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Chapter 8
Joints
Joints are classified by structure and function.
Structural Classification involves the material binding the joint together and whether a
joint cavity is present. There are three types of joints:
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Fibrous- typically immovable
Cartilaginous- slightly movable
Synovial- free movement
Functional Classification is based on the amount of movement in the joint. There are
three types of movement:
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Synarthroses-immovable
Amphiathroses-slight movement
Diathroses-free movement
I. Fibrous Joints
Bones are joined by bands of dense fibrous connective tissue. There is no joint cavity.
There are three types of fibrous joints:
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Sutures
Syndesmoses
Gomphoses
1. Sutures- found connecting the plates of the skull. Examples include the coronal,
lamboid and squamous sutures of the skull. As the plates fuse in the adult, the sutures are
called synostoses and result in immovable joints.
2. Syndesmoses- bones connected by ligaments or other connective tissue. Movement is
limited and depends on the length of the connecting fibers. Examples include the joint at
the distal end of the tibia and fibula.
3. Gomphoses- This is a peg and socket joint. The tooth in its boney alveolar socket is
the only example. The periodontal ligament connects the tooth to the socket.
II. Cartilaginous Joints
These are joints where the articulating bones are united by cartilage. There is no joint
cavity and mobility is limited. There are 2 types:
1. Synchondroses- These are joints where hyaline cartilage unites the bones. The most
common example is the epiphyseal plates of the long bones. These eventually form
synostoses. The costal cartilage and first rib is a second example of a cartilaginous joint.
2. Symphyses- Articular surfaces of bones are covered with hyaline cartilage which fuses
fibroelastic cartilage. These joints are amphiarthrotic and have strength and flexibility.
Examples include intervertebral joints and the pubic symphyses.
III. Synovial Joints
The articulating bones are separated by a fluid containing cavity. These joints are
diarthrotic and provide the greatest movement. They have 6 distinguishing features.
1. Articular cartilage that is approximately 1 mm thick and composed of hyaline
cartilage.
2. Synovial cavity that is a potential space containing a small amount of liquid.
3. Articular capsule that is made of 2 layers. A dense outer fibrous capsule of dense
irregular connective tissue that is continuous with the periosteum of the bone and a
synovial membrane composed of loose connective tissue.
4. Synovial fluid occupies the free space of the joint capsule. Its contents are derived
from the blood. This provides a slippery surface that reduces friction.
5. Reinforcing ligaments provide additional structure to the joints and are of two types,
intrinsic ligaments which are part of the fibrous capsule and extrinsic ligaments that are
distinct and separate from the joint capsule.
6. Nerve and blood vessels supply nutrients and provide sensory information. Sensory
nerves can indicate pain but are also important in monitoring the stretching of a joint and
adjusting muscle tone.
Associated Structures
In addition to these 6 characteristics, other structures commonly associated with synovial
joints include fat pads and articular discs of Fibrocartilage such as the menisci in the
knee. Also found in synovial joints are bursae and tendon sheaths. They work like “ball
bearings” and reduce friction where ligaments, tendons and muscles rub against each
other. A tendon sheath is an elongated bursa that wraps completely around a tendon.
One example is the tendons running together in the wrists (carpal tunnel).
Stabilizing Factors
Three factors, shape of the articular surfaces, ligaments and muscle tone all provide
additional stability for the synovial joint. Of these, muscle tone is the most important.
Tendons are kept taught, keeping the joints together. This is important for joints like the
knee, shoulder and foot. Ligaments provide bracing and help prevent excessive motion.
They snap if their length is exceeded by 6%. Articular surfaces help to direct motion but
provide little stability.
IV. Movement of Synovial Joints
All skeletal muscles attach to bone or connective tissue by at least 2 points.
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The Origin is the muscle attachment to the less movable bone
The Insertion is the muscle attachment to the movable bone.
Motion occurs when a muscle contracts and moves towards its origin. Motion can occur
along transverse, frontal and sagittal plans.
Range of Motion of a joint is classified as follows:
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Nonaxial movement- slipping
Uniaxial movement- 1 plane
Biaxial movement- 2 planes
Multiaxial movement- many planes
These motions are seen in 3 general types of movement:
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Gliding
Angular
Rotational
1. Gliding motion is the simplest and consists of one bone sliding over another. This is
seen with the intercarpal and intertarsal joints and between vertebrae.
2. Angular movements increase or decrease the angle between 2 bones. Movements
include:
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Flexion
Extension
Hyperextension
Abduction
Adduction
Circumduction
a) Flexion is the bending along a sagittal plane; this decreases the angle of the joint and
brings the bones closer together.
b) Extension is the straightening of bones resulting in the increase angle between bones.
c) Hyperextension is the bending of a joint beyond anatomical position.
d) Abduction is moving away from the midline along the frontal plane.
e) Adduction is moving toward the midline.
f) Circumduction is moving the limb to describe a cone, for example the motion made
by a pitcher. This motion involves the motions described above.
