Use “Baby Bear” Pedicle Screws in Spinal Deformity Surgery

A nicely done propensity-matched study determines the "just right" length for pedicle screws in adult spinal deformity procedures

What’s the Claim?

A retrospective, comparative study using advanced techniques (propensity scoring) to create well-matched groups discovered three things about pedicle screw lengths in adult spinal deformity surgery:

  • Short screws are bad: Screws with an “occupancy rate” of ≤80% in the uppermost instrumented vertebral body were much more likely to have a fracture of that vertebrum than those whose occupancy rate was >80% (“occupancy rate” was defined as the ratio of the pedicle screw’s length to that of the anteroposterior dimension of the vertebral body).
  • Long screws are bad: Bicortical screws (those that perforated the anterior cortex) in the upper instrumented vertebral body also were much more likely to fracture.
  • “Baby bear” screws (those that are “just right”) — our term, not the authors’ — are good: These can be defined as screws with an occupancy rate of >80% that don’t exit the vertebrum anteriorly.

How’s It Stack Up?

We liked this study because it represents the successful clinical application of an earlier provocative but somewhat speculative laboratory project — the same author group earlier used finite-element analysis to show that stress on the uppermost instrumented vertebrum decreased the most when the occupancy rate was ≥82%. Fractures of the upper instrumented vertebrum are not uncommon — one prior study suggested they occurred in nearly 20% of patients who have these operations — and about 10% of those develop spinal cord injuries. Prior studies have not done an adequate job of addressing confounding variables — for example, patients who develop these fractures tend to be older — and so we liked this study for the substantial efforts the authors took to mitigate the issue of confounding.

What’s Our Take?

There's a lot here, so we turn to CORRelations’ advisor in Spine, Charlie Reitman, MD, for insight: