SKELETON SYSTEM

Definition

The skeleton is the structural framework of the human body. It is composed of bones and cartilages, providing shape, support, and protection while facilitating movement.

Composition

1. One-third organic material{ connective tissue }: Primarily collagen fibers, which contribute to toughness and resilience, enabling bones to withstand tensile forces (pulling or stretching).

2. Two-thirds inorganic material: Predominantly calcium salts (mainly calcium phosphate, some calcium carbonate, and trace amounts of other salts), providing hardness and rigidity. This structure allows bones to resist compressive forces like those from weight-bearing and impacts.

3. In terms of strength, bones are comparable to iron and steel.

Characteristics

1. Living Tissue:

   1. Despite its rigidity, bone is highly vascular and dynamic:

   2. Calcium turnover: Continuous exchange of calcium maintains bone health.

   3. Growth and Repair: Exhibits distinct growth patterns and excellent healing ability after fractures.

   4. Regeneration: Bones regenerate better than most tissues, second only to blood.

2. Adaptability:

   Bones can adjust to varying levels of stress and strain:

   • Disuse Atrophy: Reduction in bone mass due to lack of use.

   • Overuse Hypertrophy: Increase in bone density and size in response to repetitive stress.

3. Disease Susceptibility: Like other tissues, bones can be affected by diseases but retain significant potential for recovery.

4. Key function

   1. Bones give shape and support to the body, and resist any forms of stress (Fig. 2.1).

   2. These provide surface for the attachment of muscles, tendons, ligaments, etc.

   3. These serve as levers for muscular actions.

   4. Bone marrow manufactures blood cells.

   5. Bones store 97% of the body calcium and phosphorus.

   6. Bone marrow contains reticuloendothelial cells which are phagocytic in nature and take part in immune responses of the body.

Key function

1. Bones give shape and support to the body, and resist any forms of stress (Fig. 2.1).

2. These provide surface for the attachment of muscles, tendons, ligaments, etc.

3. These serve as levers for muscular actions.

4. Bone marrow manufactures blood cells.

5. Bones store 97% of the body calcium and phosphorus.

6. Bone marrow contains reticuloendothelial cells which are phagocytic in nature and take part in immune responses of the body.

• Shape

  • Long bones

    1. Diaphysis elongated shaft

    2. Epiphysis:- expanded, smooth, and articular ends.

    3. 3 Surfaces

       1. a central medullary cavity

       2. a nutrient foramen

       3. growing end

    4. 3 types of long bone

       1. Typical Long Bones: Humerus, radius, ulna, femur, tibia, and fibula.

       2. Miniature Long Bones: Metacarpals, metatarsals, and phalanges (with only one epiphysis).

       3. Modified Long Bones: Clavicle (lacking a medullary cavity).

  • Short Bones

    • Shape: Cuboid, cuneiform, trapezoid, or scaphoid.

    • Examples: Tarsal and carpal bones.

  • Flat Bones

    • Structure: Resemble shallow plates and form boundaries of certain body cavities.

    • Examples:

      • Skull vault bones

      • Ribs

      • Sternum

      • Scapula

  • Irregular Bones

    • : Irregularly shaped and often complex.

    • Examples:

      • Vertebra

      • Hip bone

      • Bones at the base of the skull

  • Pneumatic Bones

    • Structure: Irregular bones containing large air-filled spaces lined by epithelium.

    • Functions:

      • Reduce skull weight.

      • Enhance voice resonance.

      • Act as air-conditioning chambers for inhaled air.

    • Examples: Maxilla, sphenoid, ethmoid.

  • Sesamoid Bones

    • Small, nodular bones embedded in tendons or joint capsules.

    • Structure:

      • Lack a periosteum and ossify after birth.

      • Surfaces of contact are covered with hyaline cartilage and lubricated by a bursa or synovial membrane.

    • Examples: Patella, pisiform, fabella.

    • Functions:

      • Resist pressure.

      • Minimize friction.

      • Alter the direction of muscle pull.

      • Maintain local circulation.

  • Accessory (Supernumerary) Bones

    • Definition:

      Extra bones that are not always present, arising due to ununited epiphyses from additional centers of ossification.

