The prosthetic socket is the most important part of the prosthesis for the more than 400,000 people living with an amputation in the United States. 1,2 If the socket fits well, the person’s ability to function is similar to that of a person with an able body. In contrast, an ill-fitting socket results in chafing, bleeding, bruising, pressure sores, and pain, which can reduce the amputee’s functional ability or compromise long-term health. These problems decrease independence and increase costs to society.
The current and traditional strategy for fabricating a socket (i.e., rectified socket) is based upon the assumption that the stump or residual limb is not homogeneous in its ability to tolerate load. 3 Hence, the natural shape of the residual limb is subjectively altered by the prosthetist to produce a socket. Most research efforts have focused on improving this socket fabrication process using complex expensive technologies [i.e., computer aided design/computer aided manufacture (CAD/CAM)], or determining the most appropriate modifications to produce a well-fitting socket (i.e., stress sensors, finite element models, spiral x-ray computed tomography). 1,4–9
Recently, we diverged from the rectified socket fabrication process by shaping the socket, except for a distal end pad, to the contours of the patient’s limb (i.e., unrectified socket). Instead of using a labor-intensive casting process requiring multiple fittings, or a costly CAD/CAM system, we used a simple, fast, alginate gel process. The purpose of this pilot investigation was to objectively compare rectified and unrectified sockets in transtibial amputees.
Ten adults (mean age, 50 ± 11 years; eight men, two women; height, 172 ± 11 cm; mass, 76 ± 22 kg) with a transtibial amputation were recruited for this investigation through media advertisement. All subjects had mature residual limbs (i.e., not undergoing major changes in stump volume because of atrophy or other destabilizing factors) and had been continuously wearing a prosthesis for at least 1 year. They were independent ambulators with no health-related problems. Subjects were excluded if they had constant recurring prosthetic problems (e.g., adherent scar tissue, neuromas, bony protuberances at distal end) and required gel inserts, or other nonstandard fitting components or methods. Patients were excluded from the energy expenditure test if their health status put them at risk for performing a graded exercise test. All participants signed an informed consent approved by the Washington University Human Studies Committee.
Subject wore a prosthesis with two different sockets for approximately 4 weeks each. The order of socket wearing was randomized, and one of three different prosthetists was randomly assigned to each subject. Except for the socket shape, the prostheses were relatively the same. The prosthesis consisted of a laminated epoxy fiberglass socket, Pelite liner, aluminum pylon, and various terminal devices (e.g., Seattle lite foot). The components were switched between sockets. Each socket was fitted to wear with a three-ply sock or less.
The prosthetist fabricated the rectified socket using the traditional method. The positive mold was made from a plaster cast and modified based upon the concept that the residual limb was not uniform in its ability to tolerate load. The patient and prosthetist established when the prosthesis was acceptable, but no more than three test sockets were permitted.
In the unrectified socket fabrication process, the positive plaster mold was made using an alginate casting method. A mixer stirred alginate powder as it was poured into a pail filled with water. After the powder dissolved, the subject placed his/her residual limb into the alginate liquid and stood for approximately 5 minutes while the alginate gelled to a semi-solid state. The subject removed his/her residual limb from the alginate gel leaving a negative mold. Plaster was then poured into the negative alginate mold to make a positive mold. The positive plaster mold was removed and very slightly smoothed with sanding screen. A distal end pad was included during socket fabrication. In addition, a supracondylar wedge was used for suspension. Unlike the rectified socket, only the original socket was used, and no additional test sockets were permitted.
DATA COLLECTION AND ANALYSIS
The subjects were tested after wearing the first socket (i.e., either the rectified or unrectified, randomly selected) for a minimum of 4 weeks. The socket was then replaced with the second socket and subjects were tested a second time after another 4 weeks of wearing time. Data from three different tests were collected: gait analysis, energy expenditure during gait (6 subjects), and Prosthetic Evaluation Questionnaire (PEQ). 10 At the end of participation, each subject chose the socket he/she wished to have on the final prosthesis.
Video data from six camera HiRes Motion Analysis Corporation systems (Motion Analysis Corp., Santa Rosa, CA) captured the images of reflective surface markers during gait. 11–15 The subject walked along a 9-m walkway at his/her freely selected speed, and video data were collected during the middle 2 m. A minimum of three trials was collected for each subject. Kinetic data were also collected from a Kistler force platform.
