Predicting Reduction in Torsional Strength by Concentric/Eccentric RIA Reaming Normal and Osteoporotic Long Bones (Femurs).

Abstract

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. 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. 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). 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.