Load distribution on intervertebral cages with and without posterior instrumentation

Area of Science:

• Spine biomechanics
• Orthopedic surgery

Background:

• Posterior and transforaminal lumbar interbody fusion (PLIF, TLIF) are common spinal fusion procedures.
• Load sharing dynamics between cages, posterior constructs, and biological tissues in instrumented spines are not well understood.

Purpose of the Study:

• To quantify forces on interbody cages under axial compression.
• To compare load distribution with and without posterior instrumentation in PLIF and TLIF procedures.

Main Methods:

• Biomechanical cadaveric study on ten lumbar spinal segments.
• Uniaxial compression testing with load cell-instrumented cages (100N increments to 1000N).
• Testing included unilateral PLIF (uPLIF), bilateral PLIF (bPLIF), and TLIF, with and without posterior instrumentation.

Main Results:

• Median cage preloads ranged from 16N (uPLIF) to 35N (TLIF).
• Posterior instrumentation increased median preloads significantly (224N-328N).
• Without posterior instrumentation, cages absorbed >50% of axial load; with instrumentation, <50% (load-dependent).

Conclusions:

• Interbody cages absorb substantial axial load (>50%) without posterior instrumentation.
• Posterior instrumentation significantly reduces cage load and makes distribution load-amplitude dependent.
• Bilateral PLIF cages absorbed more force than uPLIF or TLIF cages.