The Effects of Vacuum Assisted Socket Environment on Proprioception in Transtibial Amputations

Heather Swiggum
Prosthetic Resident
August 2004

Abstract

Proprioception is the ability to sense limb motion and joint position in space. It is believed that mechanoreceptors found in capsular and extracapsular tissues are involved in creating this effect. When an amputation occurs, mechanoreceptors are eliminated about the ankle joint in transtibials, and complete loss at the knee is experienced in transfemorals. The proprioception lost, inherently affects the individual’s ability to establish and maintain his or her center of gravity, which affects balance, postural control, and gait. All of which can affect a person’s activity level, confidence in using a prosthesis, and overall mobility. One recently developed system is the vacuum assisted socket system (VASS). The purpose of this pilot study was to investigate the possible relationship between proprioception and the VASS system.

Data for the pilot study was obtained through a series tests, encompassing two-dimensional target identification trials and proprioception depth detection. Thirteen subjects, with transtibial amputations, participated. Seven participants were using the VASS suspension system, and six were using a 1-way expulsion valve suspension system. Comparisons were made between subjects using VASS suspension, and the subjects using 1-way expulsion valve suspension. Assessment compared the amputated limb of one group to the amputated limb of the opposing group, and the amputated limb to the contralateral limb of each participant. It was theorized that VASS suspension would provide an increased level of proprioception over 1-way expulsion valve suspension. Findings from the study proved to be statistically insignificant, though the information gained may prove to be beneficial for the future of prosthetic design and application.

Introduction

Proprioception, or sense of feel, can be defined as the ability to sense the position, location,orientation, and movement of the body and it’s parts in space.¹ Proprioception is the function of mechanoreceptors. These mechanoreceptors act by transmitting information regarding movement and positioning of the limb to the spine, which responds by sending signals back to the limb to adjust for the movement. It is believed that these specialized mechanoreceptors are located within capsular and extracapsular tissues, including joint capsules, ligaments,menisci, muscles, and tendons, with higher concentrations in the ankle joint,and lesser concentrations found the more proximal the joint; knee and hip.^2-4 For an individual with a lower extremity amputation, transtibial or transfemoral, it is evident that the mechanoreceptors about the ankle area have been eliminated, resulting in a substantial loss of important proprioceptive information.⁵ The loss of this sensory feedback affects the individual’s ability to establish and maintain his or her center of gravity. This deficit can then be demonstrated through gait deviations, contributing to a decreased velocity and slower cadence; functionally translating into increased energy cost during ambulation, decreased sense of confidence in using the prosthesis, and reduced overall mobility.⁶ In a survey of individual’s with lower extremity amputations, one of the questions posed was to explore the most prominent difficulties faced by individual’s in their daily lifestyles and in their activities of daily living. The leading answers included physical stamina, balance, and gross motor movement.⁷ Though studies focusing on proprioception about the knee and hip area in individual’s with transtibial and transfemoral amputations have been performed and presented, studies exploring proprioception with regard to socket design and suspension form have been very limited.

To date, the role of sensory feedback in the lower-limb prosthesis has received little attention when compared with the number of investigations for the upper-limb prosthesis. It can be seen throughout the prosthetic industry the advances in materials and state-of-the-art prosthetic designs have enabled individual’s with amputations to regain more and more of their mobility. On a daily basis, prosthetic practitioners are working with their patients in prosthetic design, material and component usage, and optimal alignment to enhance the patient’s sense of feel and function. There is, however, a limitation to how much a Prosthetist can do. The technological advances in materials, component design, and socket design we are seeing today may eventually reach a limit in their ability to emulate function of a non-amputated limb. Devices that focus on restoration of lost sensations through amputation may prove to be key aspects in future technology.

Human locomotion is a complex integration of musculoskeletal structure and neurophysiologic function.⁸ Proprioceptive and sensory information is used to select the appropriate motor responses to achieve a smooth and efficient gait. When possible, prostheses should provide the user with not only the appropriate biomechanical performance, but also the supplemental sensory information that will aid in enhancing functional use. One recently developed system that has been promoted for its ability to increase proprioception in the lower extremity amputation,primarily transtibial and transfemoral, is the Vacuum Assisted Socket System (VASS). The VASS system operates through the application of constant vacuum between the residual limb, liner, and socket wall. This created vacuum environment is believed to enhance linkage, and thus proprioception.⁹ It was theorized, and the hypothesis of this pilot study, was that participating subjects utilizing the VASS system would demonstrate an increased level of proprioception when compared with their counter-participants utilizing a non-active vacuum environment, the 1-way expulsion valve system.