The location-time data of the surface markers were tracked (digitized) and converted to three-dimensional coordinates as a function of time. The tracked data were processed using standard software (Motion Analysis Corp.). The software produced data describing the averaged joint angle as a function of the complete gait cycle for each of the three principal planes of the body. Four specific kinematic variables were determined from these data: minimum knee flexion during stance, minimum hip flexion, maximum trunk lateral flexion, and maximum transverse plane trunk rotation. In addition, linear gait variables including speed, stride length, cadence, and percentage of prosthetic stance time to nonprosthetic stance time were determined. The maximum vertical ground reaction force for each leg was determined from the force plate data. The percentage of this maximum vertical force between the prosthetic and nonprosthetic legs was calculated.
Energy expenditure was assessed during a submaximal graded exercise test (GxT) that included four 4-minute stages. The first two stages were performed at 0% incline at the rates of 3.2 km/hr (2.0 mph) and 4.0 km/hr (2.5 mph). Stages 3 and 4 repeated the same speeds at 5% incline. The stages increased in intensity from 2.5 to 4.6 metabolic equivalents [a multiple of the resting rate of O2 consumption (Vo2 rest)]. The protocol incorporated specific workloads that have successfully elicited differences in energy expenditure between prostheses. 16–19
Precision-analyzed gas mixtures and a 3-L calibration syringe (SensorMedics Corp., Yorba Linda, CA) were used to calibrate the gas analyzers and flow sensor. Oxygen uptake (L/min), pulmonary ventilation (VE), heart rate (bpm), blood pressure (mm Hg), and rate of perceived exertion (RPE) (Borg scale of 6–20) were monitored during each stage of the GxT. Equipment included a Vmax 29 metabolic cart (SensorMedics Corp.), a Marquette 2000 treadmill (GE Medical Systems, Milwaukee, WI), and a Marquette Case 800 12-lead EKG unit (GE Medical Systems).
The protocol and RPE chart were reviewed with the subject. EKG leads were attached, and a supine resting EKG was recorded. The subjects were familiarized with the treadmill (approximately 2–3 minutes) and fitted with a mouthpiece, a nose clip, and headgear apparatus. During testing, breath-by-breath measures of gas exchange and heart rate were determined and stored for future analyses. RPE was recorded during the third minute of each stage as a subjective measure of intensity. For safety reasons, the protocol was modified for two subjects. One subject completed only the first stage because of her high heart rate and blood pressure readings, and the other subject completed the entire protocol at reduced speeds of 2.4 km/hr (1.5 mph) and 3.2 km/hr (2.0 mph).
Metabolic data were assessed for accuracy and extreme outliers were removed before averaging the last 30 seconds of each stage. Vo2 was normalized by body weight and reported as mL/(kg × min).
Prosthetic Evaluation Questionnaire
The PEQ was the final test for the subject during each data collection session. It was designed to quantify the patient satisfaction of lower-limb amputees. The questionnaire is composed of nine validated scales (ambulation, appearance, frustration, perceived response, residual limb health, social burden, sounds, utility, and well-being). The scales have been validated for internal consistency and temporal stability and are scored as a unit. The PEQ has been reported to display good psychometric properties. 10 The scales are not dependent upon each other and can be used independently, depending upon the need. A composite score is also permitted by averaging the individual scale scores. The composite score was reported here. Four weeks is the minimum recommended time between assessments.
Paired t-tests were used to determine whether significant differences existed between sockets for all gait variables and between prosthetic and nonprosthetic sides for lower extremity gait kinematics and kinetics (p < .05). A χ2 test was used to determine if one socket was selected more frequently than the other as the final socket (p < .05).
There were no significant differences between socket types for any of the gait variables (Tables 1 and 2). Energy expenditure, as measured by oxygen uptake during the final stage of the GxT, was not different between socket types (Table 3). These results were supported by a subjective measure of exertion, RPE mean scores, which were identical for both socket designs (Table 3). The results for the PEQ quality of life instrument were also not significantly different (Table 3). Four of the subjects selected the rectified socket as their final prosthesis and six selected the unrectified socket (Table 3). It should be noted that one of the six persons who selected the unrectified socket actually chose both sockets. He used the rectified socket for sedentary tasks and the unrectified socket for exercise. His socket selection was included with the unrectified group because of the greater demands on the socket during exercise.
The purpose of this pilot investigation was to objectively compare rectified and unrectified sockets in transtibial amputees. At least two limitations are noteworthy. The first is that only 10 patients have been fitted (six of whom qualified for the treadmill test), and the results may not be characteristic of a larger sample. The second limitation is that the method may not be applicable to all transtibial amputees. Our strategy was to avoid patients with constant recurring prosthetic problems (e.g., adherent scar tissue, neuromas, bony protuberances at distal end) and fit only relatively uncomplicated residual limbs. It was believed that if the method could be adequately applied to the majority of amputees (i.e., 70–80%), then this was a success. Addressing the suitability of the method to difficult cases could follow.
