Cerebral Palsy Orthoses

 

 Individuals with walking disabilities as a result of cerebral palsy (CP), stroke, muscular dystrophy, brain injury, or many other conditions are often prescribed ankle foot orthoses (AFOs), as shown in Figure 1 (a), to aid in their walking. The large number of individuals with CP that use AFOs highlights the magnitude of the problem. United CP reports that an estimated 764,000 people in the United States have one or more symptoms of CP. It has been reported that more than 50% of these individuals are prescribed orthoses.
 
The prescription of AFOs usually has several treatment goals, including: (1) facilitating walking by controlling the position of the ankle and providing a base of support, (2) preventing contractures by putting muscles in a lengthened position and providing variable ranges of motion, and (3) preventing deformity by controlling the position of the foot/ankle. Previous research has found that modulation of ankle position and stiffness is a primary means for patients with CP to change performance range. Although laboratory tests – usually gait analysis of patients with CP wearing AFOs walking in clinics or camera-instrumented motion capture laboratories – provide some insight into the first goal, results in naturalistic settings are lacking and may be significantly different. Moreover, without more continuous, longitudinal measurement to help understand the long-term effects of AFO use, the efficacy of AFOs in achieving the second and third goals cannot be determined. 
 
This project seeks a novel solution for assessing the efficacy of AFOs in a continuous and non-invasive manner. Ankle joint angle is identified as an important metric for this assessment. By using wireless inertial body sensor network (BSN) based motion capture sensor, we present a promising solution that the longitudinal monitoring of performance of AFOs is possible.
 

People:

 

Medical Background:

CP is a neuromuscular disorder characterized by an injury to the immature central nervous system (CNS). There is a wide spectrum of clinical manifestations, including spasticity,abnormal reflex patterns, weakness, retention of primitive reflexes, and an abnormal increase in muscle tone. Based on the severity and location of the CNS lesion, children range from being wheelchair-bound to community ambulators. In addition, many patients develop muscle contractures during growth, resulting in joint stiffness as well as angular and rotational bony deformities. As a result, these children develop atypical gait patterns.

While there is a great deal of heterogeneity in children with CP, two gait patterns are most commonly seen: equinus and crouch gaits. An equinus gait is characterized by the individual’s ankle being plantar flexed (on one's toes) at floor contact (Figure l (b) A). In a crouch gait, the knee is excessively flexed in both swing and stance (Figure 1 (b) B). AFOs are often prescribed for both gaits. When a child has an equinus gait, the AFO promotes heel contact by maintaining the ankle in a more neutral position. Having the plantar flexors of the ankle held stretch by the AFO may help also prevent muscle contractures by putting muscles in a lengthened position, and preventing deformity by controlling the position of the foot/ankle. If a child has a crouch gait, an AFO can reduce knee flexion in stance. In this case, the AFO provides a moment at the ankle that resists ankle dorsiflexion in stance. These findings are illustrated in Figures 2 and 3, which show data collected from the Vicon® system on one of the pilot study’s CP children. In Figure 2, the ankle joint angle range is greatly limited by the AFO. In Figure 3, the AFO helps to promote a heel strike, as the gait without the AFO just has a ‘dip’ in the shank angular velocity after terminal swing, suggesting a flat-foot stomping on the ground or a forefoot landing.

 

 

Examination of the literature reveals that there has been significant research into other effects of AFO use on gait, in general finding small but significant improvements in stride length, walking speed, ankle moment, and cadence  Based on 3-D in-lab gait analysis, some research summarizes that “… the AFOs functioned successfully by limiting the range of motion at the ankle, positioning the foot appropriately prior to initial foot contact, absorbing less power following the initial foot contact, and generating a larger ankle moment during push-off.” These results are all based on short-term, in-clinic evaluations. To date none of the research has shown how AFOs impact CP gait over a longer period due to the limitations of the in-clinic evaluation technology. This lag in technology and the necessity of longitudinally assessing AFO efficacy motivates the development of methods to achieve this goal in real world situations.   

It is generally believed that a crucial factor in the development of contractures is the angular position of a joint as a function of time. The measurement of joint angle also allows determination of dorsi-/plantar-flexion mode, range of ankle motion, and ankle joint angular velocity. Therefore, continuous monitoring of the ankle joint angle for children with CP wearing AFOs is the ideal method to evaluate the effectiveness of the AFOs to prevent contractures. This information obtained would affect both the prescription of AFOs to address contractures as well as the construction of the AFOs themselves (e.g., the joint angles it maintains in the ankle).

Current gait analysis has adopted an industrial standard technology in 3D position tracking optical motion capture systems such as Vicon®. They provide accurate and precise spatial and temporal gait information such as joint angles, moment, power, and cadence; however, their immobility limits their use to in-clinic, thus preventing continuous longitudinal monitoring. Electro-goniometers have been used as a conventional portable measurement tool for joint angle, but their shortcomings in accuracy, wearability, and long-term data collection have also eliminated them from this application.

In this project, the TEMPO 3 inertial BSN research platform was employed. TEMPO provides six degrees-of-freedom sensing from an arbitrary number of wristwatch-sized nodes distributed at strategic locations on the body. Sensors are sampled at preset frequencies (120Hz in this project to enable sample-to-frame synchronization with the 120 frames per second Vicon® system) with up to 12 bit A-to-D resolution. Data can be processed on-node with the MSP430F1611 mixed signal processor and then either transmitted wirelessly via Bluetooth® to a body-worn aggregator or stored locally in an on-node flash memory. The flash solution provides significantly longer battery life – up to 18 hours with a 300mAh rechargeable lithium-ion coin cell battery and with the entire system operating in high-throughput mode. This lifetime can be extended further by using the microcontroller to use the accelerometers to detect when a subject is walking and only activate the relatively high power gyroscopes then. 

