gms | German Medical Science

A common problem after spinal surgery is the difficulty to evaluate spinal stability, the presence of bony fusion and to quantify potential micromotion between spinal segments. We used the highly accurate Radio Stereometric Analysis (RSA) to quantify micromotion between cervical spine segments following spinal fusion versus the implantation of a cervical prosthesis.

20 patients with a symptomatic cervical disc herniation were either treated with a cervical fusion (PEEK-Cages / STRYKER) + a 4 holed-plate (AESCULAP) (n=10, group 1) or with the implantation of a cervical disc (PRODISC C / SYNTHES) (n=10, group 2). Epidemiological data were comparable between both groups. To perform a RSA study, the insertion of 5-9 radio-opaque tantalum markers in each vertebra is required to determine the geometric characteristics of the vertebral anatomy. All patients were examined by RSA after each operation. The RSA x-ray setup using two 40° angulated conventional x-ray tubes was identical in all patients. All patients were positioned above the combined reference plate and calibration device with 1 mm tantalum indicators, placed at known positions in front of the film plane. X-rays were taken both in supine position at 20° reclination and in neutral spine position to put load on the lumbosacral spine. The intervertebral translations were induced by this positional change and relative movements (micromotion) of the transverse (x), vertical (y) and sagittal (z) axes were calculated by repeated x-rays using the RSA-Kinematic software. The calculated intervertebral translations allowed visualization of persisting micromotion between the cervical vertebrae. Patients were monitored post-operatively and after 6 and 12 weeks.

After surgery, the mean intervertebral micromotion in the transverse(X), vertical(Y) and sagittal (Z) axis gradually decreased over time after the fusion procedure (group 1) and were below the accuracy (0.3mm) of the RSA setup 3 months after surgery. After implantation of the cervical disc prosthesis, micromotion were above the accuracy at all times but also gradually decreased over time (see figures).

RSA is a useful and highly accurate tool to quantify in-vivo micromotion between vertebrae. A longer RSA follow-up is necessary to determine whether intervertebral mobility is further decreasing over time after the implantation of a cervical disc prosthesis.