The Military Healthcare System has provided medical, surgical, and rehabilitative care for 1645 service members with major limb loss as a result of injuries sustained during recent conflicts in Iraq and Afghanistan.1 Of those with lower-limb loss, approximately 47% had an amputation at or above the knee, and 31% lost multiple limbs.2 There are a number of unique factors that distinguish service members with combat-related amputation from civilians who undergo amputations. Notably, most combat-related amputations are a result of improvised explosive devices (IEDs),3 whereas peripheral vascular disease and/or complications associated with diabetes have been the primary cause within the civilian community.4 Because of the nature of such blast-related injury mechanisms, it is common for many service members with limb loss to also sustain comorbid injuries such as traumatic brain injury, post-traumatic stress disorder, peripheral nerve injury, fractures, and extensive soft tissue damage.1 These physical and cognitive maladies challenge traditional rehabilitative methods, especially prosthetic fitting and training. Furthermore, approximately 75% of these wounded warriors are younger than 35 years.5 Their relative youth and highly active lifestyle before injury underscore their eagerness to return to independent mobility and higher levels of physical function.6 To support this goal, the selection of prosthetic devices must be well-aligned to each individual's unique constellation of injuries.
At Walter Reed National Military Medical Center (WRNMMC), most wounded warriors with transfemoral amputation are initially fit with a variable dampening microprocessor-controlled knee (MPK) and energy storing and returning foot (ESRF). Rehabilitation plans typically involve attending rehabilitation sessions three to five times per week for 6 to 12 months; however, the regimen can vary greatly depending on patient performance and desired outcomes. Most patients are able to achieve independent ambulation, including the ability to negotiate inclines, stairs, and obstacles and even return to running within a year of injury. Although many are able to achieve high functional levels using an MPK prosthesis, kinetic and kinematic variables of gait demonstrate that asymmetry in level-ground walking and stair ascent/descent are dissimilar when compared with those of age-matched (uninjured) controls.7,8 This is of particular concern because individuals with lower-limb amputation, especially at the transfemoral level, are at an increased risk of developing a variety of secondary musculoskeletal problems, including osteopenia, low-back pain, and degenerative joint disease.9,10 These musculoskeletal complications have been attributed, in part, to the asymmetric gait patterns and abnormal biomechanics that result when utilizing a lower-limb prosthesis during daily activities.11,12 More specifically, gait analyses reveal that individuals with unilateral transfemoral amputation demonstrate increased loading on their intact limb during walking,13–15 which is amplified during sitting and standing16 and requires significant additional torque and range of motion at both the knee and hip joints.17 Of most concern is the fact that individuals with transfemoral amputation exert twice the torque on their intact limb during standing as compared with those without limb loss,12 and this may lead to future musculoskeletal complications in the intact limb ankle, knee, and hip.
Further challenging individuals with transfemoral amputation is the increase in the energy necessary for ambulation: up to 65% more exertion is required to generate approximately one-half the ambulation speed as compared to able-bodied persons,18 which contributes to a 21% decrease in walking activity per day for individuals with transfemoral amputation.19 Previous reports also indicate that individuals with amputation are at significantly increased risk for developing other medical problems such as diabetes, hypertension, and heart disease, even after controlling for risk factors such as smoking and obesity.20 These challenges may contribute to inactivity and predispose those with limb loss to even greater long-term health risks. The cumulative effect of the risks associated with aging after limb loss are particularly concerning when caring for injured military service members who, again, represent a relatively young patient population; research is needed to help identify new technologies and treatment interventions that might mitigate these lifelong increased risks of morbidity and mortality.
