REHABILITATION OF PERSONS WITH BILATERAL LOWER-LIMB LOSS
Causes of amputation, such as cardiovascular disease, trauma, and congenital defects, are often not limited to involvement of a single limb or side of the body. Accordingly, a considerable number of individuals who live with limb loss have experienced multiple limb amputations. In the United States alone, the number of persons with bilateral transtibial amputation1 has been estimated to be 11,400. Rehabilitation of patients with bilateral lower-limb amputation is challenging because of the degree of the structural loss and the need to replace both limbs with prostheses. Without a sound leg to provide balance, proprioception, and stability, prosthetic fitting and gait training are more problematic than in persons with unilateral amputation. In addition, the physiological cost of walking with bilateral prostheses is greater than with a unilateral prosthesis.2 Gait of persons with bilateral amputation has also been characterized by lower speeds, cadences, ankle moments, and knee moments as compared with able-bodied controls.1 In addition, physical capacity of persons with bilateral amputation has been reported to be lower than in persons with unilateral transtibial amputation,3 and self-reported mobility levels have been found to be similar to those with more proximal levels of unilateral amputation.4
Targeted rehabilitation regimens have been proposed as a means to address the specific challenges that present with bilateral limb loss and to mitigate the functional deficits associated with prosthetic gait.5,6 With advances in rehabilitation therapy and technology, independent ambulation can often be achieved, enabling patients to walk with little or no crutch support. The success rate of prosthetic rehabilitation in people with bilateral transtibial amputation has been reported to be in the range of 30%7,8 to 50%9 and even as high as 60% to 90%,10 albeit patients tend to use their prostheses less and have lower mobility and lower rates of employment than do persons with unilateral amputations.4 Positive long-term outcomes for ambulatory patients with bilateral amputation underscore the importance of appropriate prosthetic provision.11
SIGNIFICANCE OF LEG LATERALITY FOR SOCKET DESIGN, COMPONENT SELECTION, AND THERAPY
The term laterality describes functional asymmetry between the left and right limbs.12 It is often referred to as left- or right-handedness or, more generally, left- and right-sidedness. Natural laterality is suspected to be rooted in the brain architecture, whereas forced laterality develops as response to training or other external conditions such as unilateral limb amputations. Although leg laterality is easily assumed in persons with unilateral amputation (with the sound limb being the dominant one), it may also be present in persons with bilateral amputation. Those patients may—consciously or not—prefer one leg over the other for activities that require weightbearing, balancing, or fine-motor control.
Determining leg laterality in persons with bilateral lower-limb loss, beyond the mere subjective assessment that the respective patient provides, is important for several reasons. Knowledge of leg laterality early in the fitting process may direct selection of an appropriate socket style and related prosthetic componentry. Typically, this decision is informed by the patient’s diagnosis, weight, and activity level. The condition of the residual limb also plays a role because factors such as skin health, joint mobility, pain sensitivity, weightbearing capabilities, and controllability need to be considered.
When one residual limb of a person with bilateral amputation is more capable than the other (e.g., longer, healthier tissue, less knee instability), equipping both prostheses with the same foot type may be suboptimal. Consequently, selection of prosthetic components may be different between legs. A dynamic foot design that uses the capabilities of a long limb for swift and energy-efficient walking may jeopardize stance stability if used on a weaker contralateral leg. Conversely, a more compliable foot that provides a solid base of support on a short limb may restrict performance on a stronger contralateral limb. With information about leg laterality, the selection of prosthetic componentry could be better customized to the individual user.
Knowledge of leg laterality in a patient with bilateral amputation can also inform selection and use of other prosthetic componentry, such as torsion adapters or shock absorbers. A torsion adapter, for instance, facilitates pivoting motions that may occur frequently in various vocational and recreational activities. Persons with a unilateral prosthesis may rely on their sound leg to perform those motions, where active foot control provides safety and stability. In persons without a sound leg, a torsion adapter may allow a user to perform similar motions without causing excessive shear forces on the residual limb. Because of the weight and the cost of torsion adapters, it may be indicated to install one in only one of the two prostheses. Therefore, selection of the prosthesis in which the adapter should be installed may be informed by evidence of leg laterality.
A physical therapy regimen may also be better customized to a patient when information about leg laterality is known.13 Therapy may include strengthening activities, balance exercises, and practice of individualized strategies for stair walking and other demanding tasks of everyday life. If one leg is habitually favored, strength and range of motion may differentially develop. Such discrepancies may amplify gait asymmetries and leg laterality if not addressed. Active training of the underused leg may facilitate a more biomechanically efficient balance between limbs.
