Rod Attachment Induces Significant Strain in Lumbosacral Fixation

Area of Science:

• Spine biomechanics
• Orthopedic instrumentation
• Surgical technique

Background:

• The lumbosacral junction is prone to spinal construct failure.
• Rod fatigue fracture, sacral screw pull-out, and screw fatigue fracture are common issues.
• Rod attachment can induce significant pre-strain, potentially leading to instrumentation failure.

Purpose of the Study:

• To compare in vitro L5-S1 rod strain and S1 screw strain during rod-screw attachment (pre-strain) with strains during pure-moment bending (test-strain).
• To investigate the biomechanical consequences of rod contouring and attachment on spinal construct vulnerability.

Main Methods:

• Twelve cadaveric specimens underwent L2-ilium pedicle screw and rod instrumentation.
• Strain gauges measured rod and sacral screw strains during rod-to-screw tightening (pre-strains).
• Instrumented constructs were tested under multidirectional bending loads to measure test-strains.

Main Results:

• Rod attachment pre-strains were comparable to or greater than test-strains during flexion and extension.
• Rod attachment pre-strains significantly exceeded test-strains during lateral bending and axial rotation.
• Sacral screw pre-strain during final rod attachment was significantly greater than screw test-strains.

Conclusions:

• In vitro rod-screw attachment imposes significant pre-strains on spinal instrumentation.
• These pre-strains can be comparable to or greater than strains experienced during in vitro bending.
• Spine surgeons must recognize the biomechanical impact of rod contouring and attachment on construct vulnerability.