Our research found that 16.7% of patients with L4/5 single segment DLS had kyphosis, which was similar to the previous study.[22] The typical imaging manifestation of kyphotic spondylolisthesis is the collapse of disc in the anterior intervertebral space of the spondylolisthetic segment, which may be related to the insufficient support of anterior column caused by the vacuum of disc and the instability of vertebral body led by severe intervertebral disc degeneration (IDD).[8, 23] Some scholars believe that in the early stage of lumbar IDD, the stress concentrates in the anterior intervertebral space and the speed of IDD at the stress-concentrated area increases significantly, which leads to the anterior height collapse and the loss of lordosis of the intervertebral space.[24] Our study found that the PI of patients with DLS was significantly higher than that of normal people,[25] which was consistent with previous research conclusions.[5, 26] Higher PI results in greater stress in the lumbosacral area,[27] which will lead to a significant increase of pressure in L4/5 intervertebral space and facet joint. Degeneration acceleration, asymmetry and more prone to sagittal orientation of facet joints will increase the instability.[28, 29] Moreover, IDD and reduction of tensile strain capacity of nucleus pulposus and annulus fibrosus will lead to the intervertebral slippage.[30, 31] The change of LL plays an important role in the pathogenesis of DLS. Chen et al.[4] have indicated that the loss of ADH is an independent risk factor for the occurrence and progression of spondylolisthesis. IDD and spondylolisthesis continue to progress with age.[1] Our study found that the age of the DLS patients with kyphosis was significantly higher than that of non-kyphotic group, and the degree of IDD and anterior intervertebral space collapse were more severe, which led to the slightly higher SD of patients in kyphotic group, and that might be an important factor of kyphotic spondylolisthesis.
According to our results, PT and SVA were larger in patients with kyphosis, while LL and SS were significantly lower than those in non-kyphotic group. Previous studies mostly focused on the morphological characteristics of the spine-pelvis sagittal alignment in patients with DLS. The correlation between the abnormal anatomical sequence of spondylolisthetic segment and the spino-pelvic sagittal morphological changes was often paid less attention.[32, 33] Ferrero et al.[5] analyzed the spinal sagittal morphology of 654 patients with DLS in L4/5 segment, and suggested that the loss of lordosis angle in spondylolisthetic segment was closely related to the morphological changes of spino-pelvic sagittal alignment. Kong et al.[15] conducted a retrospective study on 53 patients with L4/5 DLS who underwent interbody fusion, and analyzed the influence of SA, SD and intervertebral space height on postoperative spino-pelvic sagittal parameters. Their results showed that the improvement of SA was the most critical to correct the deformity of spondylolisthetic segment and restore the normal spinal sagittal alignment. However, our results indicated that the soft tissue around the spondylolisthetic vertebrae should be fully released to improve the reduction of spondylolisthesis, and large fusion cage should be implanted to restore intervertebral space height and normal Cobb angle, thus to reconstruct the normal sagittal alignment and achieve better clinical effect. The normal L4/5 intervertebral space is higher in anterior part to maintain LL, however, in the process of IDD, the decrease of ADH will lead to gradual reduction of LL.[24] In addition, the loss of LL aggravates with age, especially after 50.[34] In our study, patients in kyphotic group had significantly older age, more severe loss of ADH and greater reduction of LL, which were bound to cause SVA to move forward. Meanwhile, in order to maintain the sagittal balance, the body needed to rely on pelvic retroversion to compensate, that was, the increase of PT and the decrease of SS. When the LL further decreased and exceeded the compensatory capacity of pelvic retroversion, there might be a tendency of spinal sagittal decompensation, and at that moment, the body might flex the hip and knee to keep upright.[13] This may explain our result that patients with kyphotic DLS in L4/5 segment have more obvious loss of LL, pelvic retroversion and trunk anteversion than patients in non-kyphotic group.
In addition, the VAS and ODI scores showed that the life quality of patients with kyphosis at the time of consultation was significantly lower than that of patients in non-kyphotic group. Fritz et al.[35] considered that lumbar instability and neural compression were the primary factors leading to low back pain and dysfunction in patients with DLS. Chen et al.[7] measured the sagittal displacement and rotation of the spondylolisthetic segment through flexion and extension lateral radiographs, and found that patients with kyphotic spondylolisthesis had obvious lumbar instability. Some scholars indicated that local instability caused by the loss of LL was the primary reason leading to clinical symptoms of DLS patients in L4/5 segment. Our study found that the range of motion (ROM) of the spondylolisthetic segment (including SAM, SDM) in patients with kyphosis was significantly higher than that in the non-kyphotic group, which also confirmed the above-mentioned studies, that was, the loss of disc height and the relaxation of peripheral ligaments caused the increase of ROM, thus gradually destabilizing the spine and resulting in more severe back pain and dysfunction in DLS patients with kyphosis.[30, 31] As mentioned previously, pelvic retroversion, trunk anteversion, and hip and knee flexion are compensatory methods for kyphotic DLS patients to maintain spinal sagittal balance. These postures are bound to affect the spino-pelvic biomechanical distribution of patients, resulting in excessive muscle energy consumption, muscle fatigue, and other more severe clinical symptoms.[24, 36]
The current study has several limitations. Firstly, is the relatively small sample size of kyphotic DLS. Further expanding our sample population will eliminate coincidence as much as possible, reduce measuring deviation and allow for more meaningful statistical testing. Secondly, the degree of IDD and the facet joint orientation were not compared between the two groups, so the mechanism of kyphotic DLS could not be determined. Finally, only L4/5 single segment, low-grade DLS patients were included in this study, which might make the results relatively one-sided and limited. Therefore, in future clinical work, high-quality, multicenter, large sample and wide case scope studies should be conducted to provide spine surgeons with the best evidence-based information.