Biomechanical comparison of single- and two-level cervical arthroplasty versus arthrodesis: effect on adjacent-level spinal kinematics.

Background Context: The use of motion-preserving spinal implants versus conventional arthrodesis instrumentation systems, which stabilize operative segments, necessitates improved understanding of their effect on spinal kinematics and the biomechanically optimal method for surgical reconstruction.

Purpose: The primary objective of this study was to measure operative- and adjacent-level kinematics after single- and two-level cervical arthroplasty and compare them with those after anterior cervical arthrodesis. A secondary objective was to locate the centers of intervertebral rotation at the operative and adjacent levels after arthroplasty and compare them to those after arthrodesis.

Study Design: This biomechanical study used an in vitro human cadaveric model to compare the multidirectional flexibility kinematics of single- versus two-level cervical disc arthroplasty reconstructions.

Methods: Eight cadaveric cervical spines (C2-T2) were biomechanically evaluated between Levels C4 and T1 in the intact condition and under the following reconstructions: single-level arthroplasty (C6-C7) using porous coated motion (PCM) device; single-level arthrodesis (C6-C7) using interbody cage with anterior plate; two-level arthroplasty (C5-C7) using PCM devices; two-level hybrid treatment of arthroplasty (C5-C6) using PCM device and arthrodesis (C6-C7) using cage/plate; and two-level arthrodesis (C5-C7) using cage/plate. Multidirectional flexibility testing used the Panjabi hybrid testing protocol, including pure moments for the intact condition with overall spinal motion replicated under displacement control for subsequent reconstructions. Unconstrained intact moments of +/-3.0 Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM) and neutral zone. The calculated centers of intervertebral rotation were compared for all intervertebral levels under flexion-extension conditions.

Results: Axial rotation loading demonstrated a significant decrease in the C6-C7 ROM for the single-level arthrodesis group compared with the intact spine and the single-level arthroplasty group (p<.05). No differences were observed between the intact and single-level arthroplasty groups (p>.05). For the two-level hybrid treatment group, the C5-C6 ROM significantly increased compared with the intact, single-level arthroplasty, and two-level arthrodesis groups (p<.05). Moreover, a significant increase was observed in the adjacent-level (C7-T1) ROM for the two-level arthrodesis group compared with all other treatment groups (p<.05). Under flexion-extension, no differences were observed in C6-C7 ROM between the intact spine and single-level arthroplasty groups (p>.05). However, as expected, the single-level arthrodesis and two-level hybrid treatment groups demonstrated a decreased ROM at C6-C7 versus the intact spine and arthroplasty treatments (p<.05). In terms of adjacent-level effects, two-level arthrodesis (C5-C7) led to increased ROM in the inferior level (C7-T1) in axial rotation and flexion-extension compared with the intact spine and all other treatment groups (p<0.05). Lateral bending loading conditions demonstrated no significant difference among the treatment groups (p>.05). In flexion-extension, the centers of intervertebral rotation for the intact spine and single-level arthroplasty groups were localized in the central to posterior one-third of the inferior vertebral body for each motion segment: C5-C6, C6-C7, and C7-T1. The single-level arthrodesis group produced more diffuse centers of rotation, particularly at the operative (C6-C7) and inferior adjacent levels (C7-T1).

Conclusions: This study highlights the biomechanical effects of single- and two-level cervical arthroplasty versus single- and two-level arthrodesis on four functional spinal levels (C4-T1). Operative-level ROM was preserved with single- and two-level arthroplasty under all loading modes. The distal adjacent level (C7-T1) demonstrated the greatest increase among the four levels in ROM compared with the intact condition after two-level arthrodesis. These kinematic findings were corroborated by changes in the adjacent-level centers of rotation after arthrodesis and may suggest a biomechanical cause of adjacent-level disease secondary to cervical arthrodesis.