I. Introduction
1.1 Transtibial Prostheses
Over 40,000 transtibial, or below-knee, amputations are performed annually in the United States.
While prosthetic limbs help restore mobility and normal
appearance to the amputee, the skin and underlying tissue
at the residual limb do not react well in load-bearing
activities. The prosthetic socket plays a crucial role in
assisting with load transfer and providing stability at the
prosthetic interface. Thus, it is important that the socket is
designed to optimally distribute the load to the residual
limb to maximize comfort to the patient and ensure proper
movement. A typical prosthetic contains a socket, foot, and
liner as shown in Figure 1.
Many studies have been done to elucidate the
mechanism through which discomfort arises at the
prosthetic interface in an attempt to develop effective
sockets. However, patients continue to report pressure
ulcers, blisters, cysts, edema, skin irritation, and dermatitis
due to poor fitting of the socket (Polliack, Sieh, Craig,
Figure 1: A prosthetic system consists
of a foot, liner, and socket.
Landsberger, McNeil, & Ayyappa, 2000). It has been
demonstrated that knowledge of the biomechanical factors acting at the prosthetic interface is crucial to
the development of comfortable sockets, such as the patellar tendon bearing (PTB) transtibial socket that
transfers the load on pressure-sensitive regions to the pressure-tolerant regions of the residual limb
(DeLisa, 2005). Since the 1960s, the PTB has internationally been the most commonly accepted standard
transtibial socket (DeLisa, 2005). Another method aimed to increase comfort to the patient distributes
pressure more evenly across the entire limb, which is employed by the total surface-bearing (TSB) socket.
Regardless of the method, it is important to determine an accurate means of characterizing the mechanical
interactions at the interface.
Studies quantifying the stresses at the prosthetic interface have been conducted for over 40 years
(Zhang, Turner-Smith, Tanner, Roberts, 1998). Computational modeling using finite element analysis is
a common and simple method for predicting stresses and strains at the interface in that estimates of
pressures at any region of the limb can be made, and elaborate experimental setups are not required.
However, while convenient, computational models do not account for all potential factors at the interface,
1
specifically the biological tissue reactions to loading (Mak, Zhang, & Boone, 2001). Thus, computational
modeling may simply serve best as a means of verification of experimental data.
Experimental methods involving the use of sensors to quantify the stresses at the prosthetic
interface are more desirable in that measurements obtained from actual trials incorporate all factors
contributing to discomfort at the interface. However, devising an accurate measurement system remains a
challenge. Many factors, such as the size of the sensor, the type of force transducer, and differences in
pressure dynamics between types of movement, often compromise the accuracy of stress measurements
(Mak et al., 2001). For example, too large of a sensor provides an inaccurate average of too large an area
of the limb, while sensors that are too small produce undesirable edge effects (Mak et al., 2001). Thus,
careful consideration of these factors is necessary in experimental procedures.
Researchers have been interested in quantifying two primary stresses at the prosthetic interface:
shear stress, resulting from frictional forces between the socket and skin, and interfacial pressure,
resulting from load-bearing activities. While both types of stresses contribute to patient discomfort, most
studies have predominantly focused on interfacial pressures, and few studies have successfully quantified
shear stresses due to inaccurate measurement transducers (Zhang et al., 1998). Furthermore, it is thought
that minimizing shear stresses may not be desired. One study demonstrated that smaller coefficients of
friction effectively reduced shear stresses but led to an increase in normal stresses in order to support the
same load, thus perpetuating damage to the underlying tissue of the residual limb (Mak et al., 2001).
Thus, some amount of friction may be desirable in order to help support the load on the limb.
Additionally, a gel liner, or the layer that fits in between the prosthetic socket and residual limb, is used to
help reduce shear stresses and increase comfort (DeLisa, 2005). Thus, the predominant need in the
development of effective transtibial socket lies in the accurate measurement of interfacial pressures.
1.2 The Surgical Clinic
The Surgical Clinic in Nashville, TN specializes in fabricating prosthetic devices and sockets for
amputees. Socket-fitting is conducted on a case-by-case basis, according to the patient’s qualitative
feedback and established pressure-sensitive and pressure-tolerant regions of the residual limb. Yet,
although each socket is tailored to the specific needs of the patient, reports of pain and discomfort are
common, necessitating refitting procedures that constitute a hefty portion of the costs for each prosthesis.
