Modeling of Wound Closures under Uniaxial Tensile Loading
Dan Hoang, Tefesehet Mesfin, Lindsay Pordon, Ryan Stuckert
April 28th, 2006
101A_3
Background:
This experiment combines the Instron uniaxial tensile testing lab and the imaging lab. An inch cut in the middle of a rubber sample is closed using two different closure methods, staples and sutures. These two closure methods are then compared. First uniaxial testing is used to exert a force that would ultimately result in a failure of the closure method of the wound. From the data obtained using the Instron, the force vs. displacement graph is extracted just like in the tensile testing with chicken skin shown in Graph 1 of the Appendix. The maximum point of this graph is the failure point, a point of the maximum force at which the closure method remains intact. In the case of the chicken skin this was the point where the chicken skin failed to be unable to return back to its initial state. Statistical comparison of this force across the two different closure methods is done to determine if one closure method is significantly better suited for the wound than the other. Also important to consider, however, are the two main types of failures: punching through the rubber piece and complete tearing of the piece on the sides. Imaging is used to identify the type of failure. The other types of failure, considered potential pitfalls of the experiment, are bending of the staples and breaking of the sutures. In the imaging lab, the actual displacement from initial position of the wounds was extracted from the pictures. In this experiment, qualitative data of the type of failure is obtained from the images. The successive pictures taken will be compared side by side with the force vs. displacement graph in order to determine if one type of failure is dominant over the other. The experimenter then will decide upon which type of failure is most relevant to the particular wound closure.
Hypothesis/Objectives and Aim:
The aim of this experiment is to determine the optimum closure method out of the two used, sutures and staples. The closure method that requires a higher force for failure is considered the optimum closure, because it describes the more force-resistant material. The type of failure, whether punching or tearing sideways at failure point, would be analyzed at a future time.
It is hypothesized that the suturing method would result in a more optimum closure than the stapes under uniaxial tensile stress. The threads used in the suturing technique form loops which, as a result, close the wound from the front and back side. On the contrary, the staples only close the wound from the front side. Consequently, the sutures should be able to withstand a greater force than the staples.
From the limited amount of knowledge about the material properties of the rubber, it is difficult to hypothesize the type of failure to be expected. If the material exhibits the properties of fiber-like materials aligned parallel to the direction of force applied then punching through will result. If the fibers are aligned across the wound then tearing along the wound should occur. Lastly, if the rubber has a matrix-like structure it is not possible to determine the failure type. The successive pictures taken before and after failure while under uniaxial stress would clearly show the type of failure. The dominance of one type of failure over the other closure method would be analyzed. For example, if the rubber sample punches through all the samples with staples, and tears in the sutures, it is up to the experimenter to decide which method of closure for the wounds is better.
Equipment:
Major Equipment:
The InstronTM Model 4444 table-top mechanical testing machine has a pair of clamps, holding the top and bottom of the specimen. The bottom clamp will be fixed, while the top clamp will provide an upward force on the specimen as it rises away from the bottom clamp. This machine is connected to a computer; using LabViewTM software, it will measure the applied force on the specimen. The computer also allows the user to select the fixed speed, or crosshead speed, at which the upper clamp rises away from the lower clamp. Specifically, this machine will be preset to record the current load and extension of the upper clamp. These data values will be displayed on the computer screen in a plotted graph, and the data will be saved to a file for later analysis. For this experiment, the crosshead speed, direction of movement, and name of the file for the data to be saved in must be fed to the computer before you start each run. Finally, the Instron must be calibrated using a weight set before testing.
The CCD camera will allow for the tracking of marker positions on the surface of the rubber samples. The camera records points on each image using an (x,y)-coordinate system, which is based on the total number of pixels in x and y for the image. However, since this camera will only be used for recording the type and occurrence of tears in the samples, there is no need for calibrating this system. In this setup, the camera will be held fixed for each trial, while the zoom of the camera can be adjusted prior to each trial run. The camera is also attached to the computer to take and record images in accessible files for later analysis. In this experiment; use the CCD recording system after you have started each trial run.
The Instron will be used to record force and displacement data, while the CCD camera will be used to record the type and occurrence of failure. Specifically, the CCD camera will help address some of the potential pitfalls, as described later in this report.
Lab Equipment & Supplies:
In this lab, the following materials will be required: a ruler, scissors, a scalpel, a cutting board, a weight set, and a set of string & needles. The ruler will be needed to make the measurements of the rubber samples. The scissors, scalpel, and cutting board will be used to prepare the samples with a cut of standard length described in the Protocol & Methods section. The weight set will be used to calibrate the Instron machine. Finally, the string & needles will be used to suture the cut using the technique described below in the Protocol & Methods section.
Newly Purchased Equipment:
The newly purchased equipment will include rubber from McMaster-Carr, new clamps for the Instron machine, and a surgical stapler with surgical staples. These pieces of equipment and the reasons for purchasing them will be detailed later in the Budget section of this report.
Proposed Protocol & Methods:
1. Set up the Instron and camera as described in Labs 3 & 5 of the lab manual. However, be sure to set the camera up such that the image area is large enough to account for the increased size of each specimen once it has been stretched during the testing.
