title: Biomechanical in vitro assessment of screw augmentation in locked plating of proximal humerus fractures.
abstract: BACKGROUND
  Proximal humerus fracture fixation can be difficult because of osteoporosis making it difficult to achieve stable implant anchorage in the weak bone stock even when using locking plates. This may cause implant failure requiring revision surgery. Cement augmentation has, in principle, been shown to improve stability. The aim of this study was to investigate whether augmentation of particular screws of a locking plate aimed at a region of low bone quality is effective in improving stability in a proximal humerus fracture model.
  
  
  METHODS
  Twelve paired human humerus specimens were included. Quantitative computed tomography was performed to determine bone mineral density (BMD). Local bone quality in the direction of the six proximal screws of a standard locking plate (PHILOS, Synthes) was assessed using mechanical means (DensiProbe™). A three-part fracture model with a metaphyseal defect was simulated and fixed with the plate. Within each pair of humeri the two screws aimed at the region of the lowest bone quality according to the DensiProbe™ were augmented in a randomised manner. For augmentation, 0.5 ml of bone cement was injected in a screw with multiple outlets at its tip under fluoroscopic control. A cyclic varus-bending test with increasing upper load magnitude was performed until failure of the screw-bone fixation.
  
  
  RESULTS
  The augmented group withstood significantly more load cycles. The correlation of BMD with load cycles until failure and BMD with paired difference in load cycles to failure showed that augmentation could compensate for a low BMD.
  
  
  CONCLUSIONS
  The results demonstrate that augmentation of screws in locked plating in a proximal humerus fracture model is effective in improving primary stability in a cyclic varus-bending test. The augmentation of two particular screws aimed at a region of low bone quality within the humeral head was almost as effective as four screws with twice the amount of bone cement. Screw augmentation combined with a knowledge of the local bone quality could be more effective in enhancing the primary stability of a proximal humerus locking plate because the effect of augmentation can be exploited more effectively limiting it to the degree required.
authors.affiliation: Ulm University, Department of Orthopaedic Trauma, Ulm, Germany. goetz.roederer@uniklinik-ulm.de
authors.forename: Götz
authors.initials: G
authors.lastname: Röderer
authors.forename: Alexander
authors.initials: A
authors.lastname: Scola
authors.forename: Werner
authors.initials: W
authors.lastname: Schmölz
authors.forename: Florian
authors.initials: F
authors.lastname: Gebhard
authors.forename: Markus
authors.initials: M
authors.lastname: Windolf
authors.forename: Ladina
authors.initials: L
authors.lastname: Hofmann-Fliri
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D001843:Bone Cements / Q000032:analysis*; D015519:Bone Density; D001860:Bone Plates / Q000706:statistics & numerical data*; D001863:Bone Screws / Q000706:statistics & numerical data*; D002102:Cadaver; D005260:Female; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D008297:Male; D010024:Osteoporosis / Q000150:complications; D012784:Shoulder Fractures / Q000601:surgery*
chemical_list: D001843:Bone Cements
keywords: Augmentation; Fracture; Locking plate; Osteoporosis; Proximal humerus