title: Bioabsorbable interbody magnesium-polymer cage: degradation kinetics, biomechanical stiffness, and histological findings from an ovine cervical spine fusion model.
abstract: METHODS
  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.
  
  
  BACKGROUND
  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.
  
  
  CONCLUSIONS
  In comparison with the bone graft, the bioabsorbable cage showed inferior stiffness and fusion properties. Thus, further component modifications are necessary.
  
  
  METHODS
  N/A.
authors.affiliation: *Department of Orthopedics, Spine Section, Hannover Medical School, Diakoniekrankenhaus Annastift gGmbH, Hannover, Germany †Laboratory for Biomechanics and Biomaterials, Hannover Medical School, Hannover, Germany ‡Neurosurgical Clinic, University Freiburg, Freiburg, Germany; and §Institute of Plastics Processing at RWTH Aachen, Aachen, Germany.
authors.forename: Dorothea
authors.initials: D
authors.lastname: Daentzer
authors.forename: Elmar
authors.initials: E
authors.lastname: Willbold
authors.forename: Katharina
authors.initials: K
authors.lastname: Kalla
authors.forename: Ivonne
authors.initials: I
authors.lastname: Bartsch
authors.forename: Waseem
authors.initials: W
authors.lastname: Masalha
authors.forename: Maximiliane
authors.initials: M
authors.lastname: Hallbaum
authors.forename: Christof
authors.initials: C
authors.lastname: Hurschler
authors.forename: Theresa
authors.initials: T
authors.lastname: Kauth
authors.forename: Daniel
authors.initials: D
authors.lastname: Kaltbeitzel
authors.forename: Christian
authors.initials: C
authors.lastname: Hopmann
authors.forename: Bastian
authors.initials: B
authors.lastname: Welke
mesh_terms: D020341:Absorbable Implants; D000818:Animals; D001696:Biomechanical Phenomena; D016025:Bone Transplantation; D002574:Cervical Vertebrae / Q000601:surgery*; D017586:Diskectomy / Q000295:instrumentation / Q000379:methods; D008274:Magnesium; D008422:Materials Testing; D011108:Polymers; D012756:Sheep; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods
chemical_list: D011108:Polymers; D008274:Magnesium