Total disc arthroplasty (TDA) increases the risk of adjacent segment disease (ASD). Kinematic analyses are necessary to compare the intact condition (IC) with alterations after TDA to develop better prostheses. A well-established 6D measuring apparatus (resolution < 2.4 μm; 400 positions/cycle) was used. Kinematics of the flexion and extension of 8 human cervical spine segments (cFSU) C3/C4 and C5/C6 (67.9 ± 13.2 y) were analyzed in the IC and after TDA (Bryan® Cervical Disc [B-TDA], Prestige LP® Cervical Disc [P-TDA]). The migration of the instantaneous helical axis (IHA) and the stiffness of the segments were calculated. Analyses demonstrated a stretched U-curved IHA migration in the sagittal plane. The IHA positions were significantly more cranial in cFSU C5/C6 than in C3/C4 in IC and after either TDA (IC: p < 0.001; B-TDA: p = 0.001; P-TDA: p = 0.045). In cFSU C3/C4 IHA positions shifted anteriocranially after either TDA (p < 0.001). In cFSU C5/C6, the IHA positions were significantly more anterocranial after B-TDA than in IC and after P-TDA (anterior: p < 0.001; cranial: p = 0.005). After B-TDA, the IHA migration path length was significantly longer in cFSU C3/C4 than in C5/C6 (p = 0.007) and longer than in IC in both cFSU (C3/C4: p = 0.047; C5/C6: p < 0.001). Stiffness was increased after both TDA. Various kinematic alterations were observed after both TDA. Increased translation and IHA position shifting after both TDA might indicate abnormal strain and a derogated benefit of TDA. These results imply the most abnormal strain after B-TDA. The lower cFSU might be more susceptible to alterations after TDA than the upper cFSU.
