The Biomechanical Effect on the Adjacent L4/L5 Segment Under S1 Superior Facet Arthroplasty: A Finite Element Analysis

Abstract Background. The superior facet arthroplasty is important for intervertebral foramen microscopy. To our knowledge, no study about the postoperative biomechanics of adjacent L4/L5 segments after different methods of S1 superior facet arthroplasty. To evaluate the effect of S1 superior facet arthroplasty on lumbar range of motion and disc stress of adjacent segment (L4/L5) under the intervertebral foramina plasty.Methods. Eight finite element models (FEMs) of lumbosacral vertebrae (L4/S) had been established and validated. The S1 superior facet arthroplasty was simulated with different methods. Then, the models were imported into Nastran software after optimization. 500N preload was imposed on the L4 superior endplate and 10 Nm was given to simulate flexion, extension, lateral flexion and rotation. The range of motion (ROM) and intervertebral disc stress of L4-L5 spine were recorded.Results. The ROM and disc stress of L4/L5 increased with the increasing of the proportions of S1 superior facet arthroplasty. Compared with the normal model, the ROM of L4/L5 significantly increase in most directions of motion when S1 superior facet formed greater than 3/5 from ventral to dorsal or 2/5 from apex to base. The disc stress of L4/L5 significantly increase in most directions of motion when S1 superior facet formed greater than 3/5 from ventral to dorsal or 1/5 from apex to base.Conclusion. In this study, the ROM and disc stress of L4/L5 were effected by the unilateral S1 superior facet arthroplasty. It is suggested that the forming range from ventral to dorsal should be less than 3/5 of S1 upper facet joint. It is not recommended to form from apex to base.