The process for constructing a transtibial socket at The Surgical Clinic begins with an evaluation
of the patient for several factors, including skin type and integrity, limb measurements, range of motion,
hand dexterity, fine motor skills, gross motor skills, cognition, etc. Then, the material for the gel liner is
selected. Depending on the limb shape, off-the-shelf liners or custom-made liners can be used. The most
common options include the urethane, thermoplastic elastomer, and silicone liners. After the gel liner is
selected and fit to the patient, a cast is measured and constructed over the liner, either by hand or using
2
CAD-CAM digitizing. The negative cast is then modified by hand or using computer modeling before
the positive check socket is constructed. The positive check socket can be made from a number of
materials, which have been selected by the prosthetists at The Surgical Clinic based on qualitative
characteristics of the material. For example, Polytol, a high-quality material that is polyurethane-based
and adheres well to the skin is the most ideal material for use in constructing the socket. After the
positive check socket is constructed and assessed in both static and dynamic conditions, the final
laminated socket is fitted to the patient (A. Fitzsimmons, personal communication, January 10, 2011).
The entire process is both time-consuming and costly, taking over a month and averaging around
$2500 each time. The heavy reliance on qualitative assessments of material characteristics and patient
feedback throughout the process results in patient discomfort and numerous refitting procedures down the
line. As material costs are fixed, it is worthwhile to attempt to reduce the number of refittings necessary
after a socket is made. In fact, The Surgical Clinic spends approximately $180,000 a year on refitting
procedures. Thus, transforming the socket construction process from a qualitative one to a more
quantitatively-based method is vital.
1.3 Goals
In identifying our goals for the project, it was necessary to consider what could reasonably be
accomplished in the given amount of time (around eight months). It was decided that a long-term study
with one patient, rather than few trials with multiple patients, would provide more useful results. Thus,
the overall goal was to develop a novel socket system that would optimally distribute the pressures on the
residual limb for the patient in our study. If successful, the measurement system could then be
implemented in constructing sockets for other amputees as well.
In working towards the overall goal, several objectives were established. First, an appropriate
and accurate system for acquiring pressure measurements needed to be designed. The measurement
system needed to be reliable, as several trials were to be conducted with the patient, it needed to minimize
interference with the prosthetic interface, and include an appropriate force transducer. Second, using the
measurement system, the patient would be tested under several conditions—standing on both legs,
standing on the prosthetic leg, and walking—and using all combinations of the gel liner and prosthetic
foot in order to establish a pressure distribution across the prosthetic interface. Finally, after examination
of the data, the socket system, comprised of the liner and foot that most effectively reduces the pressures
on the limb and a newly constructed socket transferring the peak pressures from the pressure-sensitive
regions to the pressure-tolerant regions of the residual limb, would be designed. The use of a systematic
measurement procedure and subsequent socket redesign should effectively reduce The Surgical Clinic’s
costs by lowering the number of refittings necessary for each amputee, while improving the utility and
comfort of the prosthetic socket.
3
II.
Methods
2.1
Determination of Data Acquisition Method
After thorough research of the technology used to acquire force measurements, Force Sensing
Resistors (FSRs) were determined to be the best device due to their thin structure and reliable decreases in
resistance with increases in force. FSRs (Model
402), produced by Interlink Electronics (See
Appendix A1.1) with a .5” diameter were chosen,
because they were the largest size that did not
contain undesirably cumbersome leads that could
cause skin abrasions or inadvertently redistribute
pressures within the socket. They were also small
enough that the contact interface between the
Figure 2: Current to Voltage Converter
active area of the FSR and the subject’s skin
would be approximately flat. This was necessary to avoid false triggering due to bending of the FSR.
A current-to-voltage converter (Figure 2) was selected as the best method to convey changes in
FSR resistance to recordable output voltages due to the direct proportionality between the two quantities,
as calculated by the following equation:
The FSR Integration Guide (“FSR Integration Guide,” 2010) stated that a VREF of 0.1V to 5.0V was
optimal for the FSRs; VREF was selected to be 5.0V in the measurement circuit to minimize the signal to
noise ratio of the recorded signal. Initially, gain resistors (RG) of 470Ω were chosen so that the expected
maximum pressure, obtained from a literature review, would not exceed the maximum possible output
voltage of the circuit (10V) (Smith, Michael, & Bowker, 2004). However, most of these gain resistors
were switched to 680Ω after a preliminary test with the patient at The Surgical Clinic revealed that most
of the regions tested in the socket needed higher gains to optimize sensitivity.
