OLIF surgery has become popular recent years. Stand-alone procedure offer patients many benefits：small incision and scar, less blood loss, less pain, less hospitalization time, faster recovery [1–5]. Nevertheless, the complications fluctuate from 3.7%to 66.7% [1–5,13–14]. Shun-wu Fan reviewed 235 patients with OLIF surgery and found 22 cases of endplate damage. The cage sedimentation incidence in the stand-alone group was higher than in the OLIF combined with posterior pedicle screw fixation . Avoiding such complications could be a major factor in deciding to use this procedure. The mechanics of endplate fracture was unclear. Whether OLIF surgery with BPSF could provide enough stability and reduce the complication was still unknown.
In this study, the OLIF model was developed using published biomechanical assessment methods. A validated lumbar FE lumbar model enabled the accuracy and reliability of the simulation results. In validation, ROMs were compared with those in the literature [10–12]. The results were in good agreement with the pre-studies. The FE model was validated successfully, and it was considered reliable for lumbar biomechanical predictions.
Based on the validated lumbar model, OLIF models including Stand-alone, BPSF at the level of FSU (L4-L5) developed. The simulation showed that both BPSF could reduce ROM of the lumbar significantly. However, OLIF with SA could not reduce the extension and axial rotation motion effetely.
The maximum stresses of L4 IEP were 49.7 MPa in extension movement, the maximum stresses of L5 SEP were 47.7 MPa in flexion movement. While the yield stress of lamellar bone was 60 MPa , and the yield stress of bone in the osteoporosis patients was less than 60 MPa. This suggested the maximum stresses of endplate in flexion and extension were close to lamellar bone’s yield point in osteoporosis patients after a stand-alone OLIF procedure, which may result in endplate fracture and cage subsidence. L4 IEP of BPSF model had 77.2% lower stress than SA model in extension moment and L5 SEP of BPSF model had 39.0% lower stress than SA model in flexion moment.
This indicated the OLIF with BPSF was safer than OLIF with SA in cage subsidence. Lumbar intervertebral fusion with BPSF are the standard for instrumentation, providing rigid fixation and increased fusion rates.
In all, the FEA revealed SA could not provide enough rigidity in OLIF surgery in osteoporosis patients. The maximum stresses of L4 IEP and L5 SED increased largely in SA model in flexion and extension moment, which may be a key risk factor of cage subsidence. Therefore, the OLIF surgery with SA is not favored for osteoporotic spine.
From the study, we also found additional BPSF could share the stresses of endplate, restrict the flexion and extension of lumbar, which may be an effective method to reduce the complication of cage subsidence. The Clinical study had proven that BPSF can decrease the ratio of cage displacement . In conclusion, additional BPSF was essential for OLIF surgery in osteoporosis patients.
The post-operative residual annular fibrous were not constructed in the stand-alone OLIF model. The risk factors of endplate fracture may be multiple, including endplate damage, obesity, high iliac crest, poor stability of lesion segments and so on .