High correlation between mechanical properties and bone mineral parameters in embalmed femurs after long-term storage

Fresh frozen human femurs are considered "the gold standard" in biomechanical studies of hip fractures, resembling the in vivo situation mostly. A more readily available alternative is formalin embalmed femurs. However, to which extent formalin affects key features of bone; its mechanical properties, bone mineral content and their mutual relationship over time, remains unknown. Accordingly, we measured the mineral parameters and related them to the mechanical properties of formalin fixed femurs after long-term storage.

36 paired femurs from human donors, fixed in formalin and stored for a mean period of 4.6 (3.5-6) years. Quantitative CT was performed to measure the bone mineral density and mass at the mainly cortical mid shaft and the center of the mainly cancellous condyles. Each pair was subjected to local tests by three-point bending and screw pullout of the shaft and lateral punch and metaphyseal cube compression of the condyles.

Neither mechanical nor bone mineral data were significantly correlated to storage time. Well-known associations for bone parameters with age and gender were retrieved. Maximum force of the cortical bone tests was highly correlated to the diaphyseal bone mass; (r = 0.80-0.87, p = 0.01), while maximum force of the cancellous bone tests correlated well to the density of the condylar bone; (r = 0.70, p = 0.01).

Our results indicate that mechanical and bone mineral data and their mutual relationship are conserved in formalin fixed femurs even after long-term storage. Formalin fixed femurs may serve as an alternative to fresh frozen femurs in biomechanical testing.