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Quantifying Interface Pressures in Below-Knee-Amputee Sockets

The purpose of this pilot investigation was to test a method of measuring pressure on the socket/residual limb interface for both clinical and research settings. Thin pressure sensing mats that could measure pressure over both large and small regions were used. The pressure sensing system was used to measure the pressure between the socket and the residual limb of two male subjects (10 and 30 years old) during standing and walking. Data were displayed on a computer screen, graphical masks were made around modification sites, and pressures were determined for regions bounded by the masks. In addition, a series of masks were made at each of two different modification regions to illustrate pressure gradients in a region. Results indicted that the mats measured the interface pressure between the socket and the residual limb for both standing and walking without utilizing a specially fabricated socket or prosthesis. Results were similar to those values previously reported. In addition, it was demonstrated that the pressure gradient varied in a modification region and was not consistent between regions. Such measuring systems could be first used to determine appropriate pressure ranges that probably exist for individual modification sites on the residual limb then used to quantitatively evaluate the fit and comfort of new sockets.

Introduction

For a below-knee amputee (BKA) the ground reaction forces during gait support are transferred from the prosthetic foot, through the prosthetic socket, to the residual limb. Despite the fact that the superficial tissues of the limb were not designed to bear the loads associated with gait it has been established that these loads are tolerable in certain regions.2,3 Hence, it is the task of the prosthetist when fabricating the prosthetic socket to direct loads or high pressures to the regions that can tolerate them and away from regions that cannot. If this is accomplished then the socket is comfortable, the residual limb is healthy, and the BKA is able to engage in activities without pain or discomfort. If it is not accomplished then the socket is uncomfortable, the residual limb is unhealthy (e.g., chafing, bruising, and pressure sores occur) and the BKA is limited in his/her activities.

Currently the fit of the socket is evaluated by the prosthetist using a clear durrplex check socket. The patient dons the socket and the prosthetist observes various levels of skin discoloration depending upon the loading or pressure in that region. (i.e., increased pressure in a particular region results in a lighter skin color). In regions of high pressure (e.g., the patellar ligament) the skin will discolor and in regions of low pressure (e.g., the crest of the tibia) the skin color will remain normal or close to normal. The current subjectivity associated with this evaluation process does not allow for the determination of specific pressure requirements between the socket and the residual limb.

Pressure between the socket and the residual limb has been measured by previous investigators. 1,3,4With respect to the present investigation, two limitations of these studies appear noteworthy. The first is that the sensors used to measure the pressure required the fabrication of special sockets and prostheses. Thus the loading conditions of the socket-residual limb interface could not be utilized in a general clinical setting with this method. The second limitation is that the sensors only measured a small localized region. Pressure gradients in the neighborhood of the sensors and pressures away from the sensors were not recorded. The purpose of this pilot investigation was to test a method of measuring pressure on the socket/residual limb interface for both clinical and research settings. Thin pressure sensing mats that could measure pressure over both large and small regions were used.

Methods

One 30 year old male BKA adult (Subject 1) and one 10 year old male BKA child (Subject 2) volunteered for this pilot investigation. The adult had been fitted with a new socket four days prior to testing and the child had been wearing his socket for two months. Each subject had a different prosthetist and became an amputee as a result of trauma.

A pressure sensing system developed by Tekscan was used to measure the pressure between the socket and the residual limb. This system consisted of a thin pressure mat (approximately 300 x 80 x 1 mm), an ampler that attached to the subjects leg, and analysis software compatible with a DOS computer. The mats were unique since they were thin enough to easily fit between the socket and the residual limb without altering the fit of the socket and they could measure pressure for both small and large regions (i.e., the mats contained an array of square pressure sensors that recorded pressure over a 25 mm square region).

A nylon stocking was placed on the residual limb of the subject and four pressure mats were taped to different positions on the limb (anterior, posterior, medial and lateral). Bony prominences determined by prosthetists to be areas of low pressure or non-loading and soft tissue locations determined to be areas of high pressure or high-loading were marked on the mats. The socket and prosthetic outer were then placed over the residual limb and mats in the usual manner. The subjects were asked to stand normally while data were collected from the Tekscan system for a two second period (50 Hz). Two trials of data were collected from each pressure mat. Next the subjects walked along a walkway while another two trails of data were collected from each mat.

For the standing condition the data from each pressure mat were averaged and displayed on the computer screen in a color coded format. A graphical mask was then made around each of the modification sites, as determined by the markings on the mat, and the software determined the pressure bounded by the mask. For walking a similar procedure was followed except that a pressure reading was taken within the mask for each sample of the gait cycle. In addition, a series of four masks were made at each of two different modification regions. These masks served to illustrate the pressure gradients in a particular region. The data for both standing and walking were then adjusted to incorporate the results of calibration tests performed on the mats.

