PHS 3504 Study Guide - Final Guide: Subscapularis Muscle, Collagen, Limb Bud
18 Sep 2019
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Chapter 9: Joints
The adult human body has 206 bones, and with the exception of the hyoid bone in the neck, each bone is
connected to at least one other bone. Joints are the location where bones come together. Many joints
allow for movement between the bones. At these joints, the articulating surfaces of the adjacent bones
can move smoothly against each other. However, the bones of other joints may be joined to each other
by connective tissue or cartilage. These joints are designed for stability and provide for little or no
movement. Importantly, joint stability and movement are related to each other. This means that stable
joints allow for little or no mobility between the adjacent bones. Conversely, joints that provide the most
movement between bones are the least stable. Understanding the relationship between joint structure
and function will help to explain why particular types of joints are found in certain areas of the body.
The articulating surfaces of bones at stable types of joints, with little or no mobility, are strongly united
to each other. For example, most of the joints of the skull are held together by fibrous connective tissue
and do not allow for movement between the adjacent bones. This lack of mobility is important, because
the skull bones serve to protect the brain. Similarly, other joints united by fibrous connective tissue allow
very little movement, which provides stability and weight-bearing support for the body. For example, the
tibia and fibula of the leg are tightly united to give stability to the body when standing. At other joints, the
bones are held together by cartilage, which permits limited movements between the bones. Thus, the
joints of the vertebral column only allow for small movements between adjacent vertebrae, but when
added together, these movements provide the flexibility that allows your body to twist, or bend to the
front, back, or side. In contrast, at joints that allow for wide ranges of motion, the articulating surfaces of
the bones are not directly united to each other. Instead, these surfaces are enclosed within a space filled
with lubricating fluid, which allows the bones to move smoothly against each other. These joints provide
greater mobility, but since the bones are free to move in relation to each other, the joint is less stable.
Most of the joints between the bones of the appendicular skeleton are this freely moveable type of joint.
These joints allow the muscles of the body to pull on a bone and thereby produce movement of that body
region. Your ability to kick a soccer ball, pick up a fork, and dance the tango depend on mobility at these
types of joints.
9.1 Classification of Joints
A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together
(articulate with each other) to form a connection. Joints are classified both structurally and functionally.
Structural classifications of joints take into account whether the adjacent bones are strongly anchored to
each other by fibrous connective tissue or cartilage, or whether the adjacent bones articulate with each
other within a fluid-filled space called a joint cavity. Functional classifications describe the degree of
movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable
joints. The amount of movement available at a particular joint of the body is related to the functional
requirements for that joint. Thus immobile or slightly moveable joints serve to protect internal organs,
give stability to the body, and allow for limited body movement. In contrast, freely moveable joints allow
for much more extensive movements of the body and limbs.
Structural Classification of Joints
The structural classification of joints is based on whether the articulating surfaces of the adjacent bones
are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces
contact each other within a fluid filled joint cavity. These differences serve to divide the joints of the body
into three structural classifications. A fibrous joint is where the adjacent bones are united by fibrous
connective tissue. At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage. At a
synovial joint, the articulating surfaces of the bones are not directly connected, but instead come into
contact with each other within a joint cavity that is filled with a lubricating fluid. Synovial joints allow for
free movement between the bones and are the most common joints of the body.
Functional Classification of Joints
The functional classification of joints is determined by the amount of mobility found between the adjacent
bones. Joints are thus functionally classified as a synarthrosis or immobile joint, an amphiarthrosis or
slightly moveable joint, or as a diarthrosis, which is a freely moveable joint (arthroun = “to fasten by a
joint”). Depending on their location, fibrous joints may be functionally classified as a synarthrosis
(immobile joint) or an amphiarthrosis (slightly mobile joint). Cartilaginous joints are also functionally
classified as either a synarthrosis or an amphiarthrosis joint. All synovial joints are functionally classified
as a diarthrosis joint.
Synarthrosis
An immobile or nearly immobile joint is called a synarthrosis. The
immobile nature of these joints provide for a strong union
between the articulating bones. This is important at locations
where the bones provide protection for internal organs. Examples
include sutures, the fibrous joints between the bones of the skull
that surround and protect the brain, and the manubriosternal
joint, the cartilaginous joint that unites the manubrium and body
of the sternum for protection of the heart.
Amphiarthrosis
An amphiarthrosis is a joint that has limited mobility. An example of this type of joint is the cartilaginous
joint that unites the bodies of adjacent vertebrae. Filling the gap between the vertebrae is a thick pad of
fibrocartilage called an intervertebral disc (Figure 9.3). Each
intervertebral disc strongly unites the vertebrae but still
allows for a limited amount of movement between them.
However, the small movements available between adjacent
vertebrae can sum together along the length of the
vertebral column to provide for large ranges of body
movements.
Another example of an amphiarthrosis is the pubic
symphysis of the pelvis. This is a cartilaginous joint in which
the pubic regions of the right and left hip bones are strongly
anchored to each other by fibrocartilage. This joint normally
has very little mobility. The strength of the pubic symphysis is important in conferring weight-bearing
stability to the pelvis.
Diarthrosis
A freely mobile joint is classified as a diarthrosis. These types of joints include all synovial joints of the
body, which provide the majority of body movements. Most diarthrotic joints are found in the
appendicular skeleton and thus give the limbs a wide range of motion. These joints are divided into three
categories, based on the number of axes of motion provided by each. An axis in anatomy is described as
the movements in reference to the three anatomical planes: transverse, frontal, and sagittal. Thus,
diarthroses are classified as uniaxial (for movement in one plane), biaxial (for movement in two planes),
or multiaxial joints (for movement in all three anatomical planes).
A uniaxial joint only allows for a motion in a single plane (around a single axis). The elbow joint, which
only allows for bending or straightening, is an
example of a uniaxial joint. A biaxial joint
allows for motions within two planes. An
example of a biaxial joint is a
metacarpophalangeal joint (knuckle joint) of
the hand. The joint allows for movement
along one axis to produce bending or
straightening of the finger, and movement
along a second axis, which allows for
spreading of the fingers away from each
other and bringing them together. A joint
that allows for the several directions of movement is called a multiaxial joint (polyaxial or triaxial joint).
This type of diarthrotic joint allows for movement along three axes. The shoulder and hip joints are
multiaxial joints. They allow the upper or lower limb to move in an anteriorposterior direction and a
medial-lateral direction. In addition, the limb can also be rotated around its long axis. This third movement
results in rotation of the limb so that its anterior surface is moved either toward or away from the midline
of the body.
9.2 Fibrous Joints
At a fibrous joint, the adjacent bones are directly connected to each other by fibrous connective tissue,
and thus the bones do not have a joint cavity between them. The gap between the bones may be narrow
or wide. There are three types of fibrous joints. A suture is the narrow fibrous joint found between most
bones of the skull. At a syndesmosis joint, the bones are more widely separated but are held together by
a narrow band of fibrous connective tissue called a ligament or a wide sheet of connective tissue called
an interosseous membrane. This type of fibrous joint is found between the shaft regions of the long bones
in the forearm and in the leg. Lastly, a gomphosis is the narrow fibrous joint between the roots of a tooth
and the bony socket in the jaw into which the tooth fits.