2.2 FSR Calibration
4
Each FSR was calibrated in order to convert the voltages obtained from the testing to pressure
values. After the necessary calibration ranges were determined from a preliminary test with the subject
amputee, ranges of fifteen different weights were selected for each gain resistor. Circuits using the 470Ω
resistor were calibrated with weights ranging from 100g to 4500g, and circuits using the 680Ω resistor
were calibrated with weights ranging
from 100g to 3500g (See Appendix
A1.2 and A1.3). These calibration
weights, borrowed from the Vanderbilt
Physics Department, were placed
sequentially in a basket weighing 250g
that was balanced on a rubber stopper
and centered on the FSR. The circular
rubber stopper had a diameter
approximately equal to that of the
circular active area of the FSRs, which
allowed the force from the calibration
weight to be evenly distributed across
Figure 3: The set-up for calibration. The basket contained
the free weights.
the region of the active region (Figure
3). A voltage reading was acquired five times at each mass value for each FSR. After the first 1200
calibration measurements demonstrated that the standard deviations between each repeated mass value
were typically less than 0.1V, only three voltage readings were taken. The mass of the weights was then
converted to pressure using the following equations:
𝐹
𝐴
𝑃 = , 𝐹 = 𝑚 ∗ 𝑔, 𝐴 = 𝜋 ∗ 𝑟 2
𝑃 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒, 𝐹 = 𝐹𝑜𝑟𝑐𝑒, 𝑚 = 𝑚𝑎𝑠𝑠, 𝐴 = 𝑎𝑟𝑒𝑎, 𝑟 = 𝑟𝑎𝑑𝑖𝑢𝑠, 𝑔 = 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 (9.8
𝑚
)
𝑠2
Pressure(kPa)
Voltage was then plotted as a function of
400
350
300
250
200
150
100
50
0
y = 4.7439x2 + 9.0666x + 29.264
R² = 0.9953
pressure for each of the FSRs (Figure X). A linear
relationship was fairly accurate for the lower values of
pressure but was less accurate at higher values. It was
determined that the pressure and voltage had a
polynomial (power of 2) relationship due to the high R2
value for this relationship (R2 was 0.9953 for the FSR in
0
2
4
Voltage(V)
6
Figure 4: Pressure as a function of Voltage.
A second degree polynomial relationship is
seen.
8
Figure 4). An equation from the calibration curve of
each FSR was determined and used to convert voltages
5
to pressures for the data acquired from testing. A calibration curve was made for each FSR to account for
the variability between each FSR.
2.3 Testing Subject
One volunteer amputee participant was used in the study. This participant was a middle-aged
male who has been assisting in the production of prosthetic sockets for several years. He was chosen due
to his prior knowledge of prosthetics and ability to easily switch between the socket and leg combinations
used in the study. The participant is a K-4 Medicare level, which means he has “the ability or potential
for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high impact, stress, or energy
levels” (“K Levels,” 2005). Due to time constraints, more subjects could not be tested. Unlike most
transtibial amputees, the test subject was capable of bearing high pressures at the distal end of the limb
and preferred to have high pressures in that region. When developing the new socket, pressures were to
be minimized in this region to make the design fundamentals applicable to a larger proportion of
transtibial amputees.
2.4 Materials
A typical lower limb prosthesis consists of three
components: a foot, liner, and socket. The three most
common feet used in industry today are the Vertical Shank
ESF, Flex-Foot & Multiaxial Ankle, and SACH foot. The
three most common liners are the thermoplastic elastomer,
urethane, and silicone liner (Figure 5). The Vertical Shank
and Flex-Foot have moderate shock absorption capacities,
and the urethane and silicone liners are thicker than the
thermoplastic elastomer liner. Before designing a new socket,
Figure 5: The three most common
liners from left to right: urethane,
thermoplastic, and silicone.
the best liner and foot combination had to be determined. In
order to improve the current design, the best option currently available to patients had to be determined as
described in section 2.8.
2.5 Regions of Interest
After researching the regions of the transtibial amputee residual limb, it was determined that data
should be obtained for both pressure tolerant and pressure sensitive regions of the limb. Eight regions
were tested due to the limitations because there are only eight available channels on the NI data
acquisition interface. Mr. Fitzsimmons aided in the selection of these regions based on his previous
experience. Four of these regions shown in Figure 6 are pressure-sensitive regions: the distal end of the
limb (2), the fibula head (6), the distal tibia (7), and the tibial tubercle (8). The other four regions shown
in Figure 6 are pressure tolerant regions: the center of the gastrocnemius muscle belly (1), the medial
6
tibial flare (3), the mid patella
tendon (4), and the tibialis
anterior (5). The FSRs were
secured to the participant’s skin
with Scotch tape during testing.
Mr. Fitzsimmons placed the
FSRs during our first trial due to
his knowledge of the anatomy of
the residual limb (See Appendix
A1.4) Pictures of the regions
(see Figure 6) were taken and
referred to in all later trials to
Figure 6: A FSR was placed on each of the eight labeled regions.
The pictures in order from left to right show the right lateral
(outer) view, the anterior view, and the left lateral (inner) view of
the residual limb. A portion of the distal end is shown in the
picture on the right. The posterior location of the gastrocnemius
muscle belly is not shown.
ensure proper placement of the
FSRs in the same regions for every trial.
