Posterior Dynamic Stabilization

Posterior dynamic stabilization, also known as soft stabilization, attempts to stabilize the spine while maintaining some intersegmental motion. The theory is that the reduction, rather than elimination, of segmental motion can result in alleviation of back pain.

The most widely used posterior dynamic stabilization device is the Dynesys system. As seen in the video below, the system decompresses the neural foramina without the need for interbody or posterolateral fusion.

The system consists of titanium pedicle screws connected vertically by a polyethylene-terephthalate cord. Between the screws, the polyethylene cord is surrounded by a polycarbonate urethane spacer.


During flexion, the cord acts as tension band and limits flexion. During extension, the spacers engage the screw heads and limit extension. The system may be a bit too good at limiting motion, however. As Kim et al have recently reported in a small series of patients, the average post-operative range of motion at the treated levels was about 4 degrees, compared to about 13 degrees pre-operatively.

The most frequently observed complication in this system is pedicle screw loosening, being seen in about 5% of screws and 20% of patients. The screw loosening tends to occur at marginal levels (above and below) rather than the middle level. The loosening, however, does not appear to change clinical outcome, with patients experiencing improvement regardless of screw loosening.

Radiographs of this system can be puzzling at first, appearing as if the surgeon put in the screws, but had to close before the rods could be placed. The reason for this is that the cord and the spacers are conveniently radiolucent. On CT, the attenuation of the cord/spacer system is almost the same as the surrounding muscle, but a subtle target appearance, representing the central cord and the outer spacer, can be appreciated. The cord has a slightly higher attenuation in this case. The T1-weighted MR image shows the structure to better advantage.

Note that the facets are preserved, one of the touted benefits of this system. Also note the absence of bone graft material, which means less donor-site morbidity from autologous iliac graft.

References

• Kim CH, Chung CK, Jahng TA. Comparisons of outcomes after single or multilevel dynamic stabilization: effects on adjacent segment. J Spinal Disord Tech. 2011 Feb;24(1):60-7.
• Murtagh RD, Quencer RM, Castellvi AE, Yue JJ. New techniques in lumbar spinal instrumentation: what the radiologist needs to know. Radiology. 2011 Aug;260(2):317-30.
• Ko CC, Tsai HW, Huang WC, Wu JC, Chen YC, Shih YH, Chen HC, Wu CL, Cheng H. Screw loosening in the Dynesys stabilization system: radiographic evidence and effect on outcomes. Neurosurg Focus. 2010 Jun;28(6):E10.