Powered prosthetic devices, such as the Össur® PowerKnee™ (PK), are a relatively new prosthetic technology aimed at providing a more normalized gait pattern, mitigating nonphysiological biomechanical forces on the musculoskeletal system, reducing metabolic cost of transport, and generally enhancing the mobility of individuals with transfemoral amputation. This device is commercially available and utilizes motorized actuators to substitute for muscles lost as a result of amputation. Although current research is limited on powered lower-limb devices, initial reports demonstrate that the PK is capable of outperforming passive prostheses for: upslope walking, improving hip and knee symmetry during sitting and standing, and reducing the power required in the contralateral limb during stair ascent.21–23 Moreover, data support that prosthetic users are able to fairly rapidly (within 20 minutes) adapt to a powered prosthetic device.24 Although these studies are generally limited to prototype powered prosthetic components and combinations of knee and feet components, their results suggest the potential biomechanical benefits of this technology,25–27 which warrant further investigation especially in the care of those with combat-related lower-limb loss. This case series describes our team's experience with offering the PK as the initial knee prosthesis (in contrast to an MPK) to four combat-injured service members with unilateral transfemoral or through-knee amputation.
Combat-injured service members were retrospectively screened for eligibility on the basis of their history using the PK as the initial knee prosthesis and subsequent use of similar prosthetic devices to enable objective comparison between modalities. Additional screening criteria included completeness of clinical records, patient geographic availability for follow-up visits, and general responsiveness and compliance for interviews. All patients sustained a unilateral transfemoral or through-knee amputation. Each patient was treated by the same prosthetist and physical therapy team at WRNMMC, including formal gait training and evaluation, per standard clinical practices, which included residual limb hand-casting and fabrication of several check sockets to accommodate for residual limb maturation; standard prosthetic alignment and programming; and physical therapy that emphasized core and limb strengthening, as well as gait training, residual limb care, standing tolerance, and stumble/fall recovery, which then progressed to stair/ramp descent and ascent with handrails and safety harnesses as necessary. As patients became independent and proficient with the PK prosthesis, each was then offered a variable dampening MPK prosthesis within 3 to 6 months. This allowed the study team to compare patient preferences between the PK and MPK. With regard to MPK fittings and training, all patients were treated following standard clinical protocols at WRNMMC and utilized ESRF. The specifics of each particular ESRF are described below.
Data for each patient were obtained from their electronic medical records and deidentified to protect privacy. All clinical care followed normal standard operating procedures at WRNMMC except for the offering of the PK as their initial prosthesis. Qualitative data regarding patient experience and preference with the PK, MPK, and other prosthetic components were obtained through provider-conducted telephonic interviews. Although the small number of patients in this case series precluded the implementation of formal statistical analysis, the intent of this was initial feasibility assessment, and all results were compiled on an individual basis and averaged to facilitate qualitative comparisons and subsequent discussion.
To provide an objective measure of meaningful patient outcomes, a focus group of experts was convened and consisted of three of each of the following professionals: prosthetists, physical therapists, physiatrists, and patients. This focus group came to a consensus opinion regarding four mobility measures that represented significant functional mobility milestones for individuals with unilateral through-knee or transfemoral amputation. These mobility milestones included the following: 1) independent ambulation within parallel bars for the length of the bars (24 ft); 2) unassisted level-ground walking 100 ft; 3) step-over-step stair/ramp ascent with or without assistance; and 4) step-over-step stair/ramp descent with or without assistance. In addition, the focus group established a consensus opinion regarding what they generally accepted as the standard time period from initial fitting to achieving these four mobility milestones (Table 2: rows 1 and 2).
For the standard clinical gait assessment, all patients walked at their self-selected walking speed across a 15-m level walkway. A cluster-based set of retro-reflective markers was used to track (120 Hz) full-body kinematics with a 23-camera motion capture system (Vicon, Oxford, UK). Ground reaction forces were simultaneously sampled (1200 Hz) from four force platforms (AMTI, Watertown, MA, USA) centrally located and embedded in the walkway. Raw marker and ground reaction force data were filtered using a second-order Butterworth filter with a cutoff frequency of 6 and 50 Hz, respectively. Selected time-distance, kinematic, and kinetic parameters, chosen to broadly represent measures related to stability, efficiency, and limb/joint loading, respectively, were calculated using a seven-segment biomechanical model developed in Visual 3D (C-Motion Inc, Germantown, MD, USA). These data were compared with an age- and anthropometrically matched convenience sample of data obtained for 13 individuals with transfemoral amputation utilizing an MPK and 37 individuals without amputation (“controls”; Table 1).
