Appendicular Skeleton and Joints: Structure and Function

Components of the Appendicular Skeleton

Clavicle

The clavicle is a slender, S-shaped bone that connects the upper limb to the trunk. It has two ends:

• Sternal end: Articulates with the manubrium of the sternum.

• Acromial end: Articulates with the acromion of the scapula.

Scapula

The scapula (shoulder blade) is a flat, triangular bone located on the posterior thorax. Key features include:

• Glenoid cavity: Shallow socket that articulates with the head of the humerus.

• Spine: Prominent ridge on the posterior surface.

• Acromion: Lateral extension of the spine; forms the acromioclavicular joint with the clavicle.

• Coracoid process: Projection for muscle attachment.

Upper Limb

Humerus

The humerus is the bone of the upper arm. It articulates:

• Superiorly: With the scapula at the glenoid cavity.

• Inferiorly: With the radius and ulna at the elbow joint.

Key anatomical features:

• Head: Fits into the glenoid cavity.

• Greater tubercle: Lateral prominence for muscle attachment.

• Surgical neck: Common site of fractures.

• Deltoid tuberosity: Attachment for the deltoid muscle.

• Radial groove: Houses the radial nerve.

• Trochlea: Articulates with the ulna.

• Capitulum: Articulates with the radius.

• Medial and lateral epicondyles: Sites for muscle attachment.

Forearm

The forearm consists of two bones:

• Ulna: Medial bone; forms the major part of the elbow joint.

• Radius: Lateral bone; important for wrist movement.

• Interosseous membrane: Connects the radius and ulna along their length.

Ulna

• Ulnar head

• Olecranon: Forms the elbow's bony prominence.

• Radial notch: Articulates with the radius.

• Trochlear notch: Articulates with the humerus.

• Styloid process: Distal projection.

• Ulnar tuberosity: Muscle attachment site.

Radius

• Head: Disc-shaped; articulates with the capitulum of the humerus.

• Radial tuberosity: Attachment for the biceps brachii.

• Styloid process: Distal projection.

Hand

The hand is divided into three regions:

• Carpus (wrist)

• Metacarpus (palm)

• Phalanges (fingers)

The Pelvic Girdle

Structure

The pelvic girdle consists of two hip bones, each formed by the fusion of three bones:

• Ilium

• Ischium

• Pubis

Key features:

• Acetabulum: Deep socket for the head of the femur.

• Obturator foramen: Large opening for nerves and blood vessels.

Pubis

• Superior and inferior pubic rami

• Pubic symphysis: Cartilaginous joint between the two pubic bones.

• Pubic arch (subpubic angle): Angle formed below the pubic symphysis.

Pelvis

• Pelvic brim: Edge separating the true and false pelvis.

• False pelvis: Superior to the pelvic brim; supports abdominal organs.

• True pelvis: Inferior to the pelvic brim; contains pelvic organs.

• Pelvic outlet: Inferior opening of the true pelvis.

General Comparison of the Male and Female Pelves

The male and female pelves differ in structure to accommodate childbirth in females.

Feature	Male Pelvis	Female Pelvis
Pelvic inlet	Narrow, heart-shaped	Wide, oval
Pubic arch	Acute angle (<90°)	Obtuse angle (>90°)
Pelvic cavity	Deeper	Shallower
Bone thickness	Thicker, heavier	Thinner, lighter
Additional info:	Female pelvis is adapted for childbirth.

Lower Limb

Thigh

The femur is the bone of the thigh. It articulates:

• Proximally: With the acetabulum of the pelvis.

• Distally: With the tibia and patella.

Key features:

• Head: Fits into the acetabulum.

• Greater and lesser trochanters: Muscle attachment sites.

• Gluteal tuberosity: Attachment for gluteal muscles.

• Linea aspera: Ridge for muscle attachment.

• Lateral and medial condyles: Articulate with tibia.

• Medial and lateral epicondyles: Sites for ligament attachment.

• Intercondylar fossa: Lies between condyles.

Leg

The leg consists of two bones:

• Tibia: Medial, weight-bearing bone.

• Fibula: Lateral, non-weight-bearing bone.

• Interosseous membrane: Connects tibia and fibula.

Tibia

• Medial and lateral condyles: Articulate with femur.

• Tibial tuberosity: Attachment for patellar ligament.

• Medial malleolus: Forms the medial ankle.

Fibula

• Head

• Lateral malleolus: Forms the lateral ankle.

Patella (Kneecap)

• Patellar ligament: Connects patella to tibia.

Foot

The foot is divided into three regions:

• Tarsus (ankle)

• Metatarsus (sole)

• Phalanges (toes)

Arches of the Foot

The foot has longitudinal and transverse arches that provide support and flexibility.

Joints

Classification by Function (Mobility Type)

Joints are classified by their degree of movement:

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

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

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

Classification by Structure (Material Attaching Bones)

Joints are also classified by the material connecting the bones:

• Fibrous joints: Bones joined by fibrous tissue (e.g., sutures).

• Cartilaginous joints: Bones joined by cartilage (e.g., pubic symphysis).

• Synovial joints: Bones separated by a fluid-filled cavity.

Types of Synovial Joints

Synovial joints allow a wide range of movements and are classified by shape:

• Hinge joint: Allows flexion and extension (e.g., elbow).

• Pivot joint: Allows rotation (e.g., proximal radioulnar joint).

• Ball-and-socket joint: Allows movement in all axes (e.g., shoulder, hip).

Accessory Structures of Synovial Joints

Synovial joints contain additional structures for stability and movement:

• Ligaments: Connect bone to bone.

• Menisci: Pads of fibrocartilage for shock absorption.

• Bursae: Fluid-filled sacs to reduce friction.

Knee Joint

The knee is a complex synovial joint with several important structures:

• Four major ligaments: Anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), lateral collateral ligament (LCL).

• Meniscus: Medial and lateral menisci provide cushioning and stability.