title: Monoaxial versus polyaxial locking systems: a biomechanical analysis of different locking systems for the fixation of proximal humeral fractures.
abstract: OBJECTIVE
  The development of locking plate systems has led to polyaxial screws and new plate designs. This study compares monoaxial head locking screws (PHILOS© by Synthes) and a new generation of polyaxial locking screws (NCB-LE© by Zimmer) with respect to biomechanical stability.
  
  
  METHODS
  On nine pairs of randomised formalin fixed humerus specimens, standardised osteotomies and osteosyntheses with nine monoaxial (group A) und nine polyaxial (group B) plate/screw systems were performed. A material testing machine by Instron (M-10 14961-DE) was used for cyclic stress tests and crash tests until defined breakup criteria as endpoints were reached.
  
  
  RESULTS
  After axial cyclic stress 200 times at 90 N, plastic deformation was 1.02 mm in group A and 1.25 mm in group B. After the next cycle using 180 N the additional deformation averaged 0.23 mm in group A and 0.39 mm in group B. The deformation using 450 N was 0.72 mm in group A compared to 0.92 mm in group B. The final full power test resulted in a deformation average of 0.49 mm in group A and 0.63 mm in group B after 2,000 cycles using 450 N. When reaching the breakup criteria the plastic deformation of the NCB plate was 9.04 mm on average. The PHILOS plate was similarly deformed by 9.00 mm. As a result of the crash test, in group A the screws pulled out of the humeral head four times whereas the shaft broke one time and another time the implant was ripped out. The gap was closed four times. In group B, there were three cases of screw cut-through, four shaft fractures/screw avulsions from the shaft and two cases of gap closure.
  
  
  CONCLUSIONS
  The two systems resist the cyclic duration tests and the increasing force tests in a similar manner. The considerable clinical benefits of the polyaxial system are enhanced by equal biomechanical performance.
authors.affiliation: Department of Trauma-, Hand- and Reconstructive Surgery, University Hospital Giessen and Marburg GmbH, Baldingerstraße, 35041 Marburg, Germany. zettl@med.uni-marburg.de
authors.forename: Ralph
authors.initials: R
authors.lastname: Zettl
authors.forename: Thorben
authors.initials: T
authors.lastname: Müller
authors.forename: Tobias
authors.initials: T
authors.lastname: Topp
authors.forename: Ulrike
authors.initials: U
authors.lastname: Lewan
authors.forename: Antonio
authors.initials: A
authors.lastname: Krüger
authors.forename: Christian
authors.initials: C
authors.lastname: Kühne
authors.forename: Steffen
authors.initials: S
authors.lastname: Ruchholtz
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D002102:Cadaver; D004548:Elasticity; D019544:Equipment Failure Analysis; D005269:Femur / Q000601:surgery; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods*; D006620:Hip Fractures / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D010027:Osteotomy; D011474:Prosthesis Design; D016474:Weight-Bearing