6.2 Bone Classification

Bones come in various shapes, each designed for specific functions in the body. Long bones provide leverage for movement, while flat bones protect organs. Short and irregular bones offer stability and specialized roles, respectively. Sesamoid bones, embedded in tendons, act as pulleys.

Bone structure is complex, with a protective periosteum covering and internal osteons for strength. Osteoblasts and osteoclasts work together in bone remodeling, constantly reshaping our skeleton throughout life. This dynamic process ensures our bones remain strong and adaptable.

Bone Classification and Function

Types of bone shapes

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• Long bones extend lengthwise and are longer than they are wide (femur, humerus, radius, ulna, metacarpals, metatarsals, phalanges)
  • Consist of a diaphysis or shaft and two epiphyses or ends
  • Contains a medullary cavity in the center of the diaphysis, which houses bone marrow
• Short bones are roughly cube-shaped (carpals in the wrist, tarsals in the ankle)
• Flat bones are thin and flat in shape (skull bones, scapula, ribs, sternum)
  • Composed of two layers of compact bone with a layer of spongy bone in between
• Irregular bones have complex shapes that do not fit into other categories (vertebrae, sacrum, coccyx, hyoid bone, turbinate bones)
• Sesamoid bones are small, round bones embedded within tendons (patella or kneecap, pisiform in the wrist, two sesamoid bones in the ball of the foot)

Relationship of shape to function

• Long bones provide leverage and allow for movement
  • Diaphysis is hollow, reducing weight while maintaining strength
  • Epiphyses provide attachment points for muscles and articulate with other bones
• Short bones have a compact shape that provides stability and strength and allows for limited movement (wrist and ankle)
• Flat bones provide protection for underlying organs and attachment points for muscles
  • Two layers of compact bone offer strength, while spongy bone layer reduces weight
• Irregular bones have unique shapes that allow for specific functions
  • Vertebrae support and protect the spinal cord while allowing for flexibility
• Sesamoid bones embedded within tendons protect them from wear and tear and act as pulleys, reducing friction and increasing mechanical advantage of muscles

Sesamoid bones in the skeleton

• Patella or kneecap anterior to the knee joint protects the joint and increases the mechanical advantage of the quadriceps muscle
• Pisiform anterior to the triquetrum in the wrist increases the mechanical advantage of the flexor carpi ulnaris muscle
• Two sesamoid bones in the first metatarsal or ball of the foot
  • Embedded within the tendons of the flexor hallucis brevis muscle
  • Increase the mechanical advantage of the muscle and protect the tendons from wear and tear during walking and running

Bone structure and cellular components

• Bones are covered by a tough connective tissue layer called the periosteum, which contains blood vessels and nerves
• Bone tissue is composed of structural units called osteons, which are cylindrical structures that run parallel to the long axis of the bone
• Bone remodeling involves two main types of cells:
  • Osteoblasts: cells responsible for bone formation and deposition of new bone matrix
  • Osteoclasts: cells that break down and resorb old bone tissue
• Ossification is the process of bone formation, which occurs throughout life for bone growth, repair, and remodeling