Foot loading in amputee stance

The relationship between trans-femoral prosthesis alignment and the center trajectory of plantar pressure in the frontal plane

[Purpose] It is difficult to identify by visual observation whether alignment abnormalities in trans-femoral prostheses in the frontal plane are attributable to the adduction angle or the abnormal alignment of the positions of the medial and lateral sides of the socket in relation to the foot. Therefore, we focused on the trajectory of the center of plantar pressure during walking, and we proposed a method for differentiating these two alignment abnormalities. [Subjects and Methods] We recruited 4 trans-femoral unilateral amputees. Bench alignment was achieved initially. We compared the amplitude of the trajectory of the center of plantar pressure when walking under 2 conditions: 1) when changing the adduction angle and 2) when changing the positional relationship between the socket and the foot. [Results] It was not possible to distinguish between the 2 types of malalignment on the prosthesis side. There was a significant difference when changing the positional relationship on the contralateral side. Thereby, the plantar pressure of the contralateral side could be used to distinguish between the 2 types of malalignment. [Conclusion] The results of this study suggested that trans-femoral prosthesis malalignment could be evaluated through the plantar pressure of the contralateral side in amputees.

Standing stability enhancement with an intelligent powered transfemoral prosthesis

The authors have developed a ground-adaptive standing controller for a powered knee and ankle prosthesis which is intended to enhance the standing stability of transfemoral amputees. The finite-state-based controller includes a ground-searching phase, a slope estimation phase, and a joint impedance modulation phase, which together enable the prosthesis to quickly conform to the ground and provide stabilizing assistance to the user. In order to assess the efficacy of the ground-adaptive standing controller, the control approach was implemented on a powered knee and ankle prosthesis, and experimental data were collected on an amputee subject for a variety of standing conditions. Results indicate that the controller can estimate the ground slope within ±1° over a range of ±15°, and that it can provide appropriate joint impedances for standing on slopes within this range.

Adaptation to altered balance conditions in unilateral amputees due to atherosclerosis: a randomized controlled study

Background

Amputation impairs the ability to balance. We examined adaptation strategies in balance following dysvascularity-induced unilateral tibial amputation in skilled prosthetic users (SPU) and first fitted amputees (FFA) (N = 28).

Methods

Excursions of center of pressure (COP) were determined during 20 s quiet standing using a stabilometry system with eyes-open on both legs or on the non-affected leg(s). Main measures: COP trajectories and time functions; distribution of reaction forces between the two legs; inclination angles obtained through second order regression analysis using stabilogram data.

Results

FFA vs SPU demonstrated 27.8% greater postural sway in bilateral stance (p = 0.0004). Postural sway area was smaller in FFA standing on the non-affected leg compared with SPU (p = 0.028). The slope of the regression line indicating postural stability was nearly identical in FFA and SPU and the direction of regression line was opposite for the left and right leg amputees.

Conclusion

Of the two adaptation strategies in balance, the first appears before amputation due to pain and fatigue in the affected leg. This strategy appears in the form of reduced postural sway while standing on the non-affected leg. The second adaptation occurs during rehabilitation and regular use of the prosthesis resulting in normal weightbearing associated with reduced postural sway on two legs and return to the normal postural stability on one leg.

Ground adaptive standing controller for a powered transfemoral prosthesis

The scope of this work is the design and verification of a new standing controller for a powered knee and ankle prosthesis. The controller is based upon a finite-state impedance control approach previously developed by the authors. The controller provides a comprehensive standing behavior that incorporates ground adaptation for unlevel terrain. An amputee subject tested the controller with a powered prosthesis for a variety of standing conditions. Results indicate that the powered prosthesis can estimate the ground slope within ±1 degree over a range of ±15 degrees, and that it can provide appropriate joint impedances for standing on slopes within this range.

Computerized pedobarography in the characterization of ankle-foot instabilities of haemophilic patients

Among the most common clinical manifestations of haemophilia are joint haemorrhages. This study aimed to verify whether repetitive ankle haemarthrosis is associated with instability of the rear foot. We evaluated haemophilic patients with repetitive bleeding in the ankles, 39 of type A and four of type B, whose mean age was 16.1 years. All presented a functional gait, without the need for motion assistance devices. The number of rear-foot and ankle haemarthrosis episodes during the 6 months prior to the study was verified from the medical records of each patient. After verifying the alignment of the rear foot of the patients, we evaluated the subjects through computerized pedobarography with the f-scan system, emphasizing the study of the trajectory of the centre of pressure (COP) with each step taken. All patients received functional orthoses according to the results of these examinations, and were re-evaluated 1 week and 6 months after being fitted. The number of haemarthrosis episodes at the rear foot and the ankle was compared with the occurrence of joint bleeding within the previous 6 months. In the first examination, the COP trajectory showed that all the 43 patients studied had some sort of instability. Six months later, a significant reduction in the frequency of spontaneous bleeding events (P<0.001) concerning the rear foot and the ankle was observed. This method of evaluation was useful to identify joint instabilities, allowing the best prescription of orthoses to improve stability in the rear foot and the ankle.

