In vivo kinematics of two types of ball-and-socket cervical disc replacements in the sagittal plane -: Cranial versus caudal geometric center

Study Design. A computer-assisted analysis of dynamic lateral radiographs of the cervical spine in flexion/extension after total disc replacement. Objective. To analyze the in vivo kinematics of 2 types of ball-and-socket cervical disc arthroplasties. Summary of Background Data. Clinical outcomes and range of motion (ROM) after cervical disc replacement have been documented in few studies. To our knowledge, no article reports the biomechanical behavior of any type of ball-and-socket arthroplasty at the cervical level in vivo in terms of center of rotation between full flexion/extension (COR-FE). Methods. The spineview software (Surgiview, Paris, France) was used to investigate the intervertebral sagittal ROM and MCR in 26 Prestige LP (Medtronic Sofamor Danek, Memphis, TN) and 25 Prodisc-C (Synthes, West Chester, PA), in reference to the measurements of 200 healthy cervical discs in vivo. The COR-FE was calculated above 3 degrees in ROM. Results. The ROM was significantly reduced with both types of arthroplasty when compared with the control group. Although the ROM was similar with both types of prosthesis, the location of the COR-FE was significantly influenced by the type of intervertebral disc despite the interindividual variability. Although the COR-FE remained within the normal range in most cases, it trended to be located more anterior and superior than normal with the 2 types of prosthesis. Conclusion. Neither the cranial or caudal types of ball-and-socket designs did fully restore the normal mobility in terms of ROM and COR in this patient's series.