Many individuals with neuromuscular and/or musculoskeletal disorders exhibit knee instability because of paresis or paralysis of lower limb muscles. These individuals cannot safely and efficiently ambulate in their homes and communities because the knee (and its supporting musculature) is a vital link in providing stable support during the stance phase of walking. A knee-ankle-foot orthosis (KAFO) with a locking knee is often prescribed to provide the needed stance stability, but because knee movement is not allowed during swing, a stiff leg gait pattern results. To walk, the individual must compensate by circumducting and/or hiking up the braced leg, or by vaulting on the contralateral leg, to clear the foot and advance the involved limb, despite the stiff knee. Walking with a locked knee has been shown to be less energy efficient than walking with free knee motion and therefore is believed to limit the distance individuals can walk before they fatigue. 1–4 Wearing a locked knee KAFO may also limit an individual’s ability to perform other functional tasks that require more toe clearance than level surfaces, such as walking up and down curbs and stairs, stepping over and around obstacles, and walking on various types of surfaces (carpet, grass, gravel). Long-term compensation via the abnormal biomechanics associated with walking with a locked knee KAFO may lead to pain and loss of motion because of soft tissue and joint dysfunction, especially in the hips and lower back.
Rehabilitation specialists have long recognized the need for an orthosis that would provide needed stability in stance while allowing the knee to move freely during stepping. Development of such a stance control orthosis (SCO) was determined to be one of the most important unmet orthotic needs during a consensus conference sponsored by the National Center for Medical Rehabilitation Research, 5 which led to renewed worldwide interest in this challenge. Kaufman et al. 1 provided data on an experimental KAFO that allowed free knee motion during swing but locked automatically in stance. Their single subject exhibited improved gait efficiency and more normal knee kinematics when walking with the free-knee versus the locked-knee KAFO.
The purpose of this study was to investigate whether individuals walk more efficiently, more symmetrically, or with fewer compensations while wearing a particular SCO incorporating the Stance Control Orthotic Knee Joint (SCOKJ®), developed by Horton’s Technology, Inc. (Little Rock, AR), than when wearing a locked knee KAFO.
STANCE CONTROL ORTHOTIC KNEE JOINT
In the mid-1990s, engineers from the National Aeronautical and Space Administration (NASA) designed a mechanical joint that locked the knee of a KAFO when it was extended fully but released automatically just before swing phase, which they called the “Selectively Lockable Knee Joint.”6 Gary Horton, CO, provided the clinical expertise for the NASA field trials and eventually licensed the NASA patent so the joints could be commercialized. When the NASA design proved too costly to manufacture, Horton redesigned the mechanism to make it safer, more versatile, and more cost-effective than the original joint. After several years of controlled field testing, the SCOKJ® was released commercially in January 2002.
The SCOKJ® design incorporates a one-way cam lock that prevents additional knee flexion during stance phase without interfering with knee extension. The SCOKJ® does not lock the knee straight but rather blocks flexion to prevent knee collapse (Figure 1). Patients report that the ability to straighten the knee during weightbearing facilitates such tasks as stepping up onto a curb or step, or rising from a seated position. This latter advantage is of particular value when bilateral KAFOs are required.
The ability of the SCOKJ® to safely support the patient even if the knee is not fully extended at initial contact means that individuals with marked hip flexor and/or extensor weakness can safely walk with such KAFOs. The stance stability is automatically released in late stance by a mechanical linkage, in response to changes in either hindfoot weightbearing forces or ankle movement. A more complete discussion of the mechanics of this joint has been published previously. 7
The patient can select from three different control modes by moving a small lever on the side of the SCOKJ® (Figure 2). In the “A” position, stance control is automatically engaged and disengaged during gait, for walking and similar ambulatory activities. For certain activities, such as driving an automobile, many patients prefer to have the knee move freely. Placing the lever in the “U” position means that the joint is always unlocked, and knee movement is unrestricted. The third mode, with the lever in the “L” position, locks the knee in full extension. Some patients prefer this setting for climbing a ladder or similar tasks where the knee must remain fully extended at all times.
