Biomechanics Quiz 5

If the stress-strain plots of the same material tested in three orthogonal directions showed three different lines, then that means that the material is:

- Orthotropic

When trabecular bone is under compression, trabeculi (small strut-like structure) break gradually and fill the pores in-between. Thus, the stress-strain curve exhibits

- Rise in the end
- Region of constant stress over a wide range of strain

Bone consists of:

- Mineral phase (70% by weight)
→ calcium phosphate (hydroxyapatite)
→ calcium carbonate
- Organic matrix (20% by weight) with cells embedded
→ Type I Collagen (90%)
→ Other glycoproteins and glycosaminoglycans (10%)
- Water (10%)

Osteoblasts

- Specialized bone cells that form new bone tissue
→ Lay down new organic matrix, calcium phosphate, hydroxyapatite crystals get incorporated into matrix

Osteoclasts

- Bone resorption (secretion of acid to de-mineralize the bone and enzyme to break down the organic matrix)
- Specialized bone cells that resorb old bone tissue

Osteocytes

- Quiescent osteoblasts that are encased in the mineralized matrix, potential role as a sensor of mechanical loading on the skeleton

Mechanical behaviors of polymers are governed mainly by:

- Change in entropy

When subjected to planar, equi-biaxial, tensile forces (same magnitude in two orthogonal directions), a heart valve tissue with dominant fiber orientation in one direction will exhibit:

- Greater extension in cross-fiber direction

A material is said to exhibit elastic behavior if:

- The length of the specimen is the same before and after a cycle of loading and unloading
- The material recovers its original shape as load is removed
- The strain response to a stress input is instantaneous

Osteoporosis

- Disorder with decreased bone mass density due to loss of balance between bone resorption (by osteoclasts) and new bone formation (by osteoblasts) in bone remodeling (constant turnover)

Name a component of bone besides hydroxyl apatite minerals

- Water

Give an example of a preventative measure for osteoporosis

- Weight-bearing/Bone loading exercise
- Running or dancing

How many independent parameters (E,G, etc.) are needed to completely describe orthotropic behavior (normal and shear)?

- 9 parameters

How many independent material parameters are needed to describe isotropic behavior?

- 2 parameters

Under small deformation, bone exhibits the following behavior(s):

- Elastic
- Linear

How many independent components of strains exist under a state of plane stress (stresses are only in xy-plane)

- 4 components

A material is said to be incompressible if the Poisson's ratio is:

0.5

The space between the loading and unloading curve is known as hysteresis and it results from:

- Energy loss

If the stress-strain plots of a material tested in three orthogonal directions showed one line that is different from the other two that are identical, then that means that the material is exhibiting which of the following behavior?

- Transverse Isotropic

When subjected to linear, equi-biaxial, tensile forces (some magnitude in two orthogonal directions), a heart valve leaflet tissue with dominant collagen fiber orientation in one direction will exhibit a greater extension in cross-fiber direction, which is a manifestation of:

- Anisotropic behavior

Mechanical behavior of collagenous soft tissues (tendon) is governed mainly by change in entropy because of ___________ when they are subject to loading.

- Untangling of chain-like molecules

How many independent parameters (E, G, etc.) are needed to completely describe the LEHI behavior (both normal and shear)?

- 2 parameters

Under small deformation, cortical bone exhibits the following behavior(s):

- Elastic
- Linear

Cortical Bone

- Compact, mineralized bone, low porosity, found in shafts of long bones and shells of all types of bone

Trabecular Bone

- Less compact bone with high porosity
- "Cancellous bone"

When trabecular bone is under compression, because of its porous structure, the plastic phase of the stress-strain curve exhibits:

- Rise in the end
- Region of constant stress over a wide range of strain

Cortical Bone and Yield Stress over time

- Yield stress decreases with age

Osteoporosis Prevention and Treatment

- Bisphosphonate (boniva, fosamax to inhibit bone resorption) and other (bone-building) drugs
- Hormone replacement/supplement (estrogen, testerone)
- Regular weight bearing exercise and high impact activities (swimming v. gymnastics)
- Adequate dietary calcium and Vitamin D supplements

Mechanical properties of metals and ceramics are governed mostly by:

- Change in internal energy