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Repair and remodeling of bone during healing and fusion require a combination of bone resorption and formation to successfully restore the bone to its previous strength. The healing process is highly responsive to the mechanical conditions of the construct, where excessive loading can cause high strains that delay healing, but moderate loading can be beneficial. Maintaining compression at the site of fracture can benefit healing by maintaining bone congruency and increasing the stability of the bone-implant construct to prevent excessive shifting. For these reasons, compressive mechanisms are employed in many orthopaedic devices, including both intramedullary (IM) nails and external fixators for ankle arthrodesis applications. Tibiotalocalcaneal (TTC) arthrodesis is a salvage procedure that fuses both the ankle and the subtalar joints. It has become the standard of care in ankle degeneration, which can be brought on by posttraumatic arthritis, failed total ankle arthroplasty, or diabetic conditions such as Charcot arthropathy. While current devices are effective in many cases, TTC arthrodesis procedures still incur failure rates as high as 22%, where failure of the bones to successfully fuse can result in amputation. Because bone healing relies upon bone resorption, the initial compression applied to the implanted constructs can be quickly lost, which may sacrifice the stability of the structure and delay or inhibit further healing.
By employing a mechanism that can sustain compression during the bone healing process, it was possible to increase the stability of the construct even during bone resorption, minimizing the failures that still occur. The focus of this study was to determine the effects of compression on the mechanical stability of the implant-bone construct found in TTC arthrodesis. A comparison was made between the torsional stability of two currently marketed intramedullary devices, as well as a prototype IM device comprised of a nickel titanium core, designed to hold constant compression for up to 9mm of resorption. Additionally, the stability of each construct over time was evaluated by correlating bone resorption to a loss in compressive force.