The Skeletal System: Structure, Function, and Clinical Relevance

The Skeletal System

Functions of the Skeletal System

The skeletal system provides the framework for the body and serves several essential functions necessary for survival and movement.

• Support: Bones form the structural framework that supports the body and cradles soft organs.

• Protection: Bones protect vital organs (e.g., the skull protects the brain, ribs protect the heart and lungs).

• Movement: Bones act as levers for muscles, enabling movement.

• Mineral Storage: Bones store minerals, especially calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Formation: Hematopoiesis (blood cell formation) occurs in red bone marrow.

Memory Tip: SPAMB — Support, Protection, Allow movement, Mineral storage, Blood cell formation.

Classification of Bones

Bones are classified by tissue type and shape, each with distinct characteristics and functions.

By Tissue Type

• Compact Bone: Dense, strong tissue found on the outer surface of bones; provides strength and support.

• Spongy Bone: Porous tissue containing trabeculae and red marrow; found mainly at the ends of long bones and inside flat bones.

By Shape

• Long Bones: Longer than they are wide (e.g., femur).

• Short Bones: Cube-shaped (e.g., carpals of the wrist).

• Flat Bones: Thin and protective (e.g., skull bones, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae).

Bone Growth and Remodeling

Bones grow and remodel throughout life to adapt to stress and maintain mineral balance.

Appositional Growth

• Increases bone thickness or diameter by adding new bone tissue to the surface.

• Strengthens bones and allows them to grow wider as the body grows.

Bone Remodeling

• Continuous process throughout life.

• Major triggers:

  • Mechanical stress (e.g., exercise, weight-bearing activities)

  • Calcium homeostasis

Hormonal Control of Calcium

Calcium levels in the blood are tightly regulated by hormones that control bone resorption and deposition.

• Parathyroid Gland: Secretes parathyroid hormone (PTH), which activates osteoclasts to break down bone and release calcium into the blood.

• Osteoblasts: Build bone tissue.

• Osteoclasts: Break down bone tissue, releasing calcium.

Memory Trick: Blasts BUILD bone, Clasts CRUSH bone.

Hypercalcemia

• When blood calcium is high, calcium is deposited in bone tissue.

• This process is not permanent; bone is constantly remodeled.

Bedridden Patients

• Lack of mechanical stress increases bone resorption, leading to bone mass loss.

Rickets

Rickets is a childhood disease characterized by soft and weak bones due to a deficiency of vitamin D.

• Cause: Lack of vitamin D impairs calcium absorption.

• Treatment: Vitamin D and calcium supplementation.

Bone Fractures

Fractures are breaks in bone that can be classified by their characteristics and require specific treatments for healing.

Types of Fractures

• Closed Fracture: Bone breaks but does not pierce the skin.

• Open Fracture: Bone breaks through the skin; higher risk of infection.

Reduction

• Definition: Realignment of fractured bone fragments.

• Closed Reduction: Bone ends are coaxed back into position by hand.

• Open Reduction: Surgical procedure using pins or wires to secure bone ends.

Bone Fracture Healing Stages

Bone healing occurs in four stages, restoring structure and function.

1. Hematoma Formation: Blood clot forms around the fracture within hours.

2. Fibrocartilage Callus: Soft callus of fibrocartilage and collagen bridges the broken bone.

3. Bony Callus: Spongy bone replaces the soft callus, strengthening the area.

4. Bone Remodeling: Bone is reshaped to its original structure; excess bone is removed.

Note: The repaired bone is usually very close to the original, but remodeling continues for months.

Facial Bones

The facial skeleton consists of 14 bones, some paired and some unpaired, forming the structure of the face.

• Nasal (2)

• Maxillae (2)

• Zygomatic (2)

• Palatine (2)

• Lacrimal (2)

• Inferior nasal conchae (2)

• Vomer (1)

• Mandible (1)

Unpaired bones: Mandible, Vomer

Bone that articulates with every facial bone: Maxilla

Ethmoid Bone

The ethmoid bone is a complex cranial bone contributing to the nasal cavity, orbit, and nasal septum.

• Key Structures: Cribriform plate, Crista galli, Perpendicular plate, Ethmoidal air cells

• Functions: Forms part of the nasal cavity, orbit, and nasal septum

Palatine Processes

Palatine processes are parts of the maxillae that form the hard palate, separating the oral and nasal cavities.

Hyoid Bone

The hyoid bone is unique because it does not articulate with any other bone. It is held in place by muscles and ligaments and supports the tongue and swallowing muscles.

Vertebrae

The vertebral column consists of individual vertebrae with specialized functions and structures.

• Atlas (C1): Allows nodding the head "yes"

• Axis (C2): Allows rotation of the head "no"

• Intervertebral Discs: Located between vertebrae; provide shock absorption, flexibility, and prevent vertebrae from rubbing together

• Fused Vertebrae: Sacrum (5 fused), Coccyx (3–5 fused)

Vertebra Types

Type	Key Features
Cervical	Small; transverse foramina
Thoracic	Have costal facets for rib attachment
Lumbar	Large body; support most weight

Rib Classification

Type	Numbers	Attachment
True ribs	1–7	Directly to sternum
False ribs	8–10	Indirectly to sternum
Floating ribs	11–12	No attachment to sternum

Sternum Landmarks

• Jugular Notch: Used as a surface landmark during exams.

• Xiphoid Process: Important during CPR; pressure can damage organs like the liver.

• Costal Cartilage: Provides flexibility to the rib cage during breathing.

• Intercostal Spaces: Spaces between ribs allow lung expansion and passage of nerves and vessels.

Appendicular Skeleton

The appendicular skeleton includes the limbs and girdles that attach them to the axial skeleton.

• Shoulder girdle (clavicle, scapula)

• Arms (humerus, radius, ulna)

• Pelvic girdle

• Legs

Clavicle

• Sternal end

• Acromial end

• Acromioclavicular joint

Scapula

• Spine

• Acromion

• Coracoid process

• Glenoid cavity

• Subscapular fossa

• Supraspinous fossa

• Infraspinous fossa

Joints (Articulations)

Joints connect bones and allow for movement and flexibility.

• Functions:

  • Hold bones together

  • Allow movement

Functional Classification of Joints

Type	Mobility
Synarthrosis	Immovable
Amphiarthrosis	Slightly movable
Diarthrosis	Freely movable

Structural Classification of Joints

• Fibrous

• Cartilaginous

• Synovial

Types of Synovial Joints

• Ball and socket

• Hinge

• Pivot

• Saddle

• Condyloid

• Plane

Fontanels

Fontanels are soft spots on an infant's skull that allow for brain growth and skull flexibility during birth. They usually ossify by 18–24 months.

Osteoporosis

Osteoporosis is a condition characterized by loss of bone density, making bones fragile and more likely to fracture. It is common in postmenopausal women.

Prevention

• Weight-bearing exercise

• Calcium intake

• Vitamin D

• Hormone balance