The main goal of this work was to compare between two different intervertebral lumbar cages with and without supplemental PSF and argue if stand-alone cage is a feasible solution for lumbar disc degeneration and hernia.
First, the intact finite element model was validated. In general, a good accordance was obtained between our results and those of the literature. For the case of axial rotation, the differences were higher. However, these disagreements were also obtained by the numerical simulations of Park et al. [57] and they can be related with the tissues involved in the cadaveric specimens of Panjabi et al. [37], which are not considered in the computational models. Moreover, the discrepancies for every movement were higher as the rotation was measured in the most caudal level of the spine. This can be explained by the fact that constraints were applied in S1. Another explanation could be related with the ligaments behaviour. We have to take into account that ligaments are rather different from upper to lower levels with different area and mechanical properties. However, due to the lack of data in the literature, we used the same definition at all levels, with exception of JC and ISL ligaments.
Attending to the operated spines [models (2) to (5)], the results revealed a drastic reduction in ROM when PSF was used whilst stand-alone cages allowed for a greater ROM. Considering that spine instability occurs if the ROM of the affected segment exceeds that of the intact segment for the same moment load [60], in our simulations, all the different models (with and without posterior fixation) showed that all the segments were stable since the stiffness increased in all cases. However, PSF achieved a much more stable union in all cases. The movement of the operated segment, although it remained stable, was high and therefore this type of surgery might compromise the complete fusion of the vertebra. Nevertheless, stand-alone cages have proven to be sufficient for intervertebral fusion when used in combination with bone graft [2, 13, 61]. Furthermore, when no graft is added, a fibrosis occurred around the implant preventing from migration and preserving some segmental motion [51].
Other authors have evaluated the ROM for a variety of cage designs and load magnitudes from the computational and experimental point of view. In vitro studies have reported ROM reductions with stand-alone cages between 6 and 70% of the intact movement in flexion–extension and LB for complete lumbar spines [17, 62] or FSUs [38]. This wide range in experimental findings may be caused by the different cages and surgical approaches used. However, all of them agreed with our results in showing that the restriction in the AR is lower and, in some cases, it was even higher than the intact movement. Moreover, all of them reported a significantly greater segmental stiffness when posterior screw fixation was added. As well as, in vitro studies, FE models from literature also reported a broad range of ROM reduction depending on the cage design and cage material in the whole lumbar spine [22, 63] and FSU [19, 21]. However, as happened in experimental works all of them showed a less stabilisation for AR and stiffer segments with the use of PSF, which is in accordance with the results of this work except in AR with stand-alone cages. In this work, the segment was stiffer in AR, which could be a consequence of ligament pre-stress, especially of the capsular ligaments. Here, the role of ligament pre-stress, due to cage insertion, in the stabilisation of the segment was considered. ALL and JC ligaments, which are dominant under extension [64], were the most affected by space distraction. Consequently, the ROM in extension was reduced with increasing distraction. The capsular ligaments also restricted flexion and AR movements, providing additional stability.
Apart from stability, the interaction between cage and endplate was studied. Cage subsidence is one of the most common causes of failure in lumbar surgeries [13, 29] and contact pressure can be related to this phenomenon [65]. The loss of disc height due to subsidence (i.e. the device sinking through the endplate) causes loss of correction in approximately 30% of the operated cases [59]. For PSF models, the contact pressures were very low and therefore it can be assumed that there would not be risk of subsidence. However, for the stand-alone constructs the behaviour was different, since the stiffest part of the model, which now corresponds to the stand-alone cage, absorbs most part of the loading. The pressure values was similar for both the designs (OLYS and NEOLIF), and this value should be compared with the yield stress of the endplates. Patel et al. [59] found that the maximum tolerable pressure of the endplates has a median value of 6.7 MPa. Our contact pressures were slightly lower than this value; however, the pressures were very near the median value and therefore, it would suggest a risk of subsidence of these implants. The OLYS cage showed a homogeneous contact pressure distributed in a large contact area. On the contrary, the NEOLIF cage exhibited concentrated contact pressures at the cage edges, as shown in other studies [52, 63]. However, whilst OLYS cage laid in the central part of the endplate, NEOLIF contact pressures were located in the outer part of the bony endplate, where its strength is higher [66]. It is known that subsidence risk depends on bone properties and is different for each patient [8, 67, 68]. Here, it was obtained that the contact pressures were near the maximum tolerable pressure of the bony endplates, so a deeper analysis including the local strength of the bony endplate would be necessary to discuss which cage would be more likely to subside.
