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Fracture Properties of Dry and Wet Chicken Bone: Three-Point Bending Test

Jing-Yuan Ma

April 25, 2007

Background:

In The Three-Point Bone Bending Experiment, chicken legs were harvested and tested for the change in yield stress while altering the deformation rate of the Instron crosshead. One concern of this previous experiment is the effect of harvesting the bones. When the tissues were removed from the bones, they are exposed to the air: dehydration begins to occur. During the original experiment, the bare bones were stored in wet paper towels to reduce the dehydration. However, some exposure and dehydration were inevitable due to long waits for the shared equipments. So, an expansion experiment is devised to test the possible effect of dehydration on the biomechanics of the chicken bones, especially yield stress. According to literature, the exposure of the bone to air will cause the bone to dehydrate and some mechanical characteristics to change significantly, such as stiffness (compressibility) in cortical bone [3] and bending strength of the skull [1]. In the original experiment, the chicken bones were tested for yield stress. When blood and other liquids in the bones evaporate, the bones loose some viscoelastic properties and become stiffer [3]. Now, this experiment is designed to see if bone dehydration may have caused the original yield stress data to have been erroneous. This follow-up experiment will also give a sense of how drastic the data may have been altered if dehydration does cause changes in yield stress of chicken bones. The results of this experiment will allow us to decide if a revision to the protocol for bone storage is in need.

Objective:

1. To determine the failure properties of yield stress of wet and dry chicken bones using a three-point bending test.

2. To determine the effects of dehydration of bones on its biomechanical properties.

3.To become familiar with measurements of strength of bones using three-point bending.

Equipment:

  1. Major equipment

Instron Model 4444 bench top materials testing machine

Customized Bending Jig (variable positions of beam supports)

Data acquisition computer

The Instron, the bending jig, and the computer will be used together to perform the three-point bone bending and the data acquisition of the experiments.

Fleetwood Heavy Duty Table Top Meat and Bone Saw 1HP motor B25

The band saw will be used (with extreme CAUTION) to cut cleanly and precisely the epiphyses off each bone. A band saw is preferred over other electronic and manual saws because the band saw can make smoother and cleaner cuts than other electronic saws, and it is also much faster and more precise in cutting than a manual bone saw.

  1. Lab equipment

Weight set (500g, 1kg, 2kg)

The weight set will be used to calibrate the Instron instrument.

Knives (sharp) and cutting board

The cutting tools will be used to harvest the chicken bones.

Electronic scale

The scale will be used to measure the mass of each bone to thousandth of a gram. Electronic is preferred over a triple-beam balance because it can acquire data faster, and it is easier to clean/maintain.

Length measuring instruments: calipers & rulers

The measuring instruments are used for measuring the necessary dimensions of the chicken bones, especially the cross-sectional area.

  1. Supplies

200 chicken legs; 10 per group

Each group will get 10 chicken bones for performing a two group comparison experiment. Each group now gets 5 more bones than original experiment to minimize the errors and discrepancies that come with collaborating with other groups.

Plastic containers

The containers will be used to store the control chicken bones in tap water.

Pipe cleaners

The pipe cleaners will be used to clean out the bone marrows. The dimensions of pipe cleaners (12”x6mm) are ideal for running through the chicken bone diaphyses and for pushing out the bone marrows.

Tap water

Tap water is used to keep the bones hydrated and to flush out the bone marrows.

Paper towels

The paper towels are for absorbing excess liquid from the bones and for cleanup uses.

Gloves and safety goggles

The gloves are needed to protect against bacteria from the raw chicken, and the goggles should protect the eyes from flying shards of bone.

Marker

The marker is used to put marks on the chicken bone.

