Introduction The Dynesys® stabilisation system (Zimmer Spine) uses transpedicular screws linked by a cord and spacers; the advantage claimed is that it allows some motion, in all directions, in the operated levels. In vitro laboratory biomechanical studies show that the movement permitted is similar to rigid fixation systems.

This study measures the changes in the lumbar spine in different postures, pre- and after insertion of the device. Methods In our study 25 patients with dominant low back pain, with or without leg pain, were treated with Dynesys system, 14 females and 11 males with mean age 43.5 yrs. Discography was done to evaluate the symptomatic painful level. All patients had a positional magnetic resonance imaging (pMRI) scans preoperatively and nine months post-operatively in standing; sitting flexion and extension, and left and right bending postures. The patients were initially divided into two groups. The first (Group A) with 14 patients in which only Dynesys was used (disc height 40–90%) and the second (group B) with 11 patients in which Dynesys was used with fusion (disc height < 40 %).

Results The operated levels were 51, 13 of which were fused. The results of the pMRI measurements showed that the mean range of movement of the lumbosacral angle reduced by 10.3° from pre-op of 39.2° to postop. of 28.9° (p= 0.016) in group A while in group B it reduced by 13.7° from 36.2° preop. to 22.5° postoperatively (p= 0.02).

The range of movement of the end plate angle at the instrumented segments in group A reduced from 5.6° preoperatively to 2.6° with a difference of 3.0° (p=0.016) while in group B it reduced from 6.7° to 2.5° postoperatively with a difference of 4.2°(p=0.008). The range of movement of the end plate angle at adjacent level in group A changed from 8.8° preop. to 7.2° with a reduction of 1.6° (p=0.427) while in group B it increased from 6.9° to 8.6°, difference of 1.7° (p=0.149)

The mean anterior disc height in group A reduced by 1.2mm (p< 0.005) from 10.1mm to 8.9mm postoperatively, and the posterior one was increased from 6.5mm to 8.9mm with a difference of 0.6mm (p= 0.013). In group B the anterior disc height was reduced by 1.1mm (pre-op 10.4mm to post-op 9.3mm; p=0.049) and the posterior one by 0.16mm (pre-op 6.98mm to post-op 6.82mm; p=0.714)

Discussion This study shows that in the Dynesys stabilising system allows a small range of movement at the instrumented levels, with no significant increased mobility in the adjacent levels. Also the device acted to compress the anterior annulus more than to distract the posterior annulus.