    • Examples:

      • Sutural Bones: Found in the sutures of the skull.

      • Os Trigonum: Located at the lateral tubercle of the talus.

      • Os Vesalianum: Found at the tuberosity of the 5th metatarsal.

    • Key Characteristics:

      • Often bilateral.

      • Surfaces are smooth and lack callus formation.

      • Commonly mistaken for fractures in medicolegal practice.

  • Heterotopic Bones

    • Definition:

      Bones that develop in soft tissues, typically in response to mechanical stress or trauma.

    • Example:

      • Rider's Bones: Develop in the adductor muscles of horse riders due to repeated stress.

• Developmental and Regional Classification of Bones

  • Membrane (Dermal) Bones

    • Formation: Ossify directly in the membrane [mesenchyme through intramembranous ossification.}

    • Origin: Derived from mesenchymal condensations.

    • Examples:

      • Bones of the vault of the skull (e.g., frontal, parietal).

      • Facial bones (e.g., maxilla, nasal bones).

  • Cartilaginous Bones

    • Formation: Ossify in cartilage through intracartilaginous or endochondral ossification.

    • Origin: Derived from preformed cartilaginous models.

    • Examples:

      • Bones of the limbs (e.g., humerus, femur).

      • Vertebral column.

      • Thoracic cage (e.g., ribs, sternum).

  • Membrano-cartilaginous Bones

    • Formation: Ossify partly in membrane and partly in cartilage.

    • Examples:

      • Clavicle.

      • Mandible.

      • Occipital, temporal, and sphenoid bones.

  • Somatic and Visceral Bones

    • Somatic Bones: Most bones of the body belong to this category.

    • Visceral Bones: Develop from pharyngeal arches.

      • Examples:Hyoid bone. Part of the mandible.Ear ossicles (e.g., malleus, incus, stapes).

• Regional Classification

  1. Axial skeleton includes the skull, vertebral column, and thoracic cage.

  2. Appendicular skeleton includes bones of the limbs.

• Structural Classifications

  • Macroscopically

    1. Compact Bone

       • Appearance: Dense in texture, resembling ivory, but highly porous.

       • Location: Primarily in the cortex of long bones.

       • Function:Adapts to bending and twisting forces, which are combinations of compression, tension, and shear stresses.

    2. Cancellous Bone/trabecular bone

       • Appearance: Spongy and porous.

       • made up of a meshwork of trabeculae (rods and plates) between which are marrow-containing spaces.

       • Location: Found in the interior of bones, particularly at the ends of long bones and in flat bones.

       • Function:Provides structural support and houses bone marrow.

  • Microscopically

    • Lamellar Bone

      • Structure:Composed of thin plates of bony tissue called lamellaeCancellous Bone: Lamellae are arranged in irregular piles.

      • Compact Bone: Lamellae are arranged in concentric cylinders forming the Haversian system (secondary osteon).

      • Characteristics: Found in most mature human bones, providing strength and resilience.

    • Woven Bone

      • Structure: Irregularly arranged collagen fibers.

      • Found in fetal bones, during fracture repair, and in pathological conditions like bone cancer.

    • Fibrous bone

      is found in young foetal bones, but are common in reptiles and amphibia.

    • Dentine and cement

CLASSIFATION OF BONES

• Shape

  • Long bones

    1. Diaphysis elongated shaft

    2. Epiphysis:- expanded, smooth, and articular ends.

    3. 3 Surfaces

       1. a central medullary cavity

       2. a nutrient foramen

       3. growing end

    4. 3 types of long bone

       1. Typical Long Bones: Humerus, radius, ulna, femur, tibia, and fibula.

       2. Miniature Long Bones: Metacarpals, metatarsals, and phalanges (with only one epiphysis).

       3. Modified Long Bones: Clavicle (lacking a medullary cavity).

  • Short Bones

    • Shape: Cuboid, cuneiform, trapezoid, or scaphoid.

    • Examples: Tarsal and carpal bones.

  • Flat Bones

    • Structure: Resemble shallow plates and form boundaries of certain body cavities.

    • Examples:

      • Skull vault bones

      • Ribs

      • Sternum

      • Scapula

  • Irregular Bones

    • : Irregularly shaped and often complex.

