones are neither long nor thick, but rather
cubelike. Your kneecaps (patellae), wrists (carpals) and some of the bones in
your feet and ankles (tarsals) are short bones. Short bones aren't designed for a
great deal of movement, but they're sturdy, compact and durable.
The short
bones in the wrist and ankle are also known as sesamoid bones. Sesamoid
bones (usually classified as short or irregular bones) are placed within tendons in
parts of the body where a tendon must cross a joint. These bones hold the
tendon slightly away from the joint to provide better range of motion when the
tendon tightens. For instance, your kneecap connects two pieces of tendon that
cross over the joint between your femur (upper leg bone) and the tibia of your
lower leg.
Flat bones. These bones are thin and flat. Flat bones have a middle layer of
spongy bone located between two protective layers of compact bone. These
bones are generally protective in nature. Flat bones make up most of the 29
bones that fuse together to form the skull and protect the brain, and also protect
the major internal organs by forming the 24 ribs (12 on each side) of the rib cage,
as well as the sternum. The shoulder blade (scapula) is also a flat bone. Flat
bones contain marrow, but aren't large enough to have marrow cavities -- marrow
is found instead in the spongy bone. The marrow in flat bones produces more red
blood cells than any other adult bone type.
Irregular bones. The bones that don't fit in the other three categories are
irregular bones. Vertebrae in the spine and the jawbone (mandible) are irregular
bones. This type of bone usually has a very specialized function and is made up
of mostly spongy bone with a thin layer of compact bone around it.
Bone Construction Zone: How Bones Grow
Right now, the bones in your body are undergoing renovation. There are
wrecking crews blasting into the bone quarry and carting off debris while an
entirely different work crew hauls bags of concrete to the blast site and patches
the newly made holes with stronger, newer, better material.
Before we talk about replacing bone with bone, we'd better learn how cartilage
turns into bone. When you're floating around in the womb, your developing body
is just beginning to take its shape, and it's creating cartilage to do so. Cartilage is
a tissue that isn't as hard as bone, but much more flexible and, in some ways,
more functional. Cartilage is pretty good stuff to use if you're going to mold a
human -- good enough for the finer work, especially, such as your nose or your
ear.
A large amount of that fetus cartilage begins transforming into bone, a process
called ossification. When ossification occurs, the cartilage (which doesn't have
salts or minerals in it) begins to calcify; that is, layers of calcium and phosphate
salts begin to accumulate on the cartilage cells. These cells, surrounded by
minerals, die off. This leaves small pockets of separation in the soon-to-be-bone
cartilage, and tiny blood vessels grow into these cavities. Specialized cells called
osteoblasts begin traveling into the developing bone by way of these blood
vessels. These cells produce a substance consisting of collagen fibers and they
also aid in the collection of calcium, which is deposited along this fibrous
substance. (One common analogy for this design is reinforced concrete, which is
a grid of metal rods covered with concrete mix.)
After a while, the osteoblasts themselves become part of the mix, turning into
lower-functioning osteocytes, sort of a retired version of osteoblasts that continue
to putter along but don't stray too far from the blood vessels. This osteocyte
network helps form the spongelike lattice of cancellous bone. Cancellous bone
isn't soft, but it does look spongy. Its spaces help transfer the stress of external
pressures throughout the bone, and these spaces also contain marrow. Little
channels called canaliculi run all throughout the calcified portions of the bone,
enabling nutrients, gases and waste to make their way through.
The Stretching of the Lambs
Baby lambs do about 90 percent of their growing
at night, and the same may hold true for growing
humans. This may be a primary reason why
growing pains are experienced mostly at night by
young children [source: BBC].
