OLIF is favored by spinal surgeons because it is associated with less bleeding, a shorter operative time, and faster recovery, compared with other lumbar disc fusion surgeries [32, 33]. Although OLIF can achieve excellent results and is widely used in practice, it can destabilize the spine and increase the risks of cage subsidence and fracture [8, 34]. In patients with osteoporosis, these risks may be greater because of the reduced bone mass and increased degradation of bone, compared with those characteristics in patients with normal bone mass; osteoporotic bone is more brittle. In earlier reports, patients with osteoporosis were prone to complications such as screw loosening and extraction, which increased internal fixation failure [35–37]. However, the effects of OLIF co-applied with various supplemental fixations in osteoporotic patients remain unknown. Therefore, we built FE models of osteoporotic and normal spines, then applied various supplemental fixations to determine their biomechanical responses.
To explore the effects of osteoporosis on various supplemental fixations that may be co-applied with OLIF, we established and verified a three-dimensional, nonlinear complete FE (L3-S1) model. Then, we endowed each part of the lumbar spine with osteoporotic features to establish a three-dimensional, nonlinear, osteoporotic FE model. Appropriate modifications were made at the L4-L5 level; we simulated five OLIF models using different fixed instruments.
After a patient has undergone OLIF surgery, the stability of the surgical segment is an important index of rehabilitation that greatly concerns clinicians [38] because instability is usually associated with various complications such as intervertebral interstitial inflammation, cage subsidence, reduced intervertebral disc height, and vertebral body non-fusion. Lu et al. [4] used an FE model to explore the biomechanical properties of four LIF surgeries (PLIF, TLIF, XLIF, and OLIF). The ROMs of the surgical segments were reduced. Chen et al. [39] established single-segment lateral interbody fusion surgical models; they found that, compared with the complete model, the ROMs decreased by 76.84–97.97%. Oxland et al. [40] reviewed the biomechanical characteristics of LIF surgeries. The maximum ROM reduction at the index level was 90%. We found that OLIF co-applied with various supplemental fixations in osteoporotic patients significantly reduced the ROM of the L4-L5 surgical segment; thus, it afforded good surgical site stability. As shown in Figure 5, the ROMs of the five supplemental fixation instruments in different positions did not differ markedly from the ROMs of normal bone, either for the surgical segment (L4-L5) or the non-fused segment. These results indicated that osteoporosis did not greatly affect the ROM of the lumbar spine, consistent with the findings in previous studies [41]. Therefore, OLIF surgery using the same fixation modality in osteoporotic patients does not affect the vertebral body ROM to a greater extent than in patients with normal bone quality. Previous studies found that a reduced spinal ROM was usually associated with disc degeneration [42, 43]. Because the intervertebral discs of the osteoporotic spine are not markedly degenerative, it is unsurprising to find that their mobility is not significantly affected.
In the osteoporotic model, the stresses on the vertebrae and supplemental fixations changed greatly, compared with stresses in the normal model). Under axial compression, the displacement of osteoporotic vertebrae is greater than the displacement of normal vertebrae; however, considering the lower strength of osteoporotic vertebrae, as well as the small displacements of the screw and rod, the load is transferred to the screw and rod. Under flexion, osteoporotic vertebrae are less stiff than normal vertebrae and exhibit deformation; screw displacement is reduced and more torque is generated between the vertebrae and the screw, imparting high-level stress to the screw. Moreover, the increased vertebral displacement enhances screw and rod displacement; the rods bend more and thus experience higher stresses. Similarly, under extension, the osteoporotic vertebral body is softer and more deformed than the normal vertebral body; a small screw displacement may increase the distance between the vertebral body and the screw, concentrating stress on the screw. Considering the softer osteoporotic vertebral body, compared with normal bone, greater stresses are imparted to the supplemental fixation instruments; this is consistent with the above findings for the rod.
Notably, during standalone OLIF, only the average stress on the vertebrae was analyzed; no supplemental fixation instrument was placed. Compared with normal lumbar spine surgery, standalone OLIF produced less average stress, similar to the results in other fixation systems. This is presumably because osteoporotic vertebrae are softer than normal vertebrae; thus, they impart less stress. The stress levels on the vertebrae and the supplemental fixations in the osteoporotic model are reduced and increased, respectively, in the various postures, compared with those values in the normal bone model; these findings are consistent with the results of previous studies [44]. Therefore, the biomechanical properties of the osteoporotic model are less robust than the properties of the normal model when the same supplemental fixation method is employed.
We found that osteoporosis affected OLIF to various extents, depending on the chosen supplemental fixation system. Fixation affords strong support and can prevent fractures. In the osteoporotic model, stresses on the proximal-junction vertebrae are reduced. However, the stress on a proximal fixation system increases; stress becomes concentrated on the contact interface between the cone and the screw, which is associated with an increased risk of internal fixation failure. Osteoporosis reduces the vertebral elastic modulus and tensile strength. Such changes may increase the relative displacement between the vertebrae and the fixation system in the same radial direction; this may cause the internal fixation device to loosen or rupture. A high pressure at the contact interface between the vertebrae and the screw can trigger bone destruction, such as a fracture. The fixation system chosen and the vertebral strength should be considered when performing OLIF in osteoporotic patients. Firm fixation and a strong vertebral body are essential for the long-term maintenance of fractures repaired after exposure to high stress; they are also essential for reducing the risk of internal fixation failure.
The structure of the lumbar spine is complex; any FE model will feature some limitations and undesirable simplifications. First, our FE model of the lumbar spine was based on geometrical information from one person. The osteoporotic FE model was constructed by ignoring individual differences and reducing the elastic moduli of the endplate, as well as the cortical, cancellous, and posterior elements, by specific proportions. Second, although FE analysis affords many advantages for assessment of biomechanics, compared with in vitro experiments, the inability to reconstruct muscles is a common problem experienced by all current lumbar FE models. Furthermore, FE methods do not closely simulate the true geometry of ligaments; these are simplified to one-dimensional non-linear springs. Finally, the results of FE analysis reflect only the post-surgical condition, rather than the long-term postoperative status. Despite these limitations, the response parameters of our lumbar spine FE model are consistent with published in vitro experimental data concerning spinal biomechanics. Therefore, clinicians may find our results useful.