You’re looking at a surgeon test a patient’s anterior cruciate ligament, or ACL, to make
sure it’s still intact.
When somebody has a healthy ACL, the tibia won’t move forward that much.
And we know that because the knee behaves as a hinge, one of the 6 types of joints in
the body.
And just like a hinge on a door, we expect it to close and open and that’s it.
That one plane of motion is exactly what we want a hinge to do.
But of course, that type of movement wouldn’t work for how we use our shoulders or hip joints.
There are multiple types of joints that let our skeletons support our squishy insides
and let us move the way we move.
So in this video, we’re going to go over the types of joints in the body and why the
demands placed on those joints use that type of joint in particular.
If you’re new here, welcome, my name is Patrick, and I love getting to look at the
body as a system of machines and levers.
This particular lesson is usually as deep as any anatomy class gets with kinesiology
or biomechanics, but if you find yourself enjoying this video, consider taking a full
kinesiology class.
You might like it.
Before we get into the applications of each joint type, we need to be super clear about
what a joint /is/.
A joint is any time a bone meets bone, and that includes more configurations than you
think.
That could be a free moving joint like your elbow, but that also includes where your pubic
bones in the pelvis meet, or the junctions of the flat bones of your skull.
The first way we can think of joint classification is based on their freedom of movement..
Like some joints barely move at all — these are synarthroses, joints like the sutures
between the flat bones of your skull or how your teeth connect to your maxilla and mandible,
what’s technically called a gomphosis.
We have these because, over our lives, our anatomy grows and tweaks itself slightly,
but after that anatomy matures, we want it to stay put.
Amphiarthroses have slightly more mobility, but only because they have dense fibrocartilage
between the two bones.
This includes your fibrocartilage of your intervertebral discs and other, mostly immobile
joints like the pubic symphysis.
This thing is really interesting in its own right, but it exists because, well, we really
don’t want these two halves separating, but we do want a little bit of wiggle room
when pushing a baby out of the birth canal.
But when you talk about joints in your day to day, you’re probably thinking of diarthroses,
also called synovial joints, the most mobile type of joint.
Usually you’ll see it represented like this because each of the six joint types are so
different.
So this picture just says “here are the two bones and here’s the stuff around them,
modify according to whatever bones you’re looking at”.
It could just as easily show a ball and socket or saddle joint, but I’m getting ahead of
myself.
Let’s break this image down.
As usual, link is in the description if you want this next part for your notes.
Synovial joints are made of two bones, coated with a super smooth, glassy looking bit of
connective tissue called articular cartilage.
That whole thing is surrounded by a fibrous capsule that ties it together with the periosteum,
the connective tissue that wraps all around the bone.
Deep to that layer is the synovial membrane which has a bunch of fibroblasts and some
stem cells and then filling the middle is synovial fluid which has some phospholipids
and glycoproteins and otherwise a good chemical environment to keep the joint moving smoothly.
So in general our example synovial joint includes the two cartilage coated bones, a fibrous
outer layer, a thinner middle layer, and special lubricating liquid inside.
Cool, now that we have the archetypal, standard synovial joint, /now/ we can transform it
into the six joint types and add ligaments and muscles.
First up, the plane joint.
Represented as two planes sliding past each other, it’s also called a gliding joint.
The two bones will usually have either flat surfaces or slightly curved surfaces that
barely move because of the connective tissue around them.
The most well known plane joints are the intercarpal and intertarsal joints and like, look at all
those ligaments keeping them together.
They might not seem like they have a ton of motion, but they can still move a bit (ask
anyone who’s dislocated their lunate before).
We also see them in the facet joints between vertebrae.
This is a totally different joint that the amphiarthroses, the discs I mentioned earlier.
The facets are these pieces of the vertebrae that glide past each other during spinal flexion
and extension, so this joint in the vertebrae is different than this joint.
Next up, the pivot joint.
This is where one bone is enclosed in some kind of soft tissue ring from another bone.
My best example is the /proximal/ radio ulnar joint, which you can super easily find on
yourself by bending your elbow, finding your lateral epicondyle of the humerus, that big
bump, then moving about an inch down the forearm.
By pronating and supinating your wrist, the radial head just pops up.
That flared end of the radius sits in a little groove in the ulna, and held in place by a
ligament called the annular ligament which makes it a pivot joint.
That’s the radial-ulnar joint at your elbow, but they form another joint at your wrist,
which is also a pivot joint, but not as mobile.
So why does it matter?
Well, since we are bipedal animals, our hands and wrists evolved more recently for tool
usage, and this control of pronation and supination, flipping our hand around the axis of the forearm,
makes more dexterity and finesse possible.