3. Rotation is the turning of the bone along its axis. This is seen with the first and second
vertebrae and the hip and shoulder joints.
4. Special movements are defined by specific joints.
a) Supination and pronation are seen with the movement of the radius over the ulna.
Supination is the radius and ulna are parallel and the palms are facing forward. Pronation
the radius and ulna form an X and the palms face posterior. (Think 3 P’s-pronation palms
posterior). This is the position the hands take when dribbling a basketball.
b) Dorsi flexion and plantar flexion is seen with the bending of the foot up and down.
Bending the foot towards the tibia is dorsi flexion and pointing the toes away is plantar
flexion. This latter motion is seen with ballet dancers.
c) Inversion/eversion is medial and lateral movement of the ankle. Overextension or
inversion is commonly seen with the sprained ankle.
d) Protraction and Retraction are the anterior and posterior movement of the jaw.
Protraction is the jutting out of the jaw.
e) Elevation and depression are the movement of the jaw in an inferior superior direction.
(Opening and closing)
f) Opposition is seen with touching of the thumb to the tips of the other phalanges.
V. Types of Synovial Joints
There are six major categories of synovial joints.
1. Plane joints- short, .nonaxial motion also known as gliding joints. Examples are the
carpals and tarsals.
2. Hinge joints- motion is a single plane. A cylindrical end of 1 bone conforms to a
trough of another. Typical examples include the elbow and phalanges.
3. Pivot joints- The rounded end of one conforms to a sleeve of another, the dens of the
axis and atlas of the cervical vertebrae.
4. Condyloid joints- both articulating surfaces are oval and this allows for angular
motion. The oval articular surface of one bone fits into the depression of the adjoining
bones; examples include phalangeal joints (knuckles) and radial carpal (wrist).
5. Saddle joints- both joints have concave and convex areas and are shaped like saddle.
This motion is seen with the twiddling of the thumbs and involves the carpometacarpal
joint of the thumb.
6. Ball and socket- Multiaxial and are the most freely moving of the synovial joints.
These joints have a spherical head fitting into a cup like socket.
VI. Selected Synovial Joints
1. Knee Joint
This is considered the most complex joint in the human body. It is actually considered
three joints working together.
These are:
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An intermediate joint between the patella and distal end of the femur
(femoropatellar joint). This is a plane joint.
A lateral and medial tibiofemoral joints between the femoral condyles and the
menisci below. The menisci help prevent lateral motion and attach to the outer
margins of the joint capsule on the tibia. They are easily torn. These are an
example of a hinge joint.
The knee is unique in that it is not completely enclosed by a capsule. The articular
capsule is found only on the lateral and posterior surfaces. The anterior surface is covered
by three ligaments going from the patella to the tibia. They are:
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The patella ligament
The medial and lateral patellar retinacula ligaments. They merge with the
articular capsule on each side.
The synovial cavity of the knee has a complicated shape and over one dozen associated
bursae. Some are easily injured such as the subcutaneous prepatellar bursa which lies
just over the patella.
Although the patellar, medial and lateral patellar retinacula cover the anterior surface of
the knee joint they do not provide extensive support. Two types of ligaments are
responsible for the stability and strength the knee joint. They consist of capsular and
extracapsular ligaments. They work to prevent hyperextension of the knee. These
ligaments are:
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The fibular and tibial(medial and lateral) collateral ligaments are extracapsular
ligaments and prevent lateral and medial rotation when the knee is extended.
The oblique popliteal ligament is an extracapsular ligament that is an extension of
the semimembranous muscle and helps to stabilize the posterior portion of the
knee.
The arcuate popliteal ligament also reinforces the posterior aspect of the knee.
The oblique popliteal and arcuate popliteal ligaments actually form an X on the posterior
aspect of the knee.
The intracapsular ligaments are the cruciate ligaments. The anterior and posterior
cruciate ligaments cross each other forming an X in the notch between the femoral
condyles. They prevent anterior and posterior displacement.
The anterior cruciate ligament attaches to the anterior intercondylar area and attaches on
the medial side of the lateral condyle of the femur. It is lax when the knee is flexed and
taut when it is extended.
The posterior cruciate ligament is stronger and is attached to the posterior intercondylar
area of the tibia and attaches to the lateral side of the medial condyle on the femur.
The knees are the most susceptible joint for sports injury because of the reliance on
nonarticular factors for joint stability. Although the knee can absorb vertical forces equal
to seven times the body, it is vulnerable to lateral blows. Common knee injuries involve
the 3 C’s, collateral ligaments, cruciate ligaments and cartilage (menisci). Lateral blows
are the most dangerous, tearing the tibial collateral ligament and the medial meniscus and
the anterior cruciate ligament.