2. Cut 12 rubber samples into 2 inch by 4 inch strips. Also cut a 1 inch slit in the center of each sample such that this slit is parallel to the 2 inch sides of the sample, as shown to the right.
3. Using 2 rubber samples standardize your testing methods. Namely, run 1 trial using a sample that has been sewn shut with the pulley stitch method using 4 equally spaced stitches, and run a second trial using a sample that has been sewn shut with 4 evenly spaced staples. In determining the standard testing methods, you must decide on the appropriate crosshead speed that you will use for this experiment, make any final adjustments to the camera image area, and decide when and how often you will take pictures to provide an accurate depiction of the tearing sequence of events. These pictures will then allow you to determine the type and process of failure for each of your trials.
4. Suture the slit with the pulley stitch method using 4 equally spaced stitches for 5 of the remaining rubber samples. Then, using your standardized methods, load and test each of these samples individually. Be sure to load each sample such that the slit lays parallel to each clamp with a 1 inch separation from each clamp to the slit, as shown above. Also, be sure to use the camera to record how and when the sample failed.
5. Repeat Step 4, but use 4 evenly spaced staples instead of 4 evenly spaced stitches. Additionally, be sure to observe any bending or failure that may occur in the staples.
6. Analyze the results using a paired t-test to compare the force at the point of failure for the stitches against the force at the point of failure for the staples. Also, be sure to describe how each of the samples failed. Particularly, how did failure for the staples occur as compared to failure for the stitches, and did every sample within each of these categories, staples and stitches, fail in the same manner? Also, be sure to describe any problems inherent to the stitches or staples, such as bending of the staples.
Anticipated Results:
The goal of this experiment is to model the failure of sutures and surgical staples under uniaxial tensile loading in order to approximate which wound closure is optimal in real world situations. There are many ways for sutures and surgical staples to fail, including punching through of the closure, further tearing of the wound, deformation of the staples, and breakage of the sutures. This experiment will guide groups to collect certain quantitative and qualitative data regarding the structural and failure properties of each wound closure and allow each group of experimenters the flexibility to critically evaluate which properties are most important in a wound closure.
A criterion which groups might consider using to evaluate the capabilities of the wound closures is the ultimate force. Large forces can cause wound closures to punch through the flesh or further tear along the wound, so wound closures with low resistance to high forces might impose limits on a patient’s physical activities. The ultimate force, which is the maximal force a surrogate is able to withstand before complete separation of the two parts, can be used as a measure of resistance to high forces. By using the Instron Model 4444, large uniaxial tensile loading can be applied to the surrogate material to obtain force versus displacement data. The ultimate force is the force at the peak of the force vs. displacement graph as shown in Graph 1 of the Appendix.
After collecting 5 ultimate forces for each of the two wound closure types, a two-tailed t-test can be performed to determine if there is a statistically significant difference between sutures and surgical staples. Since most of the variables are controlled by using a uniform rubber material as a surrogate and fixing the placement of the slit and wound closure, any measurable variance between the ultimate forces should be a result of the inherent differences between sutures and surgical stables. A p-value of less than 0.05 for this two-tailed t-test will yield a significant difference in ultimate force. Each group can determine whether a statistically significant difference in ultimate force is enough to warrant a more optimal wound closure.
Another criterion which groups are able to consider is the type of failure for each wound closure. It is not enough to consider the ultimate force. Sutures that punch through under average forces could cause more damage than tearing along the wound, because it is more difficult to repair a wound with many small tears along different axes than one big tear along the same axis. It is expected that the rubber will tear along the arrows shown in Figure 1 of the Appendix. These qualitative observations should be made while the surrogate is being stretched in order to correlate significant moments in the process of tearing with the force vs. displacement graph. Examples of significant moments include initial tearing, breaking of the sutures, bending of the staples, full tearing, and full punching through. The camera can be used to capture and record the progress of tearing at these significant moments.
Using the quantitative data collected from the Instron and the qualitative data recorded, groups should be able to choose and explain which wound closure method would produce less damage to the rubber under uniaxial tensile loading.
Potential Pitfalls & Alternative Methods/Analysis:
When initially testing the machines and the rubber, the experimenters should see whether or not there is any slipping between the clamps and the rubber by making a mark on the rubber where the clamp comes into contact with it. If there is any slipping, friction should be increased, possibly by adding a paper towel around the rubber where the clamp is. The loading rate could also be decreased so that there is less chance of slipping. If slipping still occurs with the actual samples, but there is no ripping of the sutures or staples and the rubber is still intact, the sample can be loaded again. If the slipping does cause ripping or tearing, the qualitative data should be considered when discussing the optimal type of wound closure.
It is unlikely that all samples will be exactly the same, so the types of failure need to be considered. Failure could occur by a punching through of the sutures or staples, or by a tearing of the rubber at the sides of the slit. Since in a real world application, either of these situations would not be acceptable, both will be considered to be of the same failure importance so that the samples can be compared. When performing data analysis, the force to use for comparisons would be that at which full punching through or tearing of the rubber occurs. The type of failure can then be looked at using the camera, and it can be decided which of the failure types is less optimal.