 

 

Chanllenges:

 

Inertial body sensor networks (BSNs) represent a promising platform for addressing this need. With MEMS sensor technology, ultra-low power processing and wireless communication, and high flash memory densities to reduce transmissions, the form factor and battery life of these sensor nodes minimizes invasiveness and enhances ease
of use. For longitudinal assessment of AFO efficacy, these sensors can even be molded into AFOs without affecting their function. 
 
However, while these sensors provide rich information on the kinematics of human motion, the accurate spatial information required for many gait parameters is challenging to obtain, especially for pathological gait. Intrinsic sensor noise causes integration drift, the uncertainty of mounting nodes on human bodies causes a systematic bias, and movement in multiple planes causes computational errors, all of which hinder the acquisition of accurate joint angles and other spatial features.
 

Methodology 

In this project, we use TEMPO 3.1 system, an inertial body sensor network, with 3-axis MEMS accelerometers and 3-axis MEMS gyroscope sensors embedded, capture the motion of ankle angle of subjects. Such platforms enables the portability of the motion capture hence the possibility of the longitudinal study of the ankle angle of the CP gait.

After a careful investigation by gait analysis expert, ankle angle is selected as a primary gait parameter for assessing the efficacy of AFO. The general steps for computing the ankle angle via 6 degree motion sensor is:

  • Obtaining Segment Angle
    • Integrating angular velocity obtained from gyroscope
    • Fusion with angle obtained from accelerometer
  • Ankle Angle = Shank Angle - Foot Angle

 

 

Pilot Studies on Healthy Subjects

 

 

Enabling Study Results from Children with CP

Data has been collected on four CP subjects wearing AFOs. These subjects walked on the ground within the range of the Vicon® cameras for a several trials (about 5 meters each trial). The kinematic data was taken simultaneously from Vicon® and TEMPO, based on an online synchronization procedure. Subject consent and assent was approved by the University of Virginia’s IRB and obtained for all subjects.

Some walking aid devices (walkers) block the line of sight of optical motion capture system, challenging the validation system to provide continuous monitoring. Mounting calibration can also be challenging since the required knowledge (initial sement angle) is difficult to obtain from CP subjects with crouch gait as they cannot stand straight as a healthy subject can. Therefore,with the assistance from the research staff, the subjects’ shanks were held straight for 5 seconds in order to proceed with the mounting calibration. The initial swing motion for synchronization between the Vicon® system and TEMPO system was also avoided since it was almost impossible for the CP subject. Instead, the finer offline synchronization is done by using the first shank motion of the walking trial and aligning shank and foot angle signals and foot angle signals from TEMPO with the validation system.

 

 

 

 

 

 

 

Future Plan for CP Subject Study

Among several follow-on studies of interest, a cross-over study will be performed on children with CP alternating every two weeks between wearing and not wearing their prescribed AFOs. Children will be fitted with custom AFOs made in the standard fashion, but with compartments sized for the TEMPO instrumentation, batteries, and wiring attached to the forms. A separate device will also be made at the same time on the same molds but will be designed to hold the TEMPO instrumentation while not affecting the subject‟s gait. After the AFOs and non-AFO devices are fabricated, the subjects will return to the lab and undergo the full range of laboratory tests described above. After the first two weeks, with either device, the subjects will return and repeat the in lab tests to perform a calibration for sensors. They will then wear the new devices at home for two weeks and then return for final testing. In both groups, the children will wear the TEMPO system that will continually record data during daily activity. Children wearing their prescribed AFOs will be instructed that it is important for them to wear their AFOs as they typically do, neither more nor less. Once a week, subjects will connect the devices to their computers and upload their data to the web site designed for this study. Subjects and their families will be instructed on how to upload the data to the project web site when thet pick up the AFOs. 

Future Plan for Technology Updates

The technology required for carrying out this study will also be tailored to for this specific application. The use of the AFO and the specific application requirements provide opportunities for system optimization that would otherwise be counter to principles of wearability and usability. First, a wire can be molded into the AFO (and non-AFO sensing device) connecting sensors on the foot section of the AFO to the main TEMPO node on the shank section. This eliminates the need for the full electronics on the foot section, which would have caused problems when shoes were
worn over the AFOs. Second, larger batteries can be used, as they can be spread across the surface of the AFO. Third, this application requires neither real-time data transfer nor finegrained synchronization between TEMPO nodes (other than the foot and shank sensors that will be wired together), enabling the use of an on-board flash for data storage instead of Bluetooth® transmission radio. In addition to improving data reliability, these modifications (along with firmware modifications to detect periods of walking) will also dramatically increase the system battery life, enabling the desired week-long data collections. 
 
While the molding of the TEMPO devices in the AFO will eliminate many of the mounting uncertainties that are commonly encountered wireless inertial BSNs, challenges will also arise in the technology adjustment. First, since the accelerometer and calibration will not be possible  . A new calibration algorithm must be developed for the AFO, and individual CP subjects must undergo a calibration procedure with his/her own AFO. The accelerometer will be calibrated using the Newton-Raphson method, and the gyroscope will be calibrated using a regression method directly mapping the known angle to gyroscope sensitivity. The subjects will be asked to conduct a static trial in the gait lab during their first visit to obtain the knowledge of initial node orientation. Fortunately, this procedure need only be performed once. Finally, since the operation and recharging of the devices should be extremely simple for the subjects, customized charging board and scheme will be designed. [1]

 

References

  1. Chen S, Cunninham CL, Lach J, Bennet BC.  2011.  Enabling Longitudinal Assessment of Ankle-Foot Orthosis for Children with Cerebral Palsy. Wireless Health (Best Paper Award).