Uploaded by Alex Clay

HW 1 Solutions S21

Name: _________________________
Instructions: Answer the following questions. For the questions where you will need more space (which is probably
most of them), please write on a separate sheet of paper.
Body Movements and Direction
1. (3 pts) Define the axes and planes of the body:
2. (3 pts) On the diagram above, the elbow would be ____proximal_____ (anatomical direction) to the fingers. The
neck would be ____inferior______ (anatomical direction) to the head. The toes would be ___anterior/distal____
(anatomical direction) to the heel).
3. (2 pts) What is the definition of:
Abduction away from body
Adduction toward body
Pronation rotation of the forearm so that the palm faces posteriorly (arms at side)
Supination rotation of the forearm so that the palm faces anteriorly (arms at side)
4. (2 pts) A tendon attaches ______muscle_______ to _____bone________; whereas a ligament attaches
____bone__________ to ____bone________.
5. (1 pt) During a dumbbell curl, what muscle group is the primary agonist? Antagonist?
Agonist: Biceps brachii
Antagonist: Triceps brachii
6. (1 pt) What is the function of a muscle synergist?
Aid agonist/antagonist by increasing force and reducing unwanted movements
7. (3 pts) Name at least 5 muscles (not muscle groups) that would be involved for the following movements.
Provide a brief (one sentence or less) description for the role each muscle plays during that movement. Use the
appropriate muscle names, anatomical plane and movement terms in your description when appropriate (there
are multiple correct answers for these questions):
a. One arm punch: multiple answers; need to write muscle and role that muscle plays in the action
b. Lunge: multiple answers
1. (5 pts) Sketch a long bone and label the following: epiphysis, diaphysis, cancellous bone, cortical bone,
medullary canal, periosteum, endosteum, articular cartilage.
2. (2.5 pts) What are 5 functions of bone?
place where blood cells formed
storage reservoir for calcium
supports body
provides lever system
provides protection
3. (1 pt) Define osteocytes, osteoblasts, and osteoclasts.
Osteocyte: bone cell located within the lacuna in the bone
Osteoblasts: cells that build new bone – eventually develop into osteocytes
Osteoclasts: cell that resorb bone
4. (2 pts) What is an osteon? How are the collagen fibers oriented in the lamellae of the osteons? Why is this
important from a structural standpoint?
Osteon: the basic unit of structure of compact bone, comprising a haversian canal and its concentrically
arranged lamellae. Collagen fibers are oriented in parallel fashion in concentric rings. The collagen fibers in
successive rings alternate direction, producing a more mechanically strong structure. See the diagram below:
5. (2 pts) How is apparent density different than regular density measurements? What is the difference in
apparent density between cancellous and cortical bone (a range of values is acceptable)?
The primary difference is in the degree of porosity
Dense cortical bone has a porosity of about 5% and an “apparent density of about 1.9 g/cc
Cancellous bone porosity varies considerably but is often between 60% and 95% with “apparent density” of 0.8
and 0.1 g/cc
Apparent density: the mass per unit volume of a material, taking into consideration any voids
6. (4 pts) Explain the key differences between bone modeling and remodeling. In your own words, describe the
steps of bone remodeling.
Difference 1: modeling is responsible for adding bone; remodeling for maintaining or removing bone
Difference 2: modelling bone resorption and formation occur in an uncoupled manner and on separate surfaces
remodeling: bone remodeling is based on the coupled and balanced activities of bone resorption and formation
that occur in packages of cells along specific sites on the same bone surface
1. Activation: pre-osteoclasts are stimulated and differentiate under the influence of cytokines and
growth factors into mature active osteoclasts
2. Resorption: osteoclasts digest mineral matrix (old bone)
3. Reversal: end of resorption; pre-osteoblasts infiltrate the site
4. Formation: osteoblasts synthesize new bone matrix
5. Quiescence: osteoblasts become resting bone lining cells on the newly formed bone surface
7. (3 pts) What are three assumptions that are commonly made about bone when describing their mechanical
properties (when trying to model bone as a simple material)? Discuss why each assumption is incorrect, but
why they can be justified for certain applications.
