113. Segmental contribution to total cervical flexion-extension motion before and after cervical disc arthroplasty (CDA): influence of prosthesis design

Abstract

BACKGROUND CONTEXT As a healthy cervical spine undergoes motion from extension to flexion, individual motion segments move synchronously throughout the arc of motion. The contribution of an individual spinal segment as a fraction of the total motion of the cervical spine (termed here the segmental motion fraction) depends on the flexibility of the spinal segment, but remains nearly constant in healthy segments with small variations around the average value during the arc of motion. Hypermobility, indicative of an unstable motion segment, will cause the segmental motion fraction to deviate substantially from the average value over a portion of the arc of motion. Segmental motion fraction analysis offers a bridge between motion response measured in the laboratory and in vivo kinematic assessment. PURPOSE To investigate: (1) motion contributed by individual spinal segments (C5-C6 and C6-C7) as a fraction of total cervical spine motion through the arc of motion from extension to flexion, and (2) how cervical disc prosthesis design may influence segmental motion contribution. STUDY DESIGN/SETTING Biomechanical study of CDA using Mobi-C and M6-C prostheses. PATIENT SAMPLE Sixteen human cervical spine specimens (C3-T1, 43±6 years). OUTCOME MEASURES Segmental and total cervical motion throughout the arc of motion in flexion-extension (F-E). METHODS We tested 16 human C3-T1 spine specimens under physiologic loads; first intact, after implantation of cervical disc prosthesis by an arthroplasty-trained surgeon at C5-C6 (1-level), and then at C5-C6 and C6-C7 (2-level). The Mobi-C and M6-C disc prostheses were used for 1- and 2-level implantations in eight specimens each. F-E motion was assessed before and after CDA. Quality of motion was assessed using the contribution made by each index segment toward the motion of the C3-T1 cervical spine during the F-E arc of motion. RESULTS Intact: In the intact specimens (N=16), The C5-C6 and C6-C7 segments contributed on average 23%±2.5% and 21%±3.0%, respectively, towards the total F-E motion of C3-T1 specimens. Peak deviations from the average contributions were 12%±6.1% and 14%±9.8%, respectively. M6-C (N=8): After disc replacement using the M6-C prosthesis, the contributions towards the C3-T1 F-E motion by the C5-C6 and C6-C7 segments changed to 23%±4.0% and 22%±3.8%, respectively. The peak deviations from the average contributions were 16%±3.9% and 16%±6.5%, respectively. Mobi-C (N=8): After disc replacement using the Mobi-C prosthesis, the contributions towards the C3-T1 F-E motion by the C5-C6 and C6-C7 segments changed to 23%±4.3% and 18%±6.5%, respectively. The peak deviations from the average contributions were 36%±23% and 53%±31%, respectively. CONCLUSIONS The M6-C artificial disc restored physiologic quality of motion such that the implanted C5-C6 and C6-C7 segments continued to function in harmony with the other segments of the cervical spine as before arthroplasty. The average motion contribution and the peak deviations from the average after CDA remained nearly unchanged from the healthy intact values. Conversely, the Mobi-C prosthesis, while maintaining average motion contributions similar to the pre-implantation values, demonstrated large deviations in motion contribution over the F-E arc of motion in 10 of 16 implanted segments. This behavior was attributed to the locking of the prosthesis over a portion of the arc of F-E motion in these implanted segments. Such nonphysiologic implant kinematics could lead to excessive wear of the prosthesis, and motion- and stress-shielding at adjacent segments. FDA DEVICE/DRUG STATUS Mobi C Cervical disc prosthesis (Approved for this indication), M6 C Cervical disc prosthesis (Investigational/Not approved)