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Trinity University GAIT

Memo To: Dr. Mahbub Uddin, Group Advisor

From: Gait Analysis Improvement Team

CC: Dr. Kevin Nickels, Course Administrator

Date: 5/10/10

Re: Decision Table

The goal of the Gait Analysis Improvement Team project is to design and develop an improved method to obtain quantitative measurements of the forces transmitted through below-knee prostheses. These measurements should be connected to suggested alignment changes that would improve the gait of a patient. In general, the design should provide a more scientific and cost effective procedure for the initial fitting of a prosthesis, including determination of the best alignment. Several alternatives for the design have been researched and ranked according to their adherence to the criteria described below. The intended end user of this design is a prosthetist working with an amputee in a clinical setting. Because amputees typically have more difficulty with balance than non-amputees, safety is a primary concern and a heavily-weighted criterion in the decision-making process. Any device that is incorporated into the fitting procedure must not compromise the safety of existing prostheses and related equipment. The next most important criterion is cost. The budget for this project is restricted to $1,200 and the device must also present an economical alternative to existing methods of computerized gait analysis. Additional cost guidelines were suggested by our technical consultant. The device should not exceed $150 if it is used fewer than five times. If it is used for more than five prosthesis fitting appointments, it may cost up to $500. These are rough estimates but should be considered when evaluating potential solutions. The design will not be successful unless it can be used along with existing (standard) prosthesis components, so compatibility is the third most important criterion. If force measurements are not consistent, accurate and precise, the project will fall short of the goals. Therefore measurement quality is ranked fourth among the working criteria. The final two criteria are durability and ease of use for the end-users (clinicians working with amputees). Ideally, the design will be able to withstand many uses rather than serve as one-time-use equipment. If it is not easy to install, operate and remove, it is not likely to appeal to a clinician in an average prosthetic facility. Design Alternatives The brainstorming session for this project resulted in five alternatives to be considered. The first alternative is the Smart PyramidTM by Orthocare Innovations (Smart Pyramid, 2009). The Smart PyramidTM replaces the standard pyramid in any socket and allows for computerized gait analysis with its embedded sensors. When the CompasTM Master unit is attached to the pyramid, the data collected by the pyramid is sent to a PC via Bluetooth in order to provide a prosthetist with gait

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analysis information. Figures 1 and 2 below depict the Smart PyramidTM and the CompasTM Master unit, respectively.

Figure 1. Orthocare Innovations Smart PyramidTM (Smart Pyramid, 2009).

Figure 2. Orthocare Innovations CompasTM Master unit attached to the Smart PyramidTM

(Smart Pyramid, 2009).

The second alternative is a design consisting of multiple strain gages. Strain gages are commonly used in the manufacture of pressure sensors, and the mechanical engineers in the group are familiar with the technology and have practiced its application through previous coursework. Multiple configurations could be designed to suit the force measurement needed in this project. Strain gages, such as the one shown in Figure 3, are readily available and inexpensive.

Figure 3. One-axis general purpose strain gage from Omega (Model SGD-1.5/120-LY11)

(Precision Strain Gages, 2009).

Off-the-shelf load cells are the third potential solution. They can be found as simple, single-axis sensors that resemble buttons, or as more complex, six-directional sensors (x, y and z-axis forces and torques). In both cases, they are commonly encased in stainless steel and are advertised to be very durable. Figures 4 and 5 give examples of these sensors.

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Figure 4. Stainless steel, 1-axis, miniature compression load cell by Omega (Model

LCM307-5KN) (Miniature Compression Load Cell, 2009).

Figure 5. Stainless steel, six-directional sensor from PCB Piezotronics (Model 261A01)

(Force Sensor Model 261A01, 2008).

The fourth option to consider is the Tekscan FlexiForce sensor (FlexiForce Force Sensors, 2007). This sensor is a thin-film, piezoresistive sensing device containing a flexible printed circuit. The sensing area (see circular area at right end of Figure 6) senses contact force, and silver extends from the sensing area to the connectors at the other end.

Figure 6. FlexiForce sensor model A201 from Tekscan (FlexiForce Force Sensors, 2007).

The fifth alternative is a simple, mechanical scale-type system. The displacement of springs for which the spring constant is known can be measured and correlated to the applied force. This particular option is not found pre-fabricated but could be constructed with the combined knowledge of our team members.

