Biomech
5/13/99

Flow chart from text
•	Muscles absorb a lot of stress to avoid excess stress
•	Muscles fatigue and lose the ability to absorb stress
•	Stress fractures come from muscle fatigue

Fig 1-31—ski boot
Fig 1-32—stress distribution on hip
•	Bone fractures under tension
Fig 1-33—loading—absorption at quick loading—stronger
Fig 1-30—walking gait—no muscles on tibia to help compensate for tensile stress on anterior tibia

Shape of bone
•	Resistance = bh3/12
•	Compare a 4x4 to a 2x8
•	More resistance to bending in the 2x8—less likely to break
•	Area moment of inertia
•	Most long bones are circular—most of the harder part is away from neutral (zero)
•	Tibia—bend it
•	Where does it break?
•	At the top it has a longer momentum arm, w/less bone density
•	At the bottom it has more bone, but a shorter momentum arm
•	Breaks at the bottom easier

Osteoporosis
•	Miss match b/w osteoclast and osteoblast
•	Most often found in the vertebral body
•	Looks at a grid--trabeculae
•	The weakest part is the end plate
•	When push on the top in the middle, it wants to cave in
•	Horizontal only takes a little pressure
•	If body needs calcium, it takes it from the horizontal b/c it is the least stressed
•	If only see vertical trabeculations—osteoporosis
•	If add force now, bends in more and more over time
•	Fish vertebrae
•	Lining of cortex w/ loss of horizontal trabeculations
•	Women's bone density is strongest at 25-30y/o
•	Prevent by proper diet and exercise w/gravity
•	In 1920, 0% chance of living one year after a hip fracture—bone can't heal, die of pneumonia
•	Today, 30-50% live past one year due to hip replacement—die b/c can't live through surgery

Cartilage
•	Prevent friction
•	Transfer force b/w bones
•	Knee is largest bone in body
•	Has 5mL of sinovial fluid
•	Black space is the cartilage b/w bones
•	Arthritis is decreased joint space
•	Posterior patella has thickest cartilage—highest stressed joint—5mm
•	Lumbar facets is next in stress
•	Building blocks of cartilage
•	Water—predominantly—can not compress as long as it is contained—resists compression
•	Collagen—matrix for the water—high pressure system
•	Proteoglycans—attract water to the system—water wants to leave due to the pressure
•	Cells—chondrocytes—make collagen, Proteoglycans, make enzymes that destroy collagen (collagenases) and Proteoglycans—easy cell to kill b/c has no blood supply and no nerves
•	sinovial fluid—a cellular blood (yellow, clear) that still has nutrients
•	Chondrocytes live in a bubble and it thinks that pressure around tells it to make or not
•	Kill these cells by immobilizing the joint –8wks to irreversible damage