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Synovial Joint

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The majority of articulations between bones that are present in our body are synovial joints. All synovial joints are freely movable joints and are characterized by a presence of a closed space called synovial cavity. A thin layer of hyaline cartilage, lubricated by a fluid-secreting sheath called the synovial membrane, covers the articulating surfaces of the bones. The fluid secreted is a mucopolysaccharide, highly viscous fluid that plays an important role in reducing friction between bones during joint movement. The outer layer of the joint cavity is a tough membrane of collagen fibers, which is firmly attached to the surface of the bones on either side of the joint and is continuous with the periosteum.

The strength and stability of the synovial joints, as well as the range of movements are determined by a few different factors. These are: the shape and articular surfaces of the bones, the ligaments present within the synovial joint, the muscles that extend between the two bones comprising the joint, and the presence of fibrocartilage within the synovial cavity.

Synovial joints are classified according to the shape of the articulating surfaces between the bones. With reference to the hip and the knee joints, the hip joint is a ball and socket joint whereas the knee joint is a hinge joint. The ball and socket joints are the most freely moving of all synovial joints, and are the most stable of all. The movements are allowed in all axes and planes, such as flexion/extension, abduction/adduction, circumduction and rotation. The hip joint is formed with the hemispherical head of femur fitting completely into the concavity of the cup-like depression of the acetabulum, allowing multiaxial movement. The hinge joint is a uniaxial joint thus only allows movement in one plane, such as flexion/extension. The knee joint is formed by two articulations, which is between the tibia and femur, and between the patella and femur.

Since different joints have different stability elements, both hip joint and knee joint having different articulations have different stability elements present in their synovial joints. Synovial joints are reinforced by a number of ligaments, which may be part of the fibrous capsule (intrinsic), or may be found outside (extracapsular) or deep inside the fibrous capsule (intracapsular). Since the shape of the knee joint is relatively weak, this synovial joint relies on more ligaments to ensure stability, with five extracapsular ligaments and two intracapsular ligaments. The five extracapsular ligaments include patellar ligament, fibular and tibial collateral ligament, oblique and arcuate popliteal ligament, while the two intracapsular ligaments holding the tibia and femur together are the anterior and posterior cruciate ligaments. Each of the ligaments has different attachment sites and therefore providing the joint a better stability. The collateral ligaments act to stabilize the joint by preventing any medial and lateral movement, whilst the popliteal ligaments aids to strengthen the joint capsule posteriorly. As for the hip joint, which is a more stable joint, there is only one intracapsular ligament that runs from the acetabular fossa to the fovea of the femur, and three extracapsular ligaments: iliofemoral, pubofemoral and ischiofemoral ligaments. The iliofemoral ligament is located anteriorly with a Y-shaped appearance, preventing hyperextension of the hip joint. The pubofemoral ligament prevents excessive abduction and extension and is located anteriorly and inferiorly. The ischiofemoral ligament is found posterior to the joint and prevents excessive extension of the femur at the hip joint. These three ligaments are very strong and have a very unique spiral orientation, allowing them to become tighter when the joint is extended as well as less energy is needed to maintain a standing position.

The muscles surrounding the synovial joints are important in allowing a range of movement and increasing its mobility. However, there are some other factors such as the ligaments and the shape of the articulating surface that limits the degree of movement of the joints, thus improving the stability to prevent dislocation of the joint. There are two main movements that the knee joint permits, extension and flexion. Extension is produced by quadriceps femoris, which is part of the anterior thigh muscles that inserts into the tibial tuberosity, while flexion is produced by the hamstrings, gracilis, sartorius and popliteus. The other muscles surrounding the knee joint such as biceps femoris, semimembranosus and semitendinosus permit minor movements such as medial and lateral rotation. The deep gluteal muscles such as piriformis and gemelli mainly act to laterally rotate and abduct the hip joint with the aid of other muscles from the posterior thigh region. The hip joint is the most stable during extension carried out mainly by the gluteus maximus muscle, owing to tautness of the iliofemoral ligament.

However, there are some stabilizing factors that are present in the hip joint but absent in the knee joint and vice versa. In the hip joint, there is a fibrocartilaginous collar around the acetabulum, which increases its depth. This collar is known as the acetabular labrum, providing a large articular surface thus improving the stability of the joint. As for the knee joint, the medial and lateral menisci are C-shaped fibrocartilage structures attached at both ends to the intercondylar area of the tibia that serve to absorb shock as well as to deepen the articular surface of the tibia. These structures enable the knee joint to be slightly more congruent.

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