This is an in vitro biomechanical study.
The aim of this study was to investigate the biomechanical variations of lumbar spine motor units after screw heads blocking superior-adjacent bilateral facets in cadaver specimens and to analyze the biomechanics under superior-segment different moments.
Facet joint violation by pedicle screws is not a rare adverse event in instrumented lumbar fusion surgery, and one of the most common types is the screw head blocking the superior-adjacent facet. However, its contribution to biomechanical instability at the supradjacent level is unknown.
Biomechanical testing was performed on 12 human cadaveric spines under flexion-extension, lateral bending, and axial rotation loading. After intact analysis, pedicle screws were inserted at L5, and the biomechanical testing was repeated. Full range of motion (ROM) at the proximal adjacent levels under different moments were recorded and normalized to intact (100%). The relative ROM changes were compared between the blocking and control groups.
In the blocking group, the supradjacent-level flexion-extension ROM significantly decreased under all moments (7.5, 6.0, 4.5 Nm) relative to the intact noninstrumented spine and a significant decrease in the lateral bending relative ROM was found at 4.5 Nm. In the control group, the supradjacent-level ROM did not change significantly relative to the intact noninstrumented spine at each moment. When performing flexion-extension, the relative ROM change between the two groups was significantly different at 4.5 Nm. When performing lateral bending, the relative ROM change between the two groups was significantly different at moments of 6.0 and 4.5 Nm.
When screw heads blocked superior-adjacent bilateral facets, the supradjacent-level flexion-extension ROM and lateral bending ROM decreased. From a long-term perspective, this may be a risk of persistent low-back pain due to frequent impingement.