What Is a Synovial Joint?

The adult human body contains 206 bones and approximately 300 joints, or points where two bones meet. Most joints are synovial joints, such as knees and knuckles. All synovial joints allow for movement and are susceptible to arthritis.

Common Traits of Synovial Joints

Synovial joints vary in structure—for example, the shoulder is a ball-and-socket joint and the knee is a hinge joint—but they all have the following in common:

• Synovial joints allow for movement.
• Where the bones meet to form a synovial joint, the bones' surfaces are covered with a thin layer of strong, smooth articular cartilage.
• A very thin layer of slippery, viscous joint fluid, called synovial fluid, separates and lubricates the two cartilage-covered bone surfaces. A healthy knee joint has up to 4 mL (less than a teaspoon) of synovial fluid.¹Mundt L, Shanahan K MS MT(ASCP), Graf’s Textbook of Routine Urinalysis and Body Fluids (p.255), Philadelphia, Pa: Lippincott Williams & Wilkins; 2011
• A synovial membrane encapsulates the joint surfaces and synovial fluid. The synovial membrane is very thin, often just a few cells thick (about 50 microns, or the approximate width of a human hair) and produces synovial fluid. The synovial membrane is backed by a slightly thicker subsynovial membrane.

Soft Tissues That Support Synovial Joints

In addition to this basic structure, a synovial joint is typically surrounded by soft tissue structures that support the joint and help facilitate movement. These structures typically include:

• Tendons that attach muscles to bone
• Ligaments that attach bone to bone
• Bursae, small sacs of synovial fluid that provide additional cushioning and lubrication

The health of a synovial joint is intertwined with the health of these supporting soft tissue structures. For example, damage to a ligament can skew joint alignment and eventually lead to joint degeneration (osteoarthritis) and vice versa.

See Degenerative Arthritis

Arthritis Can Affect Synovial Joints

A synovial joint's complexity allows for movement but also opens the door to a host of problems, including but not limited to:

• Rheumatoid arthritis
• Osteoarthritis
• Juvenile arthritis
• Gout
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• Conditions related to the joint’s supporting structures, such as bursitis and tendonitis

Many types of arthritis and related conditions require ongoing treatment and preventative measures to control symptoms. A doctor can help diagnose joint conditions as well as design a treatment plan.