Hybrid Strategy of Two-Level Cervical Artificial Disc and Intervertebral Cage

Abstract

This numerical study aimed to evaluate tissue and implant responses to the hybrid surgery (HS) of cervical artificial disc replacement (C-ADR) and anterior cervical discectomy and fusion (ACDF).Four hybrid strategies of two-level C-ADR and ACDF were compared in terms of adjacent segment degeneration (ASD) and implant failure.The rotary C-ADR and semirigid ACDF have been extensively used in the multilevel treatment of cervical instability and degeneration, but the constrained mobility at the ACDF segments can induce postoperative ASD problems. Hybrid surgery of C-ADR and ACDF has been an alternative to provide the optimal tradeoff between surgical cost and ASD problems. The biomechanical effects of hybrid strategies warrant thorough investigation for the two-level instrumentation.Based on computed tomography imaging, a nonlinear C2–C7 model was developed and validated by cadaveric and numerical data. Four strategies of inserting the C-ADR and ACDF into the C4–C6 segments were systematically arranged as PP (2 peek cages), AA (2 artificial discs), PA, and AP. The biomechanical behavior of these 4 strategies was evaluated in terms of motion and stresses of discs, facet forces, stresses of C-ADR and ACDF, and C-ADR motion.The constrained mobility of the ACDF segment worsened the kinematic and mechanical demands of the adjacent segments and artificial discs. The C-ADR articulation provided higher mobility than the replaced disc of the intact construct, making it an effective buffer to accommodate the compensated mobility and load from the ACDF segment. Consequently, the ASD progression of the AA construct was most restricted, followed by the PA, AP, and PP construct.The PA strategy is a tradeoff to preserve mobility and reduce cost. The C-ADR of the PA construct preserves the mobility of the C5/C6 segment and shares the transferred motion and loads of the fused C4/C5 segment. The PA construct shows optimal biomechanical results for minimizing ASD and implant failure, whereas the AP strategy is only recommended when cranial degeneration is the major concern.

Hybrid Strategy of Cervical Artificial Disc and Intervertebral Cage – Biomechanical Effects on the Adjacent Tissues and Implants

“…After slightly modifying the elastic moduli of the disc and ligaments, this study had been validated and published to achieve a good agreement (Fig. 3) [16]. For the intact model, the ROM error of the current and cadaveric results was 6.3% for flexion, 7.5% for extension, 7.2% for bending, and 3.9% for rotation (Fig.…”

“…This study used the piublished finite-elemet model by the current authors [16]. It was established an osseo-ligamentous cervicothoracic model from C2 to T1 segments using computed tomography (CT) of a 55-year-old male volunteer without any cervical disease.…”

“…The interfaces of facet joints and artificial disc were modeled as the surface-to-surface contact elements, which allow separation and slippage thereby reducing friction. 16,17 The other interfaces between implants and tissues were assumed to be bonded. All implant materials were assumed to have linearly elastic, homogeneous, and isotropic material properties throughout ( Table 1).…”

Hybrid Strategy of Cervical Artificial Disc and Intervertebral Cage – Biomechanical Effects on the Adjacent Tissues and Implants

“…This study used the published finite-element model by the current authors [16]. It was established an osseo-ligamentous cervicothoracic model from C2 to T1 segments using Computed Tomography (CT) of a 55-year-old male volunteer without any cervical disease.…”

Numerical Modelling of the Behaviour of the Cervical Spine under the Effect of a Flexion / Extension

“…While in extension the compressive force acts on the posterior part of the disc. [25][26][27][28][29] Entrainment of the nucleus pulposus on the anterior surface of the ring. The following graph (Figure 9) shows the pressure that forms in the nucleus when 2Nm is applied in flexion-extension as well as the force deflected in the fibrous ring as the end of the moment's application.…”