Synovial joints have a fluid filled joint cavity

Synovial Joints

Joints where the articulating bones are separated by a fluid-filled joint cavity, allowing substantial movement.

What Type of Movement Do Synovial Joints Allow?

They are diarthrotic, meaning they are freely movable.

Where Are Synovial Joints Found?

Nearly all joints of the limbs and most joints throughout the body.

Articular Cartilage

Smooth, glassy hyaline cartilage covering the ends of bones where they meet. It cushions the bones and absorbs compression to prevent them from being crushed.

What Is the Thickness of Articular Cartilage?

It is thin, usually 1 millimeter or less, but provides cushioning and protection.

Joint Cavity

A small, fluid-filled space between articulating bones. It is unique to synovial joints.

What Is a Potential Space?

The joint cavity is normally almost nonexistent in size but can expand when fluid accumulates, such as during inflammation.

Articular Capsule (Joint Capsule)

A two-layered structure that surrounds and encloses the joint cavity.

What Are the Two Layers of the Articular Capsule?

Fibrous layer: Tough outer layer made of dense irregular connective tissue. It strengthens the joint and prevents bones from being pulled apart.

Synovial membrane: Inner layer made of loose connective tissue that produces synovial fluid.

What Does the Synovial Membrane Cover?

It lines the inside of the fibrous layer and covers internal joint surfaces that are not covered by hyaline cartilage.

Synovial Fluid

A slippery, viscous fluid found inside the joint capsule that reduces friction between articular cartilages and helps protect the joint.

How Is Synovial Fluid Produced?

It is formed mainly from filtered blood passing through capillaries in the synovial membrane. Hyaluronic acid gives it a thick, egg-white-like consistency.

What Is the Function of Synovial Fluid?

It lubricates joint surfaces, reduces friction, absorbs pressure, and helps nourish articular cartilage.

Weeping Lubrication

The process where synovial fluid is squeezed out of articular cartilage when a joint is compressed and seeps back in when pressure is released. This lubricates and nourishes the cartilage.

How Does Synovial Fluid Protect and Clean the Joint?

It contains phagocytic cells that remove microbes and cellular debris from the joint cavity.

Reinforcing Ligaments

Strong, band-like structures that strengthen and support synovial joints.

What Are the Three Locations of Reinforcing Ligaments?

Capsular ligaments: Thickened portions of the fibrous joint capsule.

Extracapsular ligaments: Located outside the joint capsule.

Intracapsular ligaments: Located deep to the joint capsule and covered by the synovial membrane.

Why Are Some People Called Double-Jointed?

They have looser, more flexible joint capsules and ligaments than average, allowing greater movement. They still have the normal number of joints.

Nerves in Synovial Joints

Sensory nerve fibers detect pain and monitor joint position and stretch, helping the nervous system sense posture and body movements.

Blood Vessels in Synovial Joints

Blood vessels mainly supply the synovial membrane. Their capillaries provide the blood filtrate used to form synovial fluid.

Fatty Pads

Cushions found in some synovial joints, such as the hip and knee, between the fibrous layer and the synovial membrane or bone.

Articular Discs (Menisci)

Discs or wedges of fibrocartilage found in some synovial joints that partially or completely divide the joint cavity.

What Are the Functions of Articular Discs (Menisci)?

They improve the fit between articulating bones, increase joint stability, and reduce wear and tear on joint surfaces.

Where Are Articular Discs Found?

In the knees, jaw, and a few other joints.

Bursae

Flattened, fluid-filled fibrous sacs lined with synovial membrane that reduce friction between structures around joints.

Where Are Bursae Found?

Where ligaments, muscles, skin, tendons, or bones rub against one another.

Tendon Sheath

An elongated bursa that completely wraps around a tendon to reduce friction during movement.

Where Are Tendon Sheaths Commonly Found?

In narrow canals where several tendons are crowded together, such as in the wrist.

Joint Stability

The ability of a joint to resist forces that could push the bones out of alignment or cause dislocation

Why Do Synovial Joints Need Stabilization?

They are constantly stretched and compressed during movement, making them vulnerable to dislocation.

How Do Articular Surfaces Affect Joint Movement?

Their shapes determine what movements are possible at a joint.

How Much Do Articular Surfaces Contribute to Joint Stability?

They usually play only a minor role in stability. Shallow sockets or poorly fitting surfaces make joints less stable.

When Do Articular Surfaces Improve Joint Stability?

When the surfaces fit snugly together or when a joint has a deep socket.

What Is an Example of a Joint Made Stable by Its Articular Surfaces?

The hip joint, because its ball-shaped femoral head fits into a deep socket in the pelvis.

How Do Ligaments Stabilize Synovial Joints?

Joint capsules and ligaments connect bones and prevent excessive or unwanted movement.

How Does the Number of Ligaments Affect Joint Stability?

Generally, the more ligaments a joint has, the stronger it is.

What Happens When Ligaments Are Overstretched?

They stay stretched and lose some of their ability to support the joint. Ligaments can stretch only about 6% of their length before tearing or snapping.

Why Are Joints Less Stable When They Depend Mainly on Ligaments?

Too much tension can stretch the ligaments, weakening their ability to stabilize the joint.

