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Pressures at the Residual Limb-Socket Interface in Transtibial Amputees with Thigh Lacer-Slide Joints

Kazuko L. Shem, MD
James W. Breakey, PhD, CP
Peter C. Werner, MD

ABSTRACT

The residual limb-socket interface pressures were measured in 13 transtibial amputees who use prostheses with side joints and thigh lacers. The hypothesis of this study was that the interface pressures with a thigh lacer are lower than the pressures without a thigh lacer. Comparison of the interface pressures with and without thigh lacer-side joints revealed a significant difference (p < 0.05). The reduction in the interface pressures indicates that a thigh lacer redistributes the pressures from the residual limb onto the thigh, and a thigh lacer-side joint suspension may be indicated in some transtibial amputees.

Introduction

Achieving appropriate distribution of pressures at the residual limb-socket interface is critical for successful prosthetic fitting for, and rehabilitation of, transtibial amputees. Pressures placed on a residual limb in a prosthetic socket can contribute to tissue breakdown, discomfort, adventitious bursae, and poor function in transtibial amputees.^1-5 Prolonged static pressure and shear, for example, are known causes of blood flow occlusion.⁶

In this study, interface pressures were measured in transtibial amputees who use thigh lacer-side joint prostheses. The thigh corset, usually made of leather, is a form of suspension fastened around the distal two thirds of an amputee's thigh.⁷ It is attached to a transtibial socket by metal joints with vertical support bars. Prior to 1958, thigh lacer-side joints were the most common form of suspension for transtibial prostheses. With the advent of total contact sockets, thigh lacer-side joints fell out of use. Today, most transtibial amputees use patellar tendon bearing (PTB) prostheses without any suspension or with a supracondylar strap.⁸ Thigh lacer-side joints are used for amputees whose skin on the residual limb cannot tolerate applied forces or for those who have knee instability. Less than 1% of transtibial amputees are candidates to use them.^7-9 It has been assumed that the thigh lacer provides additional area over which to distribute the force. However, no prior research has been done to document how much, if any, pressure is relieved by the thigh lacer.

Several studies have been done to measure and analyze interface stresses in transtibial amputees.^5,10-18 A conventional strain gauge, a fluid-filled sensor, a capacitance bridge, a piezoelectric crystal, a magnetic inductance transducer, a printed circuit sheet, and a plunger piston-type force gauge have been used as transducers to measure the interface pressures.^5,11,16,20 In one study, a combination of strain gauges on a beam and a diaphragm strain gauge mounted in a socket was used to measure biaxial shear stress and normal stress, respectively.¹² However, most methods used to date are not practical in a clinical setting, because the measuring devices are often built into a prosthetic leg specifically made for measuring pressures. In this study, a Rincoe Socket Fitting System (SFS)a was used to measure interface pressures. With this system, sensor strips are placed in between the residual limb and the socket, and no modification of the prosthesis is necessary.

The objective of this study was to measure the amount of interface pressure relieved in transtibial amputees with thigh lacer-side joint prostheses. Static pressures at the interface with and without the thigh lacer were measured. The hypothesis of this study was that the interface pressures with the thigh lacer are lower than the pressures without the thigh lacer. The resulting reduction in pressure may indicate that the thigh lacer redistributes the pressure from the residual limb onto the thigh.

Method

All unilateral amputees (12 men and one woman) who use the thigh lacer-side joint suspensions at Breakey Prosthetics, Inc., were recruited and enrolled in this study. The subjects signed consent forms for the Santa Clara Valley Medical Center Research and Human Subjects Review Committee. Ages of the subjects ranged from 34 to 80 (average 58). Average weight was 178 pounds (Standard Deviation [SD] = 23). Average time since amputation was 26 years (SD = 21). Average use of the current prosthesis was 4.6 years (SD = 4.8). The subjects used the prosthetic leg an average of 13 hours per day (SD = 7). Residual limb length was measured from medial tibial plateau to the distal end using a ruler, and the average length was 4.9 inches (SD = 1.4). Causes of amputation were trauma (eight), osteomyelitis (two), diabetic ulcer (one), vascular (one), and burn (one). Only one subject had a minor pressure ulcer on the residual limb at the time of pressure measurement. All of the subjects were able to stand safely unassisted and had patellar tendon-bearing (PTB) sockets.