2.6 Setup
One breadboard was used for throughout the testing and calibration procedures. The breadboard
contained eight individual circuits (current-to-voltage converters), with each circuit powered by a power
supply. The output of each circuit was read out by LabVIEW using a BNC cable to connect the circuit to
the NI Instruments Box. Each FSR was connected to its corresponding circuit using seven feet of 22
AWG, RoHS compliant wire. The electrical connections to the FSR leads were made with conductive
silver epoxy from MG Chemicals, since solder would have damaged the FSRs. Since this connection was
fragile, each FSR-to-wire connection was sealed with a thin, smooth layer of high strength nonconductive
marine epoxy.
An initial attempt was made to establish
an easily transportable LabVIEW program using
the National Instruments CompactRIO system. In
addition to its mobility, the CompactRIO was
chosen based on its ability to interface with most
computers and the high number of available
channels for recording the signals from each FSR.
It was essential that our measuring system be able
to move easily because we planned on testing at
Figure 7: Output from all eight channels seen in
Labview program. The data is outputted as voltage
vs. time.
The Surgical Clinic. However, numerous
problems were encountered regarding the
7
implementation, setup, and reliability of this device. Thus, the NI BNC Connector Block used in
Vanderbilt’s Biomedical Engineering Instrumentation laboratory was used to digitally acquire the output
signals of the FSRs. As shown in Figure 7, a LabVIEW program was created to display and save the
voltage versus time data from each test. The large window of the program displayed the original signal
from the FSRs. The windows beneath displayed the inverted signals by FSR grouping so that it was
possible to view the results of a particular FSR and quickly assess if the results and determine if it
malfunctioned. These signals were recorded at 1 kHz so that there would be adequate single sampling for
the portions of walking trials where pressures changed very quickly. These trials were saved in a format
that could be opened in excel to apply the calibration curves to the voltage versus time data. An excel
template file was created to automatically convert the voltage data into pressures, create a graph of the
trial data, and calculate the necessary statistics for analysis.
2.7 FSR Replacement
Each time that an FSR broke during testing, as determined from viewing the LabVIEW module,
the particular FSR was replaced with a new FSR. Each new FSR was calibrated, and trials conducted
with a broken FSR were repeated.
2.8 Trial Procedure and Analysis
To determine the optimal combination of liner and foot to be used in the new socket system,
pressure recordings were acquired for each of the nine possible combinations of three liners and three feet.
The sockets used for each of these trials were made of the same material and had the same shape, but
minor adjustments were made due to the thickness of the liners, which was not significant enough to alter
the results. For each combination, three trials each were recorded for the patient walking two mph on a
treadmill, standing on the prosthetic limb only, standing on both legs, and standing on the normal leg only
with the prosthetic leg suspended. At least one trial was recorded with the subject wearing the liner
without the prosthetic limb attached before putting on the leg for the walking and standing tests. Since
the FSRs are prone to significant long-term drift but are stable over short periods of time, the subject
removed the prosthetic limb in between each trial to reset the FSRs. Furthermore, the standing trials were
limited to three seconds in length, and the walking trials were limited to nine seconds in length.
Once the trials were recorded, they were copied into an excel template file that converted the data
into pressures and calculated the minimum pressure, maximum pressure, and average pressure recorded
for each FSR during the trials. These summaries are included in the appendix for the standing trials. The
statistics for the walking trials were manually extracted. The peak pressures for each FSR during each
gate cycle were recorded and compiled. The average pressure was calculated from an established point
on the first recorded gait cycle to the equivalent point on the last gate cycle for each walking trial.
8
Similar procedures were followed to measure the redistribution of pressures in the sockets that
were redesigned after analyzing the data acquired from testing the first nine combinations of sockets and
liners. A minimum of five trials for each new socket were conducted with the subject standing on both
legs for three seconds and walking at two mph for nine seconds. This was also done for the optimal
socket and foot combination with an original test socket and with the socket, liner, and foot that the
subject normally wears. The average of a 0.25 second segment of the data for each standing trial was
calculated to assess the persistent distribution of forces during normal stance. A 0.25 second segment was
used to avoid large fluctuations due to the subject shifting weight while standing. The average and
maximum pressures for the walking trials were manually acquired by the same methods used when
analyzing the first nine combinations of sockets and liners.
2.9 New Socket Materials
When designing the new sockets there were two main factors of the socket that were manipulated:
contour and material. In order to relieve pressure in certain regions or add pressure in other regions the
contour can be changed by cutting some of the socket away, which will relieve pressure in that specific
area because there will no longer be material there. It will also serve to
add pressure in other areas. Typically the contour of the socket is
manipulated towards the top of the socket since cut outs cannot be made
in the middle or at the bottom of the socket while maintaining suction.
