Biomechanical study of a novel minimally-invasive pedicle screw-plate system

English Keywords:Lumbar spine  Interbody fusion  Minimally invasive surgery  Pedicle screw  Biomechanics

English Abstract:

  【Abstract】 Objectives: To evaluate the biomechanical strength of the novel minimally-invasive pedicle screw-plate system. Methods: Single screw-rod test model was developed according to a standard for destructive mechanical test published by the American Society for Testing Materials. Minimally-invasive pedicle screw-plate assembles, monoaxial pedicle screw-rod assembles and polyaxial pedicle screw-rod assembles were tested on universal material testing machine for biomechanical study. The tests included static compression bending test and fatigue test. 6 assembles of each group were tested in static test and the yield load, the bending stiffness and modes of failure were recorded. Modes of failure and maximum run out load enduring 2.5×106 cycles were reported in fatigue test. Results: The yield load and bending stiffness of the minimally-invasive pedicle screw-plate assembles were significantly less than those of the monoaxial pedicle screw-rod assembles(834.6±74.5N vs 1087.7±59.0N, 218.1±22.4N vs 300.0±14.9N, P<0.001). The yield load of the minimally-invasive pedicle screw-plate assembles was not significantly larger than that of the polyaxial pedicle screw-rod assembles(834.6±74.5N vs 762.8±88.6N, P=0.118), while the bending stiffness was significantly less than that of the other(218.1±22.4N/mm vs 389.5±37.2N/mm, P<0.001). Failure modes in monoaxial pedicle screw-rod assembles were rod plastic deformation, while the failure modes in polyaxial pedicle screw-rod assembles were loosening of the ball-in-cup mechanism． Failure modes in minimally-invasive pedicle screw-plate assembles included the plastic deformation at the plate or the plate-screw connection part. Run out load enduring 2.5×106 cycles was 450N, 450N and 350N for minimally-invasive pedicle screw-plate assembles, monoaxial pedicle screw-rod assembles and polyaxial pedicle screw-rod assembles respectively. Conclusions: The novel minimally-invasive pedicle screw-plate system has high yield strength and low construct stiffness, which can afford 450N load within 2.5×106 cycles.