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Cross Sectional Area Measurement Device for Tendon Tension Calibration
Project Description:
Project Description:
For patients with spinal cord injury, the loss of upper limb function can be one of the most debilitating effects. Some relief can be found reconstruction of certain functions through a variety of surgeries known as “tendon transfers.” These transfers can greatly impact a patient’s quality of life, allowing for improved hygiene, pain relief, and prevention of joint contractures.
For patients with spinal cord injury, the loss of upper limb function can be one of the most debilitating effects. Some relief can be found reconstruction of certain functions through a variety of surgeries known as “tendon transfers.” These transfers can greatly impact a patient’s quality of life, allowing for improved hygiene, pain relief, and prevention of joint contractures.
Because of the structure-function relationships that underlie muscle physiology, an important decision that a surgeon must make is choosing the tension at which a transfer should be set.
Because of the structure-function relationships that underlie muscle physiology, an important decision that a surgeon must make is choosing the tension at which a transfer should be set.
This tension is currently measured very crudely; the surgeon pulls on a tendon until it reaches a tension that subjectively “feels right.” tendon until it reaches a tension
This tension is currently measured very crudely; the surgeon pulls on a tendon until it reaches a tension that subjectively “feels right.” tendon until it reaches a tension
The current device, seen in the photo above, does not self calibrate from tendon to tendon. If two tendons are placed under the same load and have varying cross-sectional areas, the measurement provided by the transducer will not be the same for both tendons, despite each tendon being under the
same load.
Objective:
The objective of this project is to develop a tool, which can be used intraoperatively, to provide a
reliable and quantitative measure of the tension in a tendon.
Results of Effort:
The team developed a device that may be used to calibrate the current buckle transducer.
Figure 1: Buckle transducer for testing tendon tension A) Buckle transducer, unloaded B) Buckle transducer with a tendon under tension
Figure 2: Final prototype in use.
Originally, the buckle transducer output was calibrated using calipers to make cross-sectional area measurements (CSA). However, this method does not provide a standardized method in measuring CSA because it is subjective to the users discretion due to the "squishy" nature of tendons. To reduce human influence in the measurement, the team decided to take advantage of the elastic and malleable nature of the tendons and allow them to deform to a fully defined shape through which a CSA could be obtained.Below is a diagram to give a visual description of this method.
Figure 3: Operating Principle behind device. Tendon is constrained and forced to deform to a known area. 1: Unloaded device. 2: Tendon is inserted into device. 3: Top jaw begins to slide down under uniform standard load. 4. Tendon deforms to the square space between jaws. A measurement of final h can be used to calculate CSA of tendon.
Through the use of a high accuracy Hall Effect sensor, h may be obtained and a value for CSA calculated. Please visit Final Design for more information.
Device Performance:
Testing the device with square rods of known cross-sectional area gave the following results:
The results of this table shows that the device is accurate, with some error associated with it.
Testing of the device on tendon samples was conducted at the University Research Center at UCSD.
CSA measurements of tendons were taken using the device, calipers and a graduated cylinder.
Located below is a table describing how well the device predicted the force applied to the tendon.
Pictured above are two plots that give a visual description of "force mapping". Using the cross-sectional measurements taken from
the G-machine and the calipers, the sensitivity of the buckle transducer may be determined. Dividing the the output voltage by this sensitivity gives
the measured force in the tendon. Results varied from tendon to tendon. On some occasions, the G-machine calibrated the buckle transducer quite
accurately, as shown by the plot for tendon FPL. Other occasions, the G-machine did not improve upon the force mapping the calipers provided,
as shown by tendon EPL.
Not only did our device improve the force mapping of the buckle device, but the device improved the precision of the CSA measurement.
The graduated cylinder is considered the "gold standard" for cross-sectional area measurements. As the table above shows, the graduated cylinder had the least amount of error in force mapping.
However, this is not a practical method for a surgery setting. Therefore, looking at the calipers and our device,
we have have reduced the overall error of estimating the force applied to the tendons.
Initially, the team was also exploring the possibility of an impedance based solution. Research led the team
to believe that an impedance based measurement device will also require calibration with regards to
the cross-sectional area of the tendon. Initially, an impedance based design was the primary focus.
Due to this conclusion regarding cross-sectional area, the calibration device has become the primary focus.
The plots above is the justification for developing a CSA device, rather than a device based on impedance measurements. For the same load applied to
a tendon, the electrical impedance of the tendons undergo different changes. Therefore, without proper calibration, this device would be unable to
predict the force applied to a tendon.
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