Anisotropic: properties are directionally dependent – must be aware of the direction of loading and use the
appropriate material properties – ex. Cortical bone twice as strong in the longitudinal rather than transverse
Inhomogeneous: not uniform – but difficult to quantify and can use homogenous assumption knowing that
there is error
Viscoelastic: exhibiting both viscous and elastic behavior when deformed – typical strains are well within elastic
8. (3 pts) Draw a stress-strain curve for cortical and cancellous bone in compression, identifying the location and
appropriate values for the young’s modulus and ultimate compressive strength.
9. (8 pts) The cross-section of the distal end of a right-side femur is rigidly fixed to the top of a rigid base and
loaded with two different forces (see diagram below). Force 1 is acting vertically downward, 4 cm from the
neutral axis in the coronal plane and 0 cm from the neutral axis in the sagittal plane. Force 2 is acting directly
out of the paper, in a posterior-to-anterior direction. Point A is on the outer face of the bone in the coronal
place. Point B is on the outer face of the bone in the sagittal plane. 1.) Determine the axial stresses at A and B.
2.) Determine the bending and shear stresses (torsional and direct, potentially) at A and B.
3.) Calculate the total normal (compression or tension) and total shear stress at A and B.
Assume the bone is a thick-walled tube for your calculations. Clearly state any other assumptions that you need
to make in order to solve this problem. Use the directions in the table provided below when expressing your
Torsional Shear
Direct Shear
Axial Stress
Bending Stress
CW or CCW looking superior to inferior
Compression or Tension
Compression or Tension
10. (5 pts) What is a stress invariant? The principle stresses at point A are (Sigma1=26N/m2, Sigma2=-24N/m2,
Sigma3=2N/m2) and for point B (Sigma1=23N/m2, Sigma2=23N/m2, Sigma3=24N/m2). Calculate the hydrostatic
stress and octahedral shear stress for points A and B. The material at point A and B started out as cartilage. If A
and B are exposed to the stresses calculated above in a cyclic manner, what material will likely form in each
Stress invariant is the stress at a point that does not depend on a coordinate system. These are the principal
11. (5 pts) Now assume that points A and B lie on the edge of a long bone on a healthy adult human. Using the
calculated hydrostatic and octahedral shear stresses calculated above, calculate the maximum cyclic energy
stress for A and B. Using these values, calculate the daily stress stimulus (assuming this is the only type of
loading on this location) assuming 1500 cycles/day, and an empirical weighting constant of 4. If the lazy zone for
bone modeling/remodeling is 150±55 N/m2, is bone being resorbed or deposited slowly or quickly at A and B?
Assume Poisson’s ratio for bone is 0.15.
12. (2 pts) Describe at least 4 differences between cancellous and cortical bone.
Cancellous bone
a. No osteons
b. Arranged in trabeculae
c. Major type of tissue in short, flat, irregular bones
d. Much lighter (and less dense) than compact bone
e. Not found in diaphysis
f. Supports red bone marrow
Cortical bone
g. External layer
h. Arranged in osteons
i. Lamellae are found around periphery and between osteons
j. Central canals connected to each other by perforating canals
13. (2 pts) What are the lacuna and canaliculi?
Osteocytes are found in spaces called lacunae
Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next cell
14. (2 pts) Describe the difference between primary and secondary bone?
a. Primary bone – requires a pre-existing substrate, such as cartilaginous model
i. Seen in endochondral ossification
b. Secondary Bone – product of resorption of previously existing bone tissue and the deposition of new bone to
replace it
i. Seen in more mature bone
15. (1 pt) What is woven bone? What functions does it serve?
Woven Bone: characterized by randomly oriented collagen fibers.
i. Can be deposited without any previous hard tissue or cartilage model.
ii. Provides quick framework prior to lamellar bone
1. First step of ossification at growth plate during endochondral ossification
2. Callous formation during fracture healing
16. (3 pts) Name the fracture type for A, B, and C? Under what loading environments were each formed?
A: Spiral – torsion
B: Transverse or Oblique: Direct high impact or high off-axis compression
C. Greenstick: Bone bends and cracks instead of breaking all the way through: most commonly seen in children
b/c their bones are more flexible
17. (3 pts) In your own words, describe the steps for intramembranous ossification?
18. (3 pts) In your own words, describe the steps for endochondral ossification?
19. (3 pts) In your own words, describe the steps for secondary fracture healing?
20. (2 pts) Describe the key differences between Intramembranous and Endochondral ossification.