2. Shoulder (Glenohumeral) Joint
This is an example of a ball and socket joint. The large head of the humerus fits into the
glenoid cavity of the scapula. The cavity is extended by a fibrocartilage ring called the
glenoid labrum.
The articular capsule runs from the margins of the glenoid cavity to the anatomical neck
of the humerus. It is loose and allow for a free range of motion seen with this joint.
Connective tissue support comes from three groups of ligaments.
A) Coracohumeral ligament provides the only strong support of the upper limb. It runs
from the coracoid process to the greater tubercle of the humerus.
B) Three Glenohumeral ligaments strengthen the front of the capsule. These ligaments
are weak.
C) The Rotator Cuff is formed from four tendons and muscles that encircle the joint.
The muscles include the subscapularis, supraspinatus infraspinatus and tees minor.
Injuries
Rotator cuff injuries are common when the arm is severely circumducted as occurs with
pitchers. Dislocations of the shoulder typically occur on the anterior and inferior aspects
of the shoulder.
3. Elbow Joint
The radius rotates on the capitulum of the humerus. It allows for pronation and
supination. The articular capsule extends down to the anular ligament of the radius. Side
to side movement is prevented by the ulnar collateral ligament and radial collateral
ligament.
Dislocations to the elbow are uncommon due to the tight fitting of the ulna to the
humerus. Inflammation of the tendons and ligaments are more common. The most
notable being the so called “tennis elbow”
Tennis elbow is an inflammation, soreness, or pain on the outside (lateral) side of the
upper arm near the elbow. There may be a partial tear of the tendon fibers, which connect
muscle to bone, at or near their point of origin on the outside of the elbow. It typically
involves the lateral epicondyle. It is brought about by the repeated twisting (pronation
and supination) of the wrist.
4. Hip (Coxal Joint)
This is a ball and socket joint whose movement is limited by strong ligaments. It is
formed from the spherical head of the femur and the deeply cupped acetabulum in the
pelvis. This cup is enhanced by a fibrous ring called the acetabulum labrum. Additional
support is offered by the iliofemoral ligament. This is a triangular shaped ligament that
runs from the surface of the ischium to the greater trochanter of the femur. The
pubofemoral ligament runs from the pubis to the lesser trochanter of the femur. A third
ligament, the ischiofemoral ligament runs from the ischium to the greater trochanter.
Together they form a spiral which screws the femur into the acetabulum when a person
stands. The ligamentum teres runs from the head of the femur to the lower lip from the
acetabulum.
Common injuries to the hip joint include fractures and dislocations. Hip fractures
typically involve the neck of the femur and are the result of underlying disease such as
osteoporosis.
Motor vehicle accidents are the most common cause of hip dislocations. (Wearing a
seatbelt can greatly reduce your risk.) Falls from a height (such as a fall from a ladder) or
industrial accidents can also generate enough force to dislocate a hip.
5. Temporomandibular Joint (Jaw)
The mandibular condyle is an egg shape and fits into a complex articulation surface in the
temporal bone of the skull. Two distinct movements can occur with the jaw, a hinge like
movement and the second is a lateral movement. A lateral ligament attaches the ramus of
the mandible to the zygomatic arch of the temporal bone.
Because of the hollow socket, jaw dislocations are the most common injuries seen. A
deep yawn can dislocate the jaw.
6. Injuries to the Joints
a) Cartilage tears occur most commonly on the meniscus of the knee. These are usually
a combination of compression and shear stress. Lose pieces of cartilage can cause the
joint to lock.
b) Sprains are caused by the stretching of the ligament that can lead to tears or complete
rupture. Common sites include the knee and ankle. Ligaments will heal, but it is a slow
process. If the ligaments are severely damaged, a surgical graft may need to be done.
c) Dislocations are caused by the bones being moved out of alignment. Subluxation is a
partial dislocation.
There are many inflammatory and degenerative conditions which may affect the joints.
a) Bursitis and Tendonitis are inflammations caused by over use. A common example is
housemaid’s knee where the prepatellar bursa is inflamed; another one is student’s elbow.
b) Arthritis is a common term for over 100 conditions which describe degenerative
processes found in the joints. Major examples include osteoarthritis, rheumatoid arthritis
and gout.
Osteoarthritis is associated with wear and tear or overuse. It is commonly associated
with old age and is considered irreversible. The hips, knees spine and knuckles are
common sites for this condition. Treatment, if possible is through joint replacement.
Rheumatoid arthritis can occur at any age and like osteoarthritis affects women more than
men. It is considered an autoimmune disease and begins with inflammation of the
synovial joint.
Gout is brought about by uric acid accumulation in the joints. It usually affects only one
joint. It is more common in men. It may have a genetic component.