System IV

Biomechanics

Tests

Design

Clinical Use

Ordering information

Basic principles

The biomechanical principle of the dynamic stabilization of the lumbar spine consists in the control of uncoordinated and thus painful relative movements between vertebral bodies. This kind of implants does not replace any tissues. Depending on the chosen stiffness, structures with more or less load sharing are added. Consequently the implantation of such systems is reversible. With the insertion of the connecting rods approximately at the location of the facet joints, these facet joints are being maximally discharged. Therefore pain due to arthrosis or capsular stretching may be eliminated. With this implantation set-up the natural kinematics (instantaneous centres of rotation) is only minimally altered and the loading stimulus of the intervertebral disc is maintained. In addition the dorsal bulging of the disc may be reduced which might be beneficial in the case of nerve root irritation. Finally the global inclination of the disc may also be reduced what may diminish the danger of disc herniation.

Mimic of anatomy

The connection rods made of polymeric material mimic natural ligaments (collagenous structures) and provide comparable viscoelastic properties. They behave in a similar manner under tension and flexion loads and, in addition, are also capable to carry compression loads. The demanding anchorage, in analogy, is assured with the sophisticated intermediate pieces.

Regeneration of the intervertebral disc

Cases are reported where intervertebral discs are re-accommodating water following the implantation of dynamic stabilization systems. A plausible explanation could read as follows: With the implantation of a dynamic stabilization system the hydrostatic pressure inside the intervertebral disc may be decreased, thus increasing the oncotic pressure (water content in proteins).

Elastic coupling

With the stiff connecting rods a motion segment can be stabilised to a degree, where a spontaneous fusion could occur. The fusion of a motion segment often leads to an excessive loading of the adjacent segment, which, according to recent knowledge, should be protected. Therefore connecting rods capable to achieve fusions should provide transition features allowing an elastic coupling of adjacent segments for the protection from excessive loads.

 

Allocation of stiffness

Based on a considerable number of in-vitro tests, SpineSave has developed an envelope allowing the allocation as well as the test load derivation for dynamic stabilisation systems. In the fusion zone titanium- and PEEK-rods are allocated whereas in the dynamic zone, medium and elastic PCU-rods are allocated. In between is the transition zone, where DYNESYS® (mark of ZIMMER®) and the stiff PCU-rod are allocated. The curves shown are taken after 1 million cycles of longitudinal loads.

Fatigue testing

Since dynamic implants, not intended to lead to fusion with a later load-relief, are subject to repetitive loads for many years, they must undergo dedicated fatigue testing. The standard test of SpineSave includes 1 each million cycles at maximum load (worst-case) followed by 9 each million cycles at typical load in a longitudinal as well as in a transversal test set-up. Depending on the hypothesis ("bottom-up" or "top-down"), 1 million cycles at maximum load reflect a period of implantation in the body between 30 and 124 years.

Maximal load means forward bending with the finger tips to ground, occurring during putting on socks and shoes or washing feet. Typical load is estimated to be one third of maximal load occurring during sitting down and standing up.

 

The SpineShape System IV consists of PCU (polycarbonateurethane) rods having a dedicated cross section with plane parallel sides. Patents are filed for this cross section allowing for a safe and at the same time smooth anchorage of the rod in the unique pedicle screws made of titanium alloy. This anchorage is assured by nuts of titanium alloy and intermediate pieces of PEEK (polyetheretherketon).



The rods exist in three different stiffness (elastic, medium and stiff). The medium and stiff rod is also available as straight and pre-bent. From the medium and stiff rod a stepped version (Varistab™) is further available, allowing in the cases where a fusion is expected, the connection of an adjacent segment with reduced stiffness.



A great advantage of the Varistab™ includes the use of the same pedicle screws and nuts for the stepped end, where a only a higher intermediate piece is required.


The implants (rods, pedicle screws, nuts and intermediate pieces) are supplied in sterile condition after γ-ray sterilisation.

Indications

  • Lumbar spinal column from L1 to S1 with all the structural tissues present, but lacking in efficiency and/ or co-ordination.
  • Dynamic stenosis
  • Degenerative spondylolisthesis (grade 1 according Meyerding)
  • Hypermobility with anterolisthesis
  • Hypomobility with retrolisthesis
  • Recurrent herniated disk
  • Post-nucleotomy syndrome
  • Early degenerative (non-ridgid) scoliosis


Contraindications

  • Lack of or damaged structural tissues (e.g. vertebrae, intervertebral disk, facet joints, etc)
  • Marked idiopathic scoliosis
  • Spondylolisthesis > Meyerding grade 1
  • Isthmic spondylolisthesis
  • Bone tumour
  • Osteoporosis that could compromise the screw anchorage
  • Patient suffering from infections
  • Known allergic reactions to single elements contained in the implant materials used or combinations thereof
  • Skeletal growth


Operating technique


The implantation is performed through the classical pedicular approaches (see Operative technique for SpineShape System IV). A set of extensive especially developed instruments is available.

Pedicle screws exist in 13 sizes, intermediate pieces in 2 sizes and nuts in 1 size. They are available in sets packed with 2 each and distinguished between standard rods (normal intermediate pieces) and Varistabs (heightened intermediate pieces) with following article numbers:

Rods exist in 9 versions. Straight standard rods are packed with 1 each and all others with 2 each with following article numbers:

Changes reserved.