A 22-year-old male US Marine (180 cm, 73 kg) sustained blast injuries from a dismounted IED, resulting in a traumatic right transfemoral amputation. He was also treated for soft tissue infections in both buttocks, the left lower limb, his right hand, and his left upper limb, including an ulnar nerve transection. Approximately 3 months after his initial injury, the patient was cleared for initial prosthetic fitting and training. He was casted and fit with a PK prosthesis, which included a clear thermoplastic socket and LP Vari-Flex™ foot. After 3 days of prosthetic alignment adjustments and initial training, he was able to ambulate outside of parallel bars without an assistive device. After 3 weeks of level-ground walking with his PK prosthesis, he began stair training; however, it was discovered that his PK prosthesis had a faulty stair-climbing sensor component. This was replaced and he was subsequently able to ascend and descend stairs in a step-over-step reciprocating fashion independently. Although his initial stair negotiation required significant effort and was completed with poor biomechanical form, he vastly improved within the ensuing 7 days of training. A clinical gait laboratory analysis was performed on the patient 1 week after receipt of this initial PK prosthesis (Table 3). In a follow-up interview conducted approximately 2.5 years after fitting, the patient reported that he was using his PK prosthesis exclusively on average 18 or more hours per day and with the Vari-Flex foot. In addition, he also reported being able to run using the PK with the FlexRun™ foot, which he needed to support his current employment. The only medical complications the patient reported were residual limb sweating and heterotopic ossification, which he noted did not limit his mobility or activities. Since the replacement of the initial faulty stair-climbing sensor of the PK, he reported having no further issues or problems with his PK prosthesis. He returns to the clinic once every 3 months for liner replacements and hardware adjustments and reports being “very satisfied” with his PK prosthesis.
A 22-year-old male US Army soldier (174.5 cm, 67.9 kg) was injured by an IED blast, sustaining a traumatic right transfemoral amputation. He initially experienced a traumatic transtibial amputation, but this needed to be revised to a transfemoral amputation after an infection of his residual limb. His other injuries included left thigh fragment wounds, a right ring finger open dislocation, left index finger soft-tissue fragment wounds, and a lamellar corneal dissection of his right eye. Approximately 2 months after his amputation, the patient was cleared for initial prosthetic fitting and was offered a PK prosthesis as his initial prosthesis. He was able to ambulate outside of parallel bars without an assistive device within 5 days of prosthetic fitting. At 7 days, he demonstrated biomechanically efficient step-over-step reciprocating stair ascent and descent without an assistive device. A formal gait analysis was performed 1 week after obtaining his initial PK prosthesis (Table 3). During a follow-up interview conducted approximately 2.5 years post fitting, the patient reported that he used the PK with a low-profile Össur Talux foot as his primary lower-limb prosthesis, 14 to 16 hours per day. He denied any ongoing medical issues or any technical or mechanical problems with his PK prosthesis, although he did report some hip osteopenia that restricted him from receiving a running prosthesis. According to the patient, the major benefit of the PK prosthesis was the lack of fatigue he experiences when compared with his MPK prosthesis, particularly with long-distance ambulation. He also expressed appreciation for the function of the PK during transition to and from stair ascent and descent.
