Contribution of Epiphysial Rim Contact in Anterior Lumbar Interbody Fusion Strength: A Pilot Study on Computer Tomography Generated Bone Surrogates

Introduction
Anterior lumbar interbody fusions (ALIFs) have increased over the past 2 decades by over 24% annually. Like all interbodies, cage subsidence is a challenge that must be overcome, with reported incidences among ALIF procedures ranging from 6-23.1%. Risk factors for ALIF cage subsidence have been previously retrospectively studied in a clinical setting but only limited insights have been obtained within a biomechanical laboratory setting.
Aims/Objectives
The objective of this study was to quantify ALIF construct strength from positioning of the implant along the epiphyseal rim. Furthermore, we sought to identify if a relationship existed between the dimension of the rim and the subsidence load. We hypothesized that cage positioning along the epiphyseal rim results in increased construct strength and that the subsidence load can be estimated through the measurement of the rim dimensions.
Methods
Following Institutional Reviewer Board Approval, L3 and L4 vertebrae were reconstructed from CT scans of cadaveric specimens using 3D Slicer (www.slicer.org). The surrogates were machined in 40 PCF density for the peripheral shell to represent cortical bone of the epiphyseal ring, and an inner 15 PCF to represent the cancellous central vertebral body. A representative ALIF cage with a footprint of 36x30mm was then placed on the bone surrogates in various positions according to their contact with the epiphyseal rim, and then tested under compression from an Instron 8874 (Instron Corp, Norwood, MA). Testing was performed with 5mm/min of compression while recording force data at 100Hz (Figure 1). ALIF cages were placed along the bone surrogates with either null, partial or maximal overlap with the simulated epiphysial rim (Figure 2). Subsidence load was defined as the peak load measured within the 2mm displacement from the initial load of 5N. Linear regression was used to determine the relationship between rim dimension and subsidence load. ANOVA was used to analyze variations in strength between constructs. Paired T-test was used to identify specific differences at a significance level of 0.05.
Results
A total of 24 experiments were performed. Cage position significantly correlated to subsidence loads (p=0.02). Cages with maximal epiphyseal contact demonstrated significantly higher subsidence loads (3779N±1091, p=0.01) compared to those with partial contact (2891N±608, p=0.08). Cages with maximal rim contact demonstrated subsidence loads that were proportional with average anterior rim thickness (R^2=0.950).
Conclusions
Deliberate placement of ALIF cages along the anterior epiphyseal rim decreases subsidence in this biomechanical study. Additionally, an inverse relationship exists between dimension of the epiphyseal rim and cage subsidence. Surgeons may consider a more anterior placement of their ALIF implants to optimize outcomes in their clinical practice.