Biomechanics unit 2 - biomech of skeletal muscle tissue deck 3 Flashcards
How does muscle activity influence loading on bones?
- When a muscle contracts it will also load the bone in addition to any external loadings such as those acting at the joints.
- The load applied by the muscle alters the distribution of stress within the bone.
Muscles will often contract, not to cause movement, but to do what?
To alter the stress distribution within a bone.
If a muscle contracts, producing a compressive load on a bone, how can this reduce fracture risk?
It can eliminate any tensile loading and produce an overall compressive loading on the bone (bone stronger in compression than tension)
Give an example of how muscle contraction can alter the stress distribution within a bone
Consider when three point bending is applied to the tibia as shown in Fig. A
- One side of the tibia is in compression and the other is in tension.
- Now if the soleus muscle contracts it will produce a compressive load on the tibia by pulling downwards on the proximal end of the tibia. This compressive load is superimposed on the bending load.
- The overall effect is to reduce or completely eliminate the tensile load and produce an overall compression load throughout the cross-section of the tibia.
- This will result in a higher compressive stress on the anterior surface of the tibia. However, as bone is stronger in compression (than in tension), ==> fracture is less likely
Why are tired athletes more likely to fracture a bone than when they are fresh?
Because their muscles are fatigued and they are therefore unable to control the distribution of stress within their bones
Bone is not a dead inorganic material. It is served by a rich blood supply and has the ability to do what?
Repair itself and also to remodel itself
In what ways is a bone able to remodel itself in response to the mechanical demands placed upon it?
It is able to alter its size, shape and structure
Define Wolffs law
Bone is laid down where needed and resorbed where not needed.
Meaning that compact and cancellous bone is therefore continually gained or lost in response to the amount of stress placed on the bone
There are a number of different circumstances that result in bone remodelling, what happens in reponse to physical exercise e.g. jogging
The bones are subjected to increased levels of stress. The bones respond to this increase by laying down more collagen fibres and mineral salts to strengthen the bones.
There are a number of different circumstances that result in bone remodelling, describe what happens in response to physical inactivity
Resorption of bone tissue
Define what bone atrophy is
This is the reabsorption of bone due to physical inactivity and lack of exercise
Who may bone atrophy be a particular problem for ?
For patients who are bedridden or wheelchair bound for long periods
For patients who are bedridden or wheelchair bound for long periods - once they begin to re-use their legs, what are they liable to ?
Fracture their lower limb bones quite easily.
Bone remodelling can cause problems in fracture fixation:
Consider a fixation plate and screws used to fix the broken bone to imobilise it during healing, why does the fixation plate need to be removed soon after the fracture has healed ?
If the plate is not removed soon after the fracture has healed then the bone will weaken as unstressed bone tissue is resorbed. This is because the plate is carrying most of the load rather than the bone itself.
Bone remodelling can cause problems in fracture fixation:
Consider a fixation plate and screws used to fix the broken bone to imobilise it during healing, what happens to the bone at the points in the bone where the screws are inserted?
The bone will strengthen as the bone tissue at these sites will be carrying a greater load than normally (think as there will be shear forces acting here which wouldnt normally act)
Define what stress shielding is
Stress shielding refers to the reduction in bone density (osteopenia) due to bone reabsorption as a result of removal of typical stress on the bone by an implant e.g. fixation plate
Define what is meant by the term bone hypertrophy
This is an increase in bone tissue
What are the 2 causes of bone fractures (in terms of loading)
Bone fractures can either be caused by a single large load that exceeds the ultimate strength of the bone or by a smaller load that is applied repeatedly.
What is a fracture resulting from the repeated application of a load that is smaller than the ultimate strength of the bone is called?
A fatigue fracture
What are 2 common names also used for fatigue fractures ?
Stress fractures and march fractures
Why are fatigue fractures sometimes called march fractures ?
Due to fatigue fracture of the 2nd metatarsal of the foot which is often suffered by young army recruits after long marches
Describe the relationship of the magnitude of applied load and the number of repetitions required to cause a fatigue fracture in terms of a fatigue fracture curve
If a graph is plotted of the applied load (y-axis) against the number of repetitions required (x-axis) for that load to cause a fatigue fracture, then it will look something like the graph shown in pic.