A 22-year-old male US Marine (176.5 cm, 76.2 kg) sustained multiple injuries from an IED blast, including soft-tissue fragment wounds to both thighs and multiple other injuries to his right lower limb, including a fibular head fracture, a knee ligamentous injury, a midtalar comminuted calcaneous fracture, a midfoot dislocation, first and second cuneiform fractures, and second and third metacarpophalangeal fractures. Despite surgical attempts to preserve his right lower limb, a through-knee amputation was performed 5 days after his initial injuries. Approximately 6 weeks later, the patient was cleared for initial prosthetic fitting and was offered the PK prosthesis. Upon fitting, he was able to independently ambulate outside of the parallel bars without an assistive device within 2 days and achieved independent reciprocating step-over-step stair ascent and descent after 5 days of prosthetic usage. Formal gait analysis was performed 2 weeks after his initial fitting (Table 3). A follow-up interview was conducted approximately 3 years after his initial prosthetic fitting. Although the patient reported a great deal of satisfaction with the PK prosthesis, he noted that he preferred to use the Össur Total Knee™ because of its lesser weight, good durability, and high reliability. He reported using the Total Knee prosthesis exclusively in conjunction with the Össur Variflex XC foot for daily activities. He noted that the limitations of the PK were its heavy weight, limited battery life (<8 hours), and several technical malfunctions that made him uncertain when ascending and descending stairs, which contributed to several nonharmful stumbles. The patient also reported being an active runner, for which he utilized the Össur Cheetah prosthetic foot with his Total Knee. Aside from mild low back pain, he denied any ongoing medical problems and remained physically active at the time of the interview, routinely participating in CrossFit, Inc, and Tough Mudder competitions.
A 24-year old male US Army soldier (170.5 cm, 79.2 kg) sustained injuries to his left leg from an IED blast, resulting in surgical fusion of his knee and ankle. After 9 years of attempted limb salvage, he elected to undergo a through-knee amputation at the age of 33. Once surgically cleared for initial prosthetic fitting, the patient was offered a PK for his initial prosthesis. He was cast and fit with the PK prosthesis, to include a clear thermoplastic socket, silicone liner, suction suspension, and Össur Variflex XC foot. After static alignment and programming of the PK, he demonstrated immediate reciprocating gait in the parallel bars. Within 1 day, the patient was able to ambulate without assistive devices for greater than 100 ft. He was able to ascend and descend stairs independently within 1 week of initial fitting of his PK prosthesis. A formal gait analysis was performed 2 weeks after fitting (Table 3). Because of schedule constraints and his high level of initial performance, he quickly progressed to an independent home therapy program. A follow-up interview at 11 months indicated that the patient utilized the PK exclusively for the initial 6 weeks of his rehabilitation and subsequently transitioned to split usage between the Ottobock® X3 and Össur Total Knee prostheses. He credited the PK with helping him to initially establish a proper gait pattern. He was, however, frustrated with the limited 8-hour battery life and tendency of the system to activate the “overheating” warning light when he was ambulating while wearing long pants or walking uphill. Although such indicated “overheating” never resulted in injury to the patient, it did necessitate that he turn the PK system off to allow it to cool. He reported no falls with the PK during use but said he has experienced infrequent (quarterly) falls with his current MPK. He reported routinely utilizing alternate sport prostheses. For competitive upright cycling, he stated riding 200 miles per week with his Total Knee. He has also returned to two-track skiing utilizing the Bartlett tendon knee prosthesis (Leftside Inc, Snohomish, WA, USA).
All four patients demonstrated successful use of the PK as their initial knee prosthesis without any significant complications (e.g., skin breakdown, fall-related injuries, musculoskeletal strains). In addition, each patient was able to reach significant mobility milestones (as designated by our consensus panel of experts) sooner than what our panel of clinical experts has traditionally observed among similar patients utilizing other prosthetic knee components (Table 2, row 3). Although none of the patients reported any injuries associated with the use of the PK prosthesis, two patients did report prosthesis malfunctions. One patient reported that the PK “overheated” when wearing long pants and walking uphill, which precluded extended use of the prosthesis and prompted his transition to another prosthetic knee device. The other patient reported trouble with the PK stair function mode, which resulted in a few reported falls. However, after replacing the stair-climbing sensor, this patient encountered no subsequent problems.
