Prediction of the Biomechanical Effects of Compression Therapy on Deep Veins Using Finite Element Modelling

Three-dimensional dynamic homogenous modeling: The biomechanical influences of leg tissue stiffness on pressure performance of compression biomedical therapeutic textiles

Patient compliance and therapeutic precision of compression textiles (CTs) are frequently limited by the inaccurate pressure distributions along biological bodies in physical-based compression therapy. Therefore, the biomechanical influences of physiological tissue material characteristics of lower extremities on compression generations of CTs need to be explored systematically to improve pressure management efficacy. In this study, we developed three-dimensional (3D) homogenous finite element (FE) CT-leg systems to qualitatively compare the pressure diversities along lower limbs with different biomaterial tissue properties under each external compression level. Simultaneously, through the obtained leg circumferential displacement, a contact analysis model was applied to quantitatively explore the impact mechanisms of soft leg indentations on the pressure performance of CTs. Based on the experimental validation study, the proposed FE systems could be efficiently utilized for compression performance prediction (error ratio: 7.45%). Through the biomechanical simulation and theoretical calculations, the tissue stiffness characteristics of applied bodies showed significant correlations (p < 0.05) with the body circumferential displacements but no correlations (p > 0.05) with pressure delivery differences of CTs. This study facilitates the pressure fit design principle and leg mannequin material selection guidance for the development and experimental assessment of CTs. It also provides effective simulation methods for pressure prediction and property parametric optimization of compression materials.

Template models for simulation of surface manipulation of musculoskeletal extremities

Capturing the surface mechanics of musculoskeletal extremities would enhance the realism of life-like mechanics imposed on the limbs within surgical simulations haptics. Other fields that rely on surface manipulation, such as garment or prosthetic design, would also benefit from characterization of tissue surface mechanics. Eight homogeneous tissue models were developed for the upper and lower legs and arms of two donors. Ultrasound indentation data was used to drive an inverse finite element analysis for individualized determination of region-specific material coefficients for the lumped tissue. A novel calibration strategy was implemented by using a ratio based adjustment of tissue properties from linear regression of model predicted and experimental responses. This strategy reduced requirement of simulations to an average of under four iterations. These free and open-source specimen-specific models can serve as templates for simulations focused on mechanical manipulations of limb surfaces.

The effects of gravity and compression on interstitial fluid transport in the lower limb

Edema in the limbs can arise from pathologies such as elevated capillary pressures due to failure of venous valves, elevated capillary permeability from local inflammation, and insufficient fluid clearance by the lymphatic system. The most common treatments include elevation of the limb, compression wraps and manual lymphatic drainage therapy. To better understand these clinical situations, we have developed a comprehensive model of the solid and fluid mechanics of a lower limb that includes the effects of gravity. The local fluid balance in the interstitial space includes a source from the capillaries, a sink due to lymphatic clearance, and movement through the interstitial space due to both gravity and gradients in interstitial fluid pressure (IFP). From dimensional analysis and numerical solutions of the governing equations we have identified several parameter groups that determine the essential length and time scales involved. We find that gravity can have dramatic effects on the fluid balance in the limb with the possibility that a positive feedback loop can develop that facilitates chronic edema. This process involves localized tissue swelling which increases the hydraulic conductivity, thus allowing the movement of interstitial fluid vertically throughout the limb due to gravity and causing further swelling. The presence of a compression wrap can interrupt this feedback loop. We find that only by modeling the complex interplay between the solid and fluid mechanics can we adequately investigate edema development and treatment in a gravity dependent limb.

Acute effects of graduated and progressive compression stockings on leg vein cross-sectional area and viscoelasticity in patients with chronic venous disease

OBJECTIVE

To determine the effects of graduated and progressive elastic compression stockings (ECS) on postural diameter changes and viscoelasticity of leg veins in healthy controls and in limbs with chronic venous disease (CVD).