Before turning into osteocytes, osteoblasts produce cortical bone. One way to
imagine this process is to picture a bricklayer trapping himself inside a man-sized
brick chamber of his own construction. After forming the hard shell (cortical
bone), the bricklayer himself fills the chamber. Air makes its way through the
brick and decays the bricklayer. In bone, this part of the process is accomplished
by osteoclasts, which make their way into the calcifying cartilage and take bone
out of the middle of the shaft, leaving room for marrow to form. Osteoclasts do
this by engulfing and digesting the bone matrix using acids and hydrolytic
enzymes. So, our bricklayer (osteoblast) made the tomb (cortical bone), died
inside the tomb (became an osteocyte), decayed over time (dissolved by
osteoclasts) and left behind his remains that formed a network of mass and
space inside the brick tomb.
Eventually, all the cartilage has turned to bone, except for the cartilage on the
end of the bone (articular cartilage) and growth plates, which connect the bone
shaft on each side to the bone ends. These cartilage layers help the bone
expand, and finally calcify by adulthood.
So, right now in your body, there are osteoclasts hard at work absorbing old bone
cells and osteoblasts helping to build new bone in its place. This cycle is called
remodeling. When you're young, your osteoblasts (the builders) are more
numerous than the osteoclasts, resulting in bone gain. When you age, the
osteoblasts can't keep up with the osteoclasts, which are still efficiently removing
bone cells, and this leads to loss of bone mass (and a condition called
osteoporosis, which we'll discuss shortly).
The Breaks: Bone Fractures
Although bone is very strong material, it can break in a number of ways with
enough force pushing, pulling or twisting it. Here are some of the more common
breaks:
Stress fracture. This type of fracture is the result of sustained force on a bone,
such as that created by running or jumping. Most stress fractures occur in the
lower body, due to the accumulated weight that the bones in our legs and feet
must support. It's possible to have a stress fracture without feeling any pain.
Open fracture. Unlike closed fractures in which all portions of the broken bone
remain within the skin, open fractures result in a piece of bone puncturing and
piercing the skin.
Complete fracture. This is when the bone breaks neatly into two pieces.
Single fracture. This identifies a break in which the bone has only one damaged
area.
Comminuted fracture. Also known as "More painkillers, please," comminuted
fractures are bones that have been crushed or broken into more than two
fragments.
Greenstick fracture. With this type of fracture, the bone has cracked on one
side, but not all the way through. These breaks normally occur in children.
Pathologic fractures. These fractures may be caused by external forces, but
the underlying cause is a bone that has been weakened by disease or infection,
such as bone cancer.
Displaced fractures. The two broken ends of the bone don't line up and require
repositioning before they're set in place.
Simple transverse. This type of fracture is an even, perpendicular break to the
bone. (Imagine if someone chopped your femur bone in half by neatly striking it
from the side at a right angle.)
Oblique fracture. An oblique fracture would be a diagonal fracture running
lengthwise along the bone. (Think greenstick fracture, but all the way through the
bone.)
Spiral fracture. Spiral fractures occur when the bone has been twisted past its
maximum point of resistance.
Here are a few of the more common break locations:
Colles' fracture is just a fancy name for a broken wrist. It's also the most
common type of fracture.
Hip fractures. More than half the fractures seen in older people are hip fractures
[source: FDA]. Hip fractures are actually a break in the femur bone, right below
the joint that connects the upper portion of the femur to the pelvis.
Compression fractures. This is a fracture that affects the spine. Bad falls are
usually responsible for compression fractures, in which one or more vertebrae
are essentially crushed.
Mending the Break: Healing Broken Bones
When bones are fractured, the body immediately initiates the first phase of
healing, the reactive phase. Ruptured blood vessels gather at the site of the
break and form a clot. This clot contains fibroblasts, which are connective tissue
cells that produce collagen proteins. When this clot forms, it lays the groundwork
for what will be a full-scale restoration of the bone.
In a few days, the broken ends of the bone produce new blood vessels that grow
into the clot that now bridges the separation in bone caused by the fracture.
White blood cells arrive with these new blood vessels and begin carting away
unneeded material from the site of the break. Now, the fibroblasts begin to
multiply and secrete collagen fibers which form a matrix that replaces the blood
clot.
