INTRODUCTION: Fusion is often required when the spine is found to be or is rendered unstable, but the number of screws needed for a given construct has not been quantified. With each pedicle screw placed there is added operative time, surgical risk, and cost. This study evaluated the mechanical stiffness of a long lumbosacral construct, varyinging the number of pedicle screws. Our hypothesis was that the number of screws could be reduced from maximum without losing fixation strength.
METHODS: Six human cadaver spines were tested. Nondestructive testing was performed in extension, flexion, lateral bending and torsion. Specimens were instrumented with a titanium pedicle screw fixation system from L2 through S1. Construct A tested 5.5 mm rods fixed from L2 through S1 bilaterally. The left L3 and right L5 screws were then removed (Construct B). Next, both L3 and L5 screws were removed (Construct C). L4 pedicle screws were then removed, leaving L2 and S1 screws alone (Construct D). Finally, all screws were reinserted (Construct E) for test‐re‐test validation.
RESULTS: Omission of screws affected torsion and extension most significantly. Removing two screws (construct B) decreased stiffness in torsion by 20%, while construct C decreased it by 40%, and construct D by 60%. The results for extension were comparable: Construct B showed a 10% decrease, construct C a 35% decrease, and construct D a 60% decrease on stiffness. In flexion, non‐segmental fixation (construct D) decreased the stiffness by only 25%. Test ‐retest confirmed consruct integrity.
DISCUSSION: The most rigid construct includes bilateral screws, at every level. Our data suggest that omission of two intercalary screws had no adverse effect on construct stiffness in extension and torsion, and that omission of four screws did not effect flexion stiffness. The surgeon may reasonably reduce operative time, risk, and cost by omitting selected screws from a long lumbar PS construct.