title: Predicting Reduction in Torsional Strength by Concentric/Eccentric RIA Reaming Normal and Osteoporotic Long Bones (Femurs).
abstract: OBJECTIVE
  Reamer-irrigator-aspirator (RIA) bone graft harvesting human femurs have resulted in spiral fractures. Recommendations/studies on safe RIA diameter are noncomprehensive/inconclusive. Our purpose was to develop an analytical model to predict % reduction in torsional strength after intramedullary oversize reaming (concentric or eccentric) and to test cadaveric femurs with normal-to-osteoporotic bone mineral density to investigate its relationship to femur cross-sectional properties, bone material strength, and torsional strength reduction.
  
  
  METHODS
  An eccentric circular cross-sectional model was developed. Twenty matched cadaveric femurs (8-normal, 6-osteopenic, and 6-osteoporotic) were tested. Left femur was reamed 1.5 mm larger than isthmic endosteal diameter measured from radiographs. Right (control) and left (reamed) femurs were torsionally loaded to failure. Periosteal-endosteal circles were fit tangent to the thinnest wall on computer tomography cross-sectional image at level of spiral fracture to determine periosteal-endosteal circle diameters (dp, de) and their eccentricity (e), and to calculate cross section's size-eccentricity factor. These and torque failure were substituted into the model to calculate bone material's effective tensile strength.
  
  
  RESULTS
  Osteoporotic compared with normal femurs had higher de/dp ratio (0.71 vs. 0.47), lower cross-sectional size-eccentricity factor (-41%), bone material strength (-57%), and torsional strength (-73%). Predicted % reduction in torsional strength by either concentric over reaming or canal eccentricity exponentially increases with increase in de/dp (notably beyond 0.47 of normal bone mineral density).
  
  
  CONCLUSIONS
  Manufacturer's recommended 1.5 mm oversize RIA is conservative (<10% reduction) if concentric in femurs with isthmus de/dp < 0.60 mm and dp > 18.3 mm. An eccentric canal can significantly compromise a long bone's torsional strength, more than if reamed concentric to larger diameter having same minimum wall thickness. Other "safe/unsafe" oversize-eccentricity conditions are in quick clinical reference tables.
authors.affiliation: *Division of Orthopaedics, Section Trauma, University of Alabama at Birmingham, Birmingham, AL; †Department of Orthopaedic Surgery, University of Missouri, Columbia, MO; and ‡Comparative Orthopaedic Laboratory, University of Missouri, Columbia, MO.
authors.forename: Jason A
authors.initials: JA
authors.lastname: Lowe
authors.forename: Brett D
authors.initials: BD
authors.lastname: Crist
authors.forename: Ferris
authors.initials: F
authors.lastname: Pfeiffer
authors.forename: William L
authors.initials: WL
authors.lastname: Carson
mesh_terms: D000368:Aged; D016025:Bone Transplantation / Q000379:methods*; D019245:Compressive Strength; D003198:Computer Simulation; D055119:Elastic Modulus; D005260:Female; D005269:Femur / Q000503:physiopathology* / Q000601:surgery*; D006801:Humans; D008297:Male; D008954:Models, Biological; D010024:Osteoporosis / Q000601:surgery*; D011379:Prognosis; D013314:Stress, Mechanical; D013718:Tensile Strength; D019415:Torque