No literature could be found that discusses the method of using alginate material to cast the residual limb for the purpose of making unrectified prosthetic sockets. The method is quite simple and involves only taking an exact likeness of the residual limb. Except for the distal end pad, no attempts are made to artificially modify the tissues during casting. The method may be performed equally well by any prosthetist or technician. To continue with simplicity, few controls or restrictions are placed on the subject. The subject supports him/herself during the process by holding the backs of chairs or railings. The residual limb musculature is relaxed and remains in a relatively vertical orientation with slight knee flexion.
Literature exists quantifying many of the variables collected in this investigation. For example, gait data have been evaluated relative to different prosthetic feet, 20 movement differences between transtibial amputees and able-bodied, 21–25 and alignment of the prosthetic foot relative to the prosthetic leg. 26 However, no investigations could be found that performed gait analyses as a function of socket design.
Similarly, oxygen uptake has been accessed for a number of different scenarios, none of which include a comparison between socket designs. 16,20,27–36 The energy expenditure values reported here are in agreement with the values found in the literature for other transtibial amputees walking at similar work intensities. For example, Huang and colleagues 37 reported values between 12 and 15 mL/(kg × min) for their amputee subjects walking at 3.2 km/hr and grades of 0%, 4%, and 8%.
Undoubtedly, the most important variable of the investigation was the socket each subject chose to keep after participation in the investigation. Because this study involved a novel approach to socket design, no other studies in the literature addressed this issue. The results indicated a relatively even split (four subjects choose the rectified and six choose the unrectified socket), which suggests that the unrectified socket fit at least as well as the rectified socket. Further work is required to determine the details of why one socket was selected over the other.
The present investigation adds to the body of knowledge in at least two areas. First, there seems to be more than one paradigm for shaping a prosthetic socket for transtibial amputees. The rectified sockets of the present investigation had the typical alterations to the original shape of the residual limb to account for the inability of the residual limb to uniformly tolerate load. In contrast, the unrectified socket added only a distal end pad to the socket. Otherwise, the shape of the residual limb was retained. Despite these two different socket fabrication strategies, the results of the objective tests for gait, energy expenditure, and PEQ were not different. Furthermore, four subjects selected the rectified socket and six chose the unrectified. If these results are supported with data collected from additional subjects, at least two lines of work seem evident. The first is to understand the mechanism underlying an acceptable socket fit. Such understanding might be possible by adding additional methods to the current set used in the present investigation. These methods could include magnetic resonance imaging or computed tomography to quantify the underlying anatomical structure and tissues of the residual limb. They could include quantifying load between the socket and the residual limb over large regions. They could also include computer modeling (e.g., finite element analysis) to perhaps limit the experiments necessary to understand the mechanisms. The second area of work is the potential for creating hybrid sockets to specifically meet the lifestyle differences of patients. It is possible that one type of socket could be more suited to a person with a very active lifestyle and another type of socket more appropriate for a more sedentary lifestyle. Anecdotal evidence exists to support this second area of work, because one subject actually kept both sockets. The rectified socket was used during sedentary activities and the unrectified socket was used during exercise.
The second addition to the body of knowledge is the simplicity of fabricating the alginate socket. Such simplicity can be beneficial in countries in which prosthetists and fabrication facilities are scarce or nonexistent. With minimal input from skilled prosthetic personnel, patients could be fit with a well-fitting socket. The simplicity of the method might also be used in typical prosthetic facilities. It has been reported that three test sockets are generally used to fit each amputee with a rectified socket. 38 For our investigation, only one socket was permitted for the unrectified method. The labor costs associated with the reduction in test sockets could be substantial. These methods could potentially allow the prosthetist to delegate socket fabrication tasks to less expensive personnel, who could successfully perform these tasks with less expertise, freeing the prosthetist to address other important issues to better help the patient.
This pilot investigation used objective measures to compare rectified and unrectified sockets in transtibial amputees. Results indicated no differences between sockets for gait speed and timing, gait kinematics and kinetics, gait energy expenditure, and rate of perceived exertion. There were no differences in the PEQ; four subjects selected the rectified socket and six selected the unrectified socket. The alginate gel fabrication method was simpler and less time-consuming than the traditional method. The method could be helpful in countries in which prosthetic care is lacking and may be helpful in typical clinics to reduce costs and free the prosthetist to focus more time on patient needs. Results seem to indicate that more than one paradigm exists for shaping prosthetic sockets. Understanding the mechanisms for these paradigms could lead to customizing the socket to better meet the varying needs of individual patients.
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