Module 4 AP1
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Anatomy
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Skeletal Function
The skeleton, comprised of a network of bones held together at joints, has many functions. The skeleton protects vital internal organs. For example, the skull forms a protective encasement for the brain. The rib
cage provides protection for the heart and lungs. Flat bones, such as those of the skull, ribs, and breastbone, produce blood cells.
All bones are storage areas for inorganic calcium and phosphorus salts.
Bones also provide sites for attachment of muscle, tendons, and ligaments. The long bones, particularly those of the legs and the arms, permit flexible body movement. The large, heavy bones of the legs support the body against the pull of gravity.
There are five types of bones, each with a distinct shape and form (see
Figure 4.1
). The shape of a bone determines its function.
Long bones
are long and thin, designed to support body weight and enable movement. Examples of long bones include the humerus, ulna, radius, tibia, fibula, metacarpals, and metatarsals.
Flat bones
(such as in the cranium) form the roof of the skull to protect the brain.
Short bones
are small and cube-shaped. The carpals in the hand and tarsals in the foot are short bones.
Irregular bones
are varied in structure with ridges or irregular surfaces. The vertebrae are irregular bones designed to protect the spinal cord as well as enable spinal movements. The pelvic bones (ilium, ischium, and pubis) are also irregular bones.
Sesamoid bones
are small and round, reinforcing tendons. The patella is an example of a sesamoid bone.
Figure 4.1 The five basic bone shapes
: Long, flat, short, irregular, and sesamoid
Bone Landmarks
Bones are not simply smooth surfaces. Each bone has distinct markings, ridges, grooves, or holes called
bone landmarks
. Bone landmarks serve several functions. Some bone landmarks allow for tendons to attach. Other markings indicate where nerves and blood vessels run alongside the bone or penetrate the bone to provide blood and nervous supply. Major bone landmarks on individual bones will be discussed throughout this module. The following list of terms will help to provide a description of the types of bone landmarks:
Foramen, canal, fissure -
openings in bone to allow for nerves,
blood supply, or a passageway
Sinus -
hollow chamber in bone, usually filled with air
Process, ramus -
elevations in bone
Trochanter, tuberosity, tubercle, crest, line, spine -
processes or projections for tendon or ligament attachment
Head, neck, condyle, trochlea, facet -
processes designed for articulation with adjacent bones
Fossa, sulcus -
depressions in bone
Overview of the Skeleton
The human skeleton has two main divisions: the axial skeleton and appendicular skeleton. The
axial skeleton
lies on the midline of the
body and consists of the skull, vertebral column, sternum, laryngeal skeleton, and thoracic (rib) cage (see
Figure 4.2
and
Figure 4.3
).
Figure 4.2
The axial skeleton (blue) includes the skull, vertebral column, and ribs.
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Figure 4.3
Lateral view of the axial skeleton, appendicular skeleton removed.
The Skull
The
skull
is formed by 22 bones: the cranium (8 bones) and facial bones (14 bones). The
cranium
protects the brains and is composed of
eight bones fitted tightly together in adults. In newborns, certain bones
are not completely formed and instead are joined by membranous regions called
fontanelles
(see
Figure 4.4
), commonly called “soft spots.” Fontanelles allow the bones of the skull to compress during childbirth and expand to accommodate a rapidly growing infant brain. These regions begin to close around two months but may last up to two years.
Figure 4.4
Fontanelles are present in newborns until around two years
of age.
The large bones of the cranium have the same names as the lobes of the brain: frontal, parietal, temporal, and occipital. See
Figure 4.5
and
Figure 4.6
for views of the cranium. On the top of the cranium, the
frontal bone
(one bone) forms the forehead, the
parietal bones
(two, paired bones) extend to the sides, and the
occipital bone
curves to form the base of the skull.
Below the much larger parietal bones, each
temporal bone
has an opening that leads to the middle ear. The
sphenoid bone
not only completes the sides of the skull, it also contributes to the floors and walls of the eye sockets. Likewise, the
ethmoid bone
, which lies in front of the sphenoid, is a part of the orbital wall and, in addition, is a component of the nasal septum. The sphenoid and ethmoid bones lie largely inside the skull
(Figure 4.6)
.
