Abstract
Introduction
An in-vivo biomechanical study is presented investigating the Center of Rotation (COR) for flexion/extension before and after insertion of 3 different types of cervical disc prostheses. The aim of the study was to investigate whether the pre-operative coordinates of the COR deviate from that of Healthy Volunteers (COR-HV), and further whether and how the COR-coordinates change after insertion of the different disc prostheses.
Material and Methods
15 healthy volunteers underwent MRI-investigation of their cervical spines. Data for flexion/extension were collected in 5 different positions from maximum extension to maximum flexion for determination of the COR-HV. Further, from 30 patients with a one-level cervical disc prosthesis (10 respective patients who received a Bryan-, Discover- or Prestige-prosthesis) the routinely taken pre- and post-operative cervical flexion/extension X-rays underwent graphical analysis for determination of the respective COR from C3/4 to C6/7. The pre- and post-operative CORs were compared with the corresponding CORs-HV.
Results
Concordant to the literature a different COR-HV was determined for each respective level from C3/4 to C6/7, and it could be demonstrated that the COR for flexion/extension shows migration during motion. In the patient-study, in all 3 subgroups (Bryan-, Discover-, Prestige-prosthesis) the pre-operative COR for flexion/extension deviated considerably from the COR-HV. After prosthesis insertion the coordinates of the COR changed in all levels from C3/4 to C6/7 regardless of the operated segment; in the Bryan-group the COR was shifted toward the COR-HV; in the Discover-group it was shifted away from the COR-HV; and in the Prestige-group it was shifted partly toward and partly away from the COR-HV.
Conclusion
It must be presumed that in patients who are candidates for cervical arthroplasty the COR is not congruent anymore with the COR in healthy individuals. Further, the insertion of a disc prosthesis can change the position of the COR in all levels of the lower cervical spine, irrespective from the level of insertion. It seems that the biomechanical design of the implant can considerably influence the way how the COR changes its position: prosthesis with a simple ball-socket design could shift the already abnormal COR to an even more unphysiological position; devices with a more flexible biomechanical design however can contribute to normalize the COR toward the coordinates that were found in healthy individuals. Further studies on this topic with higher numbers of datasets should be undertaken to support the improvement of the biomechanical design of cervical disc prostheses.