Bone Tissue and Fractures

Bone Tissue

Bone Growth and Repair

• Bones can widen for additional strength and repair throughout life.
• A diet rich in proteins, vitamins A, C, and D, and minerals like calcium is necessary for bone health.
• Consume vegetables and fruits for optimal bone health.

Bone Response to Use

• Bone grows or remodels in response to forces or demands placed upon it.
• Handedness results in the bone of one upper limb being thicker and stronger than the other.
• Curved bones are thickest where they are most likely to buckle.
• Large, bony projections grow where heavy, active muscles attach.

Calcium and Phosphate Storage and Use

• Calcium and phosphate are stored in the bone matrix.
• Calcium is necessary for:
  • Transmission of nerve impulses
  • Muscle contraction
  • Blood coagulation
  • Cell division
• Calcium is moved from the bone matrix to where it is needed.

Resorption: Moving Calcium from Bone to Blood

• When calcium and phosphate are needed elsewhere in the body:
  • Osteoclasts digest bone.
    • Lysosomal enzymes digest proteins.
    • Acids solubilize calcium salts.
  • About 1% of bone minerals and proteins are returned to the blood at any one time.

Parathyroid Hormone (PTH) and Blood Calcium Regulation

• Falling blood Ca^{2+} levels (possibly due to use by muscle and nerve, or less in diet) are sensed by parathyroid glands.
• Parathyroid glands release parathyroid hormone (PTH) into the blood.
• PTH binds to osteoclasts, which release acids and enzymes to digest bone.
• Calcium is released from bone into the blood.
• Blood calcium level is restored.
• On a typical day, blood calcium rises and falls depending on diet and exercise.

Normal Control of Blood Calcium

1. Blood calcium levels drop.
2. Parathyroid glands sense low Ca^{2+}, release PTH into blood.
3. PTH travels through blood to bone and binds to osteoclasts.
4. Osteoclasts degrade bone matrix, releasing Ca^{2+} into the blood.
5. Blood Ca^{2+} levels are raised to normal.
6. Organs receive enough calcium to function.

Hypocalcemia: Abnormally Low Blood Calcium

• Can be caused by:
  • Hypoparathyroidism (understimulation of osteoclasts)
  • Oversweating
• Leads to:
  • Overexcited neurons
  • Overstimulation of skeletal muscle (cramps)
  • Cardiac muscle overworks (heart spasms)
• Excessive sweating can lead to dangerously low blood calcium, which can be fatal if not corrected quickly.

Hypercalcemia: Abnormally High Blood Calcium

• Can be caused by:
  • Taking too much calcium in the form of vitamins or antacids
  • Hyperparathyroid disease (overstimulation of osteoclasts)
  • Immobility of patient, which causes bones to release calcium into the blood
• Leads to:
  • Underexcitable neurons
  • Coma, confusion
  • Underperforming muscles
  • Heart slows or stops beating

Types of Fractures

• Nondisplaced: ends retain normal position
• Displaced: ends out of normal alignment
• Complete: broken all the way through
• Incomplete: not broken all the way through
• Linear: parallel to long axis of the bone
• Transverse: perpendicular to long axis of the bone
• Compound (open): bone ends penetrate the skin
• Simple (closed): bone ends do not penetrate skin
• Comminuted: Bone fragments into three or more pieces; common in the aged
• Compression: Bone is crushed; common in porous bones subjected to extreme trauma
• Spiral: Ragged break occurs when excessive twisting forces are applied to a bone; common sports fracture
• Epiphyseal: Epiphysis separates from the diaphysis along the epiphyseal plate; occurs where cartilage cells are dying and calcification of the matrix is occurring
• Depressed: Broken bone portion is pressed inward; typical of skull fracture
• Greenstick: Bone breaks incompletely; common in children

Stages in the Healing of a Bone Fracture

1. Hemostasis: Hematoma (clots broken vessels)
2. Inflammation: Causes swollen, painful, red area
3. Phagocytic cells clear debris
4. Fibroblasts secrete collagen fibers to connect bone ends
5. Bony callus formation: Osteoblasts form spongy bone to connect bone ends
6. Compact bone forms on the surface
7. Bone remodeling: Osteoclasts remodel and smooth the bone, approximating the original shape

Homeostatic Imbalances

• Rickets: Vitamin D deficiency or insufficient dietary calcium in children leads to bowed and weak bones
• Osteoporosis: In adults, low bone calcium due to lack of estrogen, calcium or vitamin D deficiency, immobility, thyroid disease, and diabetes can cause loss of bone mass. Spongy bone of the spine and neck of the femur becomes porous and more susceptible to fracture. Vertebrae collapse and compress.

Osteoporosis: Treatment and Prevention

• Diet with sufficient calcium
• Weight-bearing exercise throughout life