Results

Figure 1 displays the average results for three selected high pressure sites during standing (i.e., patellar ligament, posterior aspect of the calf, and posterior wall or popliteal fossa) while Figure 2 displays the average for three selected low pressure sites during standing (tibial crest, fibular head, and lateral condyle of the tibia). Except for the results of the fibular head of the BKA child, the high and low pressure sites were relatively consistent for each subject, although there were differences between subjects.

Figures 3 and 4 present the high and low pressure sites respectively, for the support phase of walking for Subject 1. Figures 5 and 6 display the same results for Subject 2. For the high pressure sites during walking it can be observed that magnitudes of the forces were larger and less homogeneous than those recorded for standing. The pressure on the posterior wall had the greatest magnitude for Subject 1 while the pressure on the patellar ligament had the greatest magnitude for Subject 2. The peak values were about twice those of standing for Subject 1 and three times for Subject 2. The maximum pressure in the low pressure regions appeared to have about the same magnitude as those for standing with Subject 1 but were slightly larger for Subject 2.

Figure 7 presents the pressure values from four different locations in the region around the distal end of the fibula. It can be observed that the shape of the pressure-time curve is quite uniform for all regions. However the magnitude is increased as the pressure is recorded farther from the modification site. Figure 8 displays the pressure-time values for the center of the patella tendon bar and additional values obtained in the modification neighborhood. In contrast to the results of Figure 7 , the shape of the curves were not uniform and the magnitudes did not always change in a uniform fashion. Since the purpose of Figures 7 and 8 was to display variation in pressure gradient in the modification region only, specific magnitudes were not included in the figures.

Discussion

The purpose of this pilot investigation was to test a method of measuring pressure on the socket/residual limb interface for both clinical and research settings. Thin pressure sensing mats that could measure pressure over both large and small regions were used. Two limitations should be noted for this investigation. The first was that the shear forces associated with the pressures and their affects are unknown. The second limitation was the compliance of the sensor. The sensor is a printed circuit consisting of a mylar substrate with insulating, conductive and resistive layers. These layers were very compliant about a transverse axis. However, when the sensor was required to conform to a curved surface along its longitudinal axis or a combination of the two axes (e.g., patellar ligament) it wrinkled and sensing capabilities were lost in particular regions. This occurred in a few instances and was compensated for by interpolating between adjacent cells to fill in the missing data.

Despite the loss of data in a few regions it is quite clear from the results that the mats measured the interface pressure between the socket and the residual limb for both standing and walking without utilizing a special socket or prosthesis. The results were quite consistent within subjects but varied between subjects. Whether the intersubject variation was due to age of the subject, age of the socket or fabrication differences between prosthetists is currently unknown.

Results obtained for the high pressure sites were slightly less than those previously reported (i.e., 890 kPA) and results for low pressure sites are within the range of those previously reported (i.e., 0-60 kPa). 3,4 It would be expected that the high pressures recorded in the present investigation would be less than those previously reported since the pressures were averaged over broader regions covering entire modification sites. The previous investigations used small individual sensors with collection areas between 12.5-32.2 mm squared.

Figures 7 and 8 demonstrate that the pressure gradient varied in the surrounding a modification region. These kinds of data have not been previously reported and could be valuable in assessing blending requirements in the fabrication of a prosthetic socket. For example, information indicating that a reduction in the area over which a modification may be blended for a low pressure region (e.g., distal end of the fibula) and still result in a comfortable socket, could allow greater regions over which to distribute load.

When the sensors are modified to enable them to conform to the shape of the residual limb interface and to improve their accuracy and reliability, pressures can be measured in a clinical setting. Such measurements in the clinical setting could be first used to determine appropriate pressure ranges that probably exist for individual modification sites on the residual limb.1 After these pressure ranges are established the system could then be used to evaluate the fit of new sockets, thereby helping the prosthetist to fabricate better fitting sockets for patients.

Acknowledgement

Funding provided by the George Reed Foundation for the Handicapped and the Variety Club of Southern Alberta-Tent 61 through the Alberta Children's Hospital.

Human Performance Laboratory, The University of Calgary, 2500 University Dr. N.W., Calgary, Alberta, Canada, T2N 1N4

References:

  1. Krouskop TA, J Brown, B Goode, D Winningham: Interface Pressures in Above Knee Sockets. Arch Phys Med Rehabil, 68:713-714, 1987.
  2. Lower Limb Prosthetics New York Medical Center, 1980.
  3. Steege JW, DS Schnur, DS Childress: Prediction of Pressure at the Below-Knee Socket interface by Finite Element Analysis. Biomechanics of Normal and Prosthetic Gait. Edited by JL Stein, ASME Winter Annual Meeting, 1987.
  4. Sanders JE, CH Daly, EM Burgess, DA Boone: An Important Prosthetic Socket Design Consideration Determined from Interface Shear Stress Measurement. Proceedings from the Seventh World Congress of the International Society of Prosthetics and Orthotics 1992.