title: Variability of the pullout strength of cancellous bone screws with cement augmentation.
abstract: BACKGROUND
  Orthopaedic surgeons often face clinical situations where improved screw holding power in cancellous bone is needed. Injectable calcium phosphate cements are one option to enhance fixation.
  
  
  METHODS
  Paired screw pullout tests were undertaken in which human cadaver bone was augmented with calcium phosphate cement. A finite element model was used to investigate sensitivity to screw positional placement.
  
  
  RESULTS
  Statistical analysis of the data concluded that the pullout strength was generally increased by cement augmentation in the in vitro human cadaver tests. However, when comparing the individual paired samples there were surprising results with lower strength than anticipated after augmentation, in apparent contradiction to the generally expected conclusion. Investigation using the finite element model showed that these strength reductions could be accounted for by small screw positional changes. A change of 0.5mm might result in predicted pullout force changes of up to 28%.
  
  
  CONCLUSIONS
  Small changes in screw position might lead to significant changes in pullout strength sufficient to explain the lower than expected individual pullout values in augmented cancellous bone. Consequently whilst the addition of cement at a position of low strength would increase the pullout strength at that point, it might not reach the pullout strength of the un-augmented paired test site. However, the overall effect of cement augmentation produces a significant improvement at whatever point in the bone the screw is placed. The use of polymeric bone-substitute materials for tests may not reveal the natural variation encountered in tests using real bone structures.
authors: Stryker Osteosynthesis, Selzach, Switzerland; School of Engineering and Design, Brunel University, Uxbridge, UK.
authors: P
authors: P
authors: Procter
authors: School of Engineering and Design, Brunel University, Uxbridge, UK.
authors: P
authors: P
authors: Bennani
authors: School of Engineering and Design, Brunel University, Uxbridge, UK. Electronic address: chris.brown@brunel.ac.uk.
authors: C J
authors: CJ
authors: Brown
authors: Clinic of Plastic and Hand Surgery, University Hospital Bern, Bern, Switzerland.
authors: J
authors: J
authors: Arnoldi
authors: Laboratory of Biomechanical Orthopedics, EPFL, Lausanne, Switzerland.
authors: D P
authors: DP
authors: Pioletti
authors: Department of Orthopaedics, Uppsala University Hospital, Uppsala, Sweden.
authors: S
authors: S
authors: Larsson
mesh_terms: D001696:Biomechanical Phenomena; D001843:Bone Cements / Q000627:therapeutic use*; D001863:Bone Screws; D002102:Cadaver; D002130:Calcium Phosphates / Q000627:therapeutic use*; D004868:Equipment Failure; D005269:Femur / Q000601:surgery; D020342:Finite Element Analysis; D006801:Humans; D008422:Materials Testing; D013718:Tensile Strength
chemical_list: D001843:Bone Cements; D002130:Calcium Phosphates; C020243:calcium phosphate
keywords: Bone screws; Cancellous bone; Finite element modelling; Human cadaver; Pullout testing