Presently, research studies focusing on the relationship between the vacuum socket environment and proprioception have not been presented. In addition to investigating the hypothesis, the purpose of this pilot study was to examine the possible relationship between proprioception and transtibial suspension systems, and to assess potential affects the VASS system may have on the future of the prosthetic industry.

Previous Investigations

Studies that have analyzed proprioception and how it is affected when an amputation occurs have looked primarily at the limb itself, neglecting the possibilities of how the prosthesis and form of suspension may be influencing the element of proprioception.

As far back as the 1950s, when suction socket suspension was still relatively new, clinical evidence found that in addition to improving circulation about the residual limb, one supplementary advantage was better control of the prosthesis and improved sensory feedback of the limb position and contact with the ground.¹⁰ In a 1986 survey of individuals with transtibial amputations, regarding suction socket suspension and what these individuals liked most about the suction environment, responses included increased mobility or possible activity, and better control or positive feeling of the ground.¹¹ As a result of these and previous findings, researchers have launched studies into examining proprioception about the knee joint in transtibial amputations and at the hip joint in transfemoral amputations. The methods used to examine proprioception have included testing procedures of passive motion reproduction,passive motion threshold detection, postural sway, and balance performance.^{5,12-13,25-27} Findings have been very similar throughout the studies, allowing positive conclusions to be made with regard to the valuable affects proprioception can have on an individual with an amputation.

In a study involving nine males, with transtibial amputations, and nine age-matched control subjects, outcomes revealed that subjects with the involved amputation were unable to detect passive leg motion as well as their sound limb or the limbs of the control subjects.¹² There was however, no significant difference in the angle reproduction test between the two groups.³ For further explanation into why the results of this study showed passive leg motion being more difficult than passive angle reproduction for an individual with an amputation, studies regarding the mechanoreceptors of individual’s with amputations were addressed.

Investigations looking at transtibial amputations and the responsible mechanoreceptors for proprioception have found that all of the mechanoreceptors of the knee and the mechanoreceptors of the muscles and tendons crossing the knee joint may be considered physically present. However, it is shown that knee flexor and extensor muscle strength is significantly diminished; making it conceivable that an actual deficiency exists in the number of mechanoreceptors and mechanoreceptor sites.^2-4 Research relating transfemoral amputations and proprioception concurred with similar findings.

In a study by Eakin, Quesada, and Skinner, using ten men with transfemoral amputations, subjects were tested for passive angle reproduction and threshold of passive motion. As with the transtibial results,the prosthetic limbs performed significantly lower in the passive motion test as compared to their sound side limb, but produced near equal results in the angle reproduction test. The authors concluded that in the absence of mechanoreceptors normally found about the ankle and knee, alternative mechanisms were used to provide proprioceptive clues of the residual limb. These mechanisms may have involved hip motion cues, pressure changes about the limb, and sensory cues from the surface and deep tissues of the residual limb.¹³

Through the test results of the afore mentioned, researchers have determined that when limb loss occurs, the capabilities of proprioception are greatly diminished and must be reorganized within the remaining skin, soft tissues, and the structures of the joint just proximal to the amputation site. This reorganization will help to compensate for the needed proprioceptive information in daily functional activities.

In an effort to restore lost proprioception, there have been a limited number of published attempts in the development of products that function to provide sensory feedback and proprioception for lower extremity amputations. One such device was the Sense-of-Feel (SOF) device by Sabolich and Ortega.¹⁴ This device functioned as the mechanoreceptors necessary for proprioception to occur.

The SOF device was a non-invasive sensory feedback system the provided transcutaneous electrical neural stimulation to afferent sensory organs located at the residual limb/socket interface. The sensors responded to pressures applied to the plantar surface of the prosthetic foot. Corresponding signals from the sensors were sent back in proportion to the strength of the pressure,and applied to the residual limb. What Sabolich and Ortega hypothesized was that the SOF device would significantly improve gait quality, efficiency, standing balance, and symmetry of stance phase duration and step length. They also theorized that an improvement in the above measures would help provide further insight as to the internal processes that occur during sensory feedback and the long-term affects it may have for individuals with lower extremity amputations. Their results showed in both the transtibial and transfemoral group that performance increased notably with respect to symmetry in weight distribution, step length, and stance. Sabolich and Ortega suggested that these improvements, with just a short acclimation period to the SOF device,represented a close relationship between proprioception and residual limb output. They also proposed it was possible that the sensation provided by the SOF may have increased awareness of timing issues throughout the gait cycle as well as enhanced the kinesthetic cognizance of the prosthetic foot interaction with the ground. It was concluded that with an increased sense of feel and proprioception, individuals with lower-limb amputations would have the ability to improve their gait symmetry, increase gait efficiency which would ultimately reduce the energy cost of ambulation, feel an increased level of confidence in using the prosthesis and improve overall mobility.¹⁴

When all of these studies are evaluated and their findings examined together,one will notice that with time, the connection is being made correlating proprioception of the body and it’s functional importance to individuals with lower extremity amputations. The vacuum assisted socket system may be one device that proves to significantly increase proprioception, and in doing so,provide the benefits that have been found in the studies of proprioception.

