Interbody device endplate engagement effects on motion segment biomechanics

Area of Science:

• Spinal fusion biomechanics
• Orthopedic device engineering
• Spinal instrumentation

Background:

• Stand-alone nonbiologic interbody fusion devices are used for lumbar spine fusion.
• Most devices lack the stability of circumferential constructs due to cage geometry and endplate interface.
• No published comparative biomechanical studies have evaluated endplate engagement effects.

Purpose of the Study:

• Compare the biomechanical effects of endplate engagement on lumbar motion segment rigidity.
• Investigate the additional stability provided by supplemental anterior and posterior fixation.

Main Methods:

• Cadaveric study using 18 human lumbar motion segments (L2-3, L4-5).
• Tested three interbody devices: polymer spacer, spiked cage, and dual threaded cages.
• Evaluated range of motion (ROM), neutral zone (NZ), stiffness, and disc height intact and with anterior/posterior fixation.

Main Results:

• Spiked and threaded cages significantly increased disc height compared to polymer spacers.
• All stand-alone devices reduced ROM in lateral bending; spiked and threaded cages reduced flexion-extension ROM and NZ.
• Only spiked cages significantly reduced axial torsion ROM compared to intact.
• Posterior fixation significantly reduced ROM in all directions; anterior plating had limited additional benefit.

Conclusions:

• Interbody cages with endplate spikes or threads enhance lumbar motion segment rigidity in bending.
• Spiked cages provide superior rigidity in torsion, addressing a limitation of other stand-alone devices.
• Anterior and posterior fixation further increase rigidity, with posterior fixation offering the greatest stability.