Without the pivot joint at our elbow, we’d have to rotate our arms with the clunkier
shoulder movements.
Take a look at one of the muscles that does pronation, the pronator teres at the elbow.
It originates at the medial humerus and medial ulna and inserts on the lateral radius around
the middle of the bone.
That lets the muscle pull tangential to the pivot point.
If this muscle inserted at the medial radius, this whole joint wouldn’t work.
The elbow also has a hinge joint, a type of joint that just rotates around one axis.
One of the bones is convex, one is concave and the only motions this joint can do are
flexion and extension like in the elbow, tibiofemoral joint of the knee, and between the digits.
Technically hinge joints can also dorsi and plantarflex, the talocrural joint in the ankle
counts as a hinge joint.
I like to think of hinge joints with the perspective of the big, brutish muscles that move them.
Powerful muscles like the biceps and triceps brachii, or the quad and hamstring groups,
or the calf muscles.
These are muscles with thick inserting tendons that you can easily palpate — literally,
sit down, drag your heel on the ground and your hamstring tendons will make themselves
known.
These muscles can afford to be bulky and powerful because their joint only has one plane of
motion to travel in.
While they do have /some/ stabilizers, they don’t need extreme precision or finesse,
the muscles around hinge joints just need to pull and let the constraints of the skeleton
guide their motion.
On the other hand, ball and socket joints like the shoulder and hip have all kinds of
degrees of freedom, more than any other type of joint.
Although, the socket in ball and socket can be deceptive.
Take the glenohumeral joint of the shoulder, which I need to spell out because we need
to differentiate it from the scapulothoracic joint — which is not a ball and socket.
It’s also not a true joint, but I’m going on a tangent.
Anyway!
Within the GH, the ball of the humeral head sits in the concave socket of the glenoid
fossa.
Fossa just means any kind of dip in the skeleton, and this isn’t a huge dip.
It’s more like a golf tee than a kendama.
So to increase the depth of the socket, our bodies deepen it with a structure called the
labrum, which blends in with dense irregular connective tissue to form the shoulder capsule
plus another layer of more dynamic support from the muscles and tendons of the rotator
cuff.
If I catch you saying rotator /cup/ I swear I will come smack you with a copy of Gray’s
Anatomy.
Not on my watch!
In ball and socket joints, we see a greater number of smaller muscles than hinge joints,
but we still have some big movers.
The glute max, iliopsoas, or rectus femoris are powerful muscles, but the femoral-acetabular
joint is much deeper than the shoulder, which makes it much more stable and capable of movement
from powerful muscles.
It has plenty of stability built in like a labrum of its own, muscles deep within the
hip, and super strong ligaments.
Not quite the full range as ball and socket joints are condyloid joints, sometimes called
ellipsoid joints.
Basically, you have a rounded end of one bone sitting inside a little dip in another bone.
It’s got lots of range of motion along the long axis, and a little bit along the short
axis.
We see this in the wrist joint, specifically where the proximal row of carpal bones makes
a rounded bump, and sits inside the groove made by the distal radius.
That lets us do a big range of motion in flexion and extension, but only a little bit of ulnar
and radial deviation.
We also see it in the metacarpal phalangeal joints — lots of flexion and extension,
little bit of finger spreading.
The last type of joint we have is the saddle joint.
The classic example is the thumb joint, not the thumb knuckle, but the carpo-metacarpal
joint.
Also, I never want to say the words /thumb knuckle/ again, it just felt weird coming
out.
Right, by using a saddle joint here, it gives us way more range of motion compared the the
other CMC joints, which are plane joints which means /they/ have fewer planes of motion they
can move through.
This saddle joint ultimately gives us our human opposable thumbs.
So we can expect a level of muscular control that matches this level of precision and finesse
too.
There are a few other important pieces in synovial joints that don’t fit neatly into
our six types of joints collection.
Like some of our joints have protective bursa that give them some cushion, or they have
bones in their ligaments — what are called sesamoid bones which I’ll have a video about.
And some have big chunks of fibrocartilage like the meniscus of the knee.
But one more thing that’s worth pointing out here is our conventional names for joints,
like the ankle, can actually be multiple joints.
The ankle includes the talocrural joint which is a hinge, and the subtalar joint which is
a gliding, or plane joint.
So while we usually say that the /ankle/ can dorsiflex and plantarflex, invert and evert,
as students of kinesiology, we have to consider this new layer of granularity.
I bestow upon thee the title of level two anatomy student.
If you want more of these musculoskeletal and kinesiology focused videos, I’ve got
a whole playlist of them right here.
One of my favorites is this video about why the human spine is terribly designed, so check
that out here.
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Have fun, be good, thanks for watching.