Does Cage Position Affect the Risk of Cage Subsidence After Oblique Lumbar Interbody Fusion in the Osteoporotic Lumbar Spine: A Finite Element Analysis

OBJECTIVE: This study aimed to evaluate the biomechanical effects of different cage positions with standalone (SA) methods and bilateral pedicle screw fixation (BPSF) in the osteoporotic lumbar spine after oblique lumbar interbody fusion (OLIF). METHODS: A finite element model of an intact L3-L5 lumbar spine was constructed. After validation, an osteoporosis model (OP) was constructed by assigning osteoporotic material properties. SA models (SA1, SA2, SA3) and BPSF models (BPSF1, BPSF2, BPSF3) in which a cage was placed in the anterior, middle, and posterior third of the L5 superior endplate (SEP) were constructed at the L4-L5 segment of the OP. The L4-L5 range of motion (ROM), the stress of the L5 SEP, the stress of the cage, and the stress of fixation were compared among the different models. RESULTS: According to the degree of ROM of L4-L5, the stress of the L5 SEP and the stress of the cage for most physiological motions, the SA and BPSF models were ranked as follows: SA2 < SA1 < SA3, BPSF2 < BPSF1 < BPSF3. In BPSF2, the stress of fixation was minimal in most motions. At the same cage position, the ROM of L4-L5, the stress of the L5 SEP, and the stress of the cage in the BPSF models were significantly reduced compared with those in SA models; compared with SA2, BPSF2 had a maximum reduction of 83.24%, 70.71%, and 73.52% in these parameters, respectively. CONCLUSIONS: Placing the cage in the middle third of the L5 SEP for OLIF could reduce the maximum stresses of the the L5 SEP, the cage, and the fixation, which may reduce the risk of postoperative cage subsidence, endplate collapse, and fixation fracture in the osteoporotic lumbar spine. Compared with SA OLIF, BPSF could provide sufficient stability for the surgical segment and may reduce the incidence of the aforementioned complications.