Appendicular Skeleton and Articulations: Structure and Function

Unit 7: The Appendicular Skeleton

Overview of the Skeleton

The human skeleton is divided into two major parts: the axial skeleton and the appendicular skeleton. The appendicular skeleton consists of the bones of the limbs and the girdles that attach them to the axial skeleton. In anatomical diagrams, the appendicular skeleton is often highlighted in green.

• Pectoral girdle: Attaches the upper limbs to the trunk.

• Upper extremity: Includes the arm, forearm, and hand.

• Pelvic girdle: Attaches the lower limbs to the trunk.

• Lower extremity: Includes the thigh, leg, and foot.

Key Terms: Appendicular skeleton, axial skeleton, girdle, extremity

Pectoral Girdle

The pectoral girdle consists of the clavicle and scapula, which serve to attach the upper limb to the body. These bones provide mobility and support for the arms.

• Clavicle: Also known as the collarbone, it is a long bone that runs horizontally between the sternum and the scapula.

• Scapula: Also known as the shoulder blade, it is a flat, triangular bone located on the posterior side of the ribcage.

• Distinguishing Right from Left: The orientation of anatomical landmarks (e.g., acromion, coracoid process, glenoid cavity) helps identify right vs. left bones.

Example: The acromion process of the scapula is lateral and superior, helping to distinguish right from left scapulae.

Upper Limb

The upper limb is composed of three major bones: the humerus, radius, and ulna. These bones form the arm and forearm, allowing for a wide range of motion.

• Humerus: The single bone of the upper arm, articulating proximally with the scapula and distally with the radius and ulna.

• Radius: Located on the lateral (thumb) side of the forearm; allows for rotation of the wrist.

• Ulna: Located on the medial (pinky) side of the forearm; forms the major part of the elbow joint.

• Distinguishing Right from Left: The position of the head, tuberosities, and styloid processes are used to identify right vs. left bones.

Example: The radial tuberosity is medial and anterior, helping to distinguish the right radius from the left.

Hands and Wrist

The hand consists of the carpal bones (wrist), metacarpals (palm), and phalanges (fingers). The arrangement of these bones allows for dexterity and fine motor skills.

• Carpals: Eight small bones arranged in two rows forming the wrist. The mnemonic "So Long To Pinky, Here Comes The Thumb" helps remember the order: Scaphoid, Lunate, Triquetrum, Pisiform, Hamate, Capitate, Trapezoid, Trapezium.

• Metacarpals: Five bones forming the palm, numbered I-V from thumb to pinky.

• Phalanges: Fourteen bones in the fingers; each finger has three (proximal, middle, distal) except the thumb, which has two (proximal, distal).

Example: The scaphoid is the most lateral carpal bone in the proximal row, adjacent to the radius.

Unit 8: Articulations

Classification of Joints

Joints, or articulations, are classified by their structure and the amount of movement they allow. The three main functional classifications are:

• Synarthroses: Immovable joints (e.g., sutures in the skull).

• Amphiarthroses: Joints allowing slight movement (e.g., intervertebral discs).

• Diarthroses: Freely movable joints (e.g., shoulder, knee).

Key Terms: Synarthrosis, amphiarthrosis, diarthrosis

Structural Classification of Joints

Joints are also classified by the type of connective tissue or the presence of a fluid-filled cavity:

• Fibrous joints: Bones joined by dense connective tissue; no joint cavity. Types include:

  • Suture: Only in the skull; immovable.

  • Gomphosis: Tooth in socket; immovable.

  • Syndesmosis: Bones connected by a ligament; may allow slight movement (e.g., distal tibiofibular joint).

• Cartilaginous joints: Bones joined by cartilage; no joint cavity. Types include:

  • Synchondrosis: Hyaline cartilage; immovable (e.g., costochondral joints).

  • Symphysis: Fibrocartilage; slightly movable (e.g., pubic symphysis).

• Synovial joints: Bones separated by a fluid-filled synovial cavity; freely movable.

Types of Synovial Joints

Synovial joints are classified by the number of axes of movement:

• Nonaxial: No movement around an axis; only slipping or gliding (e.g., intercarpal joints).

• Uniaxial: Movement in one plane (e.g., elbow).

• Biaxial: Movement in two planes (e.g., wrist).

• Multiaxial: Movement in all three axes (e.g., shoulder).

Example: The shoulder joint is a multiaxial synovial joint, allowing flexion, extension, abduction, adduction, and rotation.

Major Synovial Joints: Knee and Shoulder

The knee and shoulder are examples of complex synovial joints with specialized structures for stability and movement.

• Knee Joint: Contains ligaments such as the anterior cruciate ligament (ACL) and menisci for stability.

• Shoulder Joint: Features a shallow glenoid cavity and a loose capsule, allowing a wide range of motion but less stability.

Example: Injury to the ACL in the knee can result in instability and is a common sports injury.

Table: Functional and Structural Classification of Joints

Functional Classification	Structural Classification	Example
Synarthrosis (Immovable)	Fibrous (Suture)	Skull sutures
Synarthrosis (Immovable)	Cartilaginous (Synchondrosis)	Epiphyseal plate
Amphiarthrosis (Slightly movable)	Fibrous (Syndesmosis)	Distal tibiofibular joint
Amphiarthrosis (Slightly movable)	Cartilaginous (Symphysis)	Pubic symphysis
Diarthrosis (Freely movable)	Synovial	Shoulder, knee

Additional info:

• Mnemonic for carpal bones: "So Long To Pinky, Here Comes The Thumb" (Scaphoid, Lunate, Triquetrum, Pisiform, Hamate, Capitate, Trapezoid, Trapezium).

• Distinguishing right from left bones is essential for anatomical orientation and clinical practice.

• Synovial joints are the most common and allow the greatest range of movement in the body.