Types of Joints – Medical Imaging Anatomy Course


Hi, I am Dr Andrew Dixon from Radiopaedia.org
and in this video tutorial, which is a pre-course video for our Medical Imaging Anatomy course, I am going to be talking to you about types of joints. The Anatomy course is coming up in Melbourne
on April 9 and is going to be live recorded and released on Video on Demand on 1st May.
If you haven’t registered for the full six hour course, then visit radiopaedia.org/courses. Joints are defined as sites of close physical
connection between bones. Some joints are temporary, found only in the developing skeleton
whilst many, such as these three examples, are found in the mature skeleton. In this
tutorial we are going to concentrate on learning the three structural types of joints: the
fibrous joints such as the skull sutures, the cartilaginous joints such as the pubic
symphysis and the synovial joints such as the hip joint. These three structural types
of joint are defined by the tissue that binds the bones together. We are going to look at
each of these in turn beginning with the fibrous joints. A fibrous joint is joined by strong fibrous
tissue rich in collagen. These joints are defined functionally as being synarthroses
in that they allow very little movement, if any Examples of fibrous joints include the
skull sutures and the sutures between the facial bones; here we have the zygoma having
a suture with the frontal bone, the temporal bone and the maxilla. The syndesmosis is another
example of a fibrous joint. Syndesmoses exist between the radius and the ulna in the forearm
and, while it binds the radius and the ulna together strongly, it does allow the radius
to rotate around the ulna to perform supination and pronation. A syndesmosis is also present
between the tibia and fibula within the leg and it is particularly strong distally at
the distal tibiofibular joint. If this syndesmosis is disrupted such as in this case where there
is a Weber C fracture with widening of the syndesmosis and the medial ankle clear space,
then this creates a very unstable ankle joint and requires surgical fixation. The gomphosis, one of my favourite joints,
is another example of a fibrous joint. This is where the tooth is held tightly to the
alveolar bone of the maxilla or the mandible. The next type of joint is the cartilaginous
joint and these joints are joined entirely by cartilage. The word entirely is important
here because, as we will see, synovial joints do contain some cartilage. Cartilaginous joints
can be divided into primary and secondary. The primary cartilaginous joints are fused
by hyaline cartilage and are termed synchondroses. Synchondroses are almost always found in the
developing or immature skeleton and exist between separate centres of ossification,
often within the same bone. For example, the epiphyseal plate or growth plate is a primary
cartilaginous joint between the ossification centre here and the primary bone. Similarly
at apophyses, which are secondary centres of ossification, often associated with tendon
insertions, the same kind of thing happens. So if we look at this child’s pelvis you
can see here a synchondrosis where bone is ossifying within the iliac crest, synchondrosis
joining it to the remainder of the iliac wing. Same thing down here, the greater trochanter
ossification centre with a synchondrosis joining that ossification centre to the remainder
of the femur. Apophysis for the ischium here attaching via a synchondrosis to the remainder
of the ischium and, in the even less mature pelvis, the bones of the pelvis itself, the
pubic bone, the ischium and the ilium come together at the tri-radiate cartilage, another
example of a synchondrosis in the immature skeleton. A primary cartilaginous joint composed
of hyaline cartilage. Only a few primary cartilaginous joints persist in the adult, the most important
one is the petro-occipital synchondrosis between the clivus of the occipital bone and the apex
of the petrous temporal bone and this is important radiologically because it is a site where
chondrosarcomas can occur. The first sternocostal joint is another example of a primary cartilaginous
joint composed of hyaline cartilage that persists in the mature skeleton. Primary cartilaginous joints can also be present
as abnormalities, for example here is a synchondrosis between the calcaneum and the navicular. A
calcaneonavicular coalition. So, we have dealt with the primary cartilaginous
joints now let’s look at the secondary cartilaginous joints. These ones are composed entirely of
fibrocartilage and are termed symphyses. These joints, functionally speaking, are amphiarthroses
in that they allow a very small amount of movement but definitely do allow movement. All the symphyses are found in the midline
of the skeleton and the stereotypical example would be the pubic symphysis between the two
pubic bones in the midline and the intervertebral discs which exist between the vertebral bodies
in the vertebral column. The manubriosternal joint is another example of a secondary cartilaginous
joint, a symphysis between the manubrium and the sternal body. This patient also has a fracture
of their sternum. So that is cartilaginous joints, now let’s
move on to the final form of structural joint, the synovial joint. Synovial joints are joined by an articular
capsule which is lined by a synovial membrane which secretes fluid into the joint cavity.