All patients reported that the PK prosthesis was very effective for long-distance ambulation, noting that they perceived it as less energy demanding than other knees they subsequently utilized. In addition, all patients reported that the PK was helpful during their initial rehabilitation, as they perceived the device allowed them to walk independently with minimal training almost immediately after being fit, which helped with their confidence and early independence. However, in general, all patients reported dissatisfaction with the PK's heavy weight relative to other devices (mean mass: 2.7 kg25 vs. 1.7 kg26). Although patients reported that this increase in weight was not noticeable during ambulation, it was particularly troubling when lifting or shifting their leg while sitting (e.g., transitioning in and out of a car). Patients were also dissatisfied with the PK's limited (8-hour) battery life and the inability to run (with the exception of patient 1) or play sports on the knee. Ultimately, 50% of this limited case series preferred using an MPK and/or Total Knee as their primary knee prosthesis.
Biomechanical outcomes of the gait assessment for each of the four patients are summarized in Table 3. Patient self-selected walking speeds were generally similar between the PK users and our laboratory norms for individuals using MPK prostheses, except for patient 3, who demonstrated a significant slower self-selected walking speed. Because of this decreased speed, meaningful between-subject comparisons in biomechanical outcomes were more difficult. In general, however, PK users demonstrated a gait profile with the following characteristic features of the prosthetic versus intact limb: 1) longer step length, 2) shorter stance duration, and 3) less knee flexion in stance and swing. PK users also demonstrated larger trunk lateral flexion during prosthetic stance, although these peaks were similar to data for MPK prosthetic users obtained in the same laboratory. Kinetic parameters were generally similar between both the PK and MPK knee component users; however, intact hip and knee joint powers among the PK users tended to be more comparable with uninjured controls and generally lower than those in the MPK group. No apparent (immediate) negative consequences of initial fit with the PK, as compared with those with the MPK, were observed.
This case series provides some preliminary data on the performance of individuals with unilateral transfemoral amputation after initial fitting with the PK prosthesis. Qualitative mobility assessments showed high-level function and ambulation progression when compared with historical averages for similarly injured patients. All four patients in this case series were able to achieve independent short- and long-distance ambulation without an assistive device, as well as independent, biomechanically efficient, reciprocating step-over-step stair ascent and descent within days to weeks after initial fitting with the PK prosthesis. Moreover, their biomechanical features of gait measured within the first 2 weeks of fitting were comparable with data of similar patients using an MPK prosthesis for an average of 2.8 years.
In addition to describing our patients' experience with the PK prosthesis, this report is the first that we have found in the literature that discusses “time to achieving mobility milestones” as a meaningful rehabilitation outcome. Most previously published studies report outcomes that emphasize scores attributed to either functional activity or specific biomechanical parameters. For example, the timed up and go, 6-minute walk test, Amputee Mobility Predictor or Comprehensive High-Level Activity Mobility Predictor provide important indicators of an individual's overall mobility function on key activities of daily living.28,29 Formal instrumented gait evaluations measure kinetic and kinematic data and provide important information regarding gait symmetry and efficiency. Although these outcomes are most certainly valuable when assessing the impact that various prosthetic components may have on mobility, we believe that the timely achievement of basic independent ambulation activities may also serve as a significant clinical outcome measure that has heretofore been under appreciated.
From an economic perspective, longer hospital stays and protracted outpatient therapy programs after amputation may have serious negative social and financial implications. Most insurance companies support only a limited number of outpatient therapy sessions following limb loss.30 In addition, the Centers of Disease Control and Prevention has reported a steady decline in the average length of stay for individuals with nontraumatic lower-limb amputation, from a high of over 20 days in 1990 to less than 10 in 2009.31 As a result, rehabilitation programs have become increasingly compressed and professionals have less time to spend with their patients. Therefore, if new prosthetic technologies can produce similar or even better results in a shorter timeline, this could have a positive impact for not only the individual patient but also for the entire field of rehabilitation for individuals with amputation.