The materials can be manipulated throughout the socket. The socket
can have rigid and soft regions in specific areas in order to add or
relieve pressure. Our new sockets included manipulation of both
contour and material.
2.9.1 New Socket 1 (NS1)
The data acquired from the trials with the participant’s current
prosthesis and the combination that was determined the best from the
foot and liner testing showed that the pressure-sensitive regions had
some of the highest pressures. Therefore, the goal was to decrease
Figure 8: New Socket 1
(NS1) with void visible
in the lower left area of
the socket.
pressure in the pressure-sensitive regions by transferring the pressure to
the pressure-tolerant regions. In the first design, a void was created at the distal end of the limb (Figure 8)
due to the high pressures that were consistently acquired in this region throughout the trials, which would
give the limb more area to dissipate pressure. Also, the socket had both an inner flexible region made of
thermaline soft and an outer region consisting of carbon fiber.
2.9.2 New Socket 2 (NS2)
9
The second socket was designed with the same goal as the first socket: to decrease the pressure in
the pressure sensitive regions. However, different materials were used this time, with less rigid material
at the top (See Appendix A1.5). The material in NS2 was not cut in the same manner as in NS1 in the
fibula head, as the first socket had significantly increased the pressure on that region in an attempt to
relieve some of the pressure on the distal end of the limb. The flexible material in this socket was proflex
with silicone, which is much more flexible than the thermaline soft used in the first socket. This design
also had an extra “cushion” layer of a urethane insert that would be laminated in the final design between
the soft and rigid layers.
2.9.3 New Socket 3 (NS3)
The third socket established the importance of the contour of the socket. It was constructed using
the same material as was used in NS1, but a different liner was used. However, it was found that the
urethane and the silicone liners were basically the same. In NS1 the rigid layer extends to almost the
topmost point on the socket, but in NS3 the rigid layer stops much lower down the socket (See Appendix
A1.6).
III. Results
Figure 9: A sample graph automatically generated by the excel template file shows pressure (kPa)
over time (s) for each of the eight FSRs during a trial with the subject walking two mph.
As shown in Figure 9, the devised pressure measurement system was able to record rapidly
changing pressure values. The sampling rate of the LabVIEW program and the NI box was set at 1 kHz,
10
and the rise time of the FSRs is less than 3 microseconds. Thus, the pressure measurement system was
able to acquire maximum forces in regions where the peak values lasted for only a few milliseconds. It
was important to attain the true peaks without under-sampling, since the comparison of maximum
recorded pressures was a key component for assessing the differences between the transtibial prosthetic
Pressure (kPa)
systems.
450
400
350
300
250
200
150
100
50
0
Urethane
Silicone
Thermoplastic Elastomer
3.1 Determination of the
Optimal Combination of Foot
and Liner
The maximum
pressures for each gait cycle
were recorded for all 27
walking trials with the nine
combinations of three feet and
three liners. The averages of
all the peak pressures for a
Figure 10: Averages of Maximum Walking Pressures (kPa) for Each
Liner. Error bars depict one standard deviation. Numeric values in
Appendix Table A2.1
particular liner with the three
different feet were compiled
and compared. From Figure
10, it is evident that the thin thermoplastic elastomer liner caused significantly higher pressures in the
distal tibia. The comparison showed that the urethane and silicone liners had similar values in each
anatomical region, and that
400
the thermoplastic liner also
The averages of all
the peak pressures for a
particular foot with the
three different values were
analyzed by the same
Pressure (kPa)
caused higher pressures to
be recorded in the MPT.
Flex-Foot & Multiaxial Ankle
Sach
Vertical Shank ESF
350
300
250
200
150
100
50
0
procedure. Based upon the
standard deviations, there
were no significant
differences between the
feet. The large standard
Figure 11: Averages of Maximum Walking Pressures (kPa) for Each
Foot. Error bars depict 1 standard deviation. Numeric values in
Appendix Table A2.2
11
deviations displayed in Figure 11 were primarily due to variations caused by the three liners used with
each foot. This can be confirmed by viewing the results in the Appendix for each of the nine
combinations.
Since the optimal combination of foot and liner would reduce pressures in the pressure-sensitive
regions of the residual limb, the combination with the lowest total pressure for the distal end of the limb,
fibula head, distal tibia, and tibial tubercle was chosen. The total pressures from the silicone liner and
flex foot were lower than the total pressures with the other feet and liners.
As persistent pressure is responsible for many of the problems such as rashes that can develop
within a socket, the average pressures in each region were also analyzed. The results of the average
pressures during walking were concordant with the maximum pressures results. Once again, the
thermoplastic liner was associated with much higher pressures in the distal tibia and mid patella tendon.
