title: Is the rotation of the femoral head a potential initiation for cutting out? A theoretical and experimental approach.
abstract: BACKGROUND
  Since cut-out still remains one of the major clinical challenges in the field of osteoporotic proximal femur fractures, remarkable developments have been made in improving treatment concepts. However, the mechanics of these complications have not been fully understood.We hypothesize using the experimental data and a theoretical model that a previous rotation of the femoral head due to de-central implant positioning can initiate a cut-out.
  
  
  METHODS
  In this investigation we analysed our experimental data using two common screws (DHS/Gamma 3) and helical blades (PFN A/TFN) for the fixation of femur fractures in a simple theoretical model applying typical gait pattern on de-central positioned implants. In previous tests during a forced implant rotation by a biomechanical testing machine in a human femoral head the two screws showed failure symptoms (2-6Nm) at the same magnitude as torques acting in the hip during daily activities with de-central implant positioning, while the helical blades showed a better stability (10-20Nm).To calculate the torque of the head around the implant only the force and the leverarm is needed (N [Nm] = F [N] * × [m]). The force F is a product of the mass M [kg] multiplied by the acceleration g [m/s2]. The leverarm is the distance between the center of the head of femur and the implant center on a horizontal line.
  
  
  RESULTS
  Using 50% of 75 kg body weight a torque of 0.37Nm for the 1 mm decentralized position and 1.1Nm for the 3 mm decentralized position of the implant was calculated. At 250% BW, appropriate to a normal step, torques of 1.8Nm (1 mm) and 5.5Nm (3 mm) have been calculated.Comparing of the experimental and theoretical results shows that both screws fail in the same magnitude as torques occur in a more than 3 mm de-central positioned implant.
  
  
  CONCLUSIONS
  We conclude the center-center position in the head of femur of any kind of lag screw or blade is to be achieved to minimize rotation of the femoral head and to prevent further mechanical complications.
authors.affiliation: Department of Trauma Surgery, Klinikum Augsburg, Augsburg, Germany. lenich@uchir.me.tum.de
authors.forename: Andreas
authors.initials: A
authors.lastname: Lenich
authors.forename: Samuel
authors.initials: S
authors.lastname: Bachmeier
authors.forename: Lukas
authors.initials: L
authors.lastname: Prantl
authors.forename: Michael
authors.initials: M
authors.lastname: Nerlich
authors.forename: Jochen
authors.initials: J
authors.lastname: Hammer
authors.forename: Edgar
authors.initials: E
authors.lastname: Mayr
authors.forename: Amir Andreas
authors.initials: AA
authors.lastname: Al-Munajjed
authors.forename: Bernd
authors.initials: B
authors.lastname: Füchtmeier
mesh_terms: D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D001835:Body Weight; D001863:Bone Screws; D002102:Cadaver; D019544:Equipment Failure Analysis; D005270:Femur Head / Q000293:injuries / Q000601:surgery*; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006620:Hip Fractures / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D008297:Male; D008422:Materials Testing; D008962:Models, Theoretical; D011474:Prosthesis Design; D011475:Prosthesis Failure; D016059:Range of Motion, Articular; D012399:Rotation; D019415:Torque; D016474:Weight-Bearing