title: Polymer-on-metal or metal-on-polymer total disc arthroplasty: does it make a difference?
abstract: METHODS
  Mechanical testing of total disc arthroplasty (TDA).
  
  
  OBJECTIVE
  To compare the friction between a polymer socket-on-metal ball and metal socket-on-polymer ball TDA.
  
  
  BACKGROUND
  A degenerate intervertebral disc can be replaced by TDA. The most common designs have a ball and socket articulation; the contact between the surfaces leads to friction. Friction needs to be minimized to prevent loosening and wear. One of the common material combinations in disc arthroplasty devices is the articulation of a metal socket on polymer ball. However, the combination of a polymer socket on metal ball (which is used in hip arthroplasty) has not been investigated for TDA.
  
  
  METHODS
  TDA models with either a polymer socket/metal ball or a metal socket/polymer ball were manufactured with ball radii of 10 and 14 mm, each with a radial clearance of 0.35 mm. Samples were tested using a spine simulator with a lubricant of diluted newborn calf serum. Each sample was subjected to an axial load of 1200 N; motions of flexion-extension, lateral bending, and axial rotation were then applied at frequencies of 0.25 to 2 Hz. Frictional torque was measured to compare the performance of the TDAs.
  
  
  RESULTS
  The frictional torque was found to be significantly higher for a disc with a metal socket/polymer ball than for a disc with a polymer socket/metal ball for both 10 and 14 mm radii in axial rotation, lateral bend, and extension. The frictional torque in flexion (0°-6°) was not found to be significantly different between the 2 different material combinations. However, when the flexion motion was reduced to 0° to 2°, frictional torque in the metal socket/polymer ball was found to be significantly higher than the polymer socket/metal ball.
  
  
  CONCLUSIONS
  TDA with a combination of a polymer socket/metal ball has lower friction than the conventional TDA with metal socket/polymer ball. This conclusion has implications in the design of TDA.
authors.affiliation: School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, United Kingdom. MoghadaP@adf.bham.ac.uk
authors.forename: Parshia M
authors.initials: PM
authors.lastname: Moghadas
authors.forename: Duncan E T
authors.initials: DE
authors.lastname: Shepherd
authors.forename: David W L
authors.initials: DW
authors.lastname: Hukins
authors.forename: Aziza
authors.initials: A
authors.lastname: Mahomed
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D002417:Cattle; D017276:Friction; D006801:Humans; D055959:Intervertebral Disc Degeneration / Q000601:surgery*; D008422:Materials Testing / Q000379:methods; D008670:Metals / Q000737:chemistry; D011108:Polymers / Q000737:chemistry; D019736:Prostheses and Implants / Q000592:standards*; D011474:Prosthesis Design / Q000379:methods*; D044967:Serum / Q000737:chemistry; D013131:Spine / Q000601:surgery; D019415:Torque; D059145:Total Disc Replacement / Q000295:instrumentation* / Q000379:methods
chemical_list: D008670:Metals; D011108:Polymers