Children and Bone

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Children and Bone

• Jennifer Brittain
• October 24, 2002

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Osteoporosis

• 13.8 billion/year in healthcare costs (U.S.)
• pediatric disease with a geriatric outcome
• Key is PREVENTION
• Peak Bone Mass (PBM)
• Optimize bone mineral gains throughout childhood
  and adolescence

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Bone Physiology

• Osteoblasts(OBs) cells responsible for
  formation of bone tissue
• Osteoclasts(OCs) cells responsible for the
  breakdown of bone tissue
• Bone growth activity of OBs greater than that
  of OCs
• Bone resorption activity of OCs greater than
  that of OBs

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Bone Physiology

• Proper skeletal formation, growth and repair are
  critically dependent on the accurate
  orchestration of all the processes participating
  in the formation of endochondral bone at the
  growth plate.
• Bone Biology, Chapter 3

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Basis of Bone Growth

• Bones grow in
• Width/diameterinvolves formation externally and
  resorption internally
• Lengthenlargement of chondrocytes (cartilage
  cells) at the growth plate, followed by
  replacement with bone cells for mineralization

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Growth Plate

• The epiphyseal plate is the cartilaginous region
  separating bone into two distinct parts.
• The plate survives if the cartilage growth keeps
  pace with osteoblast invasion.
• When osteoblast activity increases beyond
  cartilage growth, the plate disappears and the
  epiphyseal line is seen.

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Bone Growth in Childhood

• Process begins approximately 6 weeks after
  fertilization and continues through adolescence.
  
• Some parts of the skeleton are developing through
  approximately 25 years of life.

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Growth Rate

• The most rapid skeletal growth is seen in
  infancy.
• A slow deceleration in the rate is seen around
  the age of 3 and on through puberty.
• Slight growth spurt appears around 8 years of age
  and distinctly decelerates immediately before
  puberty.

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Gains in Mass

• Most rapid gains in bone mass seen during
  adolescence
• 25 of PBM acquired within the 2-year period at
  peak height velocity(rate).
• Maximal rates of bone mineral gains follow behind
  the peak height velocity (6-12months)

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Bone Mineral

• Accrual of bone mineral continues after
  longitudinal growth has occurred.
• At peak growth velocity, 90 adult stature
  attained and only 57 of total bone mineral
  content (BMC)
• 90 of peak bone mass is gained by 18 yrs
• 5-12 bone mineral density gained during third
  decade

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Terminology

• Bone mineral content (BMC) is the measurement of
  amount of mineral in the bone (grams).
• Bone mineral density (BMD) is the measurement of
  mineral per area (grams per cm2)

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Bone mineral determinants

• 60-80 PBM is genetic
• Likely that the effects are greater in mineral
  acquisition than in loss
• Racial differences (reduced risk of osteoporosis
  in blacks)
• Some seen before puberty
• Others emerge in late adolescence

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Determinants cont.

• Hormonal status (systemic)
• Growth hormone (GH)
• Major influence before puberty
• Longitudinal bone growth impaired with deficiency
• Deficiency leads to reduced bone mineral/failure
  to acquire at expected rate
• Acts at growth plate by enhancing production of
  cartilage cells and then inducing IGF1 synthesis
  (a local regulator responsible for expansion of
  chondrocytes at growth plate)

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More bone mineral determinants

• Hormones cont.
• Thyroid
• Deprived levels lead to deleterious effects on
  bone growth
• Direct effect on bone
• Indirect effect on GH and IGF-1
• High levels increase growth rate and advance bone
  age

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Hormone Regulation cont.

• Estrogen
• Direct effects on growth plate
• Responsible for epiphyseal growth plate fusion
  (end of puberty) in females and males
• Glucocorticoids
• Pharmacological doses cause stunted growth
• Role in chondrocyte differentiation and
  hypertrophy

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Exercise and Bone

• Weight-bearing activity below a physiological
  threshold will lead to excessive resorption. Ex.
  Bone loss in spaceflight.
• Within physiological range, bone is maintained.
• Bone is gained as load/strain increases

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Exercise Cont.

• Greatest influence before and during puberty
• Bone mass increases due to early intense activity
  are carried into adulthood
• Ex. BMD of former gymnasts, runners and dancers
  as high as 8-12 greater than age-matched
  controls ( years later)

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Diet and Bone

• Protein
• Important role in bone development
• Influences peak bone mass
• Protein malnutrition in developmental years can
  increase risk for osteoporosis and fracture later
  in life
• Low intake impairs synthesis and action of IGF-1
  (essential factor for longitudinal growth)
• Significant association between ingested protein
  and bone mass gain in prepubertal children
• Bonjour, JP et al. 2001

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More Dietary Elements

• Calcium
• Skeleton stores 95 of bodys calcium
• In 2001, 90 of adolescent girls and 50 of boys
  consumed less than optimal Calcium amounts.
  Bachrach, 2001
• Contributes to bone mineral accrual during early
  adulthood and helps prevent loss at maturity

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Diet Cont.

• Vitamin D
• Deficiency is a major cause of rickets in
  children and osteomalacia in adults
• Expansion of zone of growth plate coupled with
  impaired matrix calcification
• Promotes mineralization
• Suggestion that vitamin D receptors are found on
  OB and OC precursors Johnson, 1996
• Increases absorption of calcium (and phosphorous)

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Overview

• Peak bone mass is key to prevention of
  osteoporosis.
• Bones grow in length and mass due to a number of
  processes working together.
• Exercise can benefit bone health.
• Protein, calcium, and vitamin D are three
  important dietary components of bone health.

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