The biomechanical effects on facet joints after posterior fusion remain unclear and seem to be responsible for accelerated degeneration. The following biomechanical study was performed to investigate the effects on the pressure and mobility of neighbouring unfused segments after double level T12-L2 posterior stabilization.

The experimental study was performed on eighteen fresh, human, cadaveric thoracolumbal spine specimens. The specimens were cleaned and dissected from muscles and fat with care to preserve bone-ligament units intact. In a specially constructed testing machine the data of the segmental pressure and mobility of adjacent segments above and below the fusion were measured before and after double level T12-L2 posterior stabilization with an internal fixator (Universal Spine System) in flexion, extension, lateral bending, and rotation. For measuring the mobility a motion tracker (3Space Fastrak) and for direct evaluation of the pressure a quartz miniature force transducer was used. Also the bone mineral density of the specimens were measured and showed normal values.

In flexion and extension Range of Motion (ROM) of the segment above the double level T12-L2 posterior fusion was significantly increased (p< 0,05). In the adjacent segment below the fusion there was no significant increased mobility after fusion for each moment was applied. The pressure did not show any significant difference, but after posterior fusion in flexion and extension the pressure below the posterior fusion (L2/L3) was decreased and above the fusion (T11/T12) increased.

There is evidence that the adjacent segment above a double-level T12-L2 posterior fusion becomes more mobile and leads possibly to an accelerated degeneration in the facet joints due to increased stress at this point. Also the posterior fusion seems to change the load distribution in the facets of adjacent segments. These results could be responsible for symptoms like low back pain after spinal surgery.

The new distractable titanium implant (Synex) is designated for replacement of the vertebral body following fracture, posttraumatic kyphosis or tumor.

Synex was compared with the “Harms” cage (MOSS, 22x28 mm, stabilising ring) in two test series.

Test A: Measurement of the compressive strength of the vertebral body end-plate in uniaxial loading via both implants; Test B: Analysis of the bisegmental stability after corpectomy, replacement of L1 and stabilisation.

Materials and methods: In testseries A human vertebral specimens (L1) were matched according to bone mineral density (BMD). They were axially loaded (v=5mm/min) to failure via Synex (n=6) or MOSS (n=6) in an electrohydraulic testing device with load-displacement recording.

In test series B the bisegmental motion (T12-L2) of 12 spinal specimens were tested in a 3D loading simulator with moments of 0–7.5 Nm for the six directions. After testing the intact spine, we replaced L1 and stabilised with Fixateur interne (USS) or Ventrofix (VFix). Analysis of the range of motion (ROM), elastic zone (EZ) and neutral zone (NZ) for five conditions: 1) Intact specimen, 2) USS+Synex, 3) USS+MOSS, 4) VFix+Synex, 5) VFix+MOSS (randomized order).

Results: With Synex, significantly higher compression forces were recorded at 1–2 mm deformation. Ultimate compression force (Fmax) was higher (3396 N vs. 2719 N) and the distance until point of failure (Dmax) was significantly less using Synex. A significant correlation (R=0.89) between Fmax and BMD was found.

Significantly higher stability was noted with USS+Synex for extension, lateral bending, and axial rotation. No differences between Synex and MOSS were observed in combination with VFix. The combined instrumentation (USS) was superior to the anterior one (VFix).

The possibility of secondary dislocation, loss of correction, or posttraumatic kyphosis can be decreased using Synex for replacement of the vertebral body, compared with MOSS. A combined anterior-posterior stabilisation provides higher biomechanical stability compared with an anterior construct.