Introduction to Joints - Page 1 of 22 Learning Modules

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Learning Modules - Medical Gross Anatomy
Introduction to Joints - Page 1 of 22
Joints - Introduction
When you think about your joints, you most likely think immediately of
your knees, shoulders, hips, elbows, etc. Certainly these are large and
important joints, but there are many other joints in the body that you may have
never realized existed. For example, the union of the parallel borders of the
radius and the ulna and the junction between a tooth and its socket are both
types of joints.
A joint, or articulation, is defined as the junction between two or more
bones of the skeleton. Joints can be classified in one of two ways: by the
movement they permit or by the tissue joining the bones of the joint. Each of
these systems of classification provides useful information about the joint, but
the systems do not necessarily correspond.
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Learning Modules - Medical Gross Anatomy
Introduction to Joints - Page 2 of 22
Joints - Classification by Movement
Joints can be classified by how much
movement they allow.
Synarthroses: Immovable joints. Specific
examples of synarthroses are suture joints
(the joints in the skull) and synchondroses
(the type of joint found in growth plates).
Amphiarthroses: Slightly moveable joints.
A specific example of an amphiarthrosis is a
symphysis (such as the joint between two
vertebrae).
Diarthroses: Freely
moveable joints. Specific examples of
diarthroses are typical synovial joints such
as the shoulder and wrist.
We will discuss these specific examples in
more detail later in the module.
An important concept to remember is that
joint strength and flexibility are opposed. Greater
joint strength comes at the cost of less flexibility
and vice versa. The movement allowed at a
particular joint depends on the shape of the bones
and articular surfaces, the ligaments crossing the
joint, and the muscles crossing the joint.
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Introduction to Joints - Page 3 of 22
Joints - Classification by Tissue Joining Bones
Joints can also be classified by the type of tissue connecting the bones of the joint.
Fibrous Joints: The bones of these joints are connected by fibrous ligaments only.
Cartilaginous Joints: The bones involved in cartilaginous joints are joined by some type of cartilage.
Synovial Joints: These joints are freely moveable and are characterized by a joint cavity between the bones that has a synovial
membrane and is lubricated with synovial fluid.
We will discuss each of these types of joints in more detail on the following screens.
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Introduction to Joints - Page 4 of 22
Fibrous Joints
Fibrous joints are connected only by fibrous ligaments. A ligament is
dense connective tissue that connects bone to bone (as opposed to tendons,
which connect muscles to bones). Ligaments are named based on their position
or based on the bones they attach.
There are 3 distinct types of fibrous joints:
1. Suture Joints
2. Gomphoses
3. Syndesmoses
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Introduction to Joints - Page 5 of 22
Fibrous Joints - Sutures
Suture joints are one type of fibrous joint. They are
the type of joint that connect the flat bones of the skull
which meet in a tooth-like pattern. The fibrous tissue of
these joints is continuous with the periosteum, and the
joints are synarthroses (immovable). Suture joints
undergo changes throughout childhood and into early
adulthood. At birth, the sutures have broad spaces of
fibrous tissue called fontanelles. These joints close down
and undergo ossification (becoming fused with bone)
beginning in childhood and continuing into a person's
20's. A joint that has ossified over time is
called a synostosis.
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Fibrous Joints - Gomphoses
Gomphoses are another type of fibrous joint. They are the
type of joint that anchors the teeth in the alveolar processes
(sockets) of the mandible and the maxilla. These joints are
amphiarthroses (slightly moveable), although the normal
movement of this joint is not what we consider a "loose tooth".
Gomphoses (even in adult teeth) allow microscopic movements
that allow us to sense how hard we are biting and if we have
food stuck in our teeth.
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Fibrous Joints - Syndesmoses
Syndesmoses are the third
type of fibrous joint. In this
type of joint, apposed bones
are joined by a fibrous
membrane (interosseous
membrane) or a ligament.
