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

Lowe JA, Crist BD, Pfeiffer F, Carson WL

Journal of orthopaedic trauma, 2015

Abstract

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%).…

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