A convenience sample of three male subjects with significant weakness in the right lower extremity was recruited for this pilot study. This study was approved by the Institutional Review Board at the University of Central Arkansas, and all subjects gave informed consent before participating in the research. Subject 1 was a 56-year-old man with postpolio syndrome who had worn a KAFO with the SCOKJ® for 2 years. He had weakness primarily on his right side and ambulated without balance aids in his home and community. Subject 2 was a 59-year-old man with postpolio syndrome who had worn the SCOKJ® KAFO for 6 weeks. He had significant weakness on his right side and increasing weakness in his left leg. He walked without a balance aid in his home and for short distances but used a cane for longer distances. Subject 3 was a 30-year-old male with a nerve root injury (L4 level) secondary to trauma. He had significant weakness only in the right lower extremity and had been wearing the SCOKJ® for 8 months. Subject 3 ambulated without a balance aid in his home but used a cane for longer distances. Subjects 1 and 2 had worn a locked knee KAFO for an extended time before being fitted with the SCO. Subject 3 had been fitted with a locked KAFO after his injury but refused to wear it. Manual muscle test results for the three subjects are listed in Table 1.
Data were collected at the University of Central Arkansas. All three subjects completed gait analysis, then obstacle course trials, then treadmill trials within 1 day, taking a break before treadmill trials to rest and eat. Based on random selection of conditions, Subjects 1 and 2 performed first in the SCOKJ® then in the locked knee KAFO in each part of data collection (gait, obstacle course, and treadmill trials). Subject 3 performed first in the locked knee KAFO, then in the SCOKJ®. All three subjects wore the SCOKJ® on the right lower extremity. Subjects wore the same orthosis for all trials, with the stance control activated (in “A” position) in the SCOKJ® trials, and the knee locked (in the “L” position) in the locked KAFO trials.
Gait data were collected in the Motion Analysis Lab, Department of Physical Therapy. Three-dimensional data were obtained using the six-camera Vicon 460 Motion Measurement System (Oxford Metrics, Inc., Oxford, UK) and two AMTI (Advanced Mechanical Technologies, Inc., Waterbury, CT) force platforms. Subjects completed three to five walking trials (self-selected speed) in each condition (SCOKJ®v locked KAFO). Subjects wore their usual shoes and walked independently, without balance aids, during all trials in the Motion Analysis Lab. Gait analysis data were averaged across available trials for each subject in each condition.
After gait trials, subjects proceeded to the obstacle course laboratory, where they completed one practice trial, then one trial in each orthotic test condition. Trial order was randomized as presented previously. The obstacle course, modified from Means and O’Sullivan, 8 incorporates a variety of obstacles and surfaces that individuals might encounter in everyday life. Subjects must open and walk through a door, walk across different carpeted surfaces, step over obstacles of different heights, walk through pine bark mulch and sand, walk up and down steps and a ramp, maneuver around cones, and sit down on and stand up from a chair. Performance was timed for each trial. Heart rate was monitored before and during obstacle course trials using a heart rate monitor (Polar Electro Inc., Port Washington, NY). For safety reasons, subjects were accompanied through the obstacle course by one of the investigators. Standard verbal directions (eg, “Step over the obstacles,” and “Sit in the chair”) were given during each performance to keep subjects on the correct path. Subjects were allowed to rest between trials as needed to permit return of heart rate to the pre-activity level.
TREADMILL WALKING TRIALS
After a break (approximately 1 hour) to rest and eat, subjects proceeded to the Human Performance Lab in the Department of Kinesiology and Physical Education. Subjects first performed two walking trials (one for each orthotic condition) on a motor-driven treadmill (Quinton Instrument Company, Bothel, WA) to determine a subject-selected, comfortable pace and familiarize them with the apparatus. The heart rate monitor determined heart rate before, during, and after activity. After these initial trials, subjects rested until the heart rate returned to the prewalking level. Subsequent testing order was randomized. Treadmill speed (mean = 1.3 mph), grade (1 percent), and duration (5 minutes) were the same between trials for each subject. Holding the front handrail of the treadmill for support was permitted; however, subjects were instructed to use as little assistance as possible. Subjects rested between each trial until heart rate returned to near prewalking level. Peak heart rate for each trial was recorded as the average between the fourth and fifth minute. Heart rate was used as an estimate of relative walking intensity. The original 6-to-20 scale of Borg and Linderholm 9 was used to compare the rate of perceived exertion (RPE) during each walking trial. The higher the reported number value, the greater the perceived amount of exertion while performing the activity.