These joints are
amphiarthroses (slightly
moveable) and are maintained
as fibrous unions throughout
life. (Syndesmoses do not
become synostoses.) Examples
of syndesmoses are the
attachment of the borders of
the radius and ulna, which are
connected with an interosseus
membrane, the attachment of
the borders of the tibia and
fibula, which are connected
with an interosseous
membrane, and the inferior
tibiofibular joint which is
connected by a ligament.
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Cartilaginous Joints
The next broad classification of joints we will discuss are cartilaginous joints. Cartilaginous joints are joined by
either hyaline cartilage or fibrocartilage. The cartilage of cartilaginous joints is avascular and anervous except at the
margins. Hyaline cartilage is slippery and strong when compressed, but has little tensile strength (strength against being
stretched). Fibrocartilage, on the other hand, is tough and strong both when compressed and when stretched (high tensile
strength). There are two distinct types of cartilaginous joints:
1. Synchondroses
2. Symphyses
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Cartilaginous Joints Synchondroses
Synchondroses are a temporary type of
cartilaginous joint that are seen at epiphyseal plates
(growth plates) during development. They allow for
growth of long bones by flexibly joining the
epiphysis, or end, of a growing long bone to the
diaphysis, or shaft, of a long bone. This type of joint
is made of hyaline cartilage that is eventually
replaced by bone after growth is completed
(synchondroses become synostoses). You may have
heard that children are particularly susceptible to
breaking bones at their growth plates. This makes
sense because that region of the bone is actually
hyaline cartilage, which has little tensile strength.
Synchondroses are also found at the union between
the first rib and the sternum.
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Cartilaginous Joints - Symphyses
Symphyses are the second type of cartilaginous joint. These are permanent cartilage unions characterized by
fibrocartilage disks separating bones covered by hyaline cartilage. These are strong amphiarthroses (slightly
moveable joints). Examples of symphyses are the pubic symphysis and the joints between vertebral segments with
their tough intervertebral disks.
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Introduction to Joints - Page 11 of 22
Synovial Joints
Synovial joints are the most common, most moveable and most complex
type of joint. Thus, we will spend a little more time on this joint type.
Essentially, all synovial joints are diarthroses (freely moveable) thereby giving us
the flexibility to walk, throw a ball, write, etc.
Synovial joints have a general structure that is characteristic of this joint
type. All synovial joints have:
1. A joint cavity between the bones
2. A synovial membrane lining
3. Articular cartilage
Some synovial joints also have accessory structures.
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Synovial Joints - Characteristics
The following are characteristic of synovial joints:
1. Joint Cavity - The space between articulating bones that is
lined with the synovial membrane. The joint capsule
surrounding the joint cavity provides support for the delicate
synovial membrane where it is not in contact with bone.
2. Synovial Membrane Lining - This structure secretes synovial
fluid which lubricates and nourishes the joint. (Syn=like,
ovial=egg-white)
3. Articular Cartilage - This covers the ends of bones that
articulate with each other. It consists of hyaline cartilage
lubricated with synovial fluid. It is very slick and smooth to
reduce friction in the joint.
4. Accessory Structures - These do not necessarily appear in all
synovial joints, but play key roles when they are present. We
will discuss these structures more in the next screen.
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Synovial Joints - Accessory
Structures
There are several types of accessory structures that may be
present in synovial joints.
1. Accessory ligaments - these connect bone to bone and
stabilize the joint by limiting motion in unwanted directions.
Accessory ligaments may be capsular - a thickening of the joint
capsule itself; extracapsular-outside the joint capsule; or
intracapsular-inside the joint capsule.
2. Articular disks or menisci - these intervene between joint
spaces. Examples are the lateral and medial menisci in the knee
and the articular discs in the sternoclavicular joints.
3. Muscles and tendons - these can be very important for the
integrity of many joints. Examples are the rotator cuff muscles in
the shoulder which support the humerus and keep it in the glenoid
fossa, and the popliteus muscle in the knee.
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Synovial Joints - Subclassifications
There are six subclassifications of synovial joints based on their
structure and how much movement they allow. In order from the
least amount of movement to the most amount of movement
allowed they are:
1.
2.
3.
4.
5.
6.
plane joints
hinge joints
pivot joints
condyloid joints
saddle joints
ball and socket joints
We will discuss each subclass of the synovial joints and give
examples in the next several screens.
