Understanding the influence of cage and instrumentation strategies with oblique lumbar interbody fusion for grade I spondylolisthesis - A comprehensive biomechanical modeling study.

Abstract

Background context: Proper implant selection and placement in oblique lumbar intervertebral fusion (OLIF) are essential to achieve the best possible results for the patient. Key factors such as interbody cage length, height, angle, and material must all be carefully considered to achieve the intended results and minimize complications. Significant challenges remain in selecting the appropriate cage parameters to control spinal alignment while minimizing subsidence risk. Ongoing debates include how long a cage should be to optimize load distribution, as well as how variations in cage angle and placement influence the outcomes.

Purpose: This study aims to biomechanically model and investigate how variations in interbody cage dimensions, positioning, and material properties influence indirect decompression, realignment, and resulting stresses involved in cage subsidence.

Study design: Computational biomechanical study of interbody cage and OLIF influence on correction outcomes.

Methods: A pathological finite element model of the L4-L5 segment presenting a grade I spondylolisthesis was used to simulate 172 different OLIF configurations, evaluating cage position (anterior, central, posterior), angle (6° or 12°), material (PEEK or titanium), length (40-60 mm), and height (10-14 mm). Bilateral pedicle screw fixation was also tested. The simulated outcomes included disc height, foraminal and spinal canal dimensions, segmental lordosis, vertebral slip, endplate stresses, and displacements under various loading conditions. Statistical comparisons were tested to analyze the influence of model, implant, and surgical parameters on correction outcomes.

Results: Longer (left-to-right dimension) cages (60 mm), which overhang on both sides of the vertebrae and sit on the apophyseal ring, significantly reduced vertebral endplate displacements and stresses by 33 % compared to shorter cages (40 mm) (p < 0.05). Posterior cage positioning improved the decompression but raised stresses by 45 % and reduced segmental lordosis by 28 %. Lowering cage height from 14 to 10 mm and increasing the angle from 6° to 12° reduced endplate stresses by 53 % and 33 %, respectively. BPS fixation decreased stresses by 36 % on average. The trends observed concurred with recently published OLIF clinical studies.

Conclusions: This study highlights the biomechanical influence of implant characteristics and positioning on OLIF results and subsidence risks. Competing factors unveil an optimization problem that can be effectively addressed with the help of accurate, robust, and reproducible numerical simulations and regression models. This study further confirms that the developed tools not only accurately simulate the surgical approach and corroborate clinical findings but also offer a relevant framework for in-depth analysis.

Clinical significance: Leveraging numerical methods, this study provides biomechanical insights into how variations in cage parameters during OLIF procedures influence outcomes. The findings aim to help clinicians refine strategies to attain desired outcomes (decompression and alignment) while understanding the consequences on the risk of subsidence. By aligning with clinical trends, our results offer valuable explanations and support for biomechanical-based surgical decision-making.