title: Resurfacing head size and femoral fracture: Are registry conclusions on head size justified?
abstract: BACKGROUND
  Joint registries report that peri-prosthetic fractures are the most common reason for early revision of a hip resurfacing arthroplasty (HRA) and are twice as likely with small implant sizes. However, a national survey found peri-prosthetic fracture to be strongly associated with surgical accuracy. We therefore asked whether the force required to induce a peri-prosthetic fracture: (1) was significantly lower when using smaller implants and (2) correlated to the size of implant used, when surgery was performed accurately.
  
  
  METHODS
  To ensure an adequate power, we calculated our sample size from pilot data. Forty-four femurs were tested in two experiments. The first experiment tested femurs with either a small (48 mm) or a large (54 mm) HRA implant. The second involved testing femurs with a range of implant sizes. A rapid prototyped femur-specific guide ensured accurate implantation. Specimens were then vertically loaded in a servo-hydraulic testing machine till fracture. Displacement (mm) and force (N) required for fracture were recorded.
  
  
  RESULTS
  A median force of 1081 N was required to fracture specimens implanted with small 48-mm heads, while 1134 N was required when a 54-mm head was used (U = 77, z = -0.054, p = 0.957). Implant head size and force required to fracture were not related, r = 0.12, p = 0.63.
  
  
  CONCLUSIONS
  The force required to induce a resurfacing peri-prosthetic fracture was not related to the size of the implant. The increased failure rate seen in all registries is unlikely to be directly the result of this single variable. Correctly performed resurfacing arthroplasty is highly resistant to fracture.
authors.affiliation: MSK Lab, Imperial College London, Charing Cross Hospital, Fullham Palace Road, London, W6 8RF, UK. a.aqil@imperial.ac.uk.
authors.forename: Adeel
authors.initials: A
authors.lastname: Aqil
authors.affiliation: MSK Lab, Imperial College London, Charing Cross Hospital, Fullham Palace Road, London, W6 8RF, UK.
authors.forename: Anatole
authors.initials: A
authors.lastname: Wiik
authors.affiliation: MSK Lab, Imperial College London, Charing Cross Hospital, Fullham Palace Road, London, W6 8RF, UK.
authors.forename: Susannah
authors.initials: S
authors.lastname: Clarke
authors.affiliation: MSK Lab, Imperial College London, Charing Cross Hospital, Fullham Palace Road, London, W6 8RF, UK.
authors.forename: Milad
authors.initials: M
authors.lastname: Masjedi
authors.affiliation: MSK Lab, Imperial College London, Charing Cross Hospital, Fullham Palace Road, London, W6 8RF, UK.
authors.forename: Justin
authors.initials: J
authors.lastname: Cobb
mesh_terms: D000077:Acetabulum / Q000502:physiology; D001696:Biomechanical Phenomena / Q000502:physiology; D005264:Femoral Fractures / Q000503:physiopathology / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D008953:Models, Anatomic; D057068:Periprosthetic Fractures / Q000209:etiology / Q000503:physiopathology*; D010865:Pilot Projects; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012086:Reoperation
keywords: Failure; Femoral fracture; Head size; Resurfacing