A Rincoe SFS was used to measure static pressures at the residual limb-socket interface. Six sensor strips were attached with 3M Health Care double-sided tape to the socket at the anterior, anterior medial, anterior lateral, posterior, posterior medial, and posterior lateral aspects of the interface (Figure 1) . Subjects were instructed to wear their usual insert or liner and then don the prosthetic leg. Thus, the sensor strips were between the socket and liner, and they did not make direct contact with a subject's skin. Each sensor strip had 10 pressure sensor dots separated by 1.5 inches. Calibrated by the manufacturer, each sensor dot can measure pressure with a resolution of 0.5 pounds per square inch (psi), up to 12 psi maximum. Although there are 60 possible sensor dots (6 sensor strips, each with 10 sensor dots) available for recording at one time, depending on a subject's socket depth, more proximal sensor dots on a strip were outside of the socket and did not record pressure (Figures 1 and 2 ).

Pressure measurements were taken with and without the thigh lacer-side joints. The subjects were asked to stand with one foot each on an analog scale, and three pressure measurements with the thigh lacer-side joints were taken when the weight placed on each scale was equal. The thigh lacer was then removed by one of the prosthetists at Breakey Prosthetics, Inc., and three measurements without the thigh lacer-side joints were recorded. The thigh lacer was then reattached by one of the prosthetists. Average pressure per sensor dot with and without the thigh lacer-side joints for each subject was calculated as follows:

Total pressure recordings from
3 trials in each subject (psi)
______________________________

3 trials x number of sensor dots
in the socket for each subject

The standard paired t-test was used to compare the pressures with and without the thigh lacer. A p value of less than 0.05 was considered to be significant.

Results

Among the 13 subjects, the average pressure per sensor dot with thigh lacer-side joints was 1.97 psi (SD = 0.61) whereas the average pressure per sensor dot without thigh lacer-side joints was 2.49 psi (SD = 0.83) (Table A) . Comparison of the interface pressures with and without thigh lacer-side joints using a paired t-test revealed a significant difference (p = 0.002). The average percentage difference was 19% (SD = 14.7) with a range of -8.5% to 39.8%. There were only two subjects whose interface pressures did not increase without the thigh lacer-side joints.

The interface pressures with and without a thigh lacer suspension were further analyzed longitudinally at anterior, anterior medial, anterior lateral, posterior, posterior medial, and posterior lateral sensor strip sites (Figure 3) . Pressure recordings were averaged for the three trials with and without the thigh lacer. Paired t-test analysis of the interface pressures along each strip site with and without the thigh lacer revealed that the interface pressures increased significantly (p < 0.05) without the thigh lacer at every sensor strip site except at the anterior and posterior lateral sites (p = 0.06 and 0.053, respectively).

Furthermore, with the Rincoe SFS system, the interface pressure recordings can be analyzed circumferentially around the residual limb. Along each sensor strip, sensor dots were numbered from 1 to 10, distally to proximally. Pressure recordings from the same-numbered sensor dots from six sensor strips were averaged from the three trials, with and without the thigh lacer (Table B) . Paired t-test analysis revealed a significant increase (p < 0.05) of the interface pressures without the thigh lacer suspension at four distal sites (i.e., sensor dots numbered 1 to 4), and no significant difference (p > 0.3) at more proximal sites.

Discussion

After an extensive literature search, no prior research was found documenting how much, if any, pressure is relieved by the thigh lacer. In some rehabilitation and prosthetic textbooks, a 25% to 60% reduction in weight bearing by a thigh lacer has been noted.^21-23 However, there is no objective documentation as to how this number was arrived at. The significance of this study is as follows: If the thigh lacer-side joint suspension is found to redistribute the pressure from the residual limb to the thigh, then it may be indicated in some transtibial amputees. Additional advantages of thigh lacer-side joints are providing mediolateral knee stability, preventing knee recurvatum, and increasing proprioceptive feedback.⁷ Some of the known disadvantages are atrophy of thigh musculature, additional weight and bulk to the prosthesis, longer fabrication time, poor cosmesis, and distal residual limb edema.