There were minimal differences noticed for the feet, since a majority of the differences were due to the
selection of the liner. The average pressures were also much lower than the maximums in most regions.
*Averages shown in bold; respective standard deviations shown in grey.
12
*Averages shown in bold; respective standard deviations shown in grey.
Pressures were also recorded for each of the nine combinations with the subject standing
stationary. The data (See Appendix Tables A2.11-A2.19) for trials with the subject standing on the
normal leg with the prosthetic leg suspended and trials with the subject standing with the liner only show
that there are near-zero pressures in these situations. The data (See Appendix Tables A2.11-A2.19) also
show that the trials for the subject standing on both legs demonstrate very similar trends to the trials for
the subjects standing on the prosthetic leg only. The primary difference between these two trials is that
higher pressures were recorded in each respective region when the testing subject stood on the prosthetic
leg only (See Appendix A2.3-A2.4) than when the subject stood on both legs (See Appendix A2.11A2.19). This was due to the prosthetic leg bearing more weight in the former situation. Figures 12 and
13 show that the distal end of the limb must accommodate much higher pressures than the other regions
of the socket when the transtibial amputee is standing. Any new socket design would have to reduce the
high pressure in this region. Once again, the thermoplastic elastomer liner was associated with higher
pressures in the distal tibia, and there was more variation due to the type of liner used than the type of foot
used. The results when standing and walking both showed that there were high pressures in the distal end
of the limb and the distal tibia. The other two regions sensitive to pressure also had pressures high
enough that they could be significantly reduced with a new socket design. The pressures in both standing
and dynamic situations were normally lower with the urethane and silicone liners than with the
thermoplastic elastomer liner. Ultimately, Table R4 was used to select the silicone liner and Flex-Foot to
be used in the redesigned socket system.
13
600
Urethane
Silicone
500
Thermoplastic Elastomer
Pressure (kPa)
400
300
200
100
0
Figure 12: Average Pressures (kPa) for Each
Liner When Standing on the Prosthetic Leg
Only. Errors bars depict one standard deviation.
Numeric values in Appendix Table A2.3
Figure 13: Average Pressures (kPa) for Each Foot
When Standing on the Prosthetic Leg Only.
Errors bars depict one standard deviation.
Numeric values in Appendix Table A2.4
3.2 Testing of New Prosthetic Systems
The test socket (SF) with
500
450
SF
CN
NS1
NS2
the silicone liner and the flexible
NS3
foot with a multi-axial ankle was
Pressure (kPa)
400
selected as a baseline to compare
350
300
improvements in force
250
redistribution for the newly
200
designed sockets. The
150
combination normally used (CN)
100
by the test subject was also used
50
as a baseline for measuring
0
Distal End of
Limb
Fibula Head
Distal Tibia
Tibial Tubercle
Figure 14: Maximum pressures in regions sensitive to pressure
when walking. Errors bars depict one standard deviation.
improvement. These two sockets
and the three new sockets (NS)
were compared on the basis of
measurements recorded with the
subject standing on both legs and with the subject walking 2 mph.
14
The averages of the maximum pressures for each gait cycle were compiled in Appendix Table
A2.20. Since the success of
200
SF
CN
NS1
NS2
the new sockets was
NS3
180
dependent on reducing the
Pressure (kPa)
160
pressure in the regions of the
140
residual limb that are
120
100
sensitive to pressure, these
80
regions were compared in
60
Figure 14. This figure
40
shows that the pressures in
20
the distal end of the limb and
0
Distal End of
Limb
Fibula Head
Distal Tibia
Tibial Tubercle
the distal tibia were reduced
dramatically by the voids in
Figure 15: Average pressures in regions sensitive to pressure when
walking. Errors bars depict one standard deviation.
those regions of socket NS1.
These decreases were
determined to be significantly reduced by observing that the pressures for these regions for socket NS1
were many standard deviations away from the pressures for these regions in the baseline sockets SF and
CN. Likewise, there was a detrimental significant increase in pressure for socket NS1 in the fibula head
region and a non-significant increase in pressure in the tibial tubercle region. Pressures for sockets NS2
and NS3 were also reduced
450
in the sensitive regions.
400
However, many of these
were noticed for the
persistent average pressures
when walking, but the
respective average pressures
were typically considerably
lower than they were for the
Pressure (kPa)
significant. Identical trends
CN
NS1
NS2
NS3
350
beneficial pressure
reductions were not
SF
300
250
200
150
100
50
0
Distal End of Limb Fibula Head
Distal Tibia
Tibial Tubercle
Figure 16: Average pressures in regions sensitive to pressure when
standing on both legs. Errors bars depict one standard deviation.
maximum pressure values.
