Biomechanical comparison of two different concepts for stand alone anterior lumbar interbody fusion

Abstract

Segmental instability in degenerative disc disease is often treated with anterior lumbar interbody fusion (ALIF). Current techniques require an additional posterior approach to achieve sufficient stability. The test device is an implant which consists of a PEEK-body and an integrated anterior titanium plate hosting four diverging locking screws. The test device avoids posterior fixation by enhancing stability via the locking screws. The test device was compared to an already established stand alone interbody implant in a human cadaveric three-dimensional stiffness test. In the biomechanical test, the L4/5 motion segment of 16 human cadaveric lumbar spines were isolated and divided into two test groups. Tests were performed in flexion, extension, right and left lateral bending, right and left axial rotation. Each specimen was tested in native state first, then a discectomy was performed and either of the test implants was applied. Finite element analysis (FE) was also performed to investigate load and stress distribution within the implant in several loading conditions. The FE models simulated two load cases. These were flexion and extension with a moment of 5 Nm. The biomechanical testing revealed a greater stiffness in lateral bending for the SynFix-LR compared to the established implant. Both implants showed a significantly higher stiffness in all loading directions compared to the native segment. In flexion loading, the PEEK component takes on most of the load, whereas the majority of the extension load is put on the screws and the screw-plate junction. Clinical investigation of the test device seems reasonable based on the good results reported here.

Fig. 1

SynFix; anterocaudal view with locking screws inserted

Fig. 2

STALIF; anterior view with two of four possible screws inserted

Fig. 3

The meshed model of vertebra for analysis of pure axial compression and tension contains 224,183 elements

Fig. 4

The meshed model of cage, plate and screw for analysis of pure axial compression and tension contains 321,586 elements

Fig. 5

Relative results of range of motion (ROM) measurements. Native segment is set as 1 (red line). The black bar indicates the median value, the edges of the box indicate first and third percentile, respectively. Error bars indicate maximum/minimum values. Outliers are defined as a data point extending greater than 1.5 box-lengths from the edge of a boxplot, extremes are defined as data points which extend greater than three box lengths from the edge of a boxplot. Significant differences between groups are indicated

Fig. 6

Relative results of neutral zone (NZ) measurements. Native segment is set as 1

Fig. 7

Oblique reconstruction showing (a) maximal tensile stress and (b) von-Mises comparison stress distribution of the cage, stabilization plate and fixation screw when tested with a flexion load of 5 Nm

Fig. 8

Oblique reconstruction showing (a) maximal tensile stress and (b) von-Mises comparison stress distribution of the cage, stabilization plate and fixation screw when tested with an extension load of 5 Nm