In the reparative phase, specialized cells -- osteoprogenitor cells -- located in
the periosteal membrane that covers most of the bone begin transforming into
different types of needed cells. Some of these cells -- chondroblasts -- produce
cartilage, while others -- osteoblasts -- produce uncalcified bone called callus.
The new cartilage and callus bridge the separated pieces of bone, and the
cartilage begins to ossify into trabecular bone.
In the third phase, the remodeling phase, osteoclasts begin removing the
trabecular bone while osteoblasts replace it with compact bone. Once this phase
is finished, the fractured bone has healed. (For more on the healing process, try
Sometimes when bones break they have to be realigned so that the fractured
ends line up correctly for healing. All pressure must be taken off the bone, so that
the ends that are attempting to fuse back together don't move out of position.
Surgery might need to be performed to hold the bone fragments together with
metal plates, rods or screws. Not only do these devices secure the bone's
position for healing, they provide a starter bridge for the calcium deposits that will
begin accumulating along the healing area.
Hooking Up: Joints
Each time you lean forward, pick up a cup of coffee, raise it to your lips and put it
back down, your bones, joints, muscles and other tissues are all working in
perfect synchronicity to make this effort possible.
There are 68 joints in the human body, and each joint is comprised of several
elements. Among them are:
Bones. Well yeah, but more precisely, the articular cartilage at the end of the
bones. This cartilage prevents the bone ends from being damaged by contact
with each other. Cartilage itself can be harmed by infection, injury, disease or
simple wear and tear. This damage may lead to pain, inflammation and stiffness,
a condition known as arthritis.
Skeletal muscle. Skeletal muscle attaches to bone and appears striped when
viewed closely, earning the name "striated muscle." Unlike your cardiac muscle
or the muscle in the walls of your stomach, skeletal muscle can be voluntarily
moved, and lie at rest when not consciously activated. These muscles connect to
bones through tendons.
Tendons. When skeletal muscle contracts or lengthens, it pulls on bone through
an attached tendon, a tough, flexible tissue.
Ligaments. These tissues are pretty similar to tendons, except they connect
bone to bone, ensuring that bones that meet together to form a joint will stay in
place.
Synovial membrane. This layer of connective tissue exists around each joint,
providing it with protection and producing synovia, a fluid that lubricates the joint
and nourishes the cartilage.
Bursa. Similar to the synovial membrane, the bursa is a small sac that provides
a lubricant to ease the movement of muscle against muscle or muscle against
bone.
Not every joint moves. The skull, for example, consists of several bone plates
that join together, but the fibrous tissue connecting these plates doesn't allow for
movement.
Open, Sesamoid: Types of Joints
Freely moveable joints fall into one of several different categories. The different
types of joints are:
Pivot joints (known also as rotary joints). These joints allow for rotation around
an axis. There is a pivot joint near the top of your spine that allows your head to
move from side to side.
Hinge joints. This type of joint can open and close like a door. Your elbow is a
hinge joint. Your biceps and triceps muscles are basically two people standing on
opposite sides of a wall (the humerus, or upper-arm bone), each with one hand
reaching over to its respective side of a door (the bones of the lower arm). The
biceps "shuts" the door, by contracting and lessening the degree of the joint
angle, and the triceps, when it pulls on its respective side of the door, "opens" the
door, as the hinge then widens.
Gliding joints (known also as plane joints). This type of joint features two bone
plates that glide against one another. The joints in your ankles and wrists are
gliding joints. (Holding your forearm steady while your hand points upward and
then waving side-to-side with your hand is an example of this joint's functioning.)
Ball-and-socket joints. This is the most maneuverable type of joint. Your
shoulder and your hip are both ball-and-socket joints. These joints feature a
connection between one bone-end equipped with a protrusion that fits into the
receptive space at the end of the other bone in the joint. These joints allow for
forward motion, backward motion and circular rotation.
