Joints: Types and Movement

• A joint, also known as an articulation, is where two bones meet.
• Arthrology is the study of joint structure, function, and dysfunction.
• Kinesiology is the study of musculoskeletal movement.
• The temporomandibular joint (TMJ) connects the temporal bone to the mandibular bone.

Joint Types

• Bony joints occur where bones are directly connected without intervening cartilage or fibrocartilage.
• Fibrous joints are where adjacent bones are strongly united by fibrous connective tissue.
• Cartilaginous joints are where bones are connected by cartilage, allowing limited movement and providing structural support.
• Synovial joints, the most common type, allow a wide range of movement.

Joint Movement

• Synarthrotic joints are immovable (e.g., sutures).
• Amphiarthrotic joints are slightly movable (e.g., symphysis pubis and vertebrae).
• Diarthrotic joints are freely movable and have the greatest range of motion; all synovial joints fall into this category.
• Joints allow for growth, binding of structures, and changes in shape.

Bony Joints

• Synostosis is a bony joint where two bones were once separate, resulting in a totally rigid, immovable joint.
• Epiphyseal lines are a normal formation of synostosis in mature long bones.
• Arthritis is an abnormality that can lead to the formation of synostosis.
• Spinal fusion is a surgical procedure that can achieve synostosis to relieve pain.

Fibrous Joints

• Sutures are fibrous joints that hold the bones of the skull together and are immobile or only slightly mobile.
• Serrate sutures have wavy lines along which adjoining bones interlock (e.g., coronal, sagittal, and lambdoidal sutures).
• Lap sutures occur where two bones have overlapping beveled edges and appear relatively smooth (e.g., squamous suture).
• Plane sutures are where two bones have straight non-overlapping edges (e.g., intermaxillary suture of the roof of the mouth).
• Gomphosis is a peg and socket joint that holds teeth in bone, allowing slight movement when chewing.
• The periodontal ligament holds the tooth in place and allows it to 'give' a little when chewing.
• Syndesmosis is a fibrous joint where two bones are bound by collagenous fibers, providing more mobility than sutures or gomphoses.
• The interosseous membrane joins the shafts of the radius and ulna in a syndesmosis joint.

Cartilaginous Joints

• Synchondrosis is a type of cartilaginous joint with hyaline cartilage, which can be amphiarthrotic or synarthrotic (e.g., epiphyseal disc, 1st rib to sternum).
• Symphysis is a type of cartilaginous joint with fibrocartilage, which is amphiarthrotic (e.g., symphysis pubis, intervertebral discs).

Synovial Joints

• Synovial joints are fluid-filled encapsulated joints.
• The joint capsule surrounds the joint and produces synovial fluid.
• Synovial membrane lines the joint capsule.
• The fluid-filled cavity is the space between bones filled with synovial fluid.
• Synovial fluid is slippery and viscous, reducing friction.
• Synovial fluid allows for an enormous decrease in friction at the joint.
• Synovial fluid provides nourishment and waste removal since the joint is poorly vascularized.
• Phagocytes are found in synovial fluid and help remove waste.
• The outer fibrous layer is continuous with the periosteum of the bone.
• When warmed up, synovial fluid becomes thinner and is more easily absorbed by articular cartilage.
• Articular cartilage swells and becomes a more effective cushion against compression when nourished by synovial fluid.
• Repetitive compression helps move nutrition in and waste out of the joint.

Synovial Joint Movements

• Flexion decreases the angle between body parts.
• Extension increases the angle between body parts.
• Abduction moves a limb away from the midline of the body.
• Adduction moves a limb toward the midline of the body.
• Elevation raises a body part.
• Depression lowers a body part.
• Protraction moves a body part forward.
• Retraction moves a body part backward.
• Circumduction involves moving a limb in a circular motion.
• Rotation involves turning around a single axis.
• Supination turns the palm upward.
• Pronation turns the palm downward.

Joint Structures

• A meniscus is a C-shaped fibrocartilage pad that cushions bones and channels synovial fluid.
• A bursa is a sac filled with synovial fluid found between tendons, ligaments, and bones.
• Tendons attach muscles to bones.
• Ligaments attach bone to bone.

Joint Conditions

• Osteoarthritis is a degenerative joint disease usually affecting people 50 and older, often due to wear and tear.
• Rheumatoid arthritis is an autoimmune disease that leads to severe inflammation in the joints.

Additional Facts

• Joint stability is the ability of a joint to maintain its position and resist dislocation.
• Diarthrotic joints are the least stable but have the greatest range of motion.
• Synarthrotic joints are the strongest type of joint and permit no movement.
• The ball and socket joint allows for the least stability and the greatest range of motion.
• Joint degeneration is the decline in joint function due to wear and tear or autoimmune disorders.