title: Where do locking screws purchase in the humeral head?
abstract: BACKGROUND
  One of the limiting factors in finding the best osteosynthesis approach in proximal humerus fractures is the current lack of information on the properties of the cancellous bone regions engaged by the implants fixing the epiphysis. The aim of this study is to assess the densitometric and mechanical characteristics of these regions when using a proximal humerus locking plate (PHLP).
  
  
  METHODS
  Nineteen PHLPs were mounted on cadaveric humeri using only their three most distal screws. Subsequently, the plates were removed and the bones were scanned using high-resolution peripheral quantitative computed tomography. Bone mineral density (BMD) was determined in the intact proximal epiphysis and in the exact locations where the six proximal screws would have been positioned concluding the instrumentation. Each plate was then repositioned on its bone and a minimally destructive local torque measurement was performed in the same six locations. A statistical analysis was performed to detect significant differences in the investigated parameters between screw positions, and to test the ability of local torque values to discriminate the bone mineral density of the entire humeral head (BMD(TOT)).
  
  
  RESULTS
  Novel data about the cancellous bone engaged by the screws of a PHLP are provided. Different epiphyseal locations showed statistically significant different properties. A local torque measurement was a good predictor of the BMD(TOT).
  
  
  CONCLUSIONS
  Position and direction of the epiphyseal screws on a locking implant are determinant to engage bone regions with significantly better bone quality. A breakaway torque measurement in a given screw position can distinguish between humeral heads with different densitometric properties.
authors.affiliation: AO Research Institute, Clavadelerstrasse 8, 7270 Davos Platz, Switzerland. stefano.brianza@aofoundation.org
authors.forename: Stefano
authors.initials: S
authors.lastname: Brianza
authors.forename: Götz
authors.initials: G
authors.lastname: Röderer
authors.forename: Damiano
authors.initials: D
authors.lastname: Schiuma
authors.forename: Ronald
authors.initials: R
authors.lastname: Schwyn
authors.forename: Alexander
authors.initials: A
authors.lastname: Scola
authors.forename: Florian
authors.initials: F
authors.lastname: Gebhard
authors.forename: Andrea E
authors.initials: AE
authors.lastname: Tami
mesh_terms: D000368:Aged; D001696:Biomechanical Phenomena; D015519:Bone Density; D001860:Bone Plates; D001863:Bone Screws; D002102:Cadaver; D005260:Female; D005593:Fracture Fixation, Internal / Q000379:methods*; D006801:Humans; D058430:Humeral Head / Q000000981:diagnostic imaging / Q000473:pathology / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011475:Prosthesis Failure; D011859:Radiography; D012784:Shoulder Fractures / Q000000981:diagnostic imaging / Q000601:surgery*; D019415:Torque