Synovial joints, functionally speaking, are considered diarthroses in that they are freely
mobile. Here is the typical example of a synovial joint, we have the articular capsule joining
the two bones at the margins, we have a synovial membrane at the inner aspect of that articular
capsule attaching to the margins of the articular cartilage, hyaline articular cartilage covers
the surface of the two articulating bones and synovial fluid sits within the joint cavity. I just want to emphasise the importance of
understanding that synovial joints still contain cartilage. They are not cartilaginous joints,
by definition they are synovial joints but they do contain cartilage. Not only do they
contain the hyaline cartilage on their articular surface, here on the femoral condyle and the
tibial plateau but also fibrocartilage often in the form of a cartilaginous disc, here
in the knee the menisci. Also over here in the shoulder you can see hyaline cartilage
on the glenoid here and on the humeral head and fibrocartilaginous labrum helping to deepen
the socket of the glenoid. So just worth remembering that cartilaginous joints and synovial joints
are different even though synovial joints contain cartilage they are not examples of
cartilaginous joints. Synovial joints scan be further classified based on their form
and the movements that they allow. The number of movements that a joint allows is often
referred to as ‘degrees of freedom’. Here are six different types that we are going
to run through: The first is the ball and socket joint and
this is where a spheroid articular surface articulates with a cup-like depression within
another bone. This joint allows 3ᵒ of freedom, flexion and extension, abduction and adduction
which together result in circumduction and, importantly, they allow axial rotation. The
hip and the shoulder are typical examples of ball-and-socket joints and here we have
the hip on MRI, a nice ball-shaped femoral head sitting within the cup-like depression
of the acetabulum. The next type of joint is the condyloid joint,
which is very similar to the ball and socket joint, however instead of having a perfect
spheroid articular surface; this has an ovoid convexity which articulates with an ellipsoidal
cavity. This still allows flexion and extension, abduction and adduction and therefore circumduction,
but it doesn’t allow axial rotation. A radiocarpal joint is a good example of a condyloid joint
with the ellipsoidal cavity of the distal radius articulating with the ovoid convexity of the scaphoid and the lunate. The metacarpophalangeal joints are also examples of condyloid joints
with the ovoid convexity of the metacarpal head articulating with the ellipsoidal cavity
of the proximal phalanx base. The saddle joint allows exactly the same types of movements
as a condyloid joint but does it by having interlocking saddles articulating by reciprocal
reception. Saddle joints allow flexion and extension, abduction and adduction and therefore
circumduction, but again, they do not allow axial rotation. The 1st carpometacarpal joint
of the thumb is the best example of a saddle joint in the human body. If we have a look
at this x-ray, we can see here that the base of the 1st metacarpal has a saddle-like depression
in it for articulating with the trapezium. If we have a look on MRI you will notice
that this joint contains a fair bit of fluid and has a very thick capsule consistent with
this joint being very mobile, and if you are a person who likes to twiddle your thumbs,
then this is the joint that allows you to do that. A hinge joint is where a convex surface articulates
directly with a perfectly corresponding concave surface and this will allow only 1ᵒ of freedom
and in the human body that is generally flexion extension. The elbow and the interphalangeal
joints of the hands and feet are classic examples of hinge joints. You can see here the distal
humerus coming down and forming the trochlea sitting within the trochlear notch of the
ulna, the convexity of the condyle perfectly fits into the concavity of the notch allowing
for only hinge-type flexion extension movement. The interphalangeal joints of the hand, again,
perfectly shaped convexity sitting within the concavity of the middle phalanx. The ankle
joint and the knee joint are primarily hinge-type joints, however the ankle joint does allow
some eversion and inversion and the knee joint allows a little bit of rotation and therefore
they are considered modified hinge joints. The next type of joint is the pivot joint
and this is where a bone axially rotates within a ring. Now in this picture it looks like
a ring is formed by bone. Normally the ring is actually a concave surface of bone and
then a complete ring formed by a ligament. There are a few excellent examples of pivot
joints within the human body, the median atlantoaxial joint is an articulation between the dens
of C2 and the C1 vertebra with the anterior arch of C1 articulating with the dens and
a complete ligamentous ring being formed by the transverse band of the cruciate ligament.
This allows the C1 or atlas to rotate axially around the dens, which remains in a fixed
position. It is this joint that allows the head to rotate to the left and to the right.
The proximal and distal radioulnar joints are also examples of pivot joints and when
they move in combination they allow pronation and supination, a very specialised movement
of the forearm. We can see here the radial head sitting within a shallow concavity on
the ulna with a complete ligamentous ring, the annular ligament, holding the head in
position and allowing it to axially rotate. The final type of joint is the plane joint.
This is another joint that only allows 1ᵒ of freedom or movement and that is a gliding
or sliding movement in the same plane as the articular surface. Examples include the intercarpal
joints within the wrists and intertarsal joints in the foot, the facet joints in the spine
and the AC joint. Here is an example with the facet joint, two surfaces which are completely
parallel to each other and allow simply a gliding movement in that plane. The main thing
to take away from this presentation is that there are three structural types of joints:
fibrous joints, cartilaginous joints and synovial joints. Examples of fibrous joints are the cranial sutures, the syndesmoses between the radius and ulna and tibia and fibula and the gomphosis joint of
the teeth. Cartilaginous joints can be divided into primary and secondary, with primary being
synchondroses and secondary cartilaginous joints being formed by fibrocartilage and
being symphyses. Synovial joints come in multiple different forms allowing varying degrees of
freedom of movement and if you have got to the end of this talk and you don’t understand
the difference between synchondrosis, symphysis, syndesmosis and synarthrosis, you are feeling
a little bit confused, then you should go back and watch this tutorial again. Just a reminder about our Medical Imaging
Anatomy course coming up in Melbourne on April 9th and being released on Video on Demand
on May 1st. Visit radiopaedia.org/courses to register.

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