METHODS

In 57 patients whose legs presented with C_1S, C₃, or C₅ CEAP classes of CVD and treated primarily with compression, and 54 healthy controls matched for age and body mass index, we recorded interface pressures at 9 reference leg levels. Cross-sectional areas of the small saphenous vein (SSV) and a deep calf vein (DCV) were measured with B-mode ultrasound with subjects supine and standing, recording the force (PF) applied on the ultrasound probe to collapse each vein with progressive ECS, and with and without graduated 15‒20 mmHg and 20‒36 mmHg elastic stockings. We chose these veins because they were free of detectable lesion and could be investigated at the same level (mid-height of the calf), while their compression by the ultrasound probe was not hampered by bone structures.

RESULTS

Interface pressures decreased from ankle to knee with graduated 15‒20 and 20‒36 mmHg, but increased with progressive ECS, and were 8.4‒13.8 mmHg lower for C_1s than for control or C₃ and C₅ limbs. Without ECS, SSV median [lower‒upper quartile] cross-sectional area was 4.9[3.6‒7.1] and 7.1[3.0‒9.9]mm² in C₃ and C₅ limbs vs. 2.9[1.8‒5.2] and 3.8[2.1‒5.4]mm² in controls (p<.01), respectively while supine and standing. It remained greater in C₃ and C₅ than in C_1S and control limbs wearing any ESC. Wearing compression, especially with progressive ECS, decreased SSV and DCV cross-sectional area only with subjects supine, thus lowering postural changes which remained highly diverse between individuals. The SSV cross-sectional area vs. PF function traced a hysteresis loop of which the area, related to viscosity, was greater in C₃ and C₅ limbs than controls, even with graduated 15‒20 or 20‒36 mmHg ECS. Progressive ECS lowered vein viscosity in the supine position whereas 20‒36 mm Hg and progressive ECS increased distensibility in the standing position.

CONCLUSION

Elastic compression stockings reduce cross-sectional area of superficial and deep calf veins with patients supine but not upright. C_1s limbs show distinctive features, especially regarding interface pressures. Graduated 20‒36 mm Hg and progressive stockings lower viscosity and increase distensibility of the small saphenous vein.

Fluid-structure interaction of blood flow around a vein valve

Introduction: Venous valves are a type of one-way valves which conduct blood flow toward the heart and prevent its backflow. Any malfunction of these organs may cause serious problems in the circulatory system. Numerical simulation can give us detailed information and point to point data such as velocity, wall shear stress, and von Mises stress from veins with small diameters, as obtaining such data is almost impossible using current medical devices. Having detailed information about fluid flow and valves' function can help the treatment of the related diseases. Methods: In the present work, the blood flow through a venous valve considering the flexibility of the vein wall and valve leaflets is investigated numerically. The governing equations of fluid flow and solid domain are discretized and solved by the Galerkin finite element method. Results: The obtained results showed that the blood velocity increases from inlet to the leaflets and then decreases passing behind the valve. A pair of vortices and the trapped region was observed just behind the valves. These regions have low shear stresses and are capable of sediment formation. Conclusion: The von Mises stress which is a criterion for the breakdown of solid materials was obtained. It was also observed that a maximum value occurred at the bottom of the leaflets.

Is a simplified Finite Element model of the gluteus region able to capture the mechanical response of the internal soft tissues under compression?

BACKGROUND

Internal soft tissue strains have been shown to be one of the main factors responsible for the onset of Pressure Ulcers and to be representative of its risk of development. However, the estimation of this parameter using Finite Element (FE) analysis in clinical setups is currently hindered by costly acquisition, reconstruction and computation times. Ultrasound (US) imaging is a promising candidate for the clinical assessment of both morphological and material parameters.

METHOD

The aim of this study was to investigate the ability of a local FE model of the region beneath the ischium with a limited number of parameters to capture the internal response of the gluteus region predicted by a complete 3D FE model. 26 local FE models were developed, and their predictions were compared to those of the patient-specific reference FE models in sitting position.

FINDINGS

A high correlation was observed (R = 0.90, p-value < 0.01). A sensitivity analysis showed that the most influent parameters were the mechanical behaviour of the muscle tissues, the ischium morphology and the external mechanical loading.

