Biomechanical assessment of a novel L4/5 level interspinous implant using three dimensional finite element analysis

Range of motion (ROM) is often restricted by conventional spinal fusion surgery, while some complications also occurred after applying posterior dynamic devices in clinic. Therefore, new surgical implant options were necessitated. The biomechanical features of a novel interspinous implant were investigated using three dimensional (3D) finite element models (FEMs).

An "H-shaped" polyether ether ketone (PEEK) interspinous implant was designed to tightly fit the upper and lower spinous processes, featuring a hollow cylindrical portion which was implanted autologous bones to enhance fusion with spinous processes. A 3D FEM of the intact L3/S segment with mild disc degeneration in L4/5 (degenerated model) was developed and subjected to flexion-extension, lateral bending, and axial rotation either with or without the implanted prosthesis (implant model) in order to examine effects on ROM, intradiscal stress, and facet joint load.

The whole lumbar ROM was altered slightly by implant insertion, and reduced end plate stress, nucleus stress, and facet joints load at the L4/5 level (implant location) were observed. L4/5 flexion-extension maximal end plate stress, nucleus stress, and facet joints load were 5.262 MPa, 0.1648 MPa, and 29.7 N, respectively, in the degenerated model and 2.323 MPa, 0.0892 MPa, and 5.4 N, respectively, in the implant model. End plate and nucleus stresses were partially alleviated at the L3/4 level. Slightly higher maximal von Mises stress in L3/4 and L5/S annuli were observed in the implant model.

The proposed novel interspinous implant effectively restored stability without producing excessive ROM limitations, meriting further clinical evaluation. Furthermore, these findings provide a useful basis for wide application of FEM in a broad variety of spinal implant assessments.