Brief reportCan a modified interspinous spacer prevent instability in axial rotation and lateral bending? A biomechanical in vitro study resulting in a new idea
Introduction
The main indications for interspinous spacers are lumbar spinal stenosis and painful facet arthrosis. In vitro, these implants were shown to reduce facet loading and to widen the neuroforamina and the spinal canal (Richards et al., 2005, Wiseman et al., 2005), which supports their effectiveness concerning the above mentioned indications. Biomechanically, the different interspinous spacers, which are on the market today, i.e. X-Stop, Wallis, Diam and Coflex, all increase stability in extension but are not able to compensate instability in axial rotation, lateral bending and in some cases in flexion (Fuchs et al., 2005, Lindsey et al., 2003, Wilke et al., 2007). This lack of stability might impair the clinical long-term success, which has so far only been reported to be good in the short-term (Anderson et al., 2006, Kondrashov et al., 2006, Zucherman et al., 2005). It would therefore be desirable to have an interspinous spacer available, which is able to compensate instability not only in extension but in all loading planes. At the same time, it would be important to not completely unload the intervertebral disc. The more stability an implant provides the more it deprives the disc of being loaded and unloaded in a physiological way. Physiological loading and unloading, however, is necessary to keep the disc viable.
To meet these requirements, a slightly modified version of the Coflex implant was developed, called “Coflex rivet” (Paradigm Spine, Wurmlingen, Germany). It differs from the original Coflex implant in that it can be more rigidly attached to the spinous processes. At the same time its “U”-shaped design is assumed to still allow some movements and, thus, some load transfer through the intervertebral disc.
The aim of the present in vitro study was to investigate whether this modified implant provides more stability than the original Coflex implant but still allows some load to be transferred through the disc.
Section snippets
Methods
Basically, the Coflex interspinous implant has the shape of a “U” with four lateral wings (Fig. 1a). It is made of titanium. The two types of the implant tested in the present study only differed in the way how they were anchored to the spinous processes while shape and thickness were the same. In case of the original Coflex implant anchorage was achieved by crimping the wings to the spinous processes (Fig. 1b). In contrast, Coflex rivet was fixed using screws, which were drilled from right to
Results
Compared to the defect condition both implants had a strong stabilising effect in extension (P < 0.05) (Fig. 3). In this loading direction, the median RoM decreased from −4.7° with the defect to −1.4° with Coflex rivet and from −4.5° to −1.9° with the original Coflex implant. Coflex rivet also strongly stabilised in flexion (P < 0.05) (Fig. 4) and was able to compensate the destabilising effect of the defect in axial rotation and lateral bending (P < 0.05) (Fig. 5, Fig. 6). However, in these two
Discussion
In this study a new version of the Coflex interspinous implant, called Coflex rivet, was tested for flexibility and load transfer and compared to the original Coflex implant. The aim was to evaluate whether Coflex rivet is able to prevent instability also in axial rotation and lateral bending while still allowing the intervertebral disc to transmit some load.
Coflex rivet only differed from the original Coflex implant in the way it was attached to the spinous processes. Screws were drilled from
Conclusions
In conclusion, the new Coflex rivet interspinous spacer was able to compensate the destabilising effect of the defect in all loading directions. At the same time, there seems to be still enough load transferred through the intervertebral disc to keep it viable. Coflex rivet might therefore become an option if additional stability is needed. The findings of this study also indicate that an interspinous spacer such as Coflex rivet might even become useful as an adjunct to interbody fusion, which
Conflict of interest statement
The authors have received or will receive benefits for professional use from a commercial party related directly or indirectly to the subject of this manuscript. Benefits have been or will be directed to a research fund, foundation, educational institution, or other nonprofit organization with which one or more of the authors are associated.
Acknowledgement
This work was financially supported by Paradigm Spine, Wurmlingen, Germany, who also provided the implants.
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