Saddle joints. These joints allow for two different types of movement. For
instance, a saddle joint allows your thumb to move toward and away from your
forefinger (as when you spread all five digits out, then bring them all together
side-by-side) as well as cross over the palm of your hand toward your little finger.
Conyloid joints. These joints are similar to ball-and-socket joints, just without
the socket (the "ball" simply rests against another bone end).
Bone Loss
Bones, like any other part of the body, are susceptible to disease, the most
common being osteoporosis. Osteoporosis is the diminishing of bone mass,
leaving it structurally brittle and physically porous. One in six Americans has
osteoporosis or early signs of the disease [source: National Institute of Arthritis
and Musculoskeletal and Skin Diseases]. In especially serious cases, a bone can
be broken by as little as a sneeze.
Women are greatly affected by this condition -- four out of five cases of
osteoporosis occur in women, and half of all women over the age of 50 will have
a fracture related to osteoporosis [source: National Osteoporosis Foundation].
But it also affects men and the young -- in fact, one quarter of all men over 50 will
suffer an osteoporosis-related fracture. Though any bone can be affected by
osteoporosis, the hip, spine and wrist are the most common. Fractures of the hip
and spine are especially problematic, resulting in immobility, severe and lasting
pain and even death.
Contributing factors of osteoporosis include:
Sedentary life. It's important to get exercise, since any weight-bearing activity
will improve the strength of your bones. Additionally, exercise will prompt glands
in the body to produce hormones -- such as growth hormone, testosterone or
estrogen -- that help prevent the deterioration of your bones as you age. Other
lifestyle choices, such as smoking and possibly high alcohol intake, adversely
affect bone mass.
Malnutrition. A poor diet will result in not getting enough vitamin C, calcium,
phosphorus, magnesium and vitamin D -- all important elements of good bone
health and the sustained ability of your bones to produce new bone matrix. Talk
to your doctor about your diet and vitamin intake.
Underdevelopment of bone mass before the age of 20. You most likely will
have achieved the majority of your peak bone density by about age 20, though
you can still gain bone mass until you're 30 or so. Living a healthy lifestyle and
gaining bone mass early on will pay dividends down the road as you age.
Low estrogen. Women with higher estrogen levels tend to have higher bone
density.
Other Bone Diseases
Osteoporosis can lead to a condition called dowager's hump. This occurs when
the spine, due to diminished mass and strength, begins to compress, resulting in
an outward curve of the upper vertebrae in the spine.
Another disease that can affect the bones is bone cancer. Bone cancer most
often spreads to the bone from other parts of the body, but it can also start in the
bone. When it begins in the bone, it's known as primary bone cancer.
Fortunately, primary bone cancer is pretty uncommon -- there are about 2,300
new cases discovered each year [source: National Cancer Institute]. When
found, bone cancer can be treated by surgically removing the tumor from the
bone or using chemotherapy, radiation therapy or cryosurgery (killing cancerous
cells by freezing them with liquid nitrogen).
Osteonecrosis is a condition in which bones no longer receive the blood they
need to survive, leading to bone death and degeneration. The cause of the
disease isn't currently known. Most cases require surgical intervention, and
doctors can graft healthy bone onto the diseased portions, attempt to restore
blood flow or replace joints with mechanical joints.
Osteogenesis imperfecta is an inherited disease that causes bones to be
especially brittle. A faulty gene leaves the body unable to produce collagen
normally. (For more, read How Osteogenesis Imperfecta Works.)
Paget's disease of bone affects about 1 million people in the United States and
tends to show up more often in people with Northern European ancestry [source:
National Institute of Arthritis and Musculoskeletal and Skin Diseases]. Paget's
disease of bone affects seemingly random bones in the body, making them grow
too large and structurally unstable. While the disease can affect any bone, it most
often affects the pelvis, skull, spine and leg bones.
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