Figure 4.5 Lateral view of the cranial bones
. The ethmoid bone is highlighted in blue.
Figure 4.6 Interior view of the of the cranial cavity
. The parietal bones and frontal bone have been removed to show the interior of the
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cranium. The sphenoid bone is highlighted in blue to note its unique shape. The zygomatic bone, part of the facial skeleton, is prominent in this view.
The occipital bone contains a large opening, the
foramen magnum
, through which the
spinal cord passes to become the brain stem. Note the bone landmarks in
Figure 4.7
,
below.
Figure 4.7
Posterior view of the occipital bone with major bone landmarks.
The bones of the cranium contain the
sinuses
, air spaces lined by mucous membrane (see
Figure 4.8
). Sinuses reduce the weight of the
skull and give a resonant sound to the voice. Two sinuses called the mastoid sinuses drain into the middle ear.
Mastoiditis
, a condition that can lead to deafness, is an inflammation of the mastoid sinuses. A sinus infection (
sinusitis
) occurs when the soft tissues inside the sinuses become inflamed from a virus, bacteria, or allergy.
Figure 4.8 Anterior view of the skull.
The frontal bone is faded to show the frontal sinuses, hollow spaces within bones of the skull.
The
foramina
of the skull allow for many functions, such as passage for blood vessels, nerves, and the spinal cord (see
Figure 4.9
). The
foramen magnum
allows for passage of the spinal cord into the skull. The
carotid canal
is an opening of the temporal bone for the internal carotid artery. The
external acoustic meatus
(
Figure 4.9, Figure 4.12
) is for transmission of sound, also located within the temporal bone. Note the locations of the other highlighted foramina in
Figure 4.9
below.
Figure 4.9 Inferior view of the foramina of the skull
. The major foramina are in blue. All foramina above are paired except for the foramen magnum.
There are fourteen facial bones. The
mandible
, lower jaw, is the only movable portion of the skull
(Figure 4.10)
. The mandible and vomer (
Figure 4.11, Figure 4.12
) are the only non-paired bones of the facial skeleton; all other facial bones are paired. The
maxillae
, the upper jaw, forms the anterior portion of the hard palate and contains the infraorbital foramen. Tooth sockets are found in both the mandible and maxillae
(Figure 4.10)
. The
zygomatic bones
give us our cheekbone prominences, and the
nasal bones
form the bridge of the nose
(Figure 4.10, Figure 4.11)
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Figure 4.10
Lateral view of the facial bones with foramen.
The
palatine bones
make up the posterior portion of the hard palate and floor of the nasal cavity
(Figure 4.11)
. Each thin, scale-
like
lacrimal bone
lies between an ethmoid bone and a maxillary bone, and the thin, flat
vomer
joins with the perpendicular plate of the
ethmoid to form the nasal septum
(Figure 4.12)
. The
inferior nasal conchae
are bones located inferiorly to the middle conchae
(Figure 4.11)
. The
middle and superior nasal conchae
are formed from the
grooves of the ethmoid bone. The nasal conchae act to swirl the air as it is breathed in through the nasal passages, helping to warm and humidify the air before it enters the lower respiratory system.
Figure 4.11
Anterior view of the facial skeleton, left maxilla bone is removed. The palatine bones are highlighted in blue bilaterally.
Figure 4.12 Lateral view (left) of the skull
. The left maxilla is removed to show the bones deep to it.
The Vertebral Column
The
vertebral column
extends from the skull to the pelvis. In a typical spine, the vertebral column has four curvatures that provide more resilience and strength in an upright posture than a straight column could (see
Figure 4.13
). The various vertebrae are named according to their location in the vertebral column. The groups names (anatomical regions) of the vertebrae are:
cervical
(neck),
thoracic
(back, ribs),
lumbar
(lower back),
sacrum
, and
coccyx
(tail). When the vertebrae join, they form a canal (vertebral foramen) through which the spinal cord passes. In the vertebral column there are seven cervical vertebrae (C1-C7), twelve thoracic vertebrae (T1-T12), five lumbar vertebrae (L1-L5), one sacrum, and one coccyx.
