title: Do Bone Graft and Cracking of the Sclerotic Cavity Improve Fixation of Titanium and Hydroxyapatite-coated Revision Implants in an Animal Model?
abstract: BACKGROUND
  We previously introduced a manual surgical technique that makes small perforations (cracks) through the sclerotic bone shell that typically forms during the process of aseptic loosening ("crack" revision technique). Perforating just the shell (without violating the proximal cortex) can maintain overall bone continuity while allowing marrow and vascular elements to access the implant surface. Because many revisions require bone graft to fill defects, we wanted to determine if bone graft could further increase implant fixation beyond what we have experimentally shown with the crack technique alone. Also, because both titanium (Ti6Al4V) and hydroxyapatite (HA) implant surfaces are used in revisions, we also wanted to determine their relative effectiveness in this model.
  
  
  OBJECTIVE
  We hypothesized that both (1) allografted plasma-sprayed Ti6Al4V; and (2) allografted plasma-sprayed HA-coated implants inserted with a crack revision technique have better fixation compared with a noncrack revision technique in each case.
  
  
  METHODS
  Under approval from our Institutional Animal Care and Use Committee, a female canine animal model was used to evaluate the uncemented revision technique (crack, noncrack) using paired contralateral implants while implant surface (Ti6Al4V, HA) was qualitatively compared between the two (unpaired) series. All groups received bone allograft tightly packed around the implant. This revision model includes a cylindrical implant pistoning 500 μm in a 0.75-mm gap, with polyethylene particles, for 8 weeks. This engenders a bone and tissue response representative of the metaphyseal cancellous region of an aseptically loosened component. At 8 weeks, the original implants were revised and followed for an additional 4 weeks. Mechanical fixation was assessed by load, stiffness, and energy to failure when loaded in axial pushout. Histomorphometry was used to determine the amount and location of bone and fibrous tissue in the grafted gap.
  
  
  RESULTS
  The grafted crack revision improved mechanical shear strength, stiffness, and energy to failure (for Ti6Al4V 27- to 69-fold increase and HA twofold increases). The histomorphometric analysis demonstrated primarily fibrous membrane ongrowth and in the gap for the allografted Ti6Al4V noncrack revisions. For allografted HA noncrack revisions, bone ongrowth at the implant surface was observed, but fibrous tissue also was present in the inner gap. Although both Ti6Al4V and HA surfaces showed improved fixation with grafted crack revision, and Ti6Al4V achieved the highest percent gain, HA demonstrated the strongest overall fixation.
  
  
  CONCLUSIONS
  The results of this study suggest that novel osteoconductive or osteoinductive coatings and bone graft substitutes or tissue-engineered constructs may further improve bone-implant fixation with the crack revision technique but require evaluation in a rigorous model such as presented here.
  
  
  CONCLUSIONS
  This experimental study provides data on which to base clinical trials aimed to improve fixation of revision implants. Given the multifactorial nature of complex human revisions, such a protocoled clinical study is required to determine the clinical applicability of this approach.
authors.affiliation: Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Brian
authors.initials: B
authors.lastname: Elmengaard
authors.affiliation: Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Joergen
authors.initials: J
authors.lastname: Baas
authors.affiliation: Department of Orthopaedic Surgery, Aalborg University Hospital, Aalborg, Denmark.
authors.forename: Thomas
authors.initials: T
authors.lastname: Jakobsen
authors.affiliation: Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Soren
authors.initials: S
authors.lastname: Kold
authors.affiliation: Department of Plastic Surgery, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Jensen
authors.affiliation: Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Aarhus University Hospital, Aarhus, Denmark. bechtold@umn.edu.|Department of Orthopaedic Surgery, Minneapolis Medical Research Foundation, University of Minnesota, Minneapolis, MN, USA. bechtold@umn.edu.|Orthopaedic Biomechanics Laboratory, Minneapolis Medical Research Foundation, Life Sciences Building, Suite 118, 700 South 10th Avenue, Minneapolis, MN, 55415, USA. bechtold@umn.edu.
authors.forename: Joan E
authors.initials: JE
authors.lastname: Bechtold
authors.affiliation: Orthopaedic Research Laboratory, Department of Orthopaedic Surgery, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Kjeld
authors.initials: K
authors.lastname: Soballe
mesh_terms: D000818:Animals; D001178:Arthroplasty / Q000379:methods*; D018786:Bone Substitutes; D016025:Bone Transplantation / Q000379:methods*; D020099:Coated Materials, Biocompatible; D004285:Dogs; D017886:Durapatite; D005260:Female; D023421:Models, Animal; D016348:Osseointegration; D012086:Reoperation; D014025:Titanium
chemical_list: D018786:Bone Substitutes; D020099:Coated Materials, Biocompatible; D017886:Durapatite; D014025:Titanium