The effect of polymethylmethacrylate augmentation on the primary stability of stand-alone implant construct versus posterior stabilization in oblique lumbar interbody fusion with osteoporotic bone quality- a finite element study.

Background Context: Oblique lumbar interbody fusion (OLIF) can provide an ideal minimally invasive solution for achieving spinal fusion in an older, more frail population where decreased bone quality can be a limiting factor. Stabilization can be achieved with bilateral pedicle screws (BPS), which require additional incisions and longer operative time. Alternatively, a novel self-anchoring stand-alone lateral plate system (SSA) can be used, where no additional incisions are required. Based on the relevant literature, BPS constructs provide greater primary biomechanical stability compared to lateral plate constructs, including SSA. This difference is further increased by osteoporosis. Screw augmentation in spinal fusion surgeries is commonly used; however, in the case of OLIF, it is a fairly new concept, lacking a consensus-based guideline.

Purpose: This comparative finite element (FE) study aimed to investigate the effect of PMMA screw augmentation on the primary stability of a stand-alone implant construct versus posterior stabilization in OLIF with osteoporotic bone quality.

Study Design: The biomechanical effect of screw augmentation was studied inside an in-silico environment using computer-aided FE analysis.

Methods: A previously validated and published L2-L4 FE model with normal and osteoporotic bone material properties was used. Geometries based on the OLIF implants (BPS, SSA) were created and placed inside the L3-L4 motion segment with increasing volumes (1-6 cm) of PMMA augmentation. A follower load of 400 N and 10 Nm bending moment (in the three anatomical planes) were applied to the surgical FE models with different bone material properties. The operated L3-L4 segmental range of motion (ROM), the inserted cage's maximal caudal displacements, and L4 cranial bony endplate principal stress values were measured.

Results: The nonaugmented values for the BPS construct were generally lower compared to SSA, and the difference was increased by osteoporosis. In osteoporotic bone, PMMA augmentation gradually decreased the investigated parameters and the difference between the two constructs as well. Between 3 cm and 4 cm of injected PMMA volume per screw, the difference between augmented SSA and standard BPS became comparable.

Conclusions: Based on this study, augmentation can enhance the primary stability of the constructs and decrease the difference between them. Considering leakage as a possible complication, between 3 cm and 4 cm of injected PMMA per screw can be an adequate amount for SSA augmentation. However, further in silico, and possibly in vitro and clinical testing is required to thoroughly understand the investigated biomechanical aspects.

Clinical Significance: This study sheds light on the possible biomechanical advantage offered by augmented OLIF implants and provides a theoretical augmentation amount for the SSA construct. Based on the findings, the concept of an SSA device with PMMA augmentation capability is desirable.