CHALLENGES OF COMPARING LEG BIOMECHANICS
Despite the importance of leg laterality to clinical decision making, measurement of related variables is challenging. Traditionally, leg laterality is assessed through differences and/or asymmetries in temporal-spatial, kinetic, and kinematic variables measured using goniometers,14 motion analysis,15 and force plates.16 Although these methods are useful for measuring symmetry (and leg laterality) in a laboratory setting, these are limited in their ability to measure outcomes under nonlevel terrain (e.g., stairs) or outdoor conditions (e.g., uneven walking surfaces). Force plate–based measurements also require numerous trials to collect a sufficient sample of steps, and consecutive steps can be recorded only by using multiple force plates.
A novel way to measure leg laterality in persons with bilateral amputation is with prosthesis-integrated load cells. Integrated load cells allow for continuous data collection, independent of the environment or the activity in which the user may be engaged.17 Integrated load cells have been used previously to study gait biomechanics of persons with unilateral transtibial amputation.18–22 However, to the authors’ knowledge, integrated sensors have not been used to study persons with bilateral amputations. When both legs are instrumented with load cells, differences in temporal and kinetic outcomes can be assessed when users ambulate over a variety of terrains. This may allow for situational assessment of leg laterality that is unique from other measurement methodologies. The purpose of this study was to explore this potential application of integrated load cells and to quantify differences in gait variables between the legs of persons with bilateral transtibial amputation. The authors hypothesized that significant difference in gait variables would be measured between the legs, even when prosthetic design and componentry were identical. Thus, observed significant differences in the measured outcomes would be indicative of leg laterality. Information from these observations could be used to understand functional differences in laterality and to enhance rehabilitation intervention strategies.
Individuals with bilateral transtibial amputations were recruited to participate in this pilot study. The inclusion criteria were ages 18–80 years, use of endoskeletal prostheses on both limbs, an ability to walk at least 30 minutes per day without pain, and an ability to walk without the use of assistive devices for short distances. Patients whose prostheses did not provide enough space between the socket and the prosthetic foot to fit the integrated load cell were excluded from participation in this study. An initial screening was conducted to ensure eligibility. Informed consent was obtained before the data collection. The study procedures were reviewed and approved by the Human Research Protection Program of the University of Wisconsin–Milwaukee. Informed written consent was obtained from the study subjects.
The existing prostheses of the study subjects were modified by replacing the prosthetic pylons with portable force sensors (iPecs Lab; College Park Industries, Warren, MI, USA) with six degrees of freedom (i.e., three forces and three moments) and shorter pylons. The iPecs Lab is a 277-g, research-grade measurement tool with an accuracy of 1.0% to 1.5%.23 Two iPecs units were installed (i.e., one in each leg) to simultaneously collect data from both legs (Figure 1).
To control static alignment of the prostheses throughout the study protocol, the foot of the doffed prosthesis was fixed within an alignment jig and plumb lines were temporarily transferred to the subjects’ prosthetic sockets. These markings allowed the investigators to preserve the original alignment and length of the prosthesis after insertion of the iPecs unit and to restore the prostheses to their original alignment upon conclusion of data collection. The iPecs unit was installed so that the iPecs x axis aligned with the frontal plane of the subject; the y axis, with the line of progression; and the z axis, with the long axis of the prosthetic pylon. This installation was consistent with the manufacturer’s instructions and allowed for direct measurement of axial force (i.e., load through the prosthetic pylon), anterior-posterior (A-P) force, and medial-lateral (M-L) force at the sensor location. Positions of the iPecs center with respect to the subjects’ knees, and ankle joint axes were measured with a ruler to allow for computation of joint moments by rigid body dynamics, using the respective distances together with the measured forces and moments. Special care was applied to define joint axes consistently within both prostheses of the same subject. Installation and removal of the iPecs unit were performed by the lead investigator (G.F.), a credentialed and experienced prosthetist. The subjects donned the modified prostheses in their usual fashion and acclimated to the test prostheses by walking for at least 10 minutes to ensure that inertial differences in the prostheses were accommodated. Anthropometric data, such as limb dimensions, subject height, and mass when wearing the modified prostheses, were measured for each subject.
