The non-fusion surgery can minimize the influence on adjacent segments by preserving the motion of the lesion segments to prevent the occurrence of ASDeg. However, when faced with severe clinical situation of lumbar instability, osteoporosis and severe spinal stenosis, fusion is usually needed[5, 6, 13, 18]. The increase of movement and stress of adjacent segments after fusion is the main cause of ASDeg, moreover, for the degenerated adjacent disc , fusion may accelerate degeneration process, even result in symptomatic degeneration[19, 20], especially for those with indications of fusion and moderate degeneration in the superior adjacent disc(Pfirrmann grade II-IV), the fusion segments should be minimized while achieving good clinical results. As a hybrid internal fixation technique, Topping-off technique may be a fair way to solve the situation[10, 22, 23].
Limited to the fact that the internal mechanical environment of the human body cannot be measured directly, the three-dimensional finite element analysis method is used to simulate the internal mechanical environment of the human body through the establishment of effective lumbar spine models. The biomechanical analysis of the entire lumbar after Topping-off were performed in the lumbosacral junction region where the biomechanical environment of lumbosacral region changed into a rigid lever consisted of pelvis, sacrum and L1-L5 segments together after L5-S1 fusion and then the stress and mobility of upper segments increased due to the relative stability of the pelvis and sacrum. There are a few studies on the changes of mechanical environment after Topping-off technique at present, nevertheless, the changes of mechanical environment of lumbosacral junction region with relatively concentrated stress and the influence of topping-off on the whole lumbar mechanical environment have rarely been referred. In addition, This study showed that, compared with the healthy model, the stress of annulus fibrosus and nucleus pulposus of L4-L5 in degenerated model increased in flexion, extension, axial rotation and bending position, while the ROM of each segment and the stress of posterior joints decreased. So, early disc degeneration may result in a change in the biomechanical state of the corresponding segments. Therefore, in order to study the effects of lumbar fusion and Topping-off on the superior segments, it was rational that the fusion model and Topping-off model were created based on the degenerated model, and then compared with the degenerated model. Previous studies have shown that the degeneration of discs mainly lies in the decrease of proteoglycan concentration and collagen fibrosis, resulting in an increase in the hardness of discs[24, 25]. So, the establishment of the moderate degenerated model was mainly achieved by increasing the elastic modulus of the annulus fibrosus, reducing the volume of the elastic matrix of the annulus fibrosus and reducing the elastic modulus of the nucleus pulposus.
Those results showed an significant increase in ROM of L4-L5 in the fusion model under different positions, especially in flexion, but no significant changes were observed in other segments. Therefore, the compensatory effect of lumbar motion after fusion mainly focused on the L4-L5 segment. Excessive activity results in the change of rotation center in the corresponding segment, which may not only tend to impair the annular fiber and endplate and lead to poor blood supplying, lower nutrition diffusivity and hydraulic permeability, but also influence the resulting forces in the facet joints, making for the resultant apoptosis and accelerated degeneration.[26-29] Several studies have shown that mechanical stimulation plays an important role in the regulation of disc biology and this has indicated that mechanical overloading is a risk factor for disc degeneration.[30, 31] As revealed in the results, Topping-off surgery significantly reduced the mobility of L4-L5 in the flexion and, to some extent, increased the ROM of L2-L4 segments, especially in flexion and extension position. Considering of the slight decrease in ROM of intact lumbar, it indicated that Coflex could not only limit the hyperactivity of the adjacent segments, but also distribute the compensatory effect of lumbar spine motion to the upper segments after fusion. And the intradiscal pressure was largest in the anteflexion position, which explained that thoracic disc frequently occurs in the anteflexion position in the clinic, and indirectly proved the validity of models.
In the lumbosacral junction region where the stress is relatively concentrated, increased disc and facet joints stress of the superior adjacent segment after L5-S1 fusion may lead to changes of biomechanical environment and structural disorders of disc, and make the intervertebral space narrow gradually, especially for the disc that has already degenerated. Facet joints and disc are involved in maintaining stability and in the coupling movement of the spine in different directions. Hyperactivity may result in chronic pressure overload of disc and facet joints. Compared with degenerated model, pressure overload may result in pressure concentration, and then joints wear and remolding[34, 35]. Eventually, under the sustained influence of hyperactivity and pressure overload, moderate degenerated discs gradually develop into the degeneration of the whole segment. In this study, decreased ROM and stress of upper adjacent level indicated that Topping-off could protect facet joints and degenerated disc from hyperactivity and excessive stress,the hyperactivity of adjacent segments, but also reduce the stress of discs and facet joints and delay the progress of degenerated disc by compensating the lost motion of lumbar spine through other adjacent segments over time. In addition, in order to prevent the occurrence of ASDeg, clinicians should improve the surgical skills as much as possible, cause less damage to the superior articular capsule , and restore the lumbar kyphosis as far as possible.