Sports Medicine: Just the Facts

OSTEOCLASTS

• Multinucleated bone resorbing cells controlled by
  hormonal and cellular mechanisms


• Function in groups termed cutting conesthat attach to
  bare bone surfaces and dissolve inorganic and organic
  matrices of bone and calcified cartilage through the
  use of hydrolytic enzymes.


• Process results in the formation of shallow pits on the
  bone surface called Howship’s Lacunae (Recker, 1992).

HEMATOPOIETICELEMENTS

• Cells primarily responsible for the proliferation of the
  cellular element of blood

TYPES OF BONE

WOVENBONE

• Formed during embryonic development, during fracture
  healing and in some pathologic states such as hyper-
  parathyroidism and Paget’s disease (Recker, 1992).


• Composed of randomly arranged collagen bundles
  and irregularly shaped vascular spaces.

CORTICALBONE(COMPACT ORLAMELLARBONE)

• Remodeled from woven bone by means of vascular
  channels that invade the embryonic bone from its
  periosteal and endosteal surfaces.


• The primary structural unit of cortical bone is an
  osteon, also known as a Haversian system.
  a. Consists of cylindrical shaped lamellar bone that
  surrounds longitudinally oriented vascular chan-
  nels called Haversian canals.
  b. Horizontally oriented canals (Volkmann) connect
  adjacent osteons.
  c. Mechanical strength of cortical bone is dependent
  on the concentration of the osteons.

CANCELLOUSBONE(TRABECULAR)

• Lies between cortical bone surfaces and consists of a
  network of honeycombed interstices containing
  hematopoietic elements and bony trabeculae.
  •Trabeculae are oriented perpendicular to external
  forces to provide structural support (White and
  Hirsch, 1971).

BONE BIOCHEMISTRY (RECKER, 1992)

• Bone is composed of organic and inorganic elements.
  a. Inorganic elements: Dry bone is made up of cal-
  cium phosphate (65–70% of the weight).
  b.Organic matrix: Fibrous protein and collagen
  (30–35% of the weight).
  c. Osteoid: (1) Unmineralized organic matrix secreted
  by osteoblasts. (2) Composed of 90% type I collagen

and 10% ground substance (noncollagenous pro-

teins, glycoproteins proteoglycans, peptides, carbo-

hydrates, and lipids). (3) Mineralization of this

substance by inorganic mineral salts provides bone

with its strength and rigidity.

d. Inorganic bone contents: (1) Primarily calcium

phosphate and calcium carbonate with small quan-

tities of magnesium, chloride, and sodium. (2)

Mineral crystals form hydroxyapatite, an orderly

precipitate around the collagen fibers of the osteoid.

REGULATORS OF BONE METABOLISM (BODEN

AND KAPLAN, 1990; REICHEL, 1989)

• Three of the calcitropic hormones that have the most
  effect on metabolism are parathyroid hormone, vita-
  min D, and calcitonin.
  
  
  1. Parathyroid hormone increases the flow of calcium
     into the calcium pool and maintains the body’s
     extracellular calcium level relatively constant.
     a. Osteoblasts are the only bone cells that have
     parathyroid hormone receptors.
  
  
  2. Vitamin D stimulates intestinal and renal calcium
     binding proteins and facilitates active calcium
     transport.
  
  
  3. Calcitonin is secreted by the parafollicular cells of
     the thyroid gland in response to rising plasma cal-
     cium level. Calcitonin serves to inhibit calcium
     dependent cellular metabolic activity.
  
  
  4. Miscellaneous proteins: (1) Released from platelets,
     macrophages, and fibroblasts. (2) Effect healing
     bone to vascularize, solidify, incorporate, and func-
     tion mechanically. (3) Induce mesenchymal derived
     cells such as monocytes and fibroblasts to migrate,
     proliferate and differentiate into bone cells (Hruska
     et al, 1993; Hynes, 1992).
  
  
  
  


• Proteins that enhance bone healing include the bone
  morphogenic proteins (BMPs), insulin-like growth
  factors, transforming growth factors, platelet derived
  growth factor, and fibroblast growth factor, among
  others (Reddi and Sampath, 1996).

BMPS

• A family of glycoproteins derived from bone matrix.
  These proteins produce mesenchymal cells to differ-
  entiate into bone cells.


• Present in only minute quantities in the body.
  •Several BMPs have been synthesized using recombi-
  nant DNA technology. Clinical trials are currently
  underway to assess their potential to facilitate bone
  fusion (Boden et al, 2000; Geesink, Hoefnagels, and
  Bulstra, 1999).

62 SECTION 1 • GENERAL CONSIDERATIONS IN SPORTS MEDICINE