Finally, the stresses’ distributions in the affected and adjacent segments were analysed. The addition of PSF reduced the maximal and minimal principal stresses in the operated annulus by more than 50%. The stand-alone construct also caused a reduction in the maximal stresses in the AF around 30%; however, the minimal principal stresses were increased in some movements. Additionally, it has been hypothesised that the addition of PSF could lead to the development of IVD degeneration in the segments adjacent to the fused level due to alterations in the stress–strain distribution [69,70,71,72]. In this work, a greater increase in tension and compression stresses was reported for the finite element models with PSF, whilst the stand-alone cages slightly altered the stress distribution in the adjacent segments, which has been also seen in the literature [73]. Moreover, the influence of PSF was higher in the cranial segment than in the caudal one, because the changes on stresses were more significant in the upper segment. Changes in the biomechanical environment of biological tissues (by means of changes on the value or/and distribution of the stresses) can cause damage to these tissues [74]. This result is in accordance with Sears el al. [75], who found that the reoperation for adjacent segment disease occurred more frequently at levels cranial rather than caudal to L4–L5 fused segments.
Although special care has been taken in computationally reproducing the physiological behaviour of the tissues and the events after the surgery, this work has several limitations. Despite spinal ligaments exhibit a non-linear, anisotropic and viscoelastic response [32, 48, 76, 77], they have been simulated as non-linear uniaxial elements. A shell or 3D model of the ligaments would allow the implementation of a more realistic material behaviour of these tissues including their preferential collagen fibre orientation. Furthermore, few experimental data are available regarding spinal ligament pretension. For deeper studies, more experimental work is needed. With respect to the exact geometry of the intervertebral cages, some simplifications were made. In the cage of OLYS implant, the top and bottom surfaces of the cage present grooves on three small zones to avoid retropulsion. These grooves were not included and a high friction coefficient was used instead, and therefore, the overall behaviour of the spine would not be affected. Regarding the PSF, a tie contact was defined at bone–screw interface. A high friction coefficient due to the threads of the screws could have been defined. However, a penalty formulation for contact definition between these elements only would affect the stress distribution in the bone around the screw, but not the movement of the screw inside the bone and, at the same time, would increase the computational time. Therefore, the assumption of a tie contact would be a valid simplification to study the intersegment movement and stresses in the intervertebral discs. Finally, only quasi-static loads were applied to the models. For a more accurate evaluation of the surgical technique, cyclic and impact loading should be considered.
To conclude, when should stand-alone cages be considered instead of traditional PLIF surgery (cage + PSF)? Some authors [78] consider that stand-alone cages would be used in young patients with discogenic pain originating from L4–L5 and/or L5–S1 and no major degenerative changes in the posterior column due to the possible risk of adjacent segment disease associated with PSF. Moreover, Costa et al. [1] argue that when placing stand-alone cage, the facet joints are preserved and the destruction of the posterior and facet joint ligaments and of the endplates is minimal, conditions that are crucial to successful bone fusion. On the other hand, long-term follow-ups [79] have obtained that PLIF stand-alone cages were associated with good clinical outcomes but although the fusion rate was excellent, maintenance of disc heights and lordotic alignment were not achieved in the long term. Therefore, there are still pending questions. Zhang et al. [80] in a review article reported that there is no relationship between radiographic fusion and recurrence of symptoms with development of subsidence. They even suggested that subsidence may be the process of bone incorporation between cages and endplates. Moreover, these authors also relate the posterior screw fixation with an increased rate of adjacent segment degeneration. The following question could be then formulated: where is the equilibrium point between intervertebral fusion and stabilisation?
In this work, a minimally invasive posterior insertion of an intervertebral cage (OLYS and 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 along the spine, the spinal movement in this case might compromise the vertebral fusion.
For more information storage cages for sale, please get in touch with us!