Biomechanical effect of different lumbar interspinous implants on flexibility and intradiscal pressure

Abstract

Interspinous implants are used to treat lumbar spinal stenosis or facet joint arthritis. The aims of implanting interspinous devices are to unload the facet joints, restore foraminal height and provide stability especially in extension but still allow motion. The aim of this in vitro study was to compare four different interspinous implants--Colfex, Wallis, Diam and X-Stop--in terms of their three-dimensional flexibility and the intradiscal pressure. Twenty-four human lumbar spine specimens were divided into four equal groups and tested with pure moments in flexion/extension, lateral bending and axial rotation: (1) intact, (2) defect, (3) after implantation. Range of motion and the intradiscal pressure were determined. In each implant-group the defect caused an increase in range of motion by about 8% in lateral bending to 18% in axial rotation. Implantation had similar effects with all four implants. In extension, Coflex, Wallis, Diam, and X-Stop all overcompensated the instability caused by the defect and allowed about 50% of the range of motion of the intact state. In contrast, in flexion, lateral bending and axial rotation the values of the range of motion stayed about the values of the defect state. Similarly the intradiscal pressure after implantation was similar to that of the intact specimens in flexion, lateral bending and axial rotation but much smaller during extension. All tested interspinous implants had a similar effect on the flexibility: they strongly stabilized and reduced the intradiscal pressure in extension, but had almost no effect in flexion, lateral bending and axial rotation.

Fig. 1

Implants tested in this study; in the upper line implants placed in the specimen for better illustration

Fig. 2

Specimen with intradiscal pressure transducer mounted in the spine tester

Fig. 3

Range of motion (ROM) in flexion and extension for the intact, the defect condition and after implantation. Values are median with minimum and maximum.*P < 0.05 for comparisons between implanted and intact condition in each of the four groups (Wilcoxon signed rank test). The border between the light and the darkbars represents the median segmental tilt caused by defect (red) and by implantation (blue)

Fig. 4

Range of motion (ROM) in lateral bending for the intact, the defect condition and after implantation. Values are median with minimum and maximum. *P < 0.05 for comparisons between implanted and intact condition in each of the four groups (Wilcoxon signed rank test)

Fig. 5

Range of motion (ROM) in axial rotation for the intact, the defect condition and after implantation. Values are median with minimum and maximum. *P < 0.05 for comparisons between implanted and intact condition in each of the four groups (Wilcoxon signed rank test)

Fig. 6

Intradiscal pressure (IDP) in flexion and extension for the intact condition the defect condition and after implantation. Exemplary curve for Diam-Implant