A unified approach to model peripheral nerves across different animal species

Some Mechanical Constraints to the Biomimicry with Peripheral Nerves

Novel high technology devices built to restore impaired peripheral nerves should be biomimetic in both their structure and in the biomolecular environment created around regenerating axons. Nevertheless, the structural biomimicry with peripheral nerves should follow some basic constraints due to their complex mechanical behaviour. However, it is not currently clear how these constraints could be defined. As a consequence, in this work, an explicit, deterministic, and physical-based framework was proposed to describe some mechanical constraints needed to mimic the peripheral nerve behaviour in extension. More specifically, a novel framework was proposed to investigate whether the similarity of the stress/strain curve was enough to replicate the natural nerve behaviour. An original series of computational optimizing procedures was then introduced to further investigate the role of the tangent modulus and of the rate of change of the tangent modulus with strain in better defining the structural biomimicry with peripheral nerves.

The effect of injurious compression on the elastic, hyper-elastic and visco-elastic properties of porcine peripheral nerves

The aim of this study was to characterise the viscoelastic and hyper-elastic properties of the ulnar nerve before and after compression has been induced, in order to aid the understanding of how the mechanical properties of nerves are altered during nerve compression, a contributing factor to cubital tunnel syndrome. Ulnar nerves were dissected from porcine legs and tensile tested to 10% strain. The Young's modulus and Yeoh hyper-elastic model were used to evaluate the materials elastic and hyper-elastic properties respectively. Dynamic mechanical analysis (DMA) was used to evaluate the viscoelastic properties over a range of frequencies between 0.5 Hz and 38 Hz. The nerves were then compressed to 40% for 60 s and the same tests were carried out after compression. The nerves were stiffer after compression, the mean Young's modulus before was 0.181 MPa and increased to 0.601 MPa after compression. The mean shear modulus calculated from the Yeoh hyper-elastic model was also higher after compression increasing from 5 kPa to 7 kPa. After compression, these properties had significantly increased (p < 0.05). The DMA results showed that the nerves exhibit frequency dependent viscoelastic behaviour across all tested frequencies. The median values of storage modulus before compression ranged between 0.605 and 0.757 MPa across the frequencies and after compression between 1.161 MPa and 1.381 MPa. There was a larger range of median values for loss modulus, before compression, median values ranged between 0.073 MPa and 0.216 MPa and after compression from 0.165 MPa to 0.410 MPa. There was a significant increase in both storage and loss modulus after compression (p < 0.05). The mechanical properties of the nerve change following compression, however the response to decompression in vivo requires further evaluation to determine whether the observed changes persist, which may have implications for clinical recovery after surgical decompression in entrapment neuropathy.

Morphological and hemodynamic changes of sciatic nerves and their vasa nervorum during circular compression and relaxation

The main aim of this study was to evaluate the biomechanical and hemodynamic responses of vasa nervorum under transverse circular compression. In situ compress-and-hold experiments were performed on the sciatic nerves of healthy and diabetic rats, and the blood flow within the vasa nervorum was observed using Doppler-optical coherence tomography. A new technique was developed to obtain the time-course of the cross sectional area and the morphology of the vasa nervorum from the tomographic images. A quasi-linear viscoelastic model was used to investigate the overall biomechanical properties of the nerves, and a two-dimensional three-layered finite element model was constructed to analyze the distribution of stress and the morphological changes during the compression-relaxation process. The results showed that the lumenal area of vasa nervorum was reduced in the compression stage, especially for the diabetic nerves. The reduction was greater than 70% when the reduction of the nerve diameter was only 10%. The quasi-linear viscoelastic model showed that normal nerves were more elastic but less viscous than the diabetic nerves. The finite element analyses demonstrated that perineurium could sustain more stress than other layers, while epineurium served as a cushion to protect vasa nervora. In addition, there were regions within epineurium with less stress, so that vasa nervora in these saddle regions were less deformed. The vasa nervorum in diabetic rats was more prone to compression and reduction of blood flow than that of the normal rats. The histological studies supported the simulation results.

A data-driven polynomial approach to reproduce the scar tissue outgrowth around neural implants

Despite the huge complexity of the foreign body reaction, a quantitative assessment over time of the scar tissue thickness around implanted materials is needed to figure out the evolution of neural implants for long times. A data-driven approach, based on phenomenological polynomial functions, is able to reproduce experimental data. Nevertheless, a misuse of this strategy may lead to unsatisfactory results, even if standard indexes are optimized. In this work, an effective in silico procedure was presented to reproduce the scar tissue dynamics around implanted synthetic devices and to predict the capsule thickness for times before and after experimental detections.

Strategies for Peripheral Nerve Repair

PURPOSE OF REVIEW

This review focuses on biomechanical and cellular considerations required for development of biomaterials and engineered tissues suitable for implantation following PNI, as well as translational requirements relating to outcome measurements for testing success in patients.

RECENT FINDINGS

Therapies that incorporate multiple aspects of the regenerative environment are likely to be key to improving therapies for nerve regeneration. This represents a complex challenge when considering the diversity of biological, chemical and mechanical factors involved. In addition, clinical outcome measures following peripheral nerve repair which are sensitive and responsive to changes in the tissue microenvironment following neural injury and regeneration are required.

SUMMARY

Effective new therapies for the treatment of PNI are likely to include engineered tissues and biomaterials able to evoke a tissue microenvironment that incorporates both biochemical and mechanical features supportive to regeneration. Translational development of these technologies towards clinical use in humans drives a concomitant need for improved clinical measures to quantify nerve regeneration.

Strain stiffening of peripheral nerves subjected to longitudinal extensions in vitro

The mechanical response of peripheral nerves is crucial to understand their physiological and pathological conditions. However, their response to external mechanical solicitations is still partially unclear, since peripheral nerves could behave in a quite complex way. In particular, nerves react to longitudinal strains increasing their stiffness to keep axons integrity and to preserve endoneural structures from overstretch. In this work, the strain stiffening of peripheral nerves was investigated in vitro through a recently introduced computational framework, which is able to theoretically reproduce the experimental behaviour of excised tibial and sciatic nerves. The evolution and the variation of the tangent modulus of tibial and sciatic nerve specimens were quantitatively investigated and compared to explore how stretched peripheral nerves change their instantaneous stiffness.

Mechanical Properties of the Human Tibial and Peroneal Nerves Following Stretch With Histological Correlations

Despite the extensive literature regarding peripheral nerve stretch injuries, there are few studies that compare the nerve histology with the mechanical properties in humans. There is clinical evidence suggesting that the peroneal nerve is at greater risk for injury compared to the tibial nerve following total hip arthroplasty and hip trauma. We examined the two nerves from fresh human cadavers with or without controlled stretch. The mechanical properties, stiffness, and strain were compared with light microscopic preparations in longitudinal sections stained by the trichrome method for collagen and showing the effects of structural deformation. The tibial nerve had an average failure load 1.7× that for the peroneal nerve (P = 0.0001). Although the corresponding average stiffness showed a trend toward being larger (4.39 vs. 3.81 N/mm), the difference was not significant (P = 0.126). Histologically, the perineurium along with the underlying nerve fascicle was undulated in the control specimens and straightened out in the stretched specimens. Peroneal nerves went on to failure at lower loads and exhibited a wavy pattern on pathologic slides after failure, which shows that peroneal nerves fail mechanically before they can unfold. The tibial nerve has a biomechanical and histological advantage compared to the peroneal nerve during tensile testing, which could be the reason why it is less commonly damaged. We conclude that the perineurium is especially protective against deformation changes in human nerves relative to the respective nerve size and number of fascicles. Anat Rec, 302:2030-2039, 2019. © 2019 American Association for Anatomy.

A Quantitative Investigation on the Peripheral Nerve Response within the Small Strain Range

Peripheral nerves are very complex biological structures crucial to linking the central nervous system to the periphery of the body. However, their real behaviour is partially unknown because of the intrinsic difficulty of studying these structures in vivo. As a consequence, theoretical and computational tools together with in vitro experiments are widely used to approximate the mechanical response of the peripheral nervous tissue to different kind of solicitations. More specifically, particular conditions narrow the mechanical response of peripheral nerves within the small strain regime. Therefore, in this work, the mechanical response of nerves was investigated through the study of the relationships among strain, stress and displacements within the small strain range. Theoretical predictions were quantitatively compared to experimental evidences, while the displacement field was studied for different values of the tissue compressibility. This framework provided a straightforward computational assessment of the nerve response, which was needed to design suitable connections to biomaterials or neural interfaces within the small strain range.

Strain partitioning between nerves and axons: Estimating axonal strain using sodium channel staining in intact peripheral nerves

BACKGROUND

Peripheral nerves carry afferent and efferent signals between the central nervous system and the periphery of the body. When nerves are strained above physiological levels, conduction blocks occur, resulting in debilitating loss of motor and sensory function. Understanding the effects of strain on nerve function requires knowledge of the multi-scale mechanical behaviour of the tissue, and how this is transferred to the cellular environment.

NEW METHOD

The aim of this work was to establish a technique to measure the partitioning of strain between tissue and axons in axially loaded peripheral nerves. This was achieved by staining extracellular domains of sodium channels clustered at nodes of Ranvier, without altering tissue mechanical properties by fixation or permeabilisation.

RESULTS

Stained nerves were imaged by multi-photon microscopy during in situ tensile straining, and digital image correlation was used to measure axonal strain with increasing tissue strain. Strain was partitioned between tissue and axon scales by an average factor of 0.55.

COMPARISONS WITH EXISTING METHODS

This technique allows non-invasive probing of cell-level strain within the physiological tissue environment.

CONCLUSIONS

This technique can help understand the mechanisms behind the onset of conduction blocks in injured peripheral nerves, as well as to evaluate changes in multi-scale mechanical properties in diseased nerves.