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Figure 4.13
Typical curvatures of the vertebral column: cervical, thoracic, lumbar, and sacral curves.
The structure of an individual vertebrae varies slightly from region to region (see
Figure 4.14
). The
spinous processes
are located on the dorsal side of the vertebrae and can
be
palpated
(examined externally by touch) as bony projections along the midline of the
neck and back. The vertebral body is located on the anterior portion and is the part of the
vertebrae with the most surface area. The
articular facets
allow adjacent vertebrae to
articulate with each other. Note how the spinal cord is protected in the center of the vertebrae
and the spinal nerves exit between the vertebrae.
Figure 4.14 Superior view of a single vertebrae.
The vertebral body is located anteriorly and the spinous process posteriorly.
The vertebrae within each section of the spine have unique features. There are seven cervical vertebrae. A typical
cervical vertebra
has a long spinous process with a bifid tip that splits into two parts posteriorly (except for C1). The cervical vertebral bodies are small, and
the vertebral foramen are large. The
transverse processes
have transverse foramina for the passage of the vertebral arteries and vertebral veins. See
Figure 4.15
below to view a typical cervical vertebra.
Figure 4.15 Superior view of a typical cervical spinal vertebrae.
Note the smaller vertebral body (anteriorly) and the bifid spinous process (posteriorly). The transverse process (highlighted in blue) contain transverse foramina for the passage of blood vessels, which travel to the brain.
A typical
thoracic vertebra
has a long, thin spinous process that does not
split
(Figure 4.16)
. The spinous process points inferiorly. The vertebral bodies are
medium-sized and contain facets for rib articulations. The transverse processes also have costal
facets for rib articulations. There are twelve thoracic vertebrae,
all of which contain the facets for rib articulation on the transverse processes except for T11 and T12
.
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Figure 4.16
Superior view of a typical thoracic spinal vertebra that contains costal facets for rib articulations. The superior costal facets are highlighted in blue on the vertebral body.
A typical
lumbar vertebra
(Figure 17)
has a shorter spinous process
that is broader and points posteriorly. The vertebral bodies of the lumbar spine are the largest, enabling it to support the weight of the head, neck, trunk, and upper limbs. The
transverse processes
are shorter and have no costal facets. The vertebral foramen of the lumbar
spine are the smallest and triangular-shaped. Note the other labeled regio
ns in
Figure 17
below.
Figure 4.17
Superior view of a typical lumbar vertebrae regions.
The
sacrum
is comprised of five fused bones at the base of the spine
(Figure 4.18)
. The base of the sacrum is the widest portion, which articulates with the L5 vertebra above it. The
coccyx
is comprised of four to five fused vertebrae, which typically begin to fuse by around age 25
(Figure 4.18)
. The sacrum and coccyx provide attachment sites for many ligaments and tendons. The stability of the sacrum, coccyx, and ligaments anchoring them to the pelvis are key for pelvic stability.
Figure 4.18
Posterior view of the sacrum (blue) and coccyx with bone landmarks.
The Thoracic Cage
All twelve pairs of
ribs
connect
directly
to the thoracic vertebrae posteriorly
(Figure 4.19)
. Ribs 1-7 connect directly to the sternum. Ribs 8-10 connect to the sternum
indirectly
via shafts of cartilage to the sternum. The lower two pairs of ribs (ribs 11 and 12) are called "
floating ribs
" because they do not attach to the sternum. The
sternum
is comprised of three parts: the manubrium, body, and xiphoid process.
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Figure 4.19
Anterior view of the ribs and sternum
The Appendicular Skeleton
The
appendicular skeleton
(Figure 4.20)
consists of the bones within the pectoral and pelvic girdles and the attached limbs. The pectoral (shoulder) girdle and upper limbs (arms) are specialized for flexibility and increased range of motion, while the pelvic girdle and lower limbs are specialized for strength.