Methods

As can be seen from previous research,proprioception plays an integral part in the functions and movements of daily life, which is a necessity for individuals with lower-limb amputations. Very few devices are currently available to the lower-extremity amputation population that aid in providing proprioception. The vacuum assisted socket system may be the foundation needed to begin resolving the subject of proprioception.

Subjects

Subjects participating in the study were male and female with unilateral transtibial amputations. Causes of amputation included traumatic and vascular/disease. Eligibility of age was limited to 13 years of age to 65 years of age. All individuals were wearing a prosthesis on a daily basis for six months or more. Eligible subjects also met the following criteria:

1. Suspension through 1-way expulsion valve or vacuum assisted suction (VASS)

2. Daily wear time of four or more hours

3. A K2, K3 or K4 ambulator, as determined by the participant’s attending Prosthetist.

4. Pain-free residual limbs

5. No current evidence of tissue breakdown on the residual limb

6. A well fitting and comfortable prosthesis, fitting with 3-ply of socks or fewer

7. Assistive devices limited to single-point support

8. Contralateral limb free of any ankle, knee or hip orthoses

Experimental Protocol

All participating subjects were tested using the proceeding methods. (See Appendix 1 for photo reference)

Experimental participants were divided into 1 of 2 groups according to their suspension style. Seven subjects comprised the VASS group, and six of the 1-way expulsion group.

All subjects stood within the casting stand, facing the target board. The target board was attached to the wall, measuring 3 inches from the ground. This location remained constant for all participants in each of the groups. The casting stand was positioned 19 inches (483 millimeters) from the target board. This distance was pre-determined, allowing all subjects to successfully contact the target board. The maximum height of a given target was 11 inches from starting position on the platform. From midline, two-dimensional target points ranged from 4½ inches to 9¼ inches across on the left side, and 7 inches to 8 ½ inches on the right side. Four total two-dimensional targets. The fifth target point, proprioception depth,was positioned at midline, 4 inches above platform level.

The target board was constructed using corrugated poster board, 18”x24” in size. Elastic bands were attached to the ends of the board to securely hold the paper targets. A template of target locations was created to help ensure all subjects were identifying corresponding target locations. Construction of the target template was completed as to require the participants to identify target locations of various heights and distances from midline. This would also require the participants to use different hip and knee angles.

A universal sandal was fabricated which allowed for attachment of the marking instrument to the participant’s shoe. This sandal was suspended using Velcro straps. Two different colored marking instruments were used to identify the limb being tested, amputated or contralateral.

Testing Method

When the participant was ready for testing, verified through proper positioning within the casting platform, the sandal was attached to his or her shoe, and the marking instrument inserted to the toe of the sandal, test instruction would begin.

All participants were given verbal instructions on the testing procedures. These instructions included visual location of the 4 two-dimensional target locations, correct starting position, target identification procedures, and correct ending position of his or her foot. Each participant was given a trial run of 1 target point to ensure comprehension of testing procedures. This trial run allowed the participant to visually observe to procedure, though the actual testing process required the participant to be visually occluded from the target board at all times. The examiner did assist the participant in initial placement of his or her foot on a given target. Participants were also given similar instruction on the proprioception depth test (3D). Instruction for the proprioception depth test did not involve a trial run, as this test procedure was similar to the two-dimensional target location test.

With the participant having complete understanding of the given protocol,initiation of the test would begin. All subjects were visually occluded from the target board. The test would begin with the left side for all participants, amputated or non-amputated. The examiner would assist the participant with initial location of target 1. The participant would return his or her foot to starting position on the casting platform. With cue from the examiner, the participant would re-identify target 1 with their foot. The marking instrument inserted into the sandal would denote this location. This same procedure was applied for targets 2-4.

With completion of targets 1-4 on the left side, the participant was tested for proprioception depth. The sandal was removed for this test. Still visually occluded, the examiner placed a 4” block on the target board, at the previously determined midline location. With assistance from the examiner, the participant would bring his or her foot in contact with the block. At this point, the participant’s foot would be 4 inches from the target board. The participant would return their foot to starting position and the block would be removed from the board. Cue from the examiner would prompt the participant to identify in space the location they believed to be 4 inches from the target board. This distance was measured using a metric ruler for improved accuracy.

