Summary: While biomechanical properties are different for pediatric and adult bone, it is unknown whether changes finalize with skeletal maturity or serve as a continuum over time.
Introduction: Our objective was to clarify pedicle bone elasticity and stratify the biomechanical differences over time. We hypothesized that pedicles of increasing age would demonstrate diminished viscoelastic properties during screw placement.
Methods: Two pediatric cadaveric specimens (9 and 13 years old) were compared to three adult specimens. All specimens were instrumented bilaterally from T2‐L5. A 2‐mm gearshift, 4.75 mm tap, and screws were sequentially placed in 0.75 mm increments (5.5 ‐ 8.5 mm). This continued until: (1) we reached the largest screw of (8.5 mm), or (2) there was medial or lateral cutout. We used digital calipers/measuring devices to record the height, width, and circumference of the pedicle. We recorded measurements before instrumentation, and before/after every tap and screw placement. Pre/Post‐instrumentation radiographs and post‐instrumentation axial cuts of the vertebral bodies confirmed cortical integrity. Cutouts were determined visually.
Results: The results are summarized in table 1. Circumferential pedicle expansion occurred in all specimens in the thoracic spine. In all age groups, the most marked changes occurred in pedicle width, especially in the thoracic spine (p<0.05). Pediatric pedicles expanded circumferentially more than their adult counterparts (p<0.05). Most pedicles in all age groups were able to accept a screw much larger than their endosteal diameter (p<0.05) based upon CT measurements.
Conclusion: Our study confirms that the pediatric pedicle undergoes plastic accommodation followed by circumferential expansion in response to pedicle screw placement. Similarly, adult specimens demonstrate viscoelastic circumferential expansion in the thoracic spine, and to a less predictable degree in the thoracolumbar junction and lumbar regions. Most pedicles in all age groups were able to accept screws much larger than their endosteal diameter. These findings may support the use larger screws for instrumentation of the pediatric and adult spines.
Significance: To our knowledge, this is the first study to confirm viscoelastic accommodation and circumferential pedicle expansion in children and adults after the placement of pedicle screws.