Figure 4.20
The appendicular skeleton (blue) consists of the pectoral girdles, pelvic girdles, and all four limbs.
The
pectoral girdle
(Figure 4.21)
, also known as the shoulder girdle, is composed of two clavicles and two scapulae. Each clavicle (collarbone) connects with the sternum anteriorly and the
scapula
(shoulder blade) posteriorly
(Figure 4.21)
. The scapula is freely movable and held in place only by muscles and ligaments. The attachment of the scapula allows it to follow the many movements of the arm
Figure 4.21
Posterior view of clavicular and scapular attachment through ligaments.
Scapula Bone Landmarks
The surfaces of the scapula are important because of the many muscles and ligaments that attach to it. The major bone landmarks are noted in the figures below
(Figure 4.22, Figure 4)
. The posterior side of the scapula
(Figure 4.22)
contains the supraspinatus fossa, infraspinatus fossa,
and scapular spine. The
acromion process
projects from the scapular spine, which can be seen from both the posterior and anterior views
(Figure
4.23)
. The acromion process connects to the clavicle anteriorly. The
neck
of the scapula on the lateral side contains the
glenoid cavity
, where the head of the humerus articulates with the scapula. The anterior side of the scapula contains the subscapular fossa. The
coracoid process
projects anteriorly from the scapula, allowing for muscular attachment.
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Figure 4.22 Posterior view of right scapula.
Major bone landmarks on the posterior side of the scapula are colored for clarity.
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Figure 4.23 Anterior view of right scapula.
Major bone landmarks on the
scapula are colored for clarity.
The Humerus
The single long bone in the upper arm, the
humerus
(Figure 4.24)
, has a smoothly rounded head
(Figure 4.25)
that fits into a socket of the scapula at the glenoid cavity
(Figure 4.23)
. The glenoid cavity is very shallow and much smaller than the head of the humerus. The humerus needs to be held to the shallow glenoid cavity by the rotator cuff muscles and other ligaments.
The structure of the shoulder permits movement of the arm in almost any direction but provides little stability. The gleno-humeral joint is prone to dislocation because it is held in place primarily by muscular and ligament attachment with very little bony stability.
Dislocation
of a joint means that the bone is removed from its socket. Dislocation of the shoulder occurs when
the head of the humerus is removed from the glenoid cavity in any direction.
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Figure 4.24 Anterior view of the humerus, radius, and ulna.
Note the radius and ulna are uncrossed in anatomical position.
The humerus is comprised of the head, neck, shaft, capitulum, and trochlea
(Figure 4.25)
. The
capitulum
articulates with a small portion of the radius. The
trochlea
articulates with the ulna. The
medial epicondyle
of the humerus can be palpated on the medial side of the arm at
the elbow. The
lateral epicondyle
of the humerus can be palpated on the lateral side of the arm at the elbow.
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Figure 4.25
Anterior view of the right humerus. Bone landmarks are colored
for clarity.
The Radius and Ulna
The distal end of the humerus meets the two bones of the lower arm, the
ulna and radius
, at the elbow
(Figure 4.26)
. The
olecranon process
of the ulna is the prominent bone that can be palpated in the elbow
posteriorly
(Figure 4.26)
. When the arm is held in anatomical position (so that the palm is turned anteriorly), the radius and ulna are almost parallel to one another. When the arm is turned so that the palm is next to the body the
radius crosses in front of the ulna, a feature that contributes to the easy twisting motion of the forearm (pronation).
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Figure 4.26
Posterior view of the right humerus, radius, and ulna.
Bones of the Hand
The many bones of the hand increase its flexibility
(Figure 4.27)
. The wrist has eight
carpal bones
, which look like small pebbles. The proximal row of carpal bones (from lateral to medial) are scaphoid, lunate, triquetral, and pisiform. The distal row of carpal bones (from lateral to medial) are trapezium, trapezoid, capitate, and hamate. From the carpal bones, five
metacarpal bones
fan out to form a framework for the palm.
