An increased vacuum suspension (VS) creates a negative pressure difference by actively drawing air out of a sealed socket through a one-way valve. The resulting vacuum resists separation of the residual limb and the prosthetic socket.1 For VS, well-fitting total surface bearing sockets are used to provide a uniformly distributed vacuum environment throughout the entire socket. It is known that significant isolated voids between the socket and the residual limb can cause adverse effects. However, this type of suspension, if fabricated and fitted properly, has revealed solutions to common lower limb amputee problems. Some observed and documented benefits regarding VS include reduction in “pistoning” or movement within the socket, increased proprioception, reduced tissue breakdown, improved gait, superior linkage, and reduction in residual limb volume fluctuations.1–5 There are indications that these effects combine to result in increased patient mobility.
Two common problems with the lower limb prosthetic amputee population are the pistoning motion in the socket and daily volume fluctuations in residual limb. The reduction in pistoning motion decreases the impact and frictional forces subsequently diminishing the potential of tissue breakdown, improving gait, providing a superior linkage, improving proprioception, and reducing wear and tear on liner interface materials. This pistoning motion has been used as a measure of the efficacy of a suspension system.1 By reducing residual limb volume fluctuations, socket fit is stabilized, thus decreasing the potential for pistoning, increasing patient comfort, and reducing patient injury. Improvement or elimination of these problems as a result of the use of VS can have strong positive effects on an amputee's daily life.
Although the use of VS in the prosthetic field is growing,6 there is little understanding of the mechanics of VS: how this new type of suspension changes the loading on the soft tissues, the short-term effects of vacuum on perfusion, or the long-term effects of vacuum on the residual limb as a whole. In addition, the knowledge about what should be considered sufficient or suggested levels of vacuum is limited. At even a more basic level, there are currently no tools available to monitor the vacuum in a socket, how it varies with time, or the actual usage by a subject. This article presents a tool that allows these types of studies to be performed in a nonintrusive manner, with the intent that it will facilitate the understanding and appropriate usage of the many new VS systems now entering the prosthetic market.
VS research to date has focused mainly on the reduction in residual limb volume fluctuations. Two studies2–3 reported that vacuum minimizes or prevents residual limb volume loss, claiming a 3.7% residual limb volume gain. In attempts to explain this benefit, another study4 measured the interface force pressures between the skin and the liner. This study predicted that the vacuum-induced alterations in the fluid dynamics prevented volume loss. Regarding the other observed benefits of VS, some of the same researchers measured a 0.7-cm decrease in pistoning compared with suction suspension.2 Even though research efforts have begun to validate and understand VS, there are many questions still unanswered. Consequently, the necessity for more extensive research requires an accurate real-time measurement device.
In conjunction with the LimbLogic™ VS system, the Ohio Willow Wood Company has developed a wireless communicator. The LimbLogic VS Communicator© (The Ohio Willow Wood Company, Mt. Sterling, OH, patents pending) is capable of both controlling and collecting data from the LimbLogic VS vacuum pump. This wireless communicator collects real-time vacuum pressure values and exports the data into a viable data structure. An accurate device containing these capabilities will allow prosthetists to monitor patient's vacuum activity and remotely control settings. In addition, the vacuum pressure measurement has the potential to provide the ability to conduct accurate and feasible vacuum research. In this article, we first present basic information about the functionality of the tool, demonstrate its accuracy to the same standards as the LimbLogic VS system, and then present a simple study on the distribution of vacuum levels in a socket, the results of which may simplify the needs of many possible future experiments in the prosthetic field.
LIMBLOGIC VS COMMUNICATOR
The LimbLogic VS Communicator is a nonintrusive, wireless device capable of controlling, monitoring, recording, and exporting vacuum pressure data. It provides an easy method to control and alter the LimbLogic VS settings. In addition, it is able to monitor real-time dynamic vacuum pressure readings. The LimbLogic VS Communicator also collects patient usage data, which can then be evaluated over time or compared between different periods of usage. As a research measurement tool, it records and displays real-time vacuum pressure data at a sampling rate of 50 Hz. The data are exportable in a practical data structure, allowing it to be analyzed and compared. This unique tool conveniently provides real-time dynamic vacuum pressure data that was not previously available to the prosthetic community.
The accuracy of the LimbLogic VS Communicator vacuum pressure readings was verified against four external measurement locations on a transtibial prosthetic evaluation socket. The verification equipment included clear evaluation prosthetic sockets, elbow port screws, four-hole connector port plates, and a data collection box containing a data acquisition card (DAQ card, NI, USB-6008), a computer (Dell Precision, M4300, Dell Inc., Round Rock, TX), and LabView Software (NI, version 7.1).