The gait initiation process in unilateral lower-limb amputees when stepping up and stepping down to a new level

BACKGROUND

Unilateral lower-limb amputees lead with their intact limb when stepping up and with their prosthesis when stepping down; the gait initiation process for the different stepping directions has not previously been investigated.

METHODS

Ten unilateral amputees (5 transfemoral and 5 transtibial) and 8 able-bodied controls performed single steps up and single steps down to a new level (73 and 219 mm). Duration, a-p and m-l centre of mass and centre of pressure peak displacements and centre of mass peak velocity of the anticipatory postural adjustment and step execution phase were evaluated for each stepping direction by analysing data collected using a Vicon 3D motion analysis system.

FINDINGS

There were significant differences (in the phase duration, peak a-p and m-l centre of pressure displacement and peak a-p and m-l centre of mass velocity at heel-off and at foot-contact) between both amputee sub-groups and controls (P<0.05), but not between amputee sub-groups. These group differences were mainly a result of amputees adopting a different gait initiation strategy for each stepping direction.

INTERPRETATION

Findings indicate the gait initiation process utilised by lower-limb amputees was dependent on the direction of stepping and more particularly by which limb the amputee led with; this suggests that the balance and postural control of gait initiation is not governed by a fixed motor program, and thus that becoming an amputee will require time and training to develop alternative neuromuscular control and coordination strategies. These findings should be considered when developing training/rehabilitation programs.

Postural and movement adaptations by individuals with a unilateral below-knee amputation during gait initiation

The present study examined the compensatory strategies adopted by individuals with a unilateral below-knee amputation (BKA) during gait initiation. Eleven individuals with a unilateral BKA and 11 able-bodied subjects initiated gait at three step length conditions (+0, +25 and +50% of preferred step length). A lead-limb condition was also introduced, such that all participants were required to initiate gait with both their left and right limbs. For all step length and lead-limb conditions, it was found that individuals with a unilateral BKA required more time to initiate gait, as compared with the able-bodied. This increase in movement duration was attributed to the stability and movement limitations of the prosthetic limb. On the other hand, by prolonging the task duration, these individuals were also able to employ a 'horizontal impulse' strategy, whereby they could create a similar magnitude of horizontal impulse as the able-bodied without the need to apply a large magnitude of peak antero-posterior (A-P) force.

Comparison of computational analysis with clinical measurement of stresses on below-knee residual limb in a prosthetic socket

Interface pressures and shear stresses between a below-knee residual limb and prosthetic socket predicted using finite element analyses were compared with experimental measurements. A three-dimensional nonlinear finite element model, based on actual residual geometry and incorporating PTB socket rectification and interfacial friction/slip conditions, was developed to predict the stress distribution. A system for measuring pressures and bi-axial shear stresses was used to measure the stresses in the PTB socket of a trans-tibial amputee. The FE-predicted results indicated that the peak pressure of 226 kPa occurred at the patellar tendon area and the peak shear stress of 50 kPa at the anterolateral tibia area. Quantitatively, FE-predicted pressures were 11%, on average, lower than those measured by triaxial transducers placed at all the measurement sites. Because friction/slip conditions between the residual limb and socket liner were taken into consideration by using interface elements in the FE model, the directions and magnitudes of shear stresses match well between the FE prediction and clinical measurements. The results suggest that the nonlinear mechanical properties of soft tissues and dynamic effects during gait should be addressed in future work.

Clinical investigation of the pressure and shear stress on the trans-tibial stump with a prosthesis

A system for measuring pressures and bi-axial shear stresses at the body support interfaces has been developed. This system has been used, in five unilateral trans-tibial amputees, to investigate the stresses at multiple points on the residual limb and prosthetic socket interface during standing and walking. The subjects investigated regularly used a patellar-tendon-bearing socket. The maximum peak pressure at the measured points was 320 kPa over the popliteal area during walking. The maximum shear stress was 61 kPa over the medial tibia area. Variable wave-forms of stress during walking were observed at the different measured points. The influence of the angular alignment on the stresses was investigated on one subject. It was found that a miss-alignment of +/- 8 degrees produced a change in peak longitudinal shear stress of between 8% and 11.5%.

