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.