GAIT ANALYSIS: SPATIOTEMPORAL CHARACTERISTICS
All three subjects exhibited improved spatiotemporal parameters when walking with the SCOKJ® (Table 2). Walking speed and cadence were increased for all three subjects when walking with the SCOKJ® (Figure 3). Increased stride and step length were also noted with the SCOKJ® (Figure 4). Several parameters approached values for typical adult gait 10 (eg, opposite foot contact, foot off) with the SCOKJ®, suggesting a move toward greater symmetry of gait as seen in typical adults.
GAIT ANALYSIS: KINEMATICS
Graphs of lower extremity and trunk angular data for all trials were compared between conditions for each subject. To allow comparisons across trials and subjects, all trials were normalized based on the gait cycle, with initial contact occurring at 0 percent. When subjects used the SCOKJ®, a knee flexion peak was noted during swing, whereas the knee was maintained in extension during stance as expected. An example from one trial for each condition for Subject 1 is shown in Figure 5A and B. This subject exhibited weakness and joint laxity in the left lower extremity as well, but wore no orthosis on the left, thus explaining the hyperextension at the left knee during stance.
Subjects exhibited more symmetry and at times fewer extraneous trunk and pelvic movements when walking with the SCOKJ® than with the locked KAFO. Examples of pelvic obliquity (movement of the pelvis up/down in the frontal plane) and trunk lateral flexion are shown in Figures 6 and 7, respectively. Note the increased smoothness of pelvic and trunk movement on both sides during wearing of the SCOKJ® (Figures 6B and 7B) compared with wearing of the locked KAFO (Figures 6A and 7A).
All three subjects successfully completed the obstacle course in both orthotic conditions, with no falls and without using balance aids. The course was completed more quickly with the SCOKJ® by Subject 1 (6 seconds faster) and Subject 2 (18 seconds faster). Peak heart rate (HR) for Subject 1 was the same (117 bpm) in both conditions, whereas Subject 2 exhibited slightly lower peak HR while wearing the SCOKJ® (99 bpm locked KAFO, 97 bpm SCOKJ®). Subject 3 completed the course 8 seconds faster in the locked KAFO condition. Peak HR for Subject 3 was higher with the locked KAFO (161 bpm locked KAFO, 156 bpm SCOKJ®).
Heart rate response to treadmill walking was greater with the locked KAFO in Subject 1 (105 bpm locked KAFO, 100 bpm SCOKJ®) and Subject 2 (101 bpm locked KAFO, 96 bpm SCOKJ®). Subject 3 exhibited a lower HR response to treadmill walking while wearing the locked KAFO (153 bpm locked KAFO, 158 bpm SCOKJ®). Subjects 2 and 3 reported no difference in RPE between conditions (Subject 2, RPE = 14; Subject 3, RPE = 11). Subject 1 reported a greater RPE after walking with the locked KAFO (RPE = 16) compared with the SCOKJ® (RPE = 14).
Although no formal survey was administered, all three subjects offered spontaneous comments that they were more satisfied with the SCOKJ® than with the locked KAFO. Subjects did not like walking with the KAFO locked, even for the short distances required for the gait trials in the lab. They expressed greater comfort walking with the SCOKJ® and reported they walked more easily and were more functional when wearing the SCOKJ®.
All three subjects exhibited more symmetric walking gait with less compensatory movement when wearing the SCOKJ®. With longer steps and strides and increased cadence, their walking speed also improved when their knee was allowed to move freely during the swing phase of gait. Not only did these changes create a more “normal looking,” more aesthetic walking pattern, but the improved symmetry and decreased compensatory movements should decrease the energy cost for walking and improve function by allowing subjects to walk further without fatiguing. Although we do not have direct evidence of subjects’ energy use during the gait trials, the treadmill walking data support this assumption. Decreasing the need for excessive movement of the hips and trunk should also decrease the long-term risk of musculoskeletal pain and dysfunction that could potentially result from compensating for a stiff knee with abnormal biomechanics.
On the obstacle course, Subjects 1 and 2 completed the course more quickly while wearing the SCOKJ®. Peak HR for Subject 1 was the same in both conditions; for Subject 2, peak HR was slightly lower during wearing of the SCOKJ®. Subjects’ ability to maneuver through the obstacles faster and with the same or lower peak HR may be attributable to more efficient movement with the SCOKJ®. Subject 3 moved through the course faster with the locked KAFO. Given that he greatly disliked wearing the locked KAFO, he may have hurried to finish the course with the locked KAFO. Alternatively, he may have felt more stable with the knee locked when encountering unfamiliar obstacles. This subject (Subject 3) was acutely injured and had limited experience wearing a locked KAFO, unlike the other two subjects. He also had stronger hip musculature on the braced side, so he had more ability to increase his walking speed than did Subjects 1 and 2, who had marked hip weaknesses.