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Figure 4.27 Anterior view of the bones left hand:
eight carpal bones, five metacarpals, five proximal phalanges, four middle phalanges, and five distal phalanges. (Note: The thumb does not have a middle phalanx.)
The first metacarpal is the thumb, and the fifth metacarpal is the pinky. The metacarpal bone that leads to the thumb is placed in such a way that the thumb can reach out and touch the other digits. (
Digits
is a term that refers to either fingers or toes.) Beyond the metacarpals are the
phalanges
, the bones of the fingers and the thumb. The phalanges of the hand are long, slender, and lightweight. The thumb, or first digit, is composed of two phalanges, while all other digits have three
(Figure 4.28).
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Figure 4.28
Distal, middle, proximal phalanges; metacarpals and carpals of the left hand.
The Pelvic Girdle
The
pelvic girdle
consists of two heavy, large
coxal bones
(Figure 4.29)
. The coxal bones, also called ossa coxae or innominate bones, are anchored to the sacrum posteriorly via a network of ligaments. Together, these bones form a hollow cavity. The coxal bones are symmetrical and formed by the fusion of three bones: the ilium, ischium, and pubis. The
ilium
is the most superior of the sections and can be palpated on the lateral sides of the hips. The ischium is located posteriorly. The
ischium
bones are also known as the “sits” bones as these are the bony landmarks that can be felt when sitting upright in a chair. The
pubis
forms the center anteriorly, connected by the pubic symphysis
(Figure 4.29)
.
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Figure 4.29
Anterior view of the pelvis and several bone landmarks. The ischium are highlighted in blue.
The pelvic girdle has many important bone landmarks for the attachment of muscles for the lower limbs. The
ASIS
(anterior superior iliac spine) and
AIIS
(anterior inferior iliac spine) are located on the anterior portion of the ilium. Note the other important bone landmarks labeled in
Figure 4.29
and
Figure 4.30
.
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Figure 4.30
Interior view of the right innominate bone. Bone landmark regions are colored for clarification.
There are several differences in the male and female pelvic girdles. The
pubic arch
(Figure 4.31)
is wider in females than in males. The
pubic brim
, also known as the pelvic outlet,
(Figure 4.31)
is shaped more like a circle in males and an oval in females. These differences a
re to accommodate childbearing.
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Figure 4.31
Anatomical differences in the male and female pelvic girdle.
The Lower Extremities
The weight of the body is transmitted through the pelvis to the legs and then
onto the ground. The largest, longest single bone in the body is the
femur
(Figure 4.32)
. The head of the femur articulates with the pelvic girdle at the
acetabulum
, called the hip joint. Note the other important bone landmarks of the femur in
Figure 4.32
below
Figure 4.32 Posterior view of the right femur
. Bone landmarks are colored for clarification.
The Patella, Tibia, and Fibula
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The
patella
(Figure 4.33)
sits anteriorly to the femur, formed within the quadriceps femoris tendon. In the lower leg, there are two bones: the
tibia
(medial) and
fibula
(lateral). See
Figure 4.33
below for the major bone landmarks on the tibia and fibula. The larger of the two bones, the tibia, has a ridge that can be easily palpated anteriorly. Both bones of the lower leg have a prominence that contributes to the ankle, the
medial malleolus
of the tibia on the medial side of the ankle and the
lateral malleolus
of the fibula on the lateral side of the ankle.
Figure 4.33 Anterior view of the right lower leg
. The medial and lateral condyles are located on the tibia.
The Foot
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The foot, like the hand, is made up of many smaller bones
(Figure 4.34)
. There are seven
tarsal bones
that make up the ankle: medial cuneiform, intermediate cuneiform, lateral cuneiform, navicular, cuboid, talus, and calcaneus. Five
metatarsal
bones span the distance between the ankle and toes. As a result, the foot has longitudinal arches from the heel to the toes and a transverse arch across the foot. These provide a stable, springy base for the body. Flat feet is a condition that is likely to occur if the tissues binding the metatarsals together become weakened. The bones of the toes are called
phalanges
, just like those of the fingers. The great toe (
hallux
) is
the first metatarsal. Like the thumb, the great toe only has a distal and proximal phalanx. Metatarsals 2-5 all have a distal, middle, and proximal phalanx.