Proposed Methods & Analysis: (Total Time of Experiment ~6hours)

  1. Bone Harvesting and Preparation (~45minutes)
  1. Two people will use the knives and cutting board to remove most of the skin, muscle, and connective tissues from the chicken legs. Store the bones in a plastic container fill with water.
  2. As the bones are being harvested, another person uses thesaw to cut off the epiphyses from all ten bones (see Appendix A).
  3. The fourth person uses the pipe cleaner and water to rinse out the bone marrows and to remove the remaining flesh on the diaphyses.The bones are now prepared and are placed back into the container of water to be kept hydrated.
  4. Remove one bone from the container and touch each end of the bone on a paper towel to drain off the excess liquid. Weigh the bone on the scale. Label the bone with a marker. Place the bone on a dry paper towel to dry. Repeat this step four more times. (n=5)
  5. Five bones should be kept submerged in the container, while the other five are left out to dry for four hours (change the paper towel when the old one becomes wet).
  1. Instron Set-Up (~5minutes)
  1. One person performs the Instron calibration steps with the set of weight as described by the BE210 Lab Manual [LM].
  2. Another person uses the ruler and finds the length of the shortest bones. If the length is greater than 11cm, set the distance between the two bottom supports of the three-point bending jig to 9cm. However, if the shortest length is less than 11cm, then set the supports to be 2cm shorter than that bone length. After setting the distance between the supports, DO NOT readjust the distance for the remainder of the experiment.
  3. Set the data point acquisition rate to 20 points per secondand the deformation rate to 25.4mm/min (1in/min). Keep these two settings the same for all ten bones.
  1. Bone Dimensions Measurement (~30minutes)
  1. Using the ruler, one person will mark the approximate center of all ten bone sample. Try to minimize the exposureof the wet bones to air.
  2. Now using the dial caliper, the one person will measure the outside larger and smaller diameters of the ellipsis-like bone at the marked center of each bone (see Appendix A).
  1. Wet Bone Three-Point Bending Test (~1hour)
  1. During the four hour wait for the dry-bone samples to dehydrate, the experiment on the five wet bones can be completed.
  2. Take one bone from the water, and place it into the Instron with the larger bone diameter horizontal to the ground (see Appendix B).
  3. Align the marked center of the bone directly under the single, top crosshead.
  4. Perform the three point bending test by following the steps listed in the Procedure (A) of the LM. Make sure to wear eye protection during the Instron tests.
  5. When each bone break, take the inside larger and smaller diameter from both fragments of bones (see Appendix A). Find the average of the two diameter values.
  1. Dry Bone Three-Point Bending Test (~1hour)
  1. After 4 hours have elapsed for dry bones to become dehydrated, weigh the five bones again on the electronic scale.
  2. Now perform the three-point bending test as described above in Step (D).
  3. Remember to measure the inside diameters of each bone after it breaks into two.
  1. Data Analysis
  1. Calculate the percent change in mass of the five dry bones. The quantitative change of the drying the bones should be given by change in density. However, since the volume of the chicken bone is difficult to measure without wetting the dried bones again, the quantification will be represented by change in mass. This assumes that the volume of the bone does not change when they are dehydrated. According to Lees, bones shrink when dehydrated by nanometers [2]. This minute change allows the volume to be assumed constant.
  2. Because the shape of the bone is not symmetrical, the cross-sectional areas of the bones can only an estimate. Since the bone shape is similar to an ellipse, the area can be estimated with the equation: Cross-Sectional Area = π/4*(A*B – a*b), where A and B are the outside diameters (larger and smaller, respectively) and a and b are the two inside diameters.
  3. Find the maximum force the Instron crosshead applied to each bone before failure.
  4. Calculate the yield stress of the ten bones by the equation:

Yield Stress=Max Force/Cross Sectional Area.

  1. Use Excel to find the variance of the dry and wet bone data sets (n=5 for each set). Check if they are within a difference of 5%: if the data sets are within 5%, then assume equal variance; and if they are greater than 5% different, then assume unequal variance.
  2. Perform a t-test: two sample assuming equal/unequal variance to see if the yield stress of the dry and wet bones are significantly different.

