A biomechanical finite element analysis of posterior atlantoccipital-clivus screws in occipitocervical fixation

English Keywords:Occipitocervical fixation  Atlantoccipital transarticular-clivus screw  Internal fixation  Finite element

English Abstract:

  【Abstract】 Objectives: To study the influence on stability and stress of posterior atlantoccipital transarticular-clivus screw(TACL) in occipitocervical fixation using the finite element analysis method. Methods: Finite element models were established using the occipitocervical CT data of a volunteer, including an intact model, an unstable model, and six models of occipitocervical fixations: occipital screw(OS)+C1 lateral mass screw(C1LMS)+C2 pedicle screw(C2PS), OS+atlantoccipital transarticular screw(TA)+C2PS, OS+atlantoccipital transarticular-clivus screw(TACL)+C2PS, Occipital condyles screw(OcC)+C1LMS+C2PS, TA+C2PS, and TACL+C2PS. Material parameters were assigned to the models, and a vertical downward force of 40 N was applied to the upper surface of the occipital bone to simulate the weight of head. A torque of 1.5N·m was applied to induce flexion, lateral bending, and rotational movements. The stability, implant stress distribution, and bone stress distribution of each occipitocervical fixation model under different conditions were compared. Results: The range of motion(ROM) of the intact model under physiological conditions was similar to those reported in the literatures, and the ROMs under flexion, extension, lateral bending and rotation conditions was 19.53°, 17.37°, 10.44° and 38.74°, respectively. The ROMs of the unstable model under flexion, extension, lateral bending and rotation conditions were 26.17°, 18.90°, 10.99° and 42.30°, respectively, which were 34.00%, 8.82%, 5.34% and 9.20% higher than those of the intact model, respectively. The ROMs of the six fixation models were reduced under the same working conditions. Compared to the unstable model, TACL +C2PS cut 97.43%, 94.06%, 96.68%, and 99.01% of the ROM under the four working conditions. The peak internal fixation stresses were 241.00MPa, 241.20MPa, 166.94MPa, and 168.80MPa for TACL+C2PS, and 295.00MPa, 295.00MPa, 73.54MPa, and 81.40MPa for OS+TACL+C2PS in the same working conditions; The internal fixation stresses were concentrated in the middle of the connecting rod and the tail of the TACL, and both models applying the TACL fixation technique had less cutting stress on the bone, which had a more reasonable stress distribution than other occipital fixation techniques. Conclusions: Posterior TACL as a technique for occipital fixation can meet the mechanical strength requirements for occipitocervical fixation and has superior stability and stress distribution compared with other occipital fixation techniques applied alone, and its combined application with posterior occipital screws provides greater mechanical stability.