Covered externally by periosteum except in areas of articular cartilage
Periosteum contains osteogenic cells and is anchored to bone by Sharpey’s fibers
Internal cavity of bones covered by thin cellular layer called endosteum
Endosteum contains osteogenic cells and bone absorbing cells called osteoclasts
Macroscopic Anatomy
Ridged CT composed of collagen embedded in ground substance
Collagen and ground substance form matrix
65-70% of matrix is composed of inorganic salts most calcium phosphate and calcium carbonate
25-30% of matrix is composed of organic compounds of which 95% is collagen and other 5% are proteoglycans and glycoproteins
Osteon is structuralunit of compact bone and lie between peripheral layers of the external circumferential system and the medullary layers of the internal circumferential system
Periosteum attaches by Sharpey’s fibers to the external circumferential system
Endosteum attaches to internal circumferential system
Interstitial system- area between osteons which is mainly remnants of old osteons after bone remodeling
Haversian canal- central canal that contains blood vessels to provide nutrients to bone cells called osteocytes and remove waste products, lymphatic channels and fine nerve fibers
Volkmanns canal- run transverse to haversian canals and transport blood vessels, lymphatics, and fine nerves to the haversian canals from nutrient foramen that lies on the surface of each bone
Each haversian canal is surrounded by rings of calcified bone called lamella and between the rings are bubble like structures called lacuna that contain more osteocytes
Osteogenic cells arise from embryonic mesenchymal cells, from periosteum and endosteum of mature bone and differentiate into osteoblasts
Osteoblasts are immature bone cells that secrete collagen and non-calcified ground substance called osteoid and become osteocytes when the surrounding matrix is calcified
Osteoclasts are large multinucleated cells that remove calcified bone and osteoid and are important in bone growth, bone remodeling, fracture healing and maintain blood calcium levels.
BONE REMODELING
Remodeling during bone growth:
Surface bone remodeling involves the simultaneous process of depositing new bone in one area by osteoblasts and the reabsorption of bone in a different area by osteoclasts
Cylindrical bones- osteoblasts in subperiosteal area deposit bone while in the same region, osteoclasts in the subendosteal area reabsorb it
Conical bone- osteoblasts in subendosteal area deposit bone and osteoclasts in subperiosteal area reabsorb it
Internal bone remodeling:
Starts with removal of old lamellar bone by osteoclasts that bore out through longitudinal cylindrical cavity= cutting cone (relatively long, reaches diameter of new osteon)
Osteoclasts in cutting cone along with blood vessels, perivascular connective tissue and numerous cells in mitosis that appear to give rise to osteoblasts
Osteoblasts deposit collagen and osteoid (uncalcified bone) along walls of cone and the cutting cone becomes = closing cone (refilled from outside in with concentric circular layers of lamellar bone until the haversian canal remains)
Bone remodeling throughout life:
Old osteons and interstitial bone are removed and replaced throughout lifespan
Reasons for bone remodeling-
Removes damaged bone from microfactures due to bone fatigue or strain
Replenishes osteocytes and maintains organic and inorganic compounds of bone
incomplete- penetrates through only part of the bone
simple or closed- surrounding tissue intact
comminuted- splintering of bone
compound- bony ends are displaced and disrupt surrounding tissue and skin
fracture site
depends on distribution of spongy and compact bone in an area and the type of load applied
spongy bone is weaker than compact bone so more prone to fractures (spongy- epiphyses, tuberosities; compact- shaft)
spongy and compound bone are weaker in tension than compression so the areas receiving tension are more susceptible to fracture than those receiving compression forces
regional compression strength difference
shaft is strongest
ends of bones are weaker than shaft (more spongy, less compact) except in portion 1 of ulna (thick olecranon is mainly compact bone)
causes of fractures
trauma
pathologies
osteoporosis- disorder characterized by decreased mass of spongy and compact bone but no abnormality in composition of bone
osteomalacia- reduced mineralization of bone during remodeling which results in softer bone and appears to be associated with lack of vitamin d
pagets disease (osteitis deformans)- metabolic disorder characterized by marked bone reabsorption followed by the formation of patches of new bone that lacks the strength of normal bone even through it is thick
osteogenesis imperfect (brittle bone)- inherited disorder resulting in abnormal collagen syntrhesis and absorption making bone brittle
Fracture results in local hematoma due to ruptured blood vessels
Blood clot develops
Capillaries grown in clot and form vascular network; connective tissue grows into granulation tissue; macrophages remove dead tissue and osteoclasts remove bone fragments
Granulation tissue becomes dense CT where hyaline cartilage and Fibrocartilage develop- fibrocartilagenous callus- divided into large external callus and small internal callus
Disruption of periosteum and endosteum at fracture site stimulates osteogenic cell activity
Osteoid calcified and bony callus of fibrous bone is formed- not organized along mechanical lines of stress but layers are aligned in direction of cap
illaries
Time and return of function allow bony callus to be remodeled and fibrous bone is converted to lamellar bone
Movements that produce rotation and traction and fx site should be avoided during early healing as they displace bony ends and can prevent union of bones
Exercises that reflect functional activities will strain fx callus to direct remodeling so that remodeled bone aligns to resist appropriate mechanical loads
Factors that effect bone
Exercise
strenuous exercise results in muscle fatigue which decreases shock absorption capability of muscle and produces altered movements and abnormal loading of the bone
abnormal loading changes force distribution and strain pattern on the bone which results in microfailures and possibly fractures
jogging
compression forces on tibia at toe strike during jogging 2x greater than at heel strike during walking
tensile forces on tibia from toe strike to toe off are 4 greater than from foot flat to heel off during walking
Bone decreases in strength, stiffness and toughness
Bone stress and strain
Compact and spongy bone
Compact is stiffer and stronger than spongy in compression, tension and shear
Compact is most resistant to compression, then tension, then shear
Trabeculae of spongy bone and osteons of compact bone are aligned to resist most frequently occurring stress
Wolff’s law- bone is deposited where needed to resist stress and absorbed where not needed … seems to apply to all CT as well
During ADLs, long bones of body are subjected to multiple stresses and strains by muscle action, gravity or resistive loads
Activities stress bones in tension, compression, bending, and torsion- because the long bones differ in shape and size, the ability to resist these forces differs in each bone
Breaking load- amount of force needed to break bone without consideration of the size of that bone
