title: Locked vs. unlocked plate osteosynthesis of the proximal humerus - a biomechanical study.
abstract: BACKGROUND
  Locked plates (internal fixators) have been found to be an optimal method for the fixation in proximal humeral fractures. In a biomechanical cadaver study the difference between locked and non-locked osteosyntheses was investigated.
  
  
  METHODS
  Paired humeri were harvested, bone density measured. Locked internal fixators were mounted on one specimen; identical plate-screw-systems without locking mechanism applied to the contralateral specimen for comparison. After that, a transverse subcapital osteotomy was performed. With 7 pairs of humeri static tests with increasing axial loads and with 5 pairs dynamic tests with 10 N preload and 80 N maximal axial load for up to 1 million cycles were performed.
  
  
  RESULTS
  In the static experiments the elastic stiffness of the construct was 74% higher in the locked group (median 80 N/mm, quartile range 77-86 N/mm) compared with the non-locked group (46 N/mm, 35.5-56.5 N/mm). The difference was statistically significant (Wilcoxon test for paired samples, P<0.05). Similarly, the linear range until failure was definitely extended in the locked group by 64% (92 N, 89-98 N vs. 56 N, 36.5-73.5 N, P<0.05). Under dynamic loading the non-locked group showed fixation failures between 97,000 and 500,000 cycles. In the locked group no failure was observed until the end of the experiment at 1 million cycles (P<0.0.5). The final deformation was found to be 1 mm (median, quartil range 1.0-1.2 mm) in the non-locked group and 0.3 mm (0.2-0.3 mm) in the locked group (P<0.05). The differences were found equally in lower as well as in higher bone density specimen.
  
  
  CONCLUSIONS
  Because of the optimal load transfer between implant and cancellous bone, a locked screw plate interface will reduce fixation failure in proximal humeral fractures.
authors.affiliation: Berufsgenossenschaftliches Unfallkrankenhaus (Trauma Hospital) Hamburg, Bergedorfer Strasse 10, D-21033 Hamburg, Germany. k.seide@buk-hamburg.de
authors.forename: K
authors.initials: K
authors.lastname: Seide
authors.forename: J
authors.initials: J
authors.lastname: Triebe
authors.forename: M
authors.initials: M
authors.lastname: Faschingbauer
authors.forename: A P
authors.initials: AP
authors.lastname: Schulz
authors.forename: K
authors.initials: K
authors.lastname: Püschel
authors.forename: G
authors.initials: G
authors.lastname: Mehrtens
authors.forename: Ch
authors.initials: Ch
authors.lastname: Jürgens
mesh_terms: D001696:Biomechanical Phenomena / Q000379:methods*; D015519:Bone Density; D001860:Bone Plates; D001863:Bone Screws; D001842:Bone and Bones / Q000473:pathology; D004548:Elasticity; D004867:Equipment Design; D005593:Fracture Fixation, Internal / Q000379:methods*; D005594:Fracture Fixation, Intramedullary; D006801:Humans; D006811:Humerus / Q000473:pathology; D016268:Internal Fixators; D011475:Prosthesis Failure; D012784:Shoulder Fractures / Q000628:therapy*