title: Measurement of the migration of a focal knee resurfacing implant with radiostereometry.
abstract: OBJECTIVE
  Articular resurfacing metal implants have been developed to treat full-thickness localized articular cartilage defects. Evaluation of the fixation of these devices is mandatory. Standard radiostereometry (RSA) is a validated method for evaluation of prosthetic migration, but it requires that tantalum beads are inserted into the implant. For technical reasons, this is not possible for focal articular resurfacing components. In this study, we therefore modified the tip of an articular knee implant and used it as a marker for RSA, and then validated the method.
  
  
  METHODS
  We modified the tip of a resurfacing component into a hemisphere with a radius of 3 mm, marked it with a 1.0-mm tantalum marker, and implanted it into a sawbone marked with 6 tantalum beads. Point-motion RSA of the "hemisphere bead" using standard automated RSA as the gold standard was compared to manual measurement of the tip hemisphere. 20 repeated stereograms with gradual shifts of position of the specimen between each double exposure were used for the analysis. The tip motion was compared to the point motion of the hemisphere bead to determine the accuracy and precision.
  
  
  RESULTS
  The accuracy of the manual tip hemisphere method was 0.08-0.19 mm and the precision ranged from 0.12 mm to 0.33 mm.
  
  
  CONCLUSIONS
  The accuracy and precision for translations is acceptable when using a small hemisphere at the tip of a focal articular knee resurfacing implant instead of tantalum marker beads. Rotations of the implant cannot be evaluated. The method is accurate and precise enough to allow detection of relevant migration, and it will be used for future clinical trials with the new implant.
authors.affiliation: Department of Orthopaedics at Danderyd Hospital and Karolinska Institutet , Department of Clinical Sciences at Danderyd Hospital (KIDS) , Stockholm.
authors.forename: Olof
authors.initials: O
authors.lastname: Sköldenberg
authors.forename: Thomas
authors.initials: T
authors.lastname: Eisler
authors.forename: André
authors.initials: A
authors.lastname: Stark
authors.forename: Olle
authors.initials: O
authors.lastname: Muren
authors.forename: Nicolas
authors.initials: N
authors.lastname: Martinez-Carranza
authors.forename: Leif
authors.initials: L
authors.lastname: Ryd
mesh_terms: D019645:Arthroplasty, Replacement, Knee / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D007720:Knee Prosthesis; D009038:Motion; D011474:Prosthesis Design; D011475:Prosthesis Failure; D059869:Radiostereometric Analysis / Q000379:methods; D013499:Surface Properties