Does Lumbar Interbody Cage Size Influence Subsidence? A Biomechanical Study.

Area of Science:

• Spinal biomechanics
• Orthopedic implant research
• Interbody fusion devices

Background:

• The assumption that larger interbody cages reduce subsidence is unproven.
• Previous studies have not compared the subsidence resistance of lateral versus bullet cages.
• Quantifying the relationship between cage size and subsidence is lacking.

Purpose of the Study:

• To investigate the correlation between interbody cage size and subsidence.
• To quantify the subsidence resistance provided by larger cages.
• To compare the subsidence resistance of lateral cages versus bullet cages.

Main Methods:

• Experimental biomechanical study compressing cages between polyurethane foam blocks.
• Tested four sizes of bullet cages (TLIF) and six sizes of lateral cages (LLIF).
• Analyzed force for 5mm subsidence, axial area, and stiffness.

Main Results:

• Larger cages demonstrated significantly higher force requirements for subsidence.
• Lateral cages (LLIF) showed substantially higher force increments (136.4%–235.7%) than bullet cages (TLIF) (6.2%–14.6%).
• Lateral cages exhibited 3x greater average force and 3.6x greater average stiffness than bullet cages; strong correlation between axial area and subsidence force.

Conclusions:

• Interbody cage size is strongly correlated with the force required to induce subsidence.
• Lateral lumbar interbody fusion (LLIF) constructs are stiffer and require higher forces than transforaminal lumbar interbody fusion (TLIF) constructs.
• While longer bullet cages offer marginal increases in resistance, lateral cages provide substantially greater subsidence resistance.