Diarthrotic joints are moveable because of the presence of a joint cavity, inside what’s called a joint capsule. Synovial refers to the types of fluid-producing membranes that line all of our freely moveable joints. Let’s look deeper into the details of synovial joints since they are the most common articulations in the body—and the most commonly injured.
The Joint Cavity
The joint cavity is a space between the two articulating bones lined by the synovial membrane, and the capsule is a collective structure that links the bones and surrounds the joint cavity. At the knee, for example, the femur of the thigh meets the tibia of the shin, but the bones are not joined together at their facing articular surfaces, as they are in fibrous or cartilaginous joints.
Instead, the bones are held together at other regions by what is called the joint’s capsule. This capsule unites the bones but leaves their facing surfaces—covered by what’s called articular cartilage—free to move against each other. The joint capsule contains the joint cavity, and that space is lined by a synovial membrane.
The synovial membrane is composed of a very thin layer of cells that secrete a mucus-like lubricating fluid, which contains compounds like the protein albumin.
Synovial Fluid’s Roles
This synovial fluid has two roles. First, it nourishes the structures within the joint and also removes waste products from the joint’s tissues. While bones have their own rich blood supply, cartilage is said to be avascular—which means it lacks its own internal blood supply.
So, the synovial fluid is especially important to the nourishment of cartilage, and the motions that occur at these freely movable joints are good for the cartilage. This is because the movements squish the cartilage while they circulate the synovial fluid around, promoting the release of waste from the cartilage and the uptake of nutrients into it.
One of the reasons that cartilage is so slow to heal is that it lacks its own intrinsic blood supply that would normally bring nutrients easily into the tissue. Bone, on the other hand, heals quite well.
In addition to nourishing joint tissues, synovial fluid also creates this viscous film of lubricant within the joint cavity—and that aids the free movement possible at diarthrotic joints. These synovial membranes are only found within and around the capsules of synovial joints—thus the naming.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
The Joint Capsule
Now, what about the capsule? The capsule is an envelope-like structure that in some cases is like a connective tissue sleeve that spans the two articulating bones at a distance from the actual bony joint. The capsule is thickened in regions by ligaments and may have tendons incorporated into it, as well.
In musculoskeletal anatomy, ligaments are usually strap-like or cord-like structures that connect bone to bone, while tendons connect muscle to bone. Both ligaments and tendons contain parallel bundles of collagen fibers for strength and to resist stress in one main direction.
The capsule provides some stability by joining the two bones, while the joint cavity and its synovial membrane lining facilitate movement between the articulating bones. The ligaments reinforce aspects of the diarthrotic joint that need more support, such as on the medial and lateral sides of what we call a hinge joint. These ligaments restrict movement on the sides where they cross the joint space, helping to ensure that movement occurs only in the intended directions.
The joint’s ligaments can be thickenings in the capsule itself—termed intrinsic ligaments—or they can be outside the capsule—so-called extracapsular ligaments. In some instances, they are even inside the capsule, like the intracapsular ligaments of the knee known as the cruciate ligaments.
Cushioning the Diarthrotic Joints
The capsule is incomplete in places—such as where it allows small blood vessels into the joint or allows a bit of synovial membrane to protrude out as something called a bursa. A bursa is a small pocket of synovial membrane outside the joint capsule—it can be between bone and tendon, between skin and bone, between bone and ligament; really, between almost any two structures around a joint.
The bursa serves to cushion structures around the joint and to allow those adjacent structures—like the skin or a ligament—to pass more easily across underlying structures when the joint moves. So, while the synovial membrane lines the interior of the joint cavity, the joint’s bursae are small pockets of synovial membrane that surround the joint and may even physically communicate with the joint’s synovial lining—like an outpouching of the synovial membrane.
Infections and Inflammations
Any bursa that is structurally continuous with the joint’s synovial lining can be precarious; if a superficial injury introduces infection into a bursa just below the skin—sometimes called a subcutaneous bursa—the infection can spread from the bursa into the synovial lining of the joint.
But those connecting bursae can also serve as pressure relievers if an injury causes inflammation of the joint cavity’s synovial lining since the fluids can escape from the joint cavity into a nearby connected bursa. Bursitis is the medical term for an inflamed bursa.
Like other membranes of the body that get infected or injured, the synovial membrane’s reaction to inflammation is to oversecrete. This is similar to the way in which mucous membranes that line the nasal cavity can produce excess fluids when they get infected or are inflamed from allergies. In some cases, when a bursa is inflamed, a clinician may need to drain the joint to relieve the pressure. Not all moveable joints have bursae around them, but all have synovial capsules lined by synovial membranes, and any of those membranes can become inflamed.
Common Questions about the Main Features of Diarthrotic Joints
The joint cavity is the space between two articulating bones lined by the synovial membrane and is responsible for the movement of diarthrotic joints. The joint cavity is surrounded by the joint capsule.
The synovial fluid in diarthrotic joints has two roles. The first is to nourish the structures within the joint and also remove waste products from the joint’s tissues because the joint lacks its own blood supply. Its second role is to create a viscous film of lubricant within the joint cavity which helps with free movement.
A bursa is a small pocket of synovial membrane outside the joint capsule, located between any two structures around a diarthrotic joint.