To simultaneously collect data from both iPecs units, each was wirelessly tethered to a unique laptop computer running the manufacturer-provided data acquisition software. Distinct wireless transmission radio frequencies were defined for each iPecs unit to eliminate cross talk between devices. The iPecs sensors were zeroed in accordance to the manufacturer’s instructions as the subjects sequentially stood with either prosthesis lifted off the ground. Continuous iPecs measurements were conducted throughout the data collection session. To synchronize measurements, both shoe soles were tapped simultaneously while the subjects were asked to sit down and extend both feet into the air. This left a spike in the recorded kinetic data that could later be used to align the individual leg’s data files with each other.
The subjects were instructed to walk at their preferred speed out the laboratory, along a well-lit hallway (level surface, no turns, concrete floor), down the stairs to the ground floor (15-step staircase, concrete), across a parking lot outside the building (slightly uneven asphalt and concrete sidewalk), up a different set of stairs (13-step staircase, concrete), and back to the laboratory. The subjects were instructed to walk the circuit one time. The entire circuit length was 210 m (689 ft). Three members of the research team walked behind the subjects, carried the laptop computers, and videotaped the subjects’ feet. The video recordings were later used to identify and isolate segments of the walking circuit (i.e., terrain type) in the iPecs data files.
Transition steps, such as the first step onto the staircase and steps through doorways, were removed from the analysis so as to focus on steady-state performance of walking over each terrain. Steps that involved turns were similarly removed because these may inherently produce asymmetries in the outcomes measured. Therefore, the distances walked varied slightly by individual. Gait parameters that allowed for direct comparison between the limbs, including stance duration, peak knee flexion moment, peak ankle flexion moment, and peak axial force as measured by the iPecs device, were averaged over the steps within each segment of the walking circuit (gait indoors, stair climbing, gait outdoors). The numbers of steps averaged for each subject and activity are included in Tables 1 and 2. For each segment, mean differences between specified parameters were calculated as a measure of asymmetry. Selected outcomes included parameters (e.g., axial force, stance duration, ankle moment, knee moment) that have been used in previous studies on gait symmetry in persons with lower-limb loss.24–26 Many continuous gait parameters have a characteristic shape over a step period and can therefore be easily compared by reducing them to their peak values. To efficiently compare the multiple correlated parameters measured in each leg, a multivariate analysis of variance (MANOVA) was conducted, using the statistical package IBM Statistical Package for the Social Sciences (SPSS) 20. A critical α level of p = 0.05 was defined before the analysis.
Two male subjects (A: 61 years, 170 cm, 83.9 kg, and B: 32 years, 173 cm, 80.7 kg) participated in this study. Both subjects were similarly active prosthesis users with several years of experience. Subject A had been a bilateral amputee for 17 years. He was able to walk without assistive devices but for reasons of comfort and stability preferred a walking cane when traversing long distances. Both of his residual limbs measured 16.5 cm in length, as measured from the medial tibia plateau to the uncompressed tip of the residual limb. Subject B had experienced bilateral amputation of his legs 4 years before this study, and his residual limbs measured 16.5 cm (right) and 15 cm (left) in length. Both subjects wore modified patellar tendon-bearing sockets with silicon liners and energy-storing carbon feet. Design, age, and components of each subject’s prostheses were identical between left and right.
Subject A walked with a cane in his right hand. He used a step-to pattern when ascending stairs and an alternating step pattern when descending stairs. He completed the 210-m–long circuit (including the stairs) in 5 minutes and 55 seconds (an average speed of 0.59 m/second). Subject B walked without assistive devices. He used an alternating step pattern when both ascending and descending stairs. He completed the circuit in 3 minutes and 53 seconds (an average speed of 0.90 m/second). Both subjects used handrails when ascending and descending stairs.
Tables 1 and 2 summarize comparisons between outcomes, as measured by the iPecs Lab units. Figure 2 illustrates the bilateral differences in measured variables between the legs during level-ground walking in subject A and subject B. The same variables as measured during stair climbing are visualized in Figure 3 (ascent) and Figure 4 (descent).
Differences in walking parameters (e.g., axial force, joint moment, and stance phase duration) between the left and right legs are indicators of gait symmetry and may indicate leg laterality in persons with bilateral amputation.12 According to the data collected in this study, walking outcomes such as peak axial force, stance phase duration, peak knee moment, and peak ankle moment are characterized by considerable asymmetries between the limbs of persons with bilateral amputation. However, the specific parameters that display asymmetries seem to vary by individual. This is exemplified in the knee and angle moment trajectories observed during stair climbing (Figures 3 and 4), signifying the different strategies used by the subjects.