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This project requires a decision for which sensor type is best suited to meet the project objectives. The rating scale and the table format are defined as follows where R is the rating and W is the weight of the criterion:

9 - 10 Excellent 7 - 8 Good 5 - 6 Fair 3 - 4 Poor 0 - 2 Unsatisfactory

Decision Matrix I: Sensor Type

Criteria Weight % Smart Pyramid

Strain gages Load Cells Tekscan

FlexiForce Mechanical

Scale (Spring)

Safety 30 9 / 270 7 / 210 7 / 210 8 / 240 3 / 90

Cost 20 4 / 80 9 / 180 4 / 80 8 / 160 8 / 160

Compatibility 15 10 / 150 7 / 105 6 / 90 8 / 120 5 / 75

Measurement Quality

15 8 / 120 7 / 105 7 / 105 8 / 120 5 / 75

Durability 10 7 / 70 6 / 60 8 / 80 7 / 70 4 / 40

Ease of use 10 5 / 50 6 / 60 6 / 60 8 / 80 6 / 60

Total 100 740 720 635 790 500

Explanation of Numerical Rankings After intensive research, background investigation and brainstorming sessions, the five potential force sensing solutions were compared and ranked by the group as shown above in the decision table. The CompasTM System Smart PyramidTM was ranked excellent in safety (9) and compatibility (10) because it has already been tested on patients and is an interchangeable pyramid found in standard transtibial prosthesis units. Based on its published claims, the Smart PyramidTM is able to measure and store data for up to 9 miles of walking. This device is intended to be used indefinitely. However, the company also claims that no batteries or service are required. This feature may suggest uncertainty to the quality of the measurements acquired over time. Orthocare Innovations does not state what material the device is made of, nor does it reveal exactly how forces are being measured within the pyramid itself, giving this alternative a good but not excellent score for measurement quality (8) and durability (7). To enable gait analysis, the CompasTM Master unit clicks on to the Smart PyramidTM – providing power, microprocessor control, motion sensing, a laser guide and communications with the end user’s PC. Although this CompasTM Master unit system would make reading and translating the data simple for the clinician, it cannot be considered as an option for our project because it costs $6,500 in addition to the price of the Smart PyramidTM itself. Therefore, without the Master Unit, the overall design would have only a fair rating for its ease of use (5). The Smart PyramidTM alone costs $750, which is considerably higher than our technical consultant’s suggested limit of $500, thus resulting in a poor ranking for cost (4).

The next proposed alternative is to use multiple strain gages to measure forces and torques below the socket. This would be the most cost effective solution (9) since it would require a minimum of 4 strain gages and possibly a steel spacer or disk mounted between the pyramid and the socket. The cost ranges from $50 for a 10-pack of basic linear strain gages to $140 for a 5-pack of prefabricated rosettes. The 3” diameter steel spacer needed to mount the unit would be approximately $2. In either case, the total cost would be below our consultant’s suggested price limit. Although the purchased strain gages themselves are assumed to be very reliable, the safety

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and quality ratings are highly dependent on our entire system design. Also, we must keep in mind that we are creating an additional part for the prosthesis as opposed to replacing an already existing piece. This increases the risk of compromising the integrity of the prosthesis, giving the strain gage design not excellent scores, but fairly good ratings for safety (7), compatibility (7) and measurement quality (7). The durability (6) of this alternative depends on the placement of the strain gages, which may be exposed to the environment. If they are placed directly on the pylon of the prosthesis, the number of times the pylon needs to be cut and adjusted to fit a patient affects the life span of the strain gage system. The strain gages would not be reusable if the pylon is cut short because they would be glued on. The electrical output of the strain gages would probably be similar to that of the Smart PyramidTM. However, since we have more flexibility with developing the strain gage design, the group will not be forced to conform to the constraints of an already-existing product. This will allow the group to design a less complicated output program system for the clinician, resulting in a higher rating for its ease of use (6) than the Smart Pyramid alone (without its accompanying $6,500 system).

Although the load cells alternative did not have any excellent scores, they did receive good overall safety (7), measurement quality (7), and durability (8) ratings for the following reasons. The group agreed that the safety of this system would be at least as high as the strain gages because we would design the housing unit ourselves. These load cells are advertised as “industrial strength,” so they should not experience structural failure. High accuracy load cells can be purchased off-the-shelf, but measurement quality also depends on where they are placed on the prosthesis. The “rugged” design of the load cells suggests a long lifetime, and they are designed for industrial environments. The stainless steel casing would also increase the durability of the unit. There is less flexibility for placement of the basic load cells on the prosthesis than the strain gages. On the other hand, the 1.5” diameter is a good size, resulting in a “fair” score for compatibility (6) with the prosthesis. Once again, there is a similar electrical output to that of the strain gages. This will allow the team to convert voltage measurements with proper calibration, making it fairly easy to use (6). The cheapest single-axis load cell (found so far) that can accommodate the anticipated loads for this project is $375. We would need 4 of those to measure single-axis forces at each 90 degree angle at the base of the socket, totaling $1,500. The market price of a stainless steel-encased six-directional sensor is $3,200. Both options are considerably above our entire budget and are therefore not cost effective (4).