Well-fitting (<2-ply) total surface bearing evaluation sockets were fitted with three tube connection port screws. To accommodate these pressure ports, three holes were drilled and tapped into the evaluation socket, and the ports were then screwed in until they were flush with the inner socket wall. Epoxy was used to seal and reinforce the ports to reduce the potential for a leak. The fourth measurement site, a four-hole port plate, was located between the distal end of the socket and the vacuum pump. Vacuum grade tubing connected the port screws to the data collection box. A DAQ card (NI, USB-6008) recorded voltage readings from the four locations at a sampling rate of 50 Hz using four separate external channels. Each port or channel was calibrated separately using a vacuum gage with an National Institute of Standards and Technology (NIST) traceable calibration. The four linear voltage pressure calibration curves had a minimum R2 value of 0.996. The DAQ card measured with a resolution better than 0.001 V. The port screws on the evaluation sockets were located based on a percentage of the limb length (distance between the patellar tendon bar and the distal end). The four recording sites were as follows:
- P1: distal end of the socket (a four-hole port plate attached externally to the socket).
- P2: 25% of the limb length (distal to the patellar tendon bar) from the distal end.
- P3: 50% of the limb length from the distal end.
- P4: 75% of the limb length from the distal end.
A diagram and picture of the configuration is shown in Figure 1. The reference data collected from the four socket locations were recorded using LabView Software and were used to validate the vacuum pressure readings from the LimbLogic VS Communicator.
All trials were tested using the Ohio Willow Wood LimbLogic VS system and a LimbLogic VS Communicator. Socket fit was assessed by a certified prosthetist to be less than a 2-ply fit, where 2-ply is approximately 2.2 mm in thickness. Four transtibial amputees, with test-patient agreements, participated in the study. One subject was a bilateral amputee with slightly different limb lengths; therefore, a total of five sockets were studied.
Subjects were asked to don the prosthesis (liner, altered evaluation socket with connecting prosthetic components, and sealing sleeve) as normal while being observed by a certified prosthetist. To eliminate the potential of a tube bending or the restriction of air flow by the sealing sleeve, elbow (90°) screw ports were used. Seven different vacuum pressure settings (8 in Hg [27.09 kPa], 10 in Hg [33.86 kPa], 12 in Hg [40.64 kPa], 14 in Hg [47.41 kPa], 16 in Hg [54.18 kPa], 18 in Hg [60.96 kPa], and 20 in Hg [67.73 kPa]) were tested in a random order. After the vacuum pump was set to the designated setting, it was allowed to stabilize. Stabilization time was deemed sufficient when the pump had achieved the designated pressure setting and the time between pump activation was at least 10 seconds. The subject was asked to stand still for 1 minute while pressure data were collected simultaneously from both the LimbLogic VS Communicator and the data collection box. To synchronize the timing for both measurement devices, the subject was asked to take a step with the prosthetic side inducing a pressure spike. The rare occurrence of the pump activation was manually counted and recorded. After a short break, the process was repeated, with a different random sequence of vacuum pressures.
The accuracy of the LimbLogic VS Communicator was determined by comparing the vacuum pressure output readings with the externally measured readings taken through the vacuum ports in the socket. The acceptable level of accuracy for the LimbLogic VS Communicator was defined to be equal or less than the standardized accuracy for the LimbLogic VS system. Each LimbLogic VS system is verified in manufacturing to an accuracy of ±1 in Hg (3.39 kPa) and a resolution of 0.1 in Hg (0.34 kPa). The data collected were evaluated by calculating the root mean square error (RMSE) value. This provided the absolute vacuum pressure deviation from the LimbLogic VS Communicator to the various measuring locations in the socket. The RMSE was calculated from a 2-second stable interval of time at each vacuum pressure setting for each subject. Stable criteria were defined by at least 10 seconds after the pump activates and detectable subject movement and at least five seconds from the end of data collection. A sample of the combined output from the LimbLogic VS Communicator and data collected through the pressure ports in the socket are displayed in Figure 2.
The RMSE values of data collected with the LimbLogic VS Communicator were calculated at the four levels described in the Methods section (P1, P2, P3, and P4). The average, maximum, and minimum RMSE values at the seven different VS pressure settings are presented in Table 1. Table 2 contains the maximum and minimum RMSE values for each individual subject regardless of the VS pressure settings.
