Biomechanical Stability Analysis of a Stand-alone Cage, Static and Rotational-dynamic Plate in a Two-level Cervical Fusion Construct

Abstract

Objective: To test the following hypotheses: (i) anterior cervical discetomy and fusion (ACDF) using stand-alone interbody spacers will significantly reduce the range of motion from intact spine; and (ii) the use of a static or a rotational-dynamic plate will significantly augment the stability of stand-alone interbody spacers, with similar beneficial effect when compared to each other.

Methods: Eleven human cadaveric subaxial cervical spines (age: 48.2 ± 5.4 years) were tested under the following sequence: (i) intact spine; (ii) ACDF at C₄ -C₅ using a stand-alone interbody spacer; (iii) ACDF at C₅ -C₆ and insertion of an interbody spacer (two-level construct); and (iv) randomized placement of either a two-level locking static plate or a rotational-dynamic plate.

Results: Insertion of stand-alone cage at C₄ -C₅ and C₅ -C₆ caused a significant decrease in the range of motion compared to intact spine (P < 0.05). Placement of both the locking and the rotational dynamic plate further reduced the range of motion at C₄ -C₅ and C₅ -C₆ compared to stand-alone cage (P < 0.01). No significant differences in range of motion restriction at either C₄ -C₅ or C₅ -C₆ were found when the two plating systems were compared (P > 0.05).

Conclusions: Cervical stand-alone interbody spacers caused significant restriction in the range of motion. Both plates significantly augmented the stability of stand-alone interbody spacers, with similar stabilizing effect.

Keywords: Anterior cervical discectomy and fusion; Biomechanics; Plating; Range of motion; Stand-alone cages.

Figure 1

Schematic presentation of a traditional static locking plate (A) and Depuy–Synthes Vectra Variable Angle Plating System. After placement, this plate is capable of 28° cranial–caudal angulation and 14° medial–lateral angulation (B).

Figure 2

The experimental setup. The cervical spine specimen is fixed to the apparatus at the caudal end and is free to move in any plane at the cranial end. A moment is applied. The apparatus allows continuous cycling of the specimen in flexion, extension, lateral bending, and axial rotation. The angular motions of the C ₃ to C ₆ vertebrae relative to C ₇ are measured using an optoelectronic motion measurement system. Bi‐axial angle sensors are mounted on each vertebra for optimization of the preload path. A six‐component load cell is placed under the specimen to measure the applied compressive preload and moments (A), fluoroscopic image during flexion extension movements following anterior cervical discectomy, cage placement, and plate insertion at C ₄–C ₆ (B).

Figure 3

Two‐level anterior cervical discectomy and stand‐alone cage insertion (A), CSLP (locking) plate placement (B) and Vectra (rotational dynamic) plate placement (C).

Figure 4

Range of motion at C ₄–C ₅ in intact spine and following stand‐alone cage insertion. A significant reduction in Flexion/Extension, Lateral Bending and axial rotation was noted compared to intact.

Figure 5

Range of motion at C ₅–C ₆ in intact spine and following stand‐alone cage insertion. A significant reduction in Flexion/Extension, Lateral bending and axial rotation was noted compared to intact.

Figure 6

Flexion/Extension range of motion in intact spine and following plate placement at C ₄–C ₆. A significant reduction was noted compared to intact with no difference between the two plating systems.

Figure 7

Lateral bending range of motion in intact spine and following plate placement at C ₄–C ₆. A significant reduction was noted compared to intact with no difference between the two plating systems.

Figure 8

Axial rotation range of motion in intact spine and following plate placement at C ₄–C ₆. A significant reduction was noted compared to intact with no difference between the two plating systems.