The graph illustrates that if the load is small (A on graph), then a great number of repetitions is required to cause a fatigue fracture, and if the load is large, (B on graph), then very few repetitions are required to cause a fatigue fracture.
A fatigue fracture will therefore only result when the frequency of repetition is too fast for the remodelling process, why is this?
- If the repetitions are well spaced then the bone will have time to remodel itself and repair any damage ==> the frequency of repetition (the number of repetitions in a given time) is also important when considering fatigue fractures
- For this reason fatigue fractures are usually only sustained during a continuous period of strenuous physical activity
During strenuous physical activity as the muscles become fatigued as the activity proceeds what happens to the likelihood of fatigue fractures and why?
.
- The likelihood of a fatigue fracture increases
- As the muscles fatigue They become unable to neutralise the tensile stresses exerted on the bones.
As previously mentioned bone formation and bone reabsorption is continually happening in the body.
In young adults describe the relationship between the rates of bone formation and reabsorption
In young adults this process is balanced so that the total amount of bone tissue does not alter.
In children describe the relationship between the rate of bone formation and reabsorption
There is more bone tissue formation than resorption as they grow and develop.
Along with the fact bone formation is > bone reabsoprtion in children state another difference between childrens bones and young adults bones
Children’s bones contain a greater proportion of collagen than adult’s bones.
What is the difference in material properties due to the higher proportion of collagen in children’s bones compared to adult bones?
It gives children’s bones more flexibility i.e. children’s bones are less brittle than adults
What type of fracture is more common in children than adults due to the greater flexibility of children’s bones ?
Greenstick fractures
Describe what a greenstick fracture is
It is an incomplete fracture whereby one side of the bone is bent (with the fracture on this side) and the other side is buckled
What type of loads are greenstick fractures usually caused by ?
Excessive bending or torsional loads
Between what ages does bone tissue begins to be lost as resorption exceeds formation?
35-40 yrs old
In people aged > 35-40 where bone tissue reabsorption exceeds formation, describe the changes in bone tissue
There is some thinning of the compact bone tissue and a larger reduction in the amount of cancellous bone tissue due to the thinning of the longitudinal trabeculae and the resorption of some transverse trabeculae
(in the pic the squares are the spaces)
The smaller reduction compact bone and large reduction in cancellous bone results in bones which are…
Only slightly weaker but significantly more brittle. Meaning that an elderly person will be more likely to break their bones then a young adult when subjected to the same loading
Describe the difference in a typical stress-strain curve for an elderly bone vs a young bone
The first sections of the curves are similar, however, the curve for elderly bone terminates at about half the strain of the young bone, illustrating the brittle nature of elderly bone. Note however that the actual strength of the elderly bone is only slightly reduced.
What is cartilage ?
A connective tissue consisting of cartilage cells, fibres and ground substance.
What are the 3 types of cartilage ?
- Hyaline cartilage
- Elastic cartilage
- Fibrocartilage
Where is hyaline cartilage found in the body?
Covering the articular surfaces of bones in synovial joints (articular cartilage), and amongst other things it forms the tip of the nose.
What is the main difference in the functional properties of elastic cartilage to other types of cartilage ?
It is more elastic than the other types
What does elastic cartilage form?
Amongst other things it forms the external ear and the epiglottis which covers the opening of the respiratory tract when swallowing
What does fibrocartilage form?
The symphysis pubis and the intervertebral discs.
What type of cartilage is articular cartilage and where is it found ?
It is a form of hyaline cartilage that is found on the articulating ends of bones in synovial joints.
(Pic shows the shoulder which is a synovial joint which has articular cartilage)
Define what a synovial joint is
Synovial joints are the very mobile joints, such as the hip and knee joints, that are held together by muscles and ligaments, and are enclosed in a capsule lined with synovial membrane containing synovial fluid
What is articular cartilage adapted to be able to withstand ?
Very large loads, up to several times body weight.
State the functions of articular cartilage
It cushions the bones while at the same time providing a smooth, lubricated bearing surface with minimal wear.
However, it is not hard and has a firm but fairly pliable consistency. This gives it its shock absorbing properties and allows applied loads to be evenly distributed over a large surface area, thus reducing contact stress (remember stress is equal to force divided by area) and wear.
Describe the appearance of articular cartilage
Glassy smooth, glistening and bluish white
