Thoracic Pedicle Expansion After Pedicle Screw Placement In A Pediatric Cadaveric Spine: A Biomechanical Analysis

• b – DePuy

Background: Significant expansion of the pediatric pedicle during pedicle screw insertion has been postulated by a few authors, but not biomechanically tested to our knowledge. Studies of adult cadavers demonstrated a mild increase in pedicle dimensions prior to fracture, but to a much lesser degree than radiographic assessments of the pediatric pedicle have suggested.

Objective: To demonstrate viscoelastic expansion of the pedicles of a pediatric spine during placement of pedicle screws.

Methods: An intact, fresh-frozen cadaveric spine of a scoliotic 9-year-old male was studied between T2 and L5. After CT and radiographic imaging of the spine, the spine was disarticulated at each level and all soft tissue was carefully removed. The height, width, and circumference of each pedicle were recorded with a precision caliper at each reference point. Measurements were performed at the following stages: prior to pedicle manipulation, after placement and removal of a 2-mm pedicle finder, and after placement and removal of sequential pedicle screws (beginning at 4.75 mm, enlarging in 0.75 mm increments to a maximum of 8.5 mm). The moment of failure of the pedicle and periosteum were recorded.

Results: The average external dimensions of the pedicles were: 8.47 mm height, 5.92 mm width, and 30.1 mm circumference prior to manipulation. The pedicle diameter expanded to accept a screw with an average diameter 111% of the original external transverse diameter (range: 73% to 143%) of the pedicle. The thoracic diameter expanded 110% and the lumbar levels expanded 113%. When compared to the internal diameter of each pedicle (based upon CT scan measurements at the isthmus), the pedicle expanded an average of 198% (range: 141% to 265%) without failure. The thoracic levels expanded 196% and the lumbar levels expanded 205%. The overall circumference of the thoracic pedicle increased an average of 6.3% to accommodate the screw. The earliest pedicle failure was at the left pedicle of T4 with a 5.5 mm screw. 83% of the pedicles accepted a 6.25mm screw, 63% accepted a 7.0 mm screw, 30% accepted a 7.75 mm screw, and 17% accepted an 8.5 mm pedicle screw before failure. There were no failures of the medial pedicle wall.

Relationship Between Findings and Existing Knowledge: Prior studies suggested pediatric pedicle screw expansion, but no biomechanical studies have confirmed the viscoelasticity of the pediatric pedicle. This relatively mild change in the overall circumference in the face of a dramatic change in the transverse diameter can be explained by viscoelastic accommodation and expansion. During accommodation, the cross section of the pedicle undergoes plastic deformation and changes from a vertically-oriented oblong shape to a more rounded shape prior to expansion.

Conclusion: Our data confirm the viscoelastic properties of the pediatric pedicle are very significant, allowing almost a 200% increase in internal diameter prior to failure. A single pedicle was unable to accept a 5.5mm screw prior to failure suggesting significantly smaller screw sizes may not be necessary in the pediatric population.

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