Lumbar interbody fusion (LIF) is an effective treatment for various spinal diseases[16–17] and is the most widely used surgical procedure[18], but complications such as excessive blood loss, muscle denervation, iatrogenic muscle disorders and soft tissue injury are unavoidable[2]. OLIF, which was first reported by Silvestre et al.[19] in 2012, uses the natural space between the psoas major muscle and the abdominal aorta for the surgical approach and has been widely used in recent years for indirect decompression and interbody fusion in patients with lumbar degenerate disorders due to its advantages of yielding less trauma, less perioperative bleeding and a lower incidence of neurological complications[20–22]. Although good clinical results of SA OLIF have been reported[23–25], lumbar degenerate disorders have been reported mostly in elderly patients, and complications such as subsidence and migration of the cage after surgery should not be neglected. For patients with dual-energy X-ray absorptiometry (DEXA) T scores of less than − 1.0, posterior instrumentation is often required[26].
In recent years, scholars have paid increasing attention to the influence of cage position on postoperative cage subsidence. Studies[27–28] have reported that the rigidity of the central region of the endplate is significantly less than that of the peripheral region, which is called the epiphyseal ring, from L1 to S1. Therefore, cage position is an important factor of cage subsidence and endplate collapse. However, the best cage position remains unclear. Barsa et al.[29] found a lower cage subsidence rate near the anterior part of the intervertebral space. Abbushi et al.[30] concluded that the medio-medial cage position shows the highest migration rate, and the lowest migration and highest fusion rate are seen when the cage is in the posteromedial position. Thus, the authors suggested posterior medial cage placement for PLIF. However, Ko et al.[31] found that the rate of cage subsidence increases with a more posterior cage position. A previous study[27] shows that the inferior endplate is approximately 40% stronger than is the superior endplate, and the subsidence probability of the superior endplate is significantly higher than that of the inferior endplate[32–33]. Therefore, we analyzed the stress of the L5 SEP and cage to evaluate the effect of cage position on the biomechanics of the osteoporotic lumbar spine.
In the present study, the ROMs of the SA models and BPSF models were more than 28.29% and 80.56% smaller in all motions, respectively. This indicated that both SA and BPSF could provide stability for the surgical segment. The stresses of the L5 SEP and cage were the lowest when the cage was placed in the middle third of the L5 SEP and were the highest when the cage was placed in the posterior third in both the SA models and the BPSF models. This result may be related to the contact area between the cage and the endplate changing according to the cage position during lumbar movement; the larger the contact area was, the smaller the stress. The stress of the fixation was also the lowest in most motions when the cage was placed in the middle third of the L5 SEP, which may be related to the fact that different cage positions affect mechanical properties such as the moment arm and torque of the bilateral pedicle screw fixation. Kim et al.[34] thought that cage positioning and the application of greater compressive load are two reasons for the high risk of cage subsidence. Based on these results, placing the cage in the middle third of the L5 SEP could help reduce the maximum stresses of the L5 SEP, the cage and the fixation, which may be beneficial for reducing the risk of postoperative cage subsidence, endplate collapse and fixation fracture, regardless of whether SA or BPSF is performed. This position is consistent with the cage placement recommended in the Guidelines for the clinical application of lumbar oblique lateral interbody fusion[35]. The guidelines also point out that whether or not to apply SA technology should be decided on the basis of whether the application of a cage alone can provide sufficient stability[35]. In our study, the L4-L5 ROM, the stress of the cage and the stress of the L5 SEP were smaller under all motions in the BPSF models than those of the SA models, especially in flexion and extension. This suggested that the SA method could not provide sufficient stability under some motions. In contrast, BPSF could significantly increase the stability of the surgical segment and reduce the stresses of the cage and L5 SEP in all motions, which could improve the safety of OLIF surgery in osteoporotic lumbar spine. This may be the reason why the BPSF group had a lower incidence of cage settlement than did the SA group in the study by Zeng et al.[25]. It is also worth noting that almost all surgical models showed the largest stress values for the cage and endplate in flexion, so few bending movements should be performed after OLIF surgery.
In addition to considering the factors of subsidence when placing the cage, it is also necessary for clinicians to restore sagittal balance and induce sufficient indirect decompression. Kepler et al.[36] concluded that anteroposterior cage placement is an important intraoperative determinant of postoperative alignment; anterior placement leads to larger angles of lordosis, while middle/posterior placement has a minimal effect on sagittal alignment. The authors also thought the rate of new neurologic deficits postoperatively after placement of the cage in the posterior part of the disc space may become significant. Park et al.[37] suggested that placing the cage within the anterior third of the disk space is better for restoring the segmental angle without compromising the extent of indirect neural decompression achieved with LLIF if the height of the cage is large enough. Jin et al[38] found that cages are located mostly in the middle third of the vertebral body, significantly increasing the posterior disk space height and foraminal height compared with those in the anterior cage position. However, another study[39] proposes that increases in disk height and foraminal and canal areas are not dependent on cage positioning within the disk space; as intraoperative placement of a cage in the central portion of the disk is an easier and safer technique, central placement may be preferable in a clinical setting.
According to the clinical experience of our team, SA OLIF is preferred during the first-stage operation, and the degree of improvement in the patients' symptoms and imaging findings observed postoperatively are used to determine whether to perform the two-stage operation combined with posterior pedicle screw fixation. In this study, we recommend that if the images taken between the two operations shows that the cage has migrated toward the posterior part of the disc space, posterior pedicle screw fixation should be performed to reduce the risk of postoperative complications.
In this study, an osteoporotic lumbar spine model was created, and the results were informative. However, our study has some limitations. The FE models were simplified. The residual discs of the surgical segments and musculoskeletal segments were not modeled, and ligaments were replaced with springs, which differ from biological ligaments in terms structure. Furthermore, the lumbar spine varies across individuals, and the present study was based on a model of the lumbar spine for analysis, so the results have yet to be validated in a clinical study.