Influence of a dynamic stabilisation system on load bearing of a bridged disc: an in vitro study of intradiscal pressure

In recent years, non-fusion implants to stabilise the lumbar spine have become more and more popular. However, little is known on the load bearing of such dynamic stabilisation systems. In order to investigate the load bearing of discs bridged with rigid and dynamic stabilisation systems, six lumbar cadaver spines were mounted in a spine tester and loaded with pure moments in the three main motion planes. Four different states of the specimens were studied: intact, destabilised, stabilisation with a Dynesys(R) and stabilisation with an internal fixator. Intradiscal pressure (IDP) measurements were used to assess the load bearing of the bridged disc. In the neutral unloaded position, there were small but not significant differences in disc pressure for the four states of the treated disc (P > 0.05). Concerning the disc pressure during the course of loading, both the Dynesys(R) and internal fixator did significantly reduce the pressure change from neutral to extension in comparison to the intact state (-0.05, -0.04 and + 0.24 MPa, respectively) (P < 0.05). Compared to the intact state, there was no significant pressure change from neutral to flexion (0.14, 0.15 and 0.18 MPa, respectively) (P > 0.05). The devices apparently eliminated the pressure change from neutral to lateral bending (Dynesys 0.01 MPa, Fixator 0.01 MPa and intact 0.24 MPa), but due to large variations in the intact and defect states the differences were not significant (P>0.05). In axial rotation, the pressure change for the internal fixator was reduced compared to the intact state; however, the change was only significant in left axial rotation (P < 0.05). The Dynesys(R) showed no significant differences (P > 0.05) in axial rotation. No changes in IDP were seen in the adjacent discs for either the Dynesys or the internal fixator. Our results showed that the IDPs for both devices were similar, but altered compared to the intact disc.