Changes in interface pressures and shear stresses over time on trans-tibial amputee subjects ambulating with prosthetic limbs: comparison of diurnal and six-month differences

For trans-tibial amputees maintenance over time of a quality fit of the prosthesis to the residual limb is an important clinical challenge. The purpose of this research was to compare diurnal and long-term (5 weeks to 6 months) interface stress changes as well as variance in the change in cross-sectional area down the length of the residual limb. If long-term changes were simply accentuated diurnal fluctuations then this result would suggest similar treatment methods should be used for both conditions. Interface pressures and shear stresses at 13 sites and residual limb shape were measured on eight trans-tibial amputee subjects using patellar-tendon-bearing prostheses. Data were collected at diurnal intervals (within the same day at least 5 h apart) as well as at long-term intervals (5, 10, 15, 20, and 25 weeks apart). Absolute diurnal interface stress changes were not significantly different from those at 5-weeks intervals but were significantly smaller than those at 15, 20, and 25-weeks intervals. Mean interface stress changes increased significantly (p<0.05) for increased session-to-session intervals. Variance of the change in cross-sectional area down the length of the residual limb was significantly smaller for diurnal intervals than for 6-months intervals, indicating that long-term changes were more localized than diurnal changes. These results indicate that long-term changes are not simply accentuated diurnal fluctuations, suggesting that different treatment methods should be used to treat each condition.

Relative influence of polymer fiber diameter and surface charge on fibrous capsule thickness and vessel density for single-fiber implants

Single polypropylene microfibers plasma-coated with polymers of different surface charge [N,N-dimethylaminoethyl methacrylate (NN) (positive charge), methacrylic acid (MA) (negative charge), and hexafluoropropylene (HF) (neutral)] were implanted in the subcutaneous dorsum of Sprague-Dawley rats for 5-week intervals. Thee groups of fiber diameters were used: (I) 1.0 to 5.9 microm; (II) 6.0 to 10.9 microm; and (III) 11.0 to 15.9 microm. Fibrous capsule thickness and blood-vessel density (number of vessels within 100 microm of the fiber) were assessed in tissue sections in the planes of microfiber cross-sections. Results from a multifactorial analysis of variance demonstrated statistically significant main effects (p < 0.05) for microfiber diameter but not for surface-charge coating. The mean differences in capsule thickness among the microfiber diameter groups were: between groups II and I: 5.4 microm; between groups III and I: 10.2 microm; and between groups III and II: 4.7 microm. The mean differences in capsule thickness among surface-charge coatings were: between MA and NN: 0.7 microm; between MA and HF: 1.4 microm; and between NN and HF: 0.7 microm. Many of the 1.0 to 5.9 microm-in-diameter fibers had no capsule and no sign of a foreign-body reaction. For the vessel density analysis, neither microfiber diameter nor surface-charge coating had a statistically significant effect. Thus the geometric feature of microfiber diameter was more important than was surface charge relative to fibrous capsule formation but not relative to local vessel density. This ranking of the relative influence of design features in relation to tissue response provides useful information for prioritization in biomaterial design.

Interface pressure and shear stress changes with amputee weight loss: case studies from two trans-tibial amputee subjects

Interface pressures and shear stresses were measured at monthly intervals on two trans-tibial amputee subjects who lost more than 12% of their body weight over the course of the study. For one subject interface pressures and shear stresses during the weight-acceptance phase of gait decreased over the study interval at all 13 sites monitored, while the other subject experienced increased pressures distally but decreased pressures proximally. Subjects' stumps appeared to atrophy over the study interval, increasing distal end and patellar tendon loading, but not increasing interface shear stresses at other locations. Adding socks at the end of the study did not return interface pressures to first session values at all sites. It is expected that local stump shape changes occurred, causing a non-uniform change in interface stress patterns.

Standing interface stresses as a predictor of walking interface stresses in the trans-tibial prosthesis

Interface pressures and shear stresses within the socket, in standing and walking, were measured for two unilateral, male, trans-tibial amputee subjects, during two sessions each. The ratios of equal weight-bearing standing stresses to peak walking stresses showed regional variation, ranging from 0.24:1 for pressure over the anterior region to 1.01:1 for resultant interface shear stress over the lateral region. Interface stresses in standing were only moderate predictors of peak walking stresses. The best correlation coefficient between standing in full weight-bearing and peak walking stress was 0.88 for pressure over the lateral region. As the amputees progressed from minimal to full weight-bearing in standing, and then to walking, the interface stresses increased in a nonlinear fashion, consistent with the assumption that the anterior tibia provides much resistance to the bending moment in the sagittal plane during walking.