Completion of the proprioception depth test and the two-dimensional target identification test would conclude testing of the left side. These procedures were repeated for the right side and maintained for all subjects participating in the study.

Individual subject results and group results were analyzed. Test results from the two-dimensional target identification and proprioception depth tests were compiled using the measured distance away from the actual target location. The resultant information was evaluated for the amputated side and the contralateral side using group means, standard deviations, and t-test values of independent sample means. (Appendix 2) Figures were computed to show statistical significance and percent differences between the amputated limb and the contralateral limb of each individual, and the amputated limb of the VASS group to the amputated limb of the 1-way expulsion group. Significance was determined using a 1-tailed T distribution table at a confidence level of P=.05 level. (Appendix 3)

Results

A total of twenty-one subjects completed the study. Sixteen of the twenty-one subjects were male; five subjects were female. Eleven subjects were of traumatic amputation, and two of the subject’s amputations were vascular/disease related. The average age of the two groups was 45.4 years. The mean age of the VASS group and the 1-way expulsion group was 47.5 and 45 respectively. (Appendix 4) On average, individuals from the VASS group have had their amputation for 8.1 years, and individuals from the 1-way expulsion group have had their amputations for 7.2 years. (Appendix 4)

Applying the values from the calculated means and standard deviations, comparison of participants using VASS suspension on the amputated side to participants using 1-way expulsion suspension on the amputated side, showed those using 1-way expulsion to perform 119.7% superiorly in the proprioception depth test. Statistically, this result was significant at the P=.05 level. Evaluation of two-dimensional target identification, within the same grouping, demonstrated the VASS participants to perform 13.2% more accurate than the 1-way expulsion participants. This result was, however, was statistically insignificant at the P= .05 level. The average of these combined test results resulted in the 1-way expulsion participants performing 4.8% over the VASS participants. This also was statistically insignificant. (Appendix 3)

Analysis within the VASS group, comparing the amputated limb to the contralateral limb in proprioception depth, found the contralateral limb performing 88.1% more accurately. Statistically, this was significant at the P= .05 level. Comparison of two-dimensional target identification results also found the contralateral limb to perform over the amputated limb by 10.7%. This was statistically insignificant at the P=.05 level, as were the findings of the combined test figures, demonstrating the contralateral limb to perform 24.1% over the amputated limb. (Appendix 3)

Evaluation of the 1-way expulsion group,amputated limb versus contralateral limb in proprioception depth scores,showed the amputated side to perform 64.9% over the contralateral limb. Two-dimensional test scores supported the contralateral limb in being more accurate by 20.6%. Analysis of the test scores combined indicated the contralateral limb performing better by 7.2%. Each of these finding, according to T-test results,were statistically insignificant at the P= .05 level.

Final assessments performed on the contralateral limb of each of the 13 participants showed the VASS subjects scoring more accurately in both the proprioception depth trial and the two-dimensional target identification trial. Average combined test scores resulted in a 10.5% increased accuracy by the contralateral limb of the VASS participants. This was statistically insignificant at the P= .05 level.

As a result of insignificant findings, this research study would accept the null hypothesis; the VASS system does not provide increased proprioception when compared with the 1-way expulsion valve system.

Discussion

Research has found that improved proprioception in individuals with lower limb amputations has the capability of increasing activity levels, improving confidence in using a prosthesis, and enhancing overall mobility; characteristics that could benefit all persons with lower-limb amputations.

As seen from the results of this pilot study,findings were insignificant. This would imply that the VASS system does not provide the increased proprioception as speculated. Findings would also reveal that with exception of the proprioception depth test, VASS versus 1-way expulsion, one form of suspension did not perform superiorly over the opposing form of suspension. Evaluation of the contralateral limb to the amputated limb, the contralateral limb usually performed more accurately. The culmination of these results may be in correlation to the design of the study,and various components of the study influencing outcomes.

One of these influencing factor, affecting the insignificant results, was the small sample size of the study. Small sample sizes reduce the amount of allowable variance within the data sets yet still affording statistically significant figures. Results were compiled using the measured distance, identified by the subject, from the actual target location. This allowed for an infinite number of test results, rather than a restricted number of results. Additionally, five data points in total were collected from each participant. Amplifying the number of data points may have allowed for more significant findings from each individual and thus, greater overall findings.