Heart rate was lower for two of the subjects during SCOKJ® walking than during locked KAFO trials, whereas RPE was unchanged. Although metabolic data during treadmill walking are not available because of a lack of appropriate equipment, it is assumed that the reduction in HR reflects a reduction in the relative intensity of walking while wearing the SCOKJ®. The more normal gait patterns demonstrated by Subjects 1 and 2 may have induced an improvement in walking economy, thereby decreasing the amount of energy required to walk at a comfortable pace. It is uncertain why the HR response of Subject 3 was greater during the SCOKJ® condition, despite a lower RPE. A lower RPE suggests the subject perceived the activity as being less intense, or easier. Less intense activity typically results in lower HR response than does more intense activity, but this was not the case for Subject 3. The lower HR exhibited by Subject 3 during the locked KAFO trial may reflect a greater reliance on the handrails for support during this condition; holding on to the handrails decreases the amount of effort to walk on the treadmill. Alternatively, because all subjects had a greater HR response to the last treadmill trial performed (locked KAFO for Subjects 1 and 2, SCOKJ® for Subject 3), the HR data may reflect the cumulative results of the day’s activities. Although rest periods were provided throughout the day, subjects may have been tired by the time they performed treadmill walking trials, especially if their cardiorespiratory conditioning status was low (which is likely for these subjects). Low cardiorespiratory conditioning status could have influenced heart rate response to activity, thus causing an order effect. Random assignment was incorporated to control for this, but sample size was too small for randomization to be fully effective.
An orthotic device is useful only if it is worn by the individual on a regular basis. Historically, many KAFOs have been rejected by patients for the reasons echoed by the subjects in this study: one cannot walk comfortably, easily, or very far when wearing a locked knee KAFO. Although the comments gathered in this study were anecdotal, all subjects volunteered strong statements in favor of their SCOKJ® versus a locked KAFO. This suggests that SCOs may increase patient acceptance compared with locked knee KAFOs.
LIMITATIONS AND FUTURE RESEARCH
The generalizability of these results is limited by the small number of subjects in this pilot study, which precluded statistical analyses, and by the heterogeneity of the subjects. Subjects 1 and 2 were certainly more similar to each other (in age, diagnosis, experience with orthoses) than they were to Subject 3. Future studies are planned to increase the number of subjects and to look differently at subjects who are long-time KAFO users versus those with limited or no experience with a locked KAFO. The long-term goal of this research is to determine whether the SCOKJ® is an effective alternative to a locked KAFO and, if so, what types of patients will benefit most from use of the SCOKJ®.
Using heart rate to compare intensity of treadmill walking is admittedly a rough estimate because of the many factors (eg, emotional, environmental) that can influence heart rate. In addition to HR and RPE, future studies will incorporate the measurement of expiratory gases (indirect calorimetry), which was not available during this pilot work, to determine energy expenditure and walking economy. The use of indirect calorimetry will allow more thorough and accurate documentation of the difference in energy expenditure between the two orthotic conditions, and is expected to verify the greater energy efficiency when walking with the SCOKJ® versus the locked KAFO.
To verify these subjects’ anecdotal comments about satisfaction, surveys of SCOKJ® users (both successful and unsuccessful) are also planned. This information will provide evidence as to which patients benefit from the SCOKJ® and how their function changed with this orthosis.
All three subjects exhibited improved spatiotemporal gait characteristics and a more symmetric gait pattern when walking with the SCOKJ® versus the locked knee KAFO. Two subjects completed an obstacle course more quickly and exhibited a lower heart rate response to treadmill walking when wearing the SCOKJ®. All three subjects reported greater satisfaction with the SCOKJ® compared with the locked knee KAFO. Future studies are planned to provide additional objective evidence regarding use of the SCOKJ®. The SCOKJ® appears to be an effective alternative to the traditional locked knee KAFO for some patients with lower limb dysfunction.
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Keywords:© 2004 American Academy of Orthotists & Prosthetists
stance control orthosis; lower limb orthosis; knee-ankle-foot orthosis; lower limb paralysis; lower limb paresis