title: Finite element analysis of the initial stability of ankle arthrodesis with internal fixation: flat cut versus intact joint contours.
abstract: OBJECTIVE
  Qualitative comparison of the initial stability provided by two joint preparation techniques and various screw configurations in ankle arthrodesis, using the finite element method.Design. A three-dimensional model of a healthy ankle was developed from computed tomography images. Two groups of models were built, one with the joint contours resected to produce flat surfaces, and the second with the joint contours preserved. In each case, a variety of screw orientations were examined.
  
  
  BACKGROUND
  Despite the improved results of ankle arthrodesis, failure rates due to non-union are still reported. The initial stability of the arthrodesis construct seems important in the final outcome of the fusion.
  
  
  METHODS
  Non-linear contact finite element analyses were performed in the arthrodesis constructs subjected to internal/external torsion and dorsiflexion. Micromotions at the bone-to-bone interface were calculated for frictionless and Coulomb friction contact, and compared for the two joint preparation techniques and screw configurations.
  
  
  RESULTS
  Overall lower peak micromotions were predicted when preserving the joint contours both in torsion and dorsiflexion. For both preparation techniques, the lowest micromotions tended to occur with the screws inserted at 30 degrees with respect to the long axis of the tibia, crossing above the fusion site. Inclusion of friction in the models caused a general decrease on the magnitude of the micromotions as compared to the frictionless case, but did not affect the ranking of the models.
  
  
  CONCLUSIONS
  The finite element method can be used as a qualitative tool to study the initial stability of ankle arthrodesis, overcoming the difficulties of measuring bone-to-bone interface micromotions experimentally. Better initial stability was predicted for ankle arthrodesis when the joint contours were preserved rather than resected. Crossing the screws above the fusion site at a steeper angle also tended to increase the stability at the fusion site.
  
  
  CONCLUSIONS
  Finite element analyses can help during the pre-operative planning of ankle arthrodesis. When bone density is not compromised, preserving the joint contour and inserting the screws at less than 45 degrees to the long axis of the tibia, crossing over the arthrodesis site, may offer better initial stability.
authors.affiliation: Bioengineering Sciences Research Group, School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, UK. aalonso@soton.ac.uk
authors.forename: Ana Alonso
authors.initials: AA
authors.lastname: Vázquez
authors.forename: Henrik
authors.initials: H
authors.lastname: Lauge-Pedersen
authors.forename: Lars
authors.initials: L
authors.lastname: Lidgren
authors.forename: Mark
authors.initials: M
authors.lastname: Taylor
mesh_terms: D000843:Ankle Joint / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery; D001174:Arthrodesis / Q000009:adverse effects / Q000295:instrumentation / Q000379:methods*; D001863:Bone Screws / Q000145:classification*; D019544:Equipment Failure Analysis / Q000379:methods*; D020342:Finite Element Analysis; D017276:Friction; D006801:Humans; D016268:Internal Fixators / Q000145:classification*; D007593:Joint Instability / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control*; D008297:Male; D008954:Models, Biological; D009038:Motion; D011475:Prosthesis Failure; D011859:Radiography; D012680:Sensitivity and Specificity; D025321:Surgery, Computer-Assisted / Q000379:methods; D019415:Torque