Bioabsorbable interbody magnesium-polymer cage: degradation kinetics, biomechanical stiffness, and histological findings from an ovine cervical spine fusion model.

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.

Preliminary results in anterior cervical discectomy and fusion with an experimental bioabsorbable cage - clinical and radiological findings in an ovine animal model.

BACKGROUND: Bioabsorbable implants are not widely used in spine surgery. This study investigated the clinical and radiological findings after anterior cervical discectomy and fusion (ACDF) in an ovine animal model with an experimental bioabsorbable cage consisting of magnesium and polymer (poly-ϵ-caprolactone, PCL) in comparison to a tricortical bone graft as the gold standard procedure. MATERIALS AND METHODS: 24 full-grown sheep had ACDF of C3/4 and C5/6 with an experimental bioabsorbable implant (magnesium and PCL) in one level and an autologous tricortical bone graft in the second level. The sheep were divided into 4 groups (6 sheep each). After 3, 6, 12, or 24 weeks postoperatively, the cervical spines were harvested and conventional x-rays of each operated segment were conducted. The progress of interbody fusion was classified according to a three-point scoring system. RESULTS: There were no operation related complications except for one intraoperative fracture of the anterior superior iliac spine and two cases of screw loosening and sinking, respectively. In particular, no vascular, neurologic, wound healing or infectious problems were observed. According to the time of follow-up, both interbody fusion devices showed similar behaviour with increasing intervertebral osseointegration and complete arthrodesis in 10 of 12 (83.3%) motion segments after 24 weeks. CONCLUSIONS: The bioabsorbable magnesium-PCL cage used in this experimental animal study showed clinically no signs of incompatibility such as infectious or wound healing problems. The radiographic results regarding the osseointegration are comparable between the cage and the bone graft group.

Mechanical testing of an absorbable hybrid fusion cage for the cervical spine.

Conventional fusion devices ("cages") are often used to join two vertebrae of the human spine and generally remain in the body for a lifetime and can theoretically lead to any complications. Therefore, an absorbable hybrid fusion cage consisting of a magnesium skeleton infiltrated with an absorbable polymer [poly-ε-caprolactone (PCL)] has been developed. The primary objective of the cage is to ensure an adequate stiffness of the disc space directly after the operation and to encourage the ingrowth of the new bone tissue to secure long life stability. Once a sufficiently rigid bone connection is formed, the implant should be absorbed. The purpose of this first study on the new absorbable fusion cage was to investigate the mechanical properties in vitro. Tensile tests were performed with tensile specimens type 1BA according to DIN EN ISO 527 made of PCL foamed using controlled expansion of saturated polymers (CESP). Furthermore, cyclic compression tests and compression tests with steady movement were performed with different designs of the new cage. Compression tests were also performed with vertebral endplates of ovine cadaveric spines. Foaming of PCL resulted in a modulus of elasticity of 135 MPa, which is approximately one third of unfoamed PCL. The results indicate that the initial compression strength of the implants should be adequate for the implantation in the cervical spine.