P7. Effect of cage position and insertion angle in the oblique lumbar interbody fusion: A finite element study

BACKGROUND CONTEXT Oblique lateral interbody fusion (OLIF) is considered a useful surgical option for various lumbar degenerative diseases with favorable clinical results and few complications. However, retroperitoneal prepsoas approach compels cage to be inserted more obliquely compared to direct lateral transpsoas approach, where cage is inserted more vertically. Moreover, the effect of cage insertion angle on postoperative cage stability and subsidence has not been fully understood yet. PURPOSE The purpose of the present study was to evaluate the biomechanical effect of cage position and insertion angle on postoperative stability and subsidence following oblique lateral interbody fusion using finite element (FE) model. STUDY DESIGN/SETTING Finite element study. PATIENT SAMPLE Previously validated 3-dimensional FE model of intact lumbar spine (L4-5) was used. OUTCOME MEASURES The cage and pedicle screw – vertebral body interface behavior were assigned ‘tie’ contact condition to simulate complete postoperative bone union. In addition, the likelihood of yield of cage and the risk of the subsidence were compared and analyzed using Peak von Mises Stresses (PVMS) values occurring in each type of cage and cancellous bone, respectively. METHODS For the vertebral fusion procedures, PEEK (E=3500MPa, v=0.3) cage (L40*W22*H12*A12, GS Medical Co., Ltd., Korea) with titanium (E=110000MPa, v=0.33) pedicle screws (Ø6.5 x 45 mm, GS Medical Co., Ltd., Korea) were used. Six surgical models were created according to cage positioning and insertion angles (Type A: anterior 1/3, Type B: center, Type C: posterior 1/3, Type D: anterior 25°, Type E: Center 25°, Type F: Posterior 25°) 10Nm pure moment of flexion/extension, lateral bending and axial rotation was applied at the superior surface of L4 endplate with a compressive follower load of 400N. RESULTS All six models showed lower stress values as compared to the yield strength of PEEK (75Mpa). The 0° models demonstrated (Types A, B and C) significantly lower likelihood of cage yield compared to 25° models (Type D, E and F; 33%, 33% and 30% vs 49%, 47% and 33%, respectively), suggesting anteriorly positioned cage with less insertion angle is more stable. Similarly, in investigating the risk of subsidence, the stresses recorded in the cancellous bone for all six models were less than the reported yield strength of the cancellous bone (16.3Mpa). The 0° models demonstrated (Types A, B and C) significantly lower risk of subsidence compared to 25° models (40%, 57% and 40% vs 68%, 54% and 67.6%, respectively), suggesting anterior or posterior positioned cage with less insertion angle lowers the risk of subsidence. CONCLUSIONS Our study demonstrated that during OLIF, insertion of cage more anteriorly with less insertion angle provided better stability and less chance of subsidence. This study suggests that cage should be inserted more anteriorly and vertically during OLIF procedure to maintain postoperative stability and to reduce cage subsidence. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs. Oblique lateral interbody fusion (OLIF) is considered a useful surgical option for various lumbar degenerative diseases with favorable clinical results and few complications. However, retroperitoneal prepsoas approach compels cage to be inserted more obliquely compared to direct lateral transpsoas approach, where cage is inserted more vertically. Moreover, the effect of cage insertion angle on postoperative cage stability and subsidence has not been fully understood yet. The purpose of the present study was to evaluate the biomechanical effect of cage position and insertion angle on postoperative stability and subsidence following oblique lateral interbody fusion using finite element (FE) model. Finite element study. Previously validated 3-dimensional FE model of intact lumbar spine (L4-5) was used. The cage and pedicle screw – vertebral body interface behavior were assigned ‘tie’ contact condition to simulate complete postoperative bone union. In addition, the likelihood of yield of cage and the risk of the subsidence were compared and analyzed using Peak von Mises Stresses (PVMS) values occurring in each type of cage and cancellous bone, respectively. For the vertebral fusion procedures, PEEK (E=3500MPa, v=0.3) cage (L40*W22*H12*A12, GS Medical Co., Ltd., Korea) with titanium (E=110000MPa, v=0.33) pedicle screws (Ø6.5 x 45 mm, GS Medical Co., Ltd., Korea) were used. Six surgical models were created according to cage positioning and insertion angles (Type A: anterior 1/3, Type B: center, Type C: posterior 1/3, Type D: anterior 25°, Type E: Center 25°, Type F: Posterior 25°) 10Nm pure moment of flexion/extension, lateral bending and axial rotation was applied at the superior surface of L4 endplate with a compressive follower load of 400N. All six models showed lower stress values as compared to the yield strength of PEEK (75Mpa). The 0° models demonstrated (Types A, B and C) significantly lower likelihood of cage yield compared to 25° models (Type D, E and F; 33%, 33% and 30% vs 49%, 47% and 33%, respectively), suggesting anteriorly positioned cage with less insertion angle is more stable. Similarly, in investigating the risk of subsidence, the stresses recorded in the cancellous bone for all six models were less than the reported yield strength of the cancellous bone (16.3Mpa). The 0° models demonstrated (Types A, B and C) significantly lower risk of subsidence compared to 25° models (40%, 57% and 40% vs 68%, 54% and 67.6%, respectively), suggesting anterior or posterior positioned cage with less insertion angle lowers the risk of subsidence. Our study demonstrated that during OLIF, insertion of cage more anteriorly with less insertion angle provided better stability and less chance of subsidence. This study suggests that cage should be inserted more anteriorly and vertically during OLIF procedure to maintain postoperative stability and to reduce cage subsidence.