Maximizing mechanical advantage: surgical technique increases stiffness in spinal instrumentation.

Area of Science:

• Orthopedics
• Spine Surgery
• Biomechanical Engineering

Background:

• Adolescent idiopathic scoliosis treatment has advanced, yet sagittal deformity correction remains challenging.
• Increased rod stiffness is known to reduce thoracic flattening in scoliosis correction.
• The influence of surgical technique on rod stiffness requires further investigation.

Purpose of the Study:

• To investigate if surgical technique can increase spinal construct stiffness.
• To quantify the effect of surgical technique on rod stiffness in adolescent idiopathic scoliosis.
• To evaluate the impact of construct constraint on sagittal bending stiffness.

Main Methods:

• A mechanical study was designed to measure sagittal bending stiffness.
• Four common spinal instrumentation systems were tested with constrained over contoured rods.
• Construct stiffness was measured under varying levels of constraint, from no constraint to maximal constraint.

Main Results:

• All tested rod types demonstrated increased bending stiffness with greater construct constraint.
• Moderately and maximally constrained groups showed significantly higher stiffness than unconstrained groups (p < 0.05).
• 6.0 mm titanium circular rods exhibited the greatest stiffness increase (3.02x) between maximal and no constraint.

Conclusions:

• Surgical technique, specifically rod constraint, is a significant determinant of spinal construct stiffness.
• Locking the rod to proximal and distal anchors substantially increases sagittal bending stiffness.
• This technique enhances rod bending stiffness irrespective of rod material or shape.