Figure 4.34 Superior view of the bones left foot:
seven tarsal bones, five metatarsals, five proximal phalanges, four middle phalanges, and five distal phalanges. (Note: The great toe does not have a middle phalanx.)
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Bone Structure and Tissue
Bone is a living tissue that continually renews itself. A longitudinal section of a typical long bone
(Figure 4.35)
shows that it is not solid but has a medullary cavity filled with bone marrow.
Yellow bone marrow
is a fat storage tissue found mainly in long bones.
Red bone marrow
is found primarily in short and flat bones, primarily to produce red blood cells. Newborns have all red bone marrow, and over time it is converted to yellow bone marrow in long bones.
The medullary or marrow cavity extends throughout the
diaphysis
, or center length of the bone
(Figure 4.35)
. Surrounding the cavity is a layer of
spongy bone that is thickest at the ends of the bone. The solid outer layer of the bone is called
compact bone
. On the ends of each long bone are called proximal and distal
epiphysis
. Within the joints, there is a layer of
articular
cartilage
to help cushion joints and enable them to move freely.
Cartilage
is another type of connective tissue, but the matrix is flexible.
Articular cartilage
is cartilage found specifically at joint articulations.
Figure 4.35
Typical bone structure of a long bone. The femur is pictured above.
Compact bone
(Figure 4.36)
contains many
osteons
(formerly called Haversian systems) in which
osteocytes
(bone cells) in tiny chambers called
lacunae
are arranged in concentric circles around center canals. The
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center canals contain blood vessels and nerves. The blood vessels bring the nutrients that allow the bone to renew itself. The lacunae are separated by a matrix that contains protein fibers of collagen and mineral deposits, primarily
of calcium and phosphorus salts. Compact bone is usually found on the surface of the bone and surrounds an interior of spongy bone
(Figure 4.36)
.
Spongy bone
(Figure 4.36)
contains numerous bony bars and plates separated by irregular spaces. Although lighter than compact bone, spongy bone is still designed for strength. Just as braces are used for support in buildings, the solid portions of spongy bone follow lines of stress.
Figure 4.36
Interior of a typical long bone.
Bone Cells
In the adult, bone is continually being broken down and built up again. Bone absorbing cells, called
osteoclasts
, break down bone, remove worn cells, and deposit calcium in the blood
(Figure 4.37)
. The destruction caused by the work of osteoclasts is repaired by
osteoblasts
. As they form new bone, osteoblasts take calcium from the blood. Eventually, some of these cells get caught in the matrix they secrete and are converted to osteocytes, the cells found within the lacunae of osteons. Thus, through a process of remodeling, old bone tissue is replaced by new bone tissue. Because of continual remodeling, the thickness of bones can change. Physical use and hormone balance can also affect the thickness of bones. Adults and children alike require regular calcium in the diet to promote the work of osteoblasts.
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Figure 4.37 Bone cells:
Osteoclasts, osteocytes, and osteoblasts.
Bone Formation
Most of the bones of the human skeleton are cartilaginous during prenatal development. Since the cartilaginous structures are shaped like the future bones, they provide models of these bones. The cartilaginous models are converted to bones (called
ossification
) when calcium salts are deposited in
the matrix, first by precursor cartilaginous cells and later by bone-forming cells, called osteoblasts.
Endochondral ossification
is the ossification of long bones from hyaline cartilage. Endochondral ossification begins at the primary ossification center in the middle of the bone. Once the cartilage cells begin to die, blood vessels
start to penetrate the bone to deliver osteoblasts, forming the medullary cavity. Later, secondary centers form at the ends of the bones. A cartilaginous disk remains between the primary ossification center and each secondary center, which can increase in length. This area is commonly referred to as the
growth plate
. The rate of growth is controlled by hormones, such as growth hormones and sex hormones. During puberty, the disks become completely ossified and the bone stops growing. The individual
attains full adult height when this occurs.