Static versus dynamic prosthetic weight bearing in elderly trans-tibial amputees

The purpose of this study was to compare prosthetic weight-bearing tolerance in the standing position to the dynamic vertical ground reaction forces (VGRF) experienced during walking in elderly dysvascular trans-tibial amputees. Ten unilateral trans-tibial amputees attending an amputee clinic (mean age = 67 +/- 6.5 years) were selected as subjects. Selection criteria were the level of amputation, age, medical fitness to participate and informed consent. Each participant completed five trials of standing (static) weight bearing measurement followed by 10 walking (dynamic) trials on a 10m level walkway, five trials for each limb, Static weight bearing (SWB) was measured using standard bathroom scales. Dynamic weight bearing (DWB) was measured during gait using a Kistler multichannel force platform. T-tests for dependent means indicated that the forces borne in prosthetic single limb stance (mean = 0.97 +/- 0.03 times body weight (BW)) were significantly lower than the forces borne by the prosthetic limb during the first peak (weight acceptance) VGRF (mean = 1.08 +/- 0.08 BW; t = -4.999; p = 0.001) and significantly higher than the midstance VGRF (mean = 0.82 +/- 0.07 BW; t = 5.401; p < 0.001). However, there was no significant difference between SWB and the second peak (push-off) VGRF generated by the prosthetic limb during walking (mean = 0.96 +/- 0.03 BW). It was concluded that clinical gait training may utilise SWB as a guide to an amputees' prosthetic weight bearing tolerance and requirements during walking.

Loading of the contralateral foot in peripheral vascular insufficiency below-knee amputees

Seven ambulatory peripheral vascular insufficiency below-the-knee amputees underwent gait analysis to determine the local pressures on seven prominent plantar regions in their remaining limb. Patients occupied significantly less stance phase time on their remaining limbs. Peak, mean, and total pressures measured at each of the seven prominent areas was decreased as compared with laboratory age and sex matched controls. The results of this study suggest that the risk of contralateral limb amputation surgery is related to systemic peripheral vascular insufficiency or local factors, and not increased loading of the remaining contralateral "limb-at-risk."

Amputee walking training: a preliminary study of biomechanical measurements of stance and balance

Biomechanical parameters of stance and balance were recorded in ten unilateral lower limb amputees at the beginning and end of walking training. Measurements were carried out using a Double Video Forceplate (DVF), a machine developed at University College, London, Bioengineering Centre, Roehampton. During free standing on the DVF there was a mean increase in weight-bearing under the prosthetic foot from 32% body weight (1st session) to 41% body weight (final session), p less than 0.01. Maximum weight-bearing during leaning as far as possible onto the prosthesis increased from a mean of 54% body weight to 63% body weight, p less than 0.01. These simple measurements of weight distribution between the feet can be of value during walking training to monitor progress and can accurately record improvement for research purposes.

Foot loading characteristics of amputees and normal subjects

The foot loading characteristics of 100 consecutively attending amputees wearing their definitive prostheses were studied during their routine visits to the Artificial Limb and Appliance Centre. Results were compared with observations on 100 age and sex matched controls who were free from any locomotor disability. The parameters measured were the percentage of body weight borne on each foot, the positions of the centres of pressure under each foot and the position of the overall centre of foot pressure. The results demonstrate the range of variability of these parameters in normal subjects of different ages and provide preliminary indications of the patterns associated with different types of prostheses and different levels of amputation. The information was collected using the Double Video Forceplate (DVF) a tool developed for the rapid assessment of stance, at University College London Bioengineering Centre. It is proposed that the DVF may be useful in assisting prosthetic alignment, in clinical teaching of prosthetists, physiotherapists and doctors and in monitoring of patients with lower limb amputation.

Bouncy knee in a semi-automatic knee lock prosthesis

The Bouncy Knee concept has previously proved of value when fitted to stabilised knee units of active amputees. The stance phase flex-extend action afforded by a Bouncy Knee increased the symmetry of gait and also gave better tolerance to slopes and uneven ground. A bouncy function has now been incorporated into a knee of the semi-automatic knee lock design in a pilot laboratory trial involving six patients. These less active patients did not show consistent changes in symmetry of gait, but demonstrated an improved ability to walk on slopes and increased their walking range. Subjective response was positive, as noted in the previous trials.