Joints: Structure and Function of Synovial Joints

Joints

Introduction to Joints

Joints, also known as articulations, are points where two or more bones meet. They play a crucial role in providing mobility and flexibility to the skeleton while maintaining its structural integrity. The study of joints is essential in understanding human movement and the mechanics of the musculoskeletal system.

Synovial Joints

Function of Synovial Joints

Synovial joints are specialized structures that act as lubricating devices within the body. They are the most common and movable type of joint in the human body. Their primary function is to reduce friction between articulating bones, thereby preventing overheating and destruction of joint tissues during movement.

• Friction Reduction: Synovial joints minimize friction, which could otherwise damage joint tissues.

• Compressive Forces: These joints are subjected to compressive forces during movement and weight-bearing activities.

• Synovial Fluid: Fluid is squeezed out as opposing cartilages touch, creating a slippery film that allows cartilages to glide smoothly over each other.

Example: The shoulder and knee joints are classic examples of synovial joints, allowing a wide range of motion.

Bursae and Tendon Sheaths

Structure and Function

Bursae and tendon sheaths are accessory structures associated with synovial joints, but they are not themselves joints. Their main function is to reduce friction between moving body elements, such as tendons and bones.

• Bursa: A flattened fibrous sac lined by a synovial membrane, filled with synovial fluid. Bursae act as cushions to reduce friction between bones and surrounding soft tissues.

• Tendon Sheath: An elongated bursa that wraps around a tendon, especially where tendons are subjected to friction, such as in the wrist or ankle.

• Closed Bags of Lubricant: Both bursae and tendon sheaths are closed sacs containing synovial fluid, providing lubrication to minimize wear and tear.

Example: The subacromial bursa in the shoulder reduces friction between the acromion and the tendon of the supraspinatus muscle.

Frontal Section of the Right Shoulder Joint

The following labeled structures are visible in a frontal section of the right shoulder joint:

• Acromion of scapula

• Subacromial bursa

• Fibrous layer of articular capsule

• Tendon sheath

• Tendon of long head of biceps brachii muscle

• Joint cavity containing synovial fluid

• Articular cartilage

• Synovial membrane

• Fibrous layer

• Humerus

Additional info: The articular cartilage covers the ends of bones, providing a smooth surface for movement, while the synovial membrane produces synovial fluid for lubrication.

Movements Allowed by Synovial Joints

Types of Movement

Synovial joints allow for a variety of movements, which can be classified into three basic types:

• Gliding: One bone moves or slides across the surface of another bone. This movement is typically seen in plane joints, such as those between the carpal bones of the wrist.

• Angular Movement: Movements that change the angle between bones. Examples include flexion, extension, abduction, and adduction.

• Rotation: Movement of a bone around its own long axis. This is seen in joints such as the atlantoaxial joint (between the first and second cervical vertebrae) and the shoulder joint.

Example: The rotation of the head from side to side is an example of rotational movement at a synovial joint.