Intramembranous
ossification is the formation of flat bones from connective tissue. Flat bone formation begins with mesenchymal cells differentiating into osteoblasts at specific points within the connective tissue of the embryo. Osteoblasts begin to secrete bone tissue. Once enough bone tissue is secreted, osteoblasts develop into osteocytes.
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Bone Pathophysiology
Bone is a living tissue and able to repair itself when damaged. There are several types of common bone fractures that can be categorized based on how the bone breaks
(Figure 4.38)
.
Closed
(or simple) fractures occur when the bone breaks but remains within the skin. An
open
(or compound) fracture occurs when the bone breaks, but part of the bone shaft breaks out of the skin. A
greenstick
fracture occurs when the bone bends and breaks, but not all the way across. This type of fracture occurs commonly in children because the bone tissue is still developing and soft. A
comminuted
fracture happens when a bone is broken into more than two segments. Comminuted fractures typically are repaired surgically. An impacted fracture occurs when one end of the broken bone shaft is pushed inside the other part of the bone.
Figure 4.38
Bone fracture types
Synovial joints are subject to arthritis. In
rheumatoid arthritis
, the synovial membrane
becomes inflamed and thickens. Degenerative changes take place that make the joint almost immovable
and painful to use. In old-age arthritis, or
osteoarthritis
, the articular cartilage at the ends of the
bones disintegrates
(Figure 4.39)
. The two bones of the joint become rough and irregular so that
it becomes painful to move the joint. This type of arthritis is apt to affect the joints that have received
the greatest use over t
he years.
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Figure 4.39
Healthy cartilage in joint (left) and arthritis in joint (right)
Osteoporosis
is a bone tissue disease
(Figure 4.40)
. When bone tissue degenerates faster than is replaced, the bones become weak. Brittle bones cause increased pain and are more likely to fracture.
Figure 4.40
Health bone tissue (left) and osteoporosis bone tissue (right)
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Bones are joined at the joints, which are classified as fibrous, cartilaginous, and synovial.
Fibrous joints
, such as those between the cranial bones, are immovable.
Cartilaginous joints
, such as those between the vertebrae, are
slightly movable. The vertebrae are also separated by disks, which increase their flexibility. The pelvic bones are slightly movable because they are anteriorly joined by fibrous cartilage, the pubic symphysis. Due to hormonal changes, this joint becomes more flexible during late pregnancy, allowing the pelvis to expand during childbirth.
Most joints are freely movable synovial joints
(Figure 4.41)
, in which bones are separated by a joint cavity.
Ligaments
, composed of fibrous connective tissue, bind the bones together. The ligaments hold the bones in place as they form a
joint capsule
. The joint capsule is lined by synovial membrane, which produces
synovial fluid
, a lubricant for the joint.
Figure 4.41
Synovial joints are freely movable and are separated by a joint cavity.
There are different types of synovial (movable) joints. The knee and elbow joints are
hinge joints
, which largely permit movement in one direction only.
Ball-and-socket
joints allow movement in all planes and even a rotational movement. Examples of ball-and-socket joints are the hip (femur into the socket of the pelvis) and the shoulder joint. Another type of joint is a saddle joint, such as the thumb. This
saddle joint
allows the thumb to freely
cross over the palm. A
pivot joint
allows for rotational movement. A pivot joints in the cervical spine (C1 and C2) allow rotation of the vertebrae.
