title: Mechanical properties of bioresorbable self-reinforced posterior cervical rods.
abstract: METHODS
  A biomechanical study.
  
  
  OBJECTIVE
  To test the mechanical and physical properties of self-reinforced copolymer bioresorbable posterior cervical rods and compare their mechanical properties to commonly used Irene titanium alloy rods.
  
  
  BACKGROUND
  Bioresorbable instrumentation is becoming increasingly common in surgical spine procedures. Compared with metallic implants, bioresorbable implants are gradually reabsorbed as the bone heals, transferring the load from the instrumentation to bone, eliminating the need for hardware removal. In addition, bioresorbable implants produce less stress shielding due to a more physiological modulus of elasticity.
  
  
  METHODS
  Three types of rods were used: (1) 5.5 mm copolymer rods and (2) 3.5 mm and (3) 5.5 mm titanium alloy rods. Four tests were used on each rod: (1) 3-point bending test, (2) 4-point bending test, (3) shear test, and (4) differential scanning calorimeter test. The outcomes were recorded: Young modulus (E), stiffness, maximum load, deflection at maximum load, load at 1.0% strain of the rod's outer surface, and maximum bending stress.
  
  
  RESULTS
  The Young modulus (E) for the copolymer rods (mean range, 6.4-6.8 GPa) was significantly lower than the 3.5 mm titanium rods (106 GPa) and the 5.5 mm titanium rods (95 GPa). The stiffness of the copolymer rods (mean range, 16.6-21.4 N/mm) was also significantly lower than the 3.5 mm titanium alloy rods (43.6 N/mm) and the 5.5 mm titanium alloy rods (239.6 N/mm). The mean maximum shear load of the copolymer rods was 2735 N and they had significantly lower mean maximum loads than the titanium rods.
  
  
  CONCLUSIONS
  Copolymer rods have adequate shear resistance, but less load resistance and stiffness compared with titanium rods. Their stiffness is closer to that of bone, causing less stress shielding and better gradual dynamic loading. Their use in semirigid posterior stabilization of the cervical spine may be considered.
authors.affiliation: *Rothman Institute of Orthopedics, Thomas Jefferson University, Philadelphia, PA †Division of Orthopaedic Surgery, McGill University Health Centre, Montreal, QC, Canada ‡Inion Oy, Tampere, Finland.
authors.forename: Katherine
authors.initials: K
authors.lastname: Savage
authors.forename: Zeeshan M
authors.initials: ZM
authors.lastname: Sardar
authors.forename: Timo
authors.initials: T
authors.lastname: Pohjonen
authors.forename: Gursukhman S
authors.initials: GS
authors.lastname: Sidhu
authors.forename: Benjamin D
authors.initials: BD
authors.lastname: Eachus
authors.forename: Alexander
authors.initials: A
authors.lastname: Vaccaro
mesh_terms: D020341:Absorbable Implants; D001696:Biomechanical Phenomena; D002152:Calorimetry, Differential Scanning; D002574:Cervical Vertebrae / Q000502:physiology*; D006801:Humans; D008422:Materials Testing; D033081:Shear Strength; D014025:Titanium / Q000494:pharmacology
chemical_list: D014025:Titanium