Contributing features that may have been patient related included age, cause of amputation, and number of years since amputation. It is reported, that as people age, proprioception and general sensitivity decreases ³². Although the benefits of improved proprioception may best apply to older patients and those who are insensate, as a result of age or disease, effort was made to limit the eligible ages, and so reducing the possible threat to internal validity. Cause of amputation may also have a direct relation to level of proprioception and sensitivity. Individuals with amputations due to diabetes or vascular disease may have a pre-existing level of lowered proprioception and/or sensitivity associated with the disease. Individuals with traumatic amputations may not experience this type of side effect associated with diabetes and vascular disease. Finally,years since amputation may play an important role in proprioception. Individuals having more experience with using a prosthesis may have developed certain ‘feelings’ they are able to associate with the relation of their prosthesis in space.

Testing and data analysis revealed interesting observations. In the testing phase of each participant, scores were predisposed to being more accurate when the target was on the ipsilateral side of the limb being tested. As the targets required the individual to cross their leg over the midline of their body, accuracy declined.

Although the weight of each prosthesis was not recorded, this may have been a factor for the individuals using VASS suspension. As can be seen from the comparison contralateral limbs, VASS versus 1-way expulsion valve, participants in the VASS group performed more accurately during all tests than the 1-way expulsion valve participants. This finding may lead one to believe that weight of the prosthesis was a contributing factor. Most of the participating subjects were using the PRS pump, an older version of the VASS pump. The weight of the PRS pump is approximately two pounds. The most recently introduced VASS pump, the P2, is reported to be 40% lighter. Even though the patient wearing the VASS system may not feel the weight of the pump due to enhanced linkage with the residual limb, the additional weight may play a role in disguising tangible perception in space.

Again, although the results of this study did not prove statistical significance of increased proprioception when using the VASS suspension system, the results of this pilot study may be applied to future studies in proprioception. These findings do show that proprioceptive capabilities are lost when amputation occurs. Just as our goal as prosthetic practitioners is to restore function for daily activities in our patients, we may also need to consider restoration of proprioceptive feelings. Functionally, this could translate into patients reducing their reliance on visual cues of how the prosthesis is responding to certain movements or actions, it may ameliorate an individual’s gait pattern, seen through increased overall mobility, an increase activity levels, and reduction in energy expenditure. All of which may significantly benefit such populations as the insensate, aging, bilateral amputations, and high-level amputations.

Pertinent to the manufacturers of prosthetic componentry,proprioceptive aptitude may require advances in feet and knee components. The capability of components may need to ‘keep up’ with the capabilities of the lower-limb amputation and their sense of feel.

Areas of further research may need to focus on specific populations; the insensate, the aging population, congenital amputations, young persons and pediatrics. Future research would also need to incorporate a larger sample population. Initial intentions with this study were to incorporate findings from individuals using pin-locking systems, and sleeve suspension (i.e. pelite liners). It may also be beneficiary to future research for a comparison of persons without amputations to persons with amputations. The ability of person’s with an amputation to reproduce angles using various forms of suspension may substantiate further research, looking from a different perspective of proprioception. Additionally, research may need to investigate what possible affects specific components may play in proprioception.

Conclusion

One simple function of the human body, that provides an undo amount of information important in determining the way in which one performs certain activities, is often taken for granted until it is lost. Though advances in research are being made to solve the issue of proprioception and lower extremity amputations, little knowledge is still known as to the possible benefits provided. Most of the research had entailed subjects who are healthy, and leading a generally active lifestyle. Knowing how a suspension system may affect proprioception, and consequently gait patterns, energy expenditure, prosthetic use, and activity level, it may beseem the field of prosthetics and orthotics to further explore the path of proprioception in the atypical populations.

The purpose of this study was to explore the possible relationship between proprioception and the VASS suspension system, as well as consider any existing differences in proprioception between VASS suspension and 1-way expulsion valve suspension. Though the statistical findings were insignificant, it was apparent that various forms of suspension would provide a patient with different qualities and characteristics. Future research will only help to provide prosthetic and orthotic practitioners with the necessitated information to best fit the needs of their patients.

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Appendix 1 - Study Photos

Casting Bars / Platform	Sandal with marking instrument	Sandal with marking instrument
Target Board	Target Sheet	Casting Bars and Target Board
Sandal Attached to Shoe	Two - Dimensional Target Identification Start	Target Identification
Proprioception Depth 4" Block	Proprioception Depth Location	Proprioception Depth Identification
Proprioception Depth Measurement	Proprioception Depth Sample Results	Two-Dimensional Test Results

Two-Dimensional Test Results	Completed Test Trial