According to Foort, there are six criteria usually used to determine if a thigh lacer-side joint type suspension is indicated in a transtibial amputee: (1) The amputee needs the prosthesis stabilized against rotation around the long axis of the limb for periods when the knee is flexed. (2) The amputee must do heavy lifting for an extended period of time. (3) There are instabilities within the anatomical knee. (4) The amputee is unable to support body weight on the residual limb because of pain or skin breakdown. (5) The amputee has difficulty donning the prosthesis correctly. (6) The amputee expresses a definite preference for the thigh lacer-side joints.²

A few of the subjects in this study are long-time amputees who have always used the thigh lacer-side joint suspension. One subject is using the prosthesis with the thigh lacer-side joints for skiing only (criterion 1). Some of the subjects are very active physically and have full-time employment (e.g., a house painter, a real estate agent, and a maintenance person) involving extensive walking or stair climbing (criterion 2). One subject with a short residual limb specifically believed that the thigh lacer-side joints gave him additional "knee stability" (criterion 3). Several of them began using the thigh lacer-side joints to relieve the residual limb pressure that was causing nonhealing pressure ulcers (criterion 4). In this study, there were higher pressures at the distal ends of the residual limbs without the thigh lacer-side joints than with the thigh lacer in many of the subjects (Table B) . A probable explanation for this phenomenon is that the residual limb has a tendency to sink into the socket when the thigh lacer is removed. Thus, it seems appropriate that the thigh lacer-side joints are considered for transtibial amputees who cannot tolerate pressures on the distal residual limb and who have pressure ulcer problems.

The Rincoe SFS has been designed to assist prosthetists in fitting sockets to the residual limbs of amputees. With the Rincoe SFS, the interface pressures can be evaluated at each sensor dot as well as either circumferentially or longitudinally. No revision to the socket is necessary to measure the interface pressure, because each flexible sensor strip, 0.012 inches thick, can be placed between a socket and a liner. In several studies, a strain gauge diaphragm transducer or a silicon diaphragm with an integrated circuit was mounted into a socket to measure the interface pressures.^5,6

There was a total of 850 data points in this study, and five data points exceeded the maximum measurable pressure of 12 psi. For those data points the pressure was considered to be 12 psi for the purpose of data analysis rather than excluding those data points completely. There were only five of these, and the actual pressures were probably close to 12 psi. We felt that this assumption would not influence the data analysis significantly.

Although absolute pressure readings were used to calculate the average interface pressures, it was more important to measure the interface pressure differences with and without thigh lacer-side joints within each subject rather than to analyze the absolute pressure data for any given subject. Therefore, a paired t-test was used for statistical analysis that showed a significant difference (p < 0.01) between the interface pressures with and without the thigh lacer-side joints. The reduction in the interface pressures indicates that the thigh lacer redistributes the pressure from the residual limb onto the thigh. It was unclear why two of the subjects had a decrease in the interface pressure without the thigh lacer suspension.

Measurement of pressures during gait also may have shown pressure relief with the thigh lacer-side joints; however, dynamic measurements could not be taken. The Rincoe SFS sensor strips could only measure pressures up to 12 psi, and the interface pressures would have surpassed 12 psi during gait. In a prior study, the pressures were noted to be higher during gait than those during stance, up to 60 psi during normal gait.^5-19

In conclusion, there was a significant increase in the interface pressures without the thigh lacer-side joints. Percentage increase in the interface pressures without the thigh lacer-side joints ranged from 3.4% to 39.8%, with an average of 19.0%. Thus, the previous assumption that the thigh lacer provides additional area over which to distribute the pressure is quantitatively proven in this study. A thigh lacer-side joint suspension should be considered in transtibial amputees who meet the above criteria.

Acknowledgment

The authors acknowledge assistance by Mike Gidding and Chris Pimental of Breakey Prosthetics, Inc. and assistance in statistical analysis by Karyl Hall and Jerry Wright, Santa Clara Valley Medical Center.

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