Atlanto-Occipital Joints



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Hyaline Cartilage

  • Tensile properties (strength and elongation) show decline at 40 and decrease markedly after 50 and continues to decline

  • Compression (strength and compaction) show decline at 40, further decrease at 50 and then levels off

  • Decrease in compressive forces is less over time than tensile

Disc Fibrocartilage

  • Tensile properties

    • Breaking load is least for small cervical disk, intermediate for thoracic disk and greatest for lumbar disk

    • Breaking load corresponds to size

    • When disk size and strength and calculated, cervical disk is tronger than lumbar and thoracic is least of all

    • Elongation greatest in cervical, intermediate in lumbar and least in thoracic

    • Tensile properties decrease less than 25% with age

  • Torsional properties

    • Breaking load least with cervical disk, intermediate for thoracic, greatest for lumbar

    • Torsional breaking load 9x greater for lumbar than cervical

    • Torsional breaking load corresponds to disk size

    • When size is considered and strength is calculated, cervical and lumbar are about the same and thoracic is slightly less

    • Angle of twist for failure is highest for cervical, imtermediate for thoracic and lowest in lumbar

    • Angle of twist corresponds with amount of rotational movement allowed in that region of the spine

    • Torsional properties decrease by less than 20% with age


BONE

  • Types of bone

    • Fibrous- irregular and unorganized pattern of bone, found during embryonic bone formation and development, fracture healing and pathological bone

    • Lamellar bone- rows or concentric circles of bones, typical of skeleton

      • Spongy or cancellous bone is formed by internal network of delicate processes called trabeculae

        • Abundant at ends of long bones in epiphyses

        • Trabeculae align to resist common stresses in region of bone where they are located

        • Arranged in rows

      • Dense or compact bone is the solid bone found on the periphery of bone

        • Thin in region of epiphyses but usually thick in diaphyses of long bones

        • Consists of concentric circular layers of bone forming osteon or haversion system

  • Coverings

    • 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

      • Adapts bone to long term changes in stress

      • Makes calcium stored in bone available to body

BONE FRACTURES

  • types of fractures

    • complete- transects entire 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

  • Bone healing process

  1. Fracture results in local hematoma due to ruptured blood vessels

  2. Blood clot develops

  3. Capillaries grown in clot and form vascular network; connective tissue grows into granulation tissue; macrophages remove dead tissue and osteoclasts remove bone fragments

  4. Granulation tissue becomes dense CT where hyaline cartilage and Fibrocartilage develop- fibrocartilagenous callus- divided into large external callus and small internal callus

  5. Disruption of periosteum and endosteum at fracture site stimulates osteogenic cell activity

  6. 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

  7. illaries

  8. 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

  • immobilization


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

  • Strength- amount of force need to break bone relative to area of the bone

  • Elongation, contraction, deflection, angle of twist all describe deformation or strain




  • TENSION

    • Strength: thin long bone is stronger in tension than thick

    • Strain: think long bones elongate more than thick when loaded in tension

  • COMPRESSION

    • Thick long bones are stronger in compression than thin long bones

    • Compression strength is directly related to CSA of a bone

    • Larger CSA, greater resistance to compression

    • Thin long bones compress more than thick long bones

    • Degree of compression of long bone is inversely related to CSA

    • Smaller CSA, greater amount of compression

  • BENDING

    • Thick long bones stronger in bending than thin long bones

    • Directly related to CSA of bone- larger CSA, greater resistance to bending

    • Thin long bones bend more than thick long bones

    • Degree of bending of a long bone is inversely related to CSA- less CSA, greater amount of bending displacement

  • TORSION

    • Thick long bones stronger in torsion than thin long bones

    • Directly related to CSA, larger CSA- greater resistance to torsion

    • Thin long bones twist more than thick long bones

    • Inversely related to CSA – smaller CSA, greater angle of twist

Aging

  • max strength in men and women 20-29 y/o

  • decreases slowly in both genders 30-39 y/o

  • before 30, osteoblasts more active than osteoclasts= increased thickness of cortical bone

  • after 40, osteoblasts decrease but osteoclasts are unchanged- slow decrease in cortical bone thickness

  • less than 50, mechanical properties of bone are similar in male and females but males tend to have more bone than females

  • after 50, strength and amount of trabecullar bone decreases more rapidly in females than in males

  • after 50, tensile strength 75% of max, compressive strength 85% of max, bending strength 75% of max, torsional strength 85%

biomechanical properties and aging in vertebrae

  • tension

    • lumbar tensile breaking load 4x greater than cervical

    • lower thoracic breaking load 2x more than upper thoracic…both less than lumbar and greater than cervical

    • breaking load is a function of size

    • considering size, tensile strength is very similar among vertebrae

    • elongation is also similar among the vertebrae

    • with age, decreases in tensile properties are less than 20%

      • tensile breaking load down 20%

      • tensile strength down 10%

      • tensile strain down 15%

  • compression

    • lumbar breaking load 2x cervical

    • lower thoracic breaking load 1.5x upper thoracic but middle thoracic is only .15x greater than upper thoracic

    • all thoracic breaking loads are less than lumbar and greater than cervical

    • BL is function of size

    • Considering size- tensile strength of cervical vertebra is greatest, then thoracic, then lumbar

    • Cervical strength 2x lumbar but thoracic are only about 20-25% greater than lumbar

    • Contraction is greatest in cervical and least in lower thoracic and lumbar but difference is only 30%

    • With age, decreases in compressive properties are 40-50%

      • Breaking load down 50%

      • Strength down 45%

      • Strain down 40%

  • Torsion

    • Lumbar torsional BL 4x > than upper thor but only 35% > than lower thor

    • Lower thor BL 60% > than upper thor and middle thor BL is 45% > than upper thor

    • All thor BL are less than lumbar and differences are a function of size

    • Considering size, torsional strength of upper thor is >, strength very similar along thor and lumbar vertebra

    • Angle of twist is > in upper thor and prob cervical and least in lumbar

    • Upper and middle thor angles are 2x greater than that of lumbar but lower thor is 60% of lumbar

    • With age, large decreases of 40% occur for BL and angle of twist but torsional strength decreases by 20%

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