a synovial joint is where two bones unite and in that connection there's a fluid filled space that's basically what a synovial joint is so let's take a look at the specific structure let's start by taking a look at a structure that we're familiar with the periosteum remember how the periosteum is a membrane that surrounds the outside of the bone in a synovial joint the periosteum of one bone spans the distance between the two bones and fuses to the periosteum of the next bone this extension of the periosteum is known as the fibrous capsule let's take a look at another structure we're familiar with the end of the long bone is kept with a coating of hyaline cartilage called articular cartilage we can see it in blue in this diagram and notice how the articular cartilage is continuous with this green layer this is what's known as the synovial membrane the synovial membrane has an outer loose connective tissue coat and an inner cellular layer but the synovial membrane lining the fibrous capsule together form what's called the articular capsule and then it encloses the space between the two bones which is known as the joint cavity the synovial membrane that lines the articular capsule produces synovial fluid the synovial fluid fills the joint cavity it's a very viscous fluid that sinks into the articular cartilage it reduces friction in the area and it also provides nutrients to the articular cartilage in addition to the synovial fluid sometimes there are additional modifications in the joint that will work to reduce friction here we're looking at the knee and do you notice these wedge-shaped pieces of fibrocartilage these are menisci and a meniscus is a piece of cartilage that subdivides the joint and that works to greatly reduce the friction in this joint thread this up by taking a quick look at the three factors that work to strengthen a synovial joint all synovial joints are freely movable joints which makes them very susceptible to injury so to make this type of joint stronger ligaments reinforce the articular capsule a ligament i'm outlining one here is a band of connective tissue that reinforces the outside of the articular capsule so the more ligaments present the stronger the joint is a second factor that could play a role is the shape of the articular surfaces this is a ball and socket joint this is the hip so we can see the deep socket the acetabulum and we can see the head of the femur so just the shapes of the articular surfaces in this case stabilize the joint and then a third factor that strengthens a synovial joint is increased muscle tone muscle tone is a partial low level passive state of contraction in a muscle let me show you the way that that works i drew in a muscle attached through a tendon and if this muscle has muscle tone that is going to lead to contraction that's a shortening of the muscle so you can imagine if this muscle had a low level state of contraction that's going to place a force on the tendon it's going to pull up on the tendon which in turn will help to stabilize the bone within the synovial joint