Human Gross Anatomy: Terminology and Joints

Relative to the Midline and Surface

• Medial vs. Lateral: Medial describes a structure moving toward the midline, while lateral describes a structure moving away from it.
• Superficial vs. Deep: Superficial refers to structures close to the body surface, whereas deep refers to those farther away from the surface.
• Proximal vs. Distal: These terms are primarily used for limbs. Proximal describes a position close to the midline or the point of origin, while distal describes a position farther away.

Relative to Sides and Orientation

• Ipsilateral vs. Contralateral: Ipsilateral refers to structures on the same side of the body, while contralateral refers to structures on opposite sides.
• Anterior vs. Posterior: Anterior refers to the front of the body, and posterior refers to the back.
• Superior vs. Inferior: Superior refers to the top of the body, and inferior refers to the bottom.

Anatomical Movements

Movement is described by how joints change their angle or orientation in space.

• Flexion and Extension: Flexion is a reduction in the angle of a joint, while extension is an increase in that angle.
• Abduction and Adduction: Abduction is the movement of a structure away from the midline, while adduction is the movement toward the midline.
• Rotation: Medial rotation involves rolling a structure inward toward the midline, while lateral rotation involves rolling it outward.
• Supination and Pronation: Specific to the forearm, supination is the lateral rotation that results in the palm facing anteriorly (like holding a bowl of soup), while pronation is the medial rotation that results in the palm facing posteriorly.
• Elevation and Depression: Elevation is upward movement, and depression is downward movement.
• Circumduction: A complex rotational movement that involves a combination of two or more movements, creating a circular motion.
• Plantar Flexion and Dorsiflexion: These terms describe movements of the foot at the ankle.

Classification of Joints

A joint is defined as a connection between two rigid parts of the skeleton. Joints are classified in two ways: by their function (movement) and by their structure (what they are made of).

Functional Classification

• Synarthrosis: An immovable joint.
• Amphiarthrosis: A slightly movable joint.
• Diarthrosis: A freely movable joint.

Structural Classification

The physical makeup of the joint determines its movement capabilities:

1. Fibrous Joints: These are held together by dense irregular connective tissue. They lack a joint capsule and are largely immovable, such as the sutures in the skull.
2. Cartilaginous Joints: These are joined by cartilage and allow very little movement. They do not have a joint capsule.
   • Synchondrosis: Joined by hyaline cartilage (e.g., the growth plate in a femur).
   • Symphysis: Joined by fibrocartilage.
3. Synovial Joints: These are the most common and are freely movable (diarthrosis). They feature articular surfaces covered in hyaline cartilage, an articular capsule (consisting of a fibrous capsule and a synovial membrane), and synovial fluid that fills the joint space to reduce friction.

Exam Tip: Remember the inverse relationship between mobility and stability. As a joint becomes more mobile (like the shoulder), it generally becomes less stable. Conversely, joints with deeper articulations (like the hip) are more stable but less mobile.

Types of Synovial Joints

Synovial joints are further categorized by the shape of their articulating surfaces:

• Planar Joint: Two almost flat surfaces moving in any direction (e.g., intercarpal joints).
• Hinge Joint: Allows movement in one plane, similar to a door hinge (e.g., the elbow).
• Pivot Joint: Allows for rotation around a central axis (e.g., the articulation of the radius and ulna).
• Condyloid Joint: An oval-shaped surface fits into a complementary depression (e.g., metacarpophalangeal joints).
• Saddle Joint: Both surfaces have concave and convex areas, resembling a saddle (e.g., the joint at the base of the thumb).
• Ball and Socket Joint: A spherical head fits into a cup-like depression, allowing for the greatest range of motion (e.g., the hip joint).