Muscle Tone

Low levels of continuous contractile activity in relaxed muscles that keep them healthy and ready to respond to stimulation.

How Does Muscle Tone Stabilize Joints?

Muscle tone keeps tendons under tension, helping hold joints together and prevent unwanted movement.

What Is the Most Important Stabilizing Factor for Most Joints?

The tendons of muscles that cross the joint.

Which Areas Depend Heavily on Muscle Tone for Stability?

The shoulder, knee joints, and arches of the feet.

How Are Skeletal Muscles Attached?

Every skeletal muscle attaches to bone or other connective tissue structures at two or more points

How Does Body Movement Occur?

When muscles contract across joints, they cause the bones and body parts to move.

What Determines How Joint Movements Are Described?

Movements are described by the direction and axes around which a body part moves and the planes of space in which movement occurs.

What Are the Three Planes of Movement?

Sagittal plane: Divides the body into left and right portions.

Frontal plane: Divides the body into front and back portions.

Transverse plane: Divides the body into upper and lower portions.

What Are the Four Categories of Synovial Joint Movement?

Nonaxial, uniaxial, biaxial, and multiaxial. These describe how many axes a joint can move around.

What Is Range of Motion?

The amount of movement allowed at a joint. It varies between joints and can be greater in people such as trained gymnasts and acrobats.

What Are the Three General Types of Synovial Joint Movements?

Gliding

Angular movements

Rotation

Gliding Movement

Occurs when one flat or nearly flat bone surface slides over another without significant bending or rotation.

Where Do Gliding Movements Occur?

At the intercarpal joints (wrist), intertarsal joints (ankle), and between the flat articular processes of the vertebrae (spine).

Angular Movements

Movements that increase or decrease the angle between two bones.

What Are the Six Types of Angular Movements?

Flexion, extension, hyperextension, abduction, adduction, and circumduction.

Where Can Angular Movements Occur?

In any plane of the body.

Flexion

A bending movement that decreases the angle between two bones, bringing them closer together. It usually occurs along the sagittal plane.

What Are Examples of Flexion?

Bending the head forward toward the chest, bending the trunk forward, bending the knee, and raising the arm forward at the shoulder.

Extension

The opposite of flexion. It increases the angle between articulating bones and typically straightens a bent body part. It usually occurs along the sagittal plane.

What Are Examples of Extension?

Straightening a bent neck, body trunk, elbow, or knee.

Hyperextension

Movement that continues beyond the normal anatomical position, extending a body part past its usual straight position.

Abduction

Movement of a limb away from the body's midline, usually along the frontal planes

What Is the Midline for Finger and Toe Abduction?

The third finger (middle finger) for the hand and the second toe for the foot. Abduction means spreading away from these reference points.

What is lateral flexion?

Bending the trunk sideways, away from the body's midline, in the frontal plane. This is called lateral flexion, not abduction.

Adduction

Movement of a limb toward the body's midline. For fingers and toes, it means moving them toward the midline of the hand or foot.

Circumduction

A movement in which a limb moves in a circular pattern, creating a cone shape in space. The end of the limb moves in a circle while the shoulder or hip joint stays relatively stationary

What Movements Make Up Circumduction?

Flexion, abduction, extension, and adduction performed in succession.

Where Does Circumduction Commonly Occur?

At the shoulder and hip ball-and-socket joints.

Rotation

The movement of a bone turning around its own long axis.

Where Does Rotation Occur?

is common at the hip and shoulder joints.

Medial Rotation

Turning a bone toward the body's midline. For example, turning the thigh so the front surface of the femur moves toward the midline.

Lateral Rotation

Turning a bone away from the body's midline. It is the opposite of medial rotation.

Special Movements

Movements that do not fit into the main categories of gliding, angular movements, or rotation. They occur at only a few joints.

Supination

Rotating the forearm laterally so the palm faces forward (anteriorly) or upward (superiorly). The radius and ulna become parallel.

Pronation

Rotating the forearm medially so the palm faces backward (posteriorly) or downward (inferiorly). The radius crosses over the ulna, forming an X.

Opposition

A movement of the thumb across the palm to touch the tips of the other fingers on the same hand.

Which Joint Allows Opposition of the Thumb?

The saddle joint between metacarpal I (first metacarpal) and the trapezium bone

Why Is Opposition Important?

It allows the hand to grasp and manipulate objects with precision.

Dorsiflexion

Lifting the foot upward at the ankle so the top of the foot moves toward the shin.

Plantarflexion

Pointing the toes downward, moving the foot away from the shin. This corresponds to wrist flexion.

Inversion

Turning the sole of the foot inward, toward the body's midline (medially).

Eversion

Turning the sole of the foot outward, away from the body's midline (laterally).

Elevation

Moving a body part upward (superiorly). Example: Shrugging your shoulders raises the scapulae.

Depression

Moving a body part downward (inferiorly). Example: Lowering your shoulders or opening your jaw.

Protraction

Moving a body part forward (anteriorly) in the transverse plane.

Retraction

Moving a body part backward (posteriorly) in the transverse plane.

How Are Synovial Joints Further Classified?

By the shape of their articular surfaces, which determines the types of movements they allow.

What Are the Six Types of Synovial Joints?

Plane

Hinge

Pivot

Condylar

Saddle

Ball-and-socket