PROSTHETICS AND ORTHOTICS RESEARCH

by Sarah J. Mattes, M.S.

at Texas Scottish Rite Hospital for Children

Movement Science Laboratory

Dallas, TX  75219, USA

Amputee patient at Texas Scottish Rite Hospital for Children.

Children seeking prosthetic and/or orthotic treatment at Texas Scottish Rite Hospital for Children are often seen in the Movement Science Laboratory for participation in on-going research projects, as well as for clinical evaluations at the request of the prosthetists and the orthopaedic staff. Kinematic and kinetic data, isokinetic or isometric muscle strength, oxygen consumption, split screen video and electromyography are often collected. It is common for these children to be tested twice, one time before intervention, prosthetic component change or surgery, and then again after the treatment.

The typical data collection for our patients involves kinematic and kinetic information collected using a Vicon system with six 60 Hz infrared cameras. In addition, sagittal and coronal plane split screen video is also obtained. The Vicon Clinical Manager (VCM) marker set is usually used with additional markers at the shoulders, elbows, wrists, lateral crests and greater trochanters to calculate the centre of mass using the BodyBuilder software. The heel markers are typically kept in place during the walking trials. Natural cadence walking is collected with at least four clean force platform strikes from each foot. The data are processed using the VCM software. Strength measurements are made for both the residual limb and sound limb. Isokinetic data are collected using a Biodex machine. If the patient is too small or weak for isokinetic testing, the equipment is reconfigured to collect isometric muscle strength. If this is not possible, manual muscle grades of I-V are recorded. Energy expenditure is measured using a K2 oxygen consumption monitor during many of our research protocols.

Left to right, Cindy Smith, Sarah Mattes, Suzanne Halliday, (back row) Scott Colby, Brian Wilk.

We have found that kinematic data from an amputee’s prosthetic limb can be collected with the same confidence as kinematic data from a normal population, depending on the deformation characteristics of the patient’s prosthetic foot and ankle component and on the socket interface. However, there are more significant limitations to collecting dependable kinetic data from the prosthetic side. Because the prosthesis does not conform to the assumptions made about the mass, location of the centre of mass or the moment of inertia for the sound side, the kinetic results from the VCM software are likely to be inaccurate. In keeping with some of the research projects that are currently underway in the lab though, it is possible that the directly measured inertia characteristics of the prosthetic limb could be input to a BodyBuilder model that would then yield accurate data.

Measuring Inertia Characteristics of Prostheses

To determine the inertia characteristics of the prosthesis, the mass, location of the centre of mass and the moment of inertia must be measured directly. Mass can be measured using a standard gram scale, centre of mass can be measured using a reaction board technique and the moment of inertia is measured using the period of oscillation while swinging the prosthesis like a pendulum through a very narrow arc. For small angles of oscillation (>5 degrees), the moment of inertia (I) about the axis of rotation is related to the period of oscillation by the equation:

where M is the mass of the prosthesis, g is the acceleration due to gravity, d is the distance from the axis of rotation to the centre of mass. The moment of inertia about a transverse axis through the centre of mass was calculated by using the parallel axis theorem and the equation

After calculating the position of the centre of mass of the prosthesis, using a reaction board technique, the moment of inertia calculation can be adjusted to reflect the moment of inertia about an axis through the centre of mass.

Manipulating Prostheses Inertia Characteristics

Timing the amount of time it takes to swing the prosthetic limb through an arc of motion allows the calculation of the movement of inertia of the prosthesis about the axis of rotation.

Recent research has started looking at mass characteristics of below-knee prostheses. In general, clinicians and researchers have embraced the notion that prostheses should be as light as possible, presumably in part to minimize muscular effort and metabolic energy demand during locomotion. This is based on the premise that an important component of metabolic demand during walking and running is associated with accelerating the limbs with each stride. Thus, the demands on the musculature should be reduced as the mass and moment of inertia of the leg are reduced. A consequence of the use of lightweight prostheses, however, is that unilateral amputees often possess a substantial inertia asymmetry between their limbs. The mass of a common prosthetic limb for an adult trans-tibial amputee may range from 0.5 to 2.0 kg, whereas the intact shank and foot (for a 70 kg adult) has an estimated mass of 4.0 kg. At present, the relationships between lower extremity inertia asymmetries, gait asymmetries, and heightened energy cost are not well understood. Previous modelling studies have suggested that gait symmetry may be improved if the mass and moment of inertia of the prosthetic and intact limbs are well matched i,ii,iii.

Inertia characteristics of the intact limb can be made with commonly used and widely accepted regression equations (methods outlined by deLeva.iv), and estimations about the residual limb can be made using geometric models (Hanavanv). After the inertia characteristics are calculated for the prosthetic limb and estimated for the sound limb, manipulations of these parameters can be made to the prosthetic limb that will make the prosthetic limb more closely match the sound limb. A new research project in the Movement Science Lab will be collecting oxygen consumption and kinematic and kinetic data after a short period of accommodation to these inertia manipulations. 

