Joint Classification and Articular Motion

Functional Classification of Joints: Synarthrosis

• Definition of Synarthrosis: A synarthrosis is a functional category of joint that permits $0$ movement ($\text{no movement}$). At a synarthrosis, the bony edges are positioned very close together and may interlock. These joints are characterized by extreme strength and are located in areas where motion between bones must be prevented.
• Structural Categories and Types of Synarthrosis:
      * Suture (Bony/Fibrous): The term suture is derived from sutura, meaning "a sewing together." This synarthrotic joint is found exclusively between the bones of the skull. The bony edges are interlocked and bound together by dense fibrous connective tissue.
      * Gomphosis ($\text{gom-FŌ-sis}$) (Fibrous): Significant for its name derivation from gomphos, meaning "bolt." This joint binds the teeth to the bony sockets within the mandible and the maxillae. The specific fibrous connection between the tooth and its socket is known as the periodontal ligament (derived from peri, around + odontos, tooth).
      * Synchondrosis ($\text{sin-kon-DRO-sis}$) (Cartilaginous): Derived from syn, together + chondros, cartilage. This joint consists of a rigid, cartilaginous bridge connecting two articulating bones.
          * Examples: The connection between the manubrium of the sternum and the first pair of vertebrosternal ribs. Another example is the epiphyseal cartilage, which serves to connect the diaphysis to the epiphysis within a growing long bone.
      * Synostosis ($\text{sin-os-TO-sis}$) (Bony): This is a totally rigid and immovable joint formed when two separate bones fuse until the boundary between them completely disappears.
          * Examples: The rare frontal (metopic) suture located on the frontal bone, the fusion of the left and right mandibular bones in an infant, and the epiphyseal lines found in mature long bones.

Functional Classification of Joints: Amphiarthrosis

• Definition of Amphiarthrosis: This functional category permits "little movement." While it allows more movement than a synarthrosis, it is significantly stronger than a joint that is freely movable. Articulating bones in an amphiarthrosis are connected via cartilage or collagen fibers.
• Structural Categories and Types of Amphiarthrosis:
      * Syndesmosis ($\text{sin-dez-MO-sis}$) (Fibrous): Derived from syndesmos, meaning "ligament." In this joint, bones are connected specifically by a ligament.
          * Example: The distal joint located between the fibula and the tibia.
      * Symphysis (Cartilaginous): At a symphysis, articulating bones are connected by a pad or wedge of fibrocartilage.
          * Example: The joint between the two pubic bones, referred to as the pubic symphysis.

Functional Classification of Joints: Diarthrosis

• Definition of Diarthrosis: This category permits "free movement." These are synonymous with Synovial ($\text{si-NO-ve-ul}$) joints. They provide a much wider range of motion compared to other joint types and are typically situated at the ends of long bones, specifically in the lower and upper limbs.
• Planes of Movement for Diarthroses:
      * Monaxial: Movement occurring in only $1$ plane (e.g., ankle, elbow).
      * Biaxial: Movement occurring in $2$ planes (e.g., wrist, ribs).
      * Triaxial: Movement occurring in $3$ planes (e.g., hip, shoulder).

A Simple Model of Articular Motion

• The Pencil Analogy: To understand the three ways a bone can move, one can use a pencil held upright on a desk. The pencil represents the bone, and the desk surface represents the articular surface.
      * Possible Movement 1: Linear Motion (Gliding): If the pencil is held upright without the point being secured, the pencil can be pushed across the surface. This is called gliding. The point can slide backward, forward, side to side, or diagonally.
      * Possible Movement 2: Angular Motion: If the tip of the pencil is held in a fixed position, the shaft angle with the surface can be changed. The eraser end can move forward, backward, side to side, or at intermediate angles.
      * Circumduction: This is a complex form of angular motion. By grasping the eraser and moving the pencil so it is no longer vertical, and then swinging the eraser through a complete circle, one performs circumduction.
      * Possible Movement 3: Rotation: If the pencil shaft remains vertical and the point stays at one location, the pencil can spin around its longitudinal axis. This is defined as rotation.
• Biological Limitation of Rotation: No biological joint can rotate freely ($360^{\circ}$ continuous rotation) because such motion would tangle the muscles, nerves, and blood vessels that cross the joint.

Axes of Motion

• Joint movement is described by the number of axes around which they rotate:
      * Monaxial: Permits movement around $1$ axis.
      * Biaxial: Permits movement around $2$ axes.
      * Triaxial: Permits movement around $3$ axes.
• Specific Axes of the Body:
      * Superior-inferior axis.
      * Lateral-medial axis.
      * Anterior-posterior axis.

Structural Classification and Functional Examples of Synovial Joints

• Synovial Joint Overview: These are freely movable diarthrotic joints. They are categorized based on the shapes of their articulating surfaces, which dictates the permitted range and type of motion.

• Plane Joint (Gliding Joint):
      * Description: Flattened or slightly curved surfaces slide across each other.
      * Movement: Gliding; Slight nonaxial.
      * Examples: Acromioclavicular and sternoclavicular joints; Intercarpal joints; Vertebrocostal joints; Sacro-iliac joints; Clavicle and Manubrium.

• Hinge Joint:
      * Description: Permits angular motion in a single plane, analogous to the opening and closing of a door.
      * Movement: Angular; Monaxial.
      * Examples: Elbow joint; Knee joint; Ankle joint; Interphalangeal joints; Humerus and Ulna.

• Condylar Joint (Ellipsoid Joint):
      * Description: An oval articular face is nestled within a depression on the opposing surface.
      * Movement: Angular; Biaxial.
      * Examples: Radiocarpal joint (Radius, Ulna, and Scaphoid bone); Metacarpophalangeal joints $2-5$; Metatarsophalangeal joints.

• Saddle Joint:
      * Description: Complex articular faces that fit together like a rider in a saddle. Each face is concave along one axis and convex along the other.
      * Movement: Angular; Biaxial.
      * Examples: First carpometacarpal joint (Trapezium and Metacarpal bone of thumb).

• Pivot Joint:
      * Description: Joints that only permit rotation.
      * Movement: Rotation; Monaxial.
      * Examples: Atlanto-axial joint (Atlas and Axis); Proximal radio-ulnar joint.

• Ball-and-Socket Joint:
      * Description: The round head of one bone rests within a cup-shaped depression of another bone.
      * Movement: Angular, circumduction, and rotation; Triaxial.
      * Examples: Shoulder joint (Scapula and Humerus); Hip joint.