gms | German Medical Science

Objective: For most cases of degenerative disc disease of the cervical spine, decompression and fusion is the standard procedure. However, the implantation of total disc replacement (TDR) is used more frequent as an alternative technique to maintain segmental range of motion (ROM) and to prevent adjacent level pathology.

Methods: The biomechanical testing setup consisted of 12 cervical spine specimens from adult sheep from C2 to C5. Pure moments of ± 2 Nm were applied in flexion/extension, lateral bending and axial rotation right/left by means of a sensor-guided industrial robot. Every specimen was tested first in the intact state, then in simulated fusion C3/4 and finally with TDR C3/4. In six cadavers the Discover^TM prosthesis (P01-06) and in six specimens the activ C prosthesis (P07-12) were implanted. The analysis of the kinematics was performed by the robot with investigation of the total ROM (tROM) C2-5 and with an optical measuring system by identification of the segmental ROM C2/3, C3/4 and C4/5. For continuous monitoring of the intradiscal pressures special transducers were inserted into the discs C2/3 and C4/5. The analysis of the pressure data was performed at an angle corresponding to 90% of the maximum angle of total ROM in the fused situation. The t-test was used for statistic evaluation of the differences for all parameters in the three conditions (intact, fused, TDR) and to detect any differences between the two prosthesis. The level of significance was determined with a p < 0.05 defined as significant.

Results: In cases of simulated fusion of C3/4, tROM was statistically significantly reduced, in particular because of a significant limitation in the mobility of C3/4. The absolute values of the adjacent levels C2/3 and C4/5 were only slightly higher compared to the intact state. However, within the tROM they showed a significant percental increase of their motion. After implantation of both TDR C3/4 the tROM showed a statistically significant increase, particularly because of a marked increase in the aim segment C3/4 with the prosthesis. In contrast, the percental part of the segmental ROM C2/3 and C4/5 decreased below the initial value. In lateral bending and axial rotation we recognized a similar tendency. Concerning the fused condition, we found in both cadaver groups a statistically significant increase in intradiscal pressures in comparison to the native situation, in particular in flexion which was always higher in the lower segment C4/5 than in the upper level C2/3. A similar tendency was seen regarding the applied forces. In lateral bending the development was not so clear, since there were statistically significant differences only in lateral bending to the right side. After implantation of both types of TDR the pressure values and the applied forces decreased to similar values of the intact state. Both types of prosthesis showed no statistically significant differences regarding their kinematics.

Conclusions: These results based on an animal model confirm the previous findings. Both semi-constrained types of the prosthesis seem to fulfill the requirements. However, long-term follow-up studies over 10 to 15 years have to show, wether the biomechanical findings also have a clinical advantage over fusion.