Hello all this is Dr Alsup and we will be discussing the basics
of one of my all-time favorites: synovial joints in this learning objective.
So this will include the basic structure as well as composition of all synovial joints and then we
will finish by describing a few of the more common accessory structures that are in association with
some of the more complex synovial joints. So the synovial joints will be the focus of the majority
of our conversations during musculoskeletal anatomy due to two reasons. First they are the
most numerous type of joint ,and particularly when we're talking about the appendages.
So it's hard to avoid a synovial joint when discussing musculoskeletal anatomy. Secondly
as synovial joints are freely movable or capable of more movement, this also lends to a reduction
of stability in these joints say compared to a suture in the skull. And with a reduction of
stability clinical issues tend to arise. Synovial joints are unique in that they articulate or join
bones not by a solid sheet of connective tissue but rather by
what's referred to as an articular capsule. That is a sleeve of connective tissue that's enveloping
the articulating bone so thus connecting the sides of the articulating bones. So not this solid sheet
like you see here and say a suture which is a fibrous type of joint. This organization allows
for a greater range of motion in synovial joints than in the other types and there
are other factors that greatly affect how much motion is allowed even amongst synovial joints.
There are four structures that all synovial joints have. And this first one we just mentioned
which is the articular or synovial capsule. And this is the connective tissue that is
actually connecting the bones to one another. So it is going to be this kind of enveloping
on the sides of the synovial joint. It will provide protection to the inner portions of the joint as
well. It's going to have two parts it will have an outer more fibrous layer as any outer layer
typically is and that's actually going to be continuous with the periosteum of the bones. The
periosteum is a complex connective tissue covering a bone. Additionally there will be an inner
synovial membrane so the inner portion of the articular capsule is actually a
synovial membrane that will secrete synovial fluid into the joint cavity.
And synovial fluid is going to be a large topic of conversation. It is this extremely
slippery lubricant which is kind of viscous slippery type texture similar to what you see
in raw egg whites. And as mentioned synovial fluid will be discussed quite a bit in a clinical
context in upcoming lectures. But functionally understand that synovial fluid plays a role in
reducing friction at a joint. It also importantly provides nourishment to the
articular cartilage and will play a role in removing waste from the joint cavity region.
Now let's discuss the other two structures located within all synovial joints.
The articular or synovial cavity is the literal space
between the two articulating bones but within the articular capsule. So in illustrations like the one
that we see here you generally see some giant space all this blue right here is supposed to
indicate the articular cavity. But there really isn't a lot of space within a true joint cavity
but the fact is that there is some. And the fact that there is space allows for easier and
more movement in these types of synovial joints. Lastly all synovial joints will have articular
cartilage so this kind of white portion right here is indicating the articular cartilage. And
articular cartilage cartilage will cover the ends of the articulating bones. And I want to note
again that this cartilage is not what is actually connecting the bones that is the articular capsule.
But the cartilage does serve to provide protection and to reduce friction within a synovial joint.
As you likely know regarding cartilage it doesn't have
a very robust vascular supply especially compared to some of these other surrounding structures and
thus articular cartilage is going to be slow to heal after injury or with wear and tear. Articular
cartilage will be discussed in considerable more detail when discussing osteoarthritis or the wear
and tear type of arthritis as this area of the joint is very often affected in a way.
Now ligaments often don't make the list of structures found with every synovial
joint and I don't really know why to be honest because ligaments are associated
with pretty much every joint synovial or otherwise. And I guess they're just so important that they
really need their own spotlight. And I don't mind spending a little time talking ligaments.
So we are finally discussing true ligaments here, so not any of those peritoneal ligaments
that we talked about before, but a true ligament is connective tissue that connects bone to bone. And
the structure and locations of ligaments can vary widely from a component of the outer portion
of the articular capsule so often referred to as a thickening of the articular capsule
or it can be completely separated or extrinsic to the joint capsule. And we'll be discussing
both types as we get into the specific joints. Ligaments play a huge role along with their close
neighbors tendons which connect muscle to bone in the structural integrity
of synovial joints. Additionally ligaments can play a role in checking action so only
allowing certain actions to occur at specific joints or only allowing a certain degree
of range of motion. So we will spend quite a bit of time getting into some of the specific ligaments.
Now on to what we often call accessory structures of synovial joints.
So structures that are not located in all synovial joints but typically just the more complex ones.
But you'll find that we will actually spend a lot of our time on those more
complex synovial joints as they are the ones that tend to show up clinically.
All three of these types of structures so all three of these structures right here
are composed of fibrocartilage just how it's organized is going to be different.
A labrum is going to look like kind of a lip of fibrocartilage that is going to be located
on the socket part of a ball and socket joint. So in essence that's going to be the shoulder
in the hip joints and this allows for a slightly greater surface area associated with the socket
and thus allowing a slightly better fit for the ball portion of the ball and socket joint.
Articular discs are more substantial pads of fibrocartilage and these are going to be
located within an articular cavity and they typically serve to divide the space within
the joint. So often it will kind of compart-, so say if this is the space the articular cavity,
you can have an articular disc of fibrocartilage that kind of divides your space and
this would be the disc. And that will compartmentalize the actions that can occur at
that joint. So the classic example of an articular disc is located in the temporomandibular joint
or the TMJ but there's also an articular disc in the acromioclavicular or AC joint.
And lastly menisci are shallow and incomplete pads of fibrocartilage that often appear kind
of a crescent moon shape. And these are most commonly associated with the knee joint which
we're looking at right here. And this will play a role in absorbing shock and pressure and it will
very slightly improve the fit between the bone so you have two menisci associated with the knee.
Lastly I want to discuss bursi which are associated with synovial joints and particularly
in areas of increased friction. And these are sac like structures I kind of like to think of them as
mini synovial capsules with a small amount of fluid but missing function of articulating bones.
These can be located between adjacent muscles, it can be located where a tendon
passes over a bone or between bone and the skin, so potentially located in a multitude of areas.
There are even specific specializations of those smaller scales so here's a illustration of a bursa
so the smaller scale bursa you can have these elongated kind of bursa-like structures associated
with long tendons so these in blue right here are going to be examples of tendon or synovial
sheet so this kind of specialized bursty for long tendons. If a bursa becomes inflamed or enlarged
due to excessive friction or other injury or a multitude of other factors this is referred to as
bursitis which is a fairly common musculoskeletal clinical condition. So more to come here for sure.
Most excellent I do love discussing synovial joints and this basic understanding of the
common structures of a synovial joint will make upcoming more detailed discussions of
the major synovial joints more meaningful. Please do take time to review and reach
out with any follow-up questions. I thank you for your time and attention