Stress Reduction in Adjacent Level Discs via Dynamic Instrumentation: A Finite Element Analysis

Area of Science:

• Spine biomechanics
• Orthopedic implant technology
• Finite element analysis

Background:

• Conventional rigid spinal fusion instrumentation may accelerate adjacent disc degeneration due to increased stress from motion discontinuity.
• Dynamic instrumentation aims to reduce this effect, but its impact on adjacent disc stress is not well understood.

Purpose of the Study:

• To compare the stresses induced in adjacent-level discs by rigid versus dynamic posterior lumbar fusion instrumentation using a finite-element model.

Main Methods:

• A 3D finite-element model of the lumbar spine simulating flexion and extension was developed.
• The L5-S1 segment was fused, and the L4-L5 segment was instrumented with either rigid or dynamic constructs.
• Peak stresses in the adjacent L4-L5 disc were calculated and compared between the two instrumentation types.

Main Results:

• Dynamic instrumentation reduced peak compressive stresses by 1-2% (reduced stiffness) and 8-9% (increased axial motion).
• The area of disc tissue exposed to high stress was 47% less with dynamic instrumentation compared to rigid instrumentation.
• Overall, dynamic instrumentation resulted in an approximate 10% cumulative stress reduction per flexion cycle.

Conclusions:

• Dynamic posterior lumbar fusion instrumentation, featuring reduced stiffness and increased axial motion, significantly lowers stress on adjacent discs.
• This stress reduction may help mitigate adjacent-level disc degeneration over time.