Effects of changes in cadence, prosthetic componentry, and time on interface pressures and shear stresses of three trans-tibial amputees

OBJECTIVE

To evaluate the effects of changes in cadence, prosthetic componentry, and time on interface pressures and resultant shear stresses in trans-tibial amputee case studies.

DESIGN

Interface stresses were monitored using custom-designed instrumentation at 13 sites on three subjects with unilateral trans-tibial amputation walking with patellar-tendon-bearing prosthetic limbs.

BACKGROUND

Previous studies suggested that week-to-week residual limb changes altered interface stresses more than did alterations in prosthetic alignment. No studies investigating effects of changes in cadence or componentry on interface stress distributions nor comparing their influence with week-to-week changes have been conducted previously.

METHODS

Five different prosthetic componentry configurations were tested at each of three cadences in four sessions. Data were analysed for the magnitudes and timings of peak pressures and resultant shear stresses as well as corresponding resultant shear angles.

RESULTS

None of the three cadences or five componentry configurations consistently induced significantly (P<0.05) higher or lower interface stress magnitudes for all subjects. However, an Aluminium Pylon/SACH Foot combination compared with an AirStance (pneumatic shank)/Seattle LightFoot unit induced later peak interface stress timings as a percentage of stance phase. Higher and more frequent interface stress changes were seen between the weekly sessions than between different cadences or between different componentry configurations.

CONCLUSION

The amputees' capabilities to compensate for week-to-week residual limb changes were less than those for intra-session cadence or componentry alterations.

RELEVANCE

Results suggest that effective techniques to accommodate week-to-week residual limb fluctuations could have a greater impact on maintaining consistent interface stress distributions than do adjustments in cadence or componentry.

Finite element estimates of interface stress in the trans-tibial prosthesis using gap elements are different from those using automated contact

When compared with automated contact methods of finite element (FE) analyses, gap elements have certain inherent disadvantages in simulating large slip of compliant materials on stiff surfaces. However, automated contact has found limited use in the biomechanical literature. A non-linear, three-dimensional, geometrically accurate, FE analysis of the trans-tibial limb-socket prosthetic system was used to compare an automated contact interface model with a gap element model, and to evaluate the sensitivity of automated contact to interfacial coefficient of friction (COF). Peak normal stresses and resultant shear stresses were higher in the gap element model than in the automated contact model, while the maximum axial slip was less. Under proximally directed load, compared with automated contact, gap elements predicted larger areas of stress concentration that were located more distally. Gap elements did not predict any relative slip at the distal end, and also transmitted a larger proportion of axial load as shear stress. Both models demonstrated non -linear sensitivity to COF, with larger variation at lower magnitudes of COF. By imposing physical connections between interface surfaces, gap elements distort the interface stress distributions under large slip. Automated contact methods offer an attractive alternative in applications such as prosthetic FE modeling, where the initial position of the limb in the socket is not known, where local geometric features have high design significance, and where large slip occurs under load.

Characterisation of three-dimensional anatomic shapes using principal components: application to the proximal tibia

The objective of the research is to determine if principal component analysis (PCA) provides an efficient method to characterise the normative shape of the proximal tibia. Bone surface data, converted to analytical surface descriptions, are aligned, and an auto-associative memory matrix is generated. A limited subset of the matrix principal components is used to reconstruct the bone surfaces, and the reconstruction error is assessed. Surface reconstructions based on just six (of 1452) principal components have a mean root-mean-square (RMS) reconstruction error of 1.05% of the mean maximum radial distance at the tibial plateau. Surface reconstruction of bones not included in the auto-associative memory matrix have a mean RMS error of 2.90%. The first principal component represents the average shape of the sample population. Addition of subsequent principal components represents the shape variations most prevalent in the sample and can be visualised in a geometrically meaningful manner. PCA offers an efficient method to characterise the normative shape of the proximal tibia with a high degree of dimensionality reduction.

Material properties of commonly-used interface materials and their static coefficients of friction with skin and socks

