title: The biomechanics of locked plating for repairing proximal humerus fractures with or without medial cortical support.
abstract: BACKGROUND
  Comminuted proximal humerus fracture fixation is controversial. Locked plate complications have been addressed by anatomic reduction or medial cortical support. The relative mechanical contributions of varus malalignment and lack of medial cortical support are presently assessed.
  
  
  METHODS
  Forty synthetic humeri divided into three subgroups were osteotomized and fixed at 0 degrees, 10 degrees, and 20 degrees of varus malreduction with a locking proximal humerus plate (AxSOS, Global model; Stryker, Mahwah, NJ) to simulate mechanical medial support with cortical contact retained. Axial, torsional, and shear stiffness were measured. Half of the specimens in each of the three subgroups underwent a second osteotomy to create a segmental defect simulating loss of medial support with cortex removed. Axial, torsional, and shear stiffness tests were repeated, followed by shear load to failure in 20 degrees of abduction.
  
  
  RESULTS
  For isolated malreduction with cortical contact, the construct at 0 degrees showed statistically equivalent or higher axial, torsional, and shear stiffness than other subgroups examined. Subsequent removal of cortical support in half the specimens showed a drastic effect on axial, torsional, and shear stiffness at all varus angulations. Constructs with cortical contact at 0 degrees and 10 degrees yielded mean shear failure forces of 12965.4 N and 9341.1 N, respectively, being statistically higher (p < 0.05) compared with most other subgroups tested. Specimens failed primarily by plate bending as the humeral head was pushed down medially and distally.
  
  
  CONCLUSIONS
  Anatomic reduction with the medial cortical contact was the stiffest construct after a simulated two-part fracture. This study affirms the concept of medial cortical support by fixing proximal humeral fractures in varus, if absolutely necessary. This may be preferable to fixing the fracture in anatomic alignment when there is a medial fracture gap.
authors.affiliation: Division of Orthopaedics, Department of Surgery, University of Toronto, Toronto, ON, Canada.
authors.forename: Jon
authors.initials: J
authors.lastname: Lescheid
authors.forename: Rad
authors.initials: R
authors.lastname: Zdero
authors.forename: Suraj
authors.initials: S
authors.lastname: Shah
authors.forename: Paul R T
authors.initials: PR
authors.lastname: Kuzyk
authors.forename: Emil H
authors.initials: EH
authors.lastname: Schemitsch
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D017102:Fracture Healing; D006801:Humans; D011474:Prosthesis Design; D033081:Shear Strength; D012784:Shoulder Fractures / Q000503:physiopathology / Q000601:surgery*; D013314:Stress, Mechanical