Biomechanical and kinetic testing of two-level cervical disc replacement

Aim. Sparse biomechanical data exist regarding local kinetics of two-level total disc replacement and salvage procedures that may be required due to arthroplasty failure. The current study investigated the kinetics of two-level disc replacement and the implications of a two-level salvage procedure. Methods. Range of motion (ROM) was measured in seven human cadaveric cervical spines (C2-T1). Four conditions were evaluated: (1) intact (INT); (2) one-level disc replacement at C5-C6 (1-LEVEL); (3) two-level disc replacement at C4-05 and C5-C6 (2-LEVEL); and (4) one level disc replacement at C4-05 with simulated salvage (DISC+PLATE) accomplished by plating over a previously implanted cervical prosthesis at C5-C6. ProDisc-C prostheses and Vectra plates and screws were used. Specimens were exposed to 2 N-m moments in flexion/extension, lateral bending and axial rotation. A stereophotogrammetric system was used to quantify the total ROM. Results. Prosthesis implantation at C5-C6 (1-LEVEL) resulted in a significant reduction in lateral bending ROM at this level (P=0.002). Neither flexion/extension nor axial rotation ROM was affected relative to the INT condition. Subsequent arthroplasty at the C4-C5 level (2-LEVEL) resulted in a significant reduction in lateral bending ROM at that level compared to the INT condition (P=0.039). Simulated salvage at C5-C6 (DISC+PLATE) significantly restricted motion in flexion/extension and axial rotation ROM at that level relative to the INT, 1-LEVEL and 2-LEVEL conditions (P<0.05). Lateral bending ROM was not significantly affected by the DISC+PLATE treatment relative to the 1-LEVEL or 2-LEVEL conditions (p>0.05), but was significantly reduced relative to the INT condition (P<0.001). Conclusion. Adjacent implantation of a second cervical disc prosthesis has minimal effect on sagittal plane ROM at either replaced level relative to an intact condition. Salvage procedures which seek to avoid disc prosthesis removal following a failed arthroplasty are capable of significantly reducing flexion/extension and axial rotation at the plated level.