Compressive stiffness (CS) of different supporting materials used in prosthetics and orthotics and their static coefficients of friction (COF) with skin and socks were characterized. Materials tested included Spenco, Poron, nylon-reinforced silicone, Soft Pelite, Medium Pelite, Firm Plastazote, Regular Plastazote, and Nickelplast. A displacement-controlled testing device was constructed to assess the CS of 11.1 mm diameter material specimens under cyclic loading (1 Hz) to 220 kPa over 10- and 60-min periods. Results demonstrated local CS ranging from 687 kPa (Poron) to 3,990 kPa (Soft Pelite). To fit the cyclic stress-strain (S-S) data within an error of 4.0 percent full-scale output, the minimum order of fit required for Spenco, Poron, and nylon-reinforced silicone was a third-order polynomial; for Soft Pelite, Medium Pelite, Firm Plastazote, and Regular Plastazote, a second-order polynomial; and for Nickelplast, a linear fit. For all materials, the nonrecovered strains were related to loading time using an exponential fit. A biaxial force-controlled load applicator device was used to assess COF at skin-material, sock-material, and skin-sock interfaces for shear forces of 1 to 4 N applied to a 10.2 x 7.8 mm loading pad. COFs ranged from 0.48 (+/- 0.05) to 0.89 (+/- 0.09). COFs at skin-material interfaces were significantly (p < 0.05) higher than those at skin-sock interfaces. There was a trend of a higher COF at sock-material interfaces than at skin-sock interfaces. These data are of potential utility in finite element modeling sensitivity analysis of residual limb-prosthetic socket systems or body-orthosis systems to characterize effects of material features on interface pressure and shear stress distributions.

Mechanical characterization of biomaterials

Experimental testing and computational modeling are two useful tools for mechanical characterization of biomaterials, providing insight into material stresses and strains and the possibility of mechanical failure and fatigue. Another interesting issue to consider, however, is tissue response to mechanical stress and strain and incorporation of that knowledge into design to create materials that form mechanically effective tissue-material interfaces in vivo. Experimental testing, computational modeling, and tissue response to stress and strain are discussed in the context of enhancing biomaterial design.

A modular six-directional force sensor for prosthetic assessment: a technical note

A device designed to measure the forces and moments transmitted through prostheses of persons with lower limb amputation is presented. The sensing unit design is an advancement over previous prosthesis force measurement devices in that it is very thin (19 mm) and lightweight (527.5 g, including signal-conditioning instrumentation). The disk-shaped transducer fits between the socket and socket adapter of a standard modular prosthesis, measuring all six force and moment components at this location. Twelve strain gages were used, configured into six two-arm active Wheatstone bridge circuits. A 6X6 matrix was constructed from calibration data to relate the 6-component bridge-output vector to a 6-component force and moment vector. In a bench-test setting, the sensor was evaluated under typical load combinations encountered during the walking of a person with transtibial amputation (TTA) and shown to have errors less than 7.2% of the full-scale output for each direction. Data collected on a subject with TTA walking at different speeds are presented.

Interface mechanics in lower-limb external prosthetics: a review of finite element models

The distribution of mechanical stress at the interface between a residual limb and prosthetic socket is an important design consideration in lower-limb prosthetics. Stresses must be distributed so that the amputee is stable and comfortable, while avoiding trauma to the tissues of the residual limb. Numerical estimation of the stresses at the interface through finite element (FE) modeling can potentially provide researchers and prosthetists with a tool to aid in the design of the prosthetic socket. This review addresses FE modeling of interface stresses in lower-limb external prosthetics. The modeling methodologies adopted by analysts are described. Verification of FE estimates of interface stress against experimental data by different analysts is presented and the likely sources of error discussed. While the performance of the models is encouraging, there are definite limitations to all of them, necessitating further improvements. Parametric analysis of the sensitivity of interface stress to model parameters provides a tool to identify model weaknesses and to suggest possible refinements. Parametric analyses by different analysts are also presented and potential refinements discussed. Finally, directions for future work in prosthetic FE modeling are suggested.

Automated hexahedral mesh generation from biomedical image data: applications in limb prosthetics

A general method to generate hexahedral meshes for finite element analysis of residual limbs and similar biomedical geometries is presented. The method utilizes skeleton-based subdivision of cross-sectional domains to produce simple subdomains in which structured meshes are easily generated. Application to a below-knee residual limb and external prosthetic socket is described. The residual limb was modeled as consisting of bones, soft tissue, and skin. The prosthetic socket model comprised a socket wall with an inner liner. The geometries of these structures were defined using axial cross-sectional contour data from X-ray computed tomography, optical scanning, and mechanical surface digitization. A tubular surface representation, using B-splines to define the directrix and generator, is shown to be convenient for definition of the structure geometries. Conversion of cross-sectional data to the compact tubular surface representation is direct, and the analytical representation simplifies geometric querying and numerical optimization within the mesh generation algorithms. The element meshes remain geometrically accurate since boundary nodes are constrained to lie on the tubular surfaces. Several element meshes of increasing mesh density were generated for two residual limbs and prosthetic sockets. Convergence testing demonstrated that approximately 19 elements are required along a circumference of the residual limb surface for a simple linear elastic model. A model with the fibula absent compared with the same geometry with the fibula present showed differences suggesting higher distal stresses in the absence of the fibula. Automated hexahedral mesh generation algorithms for sliced data represent an advancement in prosthetic stress analysis since they allow rapid modeling of any given residual limb and optimization of mesh parameters.