Bioabsorbable Interbody Magnesium-Polymer Cage

Degradation Kinetics, Biomechanical Stiffness, and Histological Findings From an Ovine Cervical Spine Fusion Model

Abstract

Study Design. 

An experimental study using a sheep cervical spine interbody fusion model.

Objective. 

First, to compare anterior cervical discectomy and fusion of an experimental bioabsorbable cage consisting of a magnesium alloy and a polymer (poly-ε-caprolactone, PCL) with an autologous tricortical iliac crest bone graft. Second, to determine the degradation kinetics of the cage, assess the 2 fusion devices for biomechanical stability, and determine their histological characteristics.

Summary of Background Data. 

Bioabsorbable cages are not routinely used in spine surgery at present, due to some undesirable effects such as cracks and foreign body reactions. This study involved the manufacture of a bioabsorbable cage from a magnesium alloy and the polymer PCL, which was then used as a device for anterior cervical discectomy and fusion in a sheep cervical spine fusion model.

Methods. 

Twenty-four sheep had anterior cervical discectomy and fusion of C3–C4 and C5–C6 with an experimental bioabsorbable cage consisting of the magnesium alloy AZ31, which was infiltrated and covered with PCL at 1 level and with an autologous tricortical iliac crest bone graft at a second level. The sheep were divided into 4 groups. After 3, 6, 12, or 24 weeks postimplantation, the animals were killed and the cervical spines were harvested. The intervertebral spaces with the cage were investigated using μ-computed tomographic images to calculate degradation kinetics. Stiffness of all monosegments was determined through biomechanical testing. Histological analysis was performed to evaluate fusion status and to detect any foreign body reactions. The results from both implants were compared.

Results. 

The magnesium-PCL cage showed nonlinear degradation over time. Both implants demonstrated time-dependent increases in stability, with a significantly greater stiffness of the bone graft after 24 weeks in all directions of motion. Histologically, the cage showed no signs of fusion with progressive encapsulation over time.

Conclusion. 

In comparison with the bone graft, the bioabsorbable cage showed inferior stiffness and fusion properties. Thus, further component modifications are necessary.

Level of Evidence: N/A

In Brief

An experimental bioabsorbable cage, consisting of a magnesium alloy and a polymer, was used for anterior cervical discectomy and fusion in a sheep cervical spine interbody fusion model. Its degradation kinetics was calculated, and its biomechanical and histological characteristics were compared with an autologous bone graft. The cage showed inferior stiffness and fusion properties.