bone, tendons and ligaments

lecture 17

functions of bone

• protection

• motion and support

• sound transmission

• blood production

• mineral store

examples of bones act in a protective way

• skull

• rib cage

how do bones provide motion and support

• bones are lever arms pivoting around the joints that act as a fulcrum

• bones are attached to each other by fibrous, cartilaginous and synovial joints

which bones allow for sound transmission

auditory ossicles - malleus, incus, stapes

how do bones help in blood production

marrow produces blood cells

basic composition of bones

• mineral phase (hydroxyapatite)

• organic phase

• pores

what do pores help allow?

nutrients to be transported to bone and marrow cells

seven-fold hierarchy of bone

1. collagen and mineral crystals

2. mineralised collagen fibrils

3. fibril array

4. fibril array patterns

5. osteons

6. spongy vs compact bone

7. whole bone

compact (cortical) bone

• hard outer layer of bones

• porosity 5-30%

• consists of haversian canals and osteons

• responsible for smooth, white appearance

• about 80% of the bone in the body

trabecular (cancellous) bone

• open network of rod-like and plate-like elements allowing room for blood vessels and marrow

• network can adapt to changing strains by remodelling

• about 20% of bone

woven bone

• produced rapidly after fracture

• has a disorganised structure of collagen fibres

• weak

lamellar (mature) bone

• produced more slowly

• reorganised woven bone

• has collagen aligned into sheets (lamellae) with a plywood structure

• strong

remodelling of bone

• bones are living organs - they are constantly being remodelled to best meet the stress conditions that they are subjected to

• the remodelling is not completely efficient, and bone structure becomes less effective with age - this is a particular problem for post-menopausal women and can lead to osteoporosis

wolff’s law

• bone remodelling responds to the loadings applied (particularly to dynamic loadings)

• this produces both more bone in the system and also orientation of bone along the stress lines of the loading

• lack of sufficient loading can produce a thinning of bone

osteoblasts

• found near the surface of bones

• make osteoid, which consists mainly of collagen

• secrete alkaline phosphatase to create sites for calcium and phosphate deposition

• osteoid becomes mineralised, forming bone

osteocytes

• mature osteoblasts found in lacunae between the lamellae in bone

• maintain the right levels of oxygen and mineral

osteoclasts

• travel to sites on the bone surface and unfix the calcium by secreting acid phosphatase

bone as a fibre composite

• lightweight

• strong

• tough

• 40-45% by volume is hydroxyapatite, most of the rest is collagen

• Voigt = 39.9 GPa

• Reuss = 10.3 GPa

• Correspond to transverse moduli for bone, but longitudinal is quite a bit lower than Voigt limit

simple fracture

skin is intact

complex fracture

skin is broken

complete fracture

fragments separate

incomplete fracture

fragments partially joined

linear fracture

a fracture that runs in a straight vertical line

transverse fracture

a fracture that runs in a straight horizontal line

oblique fracture

a fracture where the bone cracks at a diagonal or angled line across its width

spiral fractures

where part of the bone is twisted

comminuted fractures

where the bone is broken into several parts

impact fractures

where bits of bone are driven into each other

avulsion fractures

where a bit of bone is separated from the rest

immediately to 5 days after a fracture

• mass of clotted blood forms at fracture site

• bone cells die

• fibroblast cells lay down fibrous scaffold for next stage

four days to three weeks after a fracture

• callus made of cartilage fills the gap - still too weak to hold the bone in place until about six weeks after the fracture (hence the use of casts)

• fibroblasts / osteoblasts migrate in to remake the bone

three weeks to about twelve weeks after a fracture

• bony callus forms using the soft callus as a scaffold

• gentle loading in the later stages encourages the bone to lay down more mineral (this is woven bone)

twelve weeks to several years after a fracture

• remodelling processes convert the woven bone to lamellar bone and reconstruct the original structure

• density increases until bone regains original strength

fibrous joints

• immovable

• connected by dense connective tissue

cartilaginous joints

• slightly movable

• joined by cartilage

synovial joints

• freely movable

• featuring a fluid-filled cavity that lubricates the joint

function of tendons

attach muscle to bone

direct tendons

have a straight course between muscle and bone

wrap-around tendons

bend around a bony pulley or through a fibrous one

function of ligaments

attach bone to bone

six-level hierarchical structure of tendons

1. collagen molecule

2. microfibrils

3. subfibrils and fibrils

4. fibr

stress-strain curve - the ‘toe’ region

stiffness increases as the collagen crimps straighten out and kinks are removed

stress-strain curve - linear / elastic region

linear, reversible behaviour up to the yield point as the collagen molecules are stretched

stress-strain curve - plastic region

irreversible extension occurs. the system shows creep as molecules glide past each other and the fibrillar structure is disrupted

stress-strain curve - failure

• the fibres themselves fail, either together or one after another showing progressive tearing

• broken fibres may still be connected but easily pull out from the structure. the tissue has failed and cannot support a significant load although it may still look in one piece

muscles and tendons

• a tendon must be stiff enough to transmit muscle forces without itself deforming a lot

• the tendon length can be either shorter or longer than that of the muscle fibre

• long tendons tend to transmit more variable forces - the increasing length tends to increase the range of the load-extension curve over which the tendon operates

tennis elbow (lateral epicondylitis) 

• caused by overuse of the muscles (wrist extensors which pull the hand back) that attach to this part of the bone. 

• tendon becomes inflamed and sometimes tears

• surgery rarely necessary - treat with drugs

surgical repair of tendons

• involves either sewing the torn or severed tendon back together or sewing the tendon to connective tissue

• may need a tendon graft from elsewhere in the body

• rarely get back a complete range of motion

repairing ligaments

• most common repair required to the anterior cruciate ligament (joins thighbone to shin bone to stabilise the knee joint)

• cannot be stitched together - can be reconstructed using another tendon (often an accessory hamstring tendon - back of the knee)

• can be done with synthetic polymer (e.g. propylene)

• success rate about 90%