The Shoulder
There are many ligaments holding together the ball-and-socket shoulder joint
(Figure 4.42)
. Remember that the clavicle connects to the manubrium
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of the sternum anteriorly and to the acromion process of the scapula posteriorly (via the acromioclavicular ligament). It is helpful to remember the
bone landmarks when naming the ligaments. Often, a ligament is named for the two bone landmarks that it connects. For example, the
sternoclavicular
ligament
joins the sternum with the clavicle. The
coracoclavicular ligament
joins the coracoid process of the scapula to the clavicle. The
coracoacromial ligament
joins the coracoid process to the acromion of the scapula. The
capsular ligament
(articular capsule) surrounds the head of the humerus and anchors it to the scapula. The
coracohumeral ligament
connects the head of the humerus to the coracoid process of the scapula. The tendons of the rotator cuff muscles also support the shoulder joint (
supraspinatus, infraspinatus, teres minor, subscapularis
). The rotator cuff tendons help to limit the movement at the shoulder so that the head of the humerus is stabilized within the joint.
Figure 4.42
Anterior view of the left shoulder joint and associated ligaments
(connecting the scapula, clavicle, and humerus)
The Hip Joint
The hip joint is a ball and socket joint, where the head of the femur articulates with the pelvis at the acetabulum
(Figure 4.43)
. There are many
ligaments that hold the pelvis together. Posteriorly, the pelvis is held to the sacrum by the iliolumbar ligament, anterior sacroiliac ligament, and posterior
sacroiliac ligament. The
ilioinguinal ligament
holds the ASIS of the ilium to
the pubic bone. The
iliofemoral ligament
attaches the femur to the ilium. The
uterosacral ligament
connects the sacrum to each side of the uterus in females
(Figure 4.43)
.
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Figure 4.43
The pelvis and femur articulate at the hip joint. The pelvis contains many ligaments to hold it to the sacrum (posteriorly) and the internal organs.
The Knee Joint
In the knee, as in other freely movable joints, the bone ends are covered by cartilage. In addition, there are also crescent-shaped pieces of cartilage called
menisci
, which give added stability for the femur to articulate with the tibia
(Figure 4.44)
. The large patellar ligament is located anteriorly. The
medial (tibial) and lateral (fibular) collateral ligaments, also known as the MCL and LCL, stabilize the sides of the knee
(Figure 4.44)
. The interior of the joint contains the anterior cruciate ligament (ACL) and posterior cruciate ligaments (PCL)
(Figure 4.45)
. The knee joint also contains thirteen fluid-
filled sacs called
bursae
, which ease friction between tendons, ligaments, and bones. Inflammation of the bursae in a joint is called bursitis.
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Figure 4.44
Anterior view of the left knee joint ligaments
Figure 4.45 Posterior view of the left knee joint.
ACL (blue) is the anterior cruciate ligament. The PCL is the posterior cruciate ligament.
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The Spine
There are many spinal ligaments that attach to the vertebrae, which support the spine
(Figure 4.46)
Anterior longitudinal ligament -
connecting the anterior vertebral bodies (also visible in
Figure 4.43
)
Posterior longitudinal ligament -
connecting the posterior the vertebral bodies
Interspinous ligaments -
connects the spinous process of two
adjacent
vertebrae
Supraspinous ligament -
connects the posterior portion of the spinous processes
There are also
intervertebral discs
(Figure 4.46)
between the vertebrae that act as a kind of padding. They prevent the vertebrae from grinding against one another and absorb shock caused by movements such as running, jumping, and even walking. The presence of the disks allows motion
between the vertebrae so that we can bend forward, backward, and from side to side without the vertebrae touching.
Disks tend to become weakened with age and may slip or even rupture. For example, if the posterior longitudinal ligament becomes weakened, the center portion of the disc may bulge into the vertebral foramen, called a
herniated disc
. Pain results when the damaged disk presses against the spinal cord or spinal nerves. The body may heal itself over time, or the disk can be removed surgically. Surgical procedures can fuse vertebrae together, but this limits the flexibility of the spine permanently. Physical therapy is another option to help strengthen the muscles of the spine. Strengthening exercises help to support the spinal ligaments and restore spinal alignment.
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Figure 4.46 Posterior view of the components of a typical vertebra
. Note the ligaments along the anterior, lateral, and posterior sides of the vertebral column.
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