Effects of rod reduction on pedicle screw fixation strength in the setting of Ponte osteotomies

Background context The use of a rod reduction device can have deleterious consequences on pedicle screw pullout strength (POS) in the thoracic spine. However, posterior-only osteotomies in the thoracic spine are often performed to improve flexibility of the spine and offset forces of deformity correction maneuvers.

Purpose To investigate the effect on pedicle screw POS caused by the rod reduction technique in the presence of facet osteotomies in the thoracic spine.

Study design/setting The study is a biomechanical study using human cadaveric spine specimens.

Methods Thoracic Ponte osteotomies were performed on 3 thoracic levels in 15 cadaveric specimens. The right rod was contoured with a 5-mm residual gap at the middle level and was reduced using a rod reduction device. On the left side (paired control), a rod with no mismatch was placed. Biomechanical testing was performed with tensile load to failure "in line" with the screw axis and POS measured in Newtons (N).

Results After rod reduction, thoracic pedicle screw POS was significantly decreased (40%) compared with the control (419±426 N vs. 708±462 N, p=.002) and remained statistically significant after adjusting for bone mineral density (BMD) (p=.05). Eleven (73%) of the pedicle screws had visible pullout/failure during the reduction attempt and occurred irrespective of BMD.

Conclusions Despite thoracic Ponte osteotomies and increased flexibility of the spinal segments, the rod reduction device still significantly decreased pedicle screw POS, typically resulting in outright failure of the screw-bone interface. Therefore, rod reduction technique of any kind should be performed with caution as it frequently results in suboptimal pedicle screw fixation.