INTERPRETATION

Given the progress of US for capturing both morphological and material parameters, these results are promising because they open up the possibility to use personalised simplified FE models for risk estimation in daily clinical routine.

A critical review on the three-dimensional finite element modelling of the compression therapy for chronic venous insufficiency

Compression therapy is an adjuvant physical intervention providing the benefits of calibrated compression and controlled stretch and consequently is increasingly applied for the treatment of chronic venous insufficiency. However, the mechanism of the compression therapy for chronic venous insufficiency is still unclear. To elaborate the mechanism of compression therapy, in recent years, the computational modelling technique, especially the finite element modelling method, has been widely used. However, there are still many unclear issues regarding the finite element modelling of compression therapy, for example, the selection of appropriate material models, the validation of the finite element predictions, the post-processing of the results. To shed light on these unclear issues, this study provides a state-of-the-art review on the application of finite element modelling technique in the compression therapy for chronic venous insufficiency. The aims of the present study are as follows: (1) to provide guidance on the application of the finite element technique in healthcare and relevant fields, (2) to enhance the understanding of the mechanism of compression therapy and (3) to foster the collaborations among different disciplines. To achieve these aims, the following parts are reviewed: (1) the background on chronic venous insufficiency and the computational modelling approach, (2) the acquisition of medical images and the procedure for generating the finite element model, (3) the definition of material models in the finite element model, (4) the methods for validating the finite element predictions, (5) the post-processing of the finite element results and (6) future challenges in the finite element modelling of compression therapy.

Lower Limb Deep Vein Diameters Beneath Medical Compression Stockings in the Standing Position

OBJECTIVES

The mechanism by which compression therapy works is still discussed, especially at calf level. Whether lower limb deep vein diameters change under compression stockings is a matter of debate: no change versus great change. New study material helps to address this question.

METHODS

This was an experimental single centre controlled study on nine selected patients with mild to moderate superficial venous disease. A total of 34 deep vein segments were examined. A new hybrid (elastic + non-elastic materials) cuff pressure device enabled the deep vein diameter changes from baseline to occlusion similar to that which could be observed under stockings. The deep vein diameters were measured through the device with the patients in a standing position and their body weight distributed equally on both legs. This was compared to a 20-35 mm Hg medical compression stocking. The diameter change when patients put their whole body weight on the tested leg was also measured.

RESULTS

A pressure of 25.3 ± 6.4 mm Hg (mean, SD) was required to ovalise lower leg deep veins and a pressure of 43.1 ± 16.2 mm Hg (mean, SD) to occlude them. Both pressures were significantly different from baseline: p = .003 and p < .0001, respectively. No diameter reduction was achieved when the stockings were worn, and occlusion of deep veins occurred when the patients transferred their body weight onto the examined leg.

CONCLUSION

In the standing position, deep vein diameter reduction is not caused by compression stockings but may be due to the isometric muscle contractions required to support the patient's body weight.

Skin stiffness determined from occlusion of a horizontally running microvessel in response to skin surface pressure: a finite element study of sacral pressure ulcers

Pressure ulcers occur following sustained occlusion of microvessels at bony prominences under skin surface pressure (SSP). However, the mechanical conditions of the surrounding soft tissue leading to microvascular occlusion are not fully understood. This study determined the stiffness of homogenized skin with microvasculature at the sacrum that occludes microvessels at an SSP of 10 kPa (consistent with a standard mattress) and recovers from occlusion at 5 kPa (consistent with a pressure-redistribution mattress). We conducted two-dimensional finite element analyses under plane stress and plane strain conditions to determine the stiffness of the skin. The results for plane stress conditions show that the microvessel was occluded with a Young's modulus of 23 kPa in response to an SSP of 10 kPa at the center of the sacrum and that the circulation recovered following a reduction in the SSP to 5 kPa. The resulting Young's modulus is consistent with reported data. Our study indicates that the critical value of the SSP for microvascular occlusion is determined not only by the stiffness of homogenized skin with microvasculature but also by the intraluminal pressure, microvascular wall stiffness, and body support conditions.