Influence of Screw Length and Bone Quality on Pedicle Screw Anchorage Under Cyclic Fatigue Loading

Study DesignCadaveric biomechanical analysis.ObjectivesThe specific vertebral region that contributes most critically to pedicle screw fixation remains controversial. This study evaluates screw anchorage under cyclic fatigue loading to determine how bone quality affects the relative contributions of the pedicle and vertebral body to stability.MethodsTwelve human lumbar vertebrae (L1-L4) from donors aged 71.3 ± 14.2 years were selected, including six with normal bone density (>120 mg/cm³) and six osteoporotic (<80 mg/cm³). Each vertebra was bilaterally instrumented with the largest self-tapping polyaxial screws fitting the inner cortical diameter. One pedicle received a 35 mm screw, and the contralateral pedicle received the longest screw without cortical breach. Screw side and specimen order were randomized. Sinusoidal cranial-caudal loading at 0.5 Hz was applied, starting at 100 N and increasing by 1 N per cycle, until 5.4 mm screw head displacement (∼20°) was reached.ResultsIn healthy vertebrae, mean fatigue loads were similar for short (315.6 ± 148.7 N) and long screws (309.0 ± 138.3 N). In osteoporotic bone, long screws (230.9 ± 55.0 N) showed significantly greater fatigue resistance than short screws (175.1 ± 45.5 N; P = .045). Short screws in osteoporotic vertebrae also failed after fewer cycles than long screws in osteoporotic vertebrae (P = .047) and short screws in healthy bone vertebrae (P = .049).ConclusionsIn healthy vertebrae, screw anchorage is mainly pedicular, and extending screw length into the vertebral body adds no benefit. In osteoporotic bone, overall fixation strength declines, but vertebral body contribution increases, so longer screws provide significantly greater stability, whereas shorter screws remain adequate in healthy bone.