Appendicular Skeleton: Structure, Function, and Clinical Relevance

Appendicular Skeleton

Overview of the Appendicular Skeleton

The appendicular skeleton consists of the bones of the limbs and the girdles that attach them to the axial skeleton. It is essential for movement and manipulation of the environment.

• Bones of the limbs: Includes the bones of the arms, forearms, wrists, hands, thighs, legs, ankles, and feet.

• Pectoral girdle: Composed of the scapula and clavicle, connecting the upper limb to the trunk.

• Pelvic girdle: Formed by two coxal (hip) bones and the sacrum, connecting the lower limb to the trunk.

Articulation and Muscle Attachment

Bones have specific regions for articulation (joint formation) and muscle attachment, which are critical for movement and stability.

• Articulation: Joints (articulations) are where two bones meet. Ligaments reinforce these joints.

• Muscle attachment: Tendons attach muscles to bones, allowing movement when muscles contract.

• Pectoral girdle: Only the clavicle attaches to the axial skeleton; the scapula moves freely, allowing high mobility but less stability.

• Shoulder joint: The glenoid cavity of the scapula is shallow, providing flexibility but less reinforcement.

Bones of the Pectoral Girdle

The pectoral girdle consists of two bones on each side:

• Clavicle (collarbone)

• Scapula (shoulder blade)

Clavicle Fractures

The clavicle acts as a brace, holding the scapula and arm laterally. When a person falls on an outstretched arm, the force is transmitted to the clavicle, making it prone to fracture. A fractured clavicle causes the shoulder region to collapse medially.

Anatomical Locations of Major Limb Bones

Bone	Anatomical Location
Humerus	Arm (brachium)
Radius, Ulna	Forearm (antebrachium)
Carpals	Wrist (carpus)
Metacarpals	Palm (metacarpus)
Phalanges	Fingers (digits)
Femur	Thigh
Tibia, Fibula	Leg
Tarsals	Ankle (tarsus)
Phalanges	Toes

Surgical Neck of the Humerus

The surgical neck of the humerus is so named because it is the most frequently fractured part of the bone, often requiring surgical intervention.

Colle's Fracture

A Colle's fracture is a break in the distal end of the radius, commonly occurring when a person falls and tries to break the fall with outstretched hands.

Metacarpal Heads (Knuckles)

The prominent heads of the metacarpal bones form the knuckles visible when making a fist.

Phalanges of the Hand and Foot

The phalanges are the bones of the fingers and toes. The thumb is specifically called the pollex.

Pelvic Girdle and Pelvis

The pelvic girdle is formed by the sacrum and paired coxal bones. The pelvis is a deep, basin-like structure formed by the hip bones, sacrum, and coccyx.

• True pelvis: The region inferior to the pelvic brim, surrounded by bone, containing pelvic organs.

• False pelvis: The region superior to the pelvic brim, bounded by the alae of the ilia and lumbar vertebrae; supports abdominal viscera and does not restrict childbirth.

Pubic Symphysis

The pubic symphysis is the joint linking the two coxal bones, composed of a fibrocartilage disc, symphyseal surfaces, and collagen ligaments.

Anatomical Relationship of Ilium, Ischium, and Pubis

The ilium joins the pubis anteriorly and the ischium inferiorly, forming the coxal bone.

Ischial Tuberosity

The ischial tuberosity serves as a major site for muscle attachment and bears the body's weight when sitting.

Pelvic Landmarks

The greater sciatic notch, obturator foramen, and ischial spine are important landmarks for identifying structures on radiographs (X-rays).

Acetabulum

The acetabulum is a deep, hemispherical socket formed by the ilium, ischium, and pubis, serving as the site for femoral head articulation.

Sex Differences in the Pubic Arch

Sex	Pubic Arch Angle
Female	80°-90° (broader, more rounded)
Male	50°-60° (more acute)

Sesamoid Bones

Sesamoid bones are bones that develop within tendons to reduce wear and tear. The largest sesamoid bone is the patella (kneecap). Other sesamoid bones can be found in the palms of the hands (associated with metacarpals).

Femur Articulations

In adults, the femur articulates with the hip bones (acetabulum), tibia, and patella. In fetuses, the articulations may differ due to developmental changes and the presence of cartilage.

Common Names for Lower Limb Bones

Anatomical Name	Common Name
Tibia	Shin bone
Patella	Kneecap
Calcaneus	Heel bone

Arches of the Foot

The foot has three main arches that distribute body weight and provide support:

• Medial longitudinal arch: Curves above the ground, so the footprint does not show the medial part of the sole.

• Lateral longitudinal arch: Elevates the lateral part of the foot, redistributing weight from the calcaneus to the 5th metatarsal.

• Transverse arch: Runs obliquely across the foot, supported by the longitudinal arches, forming a half-dome structure.

Together, these arches allow the foot to support and distribute approximately half of a person's weight during standing and walking.