Focal cryogen insults for inducing segmental osteonecrosis: computational and experimental assessments of thermal fields.

Orthopaedic management of femoral head osteonecrosis is a common clinical problem for which there presently is no good solution. Current animal models are inappropriate to study potential new solutions, since it has been difficult to replicate the natural history of structural collapse seen in the human disorder. Recently, progression to collapse was obtained for cryogenically induced osteonecrosis in emus, although the lesions involved were imprecisely controlled in terms of size or location. A new cryo-insult probe is here reported for the purpose of delivering well-prescribed local thermal insults in this new animal model, while minimizing damage to non-targeted regions. Finite element analysis was used to elucidate the influence of operator-controlled parameters upon the temporal/spatial variation of the thermal field. The numerical formulation includes convective heat transfer attributable to tissue bed perfusion. The computational results agreed closely with the results of thermocouple recordings in a companion bench-top experiment. The cryo-insult probe successfully produced segmental lesions in the emu model of sizes comparable to the computed freeze front diameters.