Skeletal System Anatomy and Physiology

Microscopic Bone Structure and Composition

At a microscopic level, bone is a highly specialized tissue composed of approximately 55% mineral salts, primarily calcium phosphate. Through a process called calcification, hydroxyapatite crystals form around collagen fibers to harden the bone.

Bone tissue is maintained by four specialized cell types:

1. Osteoprogenitor cells: Undifferentiated stem cells capable of mitosis that give rise to other bone cells.
2. Osteocytes: Mature bone cells that maintain bone metabolism.
3. Osteoblasts: Cells that perform bone deposition and initiate calcification.
4. Osteoclasts: Cells responsible for bone resorption, which is the breakdown of bone tissue.

Compact vs. Spongy Bone

Bone is classified into two structural types based on its density and organization:

Compact (Dense) Bone Compact bone provides protection and support and is characterized by repeating functional units called osteons (Haversian systems). Each osteon consists of concentric layers of bone called lamellae arranged around a central (Haversian) canal.

• Lacunae: Small spaces between the lamellae that house osteocytes.
• Canaliculi: Tiny channels that radiate from the lacunae, allowing extracellular fluid to reach the cells.
• Perforating (Volkmann’s) canals: These allow blood vessels and nerves to penetrate through the periosteum into the bone.

Spongy (Trabecular/Cancellous) Bone Unlike compact bone, spongy bone does not contain osteons. Instead, it is composed of irregular columns of bone called trabeculae. This structure makes the bone lighter and provides protection for the bone marrow housed within the spaces.

Classification of Bones by Shape

Adults typically possess 206 bones, which are categorized into several types based on their shape and location:

Exam Tip: When identifying bones, look for the ratio of length to width. If the bone is significantly longer than it is wide, it is a long bone. If it is embedded within a tendon (like the kneecap), it is a sesamoid bone.

Joints and Movement

A joint is defined as the site where two skeletal elements meet. Joints are broadly categorized into two types: synovial joints, which possess a joint cavity, and solid joints, where bones are held together by connective tissue without a cavity.

Synovial Joints

Synovial joints are highly mobile and are characterized by the presence of a joint capsule and synovial fluid, which lubricates the joint. They can be classified by their movement capabilities:

• Uniaxial: Movement in one plane (e.g., elbow, knee).
• Biaxial: Movement in two planes (e.g., knuckle joints).
• Multi-axial: Movement in multiple directions (e.g., shoulder, hip).

Specific shapes of synovial joints include hinge joints (uniaxial), pivot joints (rotation), saddle joints, and ball-and-socket joints (multi-axial).

Basic Movements

The skeletal system allows for a wide range of movements, including:

• Flexion & Extension: Decreasing or increasing the angle at a joint.
• Abduction & Adduction: Moving a limb away from or toward the body's midline.
• Rotation: Turning a part of the body around its axis.
• Circumduction: A circular movement where one end remains stationary.

Clinical Considerations: Fractures and Repair

A break in a bone is known as a fracture. Fractures can be classified by their relationship to the skin:

• Closed Fracture: The skin remains intact.
• Open (Compound) Fracture: The bone breaks through the skin, creating a high risk of infection (osteomyelitis).

Fractures also undergo a specific healing process involving three phases: the reactive phase (inflammation and hematoma formation), the reparative phase (formation of a fibrocartilage callus followed by a bony callus), and the final remodelling phase.