Abstract: BACKGROUND: At present, whether the oblique lateral interbody fusion requires internal fixation is still controversial, and whether the osteoporosis will affect the choice of the type of internal fixation during the application of oblique lateral interbody fusion is also lack of biomechanical research support.  
OBJECTIVE: To study the biomechanical effects of osteoporosis and different types of posterior pedicle screw fixation on the lumbar spine during oblique lateral interbody fusion by three-dimensional finite element method.
METHODS:  CT scan data of L3-L5 segments of healthy people were obtained. The normal intact three-dimensional finite element model (M0) and osteoporosis model (N0) of L3-L5 were established by using Mimics, Geomagic, Solidworks and ANSYS software. M0 and N0 were used as the basis at L4-L5 to establish the normal oblique lateral interbody fusion stand-alone model (M1), the normal oblique lateral interbody fusion with unilateral pedicle screw fixation model (M2), the normal oblique lateral interbody fusion with bilateral pedicle screw fixation model (M3), the osteoporotic oblique lateral interbody fusion stand-alone model (N1), the osteoporotic oblique lateral interbody fusion with unilateral pedicle screw fixation model (N2) and the osteoporotic oblique lateral interbody fusion with bilateral pedicle screw fixation model (N3). A vertical load of 500 N and a moment of 7.5 N•m were applied to the L3 upper surface to simulate the physiological activities of lumbar spine in flexion, extension, right/left lateral bending and right/left axial rotation. The L4-L5 segment range of motion, stress distribution of L5 superior endplate, cage, and internal fixation under different working conditions was observed. The maximum Von Mises stresses were recorded.  
RESULTS AND CONCLUSION: (1) Whether in the normal models or the osteoporotic models, with the increase of posterior internal fixation, the L4-L5 range of motion, the L5 superior endplate stress, and the cage stress generally showed a downward trend. (2) The internal fixation stress of M2 increased by 115.26% mostly compared with that of M3. Compared with that of N3, the internal fixation stress of N2 increased by 78.87% mostly. (3) At the same type of internal fixation, the L4-L5 range of motion, L5 superior endplate stress, and cage stress of the osteoporotic models were all higher than those of the normal models. (4) Compared with that of M2, the internal fixation stress of N2 increased by 39.59% mostly. The internal fixation stress of N3 increased by 43.30% compared with that of M3. (5) It is indicated that the lumbar spine with normal bone mineral density and without significant alignment imbalance should be given priority for oblique lateral interbody fusion stand-alone or unilateral pedicle screw fixation. If there is significant alignment imbalance in the lumbar spine, bilateral pedicle screw fixation should be combined. The lumbar spine with osteoporosis should be combined with bilateral pedicle screw fixation to help reduce the incidence of cage settlement, endplate collapse and improve surgical safety.

Key words:  lumbar vertebrae, oblique lateral interbody fusion, osteoporosis, internal fixation, finite element analysis