Both walking pressure metrics depict overall pressure reductions in sockets NS1-3.
15
The average pressure results for the pressure tolerant regions for both walking and standing trials
were recorded in Appendix Tables A2.21 and A2.22, respectively. Figure 16 depicts the average
pressures observed in only the pressure-sensitive
Table 4: Sum of Recorded Pressures (kPa)
in Sensitive Regions
regions when the subject stood evenly on both legs.
Similarly to the walking trials, socket NS1 had the
Walking
Maximums
SF
916
CN
984
NS1
714
NS2
615
NS3
643
Walking
Averages
416
422
321
274
283
Standing
Averages
721
679
497
543
482
lowest average pressure of all the sockets for the distal
end of the limb, but had the highest pressures of all
the sockets for the fibula head and tibial tubercle. The
only significant reductions in pressure were for
sockets NS1 and NS3 in the distal end of the limb.
However, Table 4 demonstrates that the sum
of the pressures in the sensitive regions decreased
between the baseline sockets (SF & CN) and the new
Table 5: Percentage of Total Recorded
Pressure Located in Tolerant Regions
sockets. This reduction was consistent for the peak
pressures while walking, the average pressures while
walking, and the average pressures while standing.
The complete table for both pressure-sensitive and
pressure-tolerant regions from which Tables 4 and 5
are extracted can be found in Appendix Table A2.23.
Walking
Walking Standing
Maximums Averages Averages
SF
31.8
32.8
25.7
CN
31.0
32.8
27.7
NS1
37.3
40.7
40.4
NS2
36.8
40.9
33.2
NS3
36.6
38.1
32.8
Table 5 shows that the proportion of the total recorded
pressure in pressure-tolerant regions increased for sockets NS1-3. This indicates that a portion of the
pressure was redistributed from sensitive to tolerant regions.
The Socket Comfort Score (SCS) is a number that the patient assigns to the socket based upon his
Table 6: Socket Comfort Score
Socket
SF
CN
NS1
NS2
NS3
Comfort Score
5.0
7.0
6.5
7.0
8.0
*Scale of 0 being most uncomfortable to
10 being completely comfortable
level of comfort from 0-10, with 0 being most
uncomfortable and 10 being completely comfortable.
It has been shown that the reported SCS is a consistent and
reliable assessment method. (Hanspal, Fisher & Nieveen,
2003). Table 6 shows that the participant found one of the
new sockets (NS 3) to be more comfortable than both his
current socket and the best foot/liner combination
socket. NS2 was rated better than the best foot/liner
combination socket and comparable to the patient’s
current socket. NS3 was rated more comfortable than the best foot/liner combination but worse than the
patient’s current socket.
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3.3 Safety Considerations
When developing the force measurement system, design considerations were made to reduce the
risk of electrical shock to the testing subject via the powered FSRs. The power to the measurement
circuit containing the FSRs was limited to ± 12V with a power supply. The voltage from the
measurement circuit to the FSRs was limited further to ±5V by a voltage regulator. The test subject was
also protected from this minimal voltage by using insulated RoHS-compliant wires to connect the FSRs to
the measurement circuit. The conductive connection between the leads of the FSRs and readout wires
was coated in a strong, non-conductive marine epoxy.
The testing subject was protected from skin irritation by ensuring that all items placed between
the subject’s skin and the liner were very thin. It was affirmed that the subject did feel pressure from the
wires or the FSRs. The connection between the FSRs and the wires were made with smooth layers of
epoxy rather than the standard bulky lead to wire connectors. Scotch tape was used to affix the FSRs to
locations on the skin rather than using other adhesives that could be irritants. The subject was also
protected from falling by ensuring that there were always chairs or handrails nearby for support and by
holding the FSR wires perpendicular to the direction of walking when using the treadmill.
The prosthetic system used fabrication materials and methods commonly used in the prosthetic
industry. This ensured that all of the feet, liners, and sockets were made from biocompatible materials.
These materials have proven material strength and durability properties that reduce the risk of breakage.
3.4 Costs
The new socket design is intended for transtibial amputees and will be able to be used by many
patients. Even though testing was performed on one individual, the advantages of the design can easily
be altered to apply the overall fundamentals and ideas of the design to several other patients. As current
socket design is based on a qualitative process, this design will have a significant time- and cost-benefit
for The Surgical Clinic.
The main materials needed for the project were FSRs, circuit components, and materials to
fabricate each socket. This budget reflects the production of three transtibial sockets. This does not
reflect the cost of the sockets that were already designed, the feet, or the liners that were already available
at the Surgical Clinic prior to the start of our project. Appendix A2.27 shows that the total cost involved
in our project was approximately $900.