Significant differences in knee moment and ankle moment were observed between the limbs in subject A, whereas peak axial forces during level walking were almost identical. Axial forces were significantly different when walking on uneven ground outdoors, and differences in stance phase duration were present during downstairs walking. There were consistent patterns of asymmetry observed for subject A over all four walking tasks in that the ankle moment in the right foot was greater than in the left foot, which would suggest that the right leg is dominant. Conversely, the peak knee moment was always greater in the left leg than in the right leg. A potential explanation for this finding is the subject’s use of a walking cane. The use of the cane in his right hand is likely beneficial for the subject’s gait stability but may cause the subject to relieve his left forefoot late in the stance phase. However, this altered loading pattern was also evident on the stairs when the subject used handrails instead of the cane. This could be interpreted either as a sign that the asymmetrical forefoot loading has become habitual or as a response to a physiological difference between his legs. Additional research is needed to explore differences in leg laterality that may be observed with or caused by the use of an assistive device.
The bilateral differences in the kinetic outcomes of subject B were, overall, more consistent than those of subject A. Most notable was an increased ankle moment in the right leg as compared with the left leg, which was observed across all of the investigated terrain situations. The subject spontaneously reported that he often depends more on his left leg, which was confirmed by higher mean peak forces for this side during walking. Greater moments that were measured in the right ankle might be related to his residual limb being longer than the left one. This finding illustrates the complexity of mechanisms that affect limb dominance. Although a longer limb constitutes a greater lever arm and therefore facilitates greater joint moments, the axial load may be greater on the shorter limb. Reasons for increased axial loading may be a greater confidence in or preference for using that limb. Better soft tissue coverage, better joint stability, or better sensory feedback may also play a role in leg laterality. The described method for measuring leg laterality does not provide an explanation for the observed load increase. However, it enabled detection of this load increase that, irrespective of its source, may be interpreted as an objectively quantifiable indicator of leg laterality.
Because only two subjects participated in this pilot study, the small sample size limits generalizability of the findings. The use of larger samples is recommended for subsequent studies.
A further limitation of this study is the approach of comparing gait analysis curves by peak values. Peak values cannot entirely describe the kinetic parameters of the step cycle. Judged by the data collected here (as plotted in Figures 2 and 3), differences between gait analysis curves may appear even greater if assessed by more elaborate methods, such as “norm distance”27 or principal component analysis.28 In this context, however, it may be questioned what levels of difference between the left and right sides indicate clinical significance. Does, for instance, a difference of 10 N m in peak ankle moment suggest the use of a different prosthetic foot in either leg (to balance the ankle moments between the legs), or is such a small aberration an individual peculiarity that does not call for an intervention? A more extensive study with a greater sample size and longer assessment period may be required to answer this question. It may also be beneficial for future studies to consider and/or include other biomechanical variables, such as roll-over shape,29 in the assessment of leg laterality. Studies on the effects of prosthetic componentry in persons with bilateral amputation30,31 have not investigated the (possible) influence of laterality but may be amended to include this concept in future work. Interesting research questions may include the following: Can asymmetric component prescription or adjustment counterbalance leg loading differences be attributed to laterality in persons with bilateral amputation? Which kinetic gait variables are most indicative of laterality, and does stated leg preference correlate with objective data? Is there an interaction effect of leg laterality and changes in walking environment (e.g., surface evenness, firmness, or slope)?
The findings of this study suggest that gait outcomes (e.g., peak axial force, ankle moment, knee moment, and stance duration) can be quantified by prosthesis-integrated load cell measurements and that significant differences in these variables between the legs may indicate leg laterality in persons with bilateral transtibial amputation. Interestingly, the differences measured between the legs do not seem to be directly related to the differences in residual limb length or limb dominance, as spontaneously stated by one subject. Thus, integrated load cells may allow clinicians and researchers to identify subtle but potentially important differences between the legs that may be unavailable through traditional strategies. Gait kinetics data can be statistically compared between the legs and different terrains to that end. The information gained from the use of integrated load cells in this manner could be used to develop, monitor, and justify selection of therapies and/or prosthetic interventions. The authors believe that application of integrated load cell technologies for these purposes promotes evidence-based treatment decisions and can improve the quality of care provided to persons with lower-limb amputation.
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