Potential Pitfalls & Alternative Methods/Analysis:

  1. During the three-point bending of the diaphyses, the cylindrical shaft may collapse before the whole bone snaps because the epiphyses are removed. One can stuff small rubber stoppers into the two ends to simulate the presence of the two epiphyses bearing load. Leaving the epiphyses attached is not the ideal because if the bone marrow is not washed out, the dehydration of the marrow will cause a major unwanted change in the mass of the bones.
  2. The assumption that the volume of the bones does not change when they are dehydrated may cause errors in the data because the volume does change (albeit minuscule). This will cause the quantification of thedehydration to be misrepresented. The problem with finding the volume for the bone is that easiest way requires finding the displacement of liquid. However, when the dry bones are submerged into the liquid, they are re-hydrated to some degree.
  3. Another problem is the calculation of the cross-sectional area. The bone cross-section is not symmetrical nor in a regular shape; thus,accurately finding the area is extremely difficult. By estimating the area, a huge error is incorporated into the calculation of the yield stress. One possible alternative is to create a digital model of center section of each bone by measuring and inputting many diameters around the bone. And use computer programs and advanced mathematics to generate a more accurate estimation.
  4. One potential problem is that the bones do not dehydrate enough in the four hours of exposure to the air because of damp environments. This problem can potentially thwart the entire experiment. The bones were left exposed to air in order to simulate neglect; however, for the sake of the experiment, one can use a heat source to facilitate the dehydration of the bones—although the situation is unrealistic in real life.
  5. Although the original experiment compared the yield stress of the chicken bones, this follow-up protocol does not have to be limited to yield stress. The data output and measurements taken is sufficient to find and compare the elastic modulus values of the dry and wet bones. There are two methods of finding the modulus. One way is by using MATLAB to convert the data into a stress-strain curve, and the slope of the linear region is the elastic modulus. The second method is by calculating the moment of inertia and by using the equation:

Ymax = - P*L3 / (48*E*I)

where P is maximum load and Ymax is the deformation associated with this load.

Budget:

$1,365.00 for one Fleetwood Heavy Duty Table Top Meat and Bone Saw 1HP Motor B25

from ACityDiscount Restaurant Equipment

The saw uses 110 volt power source and has cutting capacity of 8”x9”.

$ 1.58 for one pack of pipe cleaners, LEO65600 Chenille Stems 100 CT, Assorted Colors

fromAmazon.com <

Each pipe cleaner is 12" long and 6mm in diameter.

$ 80.00for 20 packs of fresh Purdue Chicken legs 10ct for about $4 each pack

from Fresh Grocer Supermarket.

$ 1446.58 Total Expenditure

References:

1.Currey, John D. "EFFECTS OF DRYING AND RE-WETTING ON SOME MECHANICAL

PROPERTIES OF CORTICAL BONE." Journal of Biomechanics 21.5 (1988): 439-441. EngineeringVillage. U of Penn, Philadelphia. 19 Apr. 2007. Keyword: dry wet bones.

2. Lees, Sidney. "Shrinkage of Bone." Mechanical Properties of Bioinspired and Biological Materials 844 (2005): 33-37. EngineeringVillage. U of Penn, Philadelphia. 19 Apr. 2007. Keyword: dry wet bones.

3. Silvennoinen, Raimo, Kaarlo Nygren, Markku Karna, and Kari Karna. "Holographic Measurements of Fresh and Dry Bone Elasticity." Optical Engineering 31.8 (1992): 1695-1697. EngineeringVillage. U of Penn, Philadelphia. 19 Apr. 2007. Keyword: dry wet bones.

Appendix A: shows the measurements of the four diameters needed to calculate the cross-sectional area of the each bone. The general positions of the cuts are also depicted.

Appendix B: shows the placement of the chicken bone with the longer diameter horizontal to the floor in the three-point bending jig of the Instron. It also shows the alignments of the top crosshead to the marked center of the bone, where the bone-dimension measurements are taken.