The typical lifespan of the socket is approximately one year. The main variables that affect this
are the age of the patient, activity level of the patient, type of skin and bony prominences, and diseases
such as diabetes, Peripheral Vascular Disease (PVD), and arthritis. In general if a patient is healthier and
more active, his/her socket will demonstrate wear sooner. The older sicker patients can have sockets
lasting five years (A. Fitzsimmons, personal communication, April 22, 2011).
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Our research could increase the lifespan of a socket for a young, active patient by double or triple
by identifying problem regions before the entire fabrication process and accommodating for them to make
the socket more comfortable for a longer period of time and decrease associated problems, such as skin
breakdown.
The area this research will affect the most is cost maintenance, as the lifespan of the socket is not
as important as maintenance. When a socket is uncomfortable, it is necessary for the patient to go back to
the Clinic and have adjustments made. While no new materials are necessary for maintenance, there is a
significant time cost in the form of gas money, time off work, and lost time. If an initial appropriate fit
with definable parameters can be established for each patient, the cost maintenance can be reduced, which
will prove beneficial for both the patient and The Surgical Clinic.
IV. Conclusions
Our final designs were successful in achieving all three of the original objectives. The FSRs,
circuit, and NI box proved to be an accurate measurement system to acquire pressures on the residual
limb of an amputee during standing and walking conditions. After testing nine combinations of standard
liners and feet used in the prosthetic industry today, it was determined that the silicone liner and the flex
foot reduced the pressures in the pressure-sensitive regions and the total pressure experienced on the
residual limb compared to the other feet and liners. After this was determined, three new sockets were
designed in an attempt to account for the shortcomings of the silicone liner, Flex foot, and socket
combination and the participant’s current liner/foot/combination.
While the study did not culminate in the design of one single socket, the three final sockets that
were designed collectively established attributes that would be beneficial in the design of future sockets.
In accordance with its design, New Socket 1, the socket made of thermaline soft and carbon fiber with a
void at the distal end, most significantly reduced the pressure on the distal end of the limb. The more
flexible proflex with silicone material and unique cut of New Socket 2 allowed for greater reduction in
three of the pressure-sensitive regions of the limb but not the distal end of the limb. While New Socket 3,
the socket made of the same material as New Socket 1 but with a different contour, did not reduce
pressures in three of the pressure-sensitive regions quite as much as New Socket 2 did, it demonstrated
the significance of contour when compared to the pressures generated from using New Socket 1. Thus,
the desirable characteristics of each socket can be taken into consideration in the design of a new socket.
For example, the material used to construct New Socket 2 can be used with the contour of New Socket 3
with a void as was used in Socket A to decrease the pressure in targeted regions. Additionally, as the
silicone gel liner and flex foot were determined to be most effective in reducing pressure on the various
regions of the residual limb, this liner-foot combination can be incorporated into future socket systems.
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The Socket Comfort Score (SCS) is a number that the patient assigns to the socket based upon his
level of comfort from 0-10 with 0 being most uncomfortable and 10 being completely comfortable.
It has been shown that the reported SCS was consistent and reliable assessment method. (Hanspal, Fisher
& Nieveen, 2003). It was found that both new socket 2 and new socket 3 had better or comparable Socket
Comfort Scores than both the patient’s current socket and the best foot and liner combination (silicone
liner with flex foot). New socket 1 had a better SCS than the best foot and liner combination (silicone
liner with flex foot), but a worse SCS than the patient’s current socket. It has been found that the new
socket designs have been successful in increasing patient comfort in addition to decreasing the pressure
experienced in the pressure sensitive regions and distributing the pressures better throughout the socket.
V. Recommendations
5.1 Future Work
This study lends itself well to future work in socket design because of the accurate pressure
measurement system in place and the success of the new sockets in various aspects. Because of the
success of new socket 1’s ability to reduce the pressure at the distal end and the success of changing the
materials in new socket 2, future studies should look to incorporate the elements of successful redesign of
the sockets in this study. By using proflex with silicone and creating additional voids, an even greater
decrease in pressure should be observed. Testing should be expanded to include additional patients. This
should include patients of varying ages and activity levels in order to be able to apply the results to a
larger general population. Testing situations should also include additional activities such as walking up
and down stairs and ramps. In addition, assessment of the comfort levels of the new sockets should occur
after the patient has used the socket for a duration of several days. Another variable that can affect the
pressures on the residual limb is the suspension system used, so further tests can analyze the difference in
pressures between different suspension systems.
5.2 Ethical and Societal Issues
All of our testing included the use of a human subject. This subject volunteered to assist in our
project and was never pressured into testing. The purpose of our project and the tasks required of him
were explained. Throughout testing the safety and comfort of the subject was our primary concern and
efforts were made to ensure that the subject was not fatigued or in any type of pain.
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