Importantly, seeking biomechanical advantages to prosthetic ambulation may have both short- and long-term benefits. Previously published reports indicate that individuals with lower-limb amputation exhibit at least one significant gait deviation as a result of poor training, improper prosthetic fit or alignment, poor habits, or compensating for a secondary physical ailment.9 Studies also indicate that the long-term consequences of improper biomechanics with prosthetic ambulation can be severe and contribute to higher rates of arthritis, limb pain, and chronic low back pain.7,9,32 A previous study of World War II casualties with lower-limb amputation reported a 61% incidence of ipsilateral hip osteoarthritis and a 23% increased incidence of contralateral hip osteoarthritis, with a threefold increase in the prevalence for those with transfemoral compared with transtibial amputation.6 In fact, difficulties with gait training, particularly regaining symmetry, are frequently reported as among some of the most significant hurdles in the rehabilitation process for individuals with lower-limb amputations.33 Therefore, coupling optimal prosthetic component selection with proper gait training may not only improve gait symmetry and proper biomechanics but could also mitigate long-term musculoskeletal health risks. Furthermore, advanced prosthetic components, such as powered devices, may be useful rehabilitative aids to help teach proper gait mechanics early during the rehabilitation process. Although this case series did not collect long-term gait biomechanical data, future studies may help to elucidate whether early fitting with the PK may translate to improved long-term gait biomechanics with the PK or any other prosthetic knee component.
In addition to mitigating the physical impact of limb amputation (including bone degradation, muscle atrophy, and general deconditioning), early ambulation, particularly independent mobility, may have significant psychological benefits for individuals with limb loss. A review of goal setting and motivation theory suggests that goal achievement is mediated by self-efficacy and that setting and attaining goals early in rehabilitation may also promote improved performance on later tasks.34 Furthermore, low self-efficacy has been found to be significantly associated with poorer physical and psychosocial function; therefore, improved self-efficacy may decrease the risk of depression in individuals with lower-limb amputation, which has been reported to occur in 34.7% of traumatic and 51.4% of surgical lower-limb amputation cases.35
Although each of the four subjects in this case series demonstrated a relatively rapid achievement of significant mobility milestones and positive gait biomechanics on par with existing MPK systems, two of the patients reported preferring an alternative prosthetic knee component within 1 to 2 years of fitting, citing the increased weight, noise, limited battery life, and inability to use while playing sports or running as major limitations of the PK prosthesis. The patients reported in this study were all young, previously fit, military service members who experienced traumatic amputation from a blast, who were characterized by long residual limbs, and participated in aggressive and frequent rehabilitation. Therefore, results from this cohort may not be widely applicable to the average individual with amputation and similar preliminary data should be collected in different patient populations to discover if the trends noted in this case report are more generalizable. Furthermore, it should be noted that qualitative comparisons herein are being made against average timelines determined by a focus group of providers for various injury types (Table 1). Although this report establishes good benchmarks for general performance and discussion comparisons, it does not capture detailed data that would be presented in prospectively designed comparative effectiveness studies directly comparing the PK with an MPK prosthesis; Despite these limitations, however, we believe that the results described within this case series warrant further investigation on the role of powered prostheses, either as an initial-fit rehabilitation tool or as a long-term prosthesis for those with transfemoral or through-knee amputation.
This case series reports the experiences of four individuals with combat related lower-limb amputation who received the PK as their initial prosthesis. Data from this limited study indicate positive qualitative and quantitative outcomes for each PK user, including the following: 1) a generally faster attainment of significant mobility milestones than what has been previously observed with other prosthetic knee users; 2) gait biomechanics that are generally comparable to experienced MPK users with similar injuries; and 3) preliminary data that may indicate decreased strain on intact limb joints for PK users. This report also highlights the novel consideration of assessing prosthetic components (particularly powered prostheses) as potential rehabilitative tools that promote more rapid recovery after injury and potentially help mitigate long-term health risks, which may lead to substantial long-term cost savings. Further prospective comparative effectiveness studies are necessary to more clearly define the role of the PK prosthesis in the care of various patient populations with transfemoral amputation.
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