Cerebral Palsy Orthoses
- Shanshan Chen (firstname.lastname@example.org)
- Christopher Cunningham (email@example.com)
- Brad Bennett (firstname.lastname@example.org)
- John Lach (email@example.com)
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.
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.
Future Plan for CP Subject Study
Future Plan for Technology Updates