Intramembranous Ossification
a. Development of flat bones in the body
b. Bone growth through apposition of bone on membrane – cartilage model is not present
Endochondral Ossification
c. Development of most bones in the body – classically represented in long bones
d. Bone growth preceded by cartilage
21. (1 pt) Why is it important for long bones to have a growth plate? What function does the growth plate serve?
Bone lengthening. Cartilage is deposited on the epiphysis side while cartilage is ossified (and bone lengthens) on
the diaphysis side
22. (2 pts) From a mechanical standpoint, why does the bichondylar angle of the femur form in walking children.
Uneven loading between the medial and lateral side of the femur due to center of mass location → high loads
on the medial side → uneven loading leads to added bone deposition and quicker ossification on the medial side
of the bone, tilting the bone out toward the lateral side, forming the bichondylar angle
23. (2 pts) In general terms, what is Wolff’s Law and why is it important?
structure is not only well designed for its function but will adapt to create a new structure in response to
changes in function
24. (2 pts) From a mechanical loading standpoint, why is the periosteum important for developing straight long
Compression by the periosteum influence progenitor cells to signal osteoclasts to resorb bone.
Tension by the periosteum induce progenitor cells to differentiate to osteoblasts, which deposit bone.
25. (2 pts) From a loading standpoint, why do long bones have abnormal cross-sectional profiles in certain areas?
Long bone cross sectional shape is determined by osteoblast/osteoclast activity on periosteal and endosteal
Influenced by local forces normal to the shaft
a. Tension causes bone deposition (e.g. ridges at muscle insertion sites)
b. Compression (e.g. contraction of muscle bellies) inhibits bone deposition
Influenced by intracortical stresses/strains created in the mineralized tissue
c. The “sensors” are probably the osteocytes that signal surface cells
d. One theory is that tiny “hairs” (cilia) on the bone cells can sense fluid movement caused by loading,
signaling the proper mechanism for bone remodeling
26. (2 pts) What would happen to the cross-sectional profile of an adult human long bone if that person were to be
placed on long-term bed-rest? Sketch the new cross section.
27. (2 pts) What would happen to the cross-sectional profile of an adult human long bone if that person were to
move to an environment with 125% the gravity of earth? Sketch the new cross section.
28. (2 pts) Would you expect to see a difference in bone density between the dominant and non-dominant arms of
a professional baseball pitcher? Would you expect to see a difference in bone density between the dominant
and non-dominate legs of the baseball pitcher? Why or why not?
Difference in arms due to uneven loading. A case could be made for or against the legs having different
density...depends on uniformity of loading. If legs unevenly loaded, then there would be a difference; if not,
then no difference expected.
29. (1.5 pts) Name three conditions needed for primary fracture healing to occur?
1) Rigid fixation
2) Direct bone to bone contact
3) Blood supply
30. (2 pts) What are two conditions that could lead to a pseudoarthrosis?
Too much movement and a poor blood supply at the site of bone fracture.
31. (2 pts) What is the definition of bone quality? What is the clinical significance of this measure?
a. Defined by the following parameters
i. Architecture:
1. Microarchitecture - including organization of trabecular microstructure and microporosity
2. Macroarchitecture – including bone geometry and cross sectional area
ii. Turnover: remodeling dynamics, increased rate of turnover or unbalanced turnover
iii. Damage accumulation: accumulation of microcracks due to unrepaired fatigue damage, insufficient
repair by remodeling
iv. Mineralization: tissue material properties, mineralization profile, extra-cellular matrix properties, and
chemical consistency
b. One way bone quality is used clinically is to determining a person’s fracture risk
32. (1 pt) What is osteoporosis? Why are women at a higher risk? What is a way to combat osteoporosis?
a. Osteoporosis – associated with aging; occurs when bone resorption occurs at a higher rate than bone
b. Women at a higher risk largely due to accelerated bone loss during menopause
c. Calcium intake & building bone mass before deterioration begins
d. Low impact over time as to not cause fractures, but utilize Wolfs Law (astronauts’ primary way to combat
33. (2 pts) Briefly explain why a simple hinge is not a good design for a human knee prosthesis.
A hinge does not reproduce the 6-degrees of freedom found in the native knee. The native knee also “locks”
when extended during walking through what is called a “screw-home” mechanism. This provides additional
stability to the joint