Biomechanical consideration of prosthesis selection in hybrid surgery for bi-level cervical disc degenerative diseases

Hybrid surgery (HS) coupling total disc replacement and fusion has been increasingly applied for multilevel cervical disc diseases (CDD). However, selection of the optimal disc prosthesis for HS in an individual patient has not been investigated. This study aimed to distinguish the biomechanical performances of five widely used prostheses (Bryan, ProDisc-C, PCM, Mobi-C, and Discover) in HS for the treatment of bi-level CDD. A finite element model of healthy cervical spine (C3-C7) was developed, and five HS models using different disc prostheses were constructed by arthrodesis at C4-C5 and by arthroplasty at C5-C6. First, the rotational displacements in flexion (Fl), extension, axial rotation, and lateral bending in the healthy model under 1.0 Nm moments combined with 73.6 N follower load were achieved, and then the maximum rotations in each direction combined with the same follower load were applied in the surgical models following displacement control testing protocols. The range of motion (ROM) of the entire operative and adjacent levels was close to that of the healthy spine for ball-in-socket prostheses, that is, ProDisc-C, Mobi-C, and Discover, in Fl. For Bryan and PCM, the ROM of the operative levels was less than that of the healthy spine in Fl and resulted in the increase in ROMs at the adjacent levels. Ball-in-socket prostheses produced similar reaction moments (92-99 %) in Fl, which were close to that of the healthy spine. Meanwhile, Bryan and PCM required greater moments (> 130 %). The adjacent intradiscal pressures (IDPs) in the models of ball-in-socket prostheses were close to that of the healthy spine. Meanwhile, in the models of Bryan and PCM, the adjacent IDPs were 25 % higher than that of the ball-in-socket models. The maximum facet stress in the model of Mobi-C was the greatest among all prostheses, which was approximately two times that of the healthy spine. Moreover, Bryan produced the largest stress on the bone-implant interface, followed by PCM, Mobi-C, ProDisc-C, and Discover. Each disc prosthesis has its biomechanical advantages and disadvantages in HS and should be selected on an individual patient basis. In general, ProDisc-C, Mobi-C, and Discover produced similar performances in terms of spinal motions, adjacent IDPs, and driving moments, whereas Bryan and PCM produced similar biomechanical performances. Therefore, HS with Discover, Bryan, and PCM may be suitable for patients with potential risk of facet joint degeneration, whereas HS with ProDisc-C, Mobi-C, and Discover may be suitable for patients with potential risk of vertebral osteoporosis.