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

Objectives: 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.