    • Examples:

      • Vertebra

      • Hip bone

      • Bones at the base of the skull

  • Pneumatic Bones

    • Structure: Irregular bones containing large air-filled spaces lined by epithelium.

    • Functions:

      • Reduce skull weight.

      • Enhance voice resonance.

      • Act as air-conditioning chambers for inhaled air.

    • Examples: Maxilla, sphenoid, ethmoid.

  • Sesamoid Bones

    • Small, nodular bones embedded in tendons or joint capsules.

    • Structure:

      • Lack a periosteum and ossify after birth.

      • Surfaces of contact are covered with hyaline cartilage and lubricated by a bursa or synovial membrane.

    • Examples: Patella, pisiform, fabella.

    • Functions:

      • Resist pressure.

      • Minimize friction.

      • Alter the direction of muscle pull.

      • Maintain local circulation.

  • Accessory (Supernumerary) Bones

    • Definition:

      Extra bones that are not always present, arising due to ununited epiphyses from additional centers of ossification.

    • Examples:

      • Sutural Bones: Found in the sutures of the skull.

      • Os Trigonum: Located at the lateral tubercle of the talus.

      • Os Vesalianum: Found at the tuberosity of the 5th metatarsal.

    • Key Characteristics:

      • Often bilateral.

      • Surfaces are smooth and lack callus formation.

      • Commonly mistaken for fractures in medicolegal practice.

    Heterotopic Bones

    • Definition:

      Bones that develop in soft tissues, typically in response to mechanical stress or trauma.

    • Example:

      • Rider's Bones: Develop in the adductormuscles of horse riders due to repeated stress

• Developmental and Regional Classification of Bones

  • Membrane (Dermal) Bones

    • Formation: Ossify directly in the membrane [mesenchyme through intramembranous ossification.}

    • Origin: Derived from mesenchymal condensations.

    • Examples:

      • Bones of the vault of the skull (e.g., frontal, parietal).

      • Facial bones (e.g., maxilla, nasal bones).

  • Cartilaginous Bones

    • Formation: Ossify in cartilage through intracartilaginous or endochondral ossification.

    • Origin: Derived from preformed cartilaginous models.

    • Examples:

      • Bones of the limbs (e.g., humerus, femur).

      • Vertebral column.

      • Thoracic cage (e.g., ribs, sternum).

  • Membrano-cartilaginous Bones

    • Formation: Ossify partly in membrane and partly in cartilage.

    • Examples:

      • Clavicle.

      • Mandible.

      • Occipital, temporal, and sphenoid bones.

  • Somatic and Visceral Bones

    • Somatic Bones: Most bones of the body belong to this category.

    • Visceral Bones: Develop from pharyngeal arches.

      • Examples:HPart of the mandible.Ear ossicles (e.g., malleus, incus, stapes).hyoid bone.

• Regional Classification

  1. Axial skeleton includes the skull, vertebral column, and thoracic cage.

  2. Appendicular skeleton includes bones of the limbs.

• Structural Classifications

  • Macroscopically

    1. Compact Bone

       • Appearance: Dense in texture, resembling ivory, but highly porous.

       • Location: Primarily in the cortex of long bones.

       • Function:Adapts to bending and twisting forces, which are combinations of compression, tension, and shear stresses.

    2. Cancellous Bone/trabecular bone

       • Appearance: Spongy and porous.

       • made up of a meshwork of trabeculae (rods and plates) between which are marrow-containing spaces.

       • Location: Found in the interior of bones, particularly at the ends of long bones and in flat bones

       • Function:Provides structural support and houses bone marrow.

  • Microscopically

    • Lamellar Bone

      • Structure:Composed of thin plates of bony tissue called lamellae .Compact Bone: Lamellae are arranged in concentric cylinders forming the Haversian system (secondary osteon). Cancellous Bone: Lamellae are arranged in irregular piles.

      • Characteristics: Found in most mature human bones, providing strength and resilience.

    • Woven Bone

      • Structure: Irregulary arrangedcollagen fibers.

      • Found in fetal bones, during fracture repair, and in pathological conditions like bone cancer.

    • Fibrous bone

      is found in young foetal bones, but are common in reptiles and amphibia.

    • Dentine and cement

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