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Synovial Joints Plane Joints
Plane joints allow only gliding or
sliding motions. This motion can be in
any direction of a single plane. Thus, it
is a uniaxial joint. The articular surfaces
in plane joints are flat or slightly curved
and the movement is restricted by a
tight fibrous capsule. Examples of plane
joints include the:
1. facet joints (those
between articulating
surfaces on the
vertebrae)
2. intercarpal joints
3. carpometacarpal joints
4. intermetacarpal joints
5. intermetatarsal joints
6. acromioclavicular joints
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Synovial Joints - Hinge Joints
Hinge (a.k.a. ginglymus) joints allow flexion and extension only.
The movement at hinge joints is around one axis that is perpendicular to
the bones of the joint. Hinge joints are another example of uniaxial
joints. In these joints, the capsule is thin and flexible on the surfaces
where bending occurs, but is reinforced with strong laterally placed
collateral ligaments. Examples of hinge joints are the:
1. elbow
2. knee
3. interphalangeal joints (the joints between finger and
toe segments)
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Synovial Joints-Pivot
Joints
Pivot (a.k.a. trochoidal) joints are also
uniaxial joints. They allow rotary movement
around one axis that is longitudinal through
the bone. In this type of joint, a process of
one bone rotates in a ring formed by the
other bone and a ligament. Examples of
pivot joints are:
1. Proximal radioulnar joint
2. Atlas-axis joint (the joint between the
first and second cervical vertebrae)
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Synovial Joints - Condyloid Joints
Condyloid joints allow flexion, extension, abduction, adduction and
circumduction (which is a combination of flexion, extension, abduction, and
adduction). These joints consist of oval surfaces which allow for movement in two
planes perpendicular to each other. Thus, this is a biaxial joint. Rotation at this joint is
not allowed due to the shape of the articulating surfaces. Examples of condyloid joints
are:
1. The wrist
2. Metacarpophalangeal joints of the fingers and metatarsophalangeal joints of the
toes.
Note: If the difference between circumduction and rotation is confusing for you,
try this: With your arm pointed straight ahead of you, trace a large circle in the air
with your finger (using your whole arm)-this is circumduction of the shoulder. Now,
with the same arm, pretend as though you are turning a screwdriver with your elbow
straight-this is rotation of the shoulder. Also, remember that the metacarpophalangeal
joints are capable of circumduction (tracing out circles), but cannot do rotation.
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Synovial Joints-Saddle Joints
Saddle (a.k.a. sellar) joints are also biaxial joints, but here, the
articulating surfaces are concavoconvex (one bone shaped like a saddle and
the other shaped like a horse's back). These joints allow flexion, extension,
abduction, adduction and circumduction. An example of a saddle joint is the
carpometacarpal joint of the thumb.
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Synovial Joints - Ball and Socket Joints
Ball and socket joints are multiaxial, allowing movement in an
almost infinite number of axes through the ball of the joint. These
joints allow flexion, extension, abduction, adduction, circumduction and
rotation. Ball and socket joints allow for more flexibility than any other
type of joint. Examples are:
1. The hip
2. The shoulder
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Joints - Blood Supply
Joints are relatively poorly supplied with blood vessels. In
general, the blood supply comes from collateral branches of
larger vessels. Many joints are surrounded by systems of
anastomoses that allow blood flow around the joint regardless of
the position of the limb.
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Joints - Nerve Supply
In contrast to blood supply, joints are very well supplied with
sensory nerves. The major sensation from joints is
proprioception, which allows us to know what position our limbs
are in. We also have pain sensation in joints carried by
branches of larger nerves in the area. Thus, joint pain is often
referred to the skin overlying the joint or the muscles around the
joint.
The major point to remember about joint innervation
is Hilton's Law: A nerve innervating muscles that act across a
joint must also supply sensory fibers to that joint. So, for
example, the femoral nerve which supplies the quadriceps
muscles also sends sensory branches to the knee joint.
Copyright© 2002 The University of Michigan. Unauthorized use prohibited.
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