Immediate stability of a novel bidirectional expandable interbody device in an oblique lateral interbody fusion model:an in vitro biomechanical studyOA

Background:Oblique lateral interbody fusion(OLIF)offers indirect decompression with minimal tissue disruption but can be limited in patients with narrow surgical corridors,where conventional cages require excessive retraction and increase neurovascular risk.To address this,we developed a bidirectional expandable titanium cage(Bid-Exp)that can be inserted in a compact form and expanded in situ to achieve a large,wedge-shaped footprint.This study evaluated its immediate biomechanical performance compared with a conventional OLIF(Conv-OLIF)cage.Methods:Fifty fresh-frozen calf L3-L4 functional spinal units were randomly assigned to five groups(n=6 each):intact control,Bid-Exp alone,Conv-OLIF alone,Bid-Exp with unilateral pedicle screws(UPS),and Bid-Exp with bilateral pedicle screws(BPS).Following standardized OLIF cage implantation,specimens were tested under pure moments of 8 Nm in flexion,extension,lateral bending,and axial rotation using a multi-axis spine simulator.Range of motion(ROM)was recorded and compared among groups.Results:All instrumented constructs significantly reduced ROM compared with the intact condition(P<0.05).Bid-Exp+BPS achieved the lowest ROM in all loading modes,with significantly greater stiffness than Bid-Exp+UPS in flexion-extension and lateral bending(P<0.05)but no significant advantage in axial rotation.As stand-alone cages,Bid-Exp demonstrated lower ROM in flexion-extension than Conv-OLIF(P<0.05)but showed no significant difference in lateral bending or rotation.UPS provided sagittal and coronal stability approaching that of BPS,with comparable torsional control.Conclusion:The bidirectional expandable OLIF cage enables safe insertion through narrow corridors while providing immediate stability comparable to conventional cages.When combined with unilateral pedicle screw fixation,it may achieve stability close to bilateral fixation,potentially reducing surgical invasiveness without compromising biomechanical integrity.