The Tekscan FlexiForce solution also did not have any excellent ratings, but scored “good” in each category. Although this device has already been thoroughly tested by the company, the design of the sensor is more fragile than the Smart PyramidTM because its components are enclosed between thin layers of substrate (polyester) film. The group gave it a higher safety score (8) than the strain gages and load cells, but a lower score than the pyramid. There are two different models of the Tekscan FlexiForce sensors, each costing the same amount ($117 for an 8-pack). The A201 model is compatible with a standard multimeter (i.e. handheld digital multimeter for $550). This price is not quite as low as the price of the strain gages, but it is still under the suggested cost given by our consultant resulting in a score of 8 for cost. While the ultra-thin structure of this device is more compatible (8) with the existing prosthesis, it is still an additional component. This highly developed sensor has been tested to be fairly consistent with very low error generating a good overall measurement quality (8). The durability (7) of this alternative is decent because it protects the unit from the environment better than the strain gage system. However, it is not as strong as the stainless steel housing unit used for the load cells. Because the Tekscan FlexiForce is compatible with a standard multimeter, its ease of use (8) was rated “good” as well.

Lastly, the mechanical scale alternative was rated according to predicted performance in these categories. The mechanical scale system is fairly simple, with no electrical components required, giving it a very good cost rating (8). Although this method of physically measuring the displacement of springs seems easy, it allows room for human error and may require more time to gather the needed data. Adding this spring scale unit to the prosthesis increases the amount of “play” in the limb, and it is more likely to cause deviation from the normal performance of the prosthesis. All of these characteristics contribute to the fair ratings given for ease of use (6),

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compatibility (5) and measurement quality (5). Again, since the design for this system would require springs, it would drastically decrease the stability of the prosthesis thus resulting in a very poor safety rating (3). We must keep in mind that the more heavy-duty the springs, the less sensitive the overall system will be, also making the durability of this design fairly low as well (4).

Conclusions and Recommendations After analyzing all the alternatives using our stated criteria and the decision matrix, we can conclude that the Tekscan FlexiForce sensor is the most promising force sensing device for this project. With this ultra thin, piezoresistive sensing device, the contact forces between the socket and pyramid on the prosthesis can be measured with little intrusion into the existing prosthetic structure. However, additional materials will likely be used to support the sensors and protect them from abrasion. The sensor can be incorporated into a force-to-voltage circuit that has a linear relationship between force and conductance (according to the manufacturer). This linearity should be helpful in establishing the correlation between the electrical output of the sensors and the forces that result from loading the prosthesis. Tekscan provides a recommendation for the drive circuit, which can be modified in order to measure higher or lower forces. Ideally, the force measurements taken at every 90° angle on the base of the socket (using four sensors) will be related to suggested alignment changes that would improve the gait of a patient. In contrast to the widely used, subjective method of gait analysis and new, expensive, computerized methods, a design incorporating the Tekscan FlexiForce sensor should provide a more scientific and cost effective procedure for the initial fitting of a prosthesis including determination of the best alignment. The next step for this project will be to design the supportive structure for these sensors and determine the best placement and drive circuit.

Bibliography FlexiForce Force Sensors. (2007). Retrieved October 2009, from Tekscan:

http://www.tekscan.com/flexiforce/flexiforce.html

Force Sensor Model 261A01. (2008). Retrieved October 2009, from PCB Piezotronics:

http://www.pcb.com/spec_sheet.asp?model=261A01&item_id=10069

Miniature Compression Load Cell. (2009). Retrieved October 2009, from Omega.com:

http://www.omega.com/ppt/pptsc.asp?ref=LC307&Nav=pref03

Precision Strain Gages. (2009). Retrieved October 2009, from Omega.com:

http://www.omega.com/ppt/pptsc.asp?ref=SGD_LINEAR1-AXIS&Nav=pree02

Smart Pyramid. (2009). Retrieved October 2009, from Orthocare Innovations:

http://www.orthocareinnovations.com/category.php?cat=1029