The RMSE value indicated the amount the LimbLogic VS Communicator vacuum pressure reading deviates from the pressures measured at different locations in the socket using the data collection system. Therefore, according to Table 1, the LimbLogic VS Communicator vacuum pressure reading at 8 in Hg (27.09 kPa) had an average difference of ±0.124 in Hg (0.42 kPa). At vacuum pressures settings greater than 12 in Hg (40.64 kPa), the LimbLogic VS Communicator tended to measure a slightly greater vacuum pressure. Table 1 contains average, maximum, and minimum RMSE values for all subjects at the seven VS pressure settings. The average RMSE values increased with increasing vacuum pressure settings; therefore, the accuracy was dependent on the VS pressure setting. The deviation between these values may result from a socket pressure gradient. The fluctuation in the maximum and minimum RMSE values in Table 2 suggested that the primary error resulted from human movement rather than the instruments. In support of this claim, the data analyzed for the LimbLogic VS Communicator had an average standard deviation of 0.025 in Hg (84.66 Pa) with a resolution of 0.01 in Hg (33.86 Pa) and for the Lab View DAQ had an average standard deviation of 0.066 in Hg (223.5 Pa) with a resolution less than 0.001 in Hg (3.39 Pa). This indicated that slight shifts in the subject's applied weight (downward force) caused slight time-dependent pressure variations, thus generating noise and fluctuation in the pressure readings.
All replications were considered when calculating the average, maximum, and minimum RMSE values. For each subject, the second replication demonstrated similar results, signifying that the order in which VS pressure settings were evaluated did not influence the results. Table 2 contains maximum and minimum RMSE values for each subject regardless of the VS pressure setting.
From Table 1, pressures less than 16 in Hg (54.18 kPa) exhibited an average RMSE of only a few tenths. Pressures more than 16 in Hg (54.18 kPa) revealed a slightly higher average RMSE, but the average values were still within the range of ±0.6 in Hg (2.03 kPa). Because the RMSE values in both Tables 1 and 2 were within the specified accuracy of ±1 in Hg (3.39 kPa), these data validate that the LimbLogic VS Communicator reports socket vacuum pressure values within same or better accuracy as that of the LimbLogic VS system. The average accuracy of the LimbLogic VS Communicator was determined to be ±0.5 in Hg (±1.69 kPa). Residual limb length, limb circumference, and weight were not correlated with the RMSE results.
DISTRIBUTION OF VACUUM PRESSURE IN A SOCKET
In conjunction with this study, the distribution of vacuum pressure in a socket was evaluated. The average RMSE values comparing the external four-hole measurement location (P1) with the three socket port locations are presented in Table 3. As expected, the most distal socket measuring location (P2) to the four-hole plate (P1) exhibited smaller RMSE values. The RMSE are directly correlated with the distance from the distal end. Even though there is a slight difference between the RMSE pressure values obtained at the socket measuring locations (P1–P4), these values differ on average by approximately 0.1 in Hg (0.34 kPa), indicating that for practical purposes the vacuum pressure within a total surface bearing socket is equally distributed and uniform. Only a small pressure gradient is present.
EXAMPLE OF LIMBLOGIC COMMUNICATOR VACUUM PRESSURE DATA
The vacuum pressure data from the LimbLogic VS Communicator are wirelessly transmitted in real-time, allowing virtually unrestricted experimental design. Figure 3 displays an example of the vacuum pressure data as a subject ambulates at different VS pressure settings. When evaluating these data, it should be noted that a positive vacuum pressure is a negative pressure relative to the local atmosphere. These data illustrate how the vacuum pressure alters as the subject transitions between the swing and stance phases of gait, producing a quasi-sinusoidal signal. As the subject lifts and swings the prosthetic leg, the vacuum pressure increases in response to the downward force of the prosthesis. In contrast, the vacuum pressure decreases as total weightbearing force is exerted on the prosthesis during the stance portion of ambulation. The data also demonstrate correlation between vacuum pressure responses and the VS pressure settings. As the VS pressure settings increase, the sinusoidal range over which the vacuum pressure varies during ambulation diminishes. With regard to this example, the variation in vacuum pressure during ambulation with a vacuum setting of 8 in Hg (27.09 kPa) is approximately ±3 in Hg (±10.17 kPa). In comparison, the variation in vacuum pressure while ambulating with a vacuum setting of 20 in Hg (67.73 kPa) is approximately ±0.25 in Hg (±0.85 kPa). This example demonstrates the potential of the LimbLogic VS Communicator to be used as a measurement tool to quantify the mechanics and benefits of prosthetic VS.
The accuracy of the LimbLogic VS Communicator has been verified to the same standards of the LimbLogic VS system. In so doing, it has been shown that the LimbLogic VS system, coupled with a LimbLogic VS Communicator, provides an accurate tool to monitor the vacuum in a prosthetic socket both statically and as it varies in time. This nonintrusive tool will facilitate the understanding and usage of VS systems. The LimbLogic VS Communicator will simplify the needs of future research experiments in the prosthetic field, especially the quantification of the benefits of VS.
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