Transition to Articulating Knees

The Vicon system has also been used in the Movement Science Laboratory to assess the success of above-knee amputee children who are transitioning to an articulating knee. It is common prosthetic prescription for a pediatric above-knee or knee disarticulation amputee to be fitted with a prosthesis as soon as he/she begins to pull to stand which generally occurs between the ages of 9 and 16 months. Typically, the child is started with a non-articulating knee prosthesis during the period when he or she first learns to walk. The child is usually transitioned to a prosthesis with a functional knee around the age of three or four (sometimes as late as six years if the child is a bilateral amputee). The reasons why children are not immediately given a functional knee are: 1) children can learn to ambulate more easily without having the prosthesis knee buckling at unwanted times, and 2) there are no commercially available knees designed for children under the age of three. Often the child is ready to receive a functional knee much earlier than the recommended age, but the available knees are simply too large for the short stature of a three year old child. As the transition age is delayed, the child learns to ambulate with the stiff prosthesis by adopting a gait pattern characterized by an abnormal increase in pelvic motion and increased circumduction of the prosthesis to clear the foot during the swing phase. This compensation pattern is often retained when the child grows large enough to receive an articulating knee. The child, therefore, continues to walk in the new prosthesis as though he or she is still unable to bend the prosthetic knee.

Average range of motion at the knee for both amputated and sound limbs for children in this study. Zero degrees knee flexion represents full extention during standing. Toe-off is represented by the vertical dashed line and occurs at 60% of the gait cycle. Stance phase is represented by the first 60% of the gait cycle, and swing phase is represented by the latter 40% of the gait cycle.

Three-dimensional kinematic data were collected from seven pediatric amputees (age ranging from 1 year, 5 months to 6 years, 1 month) at three time points: 1) initially, with their non-articulated prostheses; 2) after gait training with their new, articulated prostheses; and 3) after approximately 1 year of use with the new prostheses. Results show that children as young as 1+5 years of age were fitted with a mobile knee without any instances of increased falling. The first graph above shows that children start bending the articulating knee to a limited extent during the swing phase immediately after completing gait training with the new knee. The limitation in prosthetic design for the toddler-aged amputee is in the size of componentry. While it is preferable to fit all young children with an articulating knee joint, some are simply not tall enough to allow for the incorporation of the knee components. Currently, fitting with an articulated prosthesis as soon as the child’s height allows is recommended. While the child’s gait will not immediately normalize following training with an articulated prosthesis, significant long term improvements in knee flexion during swing phase, and resolution of increased pelvic rotation and hip abduction due to circumduction can be expected. These results suggest that a pediatric amputee can achieve a more normalized gait pattern in as little as one year’s time. The complete results of this study have been accepted for publication in the Journal of Prosthetics and Orthotics. 

Proximal Femoral Focal Deficiency

Patients with proximal femoral focal deficiency are seen in the Movement Science Laboratory so that the functional benefits of the Steel iliofemoral arthrodesis may be critically evaluated. This procedure is designed to stabilize the hip joint and allow patients to use their anatomical knee to flex and extend the prosthetic limb. These children have inherently unstable hip joints due to the head of the femur being inadequately developed. To reduce the risk of dislocating the hip and to minimize Trendelenburg lurch, the hip joint is often fused.

Because the hip joints of these children are different than normal, some problems arise in modelling the hip joint centre for the calculation of joint kinetics during walking. In patients who were not fused, motion occurs at both the anatomical knee and hip joints to flex and extend the prosthetic limb. In patients with a fused hip, motion occurs through the anatomical knee to flex and extend the prosthetic limb. During surgery, the anatomical hip is fused in 90 degrees to allow full extension of the anatomical knee to flex the prosthetic limb into 90 degrees at the hip. “Hip” motion is therefore occurring about the anatomical knee axis, which is located anterior and distal to where it is calculated by the VCM software.

Fifteen patients with this diagnosis were evaluated in the Movement Science Lab. In order to reduce error, three-dimensional radiographic measurements were taken to locate the anatomical knee centre in the pelvic coordinate system. Once the three dimensional coordinates of the anatomical knee (or Ôhip’) are found, the equations of VCM can be back computed to determine the appropriate limb length, inter ASIS distance and ASIS to trochanter distance to enter into VCM computations which will then locate the centre of rotation appropriately. In addition to the kinematic and kinetic data, energy expenditure was estimated by measuring oxygen consumption.

It was found that all of the children with this diagnosis had weak abductor strength. In comparison to transfemoral amputees, patients with PFFD tended to demonstrate more significant gait deviations utilizing increased pelvic motion and vaulting to advance the prosthetic limb. Patients with an un-stabilized hip joint tended to walk with an uncompensated Trendelenburg lurch. This was evidenced by a decreased abduction moment during stance. In addition, these patients tended to utilize more energy to freely ambulate. The results of this work will be written up for publication in orthopaedic literature in the near future.

Texas Scottish Rite Hospital for children had 358 active amputee patients in 1997, and made 304 new prosthetic limbs. The Prosthetics and Orthotics departments have 3 Certified Prosthetists and 4 Certified Prosthetist/ Orthotists, many of whom are interested in collaborating with the Movement Science Laboratory on new research projects. We are looking forward to continuing with follow-up projects to the research project mentioned in this article and to starting many new ones in the field of prosthetics and orthotics.

References

i     Tsai CS, Mansour JM. Swing phase simulation and design of above knee prostheses. J Biom Eng 1986; 108: 66-72.

ii    Bach TM, Evans OM, Robinson IGA. Optimization of inertial characteristics of transfemoral limb prostheses using a computer simulation of human walking. In Proceedings of the Eighth Biennial Conference of Canadian Society for Biomechanics, Calgary, Alberta. 1994.

iii   Mena D, Mansour JM, Simon, SR. Analysis and synthesis of human swing leg motion during gait and its clinical applications. J Biom 1981; 14(12): 823-832.

iv    deLeva P. Adjustments to Zatsiorsky-Seluyanov’s segment inertia parameters. J Biom 1996a 29(9): 1223-1230.

v      Hanavan EP. A mathematical model of the human body. Wright-Patterson AIr Force Base, Ohio. (AMRL-TR-64-102), 1964.