There are several systems available for lower-limb prosthesis suspension. Vacuum suspension systems act by drawing out air trapped between the socket and liner, while a sleeve seals the system. This removal of air molecules creates a vacuum that holds the socket firmly to the residual limb. Although this system has been commercially available since 1999,1 certain insurance companies2 continue to classify vacuum systems as experimental to deny coverage. Previous studies have examined the benefits of vacuum systems and found evidence of reduced peak pressures during ambulation, reduced distraction between loading and unloading of the residual limb, and reduced volume loss.
Studies have indicated that reduced distraction between the residual limb and socket between loading and unloading is possible using vacuum suspension. This is because of the increased suspension in a vacuum system compared with a pin or suction system.1 The fact that normal daily activities on an average residual limb do not exceed the force the vacuum uses to hold the liner to the socket (70 kg of suspension force compared with 10 to 15 kg of force in daily activities)3 confirms this statement.
In addition to a more secure suspension, studies have shown that vacuum systems produce reduced peak pressures during ambulation when compared with suction suspension.4 This is particularly important because both suction and vacuum suspension systems use a total surface bearing socket, eliminating the socket design as an explanation for the difference in pressures. In addition to decreased peak pressures, the vacuum system showed a negative, rather than positive, pressure during swing.5 This is important because it is typically positive pressure rather than negative that causes skin irritation and breakdown.4 Reduced positive pressures may be particularly beneficial to patients using anticoagulants, who thus may be prone to internal bleeding potentially caused by excessive forces.3
Decreased forces and improved skin health may also be a result of decreased volume loss. Studies demonstrate that the residual limb does not lose volume throughout the day in vacuum suspension systems, in contrast to pin locking suspension.1 Preventing volume loss is important because when the residual limb changes shape or size, the socket may apply forces in unintended ways. The residual limb may actually gain volume during ambulation in a properly fitting vacuum suspension socket.1 Although excessive volume gain would also change the fit of the socket, this increase rather than decrease in volume may be beneficial for vascular patients who cannot risk using anything that may constrict blood flow in their limb. It has been shown that undersizing the socket from 4% to 6% prevents the limb from swelling.1 Thus, proper socket modifications to control the residual limb volume in a vacuum suspension system are very important.6
This study aims to examine the reported benefits of vacuum suspension systems and to study their direct effect on patient outcomes. Specifically, this study uses the Activity Balance Confidence (ABC) Scale to investigate stability during activities and the probability of future falls. Additional topics explored include overall comfort, skin issues, volume fluctuations, ease of knee flexion, perceived pistoning, and activity level. This study examines how the reported benefits of vacuum suspension systems affect stability and daily life of patients and may provide additional evidence-based practice information to insurance companies that are currently denying coverage for these systems.
Surveys were administered to patients and their prosthetists in four different Physiotherapy offices to compare vacuum and pin suspension systems. Prosthetists were questioned about their patients' components and diagnosis to ensure accuracy. The patients' survey asked about comfort, stability, skin breakdown, falls, volume changes, knee flexion, and activity levels. The patient and practitioner surveys were linked by a number identifying the office and survey number. Thirteen of 20 replies were completed and usable. Because of various suspension systems and amputation levels, all 13 surveys were not eligible for each comparison. The inclusion criteria for patients to participate in the study were as follows:
Must be at least 18-year old and have normal cognitive abilities to properly consent to the study.
Must not have any cardiovascular problems or health problems that would prevent him or her from walking, aside from the amputation.
Must be at least at a K2 level, K3 level, or K4 level and able to walk 100 ft continuously with or without an assistive device.
Must have been an amputee for at least 6 months to reduce the effect of volume fluctuations.
Must have been using the definitive prosthesis for at least 30 days.
Participants were informed that their prosthetist would not see their survey results to help them keep answers as honest and accurate as possible. Instructions were also given to skip any questions that they did not know, understand, or were unsure of.
Stability during ambulation was measured with the ABC Scale, which is a validated study designed to measure a patient's confidence and balance during a variety of daily activities to predict a probability of future falls.7 Patients rated their confidence in performing 16 different activities on a scale of 0% to 100%. Comfort of the suspension system was measured by using questions on both the patient's perception of comfort and the amount of skin breakdown. Both questions were used, because many amputees also have decrease in sensation caused by diabetic neuropathy. To measure volume fluctuations within a single day, patients were asked how many sock plies they add or remove between the time they wake up and go to bed at night. Patients also were surveyed on their activity levels and number of falls.
The ABC scores of transtibial amputees currently using an electronic/microprocessor vacuum system when compared with their scoring of previous pin locking suspension use is shown in Table 1. Survey results were excluded for incompletion, not having used both vacuum and pin locking suspensions, using a mechanical vacuum system, and for not having a transtibial amputation level. It was necessary that users had worn each system in its definitive version for at least 30 days. The ABC score was determined by the patient rating his or her confidence performing 16 different activities on a scale of 0% to 100%. These scores were averaged to determine an overall ABC score. This comparison found a significant difference in ABC scores between vacuum and pin suspension users with 95% confidence. There was no significant difference in ABC scores between vascular and nonvascular users in the same population.
Problems experienced by vacuum and pin suspension system users were compared by amputees who had used both systems in Table 2. This comparison included both transtibial and transfemoral amputation levels and both mechanical and electronic vacuum suspension systems. All users used both pin and vacuum suspension systems for at least 30 days in a definitive socket. Issues that were experienced in both suspension systems by a single user were excluded. Volume fluctuations were quantified by asking patients whether they added or removed socks within a single day. The number of socks was not taken into account because of the subjective nature of the question and the number of patients who do not sock properly. When compared with socking results using pin suspension, the difference was not statistically significant. Although there were differences in the other problems experienced by users of each suspension system, these were not statistically significant.
Differences in falls and activity levels were investigated by surveying patients who had experience wearing both pin and vacuum suspension systems. The population used for the comparison was identical to that of Table 2. The results are summarized in Table 3. Although subjects using pin suspension had more falls, and more users increased their amount of ambulation with vacuum suspension systems, these numbers were not statistically significant.
The ABC Scale is used to predict a patient's mobility and falls by differentiating capabilities from limitations caused by a fear of falling.7 Scores were significantly higher for users of elevated vacuum suspension systems than pin locking suspension as shown in Table 1. These higher scores correlate with a lower incidence of predicted future falls.7 This may be due to the more intimate fit of the socket, more secure hold, and decrease in pistoning.1 According to the survey results, patients fell fewer times on average with vacuum suspension, which correlates with the ABC scores' predictions. Although the fall results were not statistically significant, they correlate with the ABC scale's predictions.
Table 2 compares the problems experienced by patients who had worn both pin and vacuum suspension systems. Patients reported increased pistoning using pin locking suspension compared with vacuum systems. This result is consistent with previous results reporting increased displacement in pin systems compared with vacuum.1 Skin problems were more prominent in users with pin suspension rather than vacuum. This is possibly due to decreased volume fluctuations previously reported from vacuum suspension.1 A decrease in volume fluctuations prevents the socket fit from being altered. It must be noted that this reduced volume loss was not able to be quantified in this study. However, volume fluctuations were measured by patients reporting how many socks they added or removed throughout the day. Because this is a very subjective way to measure volume changes and many patients do not do perform this task correctly, these results should be investigated further and not taken as reliable evidence.
It is interesting to note that increased blistering was experienced with pin suspension in comparison with vacuum suspension. This may be because the pin suspension systems were older and were not fitting properly by the end of the patient's use of them. Blisters may be experienced with vacuum suspension in the case of an air gap or improper fit. The lack of blisters may be taken as evidence that the newer vacuum suspension sockets fit the patients properly. Increased activity levels in some patients wearing vacuum systems in comparison with their previous pin suspension systems are reported in Table 3. Fewer skin problems and therefore increased comfort may account for a portion of the increased wearing and walking times in some participants.
Ideally, this study should be repeated with a larger sample population, which is more representative of the actual population. According to the Amputee Coalition of America, 82% of limb losses were due to vascular disease in 1996, which is slightly higher than the population surveyed in this study. Exact age distributions for users with limb loss could not be found, but it is believed that in this study, the patient population was slightly younger and more active than average. Contributing to a perceived younger-than-average population is the fact that there is a “gadget tolerance” involved with using a microprocessor/electric vacuum system because the pump must be charged on a daily basis and maintained properly.
The main concern in this study is that a bias may have been created because all users were previous pin suspension users and current vacuum suspension users. A newer socket is expected to fit better than a socket at the end of its lifespan. Depending on how long patients had been using the vacuum suspension system, some may have found it difficult to remember details about their past pin suspension system. However, given the results of this study, it is hoped more research will be done on vacuum suspension systems and their effect on activity level and stability.
The author thanks Physiotherapy Associates and all its practitioners and patients who participated in this study.
1. Board WJ, Street GM, Caspers C. A comparison of trans-tibial amputee suction and vacuum socket conditions. Prosthet Orthot Int
3. Street GM. Vacuum suspension and its effects on the limb. Orthopadie Technik
4. Beil TL, Street GM, Covey SJ. Interface pressures during ambulation using suction and vacuum-assisted prosthetic sockets.J Rehabil Res Dev
5. Beil TL, Street GM. Comparison of interface pressures with pin and suction suspension systems. J Rehabil Res Dev
6. Goswami J, Lynn R, Street G. Walking in a vacuum-assisted socket shifts the stump fluid balance. Prosthet Orthot Int
7. Powell LE, Myers AM. The activities-specific balance confidence (ABC) scale. J Gerontol Med Sci
KEY INDEXING TERMS: vacuum suspension system; prosthetics; transtibial; amputation; suspension; residual limb; skin health; Activity Balance Confidence Scale