The study of the mechanical properties of the annulus fibrosus of the intervertebral discs is significant to the study on the diseases of lumbar intervertebral discs in terms of both theoretical modelling and clinical application value. The annulus fibrosus tissue of the human intervertebral disc (IVD) has a very distinctive structure and behaviour. It consists of a solid porous matrix, saturated with water, which mainly contains proteoglycan and collagen fibres network. In this work a mathematical model for a fibred reinforced material including the osmotic pressure contribution was developed. This behaviour was implemented in a finite element (FE) model and numerical characterization and validation, based on experimental results, were carried out for the normal annulus tissue. The characterization of the model for a degenerated annulus was performed, and this was capable of reproducing the increase of stiffness and the reduction of its nonlinear material response and of its hydrophilic nature. Finally, this model was used to reproduce the degeneration of the L4L5 disc in a complete finite element lumbar spine model proving that a single level degeneration modifies the motion patterns and the loading of the segments above and below the degenerated disc.

This paper proposes a new damage index named degree of health (DoH) to efficiently tackle structural damage monitoring in real-time. As a key contribution, the proposed index relies on a pattern matching methodology that measures the time-of-flight mismatch of sequential ultrasonic guided-wave measurements using fuzzy logic fundamentals. The ultrasonic signals are generated using the transmission beamforming technique with a phased-array of piezoelectric transducers. The acquisition is carried out by two phased-arrays to compare the influence of pulse-echo and pitch-catch modes in the damage assessment. The proposed monitoring approach is illustrated in a fatigue test of an aluminum sheet with an initial notch. As an additional novelty, the proposed pattern matching methodology uses the data stemming from the transmission beamforming technique for structural health monitoring. The results demonstrate the efficiency and robustness of the proposed framework in providing a qualitative and quantitative assessment for fatigue crack damage.

Cage geometry plays a crucial role in the success of lumbar surgery. General considerations such as larger cages may be applied as a guideline, but parameters such as curvature or cage placement should be determined for each specific patient. This model provides a proof-of-concept of a tool for the preoperative evaluation of lumbar surgical outcomes.

Background Segment fusion using interbody cages supplemented with pedicle screw fixation is the most common surgery for the treatment of low back pain. However, there is still much controversy regarding the use of cages in a stand-alone fashion. The goal of this work is to numerically compare the influence that each surgery has on lumbar biomechanics. Methods A non-linear FE model of the whole lumbar spine was developed to compare between two types of cages (OLYS and NEOLIF) with and without supplementary fixation. The motion of the whole spine was analysed and the biomechanical environment of the adjacent segments to the operated one was studied. Moreover, the risk of subsidence of the cages was qualitatively evaluated. Results A great ROM reduction occurred when supplementary fixation was used. This stiffening increased the stresses at the adjacent levels. It might be hypothesised that the overloading of these segments could be related with the clinically observed adjacent disc degeneration. Meanwhile, the stand-alone cages allowed for a wider movement, and therefore, the influence of the surgery on adjacent discs was much lower. Regarding the risk of subsidence, the contact pressure magnitude was similar for both intervertebral cage designs and near the value of the maximum tolerable pressure of the endplates. Conclusions A minimally invasive posterior insertion of an intervertebral cage (OLYS or NEOLIF) was compared using a stand-alone design or adding supplementary fixation. The outcomes of these two techniques were compared, and although stand-alone cage may diminish the risk of disease progression to the adjacent discs, the spinal movement in this case could compromise the vertebral fusion and might present a higher risk of cage subsidence. Electronic supplementary material The online version of this article (10.1186/s12938-019-0684-8) contains supplementary material, which is available to authorized users.

ABSTRACT:The most conventional technique to treat the intervertebral disc degeneration consists on fusing the affected segment with a posterior screw fixation and sometimes with the insertion of a cage in the intersomatic space. However, this kind of surgeries had controversial results in the adjacent discs. The aim of this work was to prove the stabilization of the spine and the decompression of the disc and to analyze the influence over the adjacent segments. With this purpose, four different models were built and simulated under different loading conditions. The stabilization of the spine was ensured by the screw fixation which reduced dramatically the relative motion in the affected segment. On the other hand, the pore pressure showed a high fall in the operated models proving the decompression of the neural structures. In the adjacent segments, the ROM increased up to 50% in the upper disc and 70% in the lower one. The pore pressure and principal stresses also increased after both surgeries. The observed results suggested that the fusion procedure could trigger a cascade degeneration effect over the adjacent discs, while it is also seen that cage insertion helps to maintain disc height in a better way than screw fixation only. Keywords: finite element method; intervertebral disc; lumbar spine; fusion; arthrodesis; cage Low back pain is one of the most common pathologies affecting society. It is associated with high direct and indirect cost for national welfare. 1,2 Some authors 3 have showed that idiopathic low back pain may be caused by the mechanical degeneration of some structures such as intervertebral discs (IVD) or facet joints affecting its ability to support and transfer loads.Degenerative disc disease (DDD) is a progressive pathology that alters the biochemistry and morphology of the IVD. The loss of proteoglycan and water content leads to a decrease in swelling pressure in the nucleus pulposus and originates a more fibrotic and stiffer tissue. 4,5 The gradual changes in collagen type and annulus fibrosus structure suggest a mechanical influence of the degeneration. 6 When conservative treatments to relieve pain do not work, surgical interventions are required. In the last years, fusion has been the most commonly surgical procedure performed to treat DDD. 7 The aim of spinal fusion is to achieve a stable union of spinal segments maintaining disc height and restoring sagittal alignment. 8 Spinal fusion traditionally required the use of autogenous bone grafting which provides osteogenic results. 9 However, the autograft extraction gave a high incidence of morbidity; therefore, it was necessary to develop new alternatives as cages. The cages have proved to be superior to the bone graft in preserving disc height, 10 which is necessary to decompress neural structures. One of the most popular interventions is posterior lumbar interbody fusion (PLIF). It consists of inserting a cage